Hot-rolled Gigaba steel with high product of strength and elongation and production method thereof

Abstract

The invention belongs to the technical field of high-strength hot continuous rolling steel, and particularly relates to high-strength-and-ductility-product hot-rolled Gigaba steel and a production method thereof. In order to overcome the defect of complicated production process of the existing gigapa-grade high-strength steel, the hot-rolled gigapa steel with the yield strength of 600-750 MPa, the tensile strength of 900-1050 MPa, the elongation of greater than or equal to 20% and the product of strength and elongation of greater than or equal to 20 GPa% is produced by adopting a hot continuous rolling production line, and the hot-rolled gigapa steel comprises the following chemical components in percentage by mass: 0.20-0.25% of C, 1.50-2.00% of Si + Als, 2.00-2.50% of Mn, less than or equal to 0.015% of P, less than or equal to 0.005% of S and the balance of Fe and inevitable impurities. According to the invention, the stable retained austenite-containing Gigaba steel is prepared by a process of hot continuous rolling and slow cooling after rolling, and the microstructure of the stable retained austenite-containing Gigaba steel contains 8-15% of retained austenite, 40-60% of bainite and the balance of ferrite.

Description

Technical Field

[0001] The invention belongs to the technical field of high-strength hot-rolled steel, and particularly relates to a high-strength and plastic-accumulated hot-rolled GPa steel and a production method thereof. Background Art

[0002] Currently, the highest strength level of hot-rolled high-strength steel used in large-scale production in the automotive industry is 750-800 MPa. Microalloying with Nb, V, and Ti is employed to enhance strength through precipitation strengthening and grain refinement. However, these high-strength steels suffer from high yield strength ratios, insufficient elongation, and susceptibility to forming or stamping cracking. TRIP steel has achieved widespread application in cold-rolled products, but has yet to achieve mass production in hot-rolled products. This is primarily due to the high process sensitivity of hot-rolled TRIP steel. Instability in the process can easily lead to the formation of abnormal microstructures such as pearlite and martensite, which not only increases the material's brittleness but also reduces the retained austenite content, further reducing the material's elongation.

[0003] Patent CN102758133A currently discloses a 1000 MPa-grade high-strength-ductility automotive steel and its manufacturing method. The steel's composition is as follows: C: 0.11% to 0.30%, Si: 0.1% to 2.0%, Mn: 5% to 10%, P ≤ 0.02%, S ≤ 0.02%, Al: 0.01% to 2.0%, N ≤ 0.02%, with the remainder being Fe and unavoidable impurities. The preparation steps include smelting, casting, hot rolling, coiling, bell-type furnace annealing, cold rolling, and continuous annealing. The high-strength-ductility automotive steel prepared in this patent has a tensile strength greater than 1000 MPa and a strength-ductility product ≥ 30 GPa·%. However, the steel is produced using a post-cold rolling annealing method to produce GPa-grade high-strength steel with retained austenite, which is a complex process.

[0004] Patent CN115341130A also discloses a method for producing high-strength-ductility hot-rolled cold-formed automotive structural steel. After smelting, casting, forging, and rolling, the steel is cooled using a two-stage cooling process. This process is followed by a brief reverse austenite annealing process to partially austenitize the structural steel microstructure. The steel is then held and cooled in the ferrite-austenite (α+γ) two-phase region before being cold-stamped according to product dimensions. The product exhibits an elongation of 30% to 40% and a strength-ductility product of 30 to 40 GPa·%. However, this method employs a post-hot-rolling heat treatment method to produce GPa-grade high-strength steel containing retained austenite, which also presents the drawback of a complex process. Summary of the Invention

[0005] To overcome the complex production process of existing GPa-grade high-strength steel, the present invention provides a hot-rolled GPa steel with a low yield ratio, high elongation, and high strength, produced using a hot-rolled continuous rolling line. The steel is produced solely through a hot-rolled continuous rolling process, without subsequent cold rolling, annealing, or heat treatment, resulting in a simple production process. This enables product upgrades in hot-rolled high-strength steel for automotive stampings.

[0006] To achieve the above application objectives, the technical solutions adopted in this application are as follows: In a first aspect, the present invention provides a hot-rolled GPa steel, whose chemical composition, calculated by mass percentage, includes: C: 0.20-0.25%, Si+Als: 1.50-2.00%, Mn: 2.00-2.50%, P≤0.015%, S≤0.005%, and the balance is Fe and unavoidable impurities.

[0007] Furthermore, the microstructure of the hot-rolled GIP steel includes 8-15% retained austenite, 40-60% bainite, and the remainder is ferrite.

[0008] Furthermore, the hot-rolled GPa steel has a yield strength of 600-750 MPa, a tensile strength of 900-1050 MPa, an elongation ≥20%, and a strength-ductility product ≥20 GPa•%.

