Dual-phase steel, preparation method thereof and vehicle applying dual-phase steel

Abstract

The invention provides dual-phase steel, a preparation method of the dual-phase steel and a vehicle applying the dual-phase steel. The dual-phase steel comprises the following element components in percentage by mass: 0.07 to 0.10 percent of C, 0.6 to 0.8 percent of Si, 1.8 to 2.1 percent of Mn, 0.018 to 0.022 percent of Cr, 0.015 to 0.025 percent of Ti, 0.02 to 0.05 percent of Al and the balance of Fe and inevitable impurities. One component of the dual-phase steel can achieve different strength grades, the tensile strength of the dual-phase steel covers the strength grade range of 590-1180 MPa, the strength requirements of all parts of a body-in-white can be met, the carbon equivalent and the cost of the dual-phase steel are low, the weldability and the welding quality between body-in-white plate combinations can be improved, the body-in-white production process is simplified, and the production efficiency is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy steels, and in particular relates to a dual-phase steel, a preparation method thereof and a vehicle using the dual-phase steel. Background Art

[0002] The body-in-white of a car accounts for about 30-40% of the total vehicle weight, which is the part with the highest weight share of the vehicle, and the material cost accounts for a relatively high proportion. The metal materials used in the body-in-white mainly include low-carbon soft steel, ordinary high-strength steel, advanced high-strength steel, hot-formed steel, aluminum alloy, magnesium alloy, etc. Among them, the commonly used steels for automobile bodies include low-carbon soft steel (DC), bake-hardened steel (BH), high-strength low-alloy steel (HSLA), dual-phase steel (DP), quenching and partitioning steel (QP), martensitic steel (MS), etc. As mentioned above, each type of steel includes specific material grades with multiple strength grades and multiple surface characteristics, and the chemical composition of each material grade is different. At present, there are more than 60 types of commonly used steel materials for car bodies, and nearly 160 types of plate thickness specifications, while a single body-in-white design uses more than 30 types of material grades, and the plate thickness specifications reach nearly 100 types.

[0003] The chemical composition of traditional HC340 / 590DP cold-rolled sheet and galvanized sheet: C≤0.15%, Si≤0.6%, Mn≤2.5%, P≤0.04%, S≤0.010%; the chemical composition of HC420 / 780DP cold-rolled sheet and galvanized sheet: C≤0.18%, Si≤0.8%, Mn≤2.5%, P≤0.04%, S≤0.010%; the chemical composition of HC550 / 980DP cold-rolled sheet: C≤0.23%, Si≤1.0%, Mn≤3.0%, P≤0.04%, S≤0.0 10%; Chemical composition of HC550 / 980DP hot-dip galvanized sheet: C≤0.2%, Si≤1.0%, Mn≤2.9%, P≤0.04%, S≤0.010%; Chemical composition of HC820 / 1180DP cold-rolled sheet: C≤0.23%, Si≤1.0%, Mn≤3.0%, P≤0.08%, S≤0.015%; Chemical composition of HC820 / 1180DP hot-dip galvanized sheet: C≤0.23%, Si≤1.0%, Mn≤3.0%, P≤0.04%, S≤0.010%. Alloy elements such as Ni, Cr, and Mo can be added to the above DP steels, and the total amount of the elements is not more than 1.5%. It can be seen that the main alloy components of DP steels of different strength grades used in traditional automobiles are very different.

[0004] In view of the increasing requirements of lightweight and consumer safety demands, the proportion of high-strength steel and ultra-high-strength steel used in the whole vehicle has increased significantly. As the strength grade of high-strength steel and ultra-high-strength steel increases, the content of alloying elements such as C, Si, Mn, Cr, and Mo in the alloy usually increases, the carbon equivalent of the steel plate increases, and the weldability of the material deteriorates, which will cause the process window of resistance spot welding to become smaller, the heat-affected zone of resistance spot welding to weaken significantly, and the strength of the weld spot to decrease to a certain extent. At the same time, due to the large differences in the composition of different steel materials, the resistivity of the steel plate is significantly different, and there are many combinations of plates in the body manufacturing process. Different plate combinations require specific spot welding process parameters, which will make the welding process of the body in white more complicated.

[0005] Therefore, in response to the above problems, developing a uniform composition variable-strength DP steel with low carbon equivalent and wide strength range and applying it to body-related structural parts is of great significance for reducing the material cost of body-in-white DP steel, improving the weldability and welding quality of body panel combinations, simplifying the body-in-white production process, and improving production efficiency. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention aims to provide a dual-phase steel, a preparation method thereof and a vehicle using the dual-phase steel. The dual-phase steel can achieve different strength grades with one component, and its tensile strength covers the strength grade range of 590-1180MPa, which can meet the strength requirements of various parts of the body-in-white, and its carbon equivalent and cost are low, which helps to improve the weldability and welding quality between the body-in-white sheet combinations, simplify the body-in-white production process, and improve production efficiency.

[0007] To achieve this object, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a dual-phase steel with uniform composition and variable strength, wherein the dual-phase steel comprises the following elemental components in mass percentage:

[0009] C: 0.07-0.10%, Si: 0.6-0.8%, Mn: 1.8-2.1%, Cr: 0.018-0.022%, Ti: 0.015-0.025% and Al: 0.02-0.05%, and the balance is Fe and inevitable impurities.

[0010] In some embodiments of the present invention, the metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 10-80%, the volume content of the martensite is 20-90%, and the grain size of the ferrite is below 20 μm.

[0011] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC340 / 590DP, the yield strength is 340-430MPa, the tensile strength is 590-700MPa, the elongation is ≥20%, the metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 70-80%, the volume content of the martensite is 20-30%, and the grain size of the ferrite is below 20μm.

[0012] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC420 / 780DP, the yield strength is 420-550MPa, the tensile strength is 780-900MPa, and the elongation is ≥14%. The metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 60-70%, the volume content of the martensite is 30-40%, and the grain size of the ferrite is below 20μm.

[0013] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC550 / 980DP, the yield strength is 550-740MPa, the tensile strength is 980-1130MPa, the elongation is ≥10%, and the metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 40-50%, the volume content of the martensite is 50-60%, and the grain size of the ferrite is below 15μm.

[0014] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC820 / 1180DP, the yield strength is 820-1150MPa, the tensile strength is 1180-1250MPa, the elongation is ≥4%, and the metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 10-20%, the volume content of the martensite is 80-90%, and the grain size of the ferrite is below 10μm.

[0015] In a second aspect, the present invention provides a method for preparing the dual-phase steel as described in the first aspect, the preparation method comprising the following steps:

[0016] (1) taking raw materials according to the elemental composition of the dual-phase steel, smelting and casting the steel to obtain a slab;

[0017] (2) hot rolling: heating the slab, and then sequentially performing rough rolling and finish rolling to obtain a hot-rolled plate;

[0018] (3) cooling the hot-rolled sheet and then curling it to obtain a hot-rolled coil;

[0019] (4) cold rolling the hot-rolled coil to obtain a cold-rolled sheet;

[0020] (5) Continuously annealing the cold-rolled sheet to obtain dual-phase steel.

