阅读说明：本技术 一种低硅高氮铁素体/马氏体钢坯的制造方法 (Method for manufacturing low-silicon high-nitrogen ferrite/martensite steel billet ) 是由 李峻宏 苏喜平 杜爱兵 冯伟 任媛媛 王明政 黄晨 杨勇 刘兴民 杨孔雳 张东辉 于 2020-11-13 设计创作，主要内容包括：本发明属于冶金材料技术领域,涉及一种低硅高氮铁素体/马氏体钢坯的制造方法。所述的制造方法依次包括如下步骤：(1)低硅高氮铁素体/马氏体钢配料真空感应炉冶炼；(2)电渣重熔；(3)重熔锭锻造：采用真空感应加电渣重熔,在冶炼过程中不添加稀土元素,并在铸锭加热后先后进行快锻机锻造、精锻机锻造、锻后退火。利用本发明的低硅高氮铁素体/马氏体钢坯的制造方法,能够得到冲击性能和抗辐照脆化性能更好的铁素体/马氏体钢坯,用于后续快堆堆芯组件用六角形外套管的加工和制造。(The invention belongs to the technical field of metallurgical materials, and relates to a manufacturing method of a low-silicon high-nitrogen ferrite/martensite steel billet. The manufacturing method sequentially comprises the following steps: (1) smelting the low-silicon high-nitrogen ferrite/martensite steel by using a vacuum induction furnace; (2) electroslag remelting; (3) and (3) remelting ingot forging: remelting by adopting vacuum induction and electroslag, adding no rare earth element in the smelting process, and successively forging by a quick forging machine, forging by a fine forging machine and annealing after forging after heating the cast ingot. By utilizing the method for manufacturing the low-silicon high-nitrogen ferrite/martensite steel billet, the ferrite/martensite steel billet with better impact property and radiation embrittlement resistance can be obtained and used for the subsequent processing and manufacturing of the hexagonal outer sleeve for the fast reactor core assembly.)

1. A method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet is characterized by sequentially comprising the following steps of:

(1) smelting the low-silicon high-nitrogen ferrite/martensite steel by using a vacuum induction furnace;

(2) electroslag remelting;

(3) and (3) remelting ingot forging: remelting by adopting vacuum induction and electroslag, adding no rare earth element in the smelting process, and successively forging by a quick forging machine, forging by a fine forging machine and annealing after forging after heating the cast ingot.

2. The manufacturing method according to claim 1, characterized in that: in the step (1), the vacuum induction furnace is used for smelting and selecting raw materials,

the surface of the raw material is not allowed to have oxidized skin, oil stain, water stain and sand debris;

selecting refined steel materials with low S, P, As, Sn, Sb and Pb contents, and controlling the S content in the metal manganese to be less than 0.010 wt.%.

3. The manufacturing method according to claim 2, characterized in that: the contents of S, P, As, Sn, Sb and Pb in the refined steel material with low contents of S, P, As, Sn, Sb and Pb in percentage by weight are respectively less than or equal to 0.0040, less than or equal to 0.010, less than or equal to 0.0060, less than or equal to 0.0065, less than or equal to 0.0035 and less than or equal to 0.0010.

4. The manufacturing method according to claim 1, characterized in that: in the step (1), the vacuum induction furnace smelting comprises the working procedures of charging, melting, refining, alloying, pouring and demolding,

the ultimate vacuum of the vacuum furnace smelting chamber in an empty state is less than or equal to 1.0Pa, and the cold state gas leakage rate meets the self-checking requirement;

the smelting crucible is made of magnesium or magnesium-aluminum;

the charging sequence is as follows: carbon, nickel plates, pure iron, metal molybdenum, ferrosilicon, metal niobium, metal chromium and ferrovanadium;

transmitting power to melt to the melting point under a vacuum state, gradually increasing power supply power, adjusting the transmitted power to the melting point by referring to vacuum induction equipment, and performing component analysis at the end of a melting period;

the vacuum degree in the refining period is less than or equal to 5.0Pa, the refining temperature is controlled at 1500-1590 ℃, and the refining temperature and the refining time can be adjusted according to the contents of key elements such as O, N, Si and Al;

the early stage vacuum degree in the alloying period is less than or equal to 5.0Pa, sampling is needed in the alloying period to analyze components, fine adjustment of the components is carried out according to the analysis results of the elements of N, Si and Al, after the adjustment of the components meets the requirements, argon or other protective gas is introduced into the furnace, the pressure of the protective atmosphere meets the requirements of 5000-8000Pa, and metal manganese is added in the argon atmosphere to prevent the volatilization of the manganese;

and adjusting the pouring temperature according to the state of the mold, controlling the target temperature of the molten steel to 1530-1600 ℃ before pouring, adopting an upper pouring method as a pouring mode, and demolding after the poured electrode is completely solidified in vacuum.

5. The manufacturing method according to claim 1, characterized in that: in the step (2), the electroslag remelting comprises the working procedures of electrode and crystallizer preparation, remelting slag preparation and baking, electrode assembly, electroslag remelting, demoulding and annealing,

selecting an electroslag furnace with the capacity of 1.6-3.5 tons and selectingA crystallizer;

the nominal diameter phi of the electrode is 340mm, and the dimensional tolerance is controlled within the range of +/-20 mm; fully grinding the surface of the electrode to remove local defects such as a laminated iron sheet, an oxide skin, a flying wing, a scab and a microcrack; the head and tail of the electrode are uniform in size, the curvature is not more than 6mm/m, and the taper is not more than 10 mm/m;

