A 600MPa-grade, low-cost, high-hole-expansion-performance pickled automotive steel plate and its production method

Abstract

The invention discloses a 600MPa-grade pickled automobile steel plate with low cost and high hole expansion performance. The chemical composition and weight percentage of the pickled automobile steel plate are as follows: C 0.035%-0.065%, Si 0.30%-0.65%, Mn 1.40%-1.65%, Nb 0.020%-0.030%, Ti 0.010%-0.025%, P≤0.010%, S≤0.004%, and the remainder is Fe and unavoidable impurities. The present invention adopts reasonable chemical composition and hot rolling pickling process to produce pickled automobile steel plate, and obtains bainite + ferrite structure with fine grain size, and grain size grade ≥10; the tensile strength of the steel plate is ≥600MPa, the yield strength is ≥450MPa, and the elongation is ≥24%; the 600MPa-grade low-cost high-hole expansion performance pickled automobile steel plate of the present invention has excellent hole expansion performance, and the hole expansion rate is ≥120%. Compared with traditional 600MPa-grade high-strength steel, the hole expansion performance is significantly improved under the premise of lower alloy element addition, which greatly meets the requirements of automobile chassis parts for high hole expansion and flanging performance; the present invention adopts CSP thin slab continuous casting and rolling process to produce thin-gauge high-strength high-hole expansion pickled steel, which reduces the cold rolling annealing process and greatly reduces production and manufacturing costs.

Description

Technical Field

[0001] The invention relates to pickled high-strength automobile steel and a production method thereof, and in particular to a 600MPa-grade, low-cost, high-hole-expansion-performance pickled automobile steel plate and a production method thereof. Background Art

[0002] In the 21st century, reducing fuel consumption and CO2 and exhaust emissions has become a societal imperative for the automotive industry. To adapt to this trend, the steel industry, as a material producer, has developed numerous types of ultra-high-strength steel plates to help reduce vehicle weight and meet the industry's new requirements. A growing number of pickled automotive steel products are being used to manufacture automotive components, such as suspension and chassis parts. With the continued advancement of lightweighting, high-strength, high-hole-expansion steels with excellent hole-expansion properties are becoming increasingly common in the automotive industry.

[0003] While pursuing high strength and high plasticity, automotive steel must also meet its performance requirements, such as hole expansion and flanging performance. Many automotive parts, such as automotive suspension and chassis parts, are usually designed with a large number of circular holes for assembly and use. These circular holes need to be punched first and then expanded and flanging to obtain the final parts during the forming process. A large amount of data shows that the hole expansion performance and forming performance of a material are not completely equivalent. Generally speaking, the better the plasticity of the steel, the better the forming performance, but it does not necessarily mean better hole expansion and flanging performance. For example, traditional duplex steel has good plasticity and good forming performance, but its hole expansion and flanging performance is poor, which makes it unsuitable for the production of automotive suspension and chassis parts that require hole expansion.

[0004] A preliminary search revealed that CN104726770A discloses a steel plate with excellent hole expansion performance, exceeding 60% and a yield strength ratio of 0.8 or greater. The main method is to control the steel's chemical composition by weight to: C: 0.07-0.15%, Si: 0-0.3%, Mn: 0-1.5%, P: 0.02-0.07%, S≤0.01%, N≤0.005%, Al: 0.02-0.05%, Ti: 0.03-0.1%, B: 0-0.002%, with the remainder being Fe and unavoidable impurities. The microstructure comprises pearlite and ferrite with an area ratio of 2-10%. This steel plate has a low carbon content and a pearlite and ferrite structure, making it easy to control during production. However, this composition and structure result in a low hole expansion performance of only over 60%, with the optimal value in the embodiment being only 76%, which does not meet the requirements for high hole expansion and flanging performance. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a 600MPa-grade pickled automotive steel plate with low cost and high hole expansion performance in response to the shortcomings of the above-mentioned existing technology. The steel plate has a thickness of 1.0mm to 4.0mm, and after pickling, the steel plate has a tensile strength of ≥600MPa, a yield strength of ≥450MPa, and an elongation of ≥24%, which meets the requirements of lightweight and high strength and high plasticity of automotive structural parts. At the same time, in order to meet the requirements of automotive chassis parts for hole expansion and flanging performance, the hole expansion rate of the pickled steel plate is ≥120%.