[0021] In some embodiments of the present invention, the heating temperature in step (2) is 1210-1250° C. and the heating time is 40-60 min.

[0022] In some embodiments of the present invention, the outlet temperature of the rough rolling is 1060-1090°C.

[0023] In some embodiments of the present invention, the final rolling temperature of the finish rolling is 850-880°C.

[0024] In some embodiments of the present invention, the reduction ratio of the cold rolling is 50-65%.

[0025] In some embodiments of the present invention, the preparation method further comprises the following steps:

[0026] Before the cold rolling, the hot rolled coil is pickled to remove surface oxides.

[0027] In some embodiments of the present invention, a flattening treatment is performed after the continuous annealing.

[0028] In some embodiments of the present invention, the cooling condition in step (3) is: cooling to 550-650°C at a cooling rate of 15-50°C / s.

[0029] In some embodiments of the present invention, the continuous annealing step in step (5) comprises: keeping the cold-rolled sheet at 760-830°C for 70-150s, cooling it to 630-660°C at a cooling rate of 10-20°C / s, cooling it to 300-330°C at a cooling rate of 30-200°C / s, then cooling it to room temperature, and then tempering it at 270-290°C for 540-900s.

[0030] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC340 / 590DP, the yield strength is 340-430MPa, the tensile strength is 590-700MPa, and the elongation is ≥20%; the cooling condition in step (3) is: cooling to 630-650°C at a cooling rate of 15-20°C / s; the continuous annealing step in step (5) includes: keeping the cold-rolled sheet at 810-830°C for 70-90s, then cooling to 630-660°C at a cooling rate of 10-15°C / s, then cooling to 310-330°C at a cooling rate of 30-50°C / s, then cooling to room temperature, and then tempering at 270-290°C for 540-600s.

[0031] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC420 / 780DP, the yield strength is 420-550MPa, the tensile strength is 780-900MPa, and the elongation is ≥14%; the cooling condition in step (3) is: cooling to 600-630°C at a cooling rate of 20-30°C / s; the continuous annealing step in step (5) includes: keeping the cold-rolled sheet at 790-810°C for 90-110s, then cooling to 630-660°C at a cooling rate of 10-15°C / s, then cooling to 310-330°C at a cooling rate of 60-90°C / s, then cooling to room temperature, and then tempering at 270-290°C for 600-660s.

[0032] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC550 / 980DP, the yield strength is 550-740MPa, the tensile strength is 980-1130MPa, and the elongation is ≥10%; the cooling condition in step (3) is: cooling to 570-600°C at a cooling rate of 30-40°C / s; the continuous annealing step in step (5) includes: keeping the cold-rolled sheet at 770-790°C for 110-130s, then cooling to 620-650°C at a cooling rate of 15-20°C / s, then cooling to 300-320°C at a cooling rate of 100-150°C / s, then cooling to room temperature, and then tempering at 270-290°C for 720-780s.

[0033] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC820 / 1180DP, the yield strength is 820-1150MPa, the tensile strength is 1180-1250MPa, and the elongation is ≥4%; the cooling condition in step (3) is: cooling to 550-570°C at a cooling rate of 40-50°C / s; the continuous annealing step in step (5) includes: keeping the cold-rolled sheet at 760-780°C for 130-150s, then cooling to 620-650°C at a cooling rate of 15-20°C / s, then cooling to 300-320°C at a cooling rate of 150-200°C / s, then cooling to room temperature, and then tempering at 270-290°C for 800-900s.

[0034] In a third aspect, the present invention provides a vehicle, comprising the dual-phase steel described in the first aspect, or the dual-phase steel prepared by the preparation method described in the second aspect.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The uniform composition variable strength dual-phase steel provided by the present invention can achieve different strength grades with one composition, and its tensile strength covers the strength grade range of 590-1180MPa. It can be used for the sheet metal parts of the body-in-white structure of passenger vehicles to meet the different strength requirements of materials in various parts of the body-in-white, thereby reducing the combination of plates required for the body-in-white, simplifying the production process, and improving production efficiency.

[0037] In addition, the content of alloy elements in the uniform composition variable strength dual-phase steel provided by the present invention is low, and its carbon equivalent and cost are low. The low carbon equivalent helps to improve the weldability and welding quality between the body-in-white sheet combinations. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0039] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0040] Figure 1 A metallographic micrograph of the HC340 / 590DP grade dual-phase steel provided in Example 1 of the present invention;

[0041] Figure 2 A metallographic micrograph of the HC420 / 780DP grade dual-phase steel provided in Example 1 of the present invention;

[0042] Figure 3 A metallographic micrograph of the HC550 / 980DP grade dual-phase steel provided in Example 1 of the present invention;

[0043] Figure 4 A metallographic micrograph of the HC820 / 1180DP grade dual-phase steel provided in Example 1 of the present invention;

[0044] Figure 5 This is a graph showing the welding process window test results of four strength grade dual-phase steels provided in Example 1 of the present invention;

[0045] Figure 6 The metallographic micrograph of the welded cross section and the hardness distribution diagram of the weld spot of the conventional dual-phase steel with strength grade HC820 / 1180DP are shown;

[0046] Figure 7 The metallographic micrograph of the welded cross section and the hardness distribution diagram of the weld spot of the HC820 / 1180DP strength grade dual-phase steel provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0047] In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the scheme of the present invention will be further described below. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0048] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present invention, rather than all of the embodiments.

[0049] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0050] In a first aspect, an embodiment of the present invention provides a dual-phase steel with uniform composition and variable strength, wherein the dual-phase steel comprises the following elemental components in percentage by mass:

[0051] C: 0.07-0.10%, Si: 0.6-0.8%, Mn: 1.8-2.1%, Cr: 0.018-0.022%, Ti: 0.015-0.025% and Al: 0.02-0.05%, and the balance is Fe and inevitable impurities.

[0052] The main functions of the elements in the uniform composition variable strength dual-phase steel provided by the embodiment of the present invention are as follows:

[0053] C: C is an important strengthening element in steel, which directly affects the strength and formability of dual-phase steel. C increases the strength of steel and reduces its plasticity. Uniform composition variable strength DP steel requires a wide range of strength grade coverage, and at the same time requires as high uniform elongation and total elongation as possible. The carbon content should not be too high. Although too high a C content is beneficial to improving hardenability and martensitic transformation, it will increase the carbon equivalent of the steel and be detrimental to weldability in subsequent applications. Too low a C content is detrimental to martensitic transformation. When the C content is less than 0.07%, it is not conducive to the production of a specific proportion of martensitic structure during annealing. Therefore, the mass content of C in the uniform composition variable strength DP steel provided in the embodiment of the present invention is controlled at 0.07-0.10%; for example, it can be 0.07%, 0.072%, 0.075%, 0.078%, 0.08%, 0.082%, 0.085%, 0.088%, 0.09%, 0.092%, 0.095%, 0.098% or 0.1%, etc. However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0054] Si: ferrite forming element, plays a role in solid solution strengthening, while increasing the Ac1 and Ac3 point temperatures, expanding the two-phase region, promoting ferrite phase transformation, and can improve the tempering resistance of martensite, inhibit the precipitation of cementite, and is conducive to stabilizing austenite. It can also improve the organizational morphology of martensite, improve the continuity of the ferrite matrix, and ensure the excellent combination of strength and plasticity of DP steel. If the Si content is too high, it is easy to cause oxidation of the steel plate surface during annealing, and it is also easy to cause surface quality problems of the steel plate, and it is not conducive to the weldability of the material and the quality of surface coating. Therefore, the mass content of Si in the uniform composition variable strength DP steel provided in the embodiment of the present invention is controlled at 0.6-0.8%; for example, it can be 0.6%, 0.62%, 0.63%, 0.65%, 0.66%, 0.68%, 0.7%, 0.72%, 0.73%, 0.75%, 0.76%, 0.78% or 0.8%, etc. However, the present invention is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0055] Mn: Austenite stabilizing element, improves ferrite strength, helps expand austenite phase area, reduces critical cooling rate, increases hardenability of austenite, is conducive to obtaining a high proportion of martensite, and is a key element in the design of uniform composition variable strength DP steel. When the Mn content is lower than 1.8%, the alloy carbon equivalent is low, resulting in insufficient hardenability, and unable to guarantee the specific proportion of martensite requirements of different strength grades of 590-1180Mpa; when the Mn content is too high, the grains in the steel tend to coarsen, increasing the overheating sensitivity of the steel, while significantly increasing the carbon equivalent of the steel, deteriorating weldability, and deteriorating the delayed cracking resistance. Therefore, the mass content of Mn in the uniform composition variable strength DP steel provided in the embodiment of the present invention is controlled at 1.8-2.1%; for example, it can be 1.8%, 1.82%, 1.85%, 1.88%, 1.9%, 1.92%, 1.95%, 1.98%, 2%, 2.02%, 2.05%, 2.08% or 2.1%, etc. However, the present invention is not limited to the listed values, and other values ​​not listed in the range are also applicable.

[0056] Cr: Mainly improves the hardenability of steel, which is beneficial for the steel to have better comprehensive mechanical properties after continuous annealing. However, when the Cr content is high, the carbon equivalent of the steel will increase, which is not conducive to welding performance. At the same time, the cost of the Cr element is high, and its addition amount should not be too high. Therefore, the mass content of Cr in the uniform composition variable strength DP steel provided in the embodiment of the present invention is controlled at 0.018-0.022%; for example, it can be 0.018%, 0.0185%, 0.019%, 0.0195%, 0.02%, 0.0205%, 0.021%, 0.0215% or 0.022%, etc. However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0057] Ti: Ti is easy to combine with C and N to form TiC and TiN precipitation phases, which significantly reduces the diffusion rate of carbon in austenite, greatly reduces the austenite formation rate, and the formed TiC and TiN precipitation phases are beneficial to hinder the growth of austenite grains and play a role in grain refinement. At the same time, the TiC precipitated during the cooling process has a precipitation strengthening effect and is beneficial to tempering stability. The Ti element is a very beneficial microalloying element. Considering the effect and cost factors, the mass content of Ti in the uniform composition variable strength DP steel provided by the embodiment of the present invention is controlled at 0.015-0.025%; for example, it can be 0.015%, 0.016%, 0.017%, 0.018%, 0.019%, 0.02%, 0.021%, 0.022%, 0.023%, 0.024% or 0.025%, etc. However, the present invention is not limited to the listed values, and other values ​​not listed within this range are also applicable.

[0058] Al: mainly plays a role in deoxidation and grain refinement. The mass content of Al in the uniform composition variable strength DP steel provided in the embodiment of the present invention is controlled at 0.02-0.05%; for example, it can be 0.02%, 0.025%, 0.03%, 0.035%, 0.04%, 0.045% or 0.05%, etc. However, the present invention is not limited to the listed values, and other values ​​not listed in the range are also applicable.

[0059] In the embodiment of the present invention, the impurity elements are usually P and S. The mass content of P is preferably controlled below 0.02%, and the mass content of S is preferably controlled below 0.01%. The lower the content, the better the effect.

[0060] The present invention uses the above elements in a specific content range to match each other, thereby obtaining a uniform composition variable strength dual-phase steel. One component of the dual-phase steel can achieve different strength levels, and its tensile strength covers the strength level range of 590-1180MPa, which can be used for passenger car body-in-white structural sheet metal parts to meet the different strength requirements of various parts of the body-in-white for materials.

[0061] The uniform composition variable strength dual-phase steel provided in the embodiment of the present invention has a low content of alloy elements, and its carbon equivalent and cost are low. The low carbon equivalent helps to improve the weldability and welding quality between the body-in-white panel combinations. The welding process windows between the dual-phase steels of different strengths provided in the embodiment of the present invention are consistent, and the welding process window can reach 2.6±0.1KA, overcoming the shortcoming that the process window of traditional dual-phase steels becomes severely narrowed with the increase of strength grade. The uniform composition variable strength DP steel provided in the embodiment of the present invention particularly significantly improves the weldability of HC550 / 980DP and HC820 / 1180DP grade DP steels, and its weldability can be improved by 10-30%. At the same time, the spot welding core hardness of the uniform composition variable strength DP steel based on the embodiment of the present invention is reduced by 20-50HV compared with the traditional DP steel of the same strength grade, and the degree of weakening of the heat affected zone of the weld core is significantly reduced, and the overall strength of the weld spot can be improved by 5-15%.

[0062] The body-in-white of the uniform composition variable-strength DP steel according to the embodiment of the present invention can significantly reduce the required plate combinations, especially the two-layer plate and three-layer plate combinations. A single body-in-white can reduce 50-150 plate combinations and reduce the corresponding welding process parameter development and management work, thereby simplifying the production process and improving production efficiency.

[0063] In some embodiments of the present invention, the metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 10-80% (for example, it can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%, etc.), the volume content of the martensite is 20-90% (for example, it can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or 90%, etc.), and the grain size of the ferrite is below 20μm (for example, it can be 20μm, 18μm, 15μm, 12μm, 10μm, 8μm or 5μm, etc.). However, the present invention is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0064] In the embodiment of the present invention, by changing the contents of ferrite and martensite in the dual-phase steel and the grain size of ferrite, the dual-phase steel with the same composition can achieve different strength levels.

