阅读说明：本技术 一种用CSP生产抗拉强度为1500MPa级高塑韧性汽车结构件用钢及生产方法 (Steel for high-plasticity-toughness automobile structural part with tensile strength of 1500MPa produced by CSP and production method ) 是由 胡宽辉 孙伟华 祝洪川 陈昊 肖厚念 王俊霖 陈一鸣 于 2021-06-21 设计创作，主要内容包括：一种用CSP生产抗拉强度为1500MPa级高塑韧性汽车结构件用钢,其化学成分及重量百分wt%含量：C：0.19～0.27%,Si：1.20～1.65%,Mn：1.30～2.50%,P：0.020%以下,S：0.010%以下,Als：0.012～0.080%,Cr：0.15～0.40%,Nb:0.025～0.045%,N：0.004%以下；工艺：铁水脱硫；电炉或转炉冶炼及精炼；连铸；第一次除鳞；均热；第二次除鳞；轧制；层流冷却；卷取；开卷落料或酸洗后落料成坯料；对坯料加热；冲压成型；进行零件表面的清理、切边、切孔。本发明不仅屈服强度≥820MPa,抗拉强度≥1365MPa,延伸率≥11％,还能使强塑积在17.7～22.5GPa·%,完全满足了制造经济型轿车用结构件轻量化对1500MPa级高抗碰撞性能零件强度和碰撞吸能特性要求。(A steel for producing high-plasticity and toughness automobile structural parts with tensile strength of 1500MPa by CSP comprises the following chemical components in percentage by weight: c: 0.19 to 0.27%, Si: 1.20-1.65%, Mn: 1.30-2.50%, P: 0.020% or less, S: 0.010% or less, Als: 0.012-0.080%, Cr: 0.15-0.40%, Nb 0.025-0.045%, N: less than 0.004%; the process comprises the following steps: desulfurizing molten iron; smelting and refining in an electric furnace or a converter; continuous casting; descaling for the first time; soaking; descaling for the second time; rolling; laminar cooling; coiling; uncoiling and blanking or pickling and then blanking into blanks; heating the blank; punching and forming; and cleaning, trimming and cutting the surface of the part. The invention not only has the yield strength of more than or equal to 820MPa, the tensile strength of more than or equal to 1365MPa and the elongation of more than or equal to 11 percent, but also can ensure that the product of strength and elongation is 17.7-22.5 GPa, and completely meets the requirements of manufacturing the lightweight structure for the economical car on the strength and the collision energy absorption characteristics of 1500 MPa-level high-collision-resistance parts.)

1. A steel for producing a high-plasticity and toughness automobile structural part with tensile strength of 1500MPa by using CSP comprises the following chemical components in percentage by weight: c: 0.19 to 0.27%, Si: 1.20-1.65%, Mn: 1.30-2.50%, P: 0.020% or less, S: 0.010% or less, Als: 0.012-0.080%, Cr: 0.15-0.40%, Nb 0.025-0.045%, N: less than 0.004%, and the balance of Fe and inevitable impurities; the metallographic structure is as follows: the martensite content is 80-90%, the bainite content is 2-8%, and the residual austenite content is 8-12%.

2. The steel for manufacturing high-ductility and toughness automotive structural parts with tensile strength of 1500MPa by CSP according to claim 1, wherein: the weight percentage of the Si is 1.32-1.65%.

3. The steel for manufacturing high-ductility and toughness automotive structural parts with tensile strength of 1500MPa by CSP according to claim 1, wherein: the weight percentage of Mn is 1.41-2.50%.

4. The steel for manufacturing high-ductility and toughness automotive structural parts with tensile strength of 1500MPa by CSP according to claim 1, wherein: the weight percentage of the Cr is 0.19-0.40%.

5. The steel for manufacturing high-ductility and toughness automotive structural parts with tensile strength of 1500MPa by CSP according to claim 1, wherein: the Nb accounts for 0.031-0.045 wt%.