[0006] The technical solution adopted by the present invention to solve the above-mentioned problems is:

[0007] The invention discloses a 600MPa-grade pickled automobile steel plate with low cost and high hole expansion performance. The steel plate comprises the following components in weight percentage: C 0.035% to 0.065%, Si 0.30% to 0.65%, Mn 1.40% to 1.65%, Nb 0.020% to 0.030%, Ti 0.010% to 0.025%, P ≤ 0.010%, S ≤ 0.004%, and the remainder is Fe and unavoidable impurities.

[0008] Preferably, the weight percentage of C is 0.038% to 0.056%.

[0009] Preferably, the weight percentage of Si is 0.37% to 0.51%.

[0010] Preferably, the weight percentage of Mn is 1.45% to 1.63%.

[0011] The method for producing the above-mentioned 600 MPa grade pickled automotive steel sheet with low cost and high hole expansion performance by CSP (thin slab continuous casting and rolling) comprises the following steps:

[0012] 1) Desulfurization of molten iron, desulfurization target: S≤0.001%, the exposed surface of molten iron after slag removal is not less than 95%;

[0013] 2) Conventional smelting and conventional refining;

[0014] 3) Carry out continuous casting, control the superheat of molten steel in the tundish to be between 15 and 30°C, the thickness of the cast billet to be between 52 and 55 mm, and the casting speed to be between 3.5 and 5.0 m / min;

[0015] 4) Heating the ingot and controlling the ingot temperature at 800-950°C, heating the ingot to 1160-1200°C and keeping it at that temperature for 20-40 minutes;

[0016] 5) Perform high-pressure water descaling before rolling, and control the descaling water pressure at 280-420 bar;

[0017] 6) Rolling is performed, and the reduction rate of the first pass is controlled to be 52% to 63%, the reduction rate of the second pass is controlled to be 50% to 60%, and the reduction rate of the final pass is controlled to be 10% to 16%; the rolling speed is controlled to be 8 to 12 m / s; medium-pressure water descaling is performed between the first and second passes, and the descaling water pressure is 200 to 280 bar; and the finishing rolling temperature is controlled to be 860 to 900°C;

[0018] 7) Use front-stage cooling method for cooling, and the front-stage cooling rate is ≥120℃ / s;

[0019] 8) Coil the steel sheet and control the coiling temperature at 480-520°C;

[0020] 9) Pickling is performed, and continuous pickling is performed at a pickling speed of 100 to 120 m / min to obtain a 600 MPa grade pickled automobile steel plate with low cost and high hole expansion performance.

[0021] Preferably, the temperature of the ingot entering the furnace is controlled at 825-948° C., the ingot is heated to 1165-1193° C., and kept warm for 20-33 minutes.

[0022] Preferably, the cooling is performed by a front-stage cooling method, and the front-stage cooling rate is not less than 120°C / s, and generally does not exceed 150°C / s.

[0023] Preferably, the coiling temperature is controlled at 486-512°C.

[0024] The pickled automotive steel sheet prepared by the above method has a thickness of 1.0 mm to 4.0 mm, preferably 1.2 mm to 2.0 mm; a metallographic structure of bainite + ferrite, and a grain size grade of 10 or higher; a tensile strength of 600 MPa or higher, a yield strength of 450 MPa or higher, an elongation of 24% or higher, and excellent hole expansion performance, with a hole expansion ratio of 120% or higher. The metallographic structure, by volume, is 73% to 80% bainite and 20% to 27% ferrite.

