Ultra-high strength beam steel with excellent plate shape and yield strength ≥1000MPa and production method thereof

Abstract

A super high strength beam steel with excellent plate shape and yield strength ≥1000MPa, the chemical composition and wt% of which are: C: 0.03-0.12%, Si: 0.15-0.25%, Mn: 1.00-1.40%, V: 0.01-0.02%, Cr: 0.1-0.2%, P: ≤0.015%, S: ≤0.002%, B: 0.0015-0.0020%, Als: 0.02-0.10%; production method: casting into billets after smelting; heating the billets; hot rolling after descaling; laminar cooling; coiling; quenching; annealing; cooling in the furnace to ≤100°C and taking out of the furnace. The invention not only has excellent plate shape quality, but also has yield strength ≥1000MPa, tensile strength ≥1100MPa, elongation ≥12%, unevenness ≤1mm / m, and relatively simple chemical composition.

Description

Technical Field

[0001] The present invention relates to automobile steel and a production method, in particular to ultra-high strength beam steel with excellent plate shape and yield strength ≥1000MPa and a production method. Background Art

[0002] Automobile beam steel is mainly used in the longitudinal beams, lining beams, cross beams, front and rear axles and other structural parts 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. 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] Before the present invention was applied, there was a document with Chinese patent number CN201811126140.8, which disclosed "a 900MPa grade hot-rolled automobile beam steel and its production method", and its raw material chemical composition mass percentage is: C: 0.06-0.08%, 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. The hot-rolled steel plate produced by the method has a yield strength of only 650MPa, and because the annealing temperature after ultra-fast cooling is not high, only 300-350°C, it is not conducive to the elimination of internal stress, so the unevenness can only reach 6mm / m.

[0004] The document with Chinese patent application number CN201811124935.5 discloses "a (tensile strength) 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%, Mn 1.5-1.7%, 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 it is directly rolled by ultra-fast cooling process, without subsequent heat treatment to eliminate stress, which is prone to unevenness and excessive internal stress.

[0005] The document with Chinese patent application 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 960MPa, which still cannot meet the demand for ultra-high strength beam steel with a yield strength of 1000MPa. In addition, it contains 0.3% to 0.5% Mo element, and the alloy cost is high. The lower heating and final rolling temperatures are not conducive to high-precision plate shape control. In addition, the tempering process is adopted and the tempering holding time is only 20 minutes, which is too short and is not conducive to eliminating the internal stress of the steel plate. Summary of the invention

[0006] The present invention aims to overcome the shortcomings of the prior art and provide an ultra-high strength beam steel having not only excellent plate quality but also yield strength ≥1000MPa, tensile strength ≥1100MPa, elongation ≥12% and unevenness ≤1mm / m and a production method.

[0007] Measures to achieve the above objectives:

[0008] The invention discloses an ultra-high strength beam steel with excellent plate shape and yield strength ≥1000MPa, wherein the components and weight percentage contents thereof are as follows: C: 0.03-0.12%, Si: 0.15-0.25%, Mn: 1.00-1.40%, V: 0.01-0.02%, Cr: 0.1-0.2%, P: ≤0.015%, S: ≤0.002%, B: 0.0015-0.0020%, Als: 0.02-0.10%, and the remainder is Fe and unavoidable inclusions.

[0009] A method for producing ultra-high strength beam steel with excellent plate shape and yield strength ≥1000MPa, comprising the following steps:

[0010] 1) After converter smelting and LF+RH refining, continuous casting into billets;

[0011] 2) heating the ingot at a temperature of 1230-1280°C;

[0012] 3) After descaling, hot rolling is carried out, using traditional two-stage rolling, and the final rolling temperature is 850-900℃;

[0013] 4) laminar flow, cooling to coiling temperature at a cooling rate of 20-30°C / s;

[0014] 5) Coil the steel sheet and control the coiling temperature at 650-700°C;

[0015] 6) Perform quenching and control the quenching temperature at 880-900°C;

[0016] 7) Annealing, controlling the annealing temperature at 600-650°C, and keeping at this temperature for 3-6 hours;

[0017] 8) Cool down in the furnace to ≤100℃ and take out of the furnace.

[0018] Preferably, the heating temperature of the casting is between 1238°C and 1265°C.

[0019] Preferably, the coiling temperature is between 664 and 683°C.

[0020] Preferably, the annealing temperature is between 617°C and 638°C.

[0021] Functions and mechanisms of the components and main processes in the present invention

[0022] 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, adding too much is detrimental to the plasticity and surface quality of the steel. Therefore, the present invention selects the Si content to be 0.15-0.25%.

[0023] 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%.

[0024] 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 present invention controls the Cr content to 0.1% to 0.2%.

[0025] 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 0.01-0.02%.

[0026] 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 0.0015% to 0.0020%.

[0027] 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 the magnetic properties and strength of steel. In particular, the Mn content in the steel of the present invention is relatively high, and the control of S should be more stringent, and the P and S contents of the steel are controlled to P: ≤0.015%, S: ≤0.002%.

[0028] The reason why the present invention controls the heating temperature of the casting billet at 1230-1280°C, preferably: the billet heating temperature is at 1238-1265°C, is because it is to ensure 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.

