A medium-high carbon alloy structural steel for fine blanking and a manufacturing method thereof

Abstract

The invention belongs to the technical field of alloy structural steel, and discloses a medium-high carbon alloy structural steel for fine blanking and a manufacturing method thereof. The chemical composition and weight percentage content of the steel are: C: 0.39-0.43%, Si: 0.17-0.3%, Mn: 0.7-0.8%, P: ≤0.015%, S: ≤0.003%, ALt: 0.02-0.05%, Mo: 0.15-0.25%, Cr: 0.9-1.1%, and the balance is Fe and inevitable inclusions. The present invention develops a medium-high carbon alloy structural steel for fine blanking through innovation of components and processes, which has a hardness of 150-175 HV, a yield strength of 800-900 MPa, a tensile strength of 900-1000 MPa, an elongation of ≥15%, a D-type inclusion level of ≤1, a DS-type inclusion level of ≤0.5, a center segregation level of ≤C0.5, a banded structure level of ≤2, a spheroidization level of 3, and a spheroidization rate of ≥95%. The steel has broad application prospects in the field of automobile fine parts manufacturing.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy structural steel, and in particular relates to a medium-high carbon alloy structural steel for fine blanking and a manufacturing method thereof. Background Art

[0002] Steel is the most important metal for manufacturing auto parts. Due to the special requirements of auto parts for specifications, thickness accuracy, surface quality and mechanical properties, there are various ways to process steel, and auto parts materials that bear key functions often use fine blanking technology. Fine blanking technology is a precision blanking method developed on the basis of ordinary stamping technology. Through a specific heat treatment method, the internal structure of the material presents a spherical carburized structure, so that the downstream fine blanking process does not produce tear bands and burrs, etc., ensuring the safety performance, quality, yield and stability of the parts. The fine blanking process can obtain mechanical parts with high dimensional accuracy and good cross-sectional quality through one-time forming. Compared with traditional forging, casting and machining processes, fine blanking technology has the characteristics of high production efficiency, low cost and low energy consumption.

[0003] The performance of fine blanking steel has a great influence on the effect of fine blanking. Since the gap between the dies for fine blanking is smaller than the gap setting for ordinary blanking, the entire thickness section of the fine blanking steel plate is required to have a uniform spheroidized pearlite structure, a small decarburization layer, and uniform hardness after cold rolling annealing. In order to ensure the uniform spheroidized annealing structure before fine blanking after cold rolling annealing, the hot rolled structure is required to be uniform and the banded structure level is low. At the same time, a certain cold rolling formability is required, and striping, pickling, and cold rolling can be performed. However, the hot rolled structure of traditional fine blanking steel is prone to form bainite or martensite, with low elongation. The spheroidization rate is low after spheroidizing annealing, which is not conducive to user precision blanking and is prone to cracking. Therefore, the fine blanking steel produced by conventional manufacturing methods cannot meet the requirements of fine blanking performance. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a medium-high carbon alloy structural steel for fine blanking and a manufacturing method thereof in view of the deficiencies in the prior art. Through the innovation of composition and process, a uniform spheroidized pearlite structure is formed, the level of non-metallic inclusions, center segregation and banded structure is reduced, and the spheroidization rate after spheroidizing annealing is improved, thereby ensuring that it has excellent fine blanking performance.

[0005] In order to solve the technical problem proposed by the present invention, the present invention provides a medium-high carbon alloy structural steel for fine blanking, whose chemical composition and weight percentage content are: C: 0.39-0.43%, Si: 0.17-0.3%, Mn: 0.7-0.8%, P: ≤0.015%, S: ≤0.003%, ALt: 0.02-0.05%, Mo: 0.15-0.25%, Cr: 0.9-1.1%, and the balance is Fe and inevitable inclusions.

[0006] Preferably, the chemical composition and weight percentage content of the steel are: C: 0.4-0.42%, Si: 0.2-0.26%, Mn: 0.72-0.78%, P: ≤0.012%, S: ≤0.002%, ALt: 0.03-0.045%, Mo: 0.18-0.22%, Cr: 0.95-1.05%, and the remainder is Fe and unavoidable inclusions.

