Hot-rolled automobile steel having good surface and fatigue performance, and production method therefor

Abstract

NOT VISIBLE DUE TO STATUS OF PATENT

Description

Hot-rolled automobile steel with good surface and fatigue properties and production method thereof Technical Field

[0001] The present invention relates to steel for automobiles and a production method thereof, and in particular to hot-rolled automobile steel with good surface and fatigue properties and a production method thereof. Background Art

[0002] In the automotive industry, in order to vigorously promote the lightweighting of automobiles, thereby reducing pollutant emissions and protecting 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, 600MPa grade hot-rolled steel plates are widely used in the manufacture of automobile parts such as beams, wheels, bridge housings, and brackets. At present, although its strength can be greatly improved, its fatigue performance cannot be improved synchronously, and may even decline, making the steel plate unstable. This has hindered 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 root cause of this phenomenon is the softening of the surface structure of the steel plate, that is, the hardness of the surface layer of the steel plate is low, making the surface layer of the steel plate a weak link during the fatigue process, that is, fatigue cracks are first generated on the surface layer of the steel plate. In order to solve the problems existing in the prior art and improve the fatigue performance of the steel plate, those skilled in the art often adopt means of strengthening the surface of the steel plate, which include: surface shot peening, surface heat treatment and other technical measures. Although these 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] 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, meeting the fatigue characteristics requirements of automobile companies when reducing weight. Summary of the Invention

[0005] In view of the shortcomings of the prior art in which hot-rolled steel plates have low fatigue performance, which hinders the improvement of strength levels, or the need for heat treatment resulting in high production costs, the present invention provides a hot-rolled automotive steel with excellent surface and fatigue performance and a production method. Under the premise of ensuring yield strength ≥750 MPa, tensile strength between 800 and 950 MPa, and elongation ≥16%, the surface roughness can also be guaranteed to be ≤1.5, the microhardness HV0.025 at 30 μm of the surface layer ≥230, and the symmetrical fatigue limit of the steel plate in the state with iron oxide scale is ≥330 MPa.

[0006] The measures adopted by the present invention to achieve the above-mentioned purpose are as follows:

[0007] A hot-rolled automobile steel with good surface and fatigue properties, the components and weight percentages of which are:

[0008] 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 balance is Fe and inevitable impurities, and the Ti / C ratio is between 2.8~3.3, and the Ti / Cr ratio is between 0.4~0.9; mechanical properties: yield strength ≥750 MPa, tensile strength 800~950 MPa, elongation A ≥16%, surface roughness ≤1.5, microhardness HV0.025 ≥230 at 30μm on the surface of the steel plate, and symmetrical fatigue limit of the steel plate with iron oxide scale ≥330MPa.

[0009] Preferably, the weight percentage content of Cr is 0.13-0.46%.

[0010] The method for producing the above-mentioned hot-rolled automobile steel with good surface and fatigue properties comprises the following steps:

[0011] 1) After converter smelting, vacuum treatment is carried out, and the treatment time is controlled to be greater than 15 minutes;

[0012] 2) After continuous casting, the billet is heated, during which the following controls are applied: the air-fuel ratio is ≤1.3, the billet feeding 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;

[0013] 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;

[0014] 4) Ultra-high pressure descaling after rough rolling, with the water pressure controlled at 350~380bar and the nozzle angle at 10~25°;

[0015] 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;

[0016] 6) Descaling with high-pressure water after finishing rolling;

[0017] 7) Cooling: Cool the steel plate surface temperature to the coiling temperature at a cooling rate of 80-150°C / s;

[0018] 8) Coil the steel sheet and control the coiling temperature between 520 and 620°C.

[0019] Preferably: the working roll surface is cleaned before finishing rolling, and the oxide film on the finishing working roll surface is intact and free of foreign matter.

[0020] 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 1087-1110°C, and the surface temperature of the steel plate after the first pass is 1025-1083°C.

[0021] Preferably, the surface temperature of the steel plate after finishing rolling is controlled in the last four passes: 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.

[0022] Preferably, the coiling temperature is between 526°C and 610°C.

