A production method for improving the performance stability in the width direction of 800MPa grade complex phase steel and 800MPa grade complex phase steel produced thereby

Abstract

The application provides a production method for improving the width direction performance stability of 800MPa grade complex phase steel and 800MPa grade complex phase steel produced by the method. In the production, the use of a width setting machine increases the edge deformation and promotes the grain structure of the edge to be more refined. The optimization of the water use strategy between the stands reduces the width direction temperature unevenness during the finishing rolling. The laminar flow cooling front section fast cooling technology is used to quickly cool the strip steel to the coiling target temperature, reduce the residence time of the strip steel in the water environment, and reduce the edge temperature drop during the period. The precise control laminar flow cooling edge shielding amount technology is used to make the edge of the strip product of different widths naturally flow through the cooling of the middle area cooling water, improve the edge temperature of the strip, and reduce the difference between the edge and the middle of the strip in the laminar flow cooling and the hot coiling cooling in the structure and performance. The product width direction tensile strength range is less than or equal to 35MPa, and the strength low point is greater than or equal to 815MPa.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled multiphase high-strength steel production process, specifically relating to a production method for improving the width-direction performance stability of 800MPa grade multiphase steel and the produced 800MPa grade multiphase steel. Background Technology

[0002] Hot-rolled multiphase steel, due to its excellent strength-plasticity ratio, has been widely used by automobile manufacturers in automotive structural components such as chassis, wheels, suspension, control arms, and safety locks. With the rapid development of the automotive industry, lightweighting, energy conservation and environmental protection, and high safety have become inevitable trends in the development of the automotive industry, and the requirements for the comprehensive performance of automotive steel are becoming increasingly higher.

[0003] The width-direction performance stability of hot-rolled multiphase steel plates is an important indicator for evaluating material performance, mainly reflected in two aspects: first, the edge strength is lower than required, while other parts meet the requirements; second, the width-direction strength meets the requirements, but the differences are significant, affecting the stability of stamping. Both of these situations will reduce the overall utilization rate of the material and production efficiency to varying degrees, and increase production costs.

[0004] Low cost and high efficiency have always been the production methods desired by automotive parts manufacturers. The stability of the width direction performance is an important factor affecting the material utilization rate and efficient production, especially for hot-rolled high-strength multiphase steel, which has a large cost per unit ton. Therefore, improving the stability of the width direction performance of multiphase steel is particularly important.

[0005] Currently, relevant patents addressing this issue fall into two categories: First, temperature compensation type, such as Chinese patent CN 107824619 A, published on March 23, 2018, which discloses a method to improve the uniformity of strip steel performance. This method achieves uniform mechanical properties of the strip steel along the same width direction by controlling the laminar cooling water volume during hot rolling. While this patent proposes reducing the percentage and distance of water flow at the edge of the laminar cooling process, it does not specify the implementation method, and its practical operability is limited by equipment capabilities. On the one hand, to achieve a different percentage of water flow in the width direction compared to the middle position, different water flow control systems are needed for each group of manifolds in the width direction, increasing equipment costs. Furthermore, precise control is impossible when dealing with different finished product widths and thicknesses. Second, heat preservation and slow cooling type, such as Chinese patent CN112063815A, published on December 11, 2020, which discloses a method for post-rolling heat preservation and slow cooling to improve the uniformity of finished product performance. By applying an insulation cover to the hot-rolled coil for slow cooling, the temperature field of the steel coil is more uniform during the cooling process, which is similar to furnace cooling, thus achieving the goal of controlling performance fluctuations. This patent proposes a specific method for slow cooling of hot-rolled coils. However, on the one hand, for some steel grades that are highly sensitive to temperature, the edge of the hot-rolled coil has already experienced a temperature drop compared to other areas before it is coiled. Furthermore, due to the long time that the hot-rolled coil spends in the insulation cover after leaving the production line during mass production, it is difficult to maintain stable time control during mass production. This results in differences in the initial microstructure, and even if the same cooling rate is maintained subsequently, it is difficult to achieve ideal performance uniformity. On the other hand, in order to maintain the capacity for mass production, a large number of insulation cover equipment needs to be invested to ensure stable mass production, increasing equipment investment costs and factory storage space costs.