[0007] In the above scheme, the microstructure of the steel is ferrite and spherical pearlite, wherein the volume proportion of the spherical pearlite is ≥95%.

[0008] In the above scheme, the spheroidization level of the steel is level 3, and the spheroidization rate is ≥95%.

[0009] In the above scheme, the D-type inclusion level of the steel is ≤ level 1, and the DS-type inclusion level is ≤ level 0.5.

[0010] In the above scheme, the central segregation level of the steel is ≤C0.5, and the banded structure level is ≤2.

[0011] In the above solution, the decarburization layer depth of the steel is ≤50 μm.

[0012] In the above scheme, the hardness of the steel is 150-175 HV, the yield strength is 800-900 MPa, the tensile strength is 900-1000 MPa, and the elongation is ≥15%.

[0013] The present invention also provides a method for manufacturing medium-high carbon alloy structural steel for fine blanking, comprising the following steps:

[0014] 1) KR hot metal desulfurization: the sulfur content of hot metal after desulfurization is ≤ 0.001%;

[0015] 2) Converter smelting: the final temperature is 1650-1665℃, the final molten steel O content is 0.02-0.04%, aluminum iron is added for deoxidation, the molten steel Alt content is controlled to be 0.005-0.02%, and alloying is performed;

[0016] 3) Argon blowing: After argon blowing, the Alt content of molten steel is 0.02-0.035%;

[0017] 4) LF refining: After refining, the Alt content of the molten steel is 0.025-0.04%, and the S content is ≤0.002%;

[0018] 5) The molten steel is degassed and removed through vacuum circulation, and after alloying and fine-tuning the composition and temperature, it is continuously cast into billets;

[0019] 6) After heating, the slab is subjected to rough rolling, finish rolling, cooling and coiling to obtain medium-high carbon alloy structural steel for fine blanking.

[0020] In the above scheme, the vacuum degree of the vacuum cycle is ≤15Pa, the total cycle time is 28-35min, the net cycle time after alloying is completed is ≥10min, and the vacuum end temperature is controlled at 1521-1526°C.

[0021] In the above scheme, the continuous casting process selects a mold protection slag with a basicity of 0.8-0.9, a viscosity of 0.07-0.17 Pa·S at 1300°C, and a melting point of 1000-1060°C.

[0022] In the above scheme, the taper of the continuous casting crystallizer is 1.18-1.22%, the cooling water flow rate of the wide surface of the crystallizer is 3500-3600 L / min, and the cooling water flow rate of the narrow surface of the crystallizer is 600-610 L / min.

[0023] In the above scheme, the superheat degree of molten steel in the continuous casting process is 5-10°C, the temperature of the tundish is 1496-1501°C, the casting speed is 0.9-1m / min, and the fluctuation of the molten steel level in the crystallizer is ≤±2mm.

[0024] In the above scheme, the slab heating temperature is 1230-1280°C, the rough rolling end temperature is 1060-1100°C, the finishing temperature of the finishing rolling is 850-890°C, and the coiling temperature is 570-610°C.

[0025] The components and their content ranges of the present invention are mainly based on the following principles:

[0026] Carbon (C): It is the most important element that determines the fine blanking performance of steel. When the carbon content of steel is increased, ferrite dissolves carbon to form a solid solution structure, which increases the strength of the steel, forms cementite carbide structure, and improves the hardness and wear resistance of the steel. However, when the carbon content is too high, the plasticity and toughness of the steel will decrease, and the fine blanking performance of the steel will also decrease. The most suitable carbon addition amount is 0.39-0.43%, preferably 0.4-0.42%.

[0027] Silicon (Si): It can reduce the austenite phase area. In cementite steel, silicon will not form carbides. Silicon can reduce the thickness of the carburized layer and the concentration of carbon. Silicon has a good deoxidizing effect on molten steel and can improve the strength, hardness and fatigue strength of steel. However, when the silicon content is too high, it will reduce the plasticity and toughness of the steel, produce banded structure, and deteriorate the weldability of the steel. The more suitable silicon addition amount is 0.17-0.3%, preferably 0.2-0.26%.