[0065] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC340 / 590DP, the yield strength is 340-430MPa, the tensile strength is 590-700MPa, and the elongation is ≥20%. At this time, the metallographic structure of the dual-phase steel includes ferrite and martensite, and small islands of martensite are distributed on the ferrite matrix. The volume content of the ferrite is 70-80% (for example, it can be 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79% or 80%), the volume content of the martensite is 20-30% (for example, it can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29% or 30%), and the grain size of the ferrite is below 20μm (for example, it can be 20μm, 18μm, 15μm, 12μm, 10μm, 8μm or 5μm). However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0066] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC420 / 780DP, the yield strength is 420-550MPa, the tensile strength is 780-900MPa, and the elongation is ≥14%. At this time, the metallographic structure of the dual-phase steel includes ferrite and martensite, and small islands of martensite are distributed on the ferrite matrix. The volume content of the ferrite is 60-70% (for example, it can be 60%, 61%, 62%, 63%, The volume content of the martensite is 30-40% (for example, it can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39% or 40%), and the grain size of the ferrite is below 20 μm (for example, it can be 20 μm, 18 μm, 15 μm, 12 μm, 10 μm, 8 μm or 5 μm). However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0067] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC550 / 980DP, the yield strength is 550-740MPa, the tensile strength is 980-1130MPa, and the elongation is ≥10%. At this time, the metallographic structure of the dual-phase steel includes ferrite and martensite, and the small irregular island martensite accounts for the majority, and the volume content of the ferrite dispersed around the ferrite is 40-50% (for example, it can be 40%, 41%). , 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49% or 50%, etc.), the volume content of martensite is 50-60% (for example, it can be 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59% or 60%, etc.), and the grain size of ferrite is below 15μm (for example, it can be 15μm, 12μm, 10μm, 8μm or 5μm, etc.). However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0068] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC820 / 1180DP, the yield strength is 820-1150MPa, the tensile strength is 1180-1250MPa, and the elongation is ≥4%. At this time, the metallographic structure of the dual-phase steel includes ferrite and martensite, mainly fine irregular island martensite, and only a small amount of ferrite, and the volume content of the ferrite is 10-20% (for example, it can be 10%, 11%, 12 %, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%, etc.), the volume content of martensite is 80-90% (for example, it can be 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89% or 90%, etc.), and the grain size of ferrite is below 10μm (for example, it can be 10μm, 8μm, 7μm, 6μm, 5μm, 4μm or 3μm, etc.). However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0069] In a second aspect, the present invention provides a method for preparing the dual-phase steel as described in the first aspect, the preparation method comprising the following steps:

[0070] (1) taking raw materials according to the elemental composition of the dual-phase steel, smelting and casting the steel to obtain a slab;

[0071] (2) hot rolling: heating the slab, and then sequentially performing rough rolling and finish rolling to obtain a hot-rolled plate;

[0072] (3) cooling the hot-rolled sheet and then curling it to obtain a hot-rolled coil;

[0073] (4) cold rolling the hot-rolled coil to obtain a cold-rolled sheet;

[0074] (5) Continuously annealing the cold-rolled sheet to obtain dual-phase steel.

[0075] The embodiment of the present invention adopts the above preparation method and adjusts the process conditions of each step (especially finishing rolling, curling and continuous annealing) so as to realize the preparation of DP steels with different strengths using one component.

[0076] In some embodiments of the present invention, the heating temperature in step (2) is 1210-1250° C., for example, 1210° C., 1220° C., 1230° C., 1240° C. or 1250° C., etc.; the heating time is 40-60 min, for example, 40 min, 42 min, 45 min, 48 min, 50 min, 52 min, 55 min, 58 min or 60 min, etc. However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0077] In some embodiments of the present invention, the outlet temperature of the rough rolling is 1060-1090° C., for example, 1060° C., 1065° C., 1070° C., 1075° C., 1080° C., 1085° C. or 1090° C. However, the present invention is not limited to the listed values, and other values ​​not listed in the range are also applicable.

[0078] In some embodiments of the present invention, the final rolling temperature of the finish rolling is 850-880°C, for example, it can be 850°C, 855°C, 860°C, 865°C, 870°C, 875°C or 880°C.

[0079] In some embodiments of the present invention, the cold rolling reduction rate is 50-65%, for example, it can be 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64% or 65%, etc. However, the present invention is not limited to the listed values, and other values ​​not listed in the range are also applicable.

[0080] In some embodiments of the present invention, the preparation method further comprises the following steps:

[0081] Before the cold rolling, the hot rolled coil is pickled to remove surface oxides.

[0082] In some embodiments of the present invention, the preparation method further comprises the following steps:

[0083] After the continuous annealing, a flattening treatment is performed, and the elongation during the flattening treatment is preferably controlled at 0.3-0.7%.

[0084] In some embodiments of the present invention, the cooling condition in step (3) is: cooling to 550-650°C (for example, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, etc.) at a cooling rate of 15-50°C / s (for example, 15°C / s, 20°C / s, 25°C / s, 30°C / s, 35°C / s, 40°C / s, 45°C / s, 50°C / s, etc.). However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0085] In the embodiment of the present invention, in order to obtain high-quality uniform composition variable strength DP steel, for DP steels of different strength grades, the cooling rate and coiling temperature after finish rolling need to be specifically controlled, especially the coiling temperature has a great influence on the grain and band structure of the hot-rolled plate, and directly affects the plasticity of the final cold-rolled plate.

[0086] In some embodiments of the present invention, the continuous annealing step in step (5) comprises: keeping the cold-rolled sheet at 760-830°C (for example, 760°C, 770°C, 780°C, 790°C, 800°C, 810°C, 820°C, 830°C, etc.) for 70-150s (for example, 70s, 80s, 90s, 100s, 110s, 120s, 130s, 140s, 150s, etc.), and then heating at 10-20°C. / s (for example, 10℃ / s, 12℃ / s, 13℃ / s, 15℃ / s, 16℃ / s, 18℃ / s, 20℃ / s, etc.) to 630-660℃ (for example, 630℃, 635℃, 640℃, 645℃, 650℃, 655℃, 660℃, etc.), and then cooled at a cooling rate of 30-200℃ / s (for example, 30℃ / s, 40℃ / s, 50℃ / s, 60℃ / s, 70℃ / s, 80℃ / s). , 90°C / s, 100°C / s, 120°C / s, 130°C / s, 150°C / s, 160°C / s, 180°C / s, 200°C / s, 220°C / s, 230°C / s, 250°C / s, 260°C / s, 280°C / s or 300°C / s, etc.) to 300-330°C (for example, 300°C, 305°C, 310°C, 315°C, 320°C, 325°C, 330°C, etc.), and then lower the temperature to 300-330°C (for example, 300°C, 305°C, 310°C, 315°C, 320°C, 325°C, 330°C, etc.). to room temperature, and then tempered at 270-290° C. (for example, 270° C., 275° C., 280° C., 285° C., 290° C., etc.) for 540-900 seconds (for example, 540 seconds, 550 seconds, 560 seconds, 580 seconds, 600 seconds, 620 seconds, 630 seconds, 650 seconds, 660 seconds, 680 seconds, 700 seconds, 720 seconds, 750 seconds, 780 seconds, 800 seconds, 820 seconds, 850 seconds, 880 seconds, 900 seconds, etc.). However, the present invention is not limited to the values ​​listed, and other values ​​not listed in the range are also applicable.

[0087] In the embodiment of the present invention, the continuous annealing process is a key process to ensure the final performance of the cold-rolled sheet, and is directly related to the realization of different strength grades of uniform composition variable strength DP steel. The process mainly involves annealing temperature, holding time, slow cooling rate (annealing temperature to rapid cooling start temperature), rapid cooling start temperature, rapid cooling cooling rate (rapid cooling start temperature to martensite transformation temperature 300-330°C), tempering temperature, and tempering time.

