阅读说明：本技术 一种改善扩孔性能的冷轧双相钢及其制造方法 (Cold-rolled dual-phase steel with improved hole expanding performance and manufacturing method thereof ) 是由 彭冲 唐小勇 杨源华 赵征志 唐淑云 李世桓 康涛 杨帆 王振基 黄珍 徐细华 于 2020-05-19 设计创作，主要内容包括：本发明提供了一种改善扩孔性能的冷轧双相钢及其制造方法,成分：C：0.08％-0.10％,Si：1.10％-1.20％,Mn：1.90％-2.10％,P：≤0.015％,S：≤0.004％,N：≤0.005％,其余为Fe和其他不可避免的杂质。与现有技术相比,本发明钢有以下优点：(1)成本低廉,工艺简单,适用于对强度和成形性能都具有较高要求的汽车零部件；(2)本发明钢通过设计简单的成分体系并合理控制工艺参数,使得成品组织由铁素体、马氏体以及少量贝氏体组成。本发明钢的抗拉强度大于780MPa,屈服强度较低,范围为R<Sub>p0.2</Sub>＝380MPa-435MPa,总延伸率大于18％,屈强比小于0.51。(The invention provides a cold-rolled dual-phase steel with improved hole expanding performance and a manufacturing method thereof, and the cold-rolled dual-phase steel comprises the following components: c: 0.08% -0.10%, Si: 1.10% -1.20%, Mn: 1.90% -2.10%, P: less than or equal to 0.015 percent, S: less than or equal to 0.004%, N: less than or equal to 0.005 percent, and the balance of Fe and other inevitable impurities. Compared with the prior art, the steel has the following advantages: (1) the cost is low, the process is simple, and the method is suitable for automobile parts with higher requirements on strength and forming performance; (2) the steel of the invention ensures that the finished product structure consists of ferrite, martensite and a small amount of bainite by designing a simple component system and reasonably controlling process parameters. The tensile strength of the steel is greater than 780MPa, the yield strength is lower, and the range isIs R p0.2 380MPa-435MPa, total elongation greater than 18% and yield ratio less than 0.51.)

1. The cold-rolled dual-phase steel is characterized by comprising the following components in percentage by weight: c: 0.08% -0.10%, Si: 1.10% -1.20%, Mn: 1.90% -2.10%, P: less than or equal to 0.015 percent, S: less than or equal to 0.004%, N: less than or equal to 0.005 percent, and the balance of Fe and other inevitable impurities.

2. A method of manufacturing a cold-rolled dual-phase steel with improved hole expansibility as recited in claim 1, comprising the steps of:

1) making steel and continuously casting into a plate blank;

2) hot rolling;

3) acid washing and cold rolling;

4) annealing treatment;

5) leveling and pulling-straightening.

3. The manufacturing method according to claim 2, wherein the hot rolling in step 2) is specifically: the initial rolling temperature of rough rolling in the hot rolling process is controlled to be 1030-1110 ℃, and the final rolling temperature is controlled to be 850-950 ℃.

4. The manufacturing method according to claim 2, wherein a coiling temperature is controlled to 610 ℃ to 680 ℃ during the hot rolling.

5. The manufacturing method according to claim 2, wherein the step 4) of performing continuous annealing is: the annealing temperature is controlled at 760 ℃ and 840 ℃, then the annealing temperature is slowly cooled to 630-650 ℃, and then the annealing temperature is quickly cooled to 260-340 ℃, and the overaging treatment is carried out.

6. The method as claimed in claim 5, wherein the annealing temperature is controlled at 760-840 ℃ and the holding time is 160-180 s.

7. The manufacturing method according to claim 5, wherein the slow cooling is: slowly cooling to 630-650 ℃ at a cooling speed of 3-6 ℃/s.

8. The manufacturing method according to claim 5, wherein the rapid cooling is: the cooling speed of 25 ℃/s-40 ℃/s is fast cooled to 260-340 ℃.

9. The manufacturing method according to claim 5, wherein in the step 4), the overaging treatment is performed at 260-340 ℃ for 500-600 s.

10. Manufacturing method according to any one of claims 2 to 9, characterized in that the manufactured cold-rolled dual-phase steel has the properties: rp 0.2: 380-435MPa, Rm: 780MPa-880MPa, A80 is more than or equal to 18.0 percent, Rp0.2/Rm: less than or equal to 0.51 percent and the hole expansion rate is more than or equal to 92 percent.

Technical Field

The invention belongs to the technical field of metal material manufacturing, particularly relates to the field of automobile steel manufacturing, and particularly relates to cold-rolled dual-phase steel with improved hole expanding performance and a manufacturing method thereof.