CaF adopted for electroslag remelting₂-CaO-Al₂O₃Ternary or CaF₂-CaO-Al₂O₃-MgO quaternary slag system, wherein the weight ratio of the ternary slag system is 60: 27: 13, the weight ratio of the quaternary slag system is 67: 15: 16: 2; baking the slag at the temperature of 600 ℃ and 800 ℃ for 4-20 hours before use; the slag material smashing barrel is used and is taken out from the heating furnace until the use time is not more than 15 min;

the slagging parameters of the electroslag ingot are as follows: adding slag at 3000A for 25-40min, refining at 4500A for 20-30min, controlling the temperature of the effluent of the crystallizer at 40-60 deg.C, and controlling the temperature of the effluent of the water-cooling base at 5-45 deg.C;

when the slag is refined, metal electrodes are exchanged and the current is increased, namely, the smelting period is started, the smelting current and the smelting voltage are strictly controlled in the steady-state remelting stage, and the parameters are recorded once every 30 min; controlling the steady-state remelting time to be 8.5-10.5 h; the power input in the steady-state remelting stage is stable, the voltage is controlled to be 55-68V, and the current is controlled to be 9000-14000A;

fully cooling the ingot for 60-300min after electroslag remelting, and after discharging and demoulding, slowly cooling the electroslag remelting ingot in a protective cover for 48-300 h; annealing treatment is carried out within 3 days after the electroslag ingot is uncovered, the temperature of the electroslag ingot is raised at 5-100 ℃/h after the electroslag ingot is put into an annealing furnace, and the temperature is kept at 680 +/-20 ℃ for 20-100 h; then furnace cooling is carried out at 5-40 ℃/h, and the electroslag ingot is taken out of the furnace for air cooling at the temperature of 100-300 ℃.

6. The manufacturing method according to claim 1, characterized in that: in the step (3), the ingot heating comprises the procedures of charging, preheating, heating and soaking,

the temperature of the charging furnace is 100-;

preheating at a speed of 5-100 ℃/h to 800 +/-10 ℃, and keeping the temperature for 3-5 h;

raising the temperature of the cast ingot to the initial forging heating temperature of 1080-1170 ℃, wherein the temperature raising speed is 5-100 ℃/h;

when the temperature is heated to the initial forging temperature, soaking treatment is carried out for 3-5 h.

7. The manufacturing method according to claim 1, characterized in that: in the step (3), the forging of the quick forging machine comprises two stages of forging and hammering a clamp handle and forging,

for the forging and hammering clamp handle, when the ingot material is heated to the initial forging and heating temperature, soaking for 3-5h, forging and hammering the clamp handle part, returning to the furnace after forging and keeping the temperature for 1.5-4 h;

and after soaking, forging the intermediate blank by a quick forging machine for 2-6 times, wherein the heating temperature of the intermediate blank is 1050-.

8. The manufacturing method according to claim 1, characterized in that: in the step (3), the forging of the precision forging machine comprises three stages of temperature equalization, precision forging and cutting:

for the temperature equalization, the heat preservation temperature is 1050-;

for the precision forging, the intermediate forging blank is forged to the size of a finished bar by a precision forging machine in a mode of upsetting and stretching, and the forging ratio is 7-20;

and for cutting, after finish forging, performing hot flat tail treatment on the bar, and marking the end face of the bar.

9. The manufacturing method according to claim 1, characterized in that: in the step (3), the annealing after forging comprises three stages of furnace charging, soaking and cooling:

for soaking, soaking treatment is carried out for 30-100h after the temperature is heated to the annealing temperature of 680-860 ℃;

for cooling, the furnace is cooled to 100-300 ℃, and then the furnace is taken out for air cooling.

10. The method of claim 1, wherein the low-silicon high-nitrogen ferrite/martensite steel slab comprises the following components in percentage by weight:

si: 0.04 to 0.30 percent; n: 0.0040% -0.0700%; c: 0.17% -0.22%; mn: 0.40% -0.70%; p: less than or equal to 0.015 percent; s: less than or equal to 0.010 percent; ni: 0.40% -0.70%; cr: 11.00% -12.50%; mo: 0.80% -1.05%; v: 0.25% -0.35%; w: 0.40% -0.60%; nb: less than or equal to 0.05 percent; al: less than or equal to 0.020%; ti: less than or equal to 0.010 percent; zr: less than or equal to 0.010 percent; cu: less than or equal to 0.10 percent; sb: less than or equal to 0.0030 percent; sn: less than or equal to 0.0055 percent; as: less than or equal to 0.0050 percent; pb: less than or equal to 0.0010 percent; co: less than or equal to 0.015 percent; o: less than or equal to 0.0040 percent; h: less than or equal to 0.0005 percent; the balance being Fe.

Technical Field

The invention belongs to the technical field of metallurgical materials, and relates to a manufacturing method of a low-silicon high-nitrogen ferrite/martensite steel billet.

Background

As a fourth generation nuclear energy system, a sodium-cooled fast neutron reactor (hereinafter referred to as a fast reactor) is required to achieve higher burnup at a higher temperature than a heavy water reactor or a pressurized water reactor.

The hexagonal tube in the fast reactor fuel assembly is one of the most important parts in the reactor core, and is corroded by a coolant sodium and damaged by high-dose irradiation of more than 100dpa when working in a temperature range of 360-600 ℃ for a long time. Along with the increase of fuel burnup of fast reactor, the austenitic stainless steel has poor radiation swelling resistance under high radiation dose, and the radiation swelling can cause the reduction of the mechanical property of the material and cause the deformation of the hexagonal outer sleeve of the assembly. Swelling deformation of the outer sleeve can affect the extraction of the assembly, which is also one of the factors limiting the life of the assembly.

The ferrite/martensite steel has the advantages of high thermal conductivity and low thermal expansion coefficient, and the most prominent advantage is that the radiation swelling resistance of the material is far better than that of austenitic stainless steel due to the BCC lattice structure, and the ferrite/martensite steel can be used as a core component structural material to ensure that the material can keep better geometric dimension stability under the radiation condition. However, the mechanical properties of the material at high temperature are seriously reduced, particularly the high-temperature creep property, and the use of the material as a core cladding tube material is limited. Since the temperature of the outer sleeve in fast reactor is lower than the cladding temperature, ferrite/martensite steel can be used as the outer sleeve material of the component. Therefore, the hexagonal outer sleeve material of the second generation in the fast reactor basically adopts 9% -12% Cr ferrite/martensite steel with excellent radiation swelling resistance, such as HT9 used by the U.S. FFTF, EM10 used by the French PHENIX and European EFR, and EP450 used by the Russian BN600 and BN 800.

However, one problem with the use of ferritic/martensitic steels for the fast reactor outer jackets is that they are radiation brittle and, after in-reactor irradiation, the DBTT rises, possibly reaching temperatures at which the reactor is shut down and reloaded. If the component handling operation temperature is below the ductile-brittle transition temperature of the material, there is a risk of component breakage.