[0088] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC340 / 590DP, the yield strength is 340-430MPa, the tensile strength is 590-700MPa, and the elongation is ≥20%; the cooling condition in step (3) is: cooling to 630-650°C (for example, 630°C, 632°C, 635°C, 636°C, 637°C, 638°C, 639°C, 640°C, 641°C, 642°C, 643°C, 644°C, 645°C, 646°C, 647°C, 648°C, 649°C, 650°C) at a cooling rate of 15-20°C / s (for example, 15°C / s, 16°C / s, 17°C / s, 18°C / s, 19°C / s, 20°C / s, etc.) 8°C, 640°C, 642°C, 645°C, 648°C, 650°C, etc.); the continuous annealing step in step (5) comprises: keeping the cold-rolled sheet at 810-830°C (for example, 810°C, 812°C, 815°C, 818°C, 820°C, 822°C, 825°C, 828°C, 830°C, etc.) for 70-90s (for example, 70s, 72s, 75s, 78s, 80s, 82s, 85s, 88s, 90s, etc.), and then heating at 10-15°C / s (for example, The cooling rate may be 10°C / s, 11°C / s, 12°C / s, 13°C / s, 14°C / s, 15°C / s, etc.) to 630-660°C (for example, 630°C, 635°C, 640°C, 645°C, 650°C, 655°C, 660°C, etc.), and then cooled to 310-50°C / s (for example, 30°C / s, 32°C / s, 35°C / s, 38°C / s, 40°C / s, 42°C / s, 45°C / s, 48°C / s, 50°C / s, etc.) at a cooling rate. The steel is annealed at 330°C (for example, 310°C, 312°C, 315°C, 318°C, 320°C, 322°C, 325°C, 328°C, 330°C, etc.), then cooled to room temperature, and then tempered at 270-290°C (for example, 270°C, 272°C, 275°C, 278°C, 280°C, 282°C, 285°C, 288°C, 290°C, etc.) for 540-600s (for example, 540s, 550s, 560s, 570s, 580s, 590s, 600s, etc.). However, the present invention is not limited to the listed values, and other values ​​not listed in the range are also applicable.

[0089] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC420 / 780DP, the yield strength is 420-550MPa, the tensile strength is 780-900MPa, and the elongation is ≥14%; the cooling condition in step (3) is: cooling to 600-630°C (for example, 600°C, 605°C) at a cooling rate of 20-30°C / s (for example, 20°C / s, 22°C / s, 23°C / s, 25°C / s, 26°C / s, 28°C / s, 30°C / s, etc.) ℃, 610℃, 615℃, 620℃, 625℃, 630℃, etc.); the continuous annealing step in step (5) comprises: keeping the cold-rolled sheet at 790-810℃ (for example, 790℃, 792℃, 795℃, 798℃, 800℃, 802℃, 805℃, 808℃, 810℃, etc.) for 90-110s (for example, 90s, 92s, 95s, 98s, 100s, 102s, 105s, 108s, 110s, etc.), and then heating at 10-1 The product is cooled to 630-660°C (for example, 630°C, 635°C, 640°C, 645°C, 650°C, 655°C, 660°C, etc.) at a cooling rate of 5°C / s (for example, 10°C / s, 11°C / s, 12°C / s, 13°C / s, 14°C / s, 15°C / s, etc.), and then cooled to 310-330°C at a cooling rate of 60-90°C / s (for example, 60°C / s, 65°C / s, 70°C / s, 75°C / s, 80°C / s, 85°C / s, 90°C / s, etc.). The steel sheet is annealed at 270-290°C (for example, 270°C, 272°C, 275°C, 278°C, 280°C, 282°C, 285°C, 288°C, 290°C) for 600-660s (for example, 600s, 610s, 620s, 630s, 640s, 650s, 660s, etc.). However, the present invention is not limited to the values ​​listed, and other values ​​not listed in the range are also applicable.

[0090] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC550 / 980DP, the yield strength is 550-740MPa, the tensile strength is 980-1130MPa, and the elongation is ≥10%; the cooling condition in step (3) is: cooling to 570-600°C (for example, 570°C, 572°C, 573°C, 574°C, 576°C, 577°C, 578°C, 579°C, 580°C, 581°C, 582°C, 583°C, 584°C, 585°C, 586°C, 587°C, 588°C, 589°C, 590°C, 601°C, 611°C, 612°C, 621°C, 637°C, 643°C, 657°C, 663°C, 677°C, 687°C, 691°C, 701°C, 713°C, 723°C, 739°C, 740°C, 751°C, 761°C, 779°C, 789°C, 791°C, 801°C, 813°C, 823°C, 839°C, 840°C, 851°C 5℃, 578℃, 580℃, 582℃, 585℃, 588℃, 600℃, etc.); the continuous annealing step in step (5) comprises: keeping the cold-rolled sheet at 770-790℃ (for example, 770℃, 772℃, 775℃, 778℃, 780℃, 782℃, 785℃, 788℃, 790℃, etc.) for 110-130s (for example, 110s, 112s, 115s, 118s, 120s, 122s, 125s, 128s, 130s, etc.), Then cool to 620-650°C (for example, 620°C, 625°C, 630°C, 635°C, 640°C, 645°C, 650°C, etc.) at a cooling rate of 15-20°C / s (for example, 15°C / s, 16°C / s, 17°C / s, 18°C / s, 19°C / s, 20°C / s, etc.), and then cool to 30°C at a cooling rate of 100-150°C / s (for example, 100°C / s, 110°C / s, 120°C / s, 130°C / s, 140°C / s, 150°C / s, etc.). 0-320°C (for example, 300°C, 302°C, 305°C, 308°C, 310°C, 312°C, 315°C, 318°C, 320°C, etc.), then cooled to room temperature, and then tempered at 270-290°C (for example, 270°C, 272°C, 275°C, 278°C, 280°C, 282°C, 285°C, 288°C, 290°C, etc.) for 720-780s (for example, 720s, 730s, 740s, 750s, 760s, 770s, 780s, etc.). However, the present invention is not limited to the listed values, and other values ​​not listed in the range are also applicable.

[0091] In some embodiments of the present invention, the strength grade of the dual-phase steel is HC820 / 1180DP, the yield strength is 820-1150 MPa, the tensile strength is 1180-1250 MPa, and the elongation is ≥4%; the cooling condition in step (3) is: cooling to 550-570°C (for example, 550°C, 552°C, 555°C, 556°C, 557°C, 558°C, 559°C, 560°C, 561°C, 562°C, 563°C, 564°C, 565°C, 566°C, 567°C, 568°C, 569°C, 570°C, 571°C, 572°C, 573°C, 574°C, 575°C, 576°C, 577°C, 578°C, 579°C, 580°C, 581°C, 582°C, 583°C, 584°C, 585°C, 586°C, 587°C, 588°C The continuous annealing step in step (5) comprises: keeping the cold-rolled sheet at 760-780°C (for example, 760°C, 762°C, 765°C, 768°C, 770°C, 772°C, 775°C, 778°C, 780°C, etc.) for 130-150s (for example, 130s, 132s, 135s, 138s, 140s, 142s, 145s, 148s, 150s, etc.), and then heating at 15-20°C / s (for example, 15°C / s, 16°C / s, 17°C / s, 18°C / s, 19°C / s, 20°C / s, etc.) to 620-650°C (for example, 620°C, 625°C, 630°C, 635°C, 640°C, 645°C, 650°C, etc.), and then cooled to 300-320°C (for example, 30 The steel sheet is annealed at 270-290°C (for example, 270°C, 272°C, 275°C, 278°C, 280°C, 282°C, 285°C, 288°C, 290°C, etc.) for 800-900s (for example, 800s, 810s, 820s, 830s, 840s, 850s, 860s, 870s, 880s, 890s, 900s, etc.). However, the present invention is not limited to the values ​​listed, and other values ​​not listed in the range are also applicable.