[0029] The reason why the final rolling temperature is controlled at 850-900° C. in the present invention is to facilitate obtaining lower original internal stress and excellent rolled plate shape.

[0030] The reason why the coiling temperature is controlled at 650-700°C, preferably at 664-683°C, is to make the steel coil cool evenly through a higher coiling temperature, so as to avoid large internal stress and plate shape defects caused by uneven temperature when the steel plate is cooled to room temperature.

[0031] The quenching heating temperature of the present invention is 880-900° C., mainly to obtain finer original austenite grains, thereby obtaining a refined quenched martensite structure and improving the strength of the steel.

[0032] The reason why the present invention controls the annealing heating temperature to 600-650°C, preferably the annealing temperature is 617-638°C, and the holding time is 3-6h is that the supersaturated carbon atoms in the quenched martensite are desolvated to form fine carbide particles through the annealing process, which further improves the strength of the steel plate and improves the plasticity of the steel. At the same time, 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.

[0033] Compared with the prior art, the present invention not only has excellent plate quality, but also has yield strength ≥1000MPa, tensile strength ≥1100MPa, elongation ≥12%, unevenness ≤1mm / m, and relatively simple chemical composition. DETAILED DESCRIPTION

[0034] The present invention is described in detail below:

[0035] Table 1 is a list of chemical components of various embodiments and comparative examples of the present invention;

[0036] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0037] Table 3 is a table of performance test results of various embodiments of the present invention and comparative examples.

[0038] Each embodiment of the present invention is produced according to the following steps

[0039] A method for producing ultra-high strength beam steel with excellent plate shape and yield strength ≥1000MPa, comprising the steps of: 1) continuous casting into billets after converter smelting and LF+RH refining;

[0040] 2) heating the ingot at a temperature of 1230-1280°C;

[0041] 3) After descaling, hot rolling is carried out, using traditional two-stage rolling, and the final rolling temperature is 850-900℃;

[0042] 4) laminar flow, cooling to coiling temperature at a cooling rate of 20-30°C / s;

[0043] 5) Coil the steel sheet and control the coiling temperature at 650-700°C;

[0044] 6) Perform quenching and control the quenching temperature at 880-900°C;

[0045] 7) Annealing, controlling the annealing temperature at 600-650°C, and keeping at this temperature for 3-6 hours;

[0046] 8) Cool down in the furnace to ≤100℃ and take out of the furnace.

[0047] Table 1 List of chemical compositions of various embodiments and comparative examples of the present invention (wt%)

[0048]

[0049] Table 2 Main process parameters and performance test list of various embodiments and comparative examples of the present invention

[0050]

[0051]

[0052] Table 3 List of mechanical properties test results of various embodiments of the present invention and comparative examples

[0053]

[0054] It can be seen from the results in Table 3 above that the steel of the present invention has a yield strength of ≥1000 MPa, a tensile strength of ≥1100 MPa, an elongation of ≥12%, and a roughness of ≤1 mm / m, which can meet the demand for ultra-high-strength beam steel with excellent plate shape.

[0055] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. An ultra-high strength beam steel with excellent plate shape and yield strength ≥1000MPa, wherein the components and weight percentage contents are: C: 0.03-0.09%, Si: 0.15-0.24%, Mn: 1.00-1.40%, V: 0.01-0.019%, Cr: 0.1-0.2%, P: ≤0.015%, S: ≤0.002%, B: 0.0015-0.0020%, Als: 0.05-0.10%, and the rest is Fe and unavoidable inclusions; Its production steps: 1) After converter smelting and LF+RH refining, continuous casting into billets; 2) Heating the ingot at a temperature of 1253-1280°C; 3) After descaling, hot rolling is carried out, using traditional two-stage rolling, and the final rolling temperature is 862-894°C; 4) Perform laminar flow and cool to coiling temperature at a cooling rate of 20-30°C / s; 5) Coil the steel sheet and control the coiling temperature between 650 and 700 °C; 6) Perform quenching and control the quenching temperature at 880-900°C; 7) Perform annealing, control the annealing temperature at 600-650°C, and keep it at this temperature for 3-6 hours; 8) Cool down in the furnace to ≤100℃ and take out of the furnace.

2. A method for producing a high strength beam steel with excellent plate shape and yield strength ≥1000 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 1253-1280°C; 3) After descaling, hot rolling is carried out, using traditional two-stage rolling, and the final rolling temperature is 862-894°C; 4) Perform laminar flow and cool to coiling temperature at a cooling rate of 20-30°C / s; 5) Coil the steel sheet and control the coiling temperature between 650 and 700 °C; 6) Perform quenching and control the quenching temperature at 880-900°C; 7) Perform annealing, control the annealing temperature at 600-650°C, and keep it at this temperature for 3-6 hours; 8) Cool down in the furnace to ≤100℃ and take out of the furnace.

3. The method for producing a high strength beam steel with excellent plate shape and yield strength ≥1000 MPa as claimed in claim 2, characterized in that: The coiling temperature is between 664 and 683°C.

4. The method for producing a super high strength beam steel with excellent plate shape and yield strength ≥1000 MPa as claimed in claim 2, characterized in that: The annealing temperature is between 617 and 638°C.

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