阅读说明：本技术 一种经济型低屈强比高强度钢及其制造方法 (Economical low-yield-ratio high-strength steel and manufacturing method thereof ) 是由 孙磊磊 朱春生 吴扣根 郑磊 章传国 沈燕 于 2020-04-27 设计创作，主要内容包括：本发明公开了一种经济型低屈强比高强度钢,其含有质量百分比如下的下述化学元素：C：0.045～0.080％,Si：0.10～0.30％,Mn：1.60～1.85％,Cr：0.15～0.30％,Mo：0.06～0.24％,Nb：0.040～0.075％,Ti：0.005～0.020％,Al：0.01～0.05％,Ca：0.001～0.004％,N：0.001～0.005％。此外本发明还公开了一种经济型低屈强比高强度钢的制造方法,其包括步骤：(1)冶炼和连铸；(2)再加热；(3)轧制；(4)冷却：采用DQ+ACC两段式冷却工艺,其中开冷温度为700～750℃,在DQ段冷速为30～40℃/s,DQ停冷温度为550～620℃,ACC段冷速为10～25℃/s,ACC停冷温度为430～530℃。本发明所述的经济型低屈强比高强度钢采用合理的化学成分和工艺设计,不仅有优良的经济性,还具有高强韧性和低屈强比的特性。(The invention discloses economical low-yield-ratio high-strength steel which contains the following chemical elements in percentage by mass: c: 0.045-0.080%, Si: 0.10 to 0.30%, Mn: 1.60-1.85%, Cr: 0.15-0.30%, Mo: 0.06-0.24%, Nb: 0.040-0.075%, Ti: 0.005-0.020%, Al: 0.01-0.05%, Ca: 0.001-0.004%, N: 0.001 to 0.005%. In addition, the invention also discloses a manufacturing method of economical low-yield-ratio high-strength steel, which comprises the following steps: (1) smelting and continuous casting; (2) reheating; (3) rolling; (4) and (3) cooling: a DQ + ACC two-stage cooling process is adopted, wherein the open cooling temperature is 700-750 ℃, the cooling speed in the DQ stage is 30-40 ℃/s, the DQ stop cooling temperature is 550-620 ℃, the cooling speed in the ACC stage is 10-25 ℃/s, and the ACC stop cooling temperature is 430-530 ℃. The economical low-yield-ratio high-strength steel disclosed by the invention adopts reasonable chemical components and process design, and not only has excellent economy, but also has the characteristics of high strength and toughness and low yield ratio.)

1. An economical low-yield-ratio high-strength steel is characterized by comprising the following chemical elements in percentage by mass:

C：0.045～0.080％，Si：0.10～0.30％，Mn：1.60～1.85％，Cr：0.15～0.30％，Mo：0.06～0.24％，Nb：0.040～0.075％，Ti：0.005～0.020％，Al：0.01～0.05％，Ca：0.001～0.004％，N：0.001～0.005％。

2. the economical low yield ratio high strength steel of claim 1, wherein the chemical elements comprise, in mass percent:

c: 0.045-0.080%, Si: 0.10 to 0.30%, Mn: 1.60-1.85%, Cr: 0.15-0.30%, Mo: 0.06-0.24%, Nb: 0.040-0.075%, Ti: 0.005-0.020%, Al: 0.01-0.05%, Ca: 0.001-0.004%, N: 0.001 to 0.005% and the balance Fe and other unavoidable impurities.

3. The economical low-yield-ratio high-strength steel according to claim 1 or 2, wherein the chemical elements simultaneously satisfy the following contents in percentage by mass: c + Mn/6 is more than or equal to 0.34 percent and less than or equal to 0.38 percent, Cr + Mo is more than or equal to 0.30 percent and less than or equal to 0.40 percent, and Ti/N is more than or equal to 2.5 and less than or equal to 5.0 percent, wherein C, Mo, Cr, N, Ti and Mn all represent the mass percent of corresponding elements.