[0006] Given the current situation and needs of major steel mills in terms of production line equipment capacity and cost reduction, how to simply, reliably, and efficiently utilize existing equipment and processes to optimize the design for performance stability in the width direction is a topic with practical application significance. Summary of the Invention

[0007] The purpose of this invention is to provide a production method for improving the width-direction performance stability of 800MPa grade multiphase steel, and the produced 800MPa grade multiphase steel. During production, the use of a fixed-width mill increases edge deformation, promoting finer edge grain structure. Optimized water injection strategies between stands reduce width-direction temperature unevenness during finishing rolling. Laminar flow cooling pre-cooling technology rapidly cools the strip to the target coiling temperature, reducing the strip's residence time in the water environment and minimizing edge temperature drop. Precise control of laminar flow cooling edge shielding technology allows for natural flow of cooling water through the middle region to cool the edges of strips of different widths, further reducing edge temperature drop. This minimizes the differences in microstructure and properties between the edges and center of the strip during laminar flow cooling and hot-rolling cooling. The result is a product with a width-direction tensile strength range ≤35MPa and a low strength point ≥815MPa. Furthermore, this invention offers better economic efficiency compared to other methods that control the percentage difference in width-direction water flow and the middle position, and incorporate additional cooling equipment.

[0008] The specific technical solution of this invention is as follows:

[0009] A production method for improving the width-direction property stability of 800MPa grade multiphase steel includes the following steps:

[0010] 1) The width difference between the slab width and the finished product width is required to be +200 to +280 mm;

[0011] 2) During finishing rolling, water is used between the finishing mill stands;

[0012] 3) Use laminar flow cooling to rapidly cool the strip to the target coiling temperature.

[0013] In step 1), the use of the material width-fixing machine requires a width difference of +200 to 280 mm between the slab width and the finished product width. The width-fixing machine is used to apply side pressure to the edge of the slab, with a side pressure of 200 to 280 mm and a single-side side pressure of 100 to 140 mm. This can increase the amount of thermal deformation of the microstructure in this part of the edge, refine the grains during recrystallization, and increase the strengthening effect of fine grains in the edge. If the side pressure is too small, it will not achieve a significant improvement; if it is too large, it will exceed the capacity of the side pressure machine, which existing equipment cannot meet.

[0014] In step 2), for finishing rolled products with a thickness of 1.8 ≤ thickness < 3.0 mm, inter-stand water supply options F12, F23, F45, and F67 are used; for thicknesses of 3.0 ≤ thickness ≤ 6.0 mm, options F12, F34, and F56 are used. The main reason is that during the rolling process, the edges of the steel plate experience greater heat exchange than the center, and the flow of cooling water towards the edges results in a higher heat transfer coefficient. Furthermore, thicker products have lower rolling speeds and longer residence times in the stands compared to thinner products, leading to greater temperature drops at the edges. Therefore, different inter-stand water supply strategies are adopted for different thicknesses. In actual inter-stand water supply, automatic mode is selected, with a supply percentage ranging from 15% to 60%. Specific parameters will dynamically change based on belt speed and FDT (Front-to-Distance Flow) hit rate.

[0015] In step 3), the rapid cooling of the laminar flow cooling front section specifically means: the average cooling rate of the rapid cooling of the laminar flow cooling front section is ≥50℃ / s, preferably 60~65℃ / s; the laminar flow cooling unit of the 2250 hot rolling production line has a total of 22 sets of manifolds (20 sets for coarse adjustment and 2 sets for fine adjustment), and the front section cooling refers to the water spray cooling of the front section manifolds immediately after the strip exits the F7 stand of the finishing mill.

[0016] In step 3), the laminar flow cooling employs edge shielding. For finished strip steel with a width of 1100-1500mm, four sets of edge shielding are used. The first and second sets of laminar flow cooling strips have a shielding width of D1mm on both sides, while the third and fourth sets have a shielding width of D2mm on both sides. Since the stability of strip width performance is related to the strip width, this invention uses different shielding amounts for different widths to achieve precise control of the shielding amount. If the shielding amount is too low, it will not have an improvement effect; if the shielding amount is too high, the edge temperature will be too high, resulting in excessive strength increase, which will also fail to meet the improvement requirements. Therefore, precise control of the relationship between the shielding amount and the width is one of the core technologies of this invention.

