Secondary processing brittleness resistant transformation induced plasticity steel plate and production method thereof

Abstract

The invention relates to a secondary processing brittleness resistant transformation induced plasticity steel plate and a production method thereof. The steel comprises the following chemical components in percentage by weight: 0.10%-0.50% of C, 1.5%-4.5% of Mn, 0.15%-2.0% of Si, 0.02%-9.00% of Al, 0.0005%-0.02% of B, less than or equal to 0.005% of P, less than or equal to 0.005% of S, less than or equal to 0.005% of N, 0.005%-0.50% of Cu, 0.005%-0.50% of V, 0.01%-0.50% of Ti and the balance of Fe. And the balance of Fe and inevitable impurities. The steel plate has the characteristic of resisting secondary processing embrittlement while meeting basic performance indexes of products, the tensile strength of the steel plate is larger than or equal to 600 MPa, the percentage elongation after fracture is larger than or equal to 28%, the hole expansion rate is larger than or equal to 40%, and the density is 6.0-7.5 g / cm < 3 >; and the secondary processing brittle transition temperature is less than or equal to-150 DEG C.

Description

Technical Field

[0001] The invention belongs to the technical field of automobile steel, and particularly relates to a secondary processing resistant brittle phase transformation induced plasticity steel plate and a production method thereof. Background Art

[0002] With the rapid development of automobile lightweighting, automobile manufacturers have gradually increased their requirements for the comprehensive performance of steel products. Among them, the secondary processing brittleness of automobile steel plates has been highly valued by automobile manufacturers and steel manufacturers. Secondary processing performance. Secondary processing brittleness (SWE) refers to the low-temperature brittleness characteristics of automobile cold-rolled steel plates after stamping, which are manifested by their ability to withstand impact loads at low temperatures. When the secondary processing brittle transition temperature (SWET) is too high, it will bring hidden dangers to the safety of passenger cars. Especially for areas with cold winter climates and large temperature differences between day and night, there is a great risk of automobile plates being broken by low-temperature impact during use after stamping. How to effectively solve the secondary processing brittleness problem of automobile steel products has become a hot spot in the research and development of automobile steel products. Studies have found that adding boron to steel can significantly improve its hardenability. Boron can easily combine with sulfides and oxides to organize their further growth, and make the shape of these inclusions tend to be small spherical and evenly distributed on the grain boundaries, which enhances the grain boundary energy and reduces the concentration of internal stress, thereby reducing the possibility of cracks. It can significantly improve the low-temperature toughness of steel materials. In addition, adding a small amount of boron to steel can save a lot of precious elements. The country encourages the development of boron-containing steel and gives certain preferential export tax rebate policies. At the same time, under the background of "dual carbon", the steel industry is actively promoting energy conservation, environmental protection and green transformation and development. How to develop green and low-carbon high-strength automotive products has become a research hotspot for major steel suppliers. Based on the above research status, it is urgent to solve the problems of poor forming performance and secondary processing brittleness of automotive high-strength steel, and at the same time adapt to the design of green and low-carbon products of automotive high-strength steel under the background of dual carbon.

[0003] Therefore, the present invention aims to develop a steel plate that is resistant to secondary processing brittle phase transformation induced plasticity and a production method thereof. The invention achieves 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

[0004] The present invention provides a secondary processing brittle phase transformation induced plasticity steel plate and a production method thereof, which not only meets the basic performance indicators of the product, but also has the characteristics of secondary processing brittleness resistance. The steel plate of the present invention has a tensile strength of ≥600MPa, an elongation after fracture of ≥28%, a hole expansion rate of ≥40%, and a density of 6.0-7.5g / cm 3 ; Secondary processing brittle transition temperature ≤-150℃.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] The invention discloses a steel plate resistant to secondary processing brittle phase transformation induced plasticity. The chemical components of the steel are as follows by weight percentage: C: 0.10%-0.50%, Mn: 1.5%-4.5%, Si: 0.15%-2.0%, Al: 0.02%-9.00%, B: 0.0005%-0.02%, P≤0.005%, S≤0.005%, N≤0.005%, Cu: 0.005%-0.50%, V: 0.005%-0.50%, Ti: 0.01%-0.50%; the balance is Fe and unavoidable impurities.

[0007] The reasons for the alloy design of the present invention are as follows:

[0008] C: Carbon element ensures the strength requirement of steel through solid solution strengthening. A sufficient amount of carbon element helps stabilize austenite, thereby improving the formability of steel. If the C content is too low, the mechanical properties of the steel in the present invention cannot be obtained; if the content is too high, the steel will become brittle and there is a risk of secondary processing embrittlement. Therefore, in the present invention, the content of C element is controlled to 0.10% to 0.50%.