[0028] Manganese (Mn): It can make the structure of cementite steel uniform and refined, avoid the carbides in the carburized layer from agglomerating into blocks, and easily form spherical cementite, thereby improving the fine blanking performance of steel and the hardenability of steel. However, when the manganese content is too high, the overheating sensitivity and temper brittleness of steel are increased. The more suitable manganese addition amount is 0.7-0.8%, preferably 0.72-0.78%.

[0029] Phosphorus and sulfur (P, S): Phosphorus can easily cause cold brittleness of steel; sulfur can easily cause hot brittleness; therefore, they should be reduced as much as possible.

[0030] Aluminum (ALt): Aluminum is the main deoxidizing element in steel. Aluminum can refine the intrinsic grains of steel, reduce or eliminate the aging phenomenon of steel, and improve the toughness of steel. However, when the aluminum content is too high, the austenite grains tend to grow and coarsen. The most suitable aluminum addition amount is 0.02-0.05%, preferably 0.03-0.045%.

[0031] Molybdenum (Mo): It can improve hardenability and heat strength, and is a strong hardenability element. It can improve the tempering stability of steel, improve the plasticity of steel, reduce the tendency of carbides to form a continuous network on the grain boundary in the carburized layer, reduce the residual austenite in the carburized layer, and relatively increase the wear resistance of the surface layer. Molybdenum can also maintain a relatively stable hardness of steel and increase resistance to degeneration, cracking and wear. However, too high a molybdenum content will reduce the oxidation resistance of steel and increase the deformation resistance of steel to hot working. The most suitable molybdenum addition amount is 0.15-0.25%, preferably 0.18-0.22%.

[0032] Chromium (Cr): Chromium easily forms a continuous solid solution and reduces the austenite phase area. Chromium forms a variety of carbides with carbon, which are not easy to decarburize during heat treatment. They can increase the hardenability of steel, improve the strength and hardness of medium and high carbon steel in the rolled state, improve the wear resistance of steel, and have a certain tempering stability and toughness, while improving the surface processing quality of steel. However, as the chromium content continues to increase, the strength, hardness and plasticity of the steel will decrease, so the chromium content should not be too high. The more suitable chromium addition amount is 0.9-1.1%, preferably 0.95-1.05%.

[0033] The medium-high carbon alloy structural steel for fine blanking of the present invention must ensure its excellent hardenability and hardenability, and at the same time have certain requirements for the surface decarburization layer. In order to ensure its fine blanking performance, the fine blanking steel has very high requirements for the organization, inclusions and segregation. In order to ensure that the shear surface is smooth and flat, the internal organization of the steel is required to be spheroidized pearlite, the spheroidization level is level 3, and the spheroidization rate is ≥95%; the shear zone of the fine blanking parts will undergo severe deformation, the maximum deformation amount of the local area is greater than 50%, and the hardness after deformation is about twice the hardness of the matrix. In order to ensure the continuity of production and the life of the mold, the hardness is required to be lower than 180HV. Large particle inclusions in the steel will cause cracking in the shear area, so the steel D inclusion level is required to be ≤1 level, and the DS inclusion level is required to be ≤0.5 level. Segregation is also a cause of cracking on the fine blanking shear surface, requiring the central segregation level to be ≤C0.5 level and the banded structure to be ≤2 level.

[0034] In order to ensure the above performance, the chemical composition of the steel of the present invention has a high carbon content, and alloying elements Cr and Mo are added. The C element is the most economical strengthening element. It can improve the strength of steel through interstitial solid solution strengthening, increase the carbon content, greatly improve the hardenability of steel, reduce the addition amount of other precious alloys, and reduce production costs. Cr and C form a variety of carbides, which are not easy to decarburize during heat treatment, improve the hardenability, strength, hardness and wear resistance of steel, and have certain tempering stability and toughness. Mo can improve hardenability and thermal strength, improve the tempering stability and plasticity of steel, reduce the tendency of carbides to form a continuous network on the grain boundary in the carburized layer, reduce the residual austenite in the carburized layer, and relatively increase the wear resistance of the surface layer. Mo can also keep the steel with a relatively stable hardness and increase the resistance to degeneration, cracking and wear. C, Cr, and Mo interact with each other at the same time, which can significantly improve the hardenability, strength, hardness, wear resistance, toughness and tempering stability of steel.