[0023] The functions and mechanisms of the components and main processes in the present invention are as follows:

[0024] Carbon: Carbon is an inexpensive solid solution strengthening element. If its content is less than 0.03%, the material strength requirements cannot be met. If its content exceeds 0.065%, it will cause severe surface decarburization and soften the surface hardness. Therefore, its content is limited to the range of 0.03-0.065%.

[0025] Manganese: Manganese is an effective solid solution strengthening element that can lower the austenite transformation temperature, refine grains, and improve the hardness uniformity between the core and surface of the steel plate. If its content is less than 1.2%, it will not meet the material strength requirements; however, adding too much manganese will reduce the toughness of the steel. Therefore, the upper limit is set at 1.8%, so its content is limited to the range of 1.2-1.8%.

[0026] 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%.

[0027] 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 in performance. Therefore, to meet fatigue performance requirements, the sulfur content in steel is controlled below 0.003%.

[0028] Aluminum: Aluminum is a good deoxidizing element, reducing the austenite phase. However, this effect is not fully realized when the AlS content is less than 0.25%. On the other hand, adding too much aluminum can easily form alumina agglomerates, so the upper limit for AlS is set at 0.45%. Therefore, the AlS content is limited to the range of 0.25-0.45%.

[0029] 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 titanium-containing metal inclusions. Therefore, the Ti content is limited to 0.11~0.15%.

[0030] Chromium: Chromium is a strong solid-solution strengthening element that improves surface hardenability and hardness. It also forms ferrochromium oxide on the steel plate surface, reducing surface roughness. Chromium content below 0.1% is not very effective, while content above 0.5% reduces the toughness of the steel plate. Therefore, the chromium content is limited to 0.1-0.5%, with a preferred Cr content of 0.13-0.46% by weight.

[0031] Nitrogen: Nitrogen forms nano-scale precipitates with titanium to increase the hardness of the steel plate. Its content is limited to 0.001~0.005%.

[0032] The reason why the present invention limits the Ti / C ratio 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. Therefore, the Ti / C ratio is set to 2.8-3.3.

[0033] 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 be deteriorated; if Ti / Cr is greater than 0.9, it is difficult to achieve the effect, so Ti / Cr is set to 0.4-0.9.

[0034] In addition to limiting the range of the above chemical components, the present invention does not add precious alloy elements such as Cu, Ni, and Mo from the perspective of improving material formability and economic efficiency.

[0035] The key technical point of this invention lies in controlling the heating temperature, time, and air-fuel ratio of the ingot. The heating temperature and time must ensure sufficient solid solution of alloying elements to maintain matrix strength, while also coordinating with the air-fuel ratio to prevent the formation of excessive iron oxide scale, which worsens surface roughness, and prevent severe surface decarburization, which reduces surface hardness.

[0036] 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℃, and the surface temperature of the steel plate after the first pass is 1020~1090℃, 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℃, and the surface temperature of the steel plate after the first pass is 1025~1083℃, is to keep the surface structure consistent with the core structure, thereby improving the surface hardness.

[0037] The reason why the present invention controls the ultra-high pressure descaling water pressure after rough rolling to 350~380bar and the nozzle angle to 10~25° 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.

[0038] 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℃, the surface temperature of the steel plate in the third pass is controlled at 920~990℃, the surface temperature of the steel plate in the second pass is controlled at 890~960℃, and the surface temperature of the steel plate in the first pass is controlled at 860~940℃, is to keep the surface structure consistent with the core structure, thereby improving the surface hardness.

[0039] 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 finish rolling, thereby increasing the degree of supercooling so that the austenite undergoes a rapid medium-low temperature phase transformation into ferrite or bainite, thereby increasing the surface hardness.

[0040] The reason why the coiling temperature is controlled at 520-620° C. in the present invention is that this temperature range is suitable for the composite precipitation of titanium, carbon, niobium and the like at different thicknesses in the present invention, thereby improving the hardness of the matrix.