[0017] This invention uses a laminar flow cooling unit for a 2250 hot rolling production line with a total of 22 sets of manifolds. The first, second, third, and fourth sets represent the manifolds closest to the finishing mill exit, with the closest being the first set, the next closest being the second set, and so on.

[0018] D1 = k1b / 2, D2 = k2b / 2; b is the width of the hot-rolled finished strip, 1100-1500 mm, in mm; k1 and k2 are empirical coefficients, taken as 0.1 and 0.125 respectively.

[0019] Based on actual measurements of the temperature of hot-rolled coils with different widths, such as... Figure 2 As shown, temperature drops occur within approximately 78mm and 88mm of the edge portions for widths of 1250mm and 1410mm, respectively, with the lowest temperature drop being approximately 60℃. Different edge cooling amounts are set for different width specifications. The width section temperature measuring device is located at the winding inlet and can monitor the plate width temperature. Figure 2 The results, derived from the temperature curve analysis of the strip width, demonstrate that different strip widths require different amounts of edge shielding to improve performance. Under normal circumstances, during laminar flow cooling, the temperature drop along a certain distance along the edge of the strip varies, with the temperature drop increasing closer to the edge. Therefore, differentiated treatment is adopted when shielding the edges, and the different coefficients of K1 and K2 reflect this differentiation.

[0020] Furthermore, the production method for improving the width-direction performance stability of 800MPa grade multiphase steel also includes the use of electromagnetic rollers in the continuous casting process to achieve uniformity of the billet structure;

[0021] The production method for improving the width-direction performance stability of 800MPa grade multiphase steel also includes pre-rolling heating, with the furnace exit temperature at 1230-1270℃ and the heating time at 200-240min.

[0022] The production method for improving the width-direction performance stability of 800MPa grade multiphase steel also includes controlling the rolling temperature to be 860℃≤final rolling temperature≤900℃ and 540℃≤coiling temperature≤580℃ during rolling.

[0023] The production method for improving the width-direction performance stability of 800MPa grade multiphase steel also includes stacking and slow cooling of hot-rolled coils to room temperature.

[0024] This invention provides an 800MPa grade multiphase steel, produced using the aforementioned method for improving the width-direction performance stability of 800MPa grade multiphase steel. The 800MPa grade multiphase steel comprises the following chemical composition by weight percentage: C: 0.05%–0.10%, Si: 0.10%–0.30%, Mn: 1.50%–2.0%, Als: 0.020%–0.060%, P≤0.020%, S≤0.003%, Nb: 0.02%–0.10%, Ti: 0.05%–0.20%, Cr+Mo+Ni≤1.5%, with the remainder being Fe and unavoidable impurities. Using this composition, the width-direction performance stability of the 800MPa grade multiphase steel is achieved.

[0025] The composition of this invention is based on a low-cost design with high strength and high formability at the 800MPa level. It adopts a C, Si, Mn+Nb, Ti microalloying composite reinforcement design concept. The reinforcement effect of Nb and Ti microalloying is relatively sensitive to temperature changes, which has an adverse effect on the performance stability in the width direction. The improvement of temperature difference in the width direction of the plate in this invention can reduce or eliminate this adverse effect.

[0026] The 800MPa grade multiphase steel has a microstructure mainly composed of granular bainite and quasi-polygonal ferrite, with grain sizes of 13.5, 13.0, and 13.0 at the 30mm, 90mm, and 1 / 4 of the edge, respectively.

[0027] The 800MPa grade multiphase steel has a thickness of 1.8–6.0 mm and a width of 1100–1500 mm.

[0028] The 800MPa grade multiphase steel exhibits a width-direction tensile strength range ≤35MPa, with the lowest strength point ≥815MPa. The tensile strength at the edge 30mm is 825±10MPa, at the edge 90mm is 840±10MPa, at the 1 / 4 mark is 830±15MPa, and in the middle is 830±10MPa. The overall width-direction tensile strength range is ≤35MPa.