[0035] In order to ensure the above performance, the present invention controls the content of S and Alt in molten steel during KR desulfurization, converter smelting, argon blowing and LF refining to improve the purity of molten steel; strictly controls the cycle time and vacuum degree during the vacuum cycle process, which puts forward higher requirements than conventional vacuum treatment in order to control the level of non-metallic inclusions in steel; the liquidus temperature of the steel in the present invention is very low, and in order to control central segregation and banded structure, a low superheat is controlled during the continuous casting process. The closer the temperature of the tundish is to the liquidus temperature of the molten steel, the easier it is to cause the solidification interruption of the molten steel during the casting process. Therefore, the control of the basicity, viscosity and melting point of the crystallizer protective slag, and the matching of low superheat, low pulling speed and crystallizer cooling speed are the key.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1) The present invention adopts a medium-high carbon composition design and adds alloying elements Cr and Mo, which can significantly improve the comprehensive properties of steel such as hardenability, strength, hardness, and toughness. Combined with the design and control of the manufacturing process, a uniform spheroidized pearlite structure is formed, the level of non-metallic inclusions, center segregation and banded structure is reduced, and the spheroidization rate after spheroidizing annealing is increased, thereby ensuring that it has excellent fine blanking performance.

[0038] 2) The medium-high carbon alloy structural steel for fine blanking manufactured by the present invention has a hardness of 150-175HV, a yield strength of 800-900MPa, a tensile strength of 900-1000MPa, an elongation ≥15%, a D-type inclusion level ≤1 level, a DS-type inclusion level ≤0.5 level, a center segregation level ≤C0.5 level, a banded structure level ≤2 level, a spheroidization level of 3 level, and a spheroidization rate ≥95%, and has broad application prospects in the field of automobile fine parts manufacturing. DETAILED DESCRIPTION

[0039] In order to better understand the present invention, the content of the present invention is further explained below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0040] Examples 1-10

[0041] The chemical composition and weight percentage of the medium-high carbon alloy structural steel for fine blanking in Examples 1-10 are shown in Table 1.

[0042] Table 1 Chemical composition of steel

[0043]

[0044] Embodiment 1-10 The method for manufacturing medium-high carbon alloy structural steel for fine blanking comprises the following steps:

[0045] 1) KR hot metal desulfurization: the sulfur content of hot metal after desulfurization is ≤ 0.001%;

[0046] 2) Converter smelting: the final temperature is 1650-1665℃, the final molten steel O content is 0.02-0.04%, aluminum iron is added for deoxidation, the molten steel Alt content is controlled to be 0.005-0.02%, and alloying is performed;

[0047] 3) Argon blowing: After argon blowing, the Alt content of molten steel is 0.02-0.035%;

[0048] 4) LF refining: After refining, the Alt content of the molten steel is 0.025-0.04%, and the S content is ≤0.002%;

[0049] 5) The molten steel is degassed and removed through vacuum circulation, and after alloying and fine-tuning the composition and temperature, it is continuously cast into billets;

[0050] 6) After heating, the slab is subjected to rough rolling, finish rolling, cooling and coiling to obtain medium-high carbon alloy structural steel for fine blanking.

[0051] Table 2 Main process parameters in steps 1-4

[0052]

[0053]

[0054] Table 3 Main process parameters of vacuum cycle and continuous casting in step 5

[0055]

[0056] Table 4 Main process parameters of the mold powder crystallizer in step 5

[0057]

[0058]

[0059] Table 5 Main process parameters in step 6

[0060]

[0061] The performance parameters of the steels manufactured in Examples 1-10 and Comparative Examples 1-2 were tested, and the results are shown in Figure 6 and Table 7.