[0025] The functions and mechanisms of the raw materials and main processes in the present invention are as follows:

[0026] C: C is an inexpensive solid solution strengthening element and one of the main factors affecting yield strength and tensile strength. Carbon also has a significant impact on the material's weldability. Generally speaking, higher carbon content in steel increases yield strength and tensile strength, while decreasing weldability. To ensure that the steel plate achieves a tensile strength above 600 MPa and a hole expansion ratio exceeding 120%, the present invention requires strict limits on the carbon content. Since the present invention achieves high strength and high hole expansion ratio through a single bainite structure, the strength and hole expansion performance of bainite are related to the carbon content: a higher carbon content increases strength but reduces hole expansion ratio. Therefore, the present invention limits the carbon content to a range of 0.035% to 0.065%. Furthermore, for CSP production lines, a carbon content exceeding 0.065% can enter the peritectic region, which can easily lead to breakout risks. Therefore, considering the impact of carbon on strength, hole expansion, and the peritectic region, the carbon content in steel should be controlled between 0.035% and 0.065%. The C content has a significant effect on the hole expansion performance. In order to improve the hole expansion rate and ensure the strength, the C content is preferably 0.038% to 0.056%.

[0027] Si: Si effectively inhibits cementite precipitation, which negatively impacts hole expansion performance. Therefore, to improve hole expansion performance, the present invention adds 0.30% Si to effectively inhibit cementite precipitation and improve hole expansion performance. However, excessive Si content can affect the surface quality of the steel plate. Therefore, the Si content is controlled within a range of 0.30% to 0.65%, preferably 0.37% to 0.51%.

[0028] Mn: Mn is the most effective element for improving strength and toughness, and is also one of the important alloying elements used in the present invention. Since the addition of C content is limited by the requirements of hole expansion performance, it is necessary to further ensure that the strength reaches above 600MPa through the solid solution strengthening effect of the Mn element, and the Mn element can improve the hardenability of the plate during laminar cooling, so that the steel can quickly transform from the austenite structure at the finishing outlet to the bainite structure. Therefore, the present invention adds a Mn content of more than 1.4%. However, too high a Mn content will increase the risk of slab cracking, so the Mn content should be controlled between 1.4% and 1.65%, preferably between 1.45% and 1.63%.

[0029] S: S is an impurity element in steel. Excessive sulfur content can increase the steel's tendency to become hot brittle. S in steel often exists as manganese sulfide. These sulfide inclusions are detrimental to the steel's impact toughness and can cause anisotropy in properties. Therefore, the sulfur content in steel must be kept as low as possible. Therefore, the sulfur content in steel is typically controlled below 0.004%.

[0030] Phosphorus (P) is an impurity element in steel. Excessive phosphorus content can lead to the precipitation of Fe2P eutectic structures during solidification, resulting in cold brittleness. Therefore, the lower the phosphorus content, the better. In actual production, it is generally controlled below 0.010%.

[0031] Nb: Nb is one of the key alloying elements used in this invention. Adding Nb effectively refines the original austenite grain size, improving the strength and plasticity of the steel plate. Furthermore, the refined bainite structure effectively inhibits crack propagation during hole expansion and flanging, improving the hole expansion rate. If the Nb content exceeds 0.030%, the grain refinement effect is insignificant, and costs may increase. Therefore, the Nb content is controlled within the range of 0.020% to 0.030%.

[0032] Ti: Ti is one of the important alloying elements used in the present invention. Ti can play a role in refining grains and improving strength and toughness. In addition, for the present invention using CSP thin slab continuous casting and rolling, if Ti is not added, the high-temperature plasticity of the slab will be reduced due to the Nb element during the continuous casting process, and it will not be possible to continuously cast smoothly. Adding Ti elements can form Nb / Ti composite precipitates at high temperatures, thereby ensuring that the high-temperature plasticity of the slab is not reduced and continuous casting can be achieved smoothly. If the Ti element content is too high, the number of coarse carbide and nitride inclusions will increase, thereby affecting the comprehensive mechanical properties. Therefore, the Ti content should be controlled at 0.010% to 0.025%.

[0033] In addition to limiting the range of the above chemical components, the present invention does not add precious alloy elements such as Mo, V, Cu, and Ni from the perspective of improving material formability and economy.

