Residual austenite plasticized ferrite-bainite dual-phase steel and method for manufacturing the same

Abstract

The application discloses a residual austenite plasticizing type ferrite-bainite complex phase steel and a preparation method thereof, and belongs to the technical field of high-strength hot continuous rolling steel production. The preparation method is smelting-slab heating-coarse rolling-precision rolling-laminar flow cooling-coiling, the laminar flow cooling adopts three-section cooling, i.e., water cooling-air cooling-water cooling, the first-section water cooling is cooled at a cooling temperature A e3 ~ A e3 +20℃, the air cooling is cooled at a cooling temperature A e1 ~ A e1 +20℃, and the last-section water cooling is cooled at a final cooling temperature B s -50℃~B s The complex phase steel has the following chemical components: C 0.16% to 0.20%, Si 0.6% to 1.2%, Mn 1.6% to 1.8% and Als 0 to 0.65%, and the rest is Fe and impurities; the microstructure is ferrite, bainite and 5% to 10% residual austenite; the yield strength is 650 to 750 MPa, the tensile strength is 750 to 850 MPa, the elongation is greater than or equal to 25%, the hole expansion rate is greater than or equal to 60%, and the thickness is 3 to 6 mm.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-strength hot-rolled steel production, and particularly relates to a residual austenite plasticized ferrite-bainite composite phase steel and a preparation method thereof. BACKGROUND

[0002] In recent years, with the increasing demand of the automobile manufacturing industry for energy saving, carbon reduction, lightweight and emission reduction, the product upgrading iteration speed of high-strength hot-rolled automobile steel is rapid. For example, the steel used for automobile structural parts such as commercial vehicle frame, wheel and carriage is upgraded from the Q355 series in the early 21st century to the 510MPa and 610MPa special steel, and further upgraded to the 700MPa or even 800MPa automobile steel in the 2010s. In order to meet the application requirements of the automobile manufacturing industry, the existing hot-rolled steel is also developing towards high strength and lightweight. However, the traditional microalloyed steel has some problems in high strength and lightweight, such as poor forming and blanking performance, and liquid precipitation TiN inclusions.

[0003] CN114045441B discloses an 800MPa grade enhanced plastic dual-phase steel for continuous annealing and a preparation method thereof, which has the following chemical composition: C: 0.165-0.17%, Si: 0.65-0.80%, Mn: 1.90-2.10%, P≤0.010%, S≤0.005%, Als: 0.20-0.35%, N≤0.003%, and the balance of Fe and inevitable impurities; the preparation method comprises smelting, hot rolling, acid rolling and annealing, wherein the annealing process is three-stage step heating of the cold-rolled thin strip steel: the first stage is a preheating stage, the outlet temperature is 130-170℃; the second stage is a first heating stage, the strip steel is heated to 710-745℃; the third stage is a second heating stage, the strip steel is heated to 785-815℃; the step heating speeds are 4.52-7.78℃ / s, 1.76-3.06℃ / s and 0.30-0.54℃ / s respectively; the obtained dual-phase steel has a yield strength of 480-540MPa, a tensile strength of 815-870MPa, and an elongation A 80 of 22.5-28.0%. The microstructure thereof is composed of 45-55% ferrite with an average grain size of 8.0μm, about 10-15% island-shaped martensite with an average grain size of 2.0μm, 20-35% bainite with an average grain size of 4.5μm and about 10% residual austenite.

[0004] CN111363902A discloses a method for promoting the formation of residual austenite in hot-rolled medium manganese steel plate, which comprises heating, hot rolling, coiling and heat treatment processes; the heat treatment process adopts a twice partitioning process after annealing, the sample is placed in a muffle furnace at a heating temperature of 650-780°C for 5-15 min, then placed in a molten salt at 100-150°C for isothermal holding for 1-5 min, then moved to a muffle furnace at 450-500°C for 3-5 min for the first carbon and manganese element partitioning; then placed in a molten salt at 100-120°C for isothermal holding for 1-3 min, then moved to a muffle furnace at 400-450°C for 5-10 min for the second carbon and manganese element partitioning, and then the sample is taken out and air cooled to room temperature. The present application significantly promotes the formation of residual austenite in hot-rolled medium manganese steel plate and its stability under room temperature conditions based on composition design combined with twice partitioning process after annealing, the volume fraction of residual austenite is ≥23%, and the strength and ductility product after uniaxial tensile test is greater than 31 GPa%, fully meeting the performance requirements of the third generation of automobile steel.

[0005] In the above technical solution, multiple annealing heat treatments are used to obtain a certain content of residual austenite.

