阅读说明：本技术 一种无钴马氏体时效钢冷轧薄带的制造方法 (Manufacturing method of cobalt-free maraging steel cold-rolled thin strip ) 是由 王万林 毛松 王慧惠 吕培生 王兰坤 黄道远 周乐君 周游 于 2021-09-09 设计创作，主要内容包括：本发明公开了一种无钴马氏体时效钢冷轧薄带的制造方法,具体步骤如下：(1)冶炼成分合格的钢水；(2)钢水流入双辊薄带连铸机铸造出厚度为3.0-5.0mm的铸态薄带；(3)薄带出辊后立即进行二冷,快速冷却到室温；(4)室温冷轧；(5)退火；(6)薄带卷取。本发明利用薄带连铸铸带尺寸薄、晶粒细小的优势,免去了传统工艺多道次热轧的工序；利用冷轧产生的大量位错促进第二相析出,提升了强化效果；利用薄带连铸亚快速凝固抑制元素偏析的优势,免去了传统工艺的长时间均匀化退火、高温固溶退火等工序,大大缩短了工艺流程,同时保证了产品的强度和塑性,提高了产品的韧性。(The invention discloses a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip, which comprises the following specific steps: (1) smelting molten steel with qualified components; (2) the molten steel flows into a twin-roll strip caster to cast an as-cast strip with the thickness of 3.0-5.0 mm; (3) carrying out secondary cooling immediately after the thin strip is taken out of the roller, and rapidly cooling to room temperature; (4) cold rolling at room temperature; (5) annealing; (6) and (4) rolling the thin strip. The advantages of thin size and fine crystal grains of the thin strip continuous casting strip are utilized, and the process of multi-pass hot rolling in the traditional process is omitted; a large amount of dislocation generated by cold rolling is utilized to promote the precipitation of a second phase, so that the strengthening effect is improved; the advantage of restraining element segregation by using the sub-rapid solidification of the thin strip continuous casting is utilized, the procedures of long-time homogenization annealing, high-temperature solution annealing and the like in the traditional process are omitted, the process flow is greatly shortened, the strength and the plasticity of the product are ensured, and the toughness of the product is improved.)

1. A method for manufacturing a cobalt-free maraging steel cold-rolled thin strip is characterized by comprising the following steps:

(1) smelting molten steel: smelting to obtain molten steel, wherein the molten steel comprises the following chemical components in percentage by mass: ni: 14-18%, Mo: 3-4.5%, Ti: 0.5-1.5%, C: less than or equal to 0.010 percent, S: less than or equal to 0.006 percent, P: less than or equal to 0.020%, N: less than or equal to 0.007 percent, and the balance of Fe and inevitable impurities;

(2) strip continuous casting: casting the molten steel into an as-cast thin strip with the thickness of 3.0-5.0mm by a double-roller thin strip continuous casting machine;

(3) secondary cooling: rapidly cooling the cast strip to room temperature, wherein the cooling rate is higher than 150 ℃/s and preferably 200 and 350 ℃/s when the temperature of the strip is above 700 ℃, and the cooling rate in the interval from 700 ℃ to room temperature is higher than 10 ℃/s and is less than the cooling rate of the strip above 700 ℃, preferably 20-50 ℃/s;

(4) cold rolling at room temperature: cold rolling the thin strip prepared in the step (3) at room temperature, wherein the cold rolling temperature is not higher than 100 ℃, the cold rolling reduction rate is 40-65%, and the strain rate is 0.1-0.5s^-1；

(5) Annealing treatment: annealing the thin strip prepared in the step (4), wherein the annealing temperature is 520-550 ℃, and the annealing time is 1-1.5 h;

(6) coiling the thin strip: and (4) cooling the thin strip obtained in the step (5) to room temperature of-100 ℃ for online coiling to obtain a finished thin strip steel coil.

2. The method of manufacturing a thin cobalt-free maraging steel strip as recited in claim 1, wherein: the molten steel comprises the following chemical components in percentage by mass: ni: 15%, Mo: 3%, Ti: 0.8%, C: less than or equal to 0.010 percent, S: less than or equal to 0.006 percent, P: less than or equal to 0.020%, N: less than or equal to 0.007 percent, and the balance of Fe and inevitable impurities.

3. The method of manufacturing a thin cobalt-free maraging steel strip as recited in claim 1, wherein: in the step (1), the smelting mode is an electric furnace or converter steelmaking method, and molten steel is refined, including vacuum degassing refining and ladle refining.

4. The method of manufacturing a thin cobalt-free maraging steel strip as recited in claim 1, wherein: in the step (2), the superheat degree of the molten steel is 80-120 ℃.