Background

Dual Phase steel (commonly referred to as DP steel, DP being designated by the first letter of the english Dual Phase) means a two Phase steel in which the matrix structure is mainly ferrite and martensite. Not only has higher tensile strength level, but also has lower yield ratio and higher work hardening rate. In view of the good matching of the strength and the plasticity of the dual-phase steel, the steel grade becomes a hot spot of selecting high-strength steel cold-rolled sheets for automobile manufacturing enterprises. At present, the iron and steel enterprises at home and abroad develop dual-phase steel with strength level of more than 780 MPa.

The traditional continuous annealing process of the dual-phase steel is to obtain ferrite and a small amount of martensite structure by heating in a critical two-phase region and then rapidly cooling, and different alloy component systems and different process regimes influence the final structure state of a product. The problem of cracking often occurs in the hole expanding and flanging process because the hole expanding performance of the method is poor due to the large difference of the strength and hardness between martensite and ferrite.

Wuhan Steel Co Ltd, 2017, 10 and 19, applied for dual-phase steel with thin specification and good hole-expanding performance and a processing method thereof (authorization notice number: CN107829025A), and the dual-phase steel comprises the following components: c: 0.05% -0.08%, Si: 0.40% -0.90%, Mn: 1.10-1.60%, P is less than or equal to 0.015%, S is less than or equal to 0.004%, Cr: 0.30% -0.60%, Nb: 0.015% -0.040%, Als: 0.020-0.060% of Fe and the balance of inevitable impurities. Thin dual-phase steel with the thickness of 1.0-3.0mm is produced by adopting a thin slab continuous casting and rolling technology, and the tensile strength of a final product is more than 780 MPa. Although the technology has a short process flow, the yield strength of the final product is high (more than 500MPa), and the forming performance of the product is influenced. In addition, the hole expanding rate is only about 60 percent.

A cold-rolled dual-phase steel plate with 780 MPa-grade tensile strength and a preparation method thereof are applied by Maanshan Steel works Ltd in 2018, 5 month and 17 th (authorization notice No. CN108517466A), and the cold-rolled dual-phase steel plate comprises the following components: c: 0.08% -0.10%, Si: 0.10% -0.30%, Mn: 1.80-2.10%, P is less than or equal to 0.015%, S is less than or equal to 0.010%, Cr: 0.30% -0.50%, Nb: 0.04% -0.06%, N: less than or equal to 0.005 percent, and the balance of Fe and inevitable impurities. Through smelting, hot continuous rolling, acid pickling and cold rolling and continuous annealing processes, the 780MPa grade dual-phase steel with ferrite and martensite as metallographic structures and better mechanical properties is obtained. However, the precious alloy Nb is still adopted, the cost is improved, and in addition, the hole expansion rate is lower and is only about 40%.

Disclosure of Invention

The invention aims to provide the cold-rolled dual-phase steel with improved hole expanding performance, the manufactured product has excellent mechanical property, the hole expanding rate reaches more than 92 percent, and the cracking phenomenon in the punching operation process is avoided.

Still another object of the present invention is to provide a method for manufacturing a cold-rolled dual-phase steel with improved hole expansion performance, which significantly improves the hole expansion performance while ensuring the strength and plasticity index of the dual-phase steel through hot rolling, cold rolling, annealing, and overaging.

The specific technical scheme of the invention is as follows:

a cold-rolled dual-phase steel with improved hole expanding performance comprises the following components in percentage by weight: c: 0.08% -0.10%, Si: 1.10% -1.20%, Mn: 1.90% -2.10%, P: less than or equal to 0.015 percent, S: less than or equal to 0.004%, N: less than or equal to 0.005 percent, and the balance of Fe and other inevitable impurities.

The invention provides a manufacturing method of cold-rolled dual-phase steel with improved hole expanding performance, which comprises the following steps:

1) making steel and continuously casting into a plate blank;

2) hot rolling;

3) acid washing and cold rolling;

4) annealing treatment;

5) leveling and pulling-straightening.

The step 1) comprises the following steps: molten iron desulfurization and converter smelting, argon blowing is carried out on the smelted molten steel, refining is carried out in an LF furnace, and then the molten steel is continuously cast into a plate blank.

The hot rolling in the step 2) is specifically as follows: and heating the plate blank and then carrying out hot rolling, wherein the initial rolling temperature of rough rolling in the hot rolling process is controlled to be 1030-1110 ℃, the final rolling temperature is controlled to be 850-950 ℃, and the coiling temperature is controlled to be 610-680 ℃.

Preferably, the hot rolling is: the initial rolling temperature of rough rolling is controlled to be 1050-1100 ℃, the final rolling temperature is controlled to be 880-920 ℃, and the coiling temperature is controlled to be 630-670 ℃.

