An 850mpa grade automotive steel and method of production

Abstract

The application discloses a kind of 850MPa grade automobile steel, component and wt% are as follows: C:0.04~0.075%, Mn:1.8~3.0%, P≤0.02%, S≤0.008%, Al:0.5~0.85%, Ti:0.13~0.18%, B:0.0003~0.002%, Cu:0.1~0.5%, N:0.002~0.006;Production method: conventional smelting and continuous casting into billet;Casting billet heating;Rough rolling;Finish rolling;Laminar cooling;Coiling;Standby.This application realizes 10~30% lightweight under guaranteeing yield strength ≥750MPa, tensile strength 850~1050MPa, elongation ≥14%, carries out 90 degrees positive and negative bending and all reaches qualified, and makes steel plate carry out organization as small ferrite+bainite organization, grain size is less than 5um, and has good toughness, manufacturing cost is low, energy saving, reduces carbon emission, fully meets the higher demand of manufacturing automobile structural part.

Description

Technical Field

[0001] This invention relates to automotive steel and its production method, specifically to an 850MPa grade automotive steel and its production method. Background Technology

[0002] With increasing environmental awareness in China, major commercial vehicle manufacturers are vigorously promoting lightweighting of automobiles to meet environmental protection requirements and improve air quality. Currently, 600MPa grade hot-rolled steel sheets are still widely used in the manufacture of automotive structural components in China. These low-strength steel sheets have a microstructure primarily composed of ferrite and pearlite, offering good formability. Existing high-strength steel sheets, due to the addition of numerous strengthening alloys, exhibit altered microstructures, resulting in unstable structures and even the formation of martensite. This reduces material toughness and makes them prone to cracking during the forming process. Summary of the Invention

[0003] The present invention aims to overcome the shortcomings of the existing technology and provide an 850MPa grade automotive steel and its production method that achieves 10-30% weight reduction while ensuring yield strength ≥750MPa, tensile strength 850-1050MPa, elongation ≥14%, and meets the requirements for 90-degree forward and reverse bending when the bending diameter is half the thickness.

[0004] Measures to achieve the above objectives:

[0005] A type of 850MPa grade automotive steel has the following composition and weight percentage content: C: 0.04-0.075%, Mn: 1.8-3.0%, P≤0.02%, S≤0.008%, Al: 0.5-0.85%, Ti: 0.13-0.18%, B: 0.0003-0.002%, Cu: 0.1-0.5%, N: 0.002-0.006%, with the remainder being Fe and impurities.

[0006] Preferably, the Al content is 0.55% to 0.75% by weight.

[0007] Preferably, the weight percentage content of Ti is 0.15% to 0.175%.

[0008] Preferably, the weight percentage content of B is 0.0006 to 0.0015%.

[0009] Preferably, the weight percentage content of Cu is 0.16-0.43%.

[0010] A method for producing an 850MPa grade automotive steel, comprising the following steps:

[0011] 1) Conventional smelting and continuous casting into billets; during this process, the vacuum treatment time is controlled to be greater than 10 minutes;

[0012] 2) Heating the billet: The billet temperature entering the furnace should be 500-700℃, the heating temperature should be 12620-1320℃, and the temperature should be held at this temperature for 75-100 minutes; the air-fuel ratio should be 1.35-1.85; the surface temperature difference of the billet should be controlled within ±30℃.

[0013] 3) Perform rough rolling, and control the rough rolling end temperature at 1080-1140℃, and control the cumulative reduction rate of rough rolling at 75-85%;

[0014] 4) Perform finishing rolling, controlling the final rolling temperature at 880–920℃, and controlling the cumulative reduction rate of finishing rolling at 85–95%;

[0015] 5) Perform laminar flow cooling until the winding temperature is reached;

[0016] 6) Perform winding: Control the winding temperature between 460 and 520℃;

[0017] 7) Reserved.

[0018] The role and mechanism of each component and main process in this invention

[0019] Carbon: Carbon is an inexpensive solid solution strengthening element. If its content is less than 0.1%, the material strength requirements cannot be met; if its content is greater than 0.16%, the material toughness is reduced. Therefore, its content is limited to the range of 0.1% to 0.16%.