At present, the components and the heat treatment process of ferrite/martensite steel are researched extensively internationally, so as to improve the high-temperature strength and the durability of the material and reduce the ductile-brittle transition temperature of the material. The domestic fast reactor core assembly low-silicon high-nitrogen ferrite/martensite steel (code number CN-FMS) is characterized in that N is added, Si is controlled, and a plurality of microalloying elements (C, Mn, Ni, Cr, Mo, V, W, Nb, Al, Ti and the like) are added; the method optimizes the preparation processes of billet smelting and forging, adopts a duplex process of vacuum induction smelting (VIM) and electroslag remelting (ESR), defines the process parameters of heating temperature, deformation ratio, post-forging annealing and the like of forging (fast forging and finish forging), reduces O, S, P and other harmful elements, simultaneously controls the level of A, B, C, D non-metallic inclusions to be less than or equal to 1.0 level, and controls the level of TiN inclusions to be less than or equal to 1.0 level. By controlling the content of low Si and high N elements and increasing the forging ratio (more than or equal to 7), the impact property and the radiation embrittlement resistance of the material can be obviously improved.

Disclosure of Invention

The invention aims to provide a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet, so that the ferrite/martensite steel billet with better impact property and radiation embrittlement resistance can be obtained and is used for processing and manufacturing a hexagonal outer sleeve for a subsequent fast reactor core assembly.

To achieve this object, in a basic embodiment, the present invention provides a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel slab, the manufacturing method comprising the following steps in order:

(1) smelting the low-silicon high-nitrogen ferrite/martensite steel by using a vacuum induction furnace;

(2) electroslag remelting;

(3) and (3) remelting ingot forging: remelting by adopting vacuum induction and electroslag, adding no rare earth element in the smelting process, and successively forging by a quick forging machine, forging by a fine forging machine and annealing after forging after heating the cast ingot.

In a preferred embodiment, the present invention provides a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet, wherein in step (1), the vacuum induction furnace smelting selects raw materials,

the surface of the raw material is not allowed to have oxidized skin, oil stain, water stain and sand debris;

selecting refined steel materials with low S, P, As, Sn, Sb and Pb contents, and controlling the S content in the metal manganese to be less than 0.010 wt.%.

In a more preferred embodiment, the invention provides a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet, wherein the contents of S, P, As, Sn, Sb and Pb in the refined steel with low contents of S, P, As, Sn, Sb and Pb in percentage by weight are respectively less than or equal to 0.0040, less than or equal to 0.010, less than or equal to 0.0060, less than or equal to 0.0065, less than or equal to 0.0035 and less than or equal to 0.0010.

In a preferred embodiment, the invention provides a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet, wherein in the step (1), the vacuum induction furnace smelting comprises the procedures of charging, melting, refining, alloying, pouring and demolding,

the ultimate vacuum of the vacuum furnace smelting chamber in an empty state is less than or equal to 1.0Pa, and the cold state gas leakage rate meets the self-checking requirement;

the smelting crucible is made of magnesium or magnesium-aluminum;

the charging sequence is as follows: carbon, nickel plates, pure iron, metal molybdenum, ferrosilicon, metal niobium, metal chromium and ferrovanadium;

transmitting power to melt to the melting point under a vacuum state, gradually increasing power supply power, adjusting the transmitted power to the melting point by referring to vacuum induction equipment, and performing component analysis at the end of a melting period;

the vacuum degree in the refining period is less than or equal to 5.0Pa, the refining temperature is controlled at 1500-1590 ℃, and the refining temperature and the refining time can be adjusted according to the contents of key elements such as O, N, Si and Al;

the early stage vacuum degree in the alloying period is less than or equal to 5.0Pa, sampling is needed in the alloying period to analyze components, fine adjustment of the components is carried out according to the analysis results of the elements of N, Si and Al, after the adjustment of the components meets the requirements, argon or other protective gas is introduced into the furnace, the pressure of the protective atmosphere meets the requirements of 5000-8000Pa, and metal manganese is added in the argon atmosphere to prevent the volatilization of the manganese;

and adjusting the pouring temperature according to the state of the mold, controlling the target temperature of the molten steel to 1530-1600 ℃ before pouring, adopting an upper pouring method as a pouring mode, and demolding after the poured electrode is completely solidified in vacuum.

In a preferred embodiment, the invention provides a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet, wherein in the step (2), the electroslag remelting comprises the procedures of preparing an electrode and a crystallizer, preparing and baking remelting slag, assembling the electrode, remelting electroslag, demoulding and annealing,

selecting an electroslag furnace with the capacity of 1.6-3.5 tons and selectingA crystallizer;

the nominal diameter phi of the electrode is 340mm, and the dimensional tolerance is controlled within the range of +/-20 mm; fully grinding the surface of the electrode to remove local defects such as a laminated iron sheet, an oxide skin, a flying wing, a scab and a microcrack; the head and tail of the electrode are uniform in size, the curvature is not more than 6mm/m, and the taper is not more than 10 mm/m;

CaF adopted for electroslag remelting₂-CaO-Al₂O₃Ternary or CaF₂-CaO-Al₂O₃-MgO quaternary slag system, wherein the weight ratio of the ternary slag system is 60: 27: 13, the weight ratio of the quaternary slag system is 67: 15: 16: 2; baking the slag at the temperature of 600 ℃ and 800 ℃ for 4-20 hours before use; the slag material smashing barrel is used and is taken out from the heating furnace until the use time is not more than 15 min;

the slagging parameters of the electroslag ingot are as follows: adding slag at 3000A for 25-40min, refining at 4500A for 20-30min, controlling the temperature of the effluent of the crystallizer at 40-60 deg.C, and controlling the temperature of the effluent of the water-cooling base at 5-45 deg.C;

when the slag is refined, metal electrodes are exchanged and the current is increased, namely, the smelting period is started, the smelting current and the smelting voltage are strictly controlled in the steady-state remelting stage, and the parameters are recorded once every 30 min; controlling the steady-state remelting time to be 8.5-10.5 h; the power input in the steady-state remelting stage is stable, the voltage is controlled to be 55-68V, and the current is controlled to be 9000-14000A;

fully cooling the ingot for 60-300min after electroslag remelting, and after discharging and demoulding, slowly cooling the electroslag remelting ingot in a protective cover for 48-300 h; annealing treatment is carried out within 3 days after the electroslag ingot is uncovered, the temperature of the electroslag ingot is raised at 5-100 ℃/h after the electroslag ingot is put into an annealing furnace, and the temperature is kept at 680 +/-20 ℃ for 20-100 h; then furnace cooling is carried out at 5-40 ℃/h, and the electroslag ingot is taken out of the furnace for air cooling at the temperature of 100-300 ℃.