5. The method of manufacturing a thin cobalt-free maraging steel strip as recited in claim 1, wherein: in the step (2), the thickness of the cast thin strip is 3.5-4.5 mm.

6. The method of manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 1, comprising: the method is characterized in that: and (3) in the step (3), the cast thin strip is cooled to 700 ℃ by water spraying and cooled to room temperature by air spraying.

7. The method of manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 1, comprising: the method is characterized in that: in the step (4), the cold rolling temperature is not higher than 50 ℃.

8. The method of manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 1, comprising: the method is characterized in that: in the step (6), the coiling temperature is 50-100 ℃.

9. The method of manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 1, comprising: the method is characterized in that: the thickness of the obtained finished thin strip can be 1.2-3 mm.

10. The method of manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 9, comprising: the method is characterized in that: the yield strength of the obtained finished thin strip is 1840-1890MPa, and the tensile strength is 1920-1980 MPa; the elongation is 8-10%.

Technical Field

The invention belongs to the technical field of ferrous metallurgy, and particularly relates to a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip.

Background

The maraging steel is high-strength steel with two strengthening effects of martensite transformation strengthening and aging strengthening superposed. Because of extremely high obdurability and excellent processing and welding performance, the alloy has been widely applied to important fields of aviation, aerospace, mechanical manufacturing and the like. The high cost of the alloying elements limits the range of use of maraging steels. In order to reduce costs, cobalt-free maraging steels are produced. Currently, the mainstream production method of maraging steel is double vacuum smelting, namely vacuum induction smelting and vacuum arc remelting refining. However, the production method is still limited in the traditional die casting category, and has the defects of complex process flow, high energy consumption, long production period and the like.

In the twin-roll thin strip continuous casting process, molten steel is directly cast on a water-cooled copper crystallization roll into a thin steel strip with the thickness of 1-5 mm. The double-roller thin strip continuous casting production line is greatly shortened due to the omission of repeated heating and hot rolling procedures in the traditional process. Therefore, various iron and steel companies in the world, such as Nippon Nissan iron, American NUCOR, Korea POSCO, domestic Bao Steel group and GYO group, compete with each other to develop the twin-roll strip casting technology, and have made industrial trial production practices and made important progress. But is only used for producing thin strips of carbon steel, stainless steel and silicon steel at present.

The invention patent application No. 202110321095.7 discloses a metal material forming technique for producing maraging steel by additive manufacturing with laser selective melting, which allows for direct printing of spherical powder into metal parts. However, the method has the problems of complex process, low yield, high cost and the like, and is not suitable for large-scale production.

The invention patent with the application number of 201610066981.9 discloses aluminum-reinforced maraging steel and a preparation method thereof, and the method mainly comprises the working procedures of smelting, casting, forging, solid solution, cold rolling, recrystallization, aging treatment and the like, so that the maraging steel jointly reinforced by high-density B2-NiAl intermetallic compounds, trace carbides and nanoclusters is obtained, and the method has strong innovation. However, the core element Al content is high, the core element Al is easily oxidized during smelting and casting, the formability is poor, and the thin-strip continuous casting cannot be realized.

Disclosure of Invention

In order to solve the problems in the prior art, the invention firstly tries to manufacture the cold-rolled thin strip of the cobalt-free maraging steel, utilizes the advantages of small segregation and fine crystal grains of the continuous casting of the thin strip, and is matched with proper components, rapid cooling, cold rolling and annealing to obtain the maraging steel reinforced by a fine-grain martensite matrix and a high-density second phase, and simultaneously improves the strength and the plasticity of a thin strip product. The method can directly obtain the final martensite steel strip product from the molten steel, and has the advantages of obviously shortened process, high production efficiency, low steel-making cost, low energy consumption, environmental friendliness and the like.

In order to achieve the aim, the invention discloses a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip, which comprises the following steps:

(1) smelting molten steel: smelting to obtain molten steel, wherein the molten steel comprises the following chemical components in percentage by mass: ni: 14-18%, Mo: 3-4.5%, Ti: 0.5-1.5%, C: less than or equal to 0.010 percent, S: less than or equal to 0.006 percent, P: less than or equal to 0.020%, N: less than or equal to 0.007 percent, and the balance of Fe and inevitable impurities;

(2) casting thin strips: casting the molten steel into an as-cast thin strip with the thickness of 3.0-5.0mm by a double-roller thin strip continuous casting machine;

(3) secondary cooling: rapidly cooling the cast strip to room temperature, wherein the cooling rate is higher than 150 ℃/s and preferably 200 and 350 ℃/s when the temperature of the strip is above 700 ℃, and the cooling rate in the interval from 700 ℃ to room temperature is higher than 10 ℃/s and is less than the cooling rate of the strip above 700 ℃, preferably 20-50 ℃/s; in the secondary cooling process, the cooling speed of more than or equal to 150 ℃/s is adopted in the front, and the purpose is to prevent the segregation of elements and the growth of crystal grains; the cooling speed of more than or equal to 10 ℃/s is adopted later, and the aim is to prevent the aging precipitation of the second phase in the low-temperature section.