[0092] In a third aspect, the present invention provides a vehicle, comprising the dual-phase steel described in the first aspect, or the dual-phase steel prepared by the preparation method described in the second aspect.

[0093] The uniform composition and variable strength dual-phase steel provided in the embodiment of the present invention can be used as a sheet metal part of the vehicle body-in-white structure to meet the different strength requirements of the material in various parts of the body-in-white, thereby reducing the combination of plates required for the body-in-white, simplifying the production process, and improving production efficiency.

[0094] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention will be further described in detail below in conjunction with the embodiments and drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its applications. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention.

[0095] If no specific techniques or conditions are specified in the examples, the conventional techniques or conditions in the art, or the techniques or conditions described in the literature, or the product instructions are used. If no manufacturer is specified for the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0096] Example 1

[0097] This embodiment provides a dual-phase steel with uniform composition and variable strength, including the following elemental components in percentage by mass:

[0098] C: 0.07%, Si: 0.7%, Mn: 2.0%, Cr: 0.02%, Ti: 0.02% and Al: 0.035%, impurity P: 0.01%, impurity S: 0.008%, and the balance is Fe.

[0099] The uniform composition variable strength dual-phase steel provided in this embodiment includes four strength grades: HC340 / 590DP, HC420 / 780DP, HC550 / 980DP, and HC820 / 1180DP, and the preparation methods thereof are as follows:

[0100] (1) Raw materials are selected according to the elemental composition of the dual-phase steel, smelted, and continuously cast into slabs with a thickness of 200 mm.

[0101] (2) Hot rolling: The slab was heated by induction heating at a temperature of 1230°C for 50 min. Then, rough rolling and finish rolling were carried out in sequence. The outlet temperature of the rough rolling was 1075°C and the final rolling temperature of the finish rolling was 870°C. A hot-rolled plate with a thickness of 4 mm was obtained.

[0102] (3) cooling the hot-rolled sheet and then curling it to obtain a hot-rolled coil;

[0103] For HC340 / 590DP grade dual-phase steel, the cooling conditions are: cooling to 640°C at a cooling rate of 18°C / s;

[0104] For HC420 / 780DP grade dual-phase steel, the cooling conditions are: cooling to 620°C at a cooling rate of 25°C / s;

[0105] For HC550 / 980DP grade dual-phase steel, the cooling conditions are: cooling to 590°C at a cooling rate of 35°C / s;

[0106] For HC820 / 1180DP grade dual-phase steel, the cooling condition is: cooling to 560°C at a cooling rate of 40-50°C / s.

[0107] (4) The hot-rolled coil was pickled to remove the oxide on the surface, and then cold-rolled with a reduction ratio of 60% to obtain a cold-rolled sheet with a thickness of 1.6 mm.

[0108] (5) continuously annealing the cold-rolled sheet;

[0109] For HC340 / 590DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 820°C for 80s, then cooled to 650°C at a cooling rate of 12°C / s, then cooled to 320°C at a cooling rate of 40°C / s, then cooled to room temperature, and then tempered at 280°C for 570s;

[0110] For HC420 / 780DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 800°C for 100s, then cooled to 650°C at a cooling rate of 12°C / s, then cooled to 320°C at a cooling rate of 80°C / s, then cooled to room temperature, and then tempered at 280°C for 630s;

[0111] For HC550 / 980DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 780°C for 120s, then cooled to 630°C at a cooling rate of 18°C / s, then cooled to 310°C at a cooling rate of 120°C / s, then cooled to room temperature, and then tempered at 280°C for 750s;

[0112] For HC820 / 1180DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 770°C for 140s, then cooled to 630°C at a cooling rate of 18°C / s, then cooled to 310°C at a cooling rate of 180°C / s, then cooled to room temperature, and then tempered at 280°C for 850s.

[0113] (6) The steel plate after continuous annealing is flattened, and the flattening elongation is controlled at 0.5±0.2% to obtain dual-phase steels of different strength grades.

[0114] Example 2

[0115] This embodiment provides a dual-phase steel with uniform composition and variable strength, including the following elemental components in percentage by mass:

[0116] C: 0.09%, Si: 0.8%, Mn: 1.8%, Cr: 0.022%, Ti: 0.015% and Al: 0.05%, impurity P: 0.012%, impurity S: 0.005%, and the balance is Fe.

[0117] The uniform composition variable strength dual-phase steel provided in this embodiment includes four strength grades: HC340 / 590DP, HC420 / 780DP, HC550 / 980DP, and HC820 / 1180DP, and the preparation methods thereof are as follows:

[0118] (1) Raw materials are obtained according to the elemental composition of the dual-phase steel, smelted, and continuously cast into slabs with a thickness of 200 mm.

[0119] (2) Hot rolling: The slab was heated by induction heating at a temperature of 1210°C for 60 min. Then, rough rolling and finish rolling were carried out in sequence. The outlet temperature of the rough rolling was 1060°C and the final rolling temperature of the finish rolling was 850°C. A hot-rolled plate with a thickness of 4 mm was obtained.

[0120] (3) cooling the hot-rolled sheet and then curling it to obtain a hot-rolled coil;

[0121] For HC340 / 590DP grade dual-phase steel, the cooling conditions are: cooling to 650°C at a cooling rate of 15°C / s;

[0122] For HC420 / 780DP grade dual-phase steel, the cooling conditions are: cooling to 630°C at a cooling rate of 20°C / s;

[0123] For HC550 / 980DP grade dual-phase steel, the cooling conditions are: cooling to 600°C at a cooling rate of 30°C / s;

[0124] For HC820 / 1180DP grade dual phase steel, the cooling condition is: cooling to 570°C at a cooling rate of 40°C / s.

[0125] (4) The hot-rolled coil is pickled to remove the oxide on the surface, and then cold-rolled with a reduction rate of 50% to obtain a cold-rolled sheet with a thickness of 2 mm.

[0126] (5) continuously annealing the cold-rolled sheet;

[0127] For HC340 / 590DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 810°C for 90s, then cooled to 660°C at a cooling rate of 10°C / s, then cooled to 330°C at a cooling rate of 30°C / s, then cooled to room temperature, and then tempered at 270°C for 600s;

[0128] For HC420 / 780DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 790°C for 110s, then cooled to 660°C at a cooling rate of 10°C / s, then cooled to 330°C at a cooling rate of 60°C / s, then cooled to room temperature, and then tempered at 270°C for 660s;

[0129] For HC550 / 980DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 770°C for 130s, then cooled to 650°C at a cooling rate of 15°C / s, then cooled to 320°C at a cooling rate of 100°C / s, then cooled to room temperature, and then tempered at 270°C for 780s;

[0130] For HC820 / 1180DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 760°C for 150s, then cooled to 650°C at a cooling rate of 15°C / s, then cooled to 320°C at a cooling rate of 150°C / s, then cooled to room temperature, and then tempered at 270°C for 900s.