In a preferred embodiment, the invention provides a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet, wherein in the step (3), the ingot casting heating comprises the procedures of charging, preheating, heating and soaking,

the temperature of the charging furnace is 100-;

preheating at a speed of 5-100 ℃/h to 800 +/-10 ℃, and keeping the temperature for 3-5 h;

raising the temperature of the cast ingot to the initial forging heating temperature of 1080-1170 ℃, wherein the temperature raising speed is 5-100 ℃/h;

when the temperature is heated to the initial forging temperature, soaking treatment is carried out for 3-5 h.

In a preferred embodiment, the invention provides a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet, wherein in the step (3), the quick forging machine forging comprises two stages of forging a pliers handle and forging,

for the forging and hammering clamp handle, when the ingot material is heated to the initial forging and heating temperature, soaking for 3-5h, forging and hammering the clamp handle part, returning to the furnace after forging and keeping the temperature for 1.5-4 h;

and after soaking, forging the intermediate blank by a quick forging machine for 2-6 times, wherein the heating temperature of the intermediate blank is 1050-.

In a preferred embodiment, the present invention provides a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet, wherein in the step (3), the finish forging comprises three stages of temperature equalization, finish forging and cutting:

for the temperature equalization, the heat preservation temperature is 1050-;

for the precision forging, the intermediate forging blank is forged to the size of a finished bar by a precision forging machine in a mode of upsetting and stretching, and the forging ratio is 7-20;

and for cutting, after finish forging, performing hot flat tail treatment on the bar, and marking the end face of the bar.

In a preferred embodiment, the invention provides a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet, wherein in the step (3), the post-forging annealing comprises three stages of furnace charging, soaking and cooling:

for soaking, soaking treatment is carried out for 30-100h after the temperature is heated to the annealing temperature of 680-860 ℃;

for cooling, the furnace is cooled to 100-300 ℃, and then the furnace is taken out for air cooling.

In a preferred embodiment, the present invention provides a method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel slab, wherein the low-silicon high-nitrogen ferrite/martensite steel slab comprises the following components by weight:

si: 0.04 to 0.30 percent; n: 0.0040% -0.0700%; c: 0.17% -0.22%; mn: 0.40% -0.70%; p: less than or equal to 0.015 percent; s: less than or equal to 0.010 percent; ni: 0.40% -0.70%; cr: 11.00% -12.50%; mo: 0.80% -1.05%; v: 0.25% -0.35%; w: 0.40% -0.60%; nb: less than or equal to 0.05 percent; al: less than or equal to 0.020%; ti: less than or equal to 0.010 percent; zr: less than or equal to 0.010 percent; cu: less than or equal to 0.10 percent; sb: less than or equal to 0.0030 percent; sn: less than or equal to 0.0055 percent; as: less than or equal to 0.0050 percent; pb: less than or equal to 0.0010 percent; co: less than or equal to 0.015 percent; o: less than or equal to 0.0040 percent; h: less than or equal to 0.0005 percent; the balance being Fe.

The method has the advantages that the ferrite/martensite steel blank with better impact property and radiation embrittlement resistance can be obtained by utilizing the method for manufacturing the low-silicon high-nitrogen ferrite/martensite steel blank, and the method is used for processing and manufacturing the hexagonal outer sleeve for the fast reactor core assembly.

The invention provides a novel method for manufacturing a low-silicon high-nitrogen ferrite/martensite steel billet for a hexagonal outer sleeve of a fast reactor core assembly, aiming at the problem that the conventional ferrite/martensite steel alloy components, smelting and forging processes can not meet the requirements of a hexagonal tube material of a sodium-cooled fast reactor assembly on the content of harmful element O, P, S in the steel billet, the level of inclusions, the uniformity of structure and the improvement of irradiation resistance.

The beneficial effects of the invention are embodied in that:

(1) the invention can ensure strict chemical component control on the billet by low-silicon high-nitrogen component design and combining the pure purification smelting process of vacuum induction and electroslag remelting, and the levels of non-metallic inclusions and TiN meet the requirements;

(2) the invention can ensure that the forging structure with uniform components and structure can be obtained by forging process of multiple upsetting-drawing and soaking treatment and increasing the forging ratio, thereby providing the steel billet meeting the requirements for the subsequent preparation of the hexagonal outer sleeve for the fast reactor core assembly.

Drawings

FIG. 1 is a graph showing the results of the effect of N content on the impact absorption work and ductile-brittle transition temperature of ferritic/martensitic steel.

FIG. 2 is a graph of the fitting result of nanoindentation test data before and after irradiation of low-silicon high-nitrogen ferrite/martensitic steel according to a Nix-Gao model.

Figure 3 is a plot of the number of small punch tests before and after irradiation of a ferritic/martensitic steel sample.

Detailed Description

The manufacturing method of the low-silicon high-nitrogen ferrite/martensite steel billet comprises the following steps of (by weight percentage) 0.04-0.30% of Si, 0.0040-0.0700% of N, 0.17-0.22% of C, 0.40-0.70% of Mn, less than or equal to 0.015% of P, less than or equal to 0.010% of S, 0.40-0.70% of Ni, 11.00-12.50% of Cr, 0.80-1.05% of Mo, 0.25-0.35% of V, 0.40-0.60% of W, less than or equal to 0.05% of Nb, less than or equal to 0.020% of Al, less than or equal to 0.010% of Ti, less than or equal to 0.010% of Zr, less than or equal to 0.10% of Cu, less than or equal to 0.0030% of Sb, less than or equal to 0.0055% of Sn, less than or equal to 0.0055% of As, less than or equal to 0.0050% of Ti, less than or equal to 0.010% of Co, less than or equal to 0.0005% of Pb, and less than or equal to 0..