[0006] CN102965569A discloses a hot-rolled transformation induced plasticity steel plate, which has a microstructure of ferrite + island-shaped bainite + residual austenite composite structure, and its chemical element mass percentage is: C: 0.175-0.215%, Si: 1.50-2.00%, Mn: 1.60-2.00%, Al: 0.015-0.040%, N: ≤0.006%, and the balance is Fe and other unavoidable impurities. Correspondingly, the present application also discloses a manufacturing method of the hot-rolled transformation induced plasticity steel plate, comprising the steps of smelting, casting, heating, rolling, cooling and coiling, wherein the cooling step adopts a water cooling → air cooling → water cooling segmented mode, wherein the first stage water cooling speed is ≥100°C / s, the second stage air cooling temperature is controlled at 710-750°C, the air cooling time is 4-6 s, the third stage water cooling speed is ≥75°C / s, and the final cooling temperature is controlled at 350-400°C, and then coiled at the final cooling temperature. The hot-rolled transformation induced plasticity steel plate has a yield strength >500 MPa, a tensile strength >800 MPa, and an elongation A50 >25%, and has high strength, high elongation, good formability and excellent plasticity. The technical solution adopts Si micro-alloying method, and the laminar cooling adopts a higher cooling rate (≥100°C / s), generally, the laminar cooling rate of a conventional hot continuous rolling production line is ≤50°C / s, and a higher cooling rate needs to be equipped with an ultra-fast cooling technology, which has low process adaptability and high production cost. SUMMARY

[0007] The technical problem solved by the present application is that the preparation of the existing complex microstructure steel needs to be annealed or to use ultra-fast cooling equipment, and the production cost is high.

[0008] The technical scheme adopted by the present application to solve the technical problem is: a preparation method of residual austenite plasticizing ferrite-bainite complex phase steel, a slab is smelted according to the chemical composition of target steel, and then the complex phase steel is produced according to the process flow of slab heating-rough rolling-finish rolling-laminar cooling-coiling; wherein the laminar cooling adopts three-stage cooling of 'water cooling-air cooling-water cooling':

[0009] The first-stage water cooling starts at A e3 ~ A e3 + 20 DEG C, and the cooling rate is 5 DEG C / s-15 DEG C / s.

[0010] The second-stage air cooling is at A e1 ~ A e1 + 20 DEG C, and the cooling rate is 2 DEG C / s-5 DEG C / s.

[0011] The third-stage water cooling ends at B s - 50 DEG C to B s , and the cooling rate is 5 DEG C / s-10 DEG C / s.

[0012] In the preparation method, the chemical composition of the complex phase steel includes, by weight percentage, C 0.16%-0.20%, Si 0.6%-1.2%, Mn 1.6%-1.8%, Als 0-0.65%, and the rest is Fe and inevitable impurity elements.

[0013] In the preparation method, the slab heating process is at a heating temperature of 1180 DEG C-1200 DEG C and a furnace time of 200 min-300 min.

[0014] In the preparation method, the rough rolling process is at a cumulative deformation of 75%-85%.

[0015] In the preparation method, the finish rolling process is at a cumulative deformation of 85%-95%.

[0016] In the preparation method, the laminar cooling process is at a first-stage water cooling starting temperature of 840 DEG C-910 DEG C, a second-stage air cooling temperature of 680 DEG C-720 DEG C and an air cooling time of 2-6 s, and a third-stage water cooling ending temperature of 450 DEG C-500 DEG C.

[0017] In the preparation method, the coiling process is at a coiling temperature of 450 DEG C-500 DEG C.

[0018] The residual austenite plasticized ferrite-bainite duplex phase steel prepared by the preparation method.

[0019] Further, the microstructure of the duplex phase steel is ferrite, bainite and residual austenite, wherein the volume fraction of the residual austenite is 5% to 10%.

[0020] Further, the yield strength of the duplex phase steel is 650 MPa to 750 MPa, the tensile strength is 750 MPa to 850 MPa, the elongation is greater than or equal to 25%, the hole expansion ratio is greater than or equal to 60%, and the thickness is 3 mm to 6 mm.

[0021] The present application has the beneficial effects that the present application provides a method for obtaining a residual austenite plasticized ferrite-bainite duplex phase steel by hot continuous rolling-laminar cooling. The duplex phase steel provided by the present application only adds Si, Al and other low-cost micro-alloying elements, and the alloy cost is low. The laminar cooling process method provided by the present application has a low laminar cooling rate, and the final cooling temperature / coiling temperature is a medium temperature, which can avoid the deterioration of the shape of the steel plate. The steel provided by the present application has high strength and good plasticity, mainly because a large number of high-density dislocations exist in the bainite, and the residual austenite is a soft phase structure, which is beneficial to improve the plasticity and hole expansion flange performance of the material.