[0041] Compared with existing technologies, the hot-rolled automotive steel provided by this invention not only has a yield strength of 750 MPa or higher, a tensile strength of 800-950 MPa, and an elongation of 16% or higher, but also exhibits no cracking when subjected to a cold-bending radius of half the thickness and a cold-bending angle of 180 degrees. Furthermore, the steel exhibits a surface roughness of 1.5 or lower, a microhardness (HV0.025) of 230 or higher at a depth of 30 μm, and a symmetrical fatigue limit of 330 MPa or higher even with scale. This invention achieves both low surface roughness and high surface hardness control for hot-rolled steel, achieving a superior combination of high strength and excellent fatigue performance, meeting the fatigue performance requirements of automotive manufacturers seeking lightweighting solutions. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG1 is a diagram showing the surface condition of the steel of the present invention. Modes for Carrying Out the Invention

[0043] The present invention is described in detail below:

[0044] Table 1 is a list of chemical compositions of various embodiments and comparative examples of the present invention;

[0045] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0046] Table 3 is a table of performance test results of various embodiments of the present invention and comparative examples.

[0047] Each embodiment of the present invention is produced according to the following steps:

[0048] 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;

[0049] 2) After continuous casting, the billet is heated, during which the following controls are applied: the air-fuel ratio is ≤1.3, the billet feeding temperature is between 300 and 900°C, the billet heating temperature is between 1250 and 1320°C, and the billet is kept at this heating temperature for 60 to 70 minutes, and the billet surface temperature fluctuation is within ±25°C;

[0050] 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;

[0051] 4) Ultra-high pressure descaling after rough rolling, with the water pressure controlled at 350-380 bar and the nozzle angle (i.e. the angle between the water flow and the vertical line of the ground) at 10-25°;

[0052] 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;

[0053] 6) Perform high-pressure water descaling after finishing rolling (no special requirements, high-pressure water descaling generally uses 60~100 bar water pressure);

[0054] 7) Cooling: Cool the steel plate surface temperature to the coiling temperature at a cooling rate of 80-150°C / s;

[0055] 8) Coil the steel sheet and control the coiling temperature between 520 and 620°C.

[0056] The comparative example was also produced according to the above steps of the embodiment, and the process parameters are shown in Table 2.

[0057] Table 1 Chemical composition of various embodiments and comparative examples of the present invention (wt%)

[0058] Example C %Mn %P %S %Als %Cr %Ti %N %Nb %Ti / CTi / Cr10.0341.20.0010.0010.250.130.110.0010.023.240.8520.0461.30.0070.00070.260.150.1350.0020.042.930.9030.0381.40.0030. 00210.320.30.120.0030.033.160.4040.041.50.0080.00230.30.250.1250.00430.0353.130.5050.0391.60.0120.0010.40.220.120.0050.0253. 080.5560.0631.70.0130.00130.420.40.180.0040.0452.860.4570.0531.750.010.00250.380.210.150.00230.052.830.7180.051.80.0050.0030 .450.270.1650.00350.0353.300.61Comparative Example10.071.30.010.0030.340.050.190.0030.0452.73.8Comparative Example20.021.50.0180.0060.270.60.100.0020.0650.17

[0059] Table 2 Main process parameters of various embodiments and comparative examples of the present invention

[0060] Example Billet feeding temperature ℃ Billet heating section temperature ℃ Holding time min Air-fuel ratio Billet surface section temperature fluctuation value ℃ Surface temperature of the second rough rolling pass ℃ Surface temperature of the first rough rolling pass ℃ Water pressure after rough rolling bar Nozzle angle ° Rough rolling pass times 1300 1290 60 1.310 1090 1020 375 187 2400 1300 65 1.15 1611 161090 380 223 3500 1270 61 1.225 1080 1045 360 234 44 50 1280 63 1.020 1087 10 2536525656401250611.2381095106737020367001260661.1731110108335515579001320681.07171105103536219388401310680.952111201088378215Comparison ratio18501290602.15111401100150135Comparison ratio2100012451001.84311301095180355