Compared with the materials of ASTM A826M and EN 10302, the content of Si element in the ferrite/martensite steel is lower, the content of O element is limited by adding N element, the ranges of C, P, S, Mo, Ni, Al and other elements are narrower, and therefore, the performance of the material is obviously changed. The strengthening mechanisms of ferritic/martensitic steels mainly include solid solution strengthening, precipitation strengthening and grain boundary strengthening, wherein the precipitation strengthening mainly relies on M distributed on sub-grain boundaries₂₃C₆(M is a metal element such as Cr, Fe, Mo, etc.) and MX (M is Nb, V, Ta, etc., and X is a nonmetal element C, N) phases in the subgrain. M₂₃C₆Generally about 100-200nm, mainly precipitating on lath boundaries and grain boundaries, thereby stabilizing the interface; the size of MX is about 20-80nm, and the MX is mainly distributed in the laths, plays a strong pinning effect on dislocations and prevents dislocation slippage, thereby reducing the recovery rate of a matrix. Therefore, in the design and test of material components, the content of N is increased, and more MX phase is introduced. Although high Si contents improve the high-temperature mechanical properties and the oxidation corrosion resistance in conventional ferritic/martensitic steels, they are disadvantageous in terms of structural stability (laves phase precipitation) and also in terms of the presence of a G phase (Si-rich phase) under high-irradiation use conditions. Moreover, Si does not strengthen, and the main function of the Si is to deoxidize and reduce the oxygen content during smelting. So that the Si content is particularly reduced in the material composition design and material preparation. Meanwhile, the content of the delta-ferrite of the finished product is lower through the content design of each element, the finished product is basically a martensite structure, and the comprehensive performance of the material is improved.

The content of O, P, S harmful elements is strictly controlled (O is less than or equal to 0.0040%, P is less than or equal to 0.015%, S is less than or equal to 0.010%), the level of A, B, C, D non-metallic inclusions is controlled to be less than or equal to 1.0 level, and TiN inclusions are controlled to be less than or equal to 1.0 level. Through the components, process design and test effect, the low-Si high-N pure-purification ferrite/martensite steel can obviously improve the impact property and the radiation embrittlement resistance of materials.

The ferrite/martensite steel hexagonal tube billet is manufactured according to the material components, and the main manufacturing process comprises 3-6 ton vacuum induction smelting (VIM) combined with 1.6-3.5 ton electroslag remelting (ESR) and 3 ton forging processes.

The detailed manufacturing method and the steps of the billet are as follows:

1. vacuum induction smelting

1) Raw material selection

The surface of the raw material is not allowed to have impurities such as oxidized skin, oil stain, water stain or mud and sand. In order to ensure that the S and P contents in the product meet the requirements, a refined steel material with low S and P contents should be selected, and the S content in the metal manganese should be controlled below 0.010 wt.%. In order to ensure that the contents of As, Sn, Sb and Pb in the steel meet the control requirements, fine steel with low contents of As, Sn, Sb and Pb should be selected.

2) Conditions of smelting

The ultimate vacuum of the vacuum furnace smelting chamber in an empty state is less than or equal to 1.0Pa, and the cold state gas leakage rate meets the self-checking requirement. The smelting crucible is recommended to be made of magnesium or magnesium-aluminum, when a new crucible or a previous furnace steel grade is rich in Ni, the furnace needs to be washed before smelting, and the furnace washing temperature is not lower than 1600 ℃. The inner surface of the crucible should be smooth and flat without obvious cracks, local peeling and chipping. The crucible allows for a small amount of repair, which is followed by baking or furnace washing. The portion of the hot crucible in contact with the molten steel is not allowed to be repaired.

3) Charging furnace

The weight of the furnace charge is checked again before charging, and charging is carried out after no error is confirmed.

The recommended charging sequence is: carbon, nickel plate, pure iron, molybdenum metal, ferrosilicon, niobium metal, chromium metal and ferrovanadium.

4) Period of melting

And (3) transmitting power to melt to the cleaning in a vacuum state, gradually increasing the power supply power, and adjusting the power transmission power to the cleaning with reference to the vacuum induction equipment. And component analysis is carried out at the final stage of the melting period, so that a basis is provided for smelting operation in the refining period.

5) Refining period

And (3) the vacuum degree in the refining period is less than or equal to 5.0Pa, if the vacuum degree does not meet the requirement, the power is reduced, the temperature is kept, and the refining timing is started after the vacuum degree meets the requirement. The refining temperature is controlled at 1500-. And (5) sampling and analyzing after the temperature is stable and refining is carried out for more than 45 min. And adjusting the refining temperature and the refining time according to the content of key elements such as O, N, Si, Al and the like.

6) Alloying period

And entering an alloying period after the refining period is finished. The early stage vacuum degree of the alloying period should be less than or equal to 5.0 Pa. Sampling and analyzing components in the alloying period, and finely adjusting the components according to the analysis results of elements such as N, Si, Al and the like. After the component adjustment meets the requirement, introducing argon or other protective gas into the furnace, wherein the pressure of the protective atmosphere meets the requirement of 5000-. In order to prevent the volatilization of manganese, metal manganese is added under the argon atmosphere.

7) Pouring and demolding

And adjusting the pouring temperature according to the state of the tool and the die. The target temperature of the molten steel before casting is controlled at 1530-1600 ℃. The pouring mode adopts an upper pouring method. The cast electrode needs to be completely solidified in vacuum and then demoulded.

2. Electroslag remelting

Electroslag remelting selects an electroslag furnace with the capacity of 1.6-3.5 tons and selectsA crystallizer.

1) Electrode and crystallizer preparation

The nominal diameter phi of the electrode is 340mm, and the dimensional tolerance is controlled within the range of +/-20 mm. The surface of the electrode is fully polished, and local defects such as a laminated iron sheet, an oxide skin, a flying wing, scabs, microcracks and the like are removed. The head and tail of the electrode are uniform in size, the bending degree is not more than 6mm/m, and the taper is not more than 10 mm/m. And the electrode is ensured to be clean before assembly, so that field pollution is avoided.