[0062] Table 6 Performance parameters of steel (I)

[0063]

[0064]

[0065] Table 7 Performance parameters of steel (II)

[0066]

[0067] It can be seen from Tables 6 and 7 that: 1) The microstructure is mainly spheroidal pearlite, and the spheroidization rate is greatly improved. The higher the spheroidization rate of medium and high carbon fine blanking steel, the better the fine blanking performance. The higher the spheroidization rate, the better the fine blanking performance. The higher the spheroidization rate, the better the fine blanking performance. The higher the spheroidization rate, the less coarse strips or flakes of pearlite are unevenly distributed in the ferrite matrix. This structure will reduce the elongation and plasticity of the fine blanking steel, and make the fracture of the steel rough and uneven after fine blanking, resulting in tearing or cracking. 2) The inclusions, especially D and DS inclusions, are greatly reduced, and the steel purity is higher. The D and DS inclusions in the fine blanking steel will cause cracking in the shearing area, which seriously affects the processing. D inclusions ≤1 level and DS inclusions ≤0.5 level can meet the high fine blanking processing requirements. 3) The center segregation reaches the optimal level. The center segregation has a great influence on the performance of medium and high carbon steel fine blanking steel. It will produce a low-plasticity structure such as network cementite in the center of the ingot, which will break during the subsequent processing. The center segregation will cause center looseness and center cracks, further reducing the internal density of the ingot and the subsequent fine blanking performance of the steel. 4) The banded structure reaches the optimal level. The banded structure will make the pearlite and ferrite distributed in a banded form, which will make the mechanical properties of the steel anisotropic. It will cause excessive thermal deformation and stress concentration. It is easy to crack from the junction during pressure processing, affecting the fine blanking performance of the steel. 5) The depth of the decarburization layer is greatly reduced. The decarburization layer will reduce the hardness, strength, fatigue limit, etc. of the surface layer of the fine blanking steel, seriously affecting the fine blanking performance of the steel, and early failure will occur during use. In addition, there is a large residual tensile stress in the decarburization layer, which can easily make the steel scrapped during the fine blanking process. In short, through the optimization of the above-mentioned components and processes, the comprehensive performance of the fine blanking steel described in the present invention has been greatly improved.

[0068] The above embodiments are merely examples for clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above descriptions. It is not necessary and impossible to list all the implementation methods here, and the obvious changes or modifications derived therefrom are still within the protection scope of the invention.

Claims

1. A medium-high carbon alloy structural steel for fine blanking, characterized in that: The chemical composition and weight percentage of the steel are: C: 0.39-0.43%, Si: 0.17-0.3%, Mn: 0.7-0.8%, P: ≤0.015%, S: ≤0.003%, Alt: 0.035-0.05%, Mo: 0.15-0.25%, Cr: 0.9-1.1%, and the balance is Fe and inevitable inclusions; the microstructure of the steel is ferrite and spheroidal pearlite, wherein the volume proportion of spheroidal pearlite is ≥95%; the spheroidization level of the steel is level 3, and the spheroidization rate is ≥95%; The method for manufacturing the medium-high carbon alloy structural steel for fine blanking comprises the following steps: 1) KR hot metal desulfurization: the S content of hot metal after desulfurization is ≤0.001%; 2) Converter smelting: the final temperature is 1650~1665℃, the final molten steel O content is 0.02~0.04%, aluminum iron is added for deoxidation, the molten steel Alt content is controlled to 0.005~0.02%, and alloying is performed; 3) Argon blowing: The Alt content of molten steel after argon blowing is 0.02~0.035%; 4) LF refining: After refining, the Alt content of molten steel is 0.025~0.04%, and the S content is ≤0.002%; 5) After the molten steel is degassed and removed by vacuum circulation, and the composition and temperature are fine-tuned by alloying, it is continuously cast into billets; the vacuum degree of the vacuum cycle is ≤15Pa, the total cycle time is 28~35min, the net cycle time after alloying is ≥10min, and the vacuum end temperature is controlled at 1521~1526℃; the superheat of the molten steel during the continuous casting process is 5~10℃, the temperature of the tundish is 1496~1501℃, the billet drawing speed is 0.9~1m / min, and the fluctuation of the molten steel level in the crystallizer is ≤±2mm; the continuous casting process uses a crystallizer protective slag with a basicity of 0.8~0.9, a viscosity of 0.07~0.17 Pa·s at 1300℃, and a melting point of 1000~1060℃, a crystallizer with a taper of 1.18~1.22%, a wide surface cooling water flow rate of the crystallizer of 3500~3600L / min, and a narrow surface cooling water flow rate of the crystallizer of 600~610L / min; 6) After heating, the slab is subjected to rough rolling, finish rolling, cooling and coiling. The slab heating temperature is 1230~1280℃, the rough rolling end temperature is 1060~1100℃, the finish rolling end temperature is 850~890℃, and the coiling temperature is 570~610℃ to obtain medium-high carbon alloy structural steel for fine blanking.

