Cold-rolled boron-containing dual-phase steel capable of resisting secondary machining brittleness and production method thereof

Abstract

The invention relates to cold-rolled boron-containing dual-phase steel capable of resisting secondary processing brittleness and a production method thereof. The steel comprises the following chemical components in percentage by weight: 0.10%-0.20% of C, 1.0%-2.0% of Mn, 0.1%-1.0% of Si, 0.6%-6.0% of Al, 0.02%-0.80% of Cr, 0.0005%-0.02% of B, less than or equal to 0.01% of P, less than or equal to 0.01% of S, less than or equal to 0.005% of N, 0.005%-0.50% of Mg, 0.01%-0.50% of Nb, 0.01%-0.50% of Ti and the balance of Fe and inevitable impurities, wherein Al / Si is more than or equal to 5 and less than or equal to 20; and the balance of Fe and inevitable impurities. According to the steel plate, excellent basic performance indexes such as high strength and high plasticity of a dual-phase steel product are met, meanwhile, the characteristic of secondary machining embrittlement resistance is achieved, the yield strength of the steel plate is larger than or equal to 420 MPa, the tensile strength is larger than or equal to 780 MPa, the A80 percentage elongation after fracture is larger than or equal to 15.0%, the hole expansion rate is larger than or equal to 20%, and the density is 6.5-7.5 g / cm < 3 >; and the secondary processing brittle transition temperature is less than or equal to-180 DEG C.

Description

Technical Field

[0001] The invention belongs to the technical field of cold-rolled steel, and in particular relates to a secondary processing brittle cold-rolled boron-containing dual-phase steel and a production method thereof. Background Art

[0002] Secondary working brittleness (SWE) refers to the low-temperature embrittlement of automotive cold-rolled steel sheets after stamping, affecting their ability to withstand impact loads at low temperatures. Excessively high secondary working brittle transition temperatures (SWET) pose a safety hazard to passenger vehicles, especially in regions with cold winters and large temperature swings between day and night. The risk of sheet metal fracturing during use due to low-temperature shock is significant. Effectively addressing the secondary working brittleness of automotive steel products has become a hot topic in automotive steel product research and development.

[0003] At the same time, the steel industry is actively promoting energy conservation, environmental protection, and green transformation. The development of green, low-carbon, high-strength automotive products has become a hot topic for major steel suppliers. Given the current state of research, there is an urgent need to address the poor formability and secondary processing brittleness of automotive high-strength steel, while also adapting to the design of green, low-carbon automotive high-strength steel products.

[0004] Therefore, the present invention aims to develop a cold-rolled boron-containing dual-phase steel that is resistant to secondary processing brittleness and a production method thereof. The invention realizes the low-carbon, green, and lightweight design and development of automotive high-strength steel with a short-process, low-cost process path and an extremely cost-reducing alloy design, while taking into account the personalized needs of high-strength steel for resistance to secondary processing brittleness, high plasticity, and high formability, providing a reliable technical solution for the majority of automobile manufacturers and steel companies. Summary of the Invention

[0005] In view of the above existing technical problems, the purpose of the present invention is to provide a cold-rolled boron-containing dual-phase steel resistant to secondary processing brittleness and a production method thereof, which not only meets the excellent basic performance indicators of dual-phase steel products such as high strength and high plasticity, but also has the characteristics of resistance to secondary processing brittleness. The steel plate of the present invention has a yield strength of ≥420MPa, a tensile strength of ≥780MPa, and an A 80 Elongation after fracture ≥15.0%, hole expansion rate ≥20%, density 6.5~7.5g / cm 3 ; Secondary processing brittle transition temperature ≤-180℃.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] Disclosed is a cold-rolled boron-containing duplex steel resistant to secondary processing brittleness. The chemical composition of the steel is, by weight percentage, as follows: C: 0.10%-0.20%, Mn: 1.0%-2.0%, Si: 0.1%-1.0%, Al: 0.6%-6.0%, Cr: 0.02%-0.80%, B: 0.0005%-0.02%, P≤0.01%, S≤0.01%, N≤0.005%, Mg: 0.005%-0.50%, Nb: 0.01%-0.50%, Ti: 0.01%-0.50%, and 5≤Al / Si≤20; the balance is Fe and unavoidable impurities.

