A short process 780mpa level thin gauge high surface quality high flaring performance automobile steel and its production method

Abstract

The application discloses a kind of thin gauge 780MPa grade high surface quality high flaring performance of automobile steel, its component and weight percentage content are as follows: C 0.045~0.059%, Si 0.03~0.14%, Mn 1.05~1.38%, Nb 0.012~0.020%, Ti 0.085~0.105%, Cr 0.42~0.65%, [Cr]*[Ti]≤0.045*10 ‑4 , N≤0.004%, S≤0.004%, the rest is Fe and inevitable impurities.The application adopts reasonable chemical composition, does not add Mo, V, Cu, Ni and other valuable alloy elements, but is through the component design of low Mn content low Nb content extremely low Si content, meets high surface quality demand and low cost requirement, through the fine control of Ti content on process, in combination with the optimization of casting blank thickness, drawing speed and heating temperature, finally obtains thin gauge 780MPa grade high surface quality high flaring performance of automobile steel, specifically, thickness 1.8mm~5.0mm, tensile strength ≥780MPa, yield strength ≥660MPa, elongation ≥20%, flaring rate ≥90%, with good surface quality, can meet the demand of automobile chassis parts to surface quality and high flaring performance, and further reduces production cost.

Description

TECHNICAL FIELD

[0001] The present application relates to pickling high-strength automobile steel and a production method thereof, in particular to a short-process 780MPa-grade high-surface-quality high-hole-flanging-performance automobile steel and a production method thereof. BACKGROUND

[0002] In the 21st century, reducing fuel consumption, CO2 and exhaust emissions has become a social demand in the automobile industry. In order to adapt to this trend, the steel industry as a material producer has developed many kinds of ultra-high strength steel sheets to help reduce the weight of automobiles and meet the new requirements of the automobile industry. More and more pickling automobile steel products are used to manufacture automobile parts, such as automobile suspension and chassis parts.

[0003] Many automobile parts such as front / rear control arms, spring seats, etc. are designed with more round holes for high-strength lightweight design and assembly requirements. These round holes need to be obtained through the method of punching first and then flanging and expanding in the forming process. A large amount of data shows that the hole expansion performance of the material cannot be completely equivalent to the forming performance. In general, the better the plasticity of the steel, the better the forming performance, but it does not necessarily mean that the hole expansion flanging performance is better. For example, the traditional dual-phase steel has good plasticity and good forming performance, but its hole expansion flanging performance is poor, and it is not suitable for producing automobile suspension and chassis parts that need to be expanded.

[0004] CN117144254A discloses an 800MPa-grade low-cost high-hole-expansion-performance pickling automobile steel sheet and a thin slab continuous casting and rolling production method thereof. The chemical composition and the weight percentage content are as follows: C 0.045%~0.065%, Si 0.40%~0.65%, Mn 1.40%~1.65%, Nb 0.020%~0.030%, Ti 0.020%~0.045%, Cr 0.40%~0.60%, P≤0.010%, S≤0.004%, and the rest is Fe and unavoidable impurities. However, the addition of 0.40%~0.65% Si in the composition of the steel sheet will form tiger stripes on the surface of the hot-rolled sheet, and after pickling, it will form stripe color difference, thereby affecting the surface quality and not fully meeting the requirements of automobile chassis parts for surface quality. In order to reduce the risk of tiger stripe defects caused by high Si content, high-pressure water descaling is required before rolling, which will also increase the process cost. At the same time, the addition of more than 1.4% Mn and more than 0.020% Nb in the present increasingly competitive market environment, the most cost-effective composition design can better meet market demand. SUMMARY

[0005] The present application is to overcome the deficiencies in the prior art, provide a thin gauge 780MPa grade high surface quality high hole flanging performance of automobile steel and its production method, specifically for thickness 1.8mm-5.0mm, after pickling the tensile strength of the steel plate is greater than or equal to 780MPa, the yield strength is greater than or equal to 660MPa, the elongation is greater than or equal to 20%, the hole flanging rate is greater than or equal to 90%, and the surface quality is good, which meets the requirements of automobile chassis parts on hole flanging performance.

