Hot-rolled automobile steel with good surface and fatigue performance and production method of hot-rolled automobile steel

Abstract

The invention discloses hot-rolled automobile steel with good surface and fatigue properties. The hot-rolled automobile steel comprises the following components in percentage by weight: 0.03 to 0.065 percent of C, 1.2 to 1.8 percent of Mn, less than or equal to 0.013 percent of P, less than or equal to 0.003 percent of S, 0.25 to 0.45 percent of Al, 0.11 to 0.18 percent of Ti, 0.1 to 0.5 percent of Cr, 0.02 to 0.05 percent of Nb, 0.001 to 0.005 percent of N, and the balance of Fe. The production method comprises the following steps: 1) smelting in a converter and then carrying out vacuum treatment; heating the casting blank; rough rolling and ultrahigh pressure descaling; finish rolling; carrying out descaling by using ultra-high pressure water after finish rolling; cooling is performed; and coiling. According to the hot-rolled automobile steel, the yield strength is larger than or equal to 750 MPa, the tensile strength ranges from 800 MPa to 950 MPa, the ductility is larger than or equal to 16%, the hot-rolled automobile steel does not crack when the cold bending radius is half of the thickness and the cold bending angle is 180 degrees, the surface roughness is smaller than or equal to 1.5, the microhardness HV0.025 of the position, 30 microns away from the surface layer, of the hot-rolled automobile steel is larger than or equal to 230, and the symmetric fatigue limit of a steel plate is larger than or equal to 330 MPa in the state that the steel plate is provided with oxide scales.

Description

Technical Field

[0001] The invention belongs to steel for automobile field and a production method, and specifically relates to hot-rolled automobile steel with good surface and fatigue properties and a production method. Background Art

[0002] In the automotive industry, in order to vigorously promote the lightweighting of automobiles, so as to reduce the emission of pollutants and protect the environment, various technical measures are constantly being proposed in the automotive field to meet the development needs of the automotive industry and human needs for the environment.

[0003] At present, a large number of 600MPa grade hot-rolled steel plates are used in the manufacture of automobile beams, wheels, bridge shells, brackets and other parts. At present, although its strength can be improved, its fatigue performance cannot be improved synchronously, and may even decrease, making the steel plate unstable, which hinders the use of higher strength hot-rolled plates in the automobile manufacturing field. After a lot of research and analysis, it is believed that the fundamental reason for this phenomenon is the softening of the surface structure of the steel plate, that is, the hardness of the surface of the steel plate is low, making the surface of the steel plate a weak link in the fatigue process, that is, fatigue cracks are first generated on the surface of the steel plate. In order to solve the problems existing in the prior art and improve the fatigue performance of the steel plate, the technicians in this field often use surface strengthening means for the steel plate, which include: surface shot peening strengthening, surface heat treatment and other technical measures. Although its technical means can solve the problem to a certain extent, the disadvantage is that the secondary processing of the steel plate increases the production cost.

[0004] After searching: the document with Chinese patent application number CN201910473681.6 discloses "A dispersion nano-strengthened 690 steel and its manufacturing method", whose composition is: C 0.02~0.03%, Mn 0.5~2.0%, Al 0.04~0.10%, V0.002~0.013%, Nb 0.01~0.3%, Ti 0.005~0.3%, N 0.001~0.01%, and its process is melt casting and TMCP process. Although its strength level is 690MPa, it does not involve fatigue performance, so it is still not suitable for making beam products with high forming requirements.

[0005] The present invention achieves low surface roughness and high surface hardness control of hot-rolled steel plates, so that the steel plates have excellent high fatigue performance and meet the needs of automobile companies for fatigue properties when lightweighting. Summary of the invention

[0006] In view of the obstacles to improving the strength level of hot-rolled steel plates in the prior art due to low fatigue performance, or the high production cost caused by the need for heat treatment, the present invention provides a hot-rolled automobile steel with good surface and fatigue performance and a production method. Under the premise of ensuring yield strength ≥750MPa, tensile strength between 800 and 950MPa, and elongation ≥16%, it can also ensure that the surface roughness is ≤1.5, the microhardness HV0.025 at 30μm of the surface layer is ≥230, and the symmetrical fatigue limit of the steel plate in the state with iron oxide scale is ≥330MPa.

