A method for preparing a bainite / martensite dual-phase 980mpa-grade bq&p steel based on a thin slab continuous casting and rolling process

Abstract

The application relates to a preparation method of a bainite / martensite dual-phase 980MPa-grade BQ&P steel based on a thin slab continuous casting and rolling process, and relates to a preparation method of a bainite / martensite dual-phase 980MPa-grade BQ&P steel. The application aims to solve the problems existing in the research of hot-rolled quenching and partitioning steel. The biggest feature of the application is that air cooling in the bainite transformation interval is added before quenching to the martensite starting transformation temperature, so that the austenite is first subjected to bainite transformation, a part of bainite is first formed, the austenite grains are divided, the structure is refined, and the plasticity and toughness are improved compared with Q&P steel. The multiphase structure of the bainite / martensite dual-phase 980MPa-grade BQ&P steel prepared by the application has excellent mechanical properties, is beneficial to subsequent stamping and forming processes, and has a wide application prospect. The application achieves "hot instead of cold", shortens the process, saves energy, and meets the social development trend of energy saving and emission reduction and green manufacturing.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a bainite / martensite dual-phase 980MPa-grade BQ&P steel. BACKGROUND

[0002] As the main material of automobile body, steel has developed in the direction of high strength, high formability and high strength-plastic product in recent years. Quenching and partitioning steel, as a representative of the third generation of advanced automobile steel products, has a process principle mainly including martensite transformation and carbon partitioning process, and finally obtains a ferrite, martensite and residual austenite structure, which exhibits good combination of strength and plasticity, and the strength-plastic product is more than 20GPa%. In addition, the steel grade has a low alloying design, which overcomes the smelting problem in the second generation of advanced high-strength steel represented by TWIP, and has a low alloy cost. So far, most of the quenching and partitioning steels adopt the traditional hot rolling + cold rolling + heat treatment manufacturing process, which has problems of long process flow, high energy consumption and high manufacturing cost, such as: a high-formability ultrahigh-strength cold-rolled steel sheet and a manufacturing method thereof (patent publication number CN103805838A) and a cold-rolled high-strength steel sheet with excellent phosphating performance and formability and a manufacturing method thereof (patent publication number CN106350731A) use the process of hot rolling, cold rolling, pickling, annealing and partitioning.

[0003] With the development of the steel industry, the short process technology has made great progress, and has obvious advantages in the production of thin and ultra-thin hot-rolled products. The short process technology can directly roll out hot-rolled products to replace cold-rolled products with the same strength and thickness, realizing "hot instead of cold".

[0004] So far, the method for producing a quenching and partitioning steel with a tensile strength of more than or equal to 980MPa by using a short process (CN111440987A) proposes a method for producing a 980MPa-grade quenching and partitioning steel by annealing after hot rolling by using a thin slab continuous casting and rolling process. Compared with the traditional process, the cold rolling process is reduced, but the subsequent heat treatment process is still needed, which requires a large amount of energy. A short-process hot-rolled Q&P steel sheet and a preparation method thereof (CN108504925A) proposes a short-process hot-rolled one-step partitioning Q&P steel sheet and a preparation method thereof. In order to obtain sufficient strength, the steel is directly quenched to the partitioning temperature between the martensite transformation start temperature and the martensite transformation end temperature after austenite deformation, and a hot-rolled Q&P steel sheet is obtained. Since the martensite transformation start temperature is lower than the bainite transformation start temperature, the short-process rapid cooling equipment has high requirements, and the austenite transformation into martensite will cause volume expansion. Direct quenching below the martensite transformation start temperature will easily cause excessive residual stress of the steel part, resulting in poor plate shape of the plate, and even brittle fracture.

[0005] A two-step process for obtaining carbide-free bainite / martensite dual-phase steel is proposed in a BQ&P heat treatment process for carbide-free bainite / martensite dual-phase steel (CN106893832B), but long time holding is required, the production rate is low and the energy demand is large. The steel composition in hot-rolled Fe-Mn-Al-C steel and manufacturing method of quenching to bainite region to obtain Q&P structure (CN109554622A) is: 0.22%~0.25%C, 2.8%~3.2%Mn, 1.8%~2.2%Al, and the balance is Fe; the Al element content of the composition is too high, which will block the water nozzle of the continuous casting machine, and is not conducive to the operation of the modern continuous casting equipment. SUMMARY

[0006] The present application is to solve the deficiencies in the research of hot-rolled quenching and partitioning steel, and to provide a preparation method of bainite / martensite dual-phase 980MPa grade BQ&P steel based on thin slab continuous casting and rolling process.

