Automobile frame steel with yield strength larger than or equal to 1100 MPa and production method

Abstract

The invention discloses steel for an automobile frame with yield strength larger than or equal to 1100 MPa. The steel comprises, by weight, 0.03%-0.12% of C, 0.25%-0.35% of Si, 1.0%-1.4% of Mn, 0.01%-0.02% of Nb, 0.10%-0.25% of Mo, 0.02%-0.03% of V, 0.001%-0.01% of Ti, 0.2%-0.3% of Cr, smaller than or equal to 0.015% of P, smaller than or equal to 0.002% of S, 0.0020%-0.0025% of B, 0.02%-0.10% of Als and the balance Fe. The production method comprises the following steps: continuously casting into a blank after converter smelting and LF + RH refining; heating the casting blank; performing hot rolling after descaling; carrying out online quenching; performing coiling; annealing is conducted; cooling in the furnace; and straightening. The yield strength is larger than or equal to 1100 MPa, the tensile strength is larger than or equal to 1200 MPa, the elongation is larger than or equal to 10%, the unevenness is smaller than or equal to 2 mm / m, the requirements of the market can be completely met, economy is achieved, and the automobile weight reduction effect is further enhanced.

Description

Technical Field

[0001] The present invention relates to a steel for automobile parts and a production method thereof, in particular to a steel for automobile beams with a yield strength of ≥1100MPa and a production method thereof. Background Art

[0002] Automobile beam steel is mainly used in the longitudinal beams, lining beams, cross beams and front and rear axles on the chassis of heavy trucks. Its strength is crucial to the load-bearing capacity and driving safety of the entire vehicle. At the same time, in order to reduce the energy consumption and environmental pollution of the automobile industry, the use of ultra-high-strength thin steel plates instead of low-strength thick steel plates is a trend in modern automobile plate research. The lightweighting of semi-trailer longitudinal beams is mainly achieved by replacing the original longitudinal beam materials with high-strength steel. Replacing 700MPa beam steel with 1100MPa can achieve a weight reduction of about 20%. With the increase in strength, beam steel is generally formed by welding, so there are higher requirements for the unevenness of the steel plate, but obtaining ultra-high strength and excellent plate quality at the same time is the difficulty in the development of ultra-high-strength beam steel.

[0003] After searching:

[0004] The Chinese patent application number CN201111126140.1 discloses "a 900MPa hot-rolled automobile beam steel and its production method", and its raw material chemical composition mass percentage is: C: 0.06-0.01%, Si≤0.10%, Mn: 1.5-1.7%, S≤0.005%, P≤0.015%, Nb: 0.040-0.050%, Ti: 0.07-0.09%, N≤40ppm, Als: 0.025-0.060%, H≤0.002%, and the rest is Fe. However, the yield strength of the hot-rolled steel plate produced by the method described in the document is only 650MPa, and the unevenness is only ≤3mm / m.

[0005] The Chinese patent document with the patent number CN201111124935.5 discloses "a 1000MPa grade hot-rolled automobile beam steel and its manufacturing method", and its raw material chemical composition mass percentage (%) is: C 0.10-0.20%, Si≤0.10%, Mn1.5-1.7%, S≤0.005%, P≤0.015%, Nb 0.045-0.055%, Ti 0.08-0.10%, N≤40ppm, Als 0.025-0.060%, H≤0.002%, and the rest is Fe. However, the yield strength of the hot-rolled steel plate produced by the method described in the document is only 750MPa, and the unevenness is only ≤3mm / m.

[0006] The Chinese patent document with patent number CN201511020842.4 discloses "a kind of automobile beam steel with yield strength of 960MPa and its production method", and its chemical composition by weight percentage is: C: 0.12%~0.14%; Si: 0.15%-0.35%; Mn: 1.2%~1.5%; Nb: 0.02%~0.04%; Mo: 0.3%~0.5%; V: 0.08%~0.15%; Ti: 0.01%~0.04%; Al: 0.01%~0.06%; Cr: 0.4%-0.6%; B: 0.0015%-0.0025%; P: ≤0.02%; S: ≤0.01%; N: ≤0.008%, and the rest is Fe and unavoidable impurities. However, the yield strength of the heat-treated steel plate produced by the method described in the document is only 960MPa, which still cannot meet the demand for ultra-high strength beam steel with a yield strength of 1100MPa. In addition, it contains 0.3% to 0.5% Mo element, the alloy cost is high, and the lower heating and final rolling temperatures are used, which is not conducive to high-precision plate shape control. The tempering process is used and the tempering holding time is only 20 minutes, which is short and is not conducive to eliminating the internal stress of the steel plate. Summary of the invention

[0007] The present invention aims to overcome the shortcomings of the prior art, such as the large number of alloy elements, low yield strength and large unevenness of the steel plate, and provide a steel for automobile beams with a yield strength of ≥1100MPa, a tensile strength of ≥1200MPa, an elongation of ≥10%, and an unevenness of ≤2mm / m, i.e., a steel with a yield strength of ≥1100MPa and excellent plate quality, and a production method.