[0009] Mn: Manganese is an austenite stabilizing element in steel. It can expand the austenite phase area and reduce the critical quenching rate of steel. At the same time, it can also refine the grains and help to improve the strength by solid solution strengthening. If the Mn content is too low, the supercooled austenite is not stable enough, which reduces the processing properties such as plasticity and toughness of the steel plate; if the Mn content is too high, the welding performance of the steel plate will deteriorate and the production cost will increase, which is not conducive to industrial production. Therefore, in the present invention, the Mn content is controlled to 1.5% to 4.5%.

[0010] 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. If the Si content is too low, it will not play a strengthening role; if the Si content is too high, the surface quality and welding performance of the steel plate will be reduced. Therefore, in the present invention, the content of Si is controlled to 0.15% to 2.0%.

[0011] Al: Aluminum can inhibit the decomposition of residual austenite and the precipitation of carbides, and accelerate bainite transformation to improve the coordinated deformation ability. Too high Al content will not only increase production costs, but also cause difficulties in continuous casting production, etc. When the Al content is too low, the low-density design of the material cannot be achieved. Therefore, in the present invention, the Al content is controlled within the range of 0.02% to 9.0%.

[0012] B: Adding boron to steel can significantly improve its hardenability. Due to the small radius of boron atoms, it gathers in the grain boundary in the form of free state in steel, which has a good effect of strengthening the grain boundary, enhancing the grain boundary energy, reducing the concentration of internal stress, thereby reducing the possibility of cracks, and significantly improving the low-temperature toughness of steel materials. In addition, boron can easily combine with sulfides and oxides to organize their further growth, and make the shape of these inclusions tend to be fine spherical and evenly distributed on the grain boundary. When the boron content exceeds 0.02%, the hardenability decreases and the brittleness increases due to the presence of borides in the steel. Therefore, in the present invention, the content of B element is controlled within the range of 0.0005% to 0.02%.

[0013] P: P is a harmful element in steel, which is very easy to segregate to the grain boundary and seriously reduce the plasticity and deformation performance of the steel. The lower its content, the better. Considering the cost, the content of P in the present invention is controlled to P≤0.005%.

[0014] S: S is a harmful element in steel. Sulfur and manganese are easily combined to form MnS inclusions. After rolling and deformation, the transverse properties of the material will be significantly reduced, which seriously affects the formability of the steel. The lower the content, the better. Considering the cost, the content of S in the present invention is controlled to S≤0.005%.

[0015] N: N element easily reacts with Ti to precipitate large TiN particles, which act as crack sources during deformation and are detrimental to the anti-hydrogen embrittlement performance. Therefore, the N element content in the steel must be strictly controlled. The present invention controls the N content to N≤0.005%.

[0016] Cu: Cu is a solid solution strengthening element, which can not only improve the hardenability of steel, but also effectively improve the thermodynamic stability of austenite, help to form stable residual austenite at room temperature, and further improve the material's plasticity, resistance to hydrogen embrittlement and resistance to secondary processing embrittlement, etc.; but considering the high cost of copper, the present invention controls the Cu content within the range of 0.005% to 0.50%.

[0017] V: Microalloying element vanadium mainly exists in the form of VC, which improves the strength and fatigue resistance of the material through fine grain strengthening and dispersion strengthening. During the annealing heating process, undissolved VC particles can pin the ferrite grain boundary, thereby playing a role in refining the grains; when the annealing temperature increases to the two-phase region, the VC dissolution temperature is low, so it is fully dissolved in the matrix, and at the same time, the solid solution C atoms are enriched in the austenite to improve its stability; during the annealing process, the VC in the ferrite will reprecipitate, thereby producing obvious precipitation strengthening. Therefore, in the present invention, the V element content is controlled to be 0.01-0.50%.

[0018] Ti: 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, which helps to improve the secondary processing brittleness. In the present invention, the Ti element content is controlled at 0.005% to 0.50%.

[0019] A production method of a brittle phase transformation induced plasticity steel plate resistant to secondary processing, comprising 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; wherein the continuous annealing comprises: the belt speed is controlled at 60-180 m / min, the furnace temperature of the soaking section is 760-880°C, the soaking time is 10-600s, the slow cooling outlet temperature is 700-750°C, the rapid cooling rate is greater than 25°C / s, the rapid cooling temperature is between 450-470°C, the aging temperature is 250-460°C, and the aging time is 60-1000s.