6. The method for producing a steel for a highly ductile and ductile automotive structural member having a tensile strength of 1500MPa by using a CSP according to claim 1, comprising the steps of:

1) desulfurizing molten iron, and controlling S to be less than or equal to 0.001%;

2) smelting and refining in an electric furnace or a converter;

3) continuous casting is carried out, the superheat degree of the tundish molten steel is controlled to be 15-35 ℃, the thickness of a casting blank is 50-70 mm, and the blank drawing speed is 3.0-6.0 m/min;

4) carrying out primary descaling on the casting blank, and controlling the pressure of descaling water to be not lower than 200 bar;

5) soaking the casting blank, and controlling the discharging temperature of the casting blank to be 1150-1200 ℃;

6) carrying out secondary descaling on the casting blank, and controlling the pressure of descaling water to be not lower than 250 bar;

7) rolling: and controlling the thickness of a finish rolling outlet to be 0.80-4.0 mm; controlling the first and second pass reduction rates to be not less than 50%; controlling the finish rolling temperature to be 870-910 ℃;

8) laminar cooling to coiling temperature;

9) coiling, wherein the coiling temperature is controlled to be 650-700 ℃;

10) directly uncoiling and blanking or blanking after acid cleaning to obtain a blank of the structural part to be prepared;

11) heating the blank to 830-930 ℃ at a heating speed of 12-30 ℃/s, and preserving heat for 240-500 s at the temperature;

12) stamping forming: stamping the heated blank into an automobile structural part to be prepared, wherein the cooling speed is controlled to be 15-28 ℃/s; controlling the metallographic structure of the formed part at room temperature to be that the volume content of martensite is 80-90%, the volume content of bainite is 2-8%, and the volume content of residual austenite is 8-12%;

13) and cleaning, trimming and cutting the surface of the part.

7. The method for producing high-ductility and toughness steel for automobile structural parts with tensile strength of 1500MPa by using CSP according to claim 6, wherein the method comprises the following steps: the blank is heated to 830-915 ℃ at the heating speed of 18-30 ℃/s, and the temperature is kept for 395-600 s at the temperature.

8. The method for producing high-ductility and toughness steel for automobile structural parts with tensile strength of 1500MPa by using CSP according to claim 7, wherein the method comprises the following steps: and controlling the cooling speed to be 19-28 ℃/s in the punch forming.

Technical Field

The invention relates to steel for an automobile and a production method thereof, in particular to steel for a high-plasticity and toughness automobile structural part with tensile strength of 1500MPa and a production method thereof, which are suitable for manufacturing the structural part for an economical car.

Background

With the development of the automobile industry, the continuous gliding phenomenon appears in the automobile market in China. For example, in 2020, the output and sales of Chinese automobiles are 2522.5 ten thousand and 2531.1 ten thousand respectively, and the output and sales are reduced by 2.0 percent and 1.9 percent respectively in the same ratio. Under the background of saturated production and sale in the automobile market, the improvement of automobile quality and the upgrading of industry are imminent, and the automobile manufacturing gradually develops towards the directions of light weight, intellectualization and networking, so that higher requirements are provided for the ultrahigh-strength material for the light-weight automobile body.

At present, the research surge of developing materials for ultrahigh-strength structural members for automobiles is raised worldwide, and the materials are mainly divided into two types: one type is an ultra-high strength automobile structural member formed by cold stamping, and the design of the product mainly adopts a high alloy content steel material, which adds a large amount of alloy elements into steel, improves the hardenability of the steel and simultaneously improves the room temperature stability of austenite, thereby ensuring that the developed steel has ultra-high strength and good forming performance. However, the alloy of the steel has high cost, complex process, difficult manufacturing process and low yield, and the problems of rebound, cracking, poor dimensional precision of parts and the like are easy to occur in the manufacturing process of the parts. The other type is that a proper heat treatment process mode is adopted in the part manufacturing process to realize high strengthening of the part, the alloy content of the steel for the part is lower than that of the ultrahigh-strength steel for the cold stamping forming part, and the ultrahigh-strength part is mainly obtained through the heat treatment process before or after forming. However, the steel produced by the prior art has poor strength and plasticity, the product of strength and plasticity for representing the forming capability of materials for high-strength parts for automobiles and the size of the deformation energy absorbed in the collision process is only about 5-10 GPa%, and the requirements of light weight of automobile bodies and high collision safety and energy absorption performance which are increasingly developed in the automobile industry cannot be met.

After retrieval:

chinese patent publication No. CN 101381839B discloses "a high-strength-ductility alloy steel and its heat treatment process". The high-strength-ductility alloy steel disclosed by the document comprises the following chemical components in percentage by weight: c: 0.03 to 0.05%, Mn: 30 ± 0.5%, Al: 3 ± 0.5%, Si: 3 plus or minus 0.5 percent and the balance of iron; the steel is heated to 1100 ℃ and is kept warm for 2-5 hours, and then the steel is quickly put into water for quenching. The tensile strength of the obtained workpiece is 650-700 MPa, the elongation after fracture is 55-70%, and the product of strength and elongation is 35-49 GPa. Although the product of strength and elongation of the steel is high, the alloy content is high, particularly the manganese content reaches 30%, and meanwhile, the steel needs long-time heat treatment, so that the industrial production difficulty is high, the alloy cost and the energy consumption are high, the strength is only 650-700 MPa, although the product of strength and elongation is high, the strength is low, and the requirements of lightweight automobile body design on structural parts with high impact resistance and high energy absorption performance cannot be met.