[0008] Measures to achieve the above objectives:

[0009] A steel for automobile beams with a yield strength of ≥1100MPa, the components and weight percentage contents of which are: C: 0.03-0.12, Si: 0.25-0.35, Mn: 1.0-1.4, Nb: 0.01-0.02%, Mo: 0.10-0.25, V: 0.02-0.03, Ti: 0.001-0.01, Cr: 0.2-0.3, P: ≤0.015, S: ≤0.002, B: 0.0020-0.0025, Als: 0.02-0.10, and the rest are Fe and impurities.

[0010] A method for producing a steel for automobile beams with a yield strength of ≥1100MPa, comprising the following steps:

[0011] 1) After converter smelting and LF+RH refining, continuous casting into billets;

[0012] 2) heating the ingot at a temperature of 1230-1280°C;

[0013] 3) After descaling, hot rolling is carried out, using traditional two-stage rolling, and the final rolling temperature is 850-900℃;

[0014] 4) Perform online quenching and cool to the coiling temperature at a cooling rate of 50-70°C / s;

[0015] 5) Coil the steel sheet and control the coiling temperature at 100-200°C;

[0016] 6) Annealing, controlling the annealing temperature at 550-600°C, and keeping at this temperature for 3-6 hours;

[0017] 7) Cool down in the furnace to ≤100℃ and take out of the furnace;

[0018] 8) Perform straightening.

[0019] Preferably, the heating temperature of the casting is between 1238°C and 1265°C.

[0020] Preferably, the annealing temperature is between 570°C and 600°C.

[0021] Preferably, the annealing holding time is 4.5 to 6 hours.

[0022] Functions and mechanisms of the components and main processes in the present invention

[0023] Carbon (C) of the present invention: Carbon is one of the indispensable cheap elements in steel for improving the strength of steel. Too high carbon content will reduce the toughness and welding performance of the steel, while too low carbon content cannot meet the strength requirements. Therefore, the present invention limits the carbon content to 0.03% to 0.12%.

[0024] Silicon (Si) of the present invention: Si has a good deoxidizing effect in steel, and its solid solution strengthening coefficient in ferrite is higher than that of Mn. It is an effective and inexpensive strengthening element and can effectively improve hardenability. However, excessive addition is detrimental to the plasticity and surface quality of steel. Therefore, the present invention selects the Si content to be 0.25% to 0.35%.

[0025] Manganese (Mn) in the present invention: Mn can improve hardenability, while making the quenched and tempered structure uniform and refined, and improving the yield strength and tensile strength of steel. However, excessive addition of Mn is prone to segregation, which is unfavorable to toughness and welding performance. Therefore, the present invention controls the Mn content to 1.0% to 1.4%.

[0026] Niobium (Nb) of the present invention: A certain amount of Nb can significantly refine the grains and improve the tensile strength. During the controlled rolling process, Nb can increase the recrystallization temperature of the steel and reduce the mill load. At the same time, by inhibiting recrystallization and preventing grain growth, it can refine the austenite grain size, providing a basis for obtaining a refined quenching structure. The Nb content of the present invention is 0.01% to 0.02%.

[0027] Molybdenum (Mo) of the present invention: Molybdenum exists in the solid solution and carbide of steel and has a solid solution strengthening effect. When Mo and Nb are added at the same time, Mo can increase the inhibition of austenite recrystallization during the controlled rolling process, thereby promoting the refinement of austenite microstructure. At the same time, Mo is conducive to the stability of the performance of steel after annealing. Therefore, the content of Mo in the present invention is 0.10% to 0.25%

[0028] Chromium (Cr) of the present invention: Cr slows down the decomposition rate of austenite, significantly improves the hardenability of steel, improves the strength of steel plate, and has low price. Therefore, the Cr content is controlled to 0.2% to 0.3% in the present invention.

[0029] Vanadium (V) in the present invention: when V is less than 0.03%, most of V exists in a solid solution state, which can significantly increase the hardenability in heat-treated steel and has a good strengthening effect. Therefore, the present invention controls the V content to 0.02% to 0.03%.

[0030] Titanium (Ti) in the present invention: In the present invention, micro-Ti treatment is adopted, and 0.001% to 0.01% of Ti element is added to the steel to improve the welding performance.

[0031] Boron (B) in the present invention: The main function of B is to improve the hardenability of steel. Boron, as a surface active element, is adsorbed on the austenite grain boundary, delaying the transformation of austenite to ferrite. Its segregation at the austenite grain boundary hinders the nucleation of ferrite and is conducive to the formation of martensite, thereby improving the structure strengthening effect. However, if the B content is higher than the specified value, the hardenability decreases, and a low melting point eutectic will be formed, which is concentrated at the boundary of the grains, thereby causing hot brittleness. Therefore, the boron content of the present invention is limited to 0.0020% to 0.0025%.

[0032] The phosphorus (P) content of the present invention is ≤0.015%, and the sulfur (S) content is ≤0.002%. Phosphorus is easy to cause segregation in steel, and sulfur is easy to combine with manganese to form MnS inclusions, both of which are unfavorable to strength. Therefore, the present invention should minimize the adverse effects of phosphorus and sulfur elements on strength, and control the P and S contents of steel to P: ≤0.015%, S: ≤0.002%.