Before use, the inner wall of the crystallizer is ensured to be clean, dry and pollution-free. All parts of the crystallizer are reasonably assembled, and the technical requirements of remelting are met.

2) Slag preparation and baking for remelting

CaF is recommended to be adopted in electroslag remelting₂-CaO-Al₂O₃Ternary or CaF₂-CaO-Al₂O₃-MgO quaternary slag system, the recommended ternary slag system ratio is 60: 27: 13, the recommended quaternary slag system proportion is 67: 15: 16: 2. the recommended value of the consumption of the 3-ton capacity electroslag remelting slag system is 120-180 kg.

Before use, the slag must be baked at 600-800 ℃ for not less than 4 hours.

The slag is smashed into a barrel for use, and is taken out from the heating furnace until the use time is not more than 15 min.

3) Electrode assembly

And cleaning the bottom end face of the crystallizer and the interior of the crystallizer before remelting. And (4) polishing the surface of the red copper plate on the bottom water tank by using a polishing grinding wheel, and removing the adhered substances. The electrodes are fastened up, perpendicular to the ground.

4) Electroslag remelting

The crystallizer is placed on the base, so that the crystallizer and the base are well sealed, and slag leakage is prevented. The contact area of the bottom cushion and the base is not less than 70%. The arc striking agent adopts 50 percent CaF₂-50％TiO₂The weight is 20-35 kg.

The slagging parameters of the electroslag ingot are referred as follows: adding slag at 3000A for 25-40min, refining at 4500A for 20-30 min. The water outlet temperature of the crystallizer is controlled to be 40-60 ℃, and the water outlet temperature of the water cooling base is less than 45 ℃.

When the slag is refined, the metal electrodes are exchanged and the current is increased, namely the smelting period is started. And in the steady-state remelting stage, technological parameters such as smelting current, voltage and the like are strictly controlled, and the parameters are recorded once every 30 min. The steady-state remelting time is controlled to be 8.5-10.5 h. The power input in the steady-state remelting stage should be stable, the voltage is controlled at 55-68V, and the current is controlled at 9000-14000A.

5) Stripping and annealing

And (4) fully cooling the cast ingot after electroslag remelting, wherein the cooling time is not less than 60 min. After tapping and demoulding, the electroslag remelting ingot is slowly cooled in the protective cover, and the slow cooling time is not less than 48 h.

Annealing treatment is carried out within 3 days after the electroslag ingot is uncovered. After the electroslag ingot is put into an annealing furnace, the temperature is raised according to the speed of less than or equal to 100 ℃/h, and the temperature is kept at 680 +/-20 ℃ for more than or equal to 20 h. Then furnace cooling is carried out according to the temperature of less than or equal to 40 ℃/h, and the electroslag ingot is taken out of the furnace for air cooling when the temperature is less than or equal to 300 ℃.

3. Forging of remelted ingot

1) Ingot smelting and sizing

Vacuum induction and electroslag remelting are adopted, and rare earth elements are forbidden to be added in the smelting process. The nominal diameter of the ingot is 610mm, and the surface defects should be ground or turned off so that the ingot can be forged.

2) Device

Heating equipment: a coal gas or natural gas heating furnace is adopted.

Forging equipment: a hydraulic quick forging machine above 2000T and a precision forging machine above 1000T.

3) Heating of ingots

Charging: the temperature of the charging furnace is lower than 600 ℃, the temperature is kept for 3-5h after the charging furnace is charged, and the ingot is heated slowly after being fully soaked.

Preheating: preheating at a speed of less than or equal to 100 ℃/h and at a temperature of 800 +/-10 ℃, and keeping the temperature for 3-5 h.

Heating: the temperature of the cast ingot is raised to the initial forging heating temperature of 1080-1170 ℃, and the temperature raising speed is less than or equal to 100 ℃/h.

Soaking the raw materials: when the temperature is heated to the initial forging temperature (namely the temperature of the material is consistent with the furnace temperature), soaking treatment is carried out for 3-5 h.

4) Forging by fast forging machine

Forging a forceps handle: when the ingot is heated to the initial forging heating temperature, the soaking time is 3-5h, and the part of the clamp handle is forged. The forging position is generally selected as the bottom pad end of the ingot, and the time of returning to the furnace and preserving the heat after forging is 1.5 to 4 hours.

Forging: after soaking, the intermediate blank is forged by a rapid forging machine for 2 to 6 times. The heating temperature of the intermediate blank is 1050-. The final forging temperature is more than 850 ℃ per fire. And finally, cutting off the head and the tail of the blank subjected to secondary fire forging by using a quick forging machine, wherein the cutting-off amount is more than 5% and the cutting-off amount is more than 4% relative to the steel ingot.

5) Forging of precision forging machine

Temperature equalization: returning the blank for finish forging to the heating furnace, and keeping the temperature at 1050 ℃ and 1100 ℃ for 1.5-4 h.

And (3) precision forging: and forging the intermediate forging blank to the size of a finished bar in an upsetting and stretching mode by a precision forging machine. In order to improve the impact property of the steel ingot (reduce the ductile-brittle transition temperature DBTT), the forging ratio is required to be more than or equal to 7.

Cutting: and after the finish forging is finished, performing hot flat tail treatment on the bar, and marking the end face of the bar.

6) Annealing after forging

Charging: and (5) annealing in a red sending way.

Soaking the raw materials: when the temperature is heated to the annealing temperature (680-860 ℃) (namely the temperature of the material and the temperature of the furnace are consistent), soaking treatment is carried out, and the recommended soaking time is more than or equal to 30 h.

A cooling mode: and (4) after the furnace is cooled to be less than or equal to 300 ℃, discharging and air cooling.