[0008] The reasons for the composition design of the present invention are as follows:

[0009] C: Carbon ensures the required strength of steel through solid solution strengthening. A sufficient amount of carbon helps stabilize austenite, thereby improving the steel's formability and facilitating multi-purpose design. Too low a C content will not achieve the desired mechanical properties of the steel described in this invention; too high a C content will cause embrittlement and pose the risk of hydrogen-induced delayed fracture. Therefore, in this invention, the C content is controlled to 0.10% to 0.20%.

[0010] Mn: Manganese is an austenite-stabilizing element in steel. It expands the austenite phase and reduces the critical quenching rate of steel. It also refines grain size, contributing to solid solution strengthening and improving strength. Too low a Mn content destabilizes the supercooled austenite, reducing the steel's workability, such as plasticity and toughness. Too high a Mn content impairs weldability and increases production costs, hindering industrial production. Therefore, in the present invention, the Mn content is controlled to 1.0% to 2.0%.

[0011] Si: Silicon has a certain solid solution strengthening effect in ferrite, ensuring that the steel has sufficient strength. At the same time, Si can also inhibit the decomposition of residual austenite and the precipitation of carbides, reducing inclusions in the steel. In the present invention, the Si element is used in combination with the Al element, and the design concept of replacing silicon with aluminum can significantly improve the surface quality of the steel. If the Si element content is too low, it will not have a strengthening effect; if the Si element content is too high, it will reduce the surface quality and welding performance of the steel plate. Therefore, in the present invention, the Si element content is controlled to 0.1% to 1.0%, and 5≤Al / Si≤20.

[0012] Al: The density of aluminum is much lower than that of Fe. Adding an appropriate amount of Al to steel can significantly reduce the density of steel, which is conducive to the lightweight development of steel. At the same time, Al has an antioxidant effect. Adding it in combination with Si can effectively improve the surface quality of steel, which is conducive to the design of multi-purpose products with one steel. In addition, Al can also inhibit the decomposition of residual austenite and the precipitation of carbides, and accelerate the transformation of bainite to improve the coordinated deformation ability. If the Al content is too high, it will not only increase the production cost, but also lead to difficulties in continuous casting production. When the aluminum content is too low, it is impossible to achieve low-density design of the material. Therefore, in the present invention, the content of Al is controlled within the range of 0.6% to 6.0%, and 5≤Al / Si≤20.

[0013] Cr: Chromium increases the hardenability of steel, ensuring its strength, and stabilizes retained austenite. When properly combined with Nb, it helps improve the product's crack resistance and high strength, facilitating the design of multi-purpose products using the same steel. Too low a Cr content will affect the steel's hardenability, while too high a Cr content will increase production costs. Therefore, in the present invention, the Cr content is controlled within a range of 0.02% to 0.80%.

[0014] B: Adding boron to steel significantly improves its hardenability. Due to its small atomic radius, boron, in its free form, accumulates at grain boundaries in steel, effectively strengthening the grain boundaries. This enhances grain boundary energy, reduces internal stress concentration, and thus minimizes the likelihood of cracking, significantly improving the low-temperature toughness of steel. Furthermore, boron readily combines with sulfides and oxides, preventing their further growth and causing these inclusions to become finely spherical and evenly distributed across the grain boundaries. When the boron content exceeds 0.02%, the presence of borides in the steel reduces hardenability and increases brittleness. Therefore, in the present invention, the boron content is controlled within the range of 0.0005% to 0.02%.