4. The economical low yield ratio high strength steel according to claim 1 or 2, further comprising 0 < B.ltoreq.0.0005%.

5. Economical low yield ratio high strength steel according to claim 2, characterized in that P ≦ 0.015% and/or S ≦ 0.002% among other unavoidable impurities.

6. The economical low yield ratio high strength steel according to claim 1 or 2, wherein the chemical elements thereof satisfy at least one of the following contents by mass:

C：0.050～0.075％，

Mn：1.65～1.80％，

Mo：0.06～0.18％，

Nb：0.045～0.065％。

7. the economical low yield ratio high strength steel of claim 1 or 2, wherein the microstructure is polygonal ferrite + martensite + granular bainite.

8. The economical low yield ratio high strength steel of claim 7, wherein the phase proportion of polygonal ferrite is 5-25% and the phase proportion of the martensite component is 2-10%.

9. The economical low yield ratio high strength steel of claim 7, wherein the polygonal ferrite has an average grain size of less than 10 μm; and/or the average size of the mao elements is less than 2 μm.

10. The economical low yield ratio high strength steel according to claim 1 or 2, wherein the properties satisfy at least one of: yield strength Rt_0.5560 to 680MPa, and a tensile strength Rm of 640 to760MPa, yield ratio Rt_0.5Rm is less than or equal to 0.89, and elongation A_50.8Not less than 22 percent; the Charpy impact energy AKV at-20 ℃ is more than or equal to 230J; the shearing area rate SA% of the DWTT fracture at-15 ℃ is more than or equal to 85%.

11. A method of manufacturing an economical low yield ratio high strength steel according to any one of claims 1-10, comprising the steps of:

(1) smelting and continuous casting;

(2) reheating;

(3) rolling;

(4) and (3) cooling: a DQ + ACC two-stage cooling process is adopted, wherein the open cooling temperature is 700-750 ℃, the cooling speed in the DQ stage is 30-40 ℃/s, the DQ stop cooling temperature is 550-620 ℃, the cooling speed in the ACC stage is 10-25 ℃/s, and the ACC stop cooling temperature is 430-530 ℃.

12. The manufacturing method according to claim 11, wherein in the step (1), the continuous casting drawing speed fluctuation is controlled within ± 0.3m/min, and segregation is controlled using a dynamic soft pressure.

13. The method of claim 11, wherein in the step (2), the reheating temperature is controlled to 1100 to 1160 ℃.

14. The production method as claimed in any one of claims 11 to 13, wherein in the step (3), in the rough rolling stage, the rough rolling temperature is controlled to 950 to 1080 ℃, the single pass reduction rate of the last 2 passes of the rough rolling is not less than 15%, and the final rolling temperature of the rough rolling is in the range of 950 to 990 ℃.

15. The production process according to any one of claims 11 to 13, wherein in the step (3), in the finish rolling stage, the finish rolling temperature is controlled to 770 to 860 ℃, the total reduction rate of finish rolling is 70% or more, and the finish rolling temperature is 770 to 820 ℃.

Technical Field

The invention relates to a metal material and a manufacturing method thereof, in particular to a steel grade and a manufacturing method thereof.

Background

The natural gas is taken as a main clean energy source, the ratio of the natural gas in an energy consumption structure is increased year by year, and in recent years, a large number of natural gas pipelines are paved on the global scale, which also drives the pipeline steel product technology to make continuous progress. At present, X80 high-strength pipeline steel is applied to east-west gas transmission and middle-Russian east pipeline in China. However, with the further development of the technology, in order to improve the plastic deformation tolerance and service safety of the pipeline, the pipeline designer has continuously increased the requirements on the performance index of the pipeline steel, wherein the requirement of low yield ratio has become a technological development trend. The industrial universal standard API Spec 5L requires that the upper limit of the yield ratio of an X80 pipeline pipe is 0.93, and the yield ratio required by part of pipeline engineering design is less than or equal to 0.90, so that the X80 pipeline steel needs to be redesigned to meet the yield ratio requirement of a steel pipe after the pipe is manufactured.