[0022] Compared with the existing three-stage cooling technology, the present application adopts a Si-Al micro-alloying method, further improves the phase transition point and hardenability of the steel, so that the ferrite plus bainite plus residual austenite duplex phase structure can be obtained under the condition of a lower cooling rate, the process adaptability is stronger, and the process can be realized on a conventional hot continuous rolling production line without using ultra-fast cooling equipment. Compared with the cold rolling-continuous annealing process method, the present application has the advantages of low production cost and simple operation method. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The continuous cooling transformation (CCT) curve of the steel meeting the component requirements of the present application;

[0024] Figure 2 The microstructure of the steel of Example 1;

[0025] Figure 3 The XRD result of the steel of Example 1;

[0026] Figure 4 The microstructure of the steel of Comparative Example 1;

[0027] Figure 5 The XRD result of the steel of Comparative Example 1;

[0028] Figure 6 The microstructure of the steel of Comparative Example 2. DETAILED DESCRIPTION

[0029] The technical solution of the present application can be implemented in the following manner.

[0030] The present application provides a method for laminar cooling of the above-mentioned duplex steel, and the following explains the reasons for limiting the main alloying elements in the duplex steel of the present application.

[0031] C is an important austenite stabilizing element, and a higher C content is beneficial to improving the stability of austenite, promoting the stability of residual austenite in the laminar cooling process, and improving the hardenability of the steel, thereby promoting the occurrence of bainite phase transformation in the laminar cooling process. The present application limits the C content to 0.16% to 0.20%.

[0032] Mn element also has a strong effect of stabilizing austenite, and appropriately increasing the Mn content is beneficial to retaining more residual austenite in the final microstructure. However, when the Mn content is too high, it is easy to cause segregation of the casting blank, affecting the uniformity of the structure, therefore, the Mn content is controlled to be 1.6% to 1.8%.

[0033] Si can increase the activity of C in the phase transformation process, promote the diffusion of C from ferrite to residual austenite, play a role of purifying ferrite and making austenite rich in carbon, thereby improving the stability of residual austenite. Similarly, Al element has the similar effect of inhibiting cementite formation and improving the stability of residual austenite as Si. Therefore, the present application requires adding 0.6% to 1.2% of Si and 0 to 0.65% of Al.

[0034] The following explains the reasons for limiting the production process, especially the laminar cooling process, in combination with the control requirements of the residual austenite structure in the duplex steel of the present application.

[0035] Since a high content of Si is added to the steel, Si is easy to produce FeSi2O4 olivine phase, which has a low melting point of about 1180℃, and the phase is easy to form sticky material at the irregular interface between the iron base and the iron oxide scale after solidification, thereby reducing the high-pressure water descaling effect and causing red iron oxide scale defects on the surface of the finished steel plate. Therefore, the present application limits the slab heating temperature to be higher than the melting point of the FeSi2O4 olivine phase, i.e. 1180℃ to 1200℃, and the heating time is limited to 200min to 300min.

[0036] The rolling process adopts two-stage rolling of rough rolling and finish rolling, wherein a larger reduction (75% to 85%) is adopted in the rough rolling process to promote austenite recrystallization, and a larger reduction (85% to 95%) is adopted in the finish rolling process to promote the flattening and refinement of the austenite in the non-recrystallization zone, thereby promoting the refinement of the microstructure of the finished steel.

[0037] The laminar cooling process adopts three-stage cooling. The first-stage water cooling occurs γ→α phase transformation to obtain a certain content of ferrite and austenite intermediate phase, and the first-stage water cooling open cooling temperature needs to be controlled at the ferrite start transformation temperature A e3 In order to avoid rolling in the two-phase zone at the last pass of finish rolling and causing the finished steel organization mixed crystal, thereby reducing the organization uniformity, the present application limits the first-stage water cooling open cooling temperature at A e3 ~A e3 +20℃, for the duplex steel with the chemical composition of the present application, A e3 840℃~890℃, therefore the present application limits the first-stage water cooling temperature at 840℃~910℃.