[0006] To achieve the above object, the technical scheme adopted by the present application is:

[0007] A thin gauge 780MPa grade high surface quality high hole flanging performance of automobile steel, which adopts a composition design with low Mn content, low Nb content and extremely low Si content, the components and weight percentage contents are: C 0.045-0.059%, Si 0.03-0.14%, Mn 1.05-1.38%, Nb 0.012-0.020%, Ti 0.085-0.105%, Cr 0.42-0.65%, [Cr]*[Ti]≤0.045*10 -4 (wherein, [Cr], [Ti] represent the weight percentage of Cr, Ti, respectively), N≤0.004%, S≤0.004%, the rest is Fe and inevitable impurities.

[0008] Preferably, the weight percentage content of Si is 0.03-0.11%.

[0009] Preferably, the weight percentage content of Mn is 1.08-1.36%.

[0010] Preferably, the weight percentage content of Ti is 0.088-0.103%.

[0011] Preferably, the weight percentage content of Cr is 0.43-0.56%.

[0012] Preferably, [Cr]*[Ti] is in the range of 0.035*10 -4 -0.045*10 -4 .

[0013] The present application also provides a short process for producing the above-mentioned thin gauge 780MPa grade high surface quality high hole flanging performance of automobile steel, the steps are as follows:

[0014] 1) After desulphurization of molten iron, smelting and refining, the molten steel with target composition is obtained;

[0015] 2) After refining, continuous casting is carried out, the thickness of the billet is 57-65mm, and the billet speed is 3.0-4.2m / min;

[0016] 3) Heat the billet and control the billet temperature in the furnace at 800-950℃, the time in the furnace at 20-40 minutes, and the temperature at the billet outlet at 1200-1230℃;

[0017] 4) Rolling is carried out. High-pressure water descaling is not required before rolling. The reduction rate of the first pass is controlled at 52-63%, the reduction rate of the second pass is 50-60%, and the reduction rate of the last pass is 10-16%. The rolling speed is controlled at 8-12 m / s. Medium-pressure water descaling is carried out between the first and second passes. The descaling water pressure is 200-280 bar. The final rolling temperature is controlled at 880-920℃.

[0018] 5) Cooling is performed using a front-end cooling method, with a front-end cooling rate ≥140℃ / s;

[0019] 6) Perform coiling. The coiling temperature is dynamically matched with the effective Ti. The coiling temperature is controlled as 550-1035*[effective Ti], in °C. Wherein, [effective Ti] = [Ti]-3*[S]-3.42*[N], and [Ti], [S], and [N] are the weight percentage contents of Ti, S, and N in the steel composition, respectively.

[0020] 7) Perform pickling continuously at a pickling speed of 100-120 m / min.

[0021] Preferably, the furnace temperature of the billet is controlled at 825-948℃, the furnace time is 20-33 minutes, and the furnace exit temperature is 1205-1223℃.

[0022] The thin-gauge 780MPa grade automotive steel produced by the above method has a thickness of 1.8mm to 5.0mm. After pickling, the steel sheet has a tensile strength ≥780MPa, a yield strength ≥660MPa, an elongation ≥20%, an expansion rate ≥90%, and a pickling streak color difference defect degradation rate not exceeding 0.3%. More preferably, the steel sheet has a thickness of 1.8mm to 4.0mm. After pickling, the steel sheet has a tensile strength ≥800MPa, a yield strength ≥680MPa, an elongation ≥20%, an expansion rate ≥95%, and a pickling streak color difference defect degradation rate not exceeding 0.25%.

[0023] The roles and mechanisms of each raw material and main process in this invention:

[0024] C: To ensure the steel plate achieves a tensile strength of over 780 MPa, the carbon content must be at least 0.045%; otherwise, the tensile strength of the plate will be insufficient. Simultaneously, the carbon content significantly affects the hole-expanding performance. Higher carbon content makes it easier to form cementite and pearlite, both of which are detrimental to improving hole-expanding performance. To improve the hole-expanding rate and ensure strength, the carbon content range is 0.045% to 0.059%.