(4) Cold rolling at room temperature: cold rolling the thin strip prepared in the step (3) at room temperature, wherein the cold rolling temperature is not higher than 100 ℃, the cold rolling reduction rate is 40-65%, and the strain rate is 0.1-0.5s^-1；

(5) Annealing treatment: annealing the thin strip prepared in the step (4), wherein the annealing temperature is 520-550 ℃, and the annealing time is 1-1.5 h;

(6) coiling the thin strip: and (5) cooling the thin strip obtained in the step (5) to room temperature, and then carrying out online coiling to obtain a finished thin strip steel coil.

Preferably, the invention relates to a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip, and the molten steel comprises the following chemical compositions in percentage by mass: ni: 15%, Mo: 3%, Ti: 0.8%, C: less than or equal to 0.010 percent, S: less than or equal to 0.006 percent, P: less than or equal to 0.020%, N: less than or equal to 0.007 percent, and the balance of Fe and inevitable impurities.

Preferably, in the step 1), the smelting mode is an electric furnace or converter steelmaking method, and molten steel is refined, wherein the refining comprises vacuum degassing refining and ladle refining.

Preferably, in the method for manufacturing the cold-rolled thin strip of the cobalt-free maraging steel, in the step 2), the superheat degree of the molten steel is 80-120 ℃.

Preferably, in the method for manufacturing the cold-rolled strip of the cobalt-free maraging steel, in the step 2), the thickness of the cast strip is 3.5-4.5 mm.

Preferably, in the step 3), the cast strip is cooled by spraying water to 700 ℃ and spraying air to room temperature.

Preferably, in the method for manufacturing the cobalt-free maraging steel cold-rolled thin strip, in the step 4), the cold rolling temperature is not higher than 50 ℃.

Preferably, in the method for manufacturing the cobalt-free maraging steel cold-rolled strip according to the present invention, in the step 6), the coiling temperature is 50 to 100 ℃.

The invention relates to a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip, wherein the thickness of the obtained finished thin strip can be 1-3 mm.

The yield strength of the finished thin strip obtained by the invention is 1840-1890MPa, and the tensile strength is 1920-1980 MPa; the elongation is 8-10%.

The invention firstly tries to prepare the high-quality cobalt-free maraging steel cold-rolled thin strip by utilizing thin strip continuous casting under the condition of no Co. The yield strength of the finished thin strip obtained by the invention is 1840-1890MPa, and the tensile strength is 1920-1980 MPa; the elongation is 8-10%.

The average grain size of the finished thin strip obtained by the invention is 7-9 μm.

The invention adopts an ultra-low carbon route, and limits the carbon content to be below 0.01 percent. C is a very strong solid solution strengthening element and may also combine with metallic elements to form carbides to improve strength. But at the same time, the plasticity and toughness of the material are seriously influenced, and carbide forming elements such as Ti, V, Nb and the like are consumed. Through experimental optimization, the content of C in the scheme of the invention is controlled below 0.010 wt%.

Aiming at the characteristics of high cooling speed, thin size and small segregation of the continuous casting of the thin strip, the invention properly reduces the Ni content, namely controls the Ni content to be 14-18%; therefore, the cost can be saved, and the comprehensive performance of the product can be improved by matching with the preparation process.

The reason why 3-4.5% of Mo is adopted in the invention is that: mo is beneficial to improving the strength, toughness and corrosion resistance of the maraging steel. Precipitation at early stage of agingNi₃Mo、Fe₂Mo plays an important role in strengthening while maintaining the toughness of steel. The existence of Mo can also prevent a precipitated phase from being precipitated along the prior austenite grain boundary, thereby avoiding the intergranular fracture and improving the fracture toughness. However, excessive addition of molybdenum also produces retained austenite.

Titanium is the most effective strengthening alloying element in maraging stainless steels because Ti can form Ni with Ni₃Ti precipitates out of the phase, and the effect of strengthening precipitation is achieved. However, Ti is easily oxidized, and its addition amount should be limited from the viewpoint of the process, so that its content is determined as: ti: 0.5 to 1.5%, preferably 1.0 to 1.2%.