[0029] The inventors discovered that the main factor affecting the stability of performance in the width direction is the uneven cooling of the hot plate during production. Temperature differences in the width direction of the hot-rolled coil lead to inconsistencies in microstructure and properties. This invention addresses these issues by using a fixed-width mill to increase edge deformation and promote finer edge grain structure; optimizing the water injection strategy between stands to reduce temperature unevenness in the width direction during finishing rolling; employing rapid cooling technology in the pre-laminar flow cooling stage to quickly cool the strip to the target coiling temperature, reducing the strip's residence time in the water environment and minimizing edge temperature drop; and using precise control of the laminar flow cooling edge shielding technology to allow natural flow of cooling water through the middle area to cool the edges of strips of different widths, increasing the strip edge temperature and thus reducing the differences in microstructure and properties between the edges and the middle of the strip during laminar flow cooling and hot-rolling cooling. The product's tensile strength difference in the width direction is ≤35MPa, with the lowest strength point ≥815MPa.

[0030] Compared with existing technologies, the beneficial effects of this invention are as follows: 1) By using a fixed-width mill to apply lateral pressure of 100-140mm to the edge of the slab, the thermal deformation of the edge portion can be increased, the grains can be refined during recrystallization, and the edge grain strengthening effect can be enhanced; 2) By optimizing the water injection strategy between stands and using rapid cooling technology in the laminar flow cooling section, the microstructure differences caused by uneven temperature in the width direction during finishing rolling and laminar flow cooling are reduced; 3) For strip products of different widths, by precisely controlling the edge shielding amount during laminar cooling, the natural flow of cooling water in the middle area of ​​the edge is allowed to cool, increasing the edge temperature of the strip, thereby reducing the differences in microstructure and properties between the edge and the middle of the strip during laminar cooling and hot-rolled cooling. The produced products can achieve a strength difference of ≤35MPa and a low strength point of ≥815MPa in the width direction. Attached Figure Description

[0031] Figure 1Example 1: Strip width direction microstructure: a) 30mm edge microstructure; b) 90mm edge microstructure; c) 1 / 4 section microstructure;

[0032] Figure 2 Comparative Example 2: Strip width direction microstructure: a) Microstructure at the edge 30mm; b) Microstructure at the edge 90mm; c) Microstructure at 1 / 4 mark.

[0033] Figure 3 Cross-sectional temperature distribution under normal process conditions; as shown in Comparative Example 8, under normal process conditions, the water between the stands of the finishing mill is automatically put into use, while the edge shielding process is not used in the layer cooling.

[0034] Figure 4 Cross-sectional temperature distribution of the examples and comparative examples;

[0035] Figure 5 For comparison, here is a macroscopic view of the strip surface after laminar flow cooling without any shielding at the edges (Figure 6).

[0036] Figure 6 This is a macroscopic view of the strip surface after laminar flow cooling with edge shielding in Example 1. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Unless otherwise specified in the embodiments, the techniques or conditions described in the literature in this field or in accordance with the product manual may be followed.

[0039] Examples 1-3

[0040] An 800MPa grade multiphase steel comprises the following mass percentage composition as shown in Table 1, where the balance not shown in Table 1 is Fe and unavoidable impurities.

[0041] Comparative Examples 1-8

[0042] An 800MPa grade multiphase steel comprises the following mass percentage composition as shown in Table 1, where the balance not shown in Table 1 is Fe and unavoidable impurities.

[0043] Table 1. Composition and content (wt%) of 800MPa grade multiphase steel in each embodiment and comparative example.

[0044]

[0045]

[0046] The production methods for 800MPa grade multiphase steel described in the above embodiments and comparative examples include: smelting, continuous casting, hot rolling heating, rolling, laminar flow cooling, and coiling. Details are as follows:

[0047] The continuous casting process uses electromagnetic roller technology. The hot rolling heating temperature is 1250±20℃, and the heating time is 200~240min. During rolling, the final rolling target temperature is 880±20℃, and the average coiling temperature is 560±20℃. The laminar flow cooling front section uses sparse cooling at a rate of 26~28℃ / s, or the laminar flow cooling front section uses rapid cooling at a rate of 60~65℃ / s. The produced hot coils are stacked and slowly cooled to room temperature. In production, a fixed-width mill is used, requiring a width difference of +200 to 280 mm between the slab width and the finished product width. The fixed-width mill is used to apply side pressure to the slab edges, with a side pressure of 200 to 280 mm and a single-sided side pressure of 100 to 140 mm. The side pressure amounts for each embodiment and comparative example are shown in Table 2. For the use of water between the stands of the finishing mill, for finishing products with a thickness specification of 1.8 ≤ thickness < 3.0 mm, water between the stands is applied using F12, F23, F45, and F67; for thickness specifications of 3.0 ≤ thickness ≤ 6.0 mm, water between the stands is applied using F12, F34, and F56. Laminar flow cooling adopts edge shielding technology. For strip widths of 1100 to 1500 mm, four sets of edge shielding are used. The first and second sets of laminar flow cooling strips have D1 mm shielding on both sides, and the third and fourth sets of laminar flow cooling strips have D2 mm shielding on both sides.

[0048] Here, D1 = k1b / 2, D2 = k2b / 2;

[0049] b represents the width of the hot-rolled finished strip in mm, and k1 and k2 are empirical coefficients, which are 0.1 and 0.125 respectively.

[0050] The specific production processes and parameters of the above embodiments and comparative examples are shown in Table 2.

[0051] Table 2. Production process and parameters of 800MPa grade multiphase steel for each embodiment and comparative example.

[0052]

[0053]

[0054] The mechanical property test results of the strip in the width direction of Examples 1 to 3 and Comparative Examples 1 to 8 are shown in Tables 3 and 4. The mechanical specimens were longitudinal specimens (i.e., parallel to the rolling direction), and the test method adopted was the tensile test in the national standard GB / T 228.

[0055] Table 3. Strip properties (R) in the width direction of each embodiment and comparative example m / MPa)

[0056]

[0057] Table 4. Strip properties (R) in Example 3 and Comparative Example 7 m / MPa)

[0058]

[0059] In the comparative example, the tensile strength range in the width direction is 51-78 MPa, with the lowest tensile strength points being 754-782 MPa, which does not meet the grade strength requirements; in the example, the tensile strength range in the width direction is 28-32 MPa, with the lowest tensile strength points being 815-818 MPa, which meets the grade strength requirements.

[0060] The microstructure of the steel plates in each embodiment and comparative example was tested, and the results are shown in Table 5.

[0061] Table 5. Organizational status of examples and comparative examples

[0062]

[0063] The microstructure of the embodiments and comparative examples is shown in Table 5. Through the implementation of the present invention, the difference in microstructure between the edges and the middle is reduced. The microstructure of the strip in the width direction of Example 1 is shown in Table 5. Figure 1 It is mainly composed of ferrite and granular bainite. The grain size at the edge 30mm, 90mm, and 1 / 4 of the strip width direction is 13.5, 13.0, and 13.0, respectively. The microstructure of the strip width direction in Comparative Example 2 is shown in [reference needed]. Figure 2 It is mainly composed of ferrite and granular bainite. The grain size at the edge of the strip is 13.0, 12.0 and 12.5 at 30mm, 90mm and 1 / 4 of the width direction, respectively, with a large difference between the edge and the middle structure.

[0064] The cross-sectional temperature distributions of Examples 1, 2, and 3 and Comparative Examples 7 and 8 are as follows: Figure 4 As shown, the temperature of the strip edge is increased after adopting the process of this application.

[0065] Comparative Examples 1-2 suffered from insufficient edge microstructure refinement due to the side press not being in operation or the side press volume being too small; Comparative Examples 3-4 suffered from increased edge temperature drop due to excessive water usage between racks; Comparative Example 5 suffered from increased edge temperature drop due to sparse cooling in laminar cooling, resulting in a low cooling rate and prolonged water retention time in the strip; Comparative Examples 6-7 suffered from increased edge temperature drop due to the lack of edge shielding in laminar cooling; Comparative Example 8 suffered from insufficient edge microstructure refinement and increased edge temperature drop due to insufficient side press volume, excessive water usage between racks, low laminar cooling rate, and lack of edge shielding. Therefore, to achieve stable strip width direction performance, the process parameters involved in this invention need to work synergistically to achieve the purpose of this invention.