2. The medium-high carbon alloy structural steel for fine blanking according to claim 1, characterized in that: The chemical composition and weight percentage content of the steel are: C: 0.4-0.42%, Si: 0.2-0.26%, Mn: 0.72-0.78%, P: ≤0.012%, S: ≤0.002%, Alt: 0.035-0.045%, Mo: 0.18-0.22%, Cr: 0.95-1.05%, and the balance is Fe and unavoidable inclusions.

3. The medium-high carbon alloy structural steel for fine blanking according to claim 1, characterized in that: The steel has a D-type inclusion level of ≤1, a DS-type inclusion level of ≤0.5, a central segregation level of ≤C0.5, a banded structure level of ≤2, and a decarburized layer depth of ≤50 μm.

4. The medium-high carbon alloy structural steel for fine blanking according to claim 1, characterized in that: The steel has a hardness of 150-175 HV, a yield strength of 800-900 MPa, a tensile strength of 900-1000 MPa, and an elongation of ≥15%.

5. A method for manufacturing a medium-high carbon alloy structural steel for fine blanking according to any one of claims 1 to 4, characterized in that: The following steps are involved: 1) KR hot metal desulfurization: the S content of hot metal after desulfurization is ≤0.001%; 2) Converter smelting: the final temperature is 1650~1665℃, the final molten steel O content is 0.02~0.04%, aluminum iron is added for deoxidation, the molten steel Alt content is controlled to 0.005~0.02%, and alloying is performed; 3) Argon blowing: The Alt content of molten steel after argon blowing is 0.02~0.035%; 4) LF refining: After refining, the Alt content of molten steel is 0.025~0.04%, and the S content is ≤0.002%; 5) After the molten steel is degassed and removed by vacuum circulation, and the composition and temperature are fine-tuned by alloying, it is continuously cast into billets; the vacuum degree of the vacuum cycle is ≤15Pa, the total cycle time is 28~35min, the net cycle time after alloying is ≥10min, and the vacuum end temperature is controlled at 1521~1526℃; the superheat of the molten steel during the continuous casting process is 5~10℃, the temperature of the tundish is 1496~1501℃, the billet drawing speed is 0.9~1m / min, and the fluctuation of the molten steel level in the crystallizer is ≤±2mm; the continuous casting process uses a crystallizer protective slag with a basicity of 0.8~0.9, a viscosity of 0.07~0.17 Pa·s at 1300℃, and a melting point of 1000~1060℃, a crystallizer with a taper of 1.18~1.22%, a wide surface cooling water flow rate of the crystallizer of 3500~3600L / min, and a narrow surface cooling water flow rate of the crystallizer of 600~610L / min; 6) After heating, the slab is subjected to rough rolling, finish rolling, cooling and coiling. The slab heating temperature is 1230~1280℃, the rough rolling end temperature is 1060~1100℃, the finish rolling end temperature is 850~890℃, and the coiling temperature is 570~610℃ to obtain medium-high carbon alloy structural steel for fine blanking.

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