[0025] Si: Si can effectively inhibit cementite precipitation, and cementite has an adverse effect on hole-expanding performance. Therefore, high-hole-expanding steel usually adds 0.30% Si to effectively inhibit cementite precipitation and improve hole-expanding performance. However, when the Si content exceeds 0.15%, hot-rolled steel sheets are prone to tiger-skin patterns, which appear even after pickling, affecting the surface quality of the steel sheet. The important innovation of this invention lies in adding a small or trace amount of Si element, which enables the steel sheet to obtain good surface quality. The Si content of this invention is controlled at 0.03-0.14%, preferably 0.03-0.11%.

[0026] Mn: Mn is the most effective element for improving strength and toughness. Typical 780MPa high-strength steel has an Mn content of over 1.4%, generally around 1.5%. This invention employs a low-cost composition design, controlling the Mn content at 1.05–1.38%, preferably 1.08–1.36%. Compared to the traditional 1.5% Mn content of 780MPa high-strength steel, this invention can reduce the cost per ton of steel by 30–80 yuan.

[0027] S and N: N is an impurity element in steel. If the content of S and N is too high, they will form large particles of TiS and TiN with Ti in the steel, thereby reducing the precipitation strengthening and grain refinement strengthening effects of Ti. Therefore, the lower the content of sulfur and nitrogen, the better. Moreover, when the strength level of the material is higher, these large particles of TiS and TiN will also have an adverse effect on the fatigue performance of the material, and must be controlled below 0.004%.

[0028] Nb: Nb is one of the key alloying elements used in this invention. Adding Nb can effectively refine the original austenite grain size, improving the strength and plasticity of the steel plate. Simultaneously, the combined precipitation of Nb and Ti can further refine the size of the precipitates, enhancing the material's strength. If the Nb content exceeds 0.020%, it will lead to increased alloy costs; therefore, the Nb content is controlled between 0.012% and 0.020%.

[0029] Ti and Cr: Ti and Cr are important alloying elements used in this invention. Ti can play a role in grain refinement and precipitation strengthening, while Cr can play a role in solid solution strengthening. Since Ti alloys are very inexpensive, they can replace other expensive alloying elements, achieving low-cost design. In this invention, coiling is performed in a medium-low temperature range. The precipitation strengthening effect of Ti cannot fully meet the required strength. Therefore, a certain amount of Cr must be added to ensure that the tensile strength of the steel plate exceeds 780 MPa. Because the contribution of Cr to strength is stable and controllable, while Ti is a reactive element, and its performance fluctuates greatly when the Ti content is too high, this invention limits the Ti content based on the Cr content, i.e., [Cr]*[Ti]≤0.045. Therefore, the Cr content of this invention should be controlled between 0.42 and 0.65%, and the Ti content should be controlled between 0.085 and 0.105%. Preferably, the Cr content should be controlled between 0.43 and 0.56%, and the preferred Ti content should be controlled between 0.088 and 0.103%.

[0030] In addition to limiting the range of the above chemical compositions, from the perspective of improving material formability and economy, this invention does not add expensive alloying elements such as Mo, V, Cu, and Ni. Furthermore, it achieves extremely low cost by reducing Mn and Nb content, adds a certain amount of relatively inexpensive Ti, and limits the addition of S and N, allowing the added Ti to fully exert its precipitation strengthening and grain refinement purposes. Moreover, because this invention employs a low-Si composition design, high-pressure water descaling before rolling is unnecessary. Based on the above changes in composition design, this invention also designs the casting speed, billet heating process, and coiling temperature of the continuous casting billet.