Chinese patent publication No. CN 107587052a discloses "a cold-rolled high-strength steel for automobile structural members and a manufacturing method thereof". The weight percentage of the main components is as follows: 0.06 to 0.08 percent of C, 0.65 to 0.95 percent of Mn, less than or equal to 0.05 percent of Si, less than or equal to 0.015 percent of P, less than or equal to 0.015 percent of S, 0.030 to 0.055 percent of Als, 0.025 to 0.040 percent of Nb and 0.010 to 0.030 percent of Ti. The high-strength steel plate obtained by hot rolling, acid pickling and cold continuous rolling, continuous annealing, flattening and straightening is 400MPa in yield strength of 330-. Although the method can produce an automobile structural part, the strength is low, and the requirement of automobile light weight design on ultrahigh-strength parts cannot be met.

In short, although the prior art can manufacture the automobile structural member, the product of strength and ductility is low, and the requirements of the light-weight automobile body design on the 1500 MPa-level structural member with high impact resistance and high energy absorption performance cannot be met. Although some workpieces with high product of strength and elongation can be obtained by adding a large amount of alloy and carrying out long-time heat treatment, the workpieces have low strength, high cost and difficult industrial mass production, and the requirements of lightweight design of automobiles on structural parts with high impact resistance and high energy absorption can not be met.

Disclosure of Invention

The invention aims to overcome the defects in the prior art, and provides the steel for the high-ductility and toughness automobile structural part and the production method thereof, wherein the steel for the high-ductility and toughness automobile structural part is designed by low alloy components and matched with a proper tissue regulation and control process, the yield strength is 800-1100 MPa, the tensile strength is 1350-1650 MPa, the elongation is more than or equal to 10%, and the product of strength and ductility is more than or equal to 15 GPa%, so that the requirements of manufacturing the lightweight structural part for the economical sedan on the strength and the impact energy absorption characteristics of 1500 MPa-level high-impact-resistance parts are met.

The measures for realizing the aim are as follows:

a steel for producing a high-plasticity and toughness automobile structural part with tensile strength of 1500MPa by using CSP comprises the following chemical components in percentage by weight: c: 0.19 to 0.27%, Si: 1.20-1.65%, Mn: 1.30-2.50%, P: 0.020% or less, S: 0.010% or less, Als: 0.012-0.080%, Cr: 0.15-0.40%, Nb 0.025-0.045%, N: less than 0.004%, and the balance of Fe and inevitable impurities; the metallographic structure is as follows: the martensite content is 80-90%, the bainite content is 2-8%, and the residual austenite content is 8-12%.

Preferably: the weight percentage of the Si is 1.32-1.65%.

Preferably: the weight percentage of Mn is 1.41-2.50%.

Preferably: the weight percentage of the Cr is 0.19-0.40%.

Preferably: the Nb accounts for 0.031-0.045 wt%.

A production method for producing high-ductility and toughness steel for automobile structural parts with tensile strength of 1500MPa by using CSP comprises the following steps:

1) desulfurizing molten iron, and controlling S to be less than or equal to 0.001%;

2) smelting and refining in an electric furnace or a converter;

3) continuous casting is carried out, the superheat degree of the tundish molten steel is controlled to be 15-35 ℃, the thickness of a casting blank is 50-70 mm, and the blank drawing speed is 3.0-6.0 m/min;

4) carrying out primary descaling on the casting blank, and controlling the pressure of descaling water to be not lower than 200 bar;

5) soaking the casting blank, and controlling the discharging temperature of the casting blank to be 1150-1200 ℃;

6) carrying out secondary descaling on the casting blank, and controlling the pressure of descaling water to be not lower than 250 bar;

7) rolling: and controlling the thickness of a finish rolling outlet to be 0.80-4.0 mm; controlling the first and second pass reduction rates to be not less than 50%; controlling the finish rolling temperature to be 870-910 ℃;

8) laminar cooling to coiling temperature;

9) coiling, wherein the coiling temperature is controlled to be 650-700 ℃;

10) directly uncoiling and blanking or blanking after acid cleaning to obtain a blank of the structural part to be prepared;

11) heating the blank to 830-930 ℃ at a heating speed of 12-30 ℃/s, and preserving heat for 240-500 s at the temperature;

12) stamping forming: stamping the heated blank into an automobile structural part to be prepared, wherein the cooling speed is controlled to be 15-28 ℃/s; controlling the metallographic structure of the formed part at room temperature to be that the volume content of martensite is 80-90%, the volume content of bainite is 2-8%, and the volume content of residual austenite is 8-12%;

13) and cleaning, trimming and cutting the surface of the part.