[0007] The measures adopted by the present invention to achieve the above-mentioned purpose are as follows:

[0008] A hot-rolled automobile steel with good surface and fatigue properties, the components and weight percentages of which are:

[0009] C: 0.03-0.065%, Mn: 1.2-1.8%, P≤0.013%, S≤0.003%, Al: 0.25-0.45%, Ti: 0.11-0.18%, Cr: 0.1-0.5%, Nb: 0.02-0.05%, N: 0.001-0.005%, the remainder is Fe and unavoidable impurities, and Ti / C is between 2.8-3.3, Ti / Cr is between 0.4-0.9; Mechanical properties: yield strength ≥750MPa, tensile strength is between 800-950MPa, elongation A≥16%, surface roughness ≤1.5, microhardness HV0.025≥230 at 30μm of the surface layer of the steel plate, and the symmetrical fatigue limit of the steel plate with iron oxide scale is ≥330MPa.

[0010] Preferably, the weight percentage of Cr is 0.13-0.46%.

[0011] The method for producing the above-mentioned hot-rolled automobile steel with good surface and fatigue properties comprises the following steps:

[0012] 1) After converter smelting, vacuum treatment is performed, and the treatment time is controlled to be greater than 15 minutes;

[0013] 2) After continuous casting, the billet is heated, during which the following controls are made: the air-fuel ratio is ≤1.3, the billet entering the furnace temperature is between 300 and 900°C, the billet heating temperature is between 1250 and 1320°C, and the billet is kept at this temperature for 60 to 70 minutes, and the billet surface temperature fluctuation is within ±25°C;

[0014] 3) Rough rolling is performed on the heated ingot: the rough rolling is performed in 3 to 7 passes, and the surface temperature of the steel plate after the last two passes is controlled: the surface temperature of the steel plate after the second pass is 1080 to 1120°C, and the surface temperature of the steel plate after the first pass is 1020 to 1090°C;

[0015] 4) Ultra-high pressure descaling after rough rolling, the water pressure is controlled at 350-380 bar, and the nozzle angle is 10-25°;

[0016] 5) Finish rolling is performed in 6 to 7 passes, and the surface temperature of the steel plate after rolling in the last four passes is controlled: the surface temperature of the steel plate in the fourth pass is controlled at 950 to 1035° C., the surface temperature of the steel plate in the third pass is controlled at 920 to 990° C., the surface temperature of the steel plate in the second pass is controlled at 890 to 960° C., and the surface temperature of the steel plate in the first pass is controlled at 860 to 940° C.;

[0017] 6) Descaling with high pressure water after finishing rolling;

[0018] 7) Cooling, cooling the surface temperature of the steel plate to the coiling temperature at a cooling rate of 80 to 150°C / s;

[0019] 8) Coil the steel sheet and control the coiling temperature between 520 and 620°C.

[0020] Preferably: the working roll surface is cleaned before finishing rolling, and the oxide film on the finishing rolling working roll surface is intact and free of foreign matter adhesion.

[0021] Preferably, the surface temperature of the steel plate after the last two passes of rough rolling is controlled: the surface temperature of the steel plate after the second pass is between 1087 and 1110°C, and the surface temperature of the steel plate after the first pass is between 1025 and 1083°C.

[0022] Preferably: the surface temperature of the steel plate after the last four passes of finishing rolling is controlled: the surface temperature of the steel plate after the fourth pass is controlled at 958-1031°C, the surface temperature of the steel plate after the third pass is controlled at 926-982°C, the surface temperature of the steel plate after the second pass is controlled at 895-953°C, and the surface temperature of the steel plate after the first pass is controlled at 864-936°C.

[0023] Preferably, the coiling temperature is between 526 and 610°C.

[0024] The functions and mechanisms of the components and main processes in the present invention are:

[0025] Carbon: Carbon is a cheap solid solution strengthening element. If its content is less than 0.03%, it cannot meet the material strength requirements; if its content is greater than 0.065%, it will cause serious surface decarburization and soften the surface hardness. Therefore, its content is limited to 0.03-0.065%.