[0066] This invention addresses the challenges of hot-rolling multiphase steel with a tensile strength ≥800MPa. The rapid temperature drop at the edges of the billet during rolling and cooling causes variations in precipitation strengthening after coiling, ultimately affecting product performance stability. This invention is a production method designed based on the production characteristics of multiphase steel with a tensile strength ≥800MPa. It utilizes a low-cost design for high strength and high formability at the 800MPa level, employing a C, Si, Mn + Nb, Ti microalloying composite strengthening design. However, the strengthening effect of Nb and Ti microalloying is highly sensitive to temperature changes. In actual production, the edges of the strip in the width direction are significantly affected by air cooling and water cooling, resulting in substantial temperature drops that are difficult to control. This invention increases edge deformation through the use of a fixed-width machine, promoting finer edge grain structure; employs rapid cooling technology in the front section of laminar flow cooling to reduce edge temperature drop during the process; and uses precise control technology for edge shielding during laminar flow cooling to reduce the differences in structure and properties between the edge and center of the strip during laminar flow cooling and hot coil cooling. The synergistic effect of each step, in addition to controlling the edge temperature of the strip width to improve performance stability, also improves edge strength and overall strip width performance stability through edge refinement.

[0067] The data underlined above do not meet the requirements of this invention.

[0068] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A production method for improving the width-direction property stability of 800MPa grade multiphase steel, characterized in that, The production method includes the following steps: 1) The width difference between the slab and the finished product should be +200~+280mm; 2) During the finishing rolling process, water is used between the finishing mill stands; 3) Use laminar flow cooling to rapidly cool the strip to the target coiling temperature.

2. The production method according to claim 1, characterized in that, In step 1), the material width fixing machine is put into use, with a single-side lateral pressure of 100~140mm.

3. The production method according to claim 1 or 2, characterized in that, In step 2), for finishing rolled products with a thickness specification of 1.8 ≤ thickness < 3.0 mm, water injection between stands is performed using F12, F23, F45, and F67; for products with a thickness specification of 3.0 ≤ thickness ≤ 6.0 mm, water injection between stands is performed using F12, F34, and F56.

4. The production method according to claim 1, characterized in that, In step 3), the rapid cooling of the laminar flow cooling front section specifically means that the average cooling rate of the rapid cooling of the laminar flow cooling front section is ≥50℃ / s.

5. The production method according to claim 1 or 4, characterized in that, In step 3), the laminar flow cooling adopts edge shielding. For the finished strip steel with a width of 1100~1500mm, four sets of edge shielding are used. The first and second sets of laminar flow cooling strips have a shielding width of D1mm on both sides, and the third and fourth sets of laminar flow cooling strips have a shielding width of D2mm on both sides. The values ​​are D1 = k1b / 2 and D2 = k2b / 2. b represents the width of the hot-rolled finished strip in mm, and k1 and k2 are empirical coefficients, which are 0.1 and 0.125 respectively.

6. The production method according to claim 1, characterized in that, The production method further includes: pre-rolling heating, with the furnace exit temperature at 1230~1270℃ and the heating time at 200~240min.

7. The production method according to claim 1, characterized in that, The production method also includes controlling the final rolling temperature to be 860℃ ≤ final rolling temperature ≤ 900℃ and the coiling temperature to be 540℃ ≤ coiling temperature ≤ 580℃ during rolling.

8. An 800MPa grade multiphase steel produced by the production method according to any one of claims 1-7.

9. The 800MPa grade multiphase steel according to claim 8, characterized in that, The 800MPa grade multiphase steel comprises the following chemical composition by weight percentage: C: 0.05%~0.10%, Si: 0.10%~0.30%, Mn: 1.50~2.0%, Als: 0.020%~0.060%, P≤0.020%, S≤0.003%, Nb: 0.02%~0.10%, Ti: 0.05%~0.20%, Cr+Mo+Ni≤1.5%, with the remainder being Fe and unavoidable impurities.

10. The 800MPa grade multiphase steel according to claim 8 or 9, characterized in that, The 800MPa grade multiphase steel has a width direction tensile strength range of ≤35MPa and a low strength of ≥815MPa.

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