[0020] The soaking temperature is 760-880℃. If the soaking temperature is too high, the ductility of the steel will be reduced due to the complete austenitization and insufficient ferrite ratio. If the soaking temperature is too low, the soft ferrite ratio of the final material will be too high, which will greatly reduce the strength of the material. The soaking time is 10-600s. 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.

[0021] 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 720°C, the rapid cooling rate is greater than 20°C / s, the rapid cooling outlet temperature is 450 to 470°C, the galvanizing temperature is 450 to 470°C, 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.

[0022] The hot-dip galvanizing bath contains 0.16wt% to 0.25wt% Al, the rest is Zn and unavoidable impurities, and the weight of the zinc layer per unit area is 60 to 200g / cm 2 .

[0023] The converter smelting uses 40wt% to 90wt% scrap steel as raw material, and the temperature of the molten steel is between 1600 and 1750°C.

[0024] The continuous casting and rolling of medium-thin slabs includes: using special protective slag for high aluminum steel for casting, preferably the Li2O content in the protective slag is in the range of 0.5wt% to 10.0wt%, the casting temperature is 1530 to 1600°C, the casting machine pulling speed is 1.0 to 5.5m / min, the thickness of the continuous casting slab is between 60 and 115mm; the starting rolling temperature is between 1000 and 1150°C, the final rolling temperature is above 900°C, and the coiling temperature is between 600 and 700°C. The thickness specification of the hot rolled coil is 2.0 to 4.5mm.

[0025] The microstructure of the steel plate after hot rolling is ferrite + pearlite + bainite + cementite; the volume percentage of each microstructure is as follows: ferrite 30% to 60%, pearlite 20% to 50%, bainite 5% to 20%, cementite 1% to 5%.

[0026] The pickling cold rolling: 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, and it will be difficult to roll to the target thickness; if the reduction rate is too low, the elongation of the cold rolled steel sheet will decrease, and the thickness of the finished product after cold rolling is 1.0 to 2.0 mm. After cold rolling, the steel sheet is subjected to continuous annealing or continuous hot dip galvanizing process.

[0027] The finishing process adopts elongation closed-loop control, and the finishing elongation is 0.2% to 0.6%.

[0028] The microstructure of the steel plate is 20% to 45% ferrite, 20% to 50% martensite, 5% to 20% retained austenite, and 10% to 40% bainite by volume, the total being 100%; and the retained austenite is in two forms, blocky and filmy, with a grain size between 0.05μm and 1.00μm. The blocky residual austenite is mainly distributed at the interface between bainite and ferrite and inside the ferrite, and the filmy residual austenite is distributed between the bainite laths.

[0029] Steel plate tensile strength ≥600MPa, elongation after fracture ≥28%, hole expansion rate ≥40%, density 6.0~7.5g / cm 3 ; The brittle transition temperature of steel plate secondary processing is ≤-150℃.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 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 relatively low.

[0032] 2) The present invention adopts a new short-process, 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.

[0033] 3) Adding a large amount of aluminum to the secondary processing brittle phase transformation induced plasticity steel plate produced by the present invention can achieve low density of high-strength steel, and adding a large amount of boron can greatly improve the secondary processing brittleness of high-strength steel, meeting the personalized needs of automobile lightweight and service resistance.

[0034] 4) The secondary processing resistant brittle phase transformation induced plasticity steel plate produced by the present invention can realize a set of alloy system 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 has multiple uses, which can significantly save product manufacturing costs.

[0035] 5) The secondary processing resistant brittle phase transformation induced plasticity steel plate produced by the present invention can achieve a tensile strength of ≥600MPa, an elongation after fracture of ≥28%, a hole expansion rate of ≥40%, and a density of 6.0-7.5g / cm 3 ; Excellent performance of secondary processing brittle transition temperature ≤-150℃. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The engineering stress-strain curves of Example 1-1 and Example 4-1. DETAILED DESCRIPTION

[0037] 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 embodiments. The following embodiments are used to specifically illustrate the contents of the present invention. These embodiments are only general descriptions of the contents of the present invention and do not limit the contents of the present invention.

[0038] Table 1 lists the chemical composition of the example steel; Table 2 lists the main process parameters of smelting, continuous casting and rolling and hot rolling structure of the example steel; Table 3 lists the process parameters of continuous annealing of the example steel; Table 4 gives the main process parameters of continuous hot-dip galvanizing of the example steel; Table 5 gives the mechanical properties of the example steel. The engineering stress-strain curves of Example 1-1 and Example 4-1 are shown in Figure 1 .