7) Forged product

The finished product obtained by the process steps has the following characteristics:

(1) forging the diameter of the bar: less than or equal to 280mm

(2) Diameter tolerance: plus or minus 1.0mm

(3) Length: 1000-6500mm

(4) Curvature: the maximum bending degree of the forged rod is allowed to be 4mm/m, and the total length (L) bending degree is not more than 0.4 percent L

(5) Out-of-roundness: the maximum allowable out-of-roundness of the forged rod is not more than 75 percent of the dimensional tolerance

(6) The end of the forged rod is straight, has no burrs, and has no deformation such as horseshoe shape, elbow and the like

(7) And (3) testing and grading the transverse macrostructure, so that macroscopic shrinkage cavities, bubbles, cracks, inclusions, peeling and white spots cannot be formed on the cross section acid-dipped macrostructure or cut test piece of the bar. The qualified grade of the center porosity, the general porosity and the segregation of the macrostructure meets the requirement of less than or equal to 1.0 grade.

(8) Non-metallic inclusions: and (4) carrying out longitudinal non-metallic inclusion inspection on the bar according to the GB/T10561 standard A method. A. The levels of B, C, D non-metallic inclusions are respectively controlled to be less than or equal to 1.0 level, and the levels of TiN inclusions are controlled to be less than or equal to 1.0 level.

(9) Grain size: and (4) carrying out grain size grading according to the GB/T6394 standard, wherein the original austenite grain size is required to be not less than 3.0 grades and is uniformly distributed.

(10) Metallographic structure: and detecting the content of delta-ferrite in the forged bar according to YB/T4402. The average delta-ferrite content in the forged bar should not exceed 5% and the worst field of view should not exceed 8% (area percentage).

(11) Ultrasonic inspection: and (4) carrying out ultrasonic detection on the forged bars one by one along the whole length according to a method specified by the GB/T4162-2008 standard, wherein the acceptance level is B level (the acceptance is carried out on single-point defects by adopting phi 2.0mm flat-bottom holes).

(12) Surface quality: the rod should be finished intact without allowing folds, hairlines, cracks, scars, inclusions, nicks or other defects which impair the use, and it should be ensured that the surface roughness (Ra) should not exceed 3.2. mu.m.

The method for manufacturing the low-silicon high-nitrogen ferrite/martensite steel slab of the present invention is exemplified as follows.

Example 1: production and inspection of low-silicon high-nitrogen ferrite/martensite steel billet

The diameter of the low-silicon high-nitrogen ferrite/martensite billet for the sodium-cooled fast reactor core hexagonal tube prepared by the embodiment is 241mm, the length of the low-silicon high-nitrogen ferrite/martensite billet is 6000mm, and the manufacturing process comprises the following steps.

1) Vacuum induction smelting

(1) And (3) smelting by using a 6-ton vacuum induction furnace, and vacuumizing to less than or equal to 1.0Pa when the smelting chamber is empty.

(2) And (5) charging raw materials into a furnace.

(3) Feeding electricity to melt till the refining is finished, controlling the refining vacuum degree to be less than or equal to 5.0Pa and the refining temperature to be 1560 ℃.

(4) And (4) entering an alloying period after refining, wherein the early vacuum degree is less than or equal to 5.0 Pa. The temperature in the alloying period is less than or equal to 1570 ℃, and the composition is finely adjusted according to the analysis results of elements such as N, Si, Al and the like. After the component adjustment meets the requirement, introducing argon or other protective gas into the furnace, wherein the pressure of the protective atmosphere meets the requirement of 5000-. Manganese metal was added under an argon atmosphere.

(5) And casting after the alloying period, wherein the casting temperature is 1530-1600 ℃. The cast electrode needs to be completely solidified in vacuum and then demoulded.

2) Electroslag remelting

(1) Remelting in an electroslag furnace using CaF₂-CaO-Al₂O₃-a quaternary slag system of MgO.

(2) The nominal diameter phi of the electrode is 340mm, and a phi 610mm crystallizer is selected.

(3)Slagging parameters of the electroslag ingot: adding slag at 3000A for 25-40min, refining at 4500A for 20-30 min.

(4) And in the steady-state remelting stage, technological parameters such as smelting current, voltage and the like are strictly controlled, and the parameters are recorded once every 30 min. The steady-state remelting time is controlled to be 8.5-10.5 h. The power input in the steady-state remelting stage should be stable, the voltage is controlled at 55-68V, and the current is controlled at 9000-14000A.

(5) And (4) fully cooling the cast ingot after electroslag remelting, wherein the cooling time is not less than 60 min. After tapping and demoulding, the electroslag remelting ingot is slowly cooled in the protective cover, and the slow cooling time is not less than 48 h.

(6) Annealing treatment is carried out within 3 days after the electroslag ingot is uncovered. After the electroslag ingot is put into an annealing furnace, the temperature is raised according to the speed of less than or equal to 100 ℃/h, and the temperature is kept at 680 +/-20 ℃ for more than or equal to 20 h. Then furnace cooling is carried out according to the temperature of less than or equal to 40 ℃/h, and the electroslag ingot is taken out of the furnace for air cooling when the temperature is less than or equal to 300 ℃.

3) Forging

(1) The blank is heated and forged by a natural gas heating furnace, and a hydraulic quick forging machine of more than 2000T and a precision forging machine of more than 1000T are used for forging.

(2) Heating the re-melted ingot to the initial forging temperature of 1170 ℃, soaking for 5h, and forging the clamp handle.

(3) After soaking, the intermediate blank is forged by a rapid forging machine by 5 times of fire. The heating temperature of the intermediate blank is 1120 ℃, and the time of returning to the furnace after forging is 4 hours. The final forging temperature is more than 850 ℃ per fire. And finally, cutting off the head and the tail of the blank subjected to secondary fire forging by using a quick forging machine, wherein the head cutting amount is more than 5%, and the tail cutting amount is more than 4%.

(4) And returning the blank for finish forging to the heating furnace, and keeping the temperature at 1100 ℃ for 4 h. And forging the intermediate forging blank to the size of a finished bar by a precision forging machine. The forging ratio was 16. And after the finish forging is finished, performing hot flat tail treatment on the bar, and marking the end face of the bar.

(5) Annealing after forging, and soaking when the material is heated to the annealing temperature of 860 ℃ (namely the material temperature and the furnace temperature are consistent), wherein the soaking time is more than or equal to 30 h. And (4) after the furnace is cooled to be less than or equal to 300 ℃, discharging and air cooling.

(6) And forging the intermediate forging blank to the size of a finished bar by a precision forging machine.