[0034] The present invention controls the superheat of the molten steel in the middle ladle during the continuous casting process to 15-30°C. Excessively low superheat can cause the molten steel in the larger ladle to solidify, making casting impossible. Therefore, the superheat must be kept constant. The superheat described in this invention is specifically designed for the CSP thin slab continuous casting and rolling process and the high-hole-expansion steel described herein. Low superheat casting can mitigate some of the shortcomings of conventional casting, such as reducing or even eliminating shrinkage cavities, porosity, sand adhesion, and air porosity. Furthermore, low superheat effectively reduces centerline segregation in the ingot, improving structural uniformity.

[0035] Experiments of the present invention have shown that the reason why the ingot is heated to 1160-1200°C and kept warm for 20-40 minutes is that it can ensure that the temperature of the steel plate during the steel rolling process meets the requirements of each link. From the perspective of oxidation kinetics and thermodynamics, if the heating temperature is too high and the holding time is too long, the original austenite grains before rolling will be coarse, which will cause the subsequent rolling process to be unable to complete the austenite complete recrystallization process, and then cause mixed crystals in the final product (i.e., the product grain size is uneven), which seriously affects the product quality. However, if the heating temperature is too low and the holding time is too short, the initial austenite grains before rolling will be too small, resulting in excessive deformation resistance in the subsequent rolling process exceeding the limit capacity of the steel rolling equipment, and the entire rolling process cannot be completed. In addition, if the heating temperature is too high and the holding time is too long, the depth of the product decarburization layer will also increase, resulting in a decline in product quality, which directly affects the key performance of the product.

[0036] The reason why the final rolling temperature is controlled at 860-900° C. in the present invention is to ensure that the finishing rolling step is carried out in the austenite region, and to ensure that the austenite is completely recrystallized during the finishing rolling process to form fine original austenite grains.

[0037] The present invention implements ultra-rapid cooling of the steel plate immediately after rolling, i.e., cooling at a cooling rate of no less than 120°C / s. This high cooling rate suppresses post-rolling grain growth in the steel, maintaining fine and uniform grains before coiling, resulting in a final steel with a fine grain size of 10. Coiling requires cooling to 480-520°C to achieve a microstructure of bainite with a small amount of ferrite. If the coiling temperature is too low, plasticity and hole expansion properties will decrease, hindering part forming and hole expansion and flanging. If the coiling temperature is too high, the bainite strength will decrease, failing to meet the tensile strength requirement of ≥ 600 MPa.

[0038] Compared with the prior art, the present invention adopts reasonable chemical composition and hot rolling pickling process to produce pickled automobile steel plates, obtaining a bainite + ferrite structure with fine grain size and a grain size grade ≥10; the tensile strength of the steel plate is ≥600MPa, the yield strength is ≥450MPa, and the elongation is ≥24%; the 600MPa-grade low-cost, high-hole-expansion performance pickled automobile steel plate of the present invention has excellent hole expansion performance and a hole expansion rate of ≥120%. Compared with traditional 600MPa-grade high-strength steel, the hole expansion performance is significantly improved under the premise of lower alloy element addition. For example, the traditional automobile steel DP600 has a hole expansion rate of only about 60%. The high-hole expansion steel of the present invention greatly meets the requirements of automobile chassis parts for high hole expansion and flanging performance; the present invention adopts the CSP thin slab continuous casting and rolling process to produce thin-gauge high-strength and high-hole expansion pickled steel, which reduces the cold rolling annealing process and greatly reduces production and manufacturing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the metallographic structure diagram (bainite + ferrite) of the high-strength and high-hole-expansion performance pickled steel plate of the present invention. DETAILED DESCRIPTION

[0040] In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with examples, but the present invention is not limited to the following examples.