[0031] The reason why the billet thickness is controlled at 57-65mm and the billet pulling speed at 3.0-4.2m / min in this invention is that the invention adopts a composition design with low Mn content, low Nb content and extremely low Si content, and adds a certain amount of relatively inexpensive Ti. Excessive pulling speed can easily cause billet cracks. In order to ensure steel throughput, there are certain requirements for billet thickness and pulling speed.

[0032] The reason this invention heats the billet to 1200–1240°C for 20–40 minutes in the furnace is to ensure that the temperature of the steel plate meets the requirements of each stage of the rolling process. To complement the reduction of Mn and increase of Ti, and to ensure the precipitation of Ti and its grain refinement strengthening effect, this invention specifically studies the billet heating temperature. The heating temperature has a very important influence on the various properties of the finished product, and a heating temperature above 1200°C must be ensured. When the heating temperature is too low and the furnace time is too short, the Ti in the billet cannot be completely dissolved. During the subsequent rolling and cooling processes, not enough fine and dispersed precipitates can be precipitated, failing to meet the requirements for strength and porosity. However, when the heating temperature is too high and the furnace time is too long, it leads to coarse austenite grains before rolling. This prevents the subsequent rolling process from completing the austenite recrystallization process, resulting in mixed grains (i.e., uneven grain size) in the final product, which seriously affects product quality.

[0033] The reason why this invention uses a front-end cooling method to immediately perform ultra-fast cooling on the steel plate after rolling, that is, cooling at a cooling rate of not less than 140℃ / s, is to suppress the growth of grains in the steel after rolling through a high cooling rate, so that it maintains a fine and uniform grain state before coiling, so that the final steel has a fine grain size and a grain size of 10 or above. The coiling temperature of this invention is dynamically matched with the effective Ti content. This is because Ti is a reactive element that significantly affects mechanical properties, including strength and porosity. Ti readily combines with S and N in steel to form large TiS and TiN particles. These precipitates do not contribute positively to strength and porosity. The actual effect of Ti is generally evaluated using [effective Ti], where [effective Ti] = [Ti] - 3 * [S] - 3.42 * [N]. Effective Ti is typically in the range of 0.07–0.085%. This invention sets the coiling temperature based on the actual value of [effective Ti], achieving optimal mechanical properties through dynamic matching. The coiling temperature is calculated as (550 - 1035 * [effective Ti]), in °C.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] This invention employs a reasonable chemical composition, without adding expensive alloying elements such as Mo, V, Cu, and Ni. Instead, it uses a composition design with low Mn, low Nb, and extremely low Si content to meet the requirements for high surface quality and low cost. By adding a certain amount of relatively inexpensive Ti, the performance requirements of the material are guaranteed. Based on this, through refined control of Ti content in the process, combined with optimization of billet thickness, casting speed, and heating temperature, and precise matching of coiling temperature according to effective Ti, the precipitation strengthening and fine grain strengthening effects of Ti are maximized. This achieves high strength through various strengthening effects, as well as high elongation and high porosity through fine-grained ferrite and bainite structures, while still meeting the requirements of high strength, high elongation, and high porosity. The final product is a thin-gauge 780MPa grade automotive steel with high surface quality and high porosity and flanging performance, specifically with a thickness of 1.8mm to 5.0mm, tensile strength ≥780MPa, yield strength ≥660MPa, elongation ≥20%, good surface quality, pickling streak color difference defect degradation rate not exceeding 0.3%, and porosity of pickled steel sheet ≥90%, meeting the requirements of automotive chassis components for porosity and flanging performance. Compared with traditional 780MPa grade high-expansion steel (such as FB780, which has a high alloy cost, a striped surface morphology, and an expansion rate of about 55%), this invention adopts an extremely low-cost composition design while having excellent surface quality. It can still meet the requirements of automotive chassis parts for surface quality and high expansion and flanging performance, and further reduce production costs. Attached Figure Description

[0036] Figure 1 The metallographic structure (bainite + a small amount of ferrite) of the thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance is shown in the figure. Detailed Implementation

[0037] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the present invention is not limited to the following embodiments.