[0061] Table 2

[0062] Example Surface temperature of the fourth pass of finishing rolling ℃ Surface temperature of the third pass of finishing rolling ℃ Surface temperature of the second pass of finishing rolling ℃ Surface temperature of the first pass of finishing rolling ℃ Finishing rolling pass Cooling rate ℃ / s Coiling temperature ℃ 1975923895875615061521035965925892611562039989659429166108595495892689 386071255655103198295392561325806102598496093471455307100296594692079755081032985959940780520 Comparison 1940900870820775630 Comparison 2105010201000950756510

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

[0064] Example Thickness mm Yield strength ReL MPa Tensile strength Rm MPa Elongation A % Steel plate surface roughness Ra Hardness fatigue limit at 30μm surface layer of μm steel plate MPa12.5870945171.0726135023.5815895161.2625535534.5850910180.8725236045.5780840211.5027434056.5795865201.0226334568775840190.94240350710780840171.13260345812760830181.49233330Comparative Example16.57508501520190220Comparative Example28730840151.7200250

[0065] 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.

[0066] It can be seen from Table 3 that, through the good design of composition and process, the present invention has high strength while achieving low surface roughness and high surface hardness, thereby achieving good fatigue performance.

[0067] 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 steel for automobiles with good surface and fatigue properties, and its components and weight percentage contents are as follows: 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 balance is Fe and unavoidable impurities, and Ti / C is between 2.8 - 3.3, and Ti / Cr is between 0.4 - 0.9; Mechanical properties: yield strength ≥ 750 MPa, tensile strength is between 800 - 950 MPa, elongation A ≥ 16%, surface roughness ≤ 1.5, microhardness HV0.025 at 30 μm from the steel plate surface ≥ 230, and the symmetrical fatigue limit of the steel plate in the state with mill scale ≥ 330 MPa.

2. The hot-rolled steel for automobiles with good surface and fatigue properties as described 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 steel for automobiles with good surface and fatigue properties as described in claim 1, and its steps: 1) After converter smelting, vacuum treatment is carried out, and the treatment time is controlled to be greater than 15 min; 2) After continuous casting into slabs, the slabs are heated, and during this process, the following are controlled: air-fuel ratio ≤ 1.3, the slab inlet furnace temperature is between 300 - 900 °C, the slab heating temperature is between 1250 - 1320 °C, and it is held at this temperature for 60 - 70 min, and the temperature difference fluctuation on the slab surface is within ± 25 °C; 3) Rough rolling is carried out on the heated slabs: The rough rolling is carried out in 3 - 7 passes, and the surface temperature of the steel plate after the last two passes of rolling is controlled: the surface temperature of the steel plate in the second-to-last pass is between 1080 - 1120 °C, and the surface temperature of the steel plate in the last pass is between 1020 - 1090 °C; 4) Ultra-high pressure descaling is carried out after rough rolling, the water pressure is controlled between 350 - 380 bar, and the nozzle angle is between 10 - 25°; 5) Finish rolling is carried out, and it is rolled in 6 - 7 passes, and the surface temperature of the steel plate after the last four passes of rolling is controlled: the surface temperature of the steel plate in the fourth-to-last pass is controlled between 950 - 1035 °C, the surface temperature of the steel plate in the third-to-last pass is controlled between 920 - 990 °C, the surface temperature of the steel plate in the second-to-last pass is controlled between 890 - 960 °C, and the surface temperature of the steel plate in the last pass is controlled between 860 - 940 °C; 6) Ultra-high pressure water descaling is carried out after finish rolling; 7) Cooling is carried out, and the surface temperature of the steel plate is cooled to the coiling temperature at a cooling rate of 80 - 150 °C / s; 8) Coiling is carried out, and the coiling temperature is controlled between 520 - 620 °C.

4. The method according to claim 3, wherein: During ultra-high pressure water descaling after finish rolling, it is controlled that the oxide film on the surface of the high-speed steel roll of the finish rolling work roll is intact and there is no foreign matter adhesion.

5. The method according to claim 3, wherein: The surface temperature of the steel plate after the last two passes of rough rolling: the surface temperature of the steel plate in the second-to-last pass is between 1087 - 1110 °C, and the surface temperature of the steel plate in the last pass is between 1025 - 1083 °C.

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

7. The method according to claim 3, wherein: The coiling temperature is 526 - 610 °C.