[0007] The preparation method of bainite / martensite dual-phase 980MPa grade BQ&P steel based on thin slab continuous casting and rolling process of the present application is carried out in the following steps:

[0008] I. Heating of continuous casting thin slab

[0009] The continuous casting thin slab is placed in the soaking furnace, the furnace temperature is 860℃~880℃, the heating section temperature is 1200℃~1220℃, and the holding time is 10min~15min; the heating temperature of the soaking section is 1100℃~1180℃, and the holding time of the soaking section is 10min~30min;

[0010] The mass composition of each element in the continuous casting thin slab is: C is 0.15%~0.28%, Mn is 1.5%~3%, Si is 0.3%~2%, Ti is 0.01%~0.05%, Al is 0.01%~0.06%, impurities are less than or equal to 0.012%, and the balance is Fe;

[0011] The continuous casting thin slab controls the tundish liquid superheat to be 15℃~30℃ during continuous casting, the thickness of the casting blank is 50mm~75mm, and the blank drawing speed is 4~5.5m / min;

[0012] II. Descaling: high-pressure water descaling is carried out after the continuous casting thin slab comes out of the soaking furnace, and the descaling water pressure is controlled to be 300bar~420bar;

[0013] III. Rolling: rolling is carried out, the first pass reduction rate is controlled to be 50%~65%, the second pass reduction rate is controlled to be 50%~60%, the last pass reduction rate is controlled to be 10%~15%, and the rolling speed is 6m / s~10m / s; interstand descaling is carried out between the first pass and the second pass, and the descaling water pressure is 230bar~280bar; the rough rolling temperature is 1080℃~1100℃, and the finish rolling temperature is 800℃~820℃;

[0014] IV. Cooling: cooling to 400℃~450℃ (primary quenching temperature) at a cooling rate greater than or equal to 100℃ / s and less than or equal to 300℃ / s, air cooling for 4s~8s, then cooling to 260~330℃ at a cooling rate greater than or equal to 100℃ / s and less than or equal to 180℃ / s and coiling to obtain the B / M dual-phase 980MPa-grade BQ&P steel.

[0015] The present application has the following advantages:

[0016] I. The alloy composition of the present application is simple and suitable for continuous casting production, and the controlled rolling and controlled cooling are feasible, so that the present application can be produced on the existing thin slab continuous casting and rolling production line, the thin slab continuous casting and rolling shortens the process and does not need cold rolling, improves the production efficiency, and saves the production cost;

[0017] II. The biggest feature of the present application is that air cooling in the bainite phase transformation interval (the primary quenching temperature in step IV) is added before quenching to the martensite start temperature, so that the austenite is first subjected to bainite phase transformation, a part of bainite is first formed, the austenite grains are divided, and the structure is refined, and the plasticity and toughness are improved compared with Q&P steel;

[0018] III. The present application adopts the composition design conforming to continuous casting to obtain the B / M dual-phase 980MPa-grade BQ&P steel, the multiphase structure has excellent mechanical properties, is beneficial to subsequent stamping and forming processes, and has a wide application prospect;

[0019] IV. The present application adopts the two-stage soaking furnace heating mode in step I, reduces the occurrence probability of residual iron oxide scale, and improves the surface quality of the steel coil;

[0020] V. The present application utilizes the thin slab continuous casting and rolling process, has high temperature control precision, significantly reduces the performance deviation of different parts of the strip steel, and the obtained B / M dual-phase 980MPa-grade BQ&P steel has similar performance to the steel obtained by cold rolling and then heat treatment partitioning, so that the present application can achieve "hot instead of cold", shorten the process, save energy, and meet the social development trend of energy saving and emission reduction and green manufacturing. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 The figure is a process flow diagram of the present application;

[0022] Figure 2is an SEM image of the bainite / martensite duplex 980MPa grade BQ&P steel prepared in Example 1;

[0023] Figure 3 is an SEM image of the bainite / martensite duplex 980MPa grade BQ&P steel prepared in Example 2;

[0024] Figure 4 is an SEM image of the bainite / martensite duplex 980MPa grade BQ&P steel prepared in Example 3. DETAILED DESCRIPTION

[0025] Embodiment I: The embodiment is a preparation method of a bainite / martensite duplex 980MPa grade BQ&P steel based on a thin slab continuous casting and rolling process, which is specifically carried out in the following steps:

[0026] I. Heating of the continuous casting thin slab:

[0027] The continuous casting thin slab is placed into the soaking furnace, the furnace entry temperature is 860℃~880℃, the heating section temperature is 1200℃~1220℃, and the holding time is 15min; the heating temperature of the soaking section is 1100℃~1180℃, and the holding time of the soaking section is 10min~30min;

[0028] The mass composition of each element in the continuous casting thin slab is: C is 0.15%~0.28%, Mn is 1.5%~3%, Si is 0.3%~2%, Ti is 0.01%~0.05%, Al is 0.01%~0.06%, impurities are less than or equal to 0.012%, and the balance is Fe;

[0029] The continuous casting thin slab is controlled to have a tundish liquid steel superheat of 15℃~30℃, a slab thickness of 50mm~75mm, and a casting speed of 4m / min~5.5m / min during continuous casting;

[0030] II. Descaling: After the continuous casting thin slab exits the soaking furnace, high-pressure water descaling is carried out, and the descaling water pressure is controlled to be 300bar~420bar;

[0031] III. Rolling: Rolling is carried out, the first pass reduction rate is controlled to be 50%~65%, the second pass reduction rate is controlled to be 50%~60%, the final pass reduction rate is controlled to be 10%~15%, and the rolling speed is controlled to be 6m / s~10m / s; descaling is carried out between the first pass and the second pass, and the descaling water pressure is controlled to be 230bar~280bar; the opening rolling temperature is controlled to be 1080℃~1100℃, and the finish rolling temperature is controlled to be 800℃~820℃;

[0032] Four, cooling: cooling to 400-450℃ at a cooling rate of greater than or equal to 100℃ / s and less than or equal to 300℃ / s, air cooling for 4-8s, and then cooling to 260-330℃ at a cooling rate of greater than or equal to 100℃ / s and less than or equal to 300℃ / s and coiling to obtain a bainite / martensite dual-phase 980MPa-grade BQ&P steel.

[0033] Specific embodiment two: different from the specific embodiment one, the impurities in step one are composed of S, N and P, wherein the weight percentage content of S is ≤0.001%, the weight percentage content of N is ≤0.003%, and the weight percentage content of P is ≤0.008%.

[0034] Specific embodiment three: different from the specific embodiment one or two, the opening rolling temperature in step three is 1100℃. The others are the same as the specific embodiment one or two.

[0035] Specific embodiment four: different from any one of the specific embodiments one to three, the finishing rolling temperature in step three is 800℃. The others are the same as any one of the specific embodiments one to three.

[0036] Specific embodiment five: different from the specific embodiment four, the cooling rate in step four is 152℃ / s to 420℃. The others are the same as the specific embodiment four.

[0037] The following three examples and one comparative example are used to verify the present application:

[0038] A preparation method of a bainite / martensite dual-phase 980MPa-grade BQ&P steel based on a thin slab continuous casting and rolling process, specifically performed according to the following steps:

[0039] I. Smelting (this step is a conventional technology and is not specifically limited):

[0040] The ingredients and elements are mixed according to the weight percentage (see Table 1) to form molten iron, and a converter steelmaking-LF-RH process is used during smelting. Deep desulfurization pretreatment of molten iron is used during converter steelmaking to ensure a low S content in the steel, CaMg deoxidizer is used for deoxidation to reduce O and Al in the molten steel, and to improve the morphology, size and distribution of inclusions;

[0041] The impurities in step one are composed of S, N and P, wherein the weight percentage content of S is ≤0.001%, the weight percentage content of N is ≤0.003%, and the weight percentage content of P is ≤0.008%;

[0042] II. Heating of the continuous casting thin slab:

[0043] The continuous casting thin slab is put into a soaking furnace, the entry temperature is shown in Table 2, the heating section temperature is 1200℃, and the holding time is 10 min; the soaking section heating temperature is shown in Table 2, and the holding time of the soaking section is 20 min;

[0044] The continuous casting thin slab is controlled to have a tundish liquid superheat of 20℃ during continuous casting, the slab thickness is 65 mm, and the casting speed is 5 m / min;

[0045] III. Descaling: high-pressure water descaling is performed after the continuous casting thin slab exits the soaking furnace, and the descaling water pressure is controlled to be 400 bar;

[0046] IV. Rolling: rolling is performed, the first pass reduction rate is controlled to be 60%, the second pass reduction rate is controlled to be 55%, the last pass reduction rate is controlled to be 10%, the rolling speed is 8 m / s, and there are 7 passes; inter-stand descaling is performed between the first pass and the second pass, the descaling water pressure is 250 bar; the rough rolling temperature is 1100℃, and the finish rolling temperature is 800℃;

[0047] V. Cooling: cooling is performed at a cooling rate of 152℃ / s to the temperature shown in Table 2, air cooling is performed for 6 s, then cooling is performed at a cooling rate of 152℃ / s to the temperature shown in Table 2, and coiling is performed, to obtain a BQ&P steel with a Bay / Ma duplex phase of 980 MPa grade.