[0020] Manganese: Manganese is an effective solid solution strengthening element that can lower the austenite transformation temperature, refine grains, and improve the uniformity of hardness between the core and surface of steel plates. If its content is less than 1.8%, the material strength requirements cannot be met; however, adding too much manganese will reduce the toughness of the steel. Therefore, its upper limit is set at 3.0%, and its content is limited to the range of 1.8% to 3.0%.

[0021] Phosphorus: Phosphorus is a harmful element in steel and can easily cause segregation in the center of the cast billet. In order to avoid the deterioration of cold bending performance and toughness, its content limit is set at 0.02%.

[0022] Sulfur: Sulfur is a very harmful element. Sulfur in steel often exists in the form of manganese sulfides. These sulfide inclusions are very detrimental to the fatigue performance of steel and cause anisotropy in the properties. Therefore, in order to meet the requirements of fatigue performance, the sulfur content in steel is controlled below 0.008%.

[0023] Aluminum: Aluminum is a good deoxidizing element, which can shrink the austenite phase region and stabilize the ferrite structure. When the Al content is less than 0.5%, it cannot exert its effect; on the other hand, because adding too much aluminum easily forms alumina agglomerates, the upper limit of Al is specified as 0.85%. Therefore, the Al content is limited to the range of 0.5% to 0.85%.

[0024] Titanium: Titanium is a good grain-refining strengthening element. When the Ti content is below 0.13%, it is difficult to exert its effect, and when the Ti content is above 0.18%, it is easy to form titanium-containing metallic inclusions. Therefore, the Ti content is limited to 0.13-0.18%.

[0025] Cu: Copper is a good solid solution strengthening element, which can improve surface hardenability and surface hardness. When the Cu content is below 0.1%, it is difficult to exert its effect, and when the Cu content is above 0.3%, it will reduce the toughness of the steel plate. Therefore, the copper content is limited to 0.1%–0.3%.

[0026] Nitrogen: Nitrogen forms nanoscale precipitates with titanium, which increases the hardness of the steel plate, and its content is limited to 0.002-0.006%.

[0027] The reason this invention controls the billet's furnace entry temperature to 500–700℃, the heating temperature to 1260–1320℃, and holds it at this temperature for 75–100 minutes; the air-fuel ratio to be 1.35–1.85, is that Ti can be fully dissolved, preventing the formation of a decarburized layer on the surface and ensuring high strength in the steel plate. Insufficient temperature and a low air-fuel ratio will lead to insufficient strength. Excessive temperature and a high air-fuel ratio will result in grain coarsening, severe burn-off, deterioration of material properties, and reduced yield.

[0028] The reason why the roughing end temperature is controlled at 1080-1140℃ and the cumulative reduction rate of roughing is controlled at 75-85% is that large reduction is carried out at this temperature, which can quickly refine the microstructure.

[0029] The reason this invention controls the final rolling temperature at 880–920°C and the cumulative reduction rate at 85–95% is that the microstructure can undergo large deformation in the austenite single-phase region, completing recovery and recrystallization, refining the grain size, and forming an equiaxed austenite microstructure. Excessively high temperatures lead to microstructure coarsening, while excessively low temperatures cause austenite to undergo phase transformation during rolling, producing elongated microstructures and deteriorating material properties.

[0030] The reason why the cooling rate is controlled at 60-120℃ / s in this invention is to suppress the phase transformation of austenite by rapid cooling, avoid the generation of unstable structure, and rapidly precipitate ferrite structure.

[0031] The reason why the winding temperature is controlled at 460-520°C in this invention is that within this temperature range, austenite obtains a high degree of supercooling, undergoes phase transformation, obtains a fine bainitic structure, and improves the toughness of the material.

[0032] Compared with the prior art, this invention achieves a weight reduction of 10-30% while ensuring a yield strength ≥750MPa, tensile strength 850-1050MPa, and elongation ≥14%. It also meets the requirements for 90-degree forward and reverse bending. Furthermore, it produces a fine ferrite + bainite microstructure with a grain size of less than 5µm and good toughness. The manufacturing cost is low, energy is saved, and carbon emissions are reduced, fully meeting the higher requirements for manufacturing automotive structural components. Attached Figure Description

[0033] Figure 1 This is a metallographic image of the steel of this invention. Detailed Implementation

[0034] The present invention will now be described in detail:

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

[0036] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0037] Table 3 is a list of performance test results for each embodiment and comparative example of the present invention.