[0038] In addition, the cooling rate of the first-stage water cooling also needs to be limited, as shown in the attached Figure 1 The continuous cooling transformation (CCT) curve of the steel satisfying the component requirement of the present application is shown in the attached Figure 1 It can be seen that the steel of the present application occurs A→F+P transformation when the cooling rate is ≤1℃ / s, and occurs A→F transformation when the cooling rate is ≤20℃ / s. The present application requires that the first-stage water cooling occurs A→F transformation to form ferrite (A) and supercooled austenite organization (A), and does not allow A→F+P transformation. At the same time, when the cooling rate is low, it will cause the original austenite to transform into abnormal pearlite organization, thereby reducing the stability of residual austenite, reducing the content of bainite and residual austenite in the finished steel, and affecting the toughness and plasticity of the finished steel; when the cooling rate is too high, the supercooling degree is large, and martensite organization is easy to form. Therefore, the present application limits the first-stage water cooling cooling rate at 5℃ / s~15℃ / s.

[0039] The first-stage water cooling final cooling temperature, that is, the second-stage air cooling temperature, needs to be controlled at the ferrite end transformation temperature A e1 In order to avoid entering the bainite transformation interval too early during the first-stage water cooling. For the duplex steel with the chemical composition of the present application, A e1 680℃~700℃, therefore the present application limits the first-stage final cooling temperature, that is, the second-stage air cooling temperature, at 680℃~720℃.

[0040] The second-stage air cooling process mainly occurs ferrite and austenite intermediate phase coarsening and growth, and no phase transformation occurs in this stage. The austenite coarsening and growth occurring in this stage is beneficial to improve the stability of austenite. Therefore, the second-stage air cooling temperature should not be too low, the holding time should not be too long, and the cooling rate should not be too high, so as to avoid entering the pearlite transformation zone and forming abnormal pearlite organization. Therefore, the present application limits the second-stage air cooling temperature at 680℃~720℃, the air cooling time at 2~6s, and the cooling rate at 2℃ / s~5℃ / s.

[0041] The third stage water cooling process is mainly the phase transformation of the austenite interphase to form the bainite structure and the residual austenite structure. First, the requirement of the cooling rate is considered. According to the attached Figure 1 It can be seen that the cooling rate of the bainite transformation A→B is 1℃ / s-10℃ / s, and the third stage water cooling rate of the steel should not be too high according to the present application, otherwise the martensite transformation is prone to occur, because the steel according to the present application is added with high content of C, Si and other elements, which can ensure the steel to have good hardenability, but with the increase of the water cooling rate, the supercooling degree is large in the cooling process, which is not conducive to the stable retention of the residual austenite to the finished steel, and the residual austenite may be transformed into bainite or martensite in the cooling process. At the same time, the third stage cooling rate should not be too low, otherwise the pearlite transformation is prone to occur, forming the pearlite abnormal structure, resulting in the decrease of the elongation of the finished steel. Therefore, the third stage water cooling rate is limited to 5℃ / s-10℃ / s according to the present application, and 5℃ / s is selected as the lower limit because the cooling rate of the steel plate in the air is 2℃ / s-5℃ / s, and the water cooling rate is higher than the air cooling rate.

[0042] Secondly, the final cooling temperature of the third stage water cooling, that is, the requirement of the final cooling temperature of the whole laminar cooling process, is considered. In order to obtain the bainite and residual austenite structure, the final cooling temperature should be slightly lower than the bainite transformation start temperature, but should not be too low, otherwise the martensite transformation region is prone to enter, forming the martensite structure; and should not be too high, otherwise it is impossible to enter the bainite transformation region, and the pearlite structure will be formed; therefore, the final cooling temperature is limited to B s -50℃-B s For the multiphase steel according to the present application, B s -50℃, and the final cooling temperature is limited to 450℃-500℃ according to the present application.

[0043] The technical scheme and effect of the present application are further described below through actual examples.

[0044] Examples

[0045] I. Preparation of the multiphase steel

[0046] The preparation method of the multiphase steel in the examples and comparative examples of the present application is as follows: the billet is smelted according to the chemical composition of the target steel, and then the multiphase steel is produced according to the process flow of slab heating-rough rolling-fine rolling-laminar cooling-coiling. The chemical composition of the target steel in the examples and comparative examples, and the phase transformation temperature corresponding to the chemical composition are shown in Table 1.

[0047] Table 1 Chemical composition (in mass percent, %)

[0048]

[0049] The specific production process parameters of the complex phase steel in Examples 1-2 and Comparative Examples 1-2 are shown in Table 2.

[0050] Table 2 Process parameters

[0051]

[0052]

[0053] 2. Performance testing of complex phase steel

[0054] The mechanical properties of the finished complex phase steel products obtained in Examples 1-2 and Comparative Examples 1-2 were tested, and the results are shown in Table 3.