[0048] The comparative example 1 does not have a first quenching temperature, and is directly quenched to the coiling temperature.

[0049] Table 1

[0050]

[0051] Table 2

[0052]

[0053] The mechanical properties of the products prepared in the 3 examples and 1 comparative example are tested according to GB / T228.1-2010 “Metallic Materials Tensile Testing-Part 1: Room Temperature Test Method”, the gauge length is 50 mm, and the test results are shown in Table 3.

[0054] Table 3

[0055]

[0056] Figures 2 to 4SEM images of the bainite / martensite / austenite duplex 980 MPa grade BQ&P steel prepared in Example 1-3 in sequence, from which it can be seen that a bainite, martensite and residual austenite composite structure is obtained by the method. Further analysis can be obtained that the volume fraction of bainite in the example increases relative to the conventional Q&P steel, and with the increase of the coiling temperature, the volume fraction of residual austenite in the example shows a trend of first increasing and then decreasing, and when the coiling temperature is 300°C (Example 2), the volume fraction of residual austenite in the example reaches the highest.

Claims

1. A method for producing a BQ&P steel of Bainite / Martensite dual phase 980 MPa grade based on thin slab continuous casting and rolling process, characterized in that The preparation method of the B / M complex phase 980MPa grade BQ&P steel based on the thin slab continuous casting and rolling process is carried out according to the following steps: I. Heating of the continuous casting thin slab: The continuous casting thin slab is placed into the heating furnace, the entering temperature is 860-880 DEG C, the heating section temperature is 1200-1220 DEG C, and the holding time is 10-15 min; the heating temperature of the heating section is 1100-1180 DEG C, and the holding time of the heating section is 10-30 min; The mass composition of each element in the continuous casting thin slab is: C is 0.15-0.28%, Mn is 1.5-3%, Si is 0.3-2%, Ti is 0.01-0.05%, Al is 0.01-0.06%, impurities are less than or equal to 0.012%, and the balance is Fe; The continuous casting thin slab is controlled to have a tundish liquid superheat of 15-30 DEG C, a slab thickness of 50-75 mm, and a drawing speed of 4-5.5 m / min during continuous casting; II. Descaling: high-pressure water descaling is carried out after the continuous casting thin slab exits the heating furnace, and the descaling water pressure is controlled to be 300-420 bar; III. Rolling: rolling is carried out, the first pass reduction rate is controlled to be 50-65%, the second pass reduction rate is controlled to be 50-60%, the final pass reduction rate is controlled to be 10-15%, and the rolling speed is controlled to be 6-10 m / s; descaling is carried out between the first pass and the second pass, and the descaling water pressure is controlled to be 230-280 bar; The opening rolling temperature is 1080-1100 DEG C, and the final rolling temperature is 800-820 DEG C; IV. Cooling: cooling is carried out at a cooling rate of greater than or equal to 100 DEG C / s and less than or equal to 300 DEG C / s to 400-450 DEG C, air cooling is carried out for 4-8 s, then cooling is carried out at a cooling rate of greater than or equal to 100 DEG C / s and less than or equal to 300 DEG C / s to 260-330 DEG C, and coiling is carried out, to obtain the B / M complex phase 980MPa grade BQ&P steel.

2. The method for preparing a Be / Ma duplex 980 MPa grade BQ&P steel based on a thin slab continuous casting and rolling process according to claim 1, characterized in that The impurities in step I are composed of S, N and P, wherein the weight percentage content of S is ≤0.001%, the weight percentage content of N is ≤0.003%, and the weight percentage content of P is ≤0.008%.

3. The method for preparing a Be / Ma duplex 980 MPa grade BQ&P steel based on a thin slab continuous casting and rolling process according to claim 1, characterized in that The opening rolling temperature in step III is 1100 DEG C.

4. The method for preparing a Be / Ma duplex 980 MPa grade BQ&P steel based on a thin slab continuous casting and rolling process according to claim 1, characterized in that The final rolling temperature in step III is 800 DEG C.

5. The method of producing a Be / Ma duplex phase 980 MPa grade BQ&P steel based on thin slab continuous casting and rolling process according to claim 1, characterized in that In step IV, the cooling rate is 152 DEG C / s to 420 DEG C.

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