[0039] Table 1 Chemical composition of the example steel, wt%

[0040]

[0041]

[0042] Table 2 Main process parameters and hot rolling structure of steel smelting, continuous casting and rolling according to the embodiment of the present invention

[0043]

[0044] Table 3 Main process parameters of continuous annealing of steel according to the present invention

[0045]

[0046] Table 4 Main process parameters of continuous hot-dip galvanizing of steel in the embodiment of the present invention

[0047]

[0048]

[0049] Table 5 Mechanical properties of example steel

[0050]

[0051] It can be seen from the above embodiments that the composition design, continuous casting and rolling, and continuous hot-dip galvanizing process of the present invention can produce a product with a tensile strength of ≥600MPa, an elongation after fracture of ≥28%, a hole expansion rate of ≥40%, and a density of 6.0-7.5g / cm 3 ; Excellent performance of secondary processing brittle transition temperature ≤-150℃.

Claims

1. A steel plate resistant to secondary processing and brittle phase transformation induced plasticity, characterized in that: The chemical composition of the steel by weight percentage is: C: 0.10% ~ 0.50%, Mn: 1.5% ~ 4.5%, Si: 0.15% ~ 2.0%, Al: 0.02% ~ 9.00%, B: 0.0005% ~ 0.02%, P ≤ 0.005%, S ≤ 0.005%, N ≤ 0.005%, Cu: 0.005% ~ 0.50%, V: 0.005% ~ 0.50%, Ti: 0.01% ~ 0.50%; the balance is Fe and unavoidable impurities.

2. The secondary processing resistant brittle phase transformation induced plasticity steel plate according to claim 1, characterized in that: The steel plate has a tensile strength of ≥600 MPa, an elongation after fracture of ≥28%, a hole expansion rate of ≥40%, and a density of 6.0-7.5 g / cm 3 ; The brittle transition temperature of steel plate secondary processing is ≤-150℃.

3. The secondary processing resistant brittle phase transformation induced plasticity steel plate according to claim 1, characterized in that: The microstructure of the steel plate is 20% to 45% ferrite, 20% to 50% martensite, 5% to 20% residual austenite, and 10% to 40% bainite in volume ratio, the total being 100%; and the residual austenite is in two forms, namely, block and film, with a grain size between 0.05 and 1.00 μm, the blocky residual austenite is distributed at the interface between bainite and ferrite and inside the ferrite, and the film-like residual austenite is distributed between bainite laths.

4. A method for producing a secondary processing resistant brittle phase transformation induced plasticity steel plate according to any one of claims 1 to 3, characterized in that: It 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: the belt speed is controlled at 60-180m / min, the furnace temperature in the soaking section is 760-880℃, the soaking time is 10-600s, the slow cooling outlet temperature is 700-750℃, the rapid cooling rate is greater than 25℃ / s, the rapid cooling temperature is between 450-470℃, the aging temperature is 250-460℃, and the aging time is 60-1000s.

5. The method for producing a secondary processing resistant brittle phase transformation induced plasticity steel plate 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 720°C, the rapid cooling rate is greater than 20°C / s, the rapid cooling outlet temperature is 450 to 470°C, the galvanizing temperature is 450 to 470°C, 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 resistant brittle phase transformation induced plasticity steel plate according to claim 4, characterized in that: The converter smelting uses 40wt% to 90wt% scrap steel as raw material, and the temperature of the molten steel is between 1600 and 1750°C.

7. The method for producing a secondary processing resistant brittle phase transformation induced plasticity steel plate according to claim 4, characterized in that: The continuous casting and rolling of medium-thin slabs includes: a casting temperature of 1530-1600°C, a casting machine pulling speed of 1.0-5.5m / min, a continuous casting slab thickness of 60-115mm; a starting rolling temperature of 1000-1150°C, a final rolling temperature of more than 900°C, and a coiling temperature of 600-700°C.

8. The method for producing a secondary processing resistant brittle phase transformation induced plasticity steel plate according to claim 7, characterized in that: The microstructure of the steel plate after hot rolling is ferrite + pearlite + bainite + cementite; the volume percentage of each microstructure is as follows: ferrite 30% to 60%, pearlite 20% to 50%, bainite 5% to 20%, cementite 1% to 5%.

9. The method for producing a secondary processing resistant brittle phase transformation induced plasticity steel plate 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 resistant brittle phase transformation induced plasticity steel plate according to claim 4, characterized in that: The smoothing elongation is 0.2% to 0.6%.

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