S is a harmful element in steel. The invention is very unfavorable to the toughness and the stamping property of the steel strip, and is easy to cause the anisotropy of the mechanical property of the steel strip, the lower the content of the sulfur in the steel is, the better the content is, the comprehensive consideration of the prior steelmaking level and economic factors, and the invention controls the content of the S to be below 0.006 percent.

P has the effect of improving the weather resistance of the martensitic steel, but the excessively high P content is not beneficial to the stamping performance, the welding performance, the low-temperature toughness and the like of the martensitic steel plate, and the invention focuses more on the mechanical property of the martensitic steel plate, so that the P content is not higher than 0.02 percent.

N is easy to combine with alloy elements in steel to precipitate carbonitride, and when the content of N is too high, the N is easy to form coarse nitride with the alloy elements in the steel, thereby having adverse effect on the plasticity and fatigue property of the steel. A small amount of N is beneficial to separating out fine carbonitride second phase particles and improving the strength of steel. Therefore, in the invention, N is controlled at a lower level, and the content is not higher than 0.007%.

The invention breaks through the limitation of the traditional double-vacuum smelting process, adopts the advanced thin strip continuous casting technology and is matched with cold rolling, and can realize the following effects:

a) compared with the traditional production process, the method has the advantages of short flow, high production efficiency, low energy consumption, environmental friendliness and the like.

b) The thickness of the cast strip obtained by strip continuous casting is only 3.0-5.0mm, compared with the thickness of tens of millimeters in the traditional process, the cast strip does not need to be heated and hot rolled repeatedly, the process flow is greatly shortened, meanwhile, the requirement of the strip on the hardenability of steel grade is low, the addition amount of an alloy element Ni can be reduced, the steel-making cost is reduced, the addition of Co is avoided, and the production cost is greatly reduced.

c) The solidification speed in the strip continuous casting process is high (1000-10000 ℃/s), and the segregation of alloy elements is inhibited, so that the homogenization annealing at the high temperature of 1200 ℃ is not needed; meanwhile, the crystal grains are refined, and the size of the crystal grains after heat treatment is less than 10 mu m.

The cold rolling and annealing process options of the present invention are based on the following considerations:

the main functions of cold rolling after cooling the cast thin strip are as follows: a) the surface flatness is improved; b) refining grains; c) a large number of dislocations are retained to provide sites for the precipitation of the second phase. Due to the high content of alloying elements in maraging steel, the as-cast strip obtained by twin roll strip casting has a pronounced dendritic structure, as shown in fig. 2, with primary dendrites extending inwardly perpendicular to the strip surface and fine secondary dendrites perpendicular to the primary dendrites. By the cold rolling, a large amount of deformation and dislocation are accumulated inside the as-cast thin strip, and recovery and recrystallization occur during the subsequent annealing process, forming fine grains having a grain size of less than 10 μm, as shown in fig. 3. Meanwhile, a second phase is greatly separated out along dislocation, and the effect of aging strengthening is achieved.

Drawings

FIG. 1 is a schematic process flow diagram of a twin roll strip caster set of the present invention;

FIG. 2 is an as-cast strip metallographic microstructure according to the invention;

FIG. 3 is a metallographic microstructure of the final product according to the invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The maraging steels of examples 1-4 and comparative examples 1-5 of the present invention were produced by the above-mentioned method for manufacturing a cold-rolled thin strip of cobalt-free maraging steel, and comparative example 6 was produced by conventional twin vacuum melting combined with homogenization treatment, three cold-rolling passes and aging treatment, and the molten steel compositions and process parameters of examples 1-4 and comparative examples 1-6 are listed in table 1, and are specifically described as follows:

example 1

Example 1 the steel grade had the following composition: 0 percent of Co, 15 percent of Ni, 3 percent of Mo, 0.8 percent of Ti, less than or equal to 0.010 percent of C, less than or equal to 0.007 percent of N, less than or equal to 0.020 percent of P, less than or equal to 0.006 percent of S, and the balance of Fe and inevitable impurities. Referring to fig. 1, after converter and vacuum degassing refining, molten steel in a ladle 1 enters a tundish 3 through a long nozzle 2 (superheat degree of molten steel in the tundish is 100 ℃), enters a molten pool 5 consisting of crystallizing rollers 7a and 7b and side sealing plates 6 through a water distribution nozzle 4, contacts with water-cooled copper crystallizing rollers 7a and 7b, and is cast into an as-cast thin strip 9 with the thickness of 4.5mm, and a dendritic structure of the thin strip in the direction vertical to the surfaces of the rollers is shown in fig. 2. The width of the crystallization roller is 1200mm, the diameter of the crystallization roller is 500mm, and the casting speed of the casting machine is 90 m/min. The area between the water distribution flow port 4 and the pinch roll 11 is a closed space 12 filled with inert gas, and the inert gas is nitrogen, so that the high-temperature oxidation of the thin strip can be prevented.