[0041] In the present invention, the hole expansion test is carried out according to the national standard. A center hole with a diameter of 10 mm is punched in the center of a 150 mm*150 mm square plate. A conical hole expansion punch of a specified shape and size is pushed into the punched hole of the metal dark plate sample to carry out the hole expansion test until a crack that penetrates the thickness of the sample appears at the edge of the hole. The punch is then stopped and the limit hole expansion rate is measured. Where λ is the limit hole expansion rate, %; D0 is the initial diameter of the punched hole (D0 = 10 mm); D H The average diameter of the circular holes after rupture, in millimeters (mm). The hole expansion rate of the pickled automobile steel plate prepared by the present invention can reach more than 120%.

[0042] Examples 1 to 5

[0043] Table 1 is a list of chemical compositions of various embodiments and comparative examples of the present invention;

[0044] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0045] Table 3 is a table of performance test results of various embodiments of the present invention and comparative examples.

[0046] Each embodiment of the present invention and comparative example are produced according to the following steps:

[0047] 1) Desulfurization of molten iron, desulfurization target: S≤0.001%, the exposed surface of molten iron after slag removal is not less than 95%;

[0048] 2) Conventional smelting and conventional refining;

[0049] 3) Carry out continuous casting, control the superheat of molten steel in the tundish to be between 15 and 30°C, the thickness of the cast billet to be between 52 and 55 mm, and the casting speed to be between 3.5 and 5.0 m / min;

[0050] 4) Heating the ingot and controlling the ingot temperature to be between 800 and 950°C, the ingot time to be between 20 and 40 minutes, and the outgoing temperature to be between 1160 and 1200°C;

[0051] 5) Perform high-pressure water descaling before rolling, and control the descaling water pressure at 280-420 bar;

[0052] 6) Rolling is performed, and the reduction rate of the first pass is controlled to be 52-63%, the reduction rate of the second pass is 50-60%, and the reduction rate of the final pass is 10-16%; the rolling speed is controlled to be 8-12 m / s; medium-pressure water descaling is performed between the first and second passes, and the descaling water pressure is 200-280 bar; and the finishing rolling temperature is controlled to be 860-900°C;

[0053] 7) The front section cooling method is used for cooling, and the front section cooling speed is not less than 120℃ / s, and generally does not exceed 150℃ / s;

[0054] 8) Coil the steel sheet and control the coiling temperature at 480-520°C;

[0055] 9) Perform pickling, and perform continuous pickling at a pickling speed of 100 to 120 m / min.

[0056] Table 1 Chemical composition list of various embodiments of the present invention and comparative examples (wt%)

[0057]

[0058] Table 2 List of main process parameters of various embodiments of the present invention and comparative examples

[0059]

[0060] Table 2

[0061]

[0062]

[0063] Table 3 Mechanical properties test results of various embodiments of the present invention and comparative examples

[0064]

[0065] As can be clearly seen from Table 3, the chemical composition, hot rolling, and pickling processes of the present invention result in a fine-grained bainite + ferrite structure with a grain size grade ≥10. The steel plate exhibits a tensile strength ≥600 MPa, a yield strength ≥450 MPa, an elongation ≥24%, and an ultimate hole expansion ratio ≥120%. These significantly improve the hole expansion performance compared to conventional 600 MPa-grade high-strength steels such as DP600 (which have a hole expansion ratio of only approximately 60%), greatly satisfying the requirements for high hole expansion and flanging performance for automotive chassis components. The thin-gauge, high-strength, high-hole expansion pickled steel produced using the CSP thin slab continuous casting and rolling process eliminates the need for cold rolling and annealing processes, significantly reducing production costs.

[0066] The above description is only a preferred embodiment of the present invention and is not intended to be a restrictive implementation of the technical solution of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these improvements and modifications fall within the scope of protection of the present invention.