[0015] P: P is a harmful element in steel. It easily segregates to grain boundaries and severely reduces the plasticity and deformation properties of the steel. The lower its content, the better. Considering the cost, the P content in the present invention is controlled to P≤0.01%.

[0016] S: S is a harmful element in steel. Sulfur and manganese easily combine to form MnS inclusions, which significantly degrade the material's transverse properties after rolling and deformation, severely impacting the steel's formability. The lower the S content, the better. Considering cost, the S content in this invention is controlled to S ≤ 0.01%.

[0017] N: N easily reacts with Ti to precipitate large TiN particles, which act as crack sources during deformation and are detrimental to the resistance to hydrogen embrittlement. Therefore, the N content in steel must be strictly controlled. In the present invention, the N content is controlled to N≤0.005%.

[0018] Mg: Magnesium is an excellent deoxidizer, desulfurizer, and spheroidizer in steel. It can reduce the number of inclusions in steel, reduce their size, evenly distribute them, and improve their morphology. A small amount of magnesium can improve the size and distribution of carbides in DH steel, promoting fine and uniform carbide particles and contributing to the realization of low-density material designs. To control production costs, the Mg content in this invention is controlled within a range of 0.005% to 0.50%.

[0019] Nb: The microalloying element Nb forms compounds with carbon and nitrogen, which helps to delay the recrystallization of the material during hot rolling, refines the grain size, and significantly improves the strength, toughness and fatigue failure resistance of the material. In the present invention, the Nb element content is controlled at 0.01% to 0.50%.

[0020] Ti: Adding a small amount of Ti element can refine the grain size, and the precipitates can pin dislocations to delay the expansion of crack sources and significantly improve the strength and toughness of the material. In the present invention, the Ti element content is controlled at 0.01% to 0.50%.

[0021] A production method for secondary processing brittle cold-rolled boron-containing dual-phase steel comprises converter smelting, continuous casting and rolling of medium and thin slabs, pickling and cold rolling, continuous annealing or continuous hot-dip galvanizing, and skin pass.

[0022] The converter smelting method includes selecting 40wt% to 100wt% scrap steel as raw material, smelting in a converter, and obtaining molten steel having the following composition requirements by mass percentage: C: 0.10% to 0.20%, Mn: 1.0% to 2.0%, Si: 0.1% to 1.0%, Al: 0.6% to 6.0%, Cr: 0.02% to 0.80%, B: 0.0005% to 0.02%, P≤0.01%, S≤0.01%, N≤0.005%, Mg: 0.005% to 0.50%, Nb: 0.01% to 0.50%, Ti: 0.01% to 0.50%, and 5≤Al / Si≤20; the remainder being Fe and unavoidable impurities. The molten steel temperature is between 1600°C and 1750°C.

[0023] The continuous casting and rolling of medium-thin slabs includes: using special protective slag for high-aluminum steel for casting, preferably the mass percentage of Li2O in the protective slag is 0.5% to 10.0%; the casting temperature is 1530 to 1600°C, the casting machine pulling speed is 1.0 to 5.5 m / min, and the thickness of the continuously cast slab is between 60 and 115 mm; the starting rolling temperature is between 1000 and 1150°C, the finishing rolling temperature is above 900°C, and the coiling temperature is between 600 and 700°C.

[0024] The thickness specification of hot-rolled coil is 2.0~4.5mm; the microstructure of hot-rolled steel plate is ferrite, pearlite, bainite, a small amount of cementite and impurities; calculated by volume percentage: ferrite 30%~60%, pearlite 20%~50%, bainite 5%~20%, cementite and impurities 1%~5%.

[0025] The pickling cold rolling process is as follows: before cold rolling, the hot-rolled steel coil is subjected to acid to remove the surface iron oxide scale, and the cold rolling reduction rate is 45% to 70%. If the reduction rate is too high, the deformation resistance will be too large, making it difficult to roll to the target thickness; if the reduction rate is too low, the elongation of the cold-rolled steel plate will decrease. The thickness of the finished product after cold rolling is 0.8 to 2.3 mm.