In step 3): pickling the hot rolled steel strip by using a pickling and rolling combination machine set; and (3) cold rolling, namely performing 5-pass cold rolling, wherein the cold rolling reduction rate is 50-75%.

The annealing treatment in the step 4) comprises the following steps: the annealing temperature is controlled at 760 ℃ and 840 ℃, then the annealing temperature is slowly cooled to 630-650 ℃, and then the annealing temperature is quickly cooled to 260-340 ℃, and the overaging treatment is carried out.

In the step 4), the annealing temperature is controlled at 760-;

in the step 4), the slow cooling means: slowly cooling to 630-650 ℃ at a cooling speed of 3-6 ℃/s.

In the step 4), the rapid cooling means: the cooling speed of 25 ℃/s-40 ℃/s is fast cooled to 260-340 ℃.

In the step 4), the overaging treatment refers to overaging treatment at 260-340 ℃ for 500-600 s.

Further, after overaging treatment, slowly cooling to room temperature at a cooling speed of 3 ℃/s-8 ℃/s.

Preferably, the step 4) of performing continuous annealing specifically comprises: the annealing temperature is controlled at 770-820 ℃, and the heat preservation time is 165-170 s; then slowly cooling to 630-650 ℃ at the cooling speed of 3-6 ℃/s, then quickly cooling to 280-320 ℃ at the cooling speed of 26-30 ℃/s, carrying out overaging treatment for 550-560 s, and then slowly cooling to room temperature at the cooling speed of 3-8 ℃/s.

The manufactured cold-rolled dual-phase steel has the following properties: rp 0.2: 380-435MPa, Rm: 780MPa-880MPa, A80 is more than or equal to 18.0 percent, Rp0.2/Rm: less than or equal to 0.51 percent and the hole expansion rate is more than or equal to 92 percent.

The manufacturing process of the product of the invention comprises the following steps: firstly, molten iron desulfurization and converter smelting are carried out, argon blowing and LF furnace refining are carried out on the smelted molten steel, then the molten steel is continuously cast into a plate blank, the continuously cast plate blank is continuously rolled into a steel strip through heating, the hot rolled steel strip is acid-washed by an acid rolling combination unit, then 5 times of cold rolling is carried out, and after the steps of continuous annealing, leveling and straightening, the cold-rolled dual-phase steel plate is produced.

As the strength of dual phase steel is continuously improved, formability thereof inevitably deteriorates. The cracking phenomenon often occurs in the stamping process, and particularly, the cracking is most common after the hole is reamed. Therefore, the strength and plasticity indexes of the dual-phase steel are ensured, the hole expanding performance of the product is also considered, and the good hole expanding performance is also an important index of the cold-rolled dual-phase steel. The general idea of the invention is therefore: firstly, ensuring higher strength: the content of C is controlled at a proper level to ensure that a dual-phase structure with polygonal ferrite surrounding a second phase and better welding performance are obtained; meanwhile, the higher Mn is added, so that the effects of solid solution strengthening and ferrite grain refinement are achieved; the use of more Si can play a role in 'cleaning' and 'purifying' solid solution carbon in ferrite, reduce the solid solution strengthening of gaps, inhibit the generation of coarse carbides during cooling and improve the ductility of the dual-phase steel. Secondly, improving the hole expansion performance: the overaging temperature is improved by adjusting continuous annealing process parameters of the traditional cold-rolled high-strength steel, so that a small amount of bainite structure is introduced into the structure, and the hole expansion performance of the cold-rolled dual-phase steel is improved.

The invention is produced according to the requirement organization of the dual-phase steel, and starts with the design components in order to achieve the steel belt with high strength, high plasticity, good hole expansion performance. One of the innovation points of the invention is that more Si element is added, the Si content is in the range of 1.10-1.20%, the martensite is promoted to be in a fine microscopic distribution, the good strengthening effect and the good matching of strength and plasticity of the dual-phase steel are ensured, and in addition, the yield strength of the dual-phase steel can be obviously reduced, so that the yield ratio is reduced, and the forming performance of the material is improved; simultaneously controlling the content of C in the steel to be within the range of 0.08-0.10% and the content of Mn within the range of 1.90-2.10% to ensure the strength and weldability of the dual-phase steel; p, S, N has the advantages of high purity, punching performance and surface quality of the deteriorated steel, and the content should be strictly controlled within the scope of the present invention. In addition, the invention does not add noble microalloy strengthening elements such as Nb, Ti and the like, thereby obviously reducing the production cost.