Claims

1. A 600MPa grade pickled automotive steel plate with low cost and high hole expansion performance, characterized in that: The chemical composition and weight percentage content are: C 0.035%~0.055%, Si 0.30%~0.65%, Mn 1.40%~1.65%, Nb 0.020%~0.030%, Ti0.010%~0.025%, P≤0.010%, S≤0.004%, and the rest are Fe and unavoidable impurities; The 600 MPa grade pickled automotive steel plate with low cost and high hole expansion performance is produced by thin slab continuous casting and rolling (CSP). During rolling, the first pass reduction rate is controlled at 52% to 63%, the second pass reduction rate is 50% to 60%, and the final pass reduction rate is 10% to 16%. The rolling speed is 8 to 12 m / s, and medium-pressure water descaling is performed between the first and second passes. The final rolling temperature is 860 to 900°C. After rolling, the steel plate is rapidly cooled after final rolling using a front-stage cooling method with a cooling rate of ≥120°C / s, and then coiled at a controlled coiling temperature of 505 to 520°C. Pickling is then performed continuously at a pickling speed of 100 to 120 m / min to obtain the 600 MPa grade pickled automotive steel plate with low cost and high hole expansion performance. The metallographic structure of the pickled automobile steel plate is bainite + ferrite structure, which is 73% to 80% bainite and 20% to 27% ferrite by volume percentage, and the grain size grade is ≥10; the tensile strength of the steel plate is ≥600MPa, the yield strength is ≥450MPa, the elongation is ≥24%, and the hole expansion rate is ≥120%.

2. The 600MPa grade pickled automobile steel plate with low cost and high hole expansion performance according to claim 1, characterized in that: The weight percentage content of Si is 0.37%~0.51%.

3. The 600MPa grade pickled automobile steel plate with low cost and high hole expansion performance according to claim 1, characterized in that: The weight percentage of Mn is 1.45%~1.63%.

4. The method for producing a 600 MPa grade, low-cost, high-hole-expansion-performance pickled automobile steel plate according to claim 1 comprises the following steps, characterized in that: 1) Desulfurization of molten iron, desulfurization target: S≤0.001%, the exposed surface of molten iron after slag removal is not less than 95%; 2) Smelting and refining to obtain molten steel with the target chemical composition; 3) Carry out continuous casting, control the superheat of molten steel in the tundish to be between 15 and 30°C, and the casting speed to be between 3.5 and 5.0 m / min; 4) Heat the ingot and control the ingot temperature at 800-950°C. Heat the ingot to 1160-1200°C and keep it at that temperature for 20-40 minutes. 5) High-pressure water descaling before rolling; 6) Rolling is performed, and the reduction rate of the first pass is controlled to be 52% to 63%, the reduction rate of the second pass is 50% to 60%, and the reduction rate of the final pass is 10% to 16%; the rolling speed is controlled to be 8 to 12 m / s; medium-pressure water descaling is performed between the first and second passes; and the final rolling temperature is controlled to be 860 to 900°C; 7) Use the front-stage cooling method for rapid cooling after final rolling, with a cooling rate of ≥120℃ / s; 8) Coil the steel and control the coiling temperature at 505~520℃; 9) Pickling is performed continuously at a pickling speed of 100 to 120 m / min to obtain a 600 MPa grade pickled automobile steel plate with low cost and high hole expansion performance.

5. The method for producing a 600 MPa grade pickled automobile steel plate with low cost and high hole expansion performance according to claim 4, characterized in that: The thickness of the ingot is 52-55 mm; the ingot entering the furnace temperature is controlled at 825-948°C, the furnace time is 20-33 minutes, and the furnace discharge temperature is 1165-1193°C; the descaling water pressure before rolling is 280-420 bar; during the rolling step, medium-pressure water descaling is carried out between the first and second passes, and the descaling water pressure is 200-280 bar.

6. The method for producing a 600 MPa grade pickled automobile steel plate with low cost and high hole expansion performance according to claim 4, characterized in that: The front-stage cooling method is used for rapid cooling after final rolling, with a cooling rate of not less than 120℃ / s and not more than 150℃ / s.

7. A 600 MPa grade, low-cost, high hole expansion performance pickled automobile steel sheet produced by the method according to any one of claims 4 to 6, characterized in that: The thickness of the pickled automobile steel plate is 1.0 mm to 4.0 mm.

8. A 600 MPa grade pickled automobile steel plate with low cost and high hole expansion performance produced by the method according to any one of claims 7, characterized in that: The thickness of the pickled automobile steel plate is 1.2 mm to 2.0 mm.

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