[0033] The reason why the present invention controls the heating temperature of the casting billet at 1230-1280°C is that it ensures complete solid solution and sufficient austenitization of alloy elements, while improving the uniformity of the slab temperature, reducing deformation resistance and rolling load, and laying the foundation for controlling the rolled plate shape.

[0034] The reason why the final rolling temperature is controlled at 850-900° C. in the present invention is to obtain lower original internal stress and excellent rolled plate shape, and at the same time obtain finer original austenite grains, thereby obtaining a refined quenched martensite structure and improving the strength of the steel.

[0035] The reason why the present invention controls the online quenching cooling speed to 50-70°C / s and the coiling temperature to 100-200°C is to make the organizational transformation uniform and reduce the internal stress and plate shape defects caused by organizational unevenness.

[0036] The present invention controls the annealing temperature at 550-600°C and the holding time at 3-6h. Through the annealing process, the supersaturated carbon atoms in the quenched martensite are desolvated to form fine carbide particles, further improving the strength of the steel plate and improving the plasticity of the steel. Compared with tempering, the use of a longer annealing time can better eliminate the internal stress of the steel plate to obtain the final excellent plate shape.

[0037] Compared with the prior art, the yield strength of the present invention is ≥1100MPa, the tensile strength is ≥1200MPa, the elongation is ≥10%, and the unevenness is ≤2mm / m, which can fully meet the needs of the market and is economical, further enhancing the weight reduction effect of automobiles. DETAILED DESCRIPTION

[0038] The present invention is described in detail below:

[0039] Table 1 is a list of chemical components of various embodiments and comparative examples of the present invention;

[0040] Table 2 is a list of main processes and performance test results of various embodiments and comparative examples of the present invention.

[0041] Each embodiment of the present invention is produced according to the following steps

[0042] 1) After converter smelting and LF+RH refining, continuous casting into billets;

[0043] 2) heating the ingot at a temperature of 1230-1210°C;

[0044] 3) After descaling, hot rolling is carried out, using traditional two-stage rolling, and the finishing rolling temperature is 150-900℃;

[0045] 4) Perform online quenching and cool to the coiling temperature at a cooling rate of 50-70°C / s;

[0046] 5) Coil the steel sheet and control the coiling temperature at 100-200°C;

[0047] 6) Annealing, controlling the annealing temperature at 550-600°C, and keeping at this temperature for 3-6 hours;

[0048] 7) Cool down in the furnace to ≤100℃ and take out of the furnace;

[0049] 1) Straighten.

[0050] Table 1 Chemical composition list of various embodiments and comparative examples of the present invention (wt%)

[0051]

[0052] Table 2 Main process parameters and performance test list of various embodiments and comparative examples of the present invention

[0053]

[0054]

[0055] It can be seen from the results in Table 2 above that the steel of the present invention has a yield strength of ≥1100 MPa, a tensile strength of ≥1200 MPa, an elongation of ≥10%, and a roughness of ≤2 mm / m, which can meet the demand for ultra-high-strength beam steel with excellent plate shape.

[0056] The above embodiments are only the best examples and are not all of the technical solutions of the present invention.

Claims

1. A steel for automobile beams with a yield strength of ≥1100MPa, wherein the components and weight percentages thereof are: C: 0.03-0.12, Si: 0.25-0.35, Mn: 1.0-1.4, Nb: 0.01-0.02%, Mo: 0.10-0.25, V: 0.02-0.03, Ti: 0.001-0.01, Cr: 0.2-0.3, P: ≤0.015, S: ≤0.002, B: 0.0020-0.0025, Als: 0.02~0.10, the rest is Fe and impurities.

2. A method for producing a steel for automobile beams with a yield strength of ≥1100 MPa as claimed in claim 1, comprising the following steps: 1) After converter smelting and LF+RH refining, continuous casting into billets; 2) heating the ingot at a temperature of 1230-1280°C; 3) After descaling, hot rolling is carried out, using traditional two-stage rolling, and the final rolling temperature is 850-900℃; 4) Perform online quenching and cool to the coiling temperature at a cooling rate of 50-70°C / s; 5) Coil the steel sheet and control the coiling temperature at 100-200°C; 6) Annealing, controlling the annealing temperature at 550-600°C, and keeping at this temperature for 3-6 hours; 7) Cool down in the furnace to ≤100℃ and take out of the furnace; 8) Perform straightening.

3. A method for producing a steel for automobile beams with a yield strength of ≥1100 MPa as claimed in claim 2, Features: The heating temperature of the casting is between 1238 and 1265°C.

4. A method for producing a steel for automobile beams with a yield strength of ≥1100 MPa as claimed in claim 2, Features: The annealing temperature is between 570 and 600°C.

5. A method for producing a steel for automobile beams with a yield strength of ≥1100 MPa as claimed in claim 2, Features: The annealing holding time is 4.5 to 6 hours.

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