C: c no longer dominates solid solution strengthening in dual phase steels, but still significantly affects all phase transformation processes and controls the final structure and mechanical properties. To ensure a large ferrite precipitation zone, the carbon content is low. The temperature range of critical zone treatment is enlarged by reducing the content of C, C is controlled to be enriched in a metastable austenite region to avoid the precipitation of C, and the guarantee that a dual-phase structure with polygonal ferrite surrounding a second phase is obtained. For other properties, such as weldability and the like, the content of C is required to be limited, but if C is too low (< 0.02%), a dual-phase structure is not easy to obtain, and in order to ensure that the steel has a larger ferrite precipitation zone, C is preferably controlled to be about 0.08% -0.10%.

Si: si is a solid solution strengthening element of ferrite, accelerates the segregation of carbon to austenite, has the functions of 'clearing' and 'purifying' solid solution carbon in the ferrite, reduces the interstitial solid solution strengthening, can inhibit the generation of coarse carbides during cooling, and improves the ductility of the dual-phase steel. Meanwhile, Si can enlarge Fe-Fe₃In addition, in a continuous ferrite matrix, martensite can be promoted to be in a fine microscopic distribution, and good strengthening effect and good matching of strength and plasticity of the dual-phase steel are ensured.

Mn: mn is a typical austenite stabilizing element, remarkably improves the hardenability of steel, plays a role in solid solution strengthening and ferrite grain refinement, and can remarkably delay pearlite transformation. Mn as an element for enlarging the γ phase region lowers the critical points of A3 and a1, but a high Mn content retards the pearlite transformation and also delays the precipitation of ferrite, and a too low Mn content easily causes pearlite transformation. The invention controls the Mn: 1.90 to 2.10 percent.

P: p is a harmful element in steel, although the strength of the material is increased, the cold brittleness of the steel is increased, the plasticity of the material is reduced, the lower the material is, the better the material is, and in order to ensure the excellent performance of the product, the content of P is controlled to be less than or equal to 0.015 percent.

S: s is a harmful element in steel, and can form inclusions to reduce the ductility and toughness of the steel, the lower the S content is, the better the ductility and toughness of the steel are, and the S content is controlled to be less than or equal to 0.004% in order to ensure the excellent performance of the product.

Further, the following steps: in the hot rolling process, the plate blank is heated, the rough rolling initial rolling temperature in the hot rolling process is controlled to be 1030-1110 ℃, the final rolling temperature is controlled to be 850-950 ℃, and the curling temperature is controlled to be 610-680 ℃; in the cold rolling process, 5-frame continuous rolling is adopted in the rolling step. Then continuous annealing is carried out, the fire temperature is controlled at 760-; then slowly cooling to 630-650 ℃ at the cooling speed of 3-6 ℃/s, then quickly cooling to 260-340 ℃ at the cooling speed of 25-40 ℃/s, carrying out overaging treatment for 500-600 s, and then slowly cooling to room temperature at the cooling speed of 3-8 ℃/s. The purpose of the high temperature overaging treatment is to introduce a small amount of bainite structure. Bainite has a lower hardness relative to martensite, so that the susceptibility to cracking due to differences in hardness between the different phases is reduced during local deformation of the steel sheet.

In the subsequent hot rolling, cold rolling and continuous annealing production process of the components set by the invention, the finishing rolling temperature is controlled to be 850-950 ℃, and the coiling temperature is controlled to be 610-680 ℃. The cold rolling reduction is 50-75%, the annealing temperature is 760-840 ℃, the slow cooling section ending temperature is 630-650 ℃, and the overaging temperature is 260-340 ℃, so that the cold rolling dual-phase steel with the tensile strength of more than 780MPa, the yield strength lower, the range of Rp0.2-380 MPa-435MPa, the total elongation of more than 18% and the yield ratio of less than 0.51 can be obtained. Compared with the conventional process, the invention can introduce a small amount of bainite structure while increasing the martensite tempering degree by slightly increasing and controlling the overaging temperature to be 260-340 ℃, so that the finished product structure consists of ferrite, martensite and a small amount of bainite. In the cold-rolled dual-phase steel structure manufactured by the invention, the proportion of ferrite is 55-60%, the proportion of martensite is 25-30%, and the balance is bainite. Compared with martensite, the bainite has lower hardness, so that the hole expanding performance of the steel grade is effectively improved, the forming reliability of hole expanding type and flanging type parts is further improved, and the application range of the parts is greatly enlarged.

Compared with the prior art, the invention has the advantages that (1) the component design is reasonable, noble microalloy strengthening elements such as Nb, Ti and the like are not added, and the manufacturing cost is low; (2) the process window is wider, and the production difficulty is reduced; (3) the product has low yield ratio and good hole expansion performance, and is suitable for producing parts with higher strength and complex shapes.

Drawings

FIG. 1 is a scanning photograph of the structure of the cold-rolled dual-phase steel of example 1, wherein 1 is ferrite, 2 is martensite, and 3 is bainite.

Detailed Description