After the cast thin strip 9 is taken out of the crystallization roller, the cast thin strip enters a cooling system 13 through a pinch roller 11, the cast thin strip is rapidly cooled to room temperature and then starts to be cold-rolled (the rapid cooling is that when the temperature of the thin strip is above 700 ℃, the cooling rate is 200 ℃/s, the cooling rate from 700 ℃ to the room temperature is 25 ℃/s), the cold-rolling temperature is 25 ℃, the cold-rolling reduction rate is 50 percent, and the strain rate is 0.1s^-1. The cold-rolled thin strip enters a tunnel annealing furnace 15, is annealed at 530 ℃ for 1 hour, and is then cooled to 100 ℃ through an air cooling nozzle 16. After cooling, the cast strip 9 is wound into a finished steel coil 18 (2.25 mm in thickness) by a powerful coiler under the guidance of pinch rolls 17, and the coil is naturally cooled to room temperature. Fig. 3 shows the metallographic structure of the finished steel coil 18.

Examples 2 to 4

Examples 2-4 use a similar process to example 1, except that: example 2 differs from example 1 only in that example 2 contains 18% Ni. Example 3 differs from example 1 only in that the thickness of the cast strip of example 3 is 3 mm. Example 4 differs from example 1 only in that example 4 has a cold rolling reduction of 40%.

Comparative examples 1 to 5

Comparative examples 1-5 use a similar process to example 1, except that: the only difference between comparative example 1 and example 1 is that comparative example 1 contains 10% Co. Comparative example 2 differs from example 1 only in that comparative example 2 contains 0.1% Ti. The only difference between comparative example 3 and example 1 is that the secondary cooling mode of comparative example 3 is slow cooling (slow cooling: the cooling rate is constantly 5 ℃/s). Comparative example 4 differs from example 1 only in that comparative example 4 does not undergo a cold rolling treatment. Comparative example 5 differs from example 1 only in that comparative example 5 was cold rolled without annealing.

Comparative example 6

Comparative example 6 is produced by a conventional double vacuum melting process, and the steel grade comprises the following components: 18% of Ni, 3% of Mo, 1.5% of Ti, 0.010% of C, 0.008% of N, 0.023% of P, 0.005% of S, and the balance of Fe and inevitable impurities; the thickness of the cast strip after vacuum induction melting and vacuum arc remelting refining is 40 mm; firstly carrying out homogenizing annealing on the cast strip at 1200 ℃ for 10h, and then slowly cooling to room temperature (the slow cooling is that the secondary cooling rate is constant at 5 ℃/s); then, three cold rolling passes are carried out at room temperature, the reduction rate is 80 percent, and the strain rate is 0.1s^-1(ii) a Annealing (annealing at 530 ℃ for 1h) and cooling to room temperature to obtain the final product.

Table 2 shows the product properties of examples 1-4 and comparative examples 1-6. Examples 1-4 achieved an excellent combination of mechanical properties, which varied slightly with Ni content, strip thickness, and cold rolling reduction. The comparative example 1 is added with 10% of Co, so that better performance is obtained, but the cost is greatly improved, and economic benefit is not met; comparative example 2 illustrates the necessity of adding Ti element, which is an important precipitation hardening element; comparative example 3 shows that slow cooling in the secondary cooling mode can cause grain growth, and further cause reduction of plasticity and strength; comparative example 4 shows that no cold rolling leads to an excessively large grain size and a significant reduction in strength; comparative example 5 shows that the effect of aging strengthening cannot be exerted without annealing, and although the strength is high, the plasticity is extremely poor, so that annealing treatment must be added after cold rolling; comparative example 6 the strength of the product was comparable to examples 1-4 using conventional double vacuum melting techniques, but the ductility and toughness were poor and the process was cumbersome. In conclusion, the process flow and the technical parameters adopted by the invention have higher innovativeness and advantages.

TABLE 1 chemical composition and Process parameters of molten steels of examples 1-4 and comparative examples 1-6

Note: indicates that the parameters are in full agreement with the values listed in example 1.

TABLE 2 Properties of the products of examples 1 to 4 and comparative examples 1 to 6