[0055] Table 3 Mechanical properties

[0056] Performance index Steel plate thickness Yield strength Tensile strength Elongation Hole expansion ratio Example 1 5.2 mm 683 MPa 802 MPa 26％ 66％ Example 2 3.5 mm 705 MPa 832 MPa 27％ 64％ Comparative Example 1 6.0 mm 864 MPa 987 MPa 14％ 31％ Comparative Example 2 4.4 mm 522 MPa 656 MPa 26％ 58％

[0057] Taking Example 1 as an example, the microstructure is detected as follows: Figures 2-3 As shown. Figure 2 1 is the microstructure of the steel plate in Example 1 of the present invention. It can be seen that the microstructure is mainly composed of ferrite and bainite. Figure 3 This is the XRD test result of the steel in Example 1. It can be seen that the steel in Example 1 contains 7.2% retained austenite.

[0058] As can be seen from Table 3, the steel prepared using the composition and process provided by the present invention has a yield strength in the range of 650 MPa to 750 MPa, a tensile strength in the range of 750 MPa to 850 MPa, an elongation ≥ 25%, and a hole expansion ratio ≥ 60%, showing a good match between strength and plasticity. This is due to the fine ferrite structure, bainite with a high dislocation density, and a small amount of retained austenite structure in the steel.

[0059] Attachment Figure 4 The microstructure of the steel of comparative example 1 of the present invention is shown in FIG. 1 . It can be seen that the microstructure is mainly composed of ferrite and martensite. Figure 5 Figure 2 is the XRD test result of the steel in comparative example 1. It can be seen that the steel in comparative example 1 does not contain retained austenite. Figure 6 This is the XRD test result of the steel in comparative example 2. It can be seen that the microstructure is mainly composed of ferrite and pearlite.

[0060] From Table 3, the yield strength and tensile strength of the steel of Comparative Example 1 are higher, but the elongation and the hole expansion ratio are lower, mainly because the laminar cooling rate is too high when the third section of the steel of Comparative Example 1 is water-cooled, resulting in a large amount of martensite formed in the steel, and no bainite and residual austenite formed. The tensile strength of the steel of Comparative Example 2 is lower, mainly because the second section is air-cooled at a lower temperature, and the third section is water-cooled at a higher final cooling temperature, resulting in a large amount of pearlite formed in the steel, and the contribution of the pearlite to the strength is limited compared to the bainite and martensite, so the strength of the steel of Comparative Example 2 is lower.

Claims

1. Process for the production of a residual austenite plasticized ferrite- bainite dual phase steel, characterized in that: The slab is smelted according to the chemical composition of the target steel, and then a multi-phase steel is produced according to the process flow of slab heating-rough rolling-finish rolling-laminar cooling-coiling; wherein the laminar cooling adopts a three-stage cooling of 'water cooling-air cooling-water cooling': The first-stage water cooling is performed at a cooling temperature of 840-910 DEG C and a cooling rate of 5-15 DEG C / s; The second-stage air cooling is performed at a cooling temperature of 680-720 DEG C and a cooling rate of 2-5 DEG C / s, and the air cooling time is 2-6 s; The third-stage water cooling is performed at a final cooling temperature of 450-500 DEG C and a cooling rate of 5-10 DEG C / s; The coiling temperature of the coiling process is 450-500 DEG C; The chemical composition of the multi-phase steel comprises, by weight percentage, C 0.16-0.20%, Si 0.6-1.2%, Mn 1.6-1.8%, Als 0-0.65%, and the balance of Fe and inevitable impurity elements.

2. Process for the production of residual austenite plasticity type ferritic- bainitic dual phase steels according to claim 1, characterized in that: The slab heating process is performed at a heating temperature of 1180-1200 DEG C and a furnace time of 200-300 min.

3. The method of manufacturing a residual austenite plasticity type ferrite- bainite dual phase steel according to claim 1, characterized in that: The rough rolling process is performed at a cumulative deformation of 75-85%.

4. The method of manufacturing a residual austenite plasticity type ferrite- bainite dual phase steel according to claim 1, characterized in that: The finish rolling process is performed at a cumulative deformation of 85-95%. 5.The residual austenite plasticized ferrite-bainite multi-phase steel prepared by the preparation method of any one of claims 1-4.

6. The residual austenite plasticity type ferrite- bainite dual phase steel according to claim 5, characterized in that: The microstructure of the multi-phase steel comprises ferrite, bainite and residual austenite, wherein the volume fraction of the residual austenite is 5-10%.

7. The residual austenite plasticity type ferrite- bainite dual phase steel according to claim 5, characterized in that: The yield strength of the multi-phase steel is 650-750 MPa, the tensile strength is 750-850 MPa, the elongation is greater than or equal to 25%, the hole expansion ratio is greater than or equal to 60%, and the thickness is 3-6 mm.

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