[0026] The continuous annealing process includes: a belt speed controlled between 60 and 200 m / min, a soaking zone furnace temperature of 760 to 880°C, a soaking time of 10 to 600 seconds, a slow cooling outlet temperature of 700 to 760°C, a rapid cooling rate greater than 45°C / s, a rapid cooling outlet temperature of 200 to 350°C, an aging temperature of 200 to 350°C, and an aging time of 60 to 1000 seconds. The soaking zone furnace temperature is 760 to 880°C. If the soaking temperature is too high, the austenitization will be complete and the ferrite ratio will be insufficient, which will reduce the ductility of the steel. If the soaking temperature is too low, the proportion of soft-phase ferrite in the final material will be too high, significantly reducing the strength of the material. The soaking interval is 10 to 600 seconds. If the soaking time is too long, the steel plate will have coarse grains. If the soaking time is too short, the steel plate will not have enough time to complete the annealing and recrystallization process, resulting in a decrease in the steel plate's elongation.

[0027] The continuous hot-dip galvanizing process includes: a strip speed controlled at 60-180 m / min, an annealing temperature between 760-880°C, a dew point temperature controlled between -20°C and -10°C, an annealing time between 30-300 seconds, a slow cooling outlet temperature of 680-760°C, a rapid cooling rate greater than 50°C / s, and a rapid cooling outlet temperature of 200-300°C. The galvanizing temperature is 450-470°C, and after galvanizing, the strip is first air-knife cooled to 400-420°C, followed by air cooling, with the top roller temperature of the cooling tower controlled at 250-300°C. The annealing temperature is 760-880°C. If the annealing temperature is too high, the austenitization will be complete and the ferrite content will be insufficient, which will reduce the ductility of the steel. If the soaking temperature is too low, the soft ferrite content of the final material will be too high, which will significantly reduce the strength of the material. The soaking time is 30 to 300 seconds. If the soaking time is too long, the grain size of the steel plate will be coarse. If the soaking time is too short, the steel plate will not have enough time to complete the annealing and recrystallization process, resulting in a decrease in the elongation of the steel plate and also affecting the surface quality of the galvanized surface.

[0028] The composition of the plating solution in the continuous hot-dip galvanizing process is calculated by mass percentage as follows: Al: 0.16% to 0.25%, the rest is Zn and unavoidable impurities; the weight of the zinc layer on the steel plate after continuous hot-dip galvanizing is 60 to 200 g / cm 2 .

[0029] The polishing adopts elongation closed-loop control, and the polishing elongation is 0.3% to 1.0%.

[0030] The above method can obtain a cold-rolled boron-containing dual-phase steel resistant to secondary processing brittleness: yield strength ≥420MPa, tensile strength ≥780MPa, A80 elongation ≥15.0%, hole expansion rate ≥20%, and density of 6.5-7.5g / cm 3 ; Secondary processing brittle transition temperature ≤ -180℃; meets the requirements of ultra-high strength automotive steel for resistance to secondary processing brittleness, high strength and high plasticity, and excellent formability.

[0031] The microstructure of the dual-phase steel includes ferrite and martensite, with the volume ratio being 30% to 60% of ferrite and 40% to 70% of martensite.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1) The chemical composition of the steel material of the present invention mainly includes C, Mn, Al, Si and B as main elements, and the initial cost is low.

[0034] 2) The present invention adopts a new short-process and low-cost production process of "large proportion of scrap steel + converter smelting + continuous casting and rolling of medium and thin slabs", which can greatly reduce carbon emissions and save energy consumption.

[0035] 3) The cold-rolled boron-containing dual-phase steel produced by the present invention, which is resistant to secondary processing brittleness, can achieve low density of high-strength steel by adding a large amount of aluminum elements. The addition of a large amount of boron elements can greatly improve the secondary processing brittleness of high-strength steel, meeting the personalized needs of automobile lightweighting and service resistance.