In the aspect of economy, in order to meet the requirements of strength and toughness, the conventional X80 pipeline steel generally depends on multiple alloys of Cu, Ni, Cr, Mo, Nb and V in composition to ensure the performance, the cost of the alloy is relatively high, the steel consumption of long-distance pipeline engineering is generally large, and therefore, the development of the economical X80 pipeline steel with excellent performance can certainly bring huge economic benefits and improve the market competitiveness.

Chinese patent publication No. CN102676937A, published as 2012, 9/19/2012, entitled "a process for producing a low-cost high-strength X80 steel plate for pipelines", discloses a process for producing a low-cost high-strength X80 steel plate for pipelines, which achieves high strength at a low final cooling temperature (400 ℃ or lower), contains no Mo element in chemical components, and, although achieving low cost, is difficult to resist softening of a heat affected zone caused by welding of high-strength pipeline steel by solid solution strengthening, thereby affecting engineering applications.

Chinese patent publication No. CN101768703A, published 7.2010, entitled "low yield ratio X80 grade pipeline steel and method for manufacturing the same", discloses a low yield ratio X80 grade pipeline steel and method for manufacturing the same, wherein the yield ratio is in the range of 0.748-0.845, the contents of expensive alloy elements Cu, Ni, V, and Nb in the steel are high in chemical composition design, and a high slab heating temperature (1180-1220 ℃) is adopted to ensure alloy solid solution, so that the production cost is high.

The Chinese patent document with the publication number of CN101962733A and the publication date of 2011, 2.2.3 and the name of 'X80 high-deformation-resistant pipeline steel with low cost and high toughness and production method' discloses X80 pipeline steel with low cost and high toughness and high deformation resistance and production method, which adopts two stages of air cooling and water cooling after rolling to control a cooling process, further obtains a ferrite-bainite dual-phase structure and realizes that the yield ratio of the pipeline steel is less than 0.80, but because the ferrite content is high, the yield strength of the pipeline steel is only in the range of 530-600 MPa and does not reach the yield strength requirement of X80 (not less than 555MPa), and simultaneously, the contents of expensive alloy elements Cu and Ni in the embodiment of the patent are both more than 0.15 percent, the contents are high and the production cost is high.

Disclosure of Invention

One of the objectives of the present invention is to provide an economical low yield ratio high strength steel, which is designed by alloying Mn, Cr, Mo, and Nb, does not contain Cu, Ni, and V elements, has good economy, and effectively controls the alloy cost.

In order to achieve the above object, the present invention provides an economical low yield ratio high strength steel comprising the following chemical elements in mass percent:

C：0.045～0.080％，Si：0.10～0.30％，Mn：1.60～1.85％，Cr：0.15～0.30％，Mo：0.06～0.24％，Nb：0.040～0.075％，Ti：0.005～0.020％，Al：0.01～0.05％，Ca：0.001～0.004％，N：0.001～0.005％。

further, in the economical low yield ratio high strength steel of the present invention, the chemical elements in mass percent are:

c: 0.045-0.080%, Si: 0.10 to 0.30%, Mn: 1.60-1.85%, Cr: 0.15-0.30%, Mo: 0.06-0.24%, Nb: 0.040-0.075%, Ti: 0.005-0.020%, Al: 0.01-0.05%, Ca: 0.001-0.004%, N: 0.001 to 0.005% and the balance Fe and other unavoidable impurities.

In the economical low yield ratio high strength steel of the present invention, the design principle of each chemical element is as follows:

c: in the economical low yield ratio high strength steel according to the present invention, carbon is the most basic strengthening element, and has the effects of solid solution strengthening and carbide precipitation strengthening. The proper amount of C element can effectively ensure the strength of the steel, but it is noted that too high C content increases the size and content of carbides in the structure, thereby affecting the low-temperature toughness and welding performance of the steel. Therefore, the mass percent of C in the economical low-yield-ratio high-strength steel is controlled to be 0.045-0.080%.