[0009] In a second aspect, the present invention provides a production method for the above-mentioned hot-rolled GPA steel, which comprises the following steps: smelting, continuous casting, slow cooling, slab heating, rolling, laminar cooling, coiling, and slow cooling are sequentially performed according to the set chemical composition of the steel.

[0010] Furthermore, the hot-rolled GIP steel is smelted and continuously cast to obtain a slab with a thickness of 200 to 250 mm.

[0011] Furthermore, the steel ingot after continuous casting is slowly cooled to room temperature.

[0012] Furthermore, the temperature of the steel slab after continuous casting and slow cooling is 1160-1180°C after heating, and the time in the furnace is 200-300 minutes.

[0013] Furthermore, the heated steel ingot is rolled in two stages. The rolling temperature of the first stage is 1050-1150° C., and the rolling passes are 5-6 times. After rolling, an intermediate billet with a thickness of 35-50 mm is obtained.

[0014] Furthermore, the temperature of the intermediate billet in the second stage rolling is 850-1050° C. After rolling, a steel plate with a thickness of 2.0-6.0 mm is obtained.

[0015] Furthermore, the rolled steel is rarefaction cooled to 400-450° C. at a cooling rate of 5-15° C. / s.

[0016] Furthermore, after coiling, the steel is slowly cooled, with the exit temperature being ≤ 200°C and the holding time being 45 to 60 hours.

[0017] Beneficial effects: (1) The present invention prepares GPA steel containing stable retained austenite by a process of hot rolling and slow cooling after rolling, and the microstructure includes 8-15% retained austenite, 40-60% bainite, and the balance is ferrite.

[0018] (2) Compared with the GPa steel containing retained austenite structure prepared by continuous annealing or heat treatment after cold rolling, the process of the present invention is simpler and the product has better strength and plasticity. The yield strength of the steel of the present invention is 600-750 MPa, the tensile strength is 900-1050 MPa, the elongation is ≥20%, and the strength-ductility product is ≥20 GPa•%.

[0019] (3) Compared with the cold-rolled annealed steel plate, the hot-rolled steel plate has a larger thickness and is more suitable for automotive structural parts. The production method of the present invention can also be extended to other high-strength and high-plasticity steel grades. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a microstructure diagram of the hot-rolled GIP steel of Example 1 of the present invention; Figure 2 This is a microstructure diagram of the GIP steel of Comparative Example 1 of the present invention; Figure 3 This is the microstructure diagram of the GIP steel of Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0021] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clear, the application is further described in detail below in conjunction with the embodiments. Unless otherwise defined, all scientific and technical terms used herein have the same meanings as understood by ordinary technicians in this field.

[0022] In one embodiment of the present invention, a hot-rolled GPa steel is provided, the chemical composition of which comprises, by mass percentage, C: 0.20-0.25%, Si+Als: 1.50-2.00%, Mn: 2.00-2.50%, P≤0.015%, S≤0.005%, and the balance is Fe and unavoidable impurities.

[0023] The reasons for limiting the main alloying elements in the steel of the present invention are explained below.

[0024] C is an important element to improve the stability of residual austenite, research shows that the carbon content in residual austenite directly affects the stability of residual austenite, therefore, in order to obtain the residual austenite organization with sufficient stability, the steel described in the application adopts higher C content compared with the traditional low carbon micro alloy steel, and the C content is specifically controlled in 0.20-0.25%.

[0025] Mn can also play a role in improving the stability of residual austenite and increasing the phase transition temperature, therefore, the steel described in the application adopts higher Mn content, which can improve the residual austenite content in the finished steel, at the same time, increasing the phase transition temperature is beneficial to realizing the regulation of the organization of the finished steel at a higher temperature, that is, obtaining ferrite, bainite and residual austenite organization, and the Mn content is controlled in 2.00-2.50% in the application.

[0026] Similarly, Si and Als can also play a role in improving the stability of residual austenite and increasing the phase transition temperature, in order to improve the residual austenite content in the steel, the sum of Si and Als content is limited to 1.50-2.00% in the application.

[0027] P and S are inherent impurity elements in steel, P content is too high, which is easy to segregate at the grain boundary, S content is too high, which is easy to combine with Mn to generate MnS inclusions, which will all lead to the decrease of plasticity and the increase of brittleness of the finished steel. Therefore, the P and S content is limited to ≤0.015% and ≤0.005% respectively in the application.

[0028] In another embodiment of the application, a production method of the above-mentioned hot-rolled gigapascal steel is provided, which comprises the following steps: smelting, continuous casting, slow cooling, slab heating, rolling, laminar cooling, coiling, slow cooling in sequence according to the set steel chemical composition.