[0036] 4) The cold-rolled boron-containing dual-phase steel sheet produced by the present invention, which is resistant to secondary processing brittleness, can realize a set of alloy systems to meet the two diversified product requirements of continuous annealing and continuous hot-dip galvanizing due to the special design of composition and process, that is, one steel can be used for multiple purposes, which can significantly save product manufacturing costs.

[0037] 5) The cold-rolled boron-containing dual-phase steel produced by the present invention can achieve a yield strength of ≥420MPa, a tensile strength of ≥780MPa, an A80 elongation of ≥15.0%, a hole expansion rate of ≥20%, and a density of 6.5-7.5g / cm 3 ; Excellent performance of secondary processing brittle transition temperature ≤ -180℃. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a typical engineering stress-strain curve diagram of Example 1-1. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the specific implementation methods of the present invention are further described below in conjunction with examples. The following examples are used to specifically illustrate the contents of the present invention. These examples are only general descriptions of the contents of the present invention and do not limit the contents of the present invention.

[0040] Table 1 lists the chemical composition of the steel of Example 1, Table 2 lists the continuous casting and rolling process parameters and hot rolling structure of the steel of Example 1, Table 3 lists the process parameters of continuous annealing of the steel of Example 1, Table 4 gives the process parameters of continuous hot-dip galvanizing of the steel of Example 1, and Table 5 gives the mechanical properties and steel plate structure of the steel of Example 1. The typical engineering stress-strain curve of Example 1-1 is shown in FIG. Figure 1 .

[0041] Table 1 Composition of steel according to the present invention (wt%)

[0042] Example C Mn Si Al Cr B P S N Mg Nb Ti Al / Si 1 0.17 1.87 0.25 4.51 0.32 0.0027 0.002 0.004 0.001 0.038 0.074 0.048 18.04 2 0.19 1.24 0.31 3.28 0.26 0.0068 0.004 0.002 0.003 0.085 0.092 0.182 10.58 3 0.13 1.53 0.54 3.72 0.48 0.0035 0.003 0.005 0.002 0.167 0.174 0.253 6.89 4 0.16 1.74 0.73 5.73 0.17 0.0015 0.006 0.003 0.005 0.065 0.367 0.059 7.85 5 0.12 1.42 0.16 1.86 0.56 0.0023 0.003 0.002 0.004 0.261 0.225 0.079 11.63 6 0.14 1.65 0.49 2.67 0.55 0.0054 0.001 0.001 0.002 0.192 0.053 0.327 5.45

[0043] Table 2 Main process parameters and hot rolling structure of steel smelting, continuous casting and rolling according to the embodiment of the present invention

[0044]

[0045] Table 3 Main process parameters of continuous annealing of steel according to the present invention

[0046]

[0047] Table 4 Main process parameters of continuous hot-dip galvanizing of steel according to the present invention

[0048]

[0049] Table 5 Properties and Steel Plate Structure of the Steel in the Examples of the Present Invention

[0050]

[0051] As can be seen from the above examples, the alloy composition, smelting, continuous casting and rolling, pickling and cold rolling, and continuous annealing processes of the present invention can produce a cold-rolled boron-containing dual-phase steel resistant to secondary processing brittleness with a yield strength of 420 MPa or greater, a tensile strength of 780 MPa or greater, an A80 elongation of 15.0% or greater, a hole expansion rate of 20% or greater, and a density of 6.5 to 7.5 g / cm 3 ; Excellent performance of secondary processing brittle transition temperature ≤ -180℃; meets the personalized needs of automobiles for low cost, high hole expansion and resistance to secondary processing brittleness.