[0131] (6) The steel plate after continuous annealing is flattened, and the flattening elongation is controlled at 0.5±0.2% to obtain dual-phase steels of different strength grades.

[0132] Example 3

[0133] This embodiment provides a dual-phase steel with uniform composition and variable strength, including the following elemental components in percentage by mass:

[0134] C: 0.10%, Si: 0.6%, Mn: 2.1%, Cr: 0.018%, Ti: 0.025% and Al: 0.02%, impurity P: 0.009%, impurity S: 0.006%, and the balance is Fe.

[0135] The uniform composition variable strength dual-phase steel provided in this embodiment includes four strength grades: HC340 / 590DP, HC420 / 780DP, HC550 / 980DP, and HC820 / 1180DP, and the preparation methods thereof are as follows:

[0136] (1) Raw materials are obtained according to the elemental composition of the dual-phase steel, smelted, and continuously cast into slabs with a thickness of 200 mm.

[0137] (2) Hot rolling: The slab is heated by induction heating at a temperature of 1250°C for 40 min. Then, rough rolling and finish rolling are carried out in sequence. The outlet temperature of the rough rolling is 1090°C and the final rolling temperature of the finish rolling is 880°C. A hot-rolled plate with a thickness of 4 mm is obtained.

[0138] (3) cooling the hot-rolled sheet and then curling it to obtain a hot-rolled coil;

[0139] For HC340 / 590DP grade dual-phase steel, the cooling conditions are: cooling to 630°C at a cooling rate of 20°C / s;

[0140] For HC420 / 780DP grade dual-phase steel, the cooling conditions are: cooling to 600°C at a cooling rate of 30°C / s;

[0141] For HC550 / 980DP grade dual-phase steel, the cooling conditions are: cooling to 570°C at a cooling rate of 40°C / s;

[0142] For HC820 / 1180DP grade dual-phase steel, the cooling condition is: cooling to 550°C at a cooling rate of 50°C / s.

[0143] (4) The hot-rolled coil was pickled to remove the oxide on the surface, and then cold-rolled with a reduction ratio of 65% to obtain a cold-rolled sheet with a thickness of 1.4 mm.

[0144] (5) continuously annealing the cold-rolled sheet;

[0145] For HC340 / 590DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 830°C for 70s, then cooled to 630°C at a cooling rate of 15°C / s, then cooled to 310°C at a cooling rate of 50°C / s, then cooled to room temperature, and then tempered at 290°C for 540s;

[0146] For HC420 / 780DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 810°C for 90s, then cooled to 630°C at a cooling rate of 15°C / s, then cooled to 310°C at a cooling rate of 90°C / s, then cooled to room temperature, and then tempered at 290°C for 600s;

[0147] For HC550 / 980DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 790°C for 110s, then cooled to 620°C at a cooling rate of 20°C / s, then cooled to 300°C at a cooling rate of 150°C / s, then cooled to room temperature, and then tempered at 290°C for 720s;

[0148] For HC820 / 1180DP grade dual-phase steel, the continuous annealing steps are: the cold-rolled sheet is kept at 780°C for 130s, then cooled to 650°C at a cooling rate of 20°C / s, then cooled to 300°C at a cooling rate of 200°C / s, then cooled to room temperature, and then tempered at 290°C for 800s.

[0149] (6) The steel plate after continuous annealing is flattened, and the flattening elongation is controlled at 0.5±0.2% to obtain dual-phase steels of different strength grades.

[0150] Performance Testing

[0151] 1. Metallographic structure characterization

[0152] The metallographic microstructures of the dual-phase steels provided in the examples and comparative examples were characterized using a metallographic microscope. The metallographic micrographs of the four strength grades of dual-phase steels HC340 / 590DP, HC420 / 780DP, HC550 / 980DP, and HC820 / 1180DP provided in Example 1 are shown in Figures 1 and 2, respectively. Figure 1 , Figure 2 , Figure 3 , Figure 4 shown.

[0153] It can be seen from the figure that the metallographic structure of the HC340 / 590DP grade dual-phase steel provided in Example 1 is composed of ferrite and martensite, and small islands of martensite are distributed on the ferrite matrix.

[0154] The metallographic structure of the HC420 / 780DP grade dual-phase steel provided in Example 1 is composed of two phases, ferrite and martensite, and small islands of martensite are distributed in the ferrite matrix.

[0155] The metallographic structure of the HC550 / 980DP grade dual-phase steel provided in Example 1 is composed of two phases, ferrite and martensite, and small irregular island-shaped martensite accounts for the majority and is dispersed around the ferrite.

[0156] The metallographic structure of the HC820 / 1180DP grade dual-phase steel provided in Example 1 is composed of two phases, ferrite and martensite, with fine irregular island-shaped martensite being the main phase and only a small amount of ferrite remaining.

[0157] The metallographic compositions of the dual-phase steels provided in the examples and comparative examples are shown in Table 1.

[0158] Table 1

[0159]

[0160] 2. Mechanical properties test

[0161] The tensile properties test was carried out using an Instron 5982 tensile testing machine, and the standard GB / T 228.1-2021 "Metallic Materials Tensile Test Part 1: Room Temperature Test Method" was implemented. The sample direction was transverse. The test results are shown in Table 2.

[0162] Table 2

[0163]

[0164] 3. Welding performance test

[0165] 3.1. Spot welding performance test was carried out using four strength grade duplex steels of HC340 / 590DP, HC420 / 780DP, HC550 / 980DP and HC820 / 1180DP provided in Example 1. The spot welding was carried out using a medium frequency resistance spot welding machine. The electrode pressure was 3 kN and the electrode cap specifications were The welding time is 160ms. The test piece test is carried out by increasing or decreasing the welding current value. At least 3 groups of test pieces are spot welded for each welding current value. The diameter of the molten core is measured by peeling and the molten core growth curve is obtained. The results are as follows: Figure 5 shown.

[0166] from Figure 5 It can be seen that the welding process windows of the four strength grade DP steels provided in Example 1 are basically the same, and the welding window width is 2.6kA. Under the same test conditions, the welding windows of the four strength grade traditional DP steels (from Baosteel Group) of HC340 / 590DP, HC420 / 780DP, HC550 / 980DP, and HC820 / 1180DP were measured to be 2.5kA, 2.3kA, 2.2kA, and 2.0kA, respectively. The weldable range of the four strength grade DP steels provided in Example 1 is increased by 5-30% compared with the traditional DP steels of the same strength grade; in particular, the weldability of HC550 / 980DP and HC820 / 1180DP grade DP steels is significantly improved, and the weldability can be increased by 10-30%.