[0029] The production process limitation reasons are described below in combination with the control requirements of the strength and plasticity of the steel described in the application.

[0030] As described above, the C, Mn and Si content in the steel described in the application is higher, these elements can all improve the hardenability of the steel, which leads to the easy cracking of the billet after continuous casting in the air, therefore, the cast billet is required to be slow cooled to room temperature in the slow cooling pit.

[0031] Steels with high Si content are prone to forming Fe2SiO4 olivine during slab heating. This substance has a low melting point of approximately 1180°C. When the slab temperature is high, Fe2SiO4 melts into a liquid phase, forming a bonding layer at the interface between the surface oxide scale and the steel substrate. This prevents the steel plate from being completely cleaned of surface oxides during subsequent rolling and descaling, affecting the surface quality of the finished steel. Therefore, the present invention requires the slab heating temperature to be lower than the above temperature, specifically 1160-1180°C. Furthermore, when the furnace time is short, the microalloying elements in the steel slab cannot be fully dissolved, and the size of the original austenite in the steel slab tends to coarsen, resulting in a coarsened structure in the finished steel. Therefore, the present invention limits the furnace time to 200-300 minutes.

[0032] The first stage of rolling requires deformation to occur in the recrystallization zone of austenite, so the rolling temperature must be above the critical recrystallization temperature, which is below 1000°C. Therefore, the present invention requires a first-stage rolling temperature above 1000°C, specifically 1050-1150°C. Secondly, the thickness of the intermediate bar affects the deformation during the first stage of rolling. A higher intermediate bar thickness can result in lower deformation in the recrystallization zone, hindering grain refinement of the original austenite. A lower intermediate bar thickness can result in lower deformation in the non-recrystallization zone, hindering flattening of the original austenite. Therefore, the present invention limits the intermediate bar thickness to 35-50 mm.

[0033] The second stage of rolling requires a lower rolling temperature to promote flattening of the austenite structure in the non-recrystallized zone, thereby increasing the number of nucleation sites during the subsequent phase transformation and refining the grains of the final microstructure (ferrite, bainite, etc.) in the finished steel. Excessively high rolling temperatures can lead to coarsening of the finished steel structure and even affect the morphology of the bainite and retained austenite, shifting the structure from laths or films to blocks, which in turn reduces the stability of the retained austenite. Therefore, the present invention limits the second stage rolling temperature to 850-1050°C.

[0034] The laminar cooling process adopts a sparse cooling process, the cooling rate is limited to 5-15°C / s, and the final cooling temperature is limited to 400-450°C. This is because the hardenability of the steel described in the present invention is relatively high. If the cooling rate is too high, it may lead to the formation of martensite structure during the laminar cooling process, which will cause the carbon content in the supercooled austenite to decrease during the phase transformation, which is not conducive to the nucleation of residual austenite and the improvement of its stability. If the cooling rate is too low, it may lead to the formation of pearlite structure during the laminar cooling process. The high carbon content in pearlite will also lead to a decrease in the carbon content in the supercooled austenite. The final cooling temperature is selected near the bainite transformation temperature of the steel of the present invention, that is, 400-450°C. Pearlite structure is easily formed when the final cooling temperature is high, and martensite structure is easily formed when the final cooling temperature is low.

[0035] Finally, after laminar cooling, the steel plate is coiled and then sent to a slow cooling pit for slow cooling. The exit temperature is ≤200°C, and the holding time is 45-60 hours. This is because the bainite transformation rate is slow, requiring the steel coil to be isothermally treated in the bainite isothermal range for a longer period of time to achieve a certain proportion of bainite and retained austenite. The present invention has found an alternative to isothermal heat treatment: using a slow cooling pit to keep the steel coil near the coiling temperature for a longer period of time, thus also achieving the desired bainite isothermal effect.

[0036] Specific examples will be listed below to explain the scheme of the present invention. Those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. Where specific techniques or conditions are not specified in the examples, they are carried out according to the techniques or conditions described in the literature in this area or according to the product specifications. Where the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be obtained commercially.

[0037] Preparation of hot-rolled GPA steel Table 1 shows the compositions of the embodiments of the present invention and the comparative examples, Table 2 shows the production process parameters of the embodiments of the present invention and the comparative examples, and Table 3 shows the mechanical properties of the steels of the embodiments of the present invention and the comparative examples.