In some preferred embodiments, the mass percentage of C can be controlled to be between 0.050% and 0.075%.

Si: in the economical low yield ratio high strength steel according to the present invention, Si is a solid solution strengthening element and also a deoxidizing element in the steel, but too high a mass percentage of Si in the steel deteriorates the weldability of the steel. Therefore, the mass percent of Si in the economical low yield ratio high-strength steel is controlled to be 0.10-0.30%.

Mn: in the economical low yield ratio high strength steel according to the present invention, Mn can improve the strength of the steel by solid solution strengthening, which is the most effective and economical strengthening element in the steel. The Mn element also has the function of promoting the formation of MA (martensite) in the high-strength steel, is beneficial to improving the tensile strength of the steel and further reducing the yield ratio, but the size and the content of the MA are not suitable to be too large, otherwise, the toughness of the steel is reduced. On the other hand, too high content of Mn increases the difficulty of center segregation control, resulting in a decrease in toughness of the steel. Therefore, the mass percent of Mn in the economical low-yield-ratio high-strength steel is controlled to be 1.60-1.85%.

In some preferred embodiments, the mass percent of Mn can be controlled between 1.65 and 1.80%.

Cr: in the economical low-yield-ratio high-strength steel, Cr can effectively improve the hardenability of the steel, ensure the uniformity of the structure and the performance in the thickness direction of a thick steel plate and improve the strength of the steel. However, it should be noted that if the Cr content in the steel is too high, the steel sheet is liable to form a hard phase structure during rapid cooling, which is disadvantageous in low-temperature toughness and weldability. Therefore, the economical low yield ratio high strength steel of the present invention has Cr content controlled within 0.15-0.30% by mass.

Mo: in the economical low-yield-ratio high-strength steel, Mo element can effectively improve the strength of the steel, has the function of expanding a gamma phase region, can reduce the gamma → alpha phase transition temperature of the steel, can play a role of refining a phase transition structure in the steel, and can inhibit the formation of low-toughness structures such as quasi-polygonal ferrite, pearlite and the like. In addition, the Mo element can also effectively suppress softening of the heat affected zone during the pipe welding process. However, it should be noted that Mo is expensive and should not be added in excess. Therefore, the mass percent of Mo in the economical low-yield-ratio high-strength steel is controlled to be 0.06-0.24%.

In some preferred embodiments, the mass percent of Mo can be controlled between 0.06-0.18%.

Nb: in the economical low-yield-ratio high-strength steel, Nb is an important element for grain refinement, and solid-dissolved Nb can pin deformation austenite grain boundaries through solute dragging and prevent austenite grain growth. In addition, it has the effect of raising the recrystallization temperature, which can increase the strain accumulation in the unrecrystallized region of the finish rolling process. As the temperature in the hot rolling process is reduced, the nitride and carbide of Nb are separated out, so that the ferrite grains can be inhibited from growing by pinning the grain boundary in the gamma → alpha phase transformation process, and the precipitation strengthening effect is realized. However, it should be noted that when the Nb content of the steel is too high, the solubility product of C, Nb is limited, and a higher slab heating temperature is required for heating, which results in the growth of prior austenite grains. Therefore, the mass percent of Nb in the economical low-yield-ratio high-strength steel is controlled to be 0.040-0.075%.

In some preferred embodiments, the mass percent of Nb can be controlled between 0.045 and 0.065%.