Claims

1. A cold-rolled boron-containing dual-phase steel resistant to secondary processing brittleness, characterized in that: The chemical composition of the steel is as follows by weight: C: 0.10% to 0.20%, Mn: 1.0% to 2.0%, Si: 0.1% to 1.0%, Al: 0.6% to 6.0%, Cr: 0.02% to 0.80%, B: 0.0005% to 0.02%, P≤0.01%, S≤0.01%, N≤0.005%, Mg: 0.005% to 0.50%, Nb: 0.01% to 0.50%, Ti: 0.01% to 0.50%, and 5≤Al / Si≤20; the balance is Fe and unavoidable impurities.

2. The cold-rolled boron-containing dual-phase steel resistant to secondary processing brittleness according to claim 1, characterized in that: The dual-phase steel has a yield strength of ≥420 MPa, a tensile strength of ≥780 MPa, an A80 elongation after fracture of ≥15.0%, a hole expansion rate of ≥20%, and a density of 6.5-7.5 g / cm 3 ; Secondary processing brittle transition temperature ≤-180℃.

3. The cold-rolled boron-containing dual-phase steel resistant to secondary processing brittleness according to claim 1, characterized in that: The microstructure of the dual-phase steel includes ferrite and martensite, with the volume ratio being 30% to 60% of ferrite and 40% to 70% of martensite.

4. A method for producing secondary processing brittle cold-rolled boron-containing dual-phase steel according to any one of claims 1 to 3, characterized in that: The process includes converter smelting, continuous casting and rolling of medium and thin slabs, pickling and cold rolling, continuous annealing or continuous hot-dip galvanizing, and skin pass. The continuous annealing includes: a soaking section furnace temperature of 760-880°C, a soaking time of 10-600s, a slow cooling outlet temperature of 700-760°C, a rapid cooling rate greater than 45°C / s, a rapid cooling outlet temperature between 200-350°C, an aging temperature of 200-350°C, and an aging time of 60-1000s.

5. The method for producing a secondary processing brittle cold-rolled boron-containing dual-phase steel according to claim 4, characterized in that: The continuous hot-dip galvanizing includes: the strip speed is controlled at 60 to 180 m / min, the annealing temperature is between 760 and 880°C, the dew point temperature is controlled between -20 and -10°C, the annealing time is between 30 and 300s, the slow cooling outlet temperature is 680 to 760°C, the rapid cooling rate is greater than 50°C / s, and the rapid cooling outlet temperature is 200 to 300°C; the galvanizing temperature is 450 to 470°C, and after the galvanizing is completed, the strip is first air-knife-cooled to 400 to 420°C, and then air-cooled, and the temperature of the cooling tower top roller is controlled at 250 to 300°C.

6. The method for producing a secondary processing brittle cold-rolled boron-containing dual-phase steel according to claim 4, characterized in that: The converter smelting comprises: selecting 40wt% to 100wt% scrap steel as raw material, and the temperature of molten steel is between 1600 and 1750°C.

7. The method for producing a secondary processing brittle cold-rolled boron-containing dual-phase steel according to claim 4, characterized in that: The continuous casting and rolling of medium and thin slabs includes: casting temperature of 1530-1600℃, casting machine pulling speed of 1.0-5.5m / min, continuous casting slab thickness of 60-115mm; starting rolling temperature of 1000-1150℃, finishing rolling temperature above 900℃, and coiling temperature of 600-700℃.

8. The method for producing a secondary processing brittle cold-rolled boron-containing dual-phase steel according to claim 7, characterized in that: The thickness specification of hot-rolled coil is 2.0~4.5mm; the microstructure of hot-rolled steel plate is ferrite, pearlite, bainite, a small amount of cementite and impurities; calculated by volume percentage: ferrite 30%~60%, pearlite 20%~50%, bainite 5%~20%, cementite and impurities 1%~5%.

9. The method for producing a secondary processing brittle cold-rolled boron-containing dual-phase steel according to claim 4, characterized in that: The pickling cold rolling reduction rate is 45% to 70%.

10. The method for producing a secondary processing brittle cold-rolled boron-containing dual-phase steel according to claim 4, characterized in that: The light finishing rate is 0.3% to 1.0%.

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