Preferably: the blank is heated to 830-915 ℃ at the heating speed of 18-30 ℃/s, and the temperature is kept for 395-600 s at the temperature.

Preferably: and controlling the cooling speed to be 19-28 ℃/s in the punch forming.

The action and mechanism of each element and main process in the invention

C: carbon plays a key role in obtaining ultrahigh-strength and multiphase structures, the carbon content and the distribution of the carbon content in steel have great influence on the structure form, type and performance of a final product, but the content is too high, a large amount of bainite, martensite and other hard phases are easily formed in the cooling process after finish rolling, the content is higher, the strength is higher, and the plasticity is reduced. Therefore, the carbon content is not easy to be too high on the premise of ensuring the die quenching treatment strengthening. Therefore, the content is limited to 0.19 to 0.27%.

Si: silicon has a strong solid solution strengthening effect, can improve the strength of steel, can effectively inhibit the formation of carbides, plays a key role in the distribution and the state of carbon elements in the steel, and is one of important elements of the steel with ultrahigh strength and high plasticity; meanwhile, the hardness of the steel can be reduced slowly during tempering, and the tempering stability and strength of the steel are improved. Therefore, the content is limited to 1.20 to 1.65%, and the content of Si is preferably 1.32 to 1.65%.

Mn: manganese has a solid solution strengthening effect, can reduce the phase transformation driving force, enables the C curve to move to the right, improves the hardenability of the steel, enlarges the gamma phase region, and can reduce the Ms point of the steel, so that martensite can be obtained at a proper cooling speed. Meanwhile, the Mn-Si-Mn alloy is matched with a certain silicon content and added into steel, can enhance the room temperature stability of austenite, and is an important element obtained by matching high strength and high plasticity, so that the content of the Mn-Si-Mn-content is limited in-content.

Cr: chromium can reduce the phase transformation driving force and the nucleation growth of carbide during phase transformation, thereby improving the hardenability of the steel. In addition, the chromium can improve the tempering stability and the high-temperature oxidation resistance of the steel. Therefore, the content is limited to 0.15 to 0.40%, and preferably the content of Cr is 0.19 to 0.40%.

Als, which plays a role in deoxidizing steel, should ensure that a certain amount of acid-soluble aluminum exists in the steel, otherwise the effect cannot be exerted, but too much aluminum can also cause aluminum series inclusions in the steel, and is not beneficial to smelting and casting of the steel. Meanwhile, the proper amount of aluminum is added into the steel, so that the adverse effect of nitrogen and oxygen atoms in the steel on the performance can be eliminated. Therefore, the content is limited to 0.012 to 0.080%.

P: phosphorus is a harmful element in steel and is easy to cause center segregation of a casting blank. The steel is easy to be deviated to a grain boundary in the subsequent hot continuous rolling heating process, so that the brittleness of the steel is obviously increased. Meanwhile, the content is controlled to be below 0.020 percent based on cost consideration and without influencing the performance of the steel.

S: sulfur is a very harmful element. Sulfur in steel often exists in the form of sulfide of manganese, and this sulfide inclusion deteriorates toughness of steel and causes anisotropy of properties, so that the lower the sulfur content in steel, the better. The sulfur content in steel is controlled to be less than 0.010% in consideration of the manufacturing cost.

N: nitrogen can combine with titanium in the titanium-added steel to form titanium nitride, and the second phase precipitated at high temperature is beneficial to strengthening the matrix and improving the welding performance of the steel plate. However, the content of nitrogen is higher than 0.004%, the solubility product of nitrogen and titanium is higher, titanium nitride with coarse particles can be formed in the steel at high temperature, and the plasticity and the toughness of the steel are seriously damaged; in addition, higher nitrogen content increases the amount of micro-alloying elements needed to stabilize the nitrogen element, thereby increasing costs. Therefore, the content is controlled to be less than 0.004%.

Nb is a strong C, N compound forming element and can play a role in refining austenite grains, a small amount of niobium is added into steel to form a certain amount of nano-scale fine two-phase particles, so that the austenite grains can be prevented from growing, and the ductility and toughness of the steel can be improved, therefore, the quenched martensite lath has smaller size, and the strength and the ductility and toughness of the steel are greatly improved. Therefore, the content is controlled to be 0.025-0.045%, and the preferable content of Nb is 0.031-0.045%.