Ti: in the economical low-yield-ratio high-strength steel, Ti and N have strong binding force and are strong carbonitride forming elements, TiN formed by the Ti has higher thermal stability and can prevent austenite grains from growing in the slab heating and rough rolling recrystallization processes, and in addition, the TiN can also inhibit the grains in a heat affected zone from growing in the welding process and improve the welding performance. However, it should be noted that too high a Ti content in the steel may result in formation of Ti nitrides and carbides with large sizes, which are detrimental to the toughness of the steel. Therefore, the mass percent of Ti in the economical low-yield-ratio high-strength steel is controlled to be 0.005-0.020%.

Al: in the economical low yield ratio high-strength steel, Al is a deoxidizing element, and in order to achieve the deoxidizing effect, the mass percent of Al in the economical low yield ratio high-strength steel is controlled to be 0.01-0.05%.

Ca: in the economical low yield ratio high strength steel according to the present invention, the formation of MnS inclusions can be suppressed by controlling the form of sulfides through micro Ca treatment. However, it should be noted that if the content of Ca in the steel is too high, Ca-containing inclusions are introduced, thereby affecting the properties of the steel. Therefore, the mass percent of Ca in the economical low-yield-ratio high-strength steel is controlled to be 0.001-0.004%.

N: in the economical low-yield-ratio high-strength steel, the N element and Ti can form high-melting-point TiN particles, so that the effect of inhibiting austenite grains from coarsening in the reheating process can be achieved. However, when the content of N in steel is too high, interstitial N atoms may pin dislocations, so that the yield strength and yield ratio of steel are significantly increased. Therefore, the mass percent of N in the economical low yield ratio high-strength steel is controlled to be 0.001-0.005%.

Further, in the economical low-yield-ratio high-strength steel of the invention, the mass percentages of the chemical elements simultaneously satisfy: c + Mn/6 is more than or equal to 0.34 percent and less than or equal to 0.38 percent, Cr + Mo is more than or equal to 0.30 percent and less than or equal to 0.40 percent, and Ti/N is more than or equal to 2.5 and less than or equal to 5.0 percent, wherein C, Mo, Cr, N, Ti and Mn all represent the mass percent of corresponding elements.

In the technical scheme, the economical low-yield-ratio high-strength steel can effectively ensure the strength of the economical low-yield-ratio high-strength steel by controlling the content of C + Mn/6 to be more than or equal to 0.34 and less than or equal to 0.38 and Mo + Cr to be more than or equal to 0.30 and less than or equal to 0.40 while controlling the content of a single element.

Further, the economical low yield ratio high strength steel of the present invention further contains 0 < B < 0.0005%.

In the technical scheme of the invention, the economical low-yield-ratio high-strength steel can also contain a small amount of B which is used as a strong hardenability element, and the hardenability of the steel can be improved by a proper amount of B. However, since too high a content of B adversely affects the plasticity and toughness of the steel, the mass percentage of B is controlled to be 0 < B.ltoreq.0.0005% in the economical low yield ratio high strength steel according to the present invention.

Further, in the economical low yield ratio high strength steel of the present invention, P is not more than 0.015% and/or S is not more than 0.002% among other inevitable impurities.

In the above technical solution, in the economical low yield ratio high strength steel of the present invention, both P and S are inevitable impurity elements in the steel, and the lower the content of the P and S elements in the steel, the better. S is easy to form MnS inclusions, the MnS inclusions are in a strip shape after being rolled, P is an element easy to segregate, and the performance of the steel is greatly influenced due to the fact that the content of P and S impurity elements in the steel is too high. Therefore, in the economical low-yield-ratio high-strength steel, the mass percent of P is controlled to be less than or equal to 0.015 percent, and the mass percent of S is controlled to be less than or equal to 0.002 percent.

Further, in the economical low yield ratio high strength steel of the present invention, the chemical elements thereof may satisfy at least one of the following contents by mass:

C：0.050～0.075％，

Mn：1.65～1.80％，

Mo：0.06～0.18％，

Nb：0.045～0.065％。

further, in the economical low yield ratio high strength steel according to the present invention, the microstructure thereof is polygonal ferrite + mao element + granular bainite.