[0038] The hot-rolled GPa steels described in Examples 1 to 3 of the present invention are smelted, continuously cast, slowly cooled, slab heated, rolled, laminar cooled, coiled, and slowly cooled to obtain finished steel plates. The specific composition, processing, and properties are shown in Tables 1, 2, and 3. The microstructure types F, B, RA, P, and M in Table 3 represent ferrite, bainite, retained austenite, pearlite, and martensite, respectively. As shown in Table 3, the mechanical properties of the examples all meet the requirements, namely, a yield strength of 600 to 750 MPa, a tensile strength of 900 to 1050 MPa, an elongation of ≥20%, and a strength-ductility product of ≥20 GPa·%. This is due to the presence of a large amount of bainite (45% to 50%) and retained austenite (11% to 14%) in the steel microstructure. The bainite contains a high density of dislocations, and the retained austenite can undergo a TRIP effect during deformation. The combined effects of these two factors improve the strength-ductility product of the examples.

[0039] The chemical composition of the steels in Comparative Examples 1 to 3 all meet the requirements, but the cooling rate of Comparative Example 1 is too high, reaching 19°C / s, and the coiling temperature is too low, reaching 182°C. Under these process conditions, a large amount of martensite structure is formed in Comparative Example 1 ( Figure 2 ), the formation of martensite occupies the carbon in the supercooled austenite, making it impossible for retained austenite to form, which ultimately leads to a significant decrease in the elongation of the material to only 9.0%.

[0040] The cooling rate of Comparative Example 2 is too low at 4°C / s, and the coiling temperature is too high at 654°C / s. Under these process conditions, a large amount of pearlite is formed in Comparative Example 2 ( Figure 3 ), pearlite also occupies a large amount of carbon elements, which makes it impossible for retained austenite to form. At the same time, the elongation and strength of the pearlite matrix steel are significantly lower than those of bainite steel. Therefore, the strength and plasticity of the comparative example 2 steel are significantly reduced, and the strength-plasticity product is only 15.3 GPa•%.

[0041] In Comparative Example 3, the steel coil was not slowly cooled after coiling, and the cooling rate of the steel coil was relatively fast, resulting in the appearance of some martensite in the finished steel, which in turn prevented the formation of retained austenite. The final structure of the finished steel was 65% F, 25% M, and 10% B. Although the strength met the requirements, the elongation and strength-ductility product were relatively low, at only 12.5% ​​and 14.4 GPa•%.

[0042] The Si+Als content in the steel composition of Comparative Example 4 is relatively low, only 1.02%, and the C content is relatively low, only 0.15%. The low content of the above elements leads to insufficient stability of supercooled austenite, which in turn leads to low contents of bainite and retained austenite in the finished steel, and ultimately leads to a low strength-ductility product of the finished steel, which is 15.7 GPa•%.

[0043] Table 1 Chemical composition of the steel of the present invention (%) Table 2 Process parameters of the steel of the present invention Table 2 continued Table 3 Mechanical properties of the steel described in the present invention

Claims

1. Hot rolled GIP steel, characterized in that: The chemical composition, by mass percentage, includes: C: 0.20-0.25%, Si+Als: 1.50-2.00%, Mn: 2.00-2.50%, P≤0.015%, S≤0.005%, and the balance is Fe and unavoidable impurities.

2. The hot-rolled GPA steel according to claim 1, characterized in that: The hot-rolled GIP steel microstructure comprises 8-15% retained austenite, 40-60% bainite, and the remainder is ferrite.

3. The hot-rolled GIP steel according to claim 1 or 2, characterized in that: The hot-rolled GPa steel has a yield strength of 600-750 MPa, a tensile strength of 900-1050 MPa, an elongation of ≥20%, and a strength-ductility product of ≥20 GPa•%.

4. The method for producing hot-rolled GPA steel according to any one of claims 1 to 3, characterized in that: The following steps are involved: According to the set chemical composition of the steel, smelting, continuous casting, slow cooling, slab heating, rolling, laminar cooling, coiling and slow cooling can be carried out in sequence.

5. The production method according to claim 4, characterized in that: After continuous casting, the steel ingot is slowly cooled to room temperature.

6. The production method according to claim 4 or 5, characterized in that: After continuous casting and slow cooling, the steel ingot is heated to a furnace temperature of 1160-1180°C and is kept in the furnace for 200-300 min.

7. The production method according to any one of claims 4 to 6, characterized in that: The heated steel ingot is rolled in two stages. The rolling temperature of the first stage is 1050-1150°C, and an intermediate billet with a thickness of 35-50 mm is obtained after rolling.

8. The production method according to claim 7, characterized in that: The second stage rolling temperature of the intermediate billet is 850-1050℃.

9. The production method according to any one of claims 4 to 8, characterized in that: The rolled steel is rarefaction cooled to 400-450°C at a cooling rate of 5-15°C / s.

10. The production method according to any one of claims 4 to 9, characterized in that: After coiling, slow cooling is carried out and the exit temperature is ≤200℃.

Images