[0038] The various embodiments of the present invention are produced according to the following steps.

[0039] 1) Conventional smelting and continuous casting into billets; during this process, the vacuum treatment time is controlled to be greater than 10 minutes;

[0040] 2) Heating the billet: The billet temperature entering the furnace should be 500-700℃, the heating temperature should be 1260-1320℃, and the temperature should be held at this temperature for 75-100 minutes; the air-fuel ratio should be 1.35-1.85; the surface temperature difference of the billet should be controlled within ±30℃.

[0041] 3) Perform rough rolling, and control the rough rolling end temperature at 1080-1140℃, and control the cumulative reduction rate of rough rolling at 75-85%;

[0042] 4) Perform finishing rolling, controlling the final rolling temperature at 880–920℃, and controlling the cumulative reduction rate of finishing rolling at 85–95%;

[0043] 5) Perform laminar flow cooling until the winding temperature is reached;

[0044] 6) Perform winding: Control the winding temperature between 460 and 520℃;

[0045] 7) Reserved.

[0046] Table 1. List of chemical components (wt%) of various embodiments and comparative examples of the present invention.

[0047]

[0048]

[0049] Table 2. List of main process parameters for each embodiment and comparative example of the present invention.

[0050]

[0051] Table 3. List of mechanical property test results for each embodiment and comparative example of the present invention.

[0052]

[0053] As can be seen from Table 3, the steel plate produced according to the present invention has both high strength and high toughness.

[0054] The specific embodiments described in this specification are merely best examples and are not intended to limit the implementation of the technical solutions of this invention.

Claims

1. An 850MPa grade automotive steel, the composition and weight percentage of which are: C: 0.04-0.075%, Mn: 2.4-3.0%, P≤0.02%, S≤0.008%, Al: 0.5-0.85%, Ti: 0.15-0.18%, B: 0.0003-0.0008%, Cu: 0.35-0.5%, N: 0.004-0.006%, with the remainder being Fe and impurities; Production method: 1) Conventional smelting and continuous casting into billets; during this process, the vacuum treatment time is controlled to be greater than 10 minutes; 2) Heating the billet: The billet temperature entering the furnace should be 500-700℃, the heating temperature should be 1260-1320℃, and the temperature should be held at this temperature for 75-100 minutes; the air-fuel ratio should be 1.35-1.85; the surface temperature difference of the billet should be controlled within ±30℃. 3) Perform rough rolling, and control the rough rolling end temperature at 1080–1140℃, and control the cumulative reduction rate of rough rolling at 75–85%; 4) Perform finishing rolling, controlling the final rolling temperature at 880–920℃, and controlling the cumulative reduction rate of finishing rolling at 85–95%; 5) Perform laminar flow cooling until the winding temperature is reached; 6) Perform winding: Control the winding temperature between 460 and 520℃; 7) Reserved.

2. The 850MPa grade automotive steel as described in claim 1, characterized in that: The Al content is 0.55% to 0.75% by weight.

3. A method for producing an 850MPa grade automotive steel as described in claim 1, comprising the following steps: 1) Conventional smelting and continuous casting into billets; during this process, the vacuum treatment time is controlled to be greater than 10 minutes; 2) Heating the billet: The billet temperature entering the furnace should be 500-700℃, the heating temperature should be 1260-1320℃, and the temperature should be held at this temperature for 75-100 minutes; the air-fuel ratio should be 1.35-1.85; the surface temperature difference of the billet should be controlled within ±30℃. 3) Perform rough rolling, and control the rough rolling end temperature at 1080–1140℃, and control the cumulative reduction rate of rough rolling at 75–85%; 4) Perform finish rolling, controlling the final rolling temperature at 880–920℃, and controlling the cumulative reduction rate of finish rolling at 85–95%; 5) Perform laminar flow cooling until the winding temperature is reached; 6) Perform winding: Control the winding temperature between 460 and 520℃; 7) Reserved.

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