Further, in the economical low yield ratio high strength steel of the present invention, the phase proportion of polygonal ferrite is 5 to 25%, and the phase proportion of the martensite component is 2 to 10%.

Further, in the economical low yield ratio high strength steel according to the present invention, wherein the average grain size of polygonal ferrite is less than 10 μm; and/or the average size of the mao elements is less than 2 μm.

Further, in the economical low yield ratio high strength steel according to the present invention, the properties satisfy at least one of the following: yield strength Rt_0.5560 to 680MPa, a tensile strength Rm of 640 to 760MPa, a yield ratio Rt_0.5Rm is less than or equal to 0.89, and elongation A_50.8Not less than 22 percent; the Charpy impact energy AKV at-20 ℃ is more than or equal to 230J; the shearing area rate SA% of the DWTT fracture at-15 ℃ is more than or equal to 85%.

Accordingly, another object of the present invention is to provide an economical method for manufacturing a high strength steel with a low yield ratio, the manufacturing methodThe method has low production process cost, and the yield strength Rt of the economical low-yield-ratio high-strength steel prepared by the method_0.5560 to 680MPa, a tensile strength Rm of 640 to 760MPa, a yield ratio Rt_0.5Rm is less than or equal to 0.89, and elongation A_50.8Not less than 22 percent; the Charpy impact energy AKV at-20 ℃ is more than or equal to 230J; the shearing area rate SA% of the DWTT fracture at the temperature of-15 ℃ is more than or equal to 85%, and the DWTT fracture has the characteristics of excellent toughness and low yield ratio.

In order to achieve the above object, the present invention provides a method for producing the economical low yield ratio high strength steel, comprising the steps of:

(1) smelting and continuous casting;

(2) reheating;

(3) rolling;

(4) and (3) cooling: a DQ (on-line direct quenching) and ACC (accelerated cooling) two-stage cooling process is adopted, wherein the open cooling temperature is 700-750 ℃, the cooling speed in the DQ stage is 30-40 ℃/s, the DQ stop cooling temperature is 550-620 ℃, the cooling speed in the ACC stage is 10-25 ℃/s, and the ACC stop cooling temperature is 430-530 ℃.

In the method for manufacturing the economical low-yield-ratio high-strength steel, the process conditions, particularly the cooling process parameters, are controlled, the theories of grain refinement, precipitation strengthening, phase change control and the like of the low-carbon niobium-containing steel are utilized, and the controlled rolling and controlled cooling technology is fully utilized, so that the economical low-yield-ratio high-strength steel manufactured by the method has a microstructure comprising granular bainite, fine polygonal ferrite and fine martensite, and has the characteristics of excellent obdurability and low yield ratio.

In the manufacturing method, in the step (4), the temperature of boiling and cooling the entering water is controlled to be 700-750 ℃, so that a small amount of polygonal ferrite can be ensured to be precipitated, and the low yield ratio is obtained. The DQ cooling section controls the cooling speed to be 30-40 ℃/s for fast cooling, so that the growth of ferrite can be avoided, the quasi-polygonal ferrite and the pearlite structure can be inhibited, and the strength of steel is ensured. The ACC cooling section controls a relatively low cooling speed of 10-25 ℃/s, so that a small amount of fine Mao components can be formed, the tensile strength of the steel is improved, and the toughness cannot be reduced. In addition, the ACC cooling stop temperature can influence the bainite form, and the ACC cooling stop temperature is controlled to be 430-530 ℃ in the method, so that fine granular bainite can be obtained.

Further, in the manufacturing method of the present invention, in the step (1), the continuous casting drawing speed fluctuation is controlled within the range of ± 0.3m/min, and segregation is controlled by using a dynamic soft pressure.

Further, in the manufacturing method of the present invention, in the step (2), the reheating temperature is controlled to 1100 to 1160 ℃.