[0026] Manganese: Manganese is an effective solid solution strengthening element that can reduce the austenite transformation temperature, refine the grains, and improve the hardness uniformity of the core and surface of the steel plate. If its content is less than 1.2%, it cannot meet the material strength requirements; however, adding too much manganese will reduce the toughness of the steel, so its upper limit is set at 1.8%, so its content is limited to the range of 1.2-1.8%.

[0027] Phosphorus: Phosphorus is a harmful element in steel and can easily cause center segregation of the ingot. In order to avoid deterioration of cold bending formability and toughness, the upper limit of its content is set at 0.013%.

[0028] Sulfur: Sulfur is a very harmful element. Sulfur in steel often exists in the form of manganese sulfide. This sulfide inclusion is very detrimental to the fatigue performance of steel and causes anisotropy of performance. Therefore, in order to meet the requirements of fatigue performance, the sulfur content in steel is controlled below 0.003%.

[0029] Aluminum: Aluminum is a good deoxidizing element that can reduce the austenite phase area. When the Als content is less than 0.25%, it cannot exert its effect; on the other hand, since adding a large amount of aluminum easily forms alumina agglomerates, the upper limit of Als is set at 0.45%. Therefore, the Als content is limited to the range of 0.25-0.45%.

[0030] Titanium: Titanium is a good fine-grain strengthening element. When the Ti content is less than 0.11%, it is difficult to exert its effect. When the Ti content is higher than 0.15%, it is easy to form metal inclusions containing titanium. Therefore, the Ti content is limited to 0.11-0.15%.

[0031] Chromium: Chromium is a good solid solution strengthening element that can improve surface hardenability and surface hardness. It also forms chromium iron oxide on the surface of the steel plate to reduce surface roughness. When the chromium content is less than 0.1%, it is difficult to achieve the desired effect. When the chromium content is higher than 0.5%, the toughness of the steel plate will be reduced. Therefore, the chromium content is limited to 0.1-0.5%, and preferably the weight percentage of Cr is 0.13-0.46%.

[0032] Nitrogen: Nitrogen and titanium form nano-scale precipitates to increase the hardness of the steel plate. Its content is limited to 0.001-0.005%.

[0033] The reason why the present invention limits Ti / C to 2.8-3.3 is that Ti and C elements form TiC precipitates. If the Ti / C ratio is lower than 2.8, TiC precipitation is insufficient, resulting in uneven precipitation at different thicknesses; if the ratio is higher than 3.3, the Ti element is not easily fully dissolved during heating, affecting the precipitation efficiency, so Ti / C is set to 2.8-3.3.

[0034] The reason why the present invention limits the Ti / Cr ratio to 0.4-0.9 is that the Cr element is used to slow down the growth rate of TiC and reduce the size of TiC precipitates; if Ti / Cr is less than 0.4, the performance of the steel will deteriorate; if Ti / Cr is greater than 0.9, it is difficult to exert the effect, so Ti / Cr is set to 0.4-0.9.

[0035] In addition to limiting the range of the above chemical components, from the viewpoint of improving the formability and economy of the material, the present invention does not add precious alloy elements such as Cu, Ni, and Mo.

[0036] The key technical point of the present invention is to control the heating temperature, time and air-fuel ratio of the ingot. The heating temperature and time should ensure that the alloy elements are fully dissolved to ensure the strength of the matrix, and should be coordinated with the air-fuel ratio parameters to prevent the generation of a large amount of iron oxide scale, deteriorate the surface roughness, and prevent serious surface decarburization and reduce the surface hardness.

[0037] The reason why the present invention controls the surface temperature of the steel plate after the last two passes of rough rolling: the surface temperature of the steel plate after the second pass is 1080-1120°C, and the surface temperature of the steel plate after the first pass is 1020-1090°C, preferably the surface temperature of the steel plate after the last two passes of rough rolling: the surface temperature of the steel plate after the second pass is 1087-1110°C, and the surface temperature of the steel plate after the first pass is 1025-1083°C, is to keep the surface structure consistent with the core structure, thereby improving the surface hardness.

[0038] The reason why the present invention controls the ultra-high pressure descaling water pressure after rough rolling to 350-380 bar and the nozzle angle to 10-25 degrees is that the surface quality of the rolling roller is controlled by high-pressure descaling water, thereby controlling the surface roughness of the steel plate, reducing pits on the steel plate surface, and avoiding fatigue fracture induced by them.