[0038] In this invention, the hole expansion test is conducted according to national standards. A center hole with a diameter of d=10mm is punched in the center of a 150mm*150mm square sheet. A conical hole expansion punch of a specified shape and size is inserted into the punched hole of the metal sheet sample to perform the hole expansion test until a crack penetrating the sample thickness appears at the edge of the hole. The punching is then stopped, and the limiting hole expansion rate is measured. Hole expansion rate. In the formula, λ is the limiting expansion ratio, %; D0 is the initial diameter of the punched round hole (D0 = 10 mm); D H The value represents the average diameter of the circular hole after rupture, expressed in millimeters (mm). The steel plate prepared by this invention can achieve a hole expansion rate of over 90%.

[0039] Example

[0040] The embodiments are described in detail below:

[0041] Table 1 is a list of chemical components of the various embodiments and comparative examples of the present invention;

[0042] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0043] Table 3 lists the performance test results of each embodiment and comparative example of the present invention. The testing standards for strength and elongation are GB / T 223, and the testing standard for porosity is GB / T 24524.

[0044] The embodiments of the present invention produce thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance according to the following steps:

[0045] 1) After desulfurization of molten iron, it is smelted and refined in a converter, and this is where the steel with the target composition is obtained;

[0046] 2) After smelting, continuous casting is carried out, and the superheat of the molten steel in the tundish is controlled at 15-30℃, the billet thickness is 57-65mm, and the billet pulling speed is 3.0-4.2m / min;

[0047] 3) Heat the billet and control the billet temperature in the furnace at 800-950℃, the time in the furnace at 20-40 minutes, and the temperature at the billet outlet at 1200-1230℃;

[0048] 4) Rolling is carried out. High-pressure water descaling is not required before rolling. The reduction rate of the first pass is controlled at 52-63%, the reduction rate of the second pass is 50-60%, and the reduction rate of the last pass is 10-16%. The rolling speed is controlled at 8-12 m / s. Medium-pressure water descaling is carried out between the first and second passes. The descaling water pressure is 200-280 bar. The final rolling temperature is controlled at 880-920℃.

[0049] 5) Cooling is performed using a front-end cooling method, with a front-end cooling rate ≥140℃ / s;

[0050] 6) Perform winding, with the winding temperature dynamically matched to the effective Ti. Control the winding temperature = 550 - 1035 * [effective Ti], in °C. Substitute the effective Ti from Table 1 to obtain the corresponding winding temperature (Note: When substituting [effective Ti] into the winding temperature formula, there is no need to substitute the unit %. For example, in Example 1, 0.0725 is substituted into the winding temperature formula, and the winding temperature = 550 - 1035 * 0.0725 ≈ 475 °C). Specifically, as shown in Table 3, it is within the range of 467 to 476 °C.

[0051] 7) Perform pickling continuously at a pickling speed of 100-120 m / min.

[0052] The production process for the comparative example was carried out with reference to the embodiment. The specific component values, process parameter values, and performance tests are also shown in Tables 1, 2, and 3.

[0053] Table 1. List of chemical components (wt%) of various embodiments and comparative examples of the present invention.

[0054]

[0055] Table 2. List of main process parameters for each embodiment and comparative example of the present invention.

[0056]

[0057]

[0058] Continued from Table 2

[0059]

[0060] Table 3. List of mechanical property test results for each embodiment and comparative example of the present invention.

[0061]

[0062] Note: The downgrade rate of pickling stripe color difference defects refers to the proportion of pickled steel plates that are judged as downgraded products due to severe stripe color difference on the surface.