[0167] 3.2. After spot welding tests were performed using the four strength grades of dual-phase steels HC340 / 590DP, HC420 / 780DP, HC550 / 980DP, and HC820 / 1180DP provided in Example 1 and conventional DP steels of the same strength grade, the macroscopic morphology of the cross-section of the spot welded joints of the conventional DP steel and the DP steel prepared in Example 1 was observed using an Imager.M2m metallographic microscope, and a microhardness distribution test was performed using a DuraScan50 Vickers hardness tester, with a hardness measurement point interval of 0.3 mm. The test results of the welds of the conventional DP steel with a strength grade of HC820 / 1180DP and the DP steel prepared in Example 1 are shown in Figures 1 and 2. Figure 6 , Figure 7 shown.

[0168] from Figure 6 and Figure 7 It can be seen that compared with the traditional DP steel of HC820 / 1180DP strength grade, the spot welding nugget hardness of the HC820 / 1180DP strength grade DP steel provided in Example 1 is reduced by 20-50HV, and the weakening degree of the heat affected zone of the weld nugget is significantly reduced, and the weld strength is increased by 5-15%.

[0169] The above description is only a specific embodiment of the present disclosure, so that those skilled in the art can understand or implement the present disclosure. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure will not be limited to the embodiments described herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A dual-phase steel with uniform composition and variable strength, characterized in that: The dual-phase steel comprises the following elemental compositions in mass percentage: C: 0.07-0.10%, Si: 0.6-0.8%, Mn: 1.8-2.1%, Cr: 0.018-0.022%, Ti: 0.015-0.025% and Al: 0.02-0.05%, and the balance is Fe and inevitable impurities.

2. The dual-phase steel according to claim 1, characterized in that: The metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 10-80%, the volume content of the martensite is 20-90%, and the grain size of the ferrite is below 20 μm.

3. The dual-phase steel according to claim 1 or 2, characterized in that: The strength grade of the dual-phase steel is HC340 / 590DP, the yield strength is 340-430MPa, the tensile strength is 590-700MPa, and the elongation is ≥20%. The metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 70-80%, the volume content of the martensite is 20-30%, and the grain size of the ferrite is less than 20μm; Or, the strength grade of the dual-phase steel is HC420 / 780DP, the yield strength is 420-550MPa, the tensile strength is 780-900MPa, the elongation is ≥14%, the metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 60-70%, the volume content of the martensite is 30-40%, and the grain size of the ferrite is less than 20μm; Or, the strength grade of the dual-phase steel is HC550 / 980DP, the yield strength is 550-740MPa, the tensile strength is 980-1130MPa, the elongation is ≥10%, the metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 40-50%, the volume content of the martensite is 50-60%, and the grain size of the ferrite is less than 15μm; Or, the strength grade of the dual-phase steel is HC820 / 1180DP, its yield strength is 820-1150MPa, its tensile strength is 1180-1250MPa, and its elongation is ≥4%. The metallographic structure of the dual-phase steel includes ferrite and martensite, the volume content of the ferrite is 10-20%, the volume content of the martensite is 80-90%, and the grain size of the ferrite is below 10μm.

4. A method for preparing a dual-phase steel according to any one of claims 1 to 3, characterized in that: The preparation method comprises the following steps: (1) taking raw materials according to the elemental composition of the dual-phase steel, smelting and casting the steel to obtain a slab; (2) hot rolling: heating the slab, and then sequentially performing rough rolling and finish rolling to obtain a hot-rolled plate; (3) cooling the hot-rolled sheet and then curling it to obtain a hot-rolled coil; (4) cold rolling the hot-rolled coil to obtain a cold-rolled sheet; (5) Continuously annealing the cold-rolled sheet to obtain dual-phase steel.

5. The preparation method according to claim 4, characterized in that: The heating temperature in step (2) is 1210-1250° C. and the heating time is 40-60 min; And / or, the outlet temperature of the rough rolling is 1060-1090°C; And / or, the final rolling temperature of the finish rolling is 850-880°C.

6. The preparation method according to claim 4 or 5, characterized in that: The cold rolling reduction ratio is 50-65%.

7. The preparation method according to any one of claims 4 to 6, characterized in that: The preparation method further comprises the following steps: Before the cold rolling, the hot rolled coil is pickled to remove oxides on the surface; And / or, after the continuous annealing, a flattening treatment is performed.

8. The preparation method according to any one of claims 4 to 7, characterized in that: The cooling conditions in step (3) are: cooling to 550-650° C. at a cooling rate of 15-50° C. / s; And / or, the continuous annealing step in step (5) comprises: keeping the cold-rolled sheet at 760-830°C for 70-150s, cooling it to 630-660°C at a cooling rate of 10-20°C / s, cooling it to 300-330°C at a cooling rate of 30-200°C / s, then cooling it to room temperature, and then tempering it at 270-290°C for 540-900s.

9. The preparation method according to any one of claims 4 to 8, characterized in that: The strength grade of the dual-phase steel is HC340 / 590DP, the yield strength is 340-430MPa, the tensile strength is 590-700MPa, and the elongation is ≥20%; the cooling condition in step (3) is: cooling to 630-650°C at a cooling rate of 15-20°C / s; the continuous annealing step in step (5) comprises: keeping the cold-rolled sheet at 810-830°C for 70-90s, then cooling to 630-660°C at a cooling rate of 10-15°C / s, then cooling to 310-330°C at a cooling rate of 30-50°C / s, then cooling to room temperature, and then tempering at 270-290°C for 540-600s; Or, the strength grade of the dual-phase steel is HC420 / 780DP, the yield strength is 420-550MPa, the tensile strength is 780-900MPa, and the elongation is ≥14%; the cooling condition in step (3) is: cooling to 600-630°C at a cooling rate of 20-30°C / s; the continuous annealing step in step (5) comprises: keeping the cold-rolled sheet at 790-810°C for 90-110s, then cooling to 630-660°C at a cooling rate of 10-15°C / s, then cooling to 310-330°C at a cooling rate of 60-90°C / s, then cooling to room temperature, and then tempering at 270-290°C for 600-660s; Or, the strength grade of the dual-phase steel is HC550 / 980DP, the yield strength is 550-740MPa, the tensile strength is 980-1130MPa, and the elongation is ≥10%; the cooling condition in step (3) is: cooling to 570-600°C at a cooling rate of 30-40°C / s; the continuous annealing step in step (5) comprises: keeping the cold-rolled sheet at 770-790°C for 110-130s, then cooling to 620-650°C at a cooling rate of 15-20°C / s, then cooling to 300-320°C at a cooling rate of 100-150°C / s, then cooling to room temperature, and then tempering at 270-290°C for 720-780s; Or, the strength grade of the dual-phase steel is HC820 / 1180DP, the yield strength is 820-1150MPa, the tensile strength is 1180-1250MPa, and the elongation is ≥4%; the cooling condition in step (3) is: cooling to 550-570°C at a cooling rate of 40-50°C / s; the continuous annealing step in step (5) includes: keeping the cold-rolled sheet at 760-780°C for 130-150s, then cooling to 620-650°C at a cooling rate of 15-20°C / s, then cooling to 300-320°C at a cooling rate of 150-200°C / s, then cooling to room temperature, and then tempering at 270-290°C for 800-900s.

10. A vehicle, characterized in that: The invention comprises the dual-phase steel as described in any one of claims 1 to 3, or the dual-phase steel prepared by the preparation method as described in any one of claims 4 to 9.

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