[0039] The reason why the present invention controls the surface temperature of the steel plate after the last four passes of finishing rolling: the surface temperature of the steel plate in the fourth pass is controlled at 950-1035°C, the surface temperature of the steel plate in the third pass is controlled at 920-990°C, the surface temperature of the steel plate in the second pass is controlled at 890-960°C, and the surface temperature of the steel plate in the first pass is controlled at 860-940°C, is to keep the surface structure consistent with the core structure, thereby improving the surface hardness.

[0040] The present invention controls the surface temperature of the steel plate to be cooled to the coiling temperature at a cooling rate of 80-150°C / s after fine rolling, and increases the degree of supercooling, so that austenite undergoes a rapid medium-low temperature phase transformation, transforming into a ferrite structure or bainite, thereby increasing the surface hardness.

[0041] The reason why the coiling temperature is controlled at 520-620° C. in the present invention is that the temperature range is suitable for the composite precipitation of titanium, carbon, niobium and the like at different thicknesses in the present invention to improve the hardness of the matrix.

[0042] Compared with the prior art, the hot-rolled automobile steel provided by the present invention not only has a yield strength of ≥750MPa, a tensile strength of 800-950MPa, an elongation of ≥16%, and does not crack when the cold bending radius is half of the thickness and the cold bending angle is 180 degrees, but also has a surface roughness of ≤1.5, a microhardness of HV0.025 ≥230 at 30μm on the surface, and a symmetrical fatigue limit of the steel plate with iron oxide scale of ≥330MPa. The present invention achieves low surface roughness and high surface hardness control of hot-rolled steel plates, a good combination of high strength and excellent fatigue performance, and meets the needs of automobile companies for fatigue properties when lightweighting. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a surface condition diagram of the steel of the present invention. DETAILED DESCRIPTION

[0044] The present invention is described in detail below:

[0045] Table 1 is a list of chemical components of various embodiments and comparative examples of the present invention;

[0046] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0047] Table 3 is a table of performance test results of various embodiments and comparative examples of the present invention.

[0048] Each embodiment of the present invention is produced according to the following steps:

[0049] 1) After converter smelting, vacuum treatment is performed, and the treatment time is controlled to be greater than 15 minutes, at which time molten steel with the target composition is obtained;

[0050] 2) After continuous casting, the ingot is heated, during which the following controls are applied: the air-fuel ratio is ≤1.3, the ingot furnace temperature is between 300 and 900°C, the ingot heating temperature is between 1250 and 1320°C, and the temperature is maintained at this heating temperature for 60 to 70 minutes, and the temperature difference on the ingot surface fluctuates within ±25°C;

[0051] 3) Rough rolling is performed on the heated ingot: the rough rolling is performed in 3 to 7 passes, and the surface temperature of the steel plate after the last two passes is controlled: the surface temperature of the steel plate after the second pass is 1080 to 1120°C, and the surface temperature of the steel plate after the first pass is 1020 to 1090°C;

[0052] 4) Ultra-high pressure descaling after rough rolling, the water pressure is controlled at 350-380 bar, and the nozzle angle (i.e. the angle between the water flow and the vertical line of the ground) is 10-25°;

[0053] 5) Finish rolling is performed in 6 to 7 passes, and the surface temperature of the steel plate after rolling in the last four passes is controlled: the surface temperature of the steel plate in the fourth pass is controlled at 950 to 1035° C., the surface temperature of the steel plate in the third pass is controlled at 920 to 990° C., the surface temperature of the steel plate in the second pass is controlled at 890 to 960° C., and the surface temperature of the steel plate in the first pass is controlled at 860 to 940° C.;

[0054] 6) Descaling with high-pressure water after finishing rolling (no special requirements are made, high-pressure water descaling generally uses 60-100 bar water pressure);

[0055] 7) Cooling, cooling the surface temperature of the steel plate to the coiling temperature at a cooling rate of 80 to 150°C / s;

[0056] 8) Coil the steel sheet and control the coiling temperature between 520 and 620°C.

[0057] The comparative example was also produced according to the above steps of the embodiment, and the process parameters are shown in Table 2.