The first and second pass reduction rates are controlled to be not less than 50%, and the finishing rolling temperature is controlled to be 870-910 ℃, so that the distortion energy can be effectively increased, the crystal grains can be refined, and the growth of the crystal grains can be controlled by matching with the proper rolling temperature.

The coiling temperature is controlled to be 650-700 ℃, because the uniform structure and the dispersed carbide particles can be effectively obtained, the coiling shape of the steel coil can be effectively controlled, and the batch stable manufacturing is convenient to realize.

In the invention, the blank is heated to 830-930 ℃ at a heating speed of 12-30 ℃/s, and is kept at the temperature for 240-500 s, preferably, the blank is heated to 830-915 ℃ at a heating speed of 18-30 ℃/s, and is kept at the temperature for 395-600 s. Because the method can effectively ensure that each component element in the steel is uniformly distributed on austenite, and provides favorable conditions for the regulation and control of each subsequent phase.

The cooling speed is controlled to be 15-28 ℃/s during forming, preferably the cooling speed is controlled to be 19-28 ℃/s during stamping forming, and the metallographic structure of the formed part at room temperature is 80-90% in volume of martensite, 2-8% in volume of bainite and 8-12% in volume of residual austenite.

Compared with the prior art, the invention not only has the yield strength of more than or equal to 820MPa, the tensile strength of more than or equal to 1365MPa and the elongation of more than or equal to 11 percent, but also can ensure that the product of strength and elongation is 17.7-22.5 GPa, and completely meets the requirements of manufacturing the lightweight structure for the economical car on the strength and the collision energy absorption characteristics of 1500 MPa-level high-collision-resistant parts.

Drawings

FIG. 1 is a metallographic structure diagram according to the present invention.

Detailed Description

The present invention is described in detail below:

table 1 is a list of values of the components of each example and comparative example of the present invention;

table 2 shows the values of the process parameters of the examples and comparative examples of the present invention;

table 3 is a table of the results of mechanical property measurements of the examples and comparative examples of the present invention.

The preparation method comprises the following steps:

1) desulfurizing molten iron, and controlling S to be less than or equal to 0.001%;

2) smelting and refining in an electric furnace or a converter;

3) continuous casting is carried out, the superheat degree of the tundish molten steel is controlled to be 15-35 ℃, the thickness of a casting blank is 50-70 mm, and the blank drawing speed is 3.0-6.0 m/min;

4) carrying out primary descaling on the casting blank, and controlling the pressure of descaling water to be not lower than 200 bar;

5) soaking the casting blank, and controlling the discharging temperature of the casting blank to be 1150-1200 ℃;

6) carrying out secondary descaling on the casting blank, and controlling the pressure of descaling water to be not lower than 250 bar;

7) rolling: and controlling the thickness of a finish rolling outlet to be 0.80-4.0 mm; controlling the first and second pass reduction rates to be not less than 50%; controlling the finish rolling temperature to be 870-910 ℃;

8) laminar cooling to coiling temperature;

9) coiling, wherein the coiling temperature is controlled to be 650-700 ℃;

10) directly uncoiling and blanking or blanking after acid cleaning to obtain a blank of the structural part to be prepared;

11) heating the blank to 830-930 ℃ at a heating speed of 12-30 ℃/s, and preserving heat for 240-500 s at the temperature;

12) stamping forming: stamping the heated blank into an automobile structural part to be prepared, wherein the cooling speed is controlled to be 15-28 ℃/s; controlling the metallographic structure of the formed part at room temperature to be that the volume content of martensite is 80-90%, the volume content of bainite is 2-8%, and the volume content of residual austenite is 8-12%;

13) and cleaning, trimming and cutting the surface of the part.

TABLE 1 chemical composition (wt.%) of inventive and comparative examples

TABLE 2 tabulation of values of main process parameters for each example of the invention and comparative example

TABLE 3 tabulation of mechanical Properties of each example of the invention and comparative example

In conclusion, the invention successfully realizes that the steel has high strength and high plasticity through the matching design of components and processes, can completely meet the requirements of manufacturing the lightweight structural part for the economical car on the strength and the collision energy absorption characteristic of the 1500 MPa-level high-collision-resistance part, and has great significance for promoting the development of the lightweight car.

The present embodiments are merely preferred examples, and are not intended to limit the scope of the present invention.