In the economical method for producing a low yield ratio high strength steel according to the present invention, the reheating temperature is controlled to 1100 to 1160 ℃ in the step (2). This is because: in the present invention, a reheating temperature is set as low as possible in order to prevent austenite grains of a slab from growing large, and the reheating temperature is not set too low in order to ensure sufficient solid solution of Nb. Therefore, in order to ensure the comprehensive performance of the steel, the reheating temperature is controlled to be 1100-1160 ℃.

Further, in the manufacturing method of the invention, in the step (3), in the rough rolling stage, the rough rolling temperature is controlled to be 950-1080 ℃, the single pass reduction rate of the last 2 passes of rough rolling is more than or equal to 15%, and the final pass rolling temperature of rough rolling is 950-990 ℃.

In the above scheme, in the step (3), the main function of rough rolling is to refine grains by recrystallization, and the rough rolling should be performed at a temperature above the recrystallization temperature, so that the rough rolling temperature is controlled to be 950 to 1080 ℃. In addition, in order to promote the recrystallization of the slab center, the single-pass reduction rate of the last 2 passes of rough rolling is more than or equal to 15 percent, and deformation and penetration to the slab center can be ensured. In addition, in order to control the grain growth degree of recrystallization, the final pass of rough rolling needs to be controlled in a lower temperature range of 950-990 ℃.

Further, in the manufacturing method of the present invention, in the step (3), in the finish rolling stage, the finish rolling temperature is controlled to 770 to 860 ℃, the total reduction rate of finish rolling is not less than 70%, and the finish rolling temperature is controlled to 770 to 820 ℃.

In the above aspect of the present invention, in the step (3), the finish rolling is performed in the non-recrystallization region, and increasing the finish rolling reduction ratio increases strain energy storage and a deformation zone inside the deformed austenite grains, thereby promoting phase transformation nucleation. On the other hand, the lower the rolling temperature is, the less the recovery of strain energy storage is, so the invention adopts the lower finish rolling temperature, controls the finish rolling temperature to be 770-860 ℃, controls the finish rolling temperature to be 770-820 ℃ and controls the total reduction rate of finish rolling to be more than or equal to 70%.

Compared with the prior art, the economical low-yield-ratio high-strength steel and the manufacturing method thereof have the advantages and beneficial effects as follows:

the economical low-yield-ratio high-strength steel disclosed by the invention is designed by adopting Mn, Cr, Mo and Nb alloyed chemical components, does not contain Cu, Ni and V elements, has good economical efficiency, and effectively controls the alloy cost. Yield strength Rt of the economical low yield ratio high strength steel_0.5560 to 680MPa, a tensile strength Rm of 640 to 760MPa, a yield ratio Rt_0.5Rm is less than or equal to 0.89, and elongation A_50.8Not less than 22 percent; the Charpy impact energy AKV at-20 ℃ is more than or equal to 230J; the shearing area rate SA% of the DWTT fracture at the temperature of-15 ℃ is more than or equal to 85%, and the characteristics of low yield ratio and high strength are realized while the economic performance is good.

In addition, the manufacturing method controls the process conditions, particularly the cooling process parameters, so that the microstructure of the economical low-yield-ratio high-strength steel obtained by the manufacturing method is a complex phase structure of polygonal ferrite, a martensite component and granular bainite, wherein the volume percentage content of the polygonal ferrite is 5-25%, the average grain size is less than 10 mu m, the martensite component content is 2-10%, and the average size is less than 2 mu m. Effectively ensures that the prepared economical low-yield-ratio high-strength steel has the characteristics of excellent obdurability and low yield ratio.

Drawings

FIG. 1 is a metallographic structure diagram of an economical low-yield-ratio high-strength steel according to example 1 under a microscope of 500 times.

FIG. 2 is a metallographic structure diagram of an economical low-yield-ratio high-strength steel of example 1 under a microscope 1000 times.

Detailed Description

The economical low yield ratio high strength steel and the method for manufacturing the same according to the present invention will be further explained and explained with reference to the following specific examples and the drawings attached to the specification, but the explanation and explanation do not unduly limit the technical solution of the present invention.