The final size of the tube blank prepared by the steps is 248mm multiplied by 5800 mm; A. the levels of B, C, D non-metallic inclusions are 1.0, 1.0 and 1.0 grades respectively, and the level of TiN inclusions is 1.0 grade; the grain size is 3-6 grades; the chemical components, other sizes and tolerances, macrostructures, metallographic structures, ultrasonic detection and surface quality all meet the requirements.

Example 2: production and inspection of low-silicon high-nitrogen ferrite/martensite steel billet

The low-silicon high-nitrogen ferrite/martensite steel billet (with the diameter of 241mm and the length of 6000mm) for the hexagonal tube reactor core prepared in the example 1 is respectively taken out and subjected to impact performance and ion irradiation performance tests.

The impact toughness was tested on ferritic/martensitic steels using standard test specimens (dimensions 10 x 55mm) and the results showed: reduction of Si helps to lower the ductile-to-brittle transition temperature (DBTT); the DBTT can be reduced by reducing the S content through pure smelting, but the shock high-order energy (USE) is not increased obviously; alloying with N helps to increase USE. The results of the influence of the N content on the impact absorption work and ductile-brittle transition temperature of the ferritic/martensitic steel are shown in FIG. 1. After normalizing and tempering treatment (1050 ℃/30min +780 ℃/1.5h, AC), the high-order energy and DBTT of 0.040-0.070 wt.% N ferrite/martensite steel reach the optimal values. 0.0042 to 0.02 wt.% N of the ferritic/martensitic steel DBTT is about-30 ℃ and 0.011 wt.% N of the ferritic/martensitic steel DBTT is about-25 ℃.

The ferritic/martensitic steels were subjected to ion irradiation tests. Fe with 3.5MeV¹³⁺Irradiating ion to 2.0 × 10 at normal temperature¹⁶ions/cm². Performing nano indentation test after irradiation, adopting Berkovich indenter and CSM (continuous stiffness measurement) continuous stiffness test mode (strain rate-0.05 s) of diamond material^-1Frequency-45 Hz, harmonic displacement-2.0 nm), the maximum indentation depth is chosen to be 2000 nm. 6 pressure points (distance is more than 50 μm) are selected in each irradiated/non-irradiated area of the sample surface, and the hardness curve adopts the average value of the 6 groups of data.

According to Fe ionIncrease in nanometer hardness (Δ H) by sub-irradiation₀) And percent hardness change (i.e., rate of hardening (. DELTA.H)₀/H₀) X 100%), the percentage increase in hardness that the Fe ion irradiation caused the low-silicon high-nitrogen ferrite/martensitic steel sample to harden was 24.73%, which was lower than the percentage increase in hardness of the T91 ferrous horse steel (31.41-36.87%) that was irradiated at the same time. FIG. 2 shows the fitting result of the nanoindentation test data before and after irradiation of the low-silicon high-nitrogen ferrite/martensitic steel according to the Nix-Gao model.

And (3) evaluating the radiation embrittlement resistance of the material by adopting a high-energy heavy ion irradiation combined small sample testing technology. Before irradiation, a rectangular sample is cut from a ferrite/martensite steel material, carefully ground and polished, and then a small disc with the diameter of 3mm is punched out, and the thickness is controlled to be 110 +/-2 mu m. Using high energy 36Ar¹²⁺Ions (total kinetic energy 222MeV) irradiated the sample to the same dose of 7.0x10¹⁴ions/cm²The mean displacement damage generated inside the sample was about 0.28dpa, estimated according to SRIM-2013. In the irradiation experiment process, the sample platform is refrigerated by liquid nitrogen through the upper end cold finger, the heating effect of high-energy ion beams can be effectively inhibited, and the temperature of the sample is kept near-30 ℃ in the irradiation process. The beam spot of the Ar ion beam on the target table is about phi 15 mm. In the irradiation process, the aluminum foil turntable is rotated at a constant speed, so that the energy of Ar ions reaching the surface of the sample is continuously and gradiently changed, and the displacement damage in the sample is quasi-uniformly distributed along the depth direction. Ar of 222MeV was estimated from SRIM-2013(Kinchin-Pease model, atomic dislocation threshold Ed was 40eV)¹²⁺The range of ions in ferritic/martensitic steels is about 25 μm. The vacuum of the target chamber in the ion irradiation process reaches 2 x10 at most^-5Pa。

And (3) storing the sample subjected to high-energy Ar ion irradiation in a normal-temperature low-vacuum cabinet for 3 months, and performing a small punch test together with the unirradiated sample with the same thickness after the radioactivity of the sample is reduced to a safe level. A wafer sample with the diameter phi of 3mm is fastened at the middle position by an upper clamp and a lower clamp, and Al with the diameter phi of 1mm is placed above the wafer sample₂O₃The ceramic pellet is pressed downwards by a universal experimental machine controlled by a program and adopting the same moving speed of a cross beam to push a plunger to make the sample generate plastic deformation in the middle area until the sample is brokenAnd meanwhile, a displacement extensometer is arranged below the sample and used for recording the central deformation displacement data of the sample. In a small punch test, the thickness of a ferrite/martensite steel phi 3mm wafer sample which is not irradiated/irradiated is controlled within the range of 110 +/-2 mu m, the test temperature is room temperature, and the displacement loading speed is 0.012 mm/min. Figure 3 is the small punch test number before and after irradiation of the ferritic/martensitic steel sample. The change of the fracture displacement of the sample before and after irradiation can be obtained by a small punch test loading-displacement curve, and the ductility loss rate of the material is calculated to be 0.18 percent and is lower than the ductility loss rate (1.15-1.82 percent) of T91 iron horse steel irradiated at the same time.

Therefore, the test results show that the low-silicon high-nitrogen ferrite/martensite steel billet for the fast reactor core hexagonal pipe prepared by the method obviously improves the material impact property and the radiation embrittlement resistance.

It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is intended to include such modifications and variations. The foregoing examples or embodiments are merely illustrative of the present invention, which may be embodied in other specific forms or in other specific forms without departing from the spirit or essential characteristics thereof. The described embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. The scope of the invention should be indicated by the appended claims, and any changes that are equivalent to the intent and scope of the claims should be construed to be included therein.