[0058] Table 1 Chemical composition list of various embodiments and comparative examples of the present invention (wt%)

[0059]

[0060] Table 2 List of main process parameters of various embodiments of the present invention and comparative examples

[0061]

[0062]

[0063] Table 2

[0064]

[0065] Table 3 List of mechanical properties test results of various embodiments of the present invention and comparative examples

[0066]

[0067] Note: In Table 3, strength test standard: GB / T228.1-2010, hardness test standard: GB / T 4340.1-2009, roughness test standard: GB / T 3505-2009, fatigue strength test standard: GB / T 3075-2008.

[0068] It can be seen from Table 3 that, through the good design of composition and process optimization, the present invention has low surface roughness and high surface hardness while having high strength, thereby obtaining good fatigue performance.

[0069] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. A hot-rolled automobile steel with good surface and fatigue properties, the components and weight percentages of which are: C: 0.03-0.065%, Mn: 1.2-1.8%, P≤0.013%, S≤0.003%, Al: 0.25-0.45%, Ti: 0.11-0.18%, Cr: 0.1-0.5%, Nb: 0.02-0.05%, N: 0.001-0.005%, the remainder is Fe and unavoidable impurities, and Ti / C is between 2.8-3.3, Ti / Cr is between 0.4-0.9; Mechanical properties: yield strength ≥750MPa, tensile strength is between 800-950MPa, elongation A≥16%, surface roughness ≤1.5, microhardness HV0.025≥230 at 30μm of the surface layer of the steel plate, and the symmetrical fatigue limit of the steel plate with iron oxide scale is ≥330MPa.

2. A hot-rolled automobile steel with good surface and fatigue properties as claimed in claim 1, characterized in that: The weight percentage content of Cr is 0.13-0.46%.

3. A method for producing a hot-rolled automobile steel having good surface and fatigue properties as claimed in claim 1, comprising the steps of: 1) After converter smelting, vacuum treatment is performed, and the treatment time is controlled to be greater than 15 minutes; 2) After continuous casting, the billet is heated, during which the following controls are made: the air-fuel ratio is ≤1.3, the billet entering the furnace temperature is between 300 and 900°C, the billet heating temperature is between 1250 and 1320°C, and the billet is kept at this temperature for 60 to 70 minutes, and the billet surface temperature fluctuation is within ±25°C; 3) Rough rolling is performed on the heated ingot: the rough rolling is performed in 3 to 7 passes, and the surface temperature of the steel plate after the last two passes is controlled: the surface temperature of the steel plate after the second pass is 1080 to 1120°C, and the surface temperature of the steel plate after the first pass is 1020 to 1090°C; 4) Ultra-high pressure descaling after rough rolling, the water pressure is controlled at 350-380 bar, and the nozzle angle is 10-25°; 5) Finish rolling is performed in 6 to 7 passes, and the surface temperature of the steel plate after rolling in the last four passes is controlled: the surface temperature of the steel plate in the fourth pass is controlled at 950 to 1035° C., the surface temperature of the steel plate in the third pass is controlled at 920 to 990° C., the surface temperature of the steel plate in the second pass is controlled at 890 to 960° C., and the surface temperature of the steel plate in the first pass is controlled at 860 to 940° C.; 6) Descaling with ultra-high pressure water after finishing rolling; 7) Cooling, cooling the surface temperature of the steel plate to the coiling temperature at a cooling rate of 80 to 150°C / s; 8) Coil the steel sheet and control the coiling temperature between 520 and 620°C.

4. The method according to claim 3, characterized in that: During ultra-high pressure water descaling after finishing rolling, the oxide film on the surface of the high-speed steel roll of the finishing working roll is controlled to be intact and free of foreign matter.

5. The method according to claim 3, characterized in that: The surface temperature of the steel plate after the last two passes of rough rolling is: the surface temperature of the steel plate after the second pass is 1087~1110℃, and the surface temperature of the steel plate after the first pass is 1025~1083℃.

6. The method according to claim 3, characterized in that: The surface temperature of the steel plate after the last four passes of finishing rolling is: the surface temperature of the steel plate after the fourth pass is controlled at 958-1031°C, the surface temperature of the steel plate after the third pass is controlled at 926-982°C, the surface temperature of the steel plate after the second pass is controlled at 895-953°C, and the surface temperature of the steel plate after the first pass is controlled at 864-936°C.

7. The method according to claim 3, characterized in that: The coiling temperature is between 526 and 610°C.

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