Examples 1 to 6

Table 1 shows the mass percentages of the chemical elements in the economical low yield ratio high strength steels of examples 1 to 6.

Table 1 (wt%, balance Fe and other unavoidable impurities except P, S)

The economical low yield ratio high strength steels of examples 1-6 of the present invention were all prepared by the following steps:

(1) smelting and continuous casting: wherein in the continuous casting process, the continuous casting drawing speed fluctuation is controlled within the range of +/-0.3 m/min, and the segregation is controlled by adopting dynamic soft reduction;

(2) reheating: controlling the reheating temperature to be 1100-1160 ℃;

(3) rolling: in the rough rolling stage, the rough rolling temperature is controlled to be 950-1080 ℃, the single-pass reduction rate of the last 2 passes of rough rolling is more than or equal to 15%, and the final rolling temperature of the rough rolling is 950-990 ℃. In the finish rolling stage, the finish rolling temperature is controlled to be 770-860 ℃, the total reduction rate of finish rolling is more than or equal to 70%, and the finish rolling temperature is 770-820 ℃;

(4) and (3) cooling: a DQ + ACC two-stage cooling process is adopted, wherein the open cooling temperature is 700-750 ℃, the cooling speed in the DQ stage is 30-40 ℃/s, the DQ stop cooling temperature is 550-620 ℃, the cooling speed in the ACC stage is 10-25 ℃/s, and the ACC stop cooling temperature is 430-530 ℃.

Tables 2-1 and 2-2 list specific process parameters for the method of manufacturing economical low yield ratio high strength steels of examples 1-6.

Table 2-1.

It should be noted that, since the temperature control is fluctuated and not stabilized at a fixed value during the actual operation, the rough rolling temperature and the finish rolling temperature in the step (3) in table 2-1 are assumed to be one end range values rather than point values in each example.

Table 2-2.

The economical low yield ratio high strength steels of examples 1 to 6 were subjected to various performance tests, and the results thereof are shown in Table 3.

Table 3 shows the results of mechanical property testing of economical low yield ratio high strength steels of examples 1-6.

Table 3.

As can be seen from Table 3, the yield strength Rt of each example of the present invention_0.5All in the range of 560 to 680MPa, the tensile strength Rm in the range of 640 to 760MPa, the yield ratio Rt_0.5Rm is less than or equal to 0.89, and elongation A_50.8More than or equal to 22 percent, the Charpy impact power AKV at the temperature of minus 20 ℃ is more than or equal to 230J, and the DWTT fracture shearing area rate SA at the temperature of minus 15 ℃ is more than or equal to 85 percent. The economical low-yield-ratio high-strength steel of each embodiment has excellent performances, excellent toughness and low-yield-ratio characteristics, and can be effectively used as pipeline steel to be effectively applied to the field of natural gas transportation.

FIG. 1 is a metallographic structure diagram of an economical low-yield-ratio high-strength steel according to example 1 under a microscope of 500 times.

FIG. 2 is a metallographic structure diagram of an economical low-yield-ratio high-strength steel of example 1 under a microscope 1000 times.

As can be seen from fig. 1 and 2, the microstructure of the economical low yield ratio high strength steel of example 1 is a complex phase structure of polygonal ferrite + mao element + granular bainite. In addition, it was determined that the economical low yield ratio high strength steel of example 1 had a phase ratio of polygonal ferrite of 5 to 25%, an average grain size of polygonal ferrite of less than 10 μm, a phase ratio of martensite group of 2 to 10%, and an average grain size of martensite group of less than 2 μm.

It should be noted that the above-mentioned embodiments are only specific embodiments of the present invention. It is apparent that the present invention is not limited to the above embodiments and similar changes or modifications can be easily made by those skilled in the art from the disclosure of the present invention and shall fall within the scope of the present invention.