[0063] As can be clearly seen from Table 3, the chemical composition and production process of this invention yield a thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance. Specifically, the thickness is 1.8mm to 5.0mm. After pickling, the tensile strength of the steel sheet is ≥780MPa, the yield strength is ≥660MPa, the elongation is ≥20%, and the hole expansion rate of the pickled steel sheet is ≥90%. Compared with traditional 780MPa grade high hole expansion steel (such as FB780, which has a higher alloy cost, is prone to surface stripe color difference defects, and has a hole expansion rate of around 55%), this significantly reduces production costs, improves surface quality, and the degradation rate of pickling stripe color difference defects does not exceed 0.3%. It also ensures mechanical properties, meets the requirements of automotive chassis components for surface quality and high hole expansion and flanging performance, and reduces manufacturing costs. For example, Comparative Examples 2 and 3, which added relatively high amounts of Si, easily formed tiger-skin patterns in the hot-rolled steel sheet, and after pickling, pickling stripe color difference defects were formed, which could not meet the user's high requirements for surface quality.

[0064] This specific embodiment is merely a best example and is not intended to limit the implementation of the technical solution of the present invention.

Claims

1. A method for producing thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance, characterized in that, The steps are as follows: 1) After desulfurization, smelting and refining of molten iron, steel with the target composition is obtained; 2) After refining, continuous casting is carried out, with a billet thickness of 57-65 mm and a casting speed of 3.0-4.2 m / min; 3) Heat the billet and control the billet temperature in the furnace at 800-950℃, the time in the furnace at 20-40 minutes, and the temperature at the billet outlet at 1205-1230℃; 4) Rolling is carried out. High-pressure water descaling is not required before rolling. The reduction rate of the first pass is controlled at 52-63%, the reduction rate of the second pass is 50-60%, and the reduction rate of the last pass is 10-16%. The rolling speed is controlled at 8-12 m / s. Medium-pressure water descaling is carried out between the first and second passes, with a descaling water pressure of 200-280 bar. The final rolling temperature is controlled at 880-920℃. 5) Cooling is performed using a front-end cooling method, with a front-end cooling rate ≥140℃ / s; 6) Perform coiling. The coiling temperature is dynamically matched with the effective Ti. The coiling temperature is controlled as 550-1035*[effective Ti], in °C. Wherein, [effective Ti] = [Ti]-3*[S]-3.42*[N], and [Ti], [S], and [N] are the weight percentage contents of Ti, S, and N in the steel composition, respectively. 7) Perform pickling continuously at a pickling speed of 100-120 m / min to obtain the thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance. The thickness is 1.8mm-5.0mm, tensile strength ≥780MPa, yield strength ≥660MPa, elongation ≥20%, hole expansion rate ≥95%, and pickling streak color difference defect degradation rate not exceeding 0.3%. The composition and weight percentage content of the automotive steel are as follows: C 0.045~0.059%, Si 0.03~0.09%, Mn 1.05~1.38%, Nb 0.012~0.019%, Ti 0.085~0.105%, Cr 0.42~0.65%, [Cr]*[Ti]≤0.045*10 -4 N≤0.004%, S≤0.004%, the remainder being Fe and unavoidable impurities; where [Cr] and [Ti] represent the weight percentages of Cr and Ti, respectively.

2. The method for producing thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance according to claim 1, characterized in that, The weight percentage content of Si is 0.03~0.08%.

3. The method for producing thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance according to claim 1, characterized in that, The weight percentage content of Mn is 1.08~1.36%.

4. The method for producing thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance according to claim 1, characterized in that, The weight percentage content of Ti is 0.088~0.103%.

5. The method for producing thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance according to claim 1, characterized in that, The weight percentage content of Cr is 0.43~0.56%.

6. The method for producing thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance according to claim 1, characterized in that, The temperature of the billet entering the furnace is controlled at 825-948℃, the time in the furnace is 20-33 minutes, and the temperature at the exit of the furnace is 1205-1223℃.

7. The method for producing thin-gauge 780MPa grade automotive steel with high surface quality and high hole expansion and flanging performance according to claim 1, characterized in that, The automotive steel produced has a thickness of 1.8mm to 4.0mm, tensile strength ≥800MPa, yield strength ≥680MPa, elongation ≥20%, hole expansion rate ≥95%, and pickling stripe color difference defect degradation rate not exceeding 0.25%.

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