A low cost high surface quality steel for automotive chassis with thin gauge 590mpa grade produced by csp line and method thereof

Abstract

The application discloses a kind of thin gauge 590MPa grade extremely low cost high surface quality automobile chassis steel, using low Mn content low Nb content extremely low Si content composition design, its composition and weight percentage content are as follows: C 0.034~0.059%, Si 0.03~0.14%, Mn 0.64~0.97%, Nb 0.012~0.020%, Ti 0.031~0.057%, N≤0.005%, S≤0.004%, the rest is Fe and inevitable impurities;Its production method uses CSP thin slab continuous casting and rolling process production, the tensile strength of the obtained steel plate is ≥590MPa, yield strength is ≥440MPa, elongation is ≥24%, the hole expansion ratio of pickling steel plate is ≥120%, and the degradation rate of pickling stripe color difference defect is not more than 0.2%.The application adopts extremely low cost composition design, and simultaneously has excellent surface quality, still can satisfy the demand of automobile chassis parts to surface quality and high hole expansion flanging performance, and further reduces production cost.

Description

Technical Field

[0001] This invention belongs to the field of pickled high-performance automotive steel, specifically relating to a method and product for producing thin-gauge 590MPa grade automotive chassis steel with extremely low cost and high surface quality using a CSP production line. Background Technology

[0002] In the 21st century, reducing fuel consumption, CO2, and exhaust emissions has become a societal need for the automotive industry. To adapt to this trend, the steel industry, as a material producer, has developed many types of ultra-high-strength steel plates to help reduce vehicle weight and meet the new requirements of the automotive industry.

[0003] As a crucial component of automobiles, automotive chassis parts, besides requiring high strength and thinning, often incorporate numerous round holes for assembly and use. These holes necessitate a process of punching, enlarging, and flanging during forming to obtain the final part. Extensive data shows that a material's hole-expanding performance is not entirely equivalent to its forming performance. Generally, better plasticity in steel correlates with better forming performance, but this doesn't necessarily mean better hole-expanding and flanging performance. For example, traditional duplex steel possesses good plasticity and forming performance, but its hole-expanding and flanging performance is poor, making it unsuitable for producing automotive suspension and chassis parts requiring hole expansion. Therefore, the increasingly stringent requirements for automotive chassis parts in the ever-evolving automotive industry demand not only high strength and high plasticity but also excellent hole-expanding and flanging performance and surface quality. The minimum thickness requirement for pickled steel sheets after high-strength thinning reaches 1.5mm. Furthermore, to meet the demands of fierce market competition, cost control has become paramount for all automakers.

[0004] CN117248161A discloses a 600MPa grade, low-cost, high-hole-expansion-performance pickled automotive steel sheet. The chemical composition and weight percentage content are as follows: C 0.035%~0.065%, Si 0.30%~0.65%, Mn 1.40%~1.65%, Nb 0.020%~0.030%, Ti 0.010%~0.025%, P≤0.010%, S≤0.004%, with the remainder being Fe and unavoidable impurities. However, the steel plate contains more than 1.4% Mn and more than 0.020% Nb. In today's increasingly competitive market environment, the composition design with extremely low cost can better meet market demand. Moreover, it increases the porosity by adding Si to suppress cementite precipitation. However, adding Si will affect the surface quality and form tiger stripe pattern on the surface of the hot-rolled plate, which cannot fully meet the surface quality requirements of automotive chassis parts. In addition, in order to reduce the risk of tiger stripe pattern defects caused by high Si content, high-pressure water descaling must be carried out before rolling, which will also increase the process cost. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides a thin-gauge, 590MPa grade steel for automotive chassis with extremely low cost and high surface quality. Specifically, the steel sheet has a thickness of 1.0mm to 5.0mm. After pickling, the steel sheet has a tensile strength ≥590MPa, a yield strength ≥440MPa, an elongation ≥24%, and a hole expansion rate ≥120%, exhibiting excellent surface quality and meeting the requirements of automotive chassis components for hole expansion and flanging performance.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A thin-gauge, 590MPa grade automotive chassis steel with extremely low cost and high surface quality is designed with a low Mn content, low Nb content, and extremely low Si content. Its composition and weight percentage content are as follows: C 0.034~0.059%, Si 0.03~0.14%, Mn 0.64~0.97%, Nb 0.012~0.020%, Ti 0.031~0.057%, N≤0.005%, S≤0.004%, with the remainder being Fe and unavoidable impurities.

[0008] Preferably, the weight percentage content of C is 0.038 to 0.055%.

[0009] Preferably, the weight percentage content of Si is 0.06 to 0.11%.

[0010] Preferably, the weight percentage content of Mn is 0.65 to 0.90%.

[0011] Preferably, the weight percentage content of Ti is 0.035 to 0.053%.

[0012] This invention also provides a method for producing thin-gauge 590MPa grade ultra-low cost high surface quality automotive chassis steel, using a CSP thin slab continuous casting and rolling process. The process flow includes, in sequence: hot metal desulfurization, smelting and refining, continuous casting, slab heating, rolling, cooling, coiling, and pickling, etc., with the specific steps as follows:

[0013] 1) After desulfurization, smelting and refining of molten iron, steel with the target composition is obtained;

[0014] 2) After refining, 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 4.0-4.6m / min;

[0015] 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 1200-1240℃;

[0016] 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 860-900℃.

[0017] 5) Cooling is performed using a front-end cooling method, with a front-end cooling rate ≥120℃ / s;

[0018] 6) Perform coiling. The coiling temperature is dynamically matched with the effective Ti. The coiling temperature is controlled as 500-1020*[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.

[0019] 7) Perform pickling continuously at a pickling speed of 100-120 m / min.

[0020] Preferably, the temperature of the billet entering the furnace is controlled at 835-910℃, the time in the furnace is 25-35 minutes, and the temperature of the billet exiting the furnace is 1210-1233℃.

[0021] The thin-gauge, 590MPa grade, ultra-low-cost, high-surface-quality automotive chassis steel produced by the above method has a thickness of 1.0mm to 5.0mm. The steel sheet has a tensile strength ≥590MPa, yield strength ≥440MPa, elongation ≥24%, and ultimate porosity ≥120%. The metallographic structure is a fine-grained bainite + ferrite structure with a grain size grade ≥10. More preferably, the thickness is 1.0mm to 4.0mm, the steel sheet has a tensile strength ≥615MPa, yield strength ≥490MPa, elongation ≥25%, ultimate porosity ≥125%, and the pickling streak color difference defect degradation rate does not exceed 0.2%. The metallographic structure is a fine-grained bainite + ferrite structure with a grain size grade ≥10.5.

[0022] The roles and mechanisms of each raw material and main process in this invention:

[0023] C: To ensure the steel plate achieves a tensile strength of over 590 MPa, the carbon content must be at least 0.034%; 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 is controlled within the range of 0.034–0.059%, preferably 0.038–0.055%.

[0024] 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%.

[0025] Mn: Mn is the most effective element for improving strength and toughness, and it is also one of the important alloying elements used in this invention. Mn can stabilize austenite and reduce the critical quenching rate of steel, thereby improving the hardenability of the material. Typically, 590MPa grade high-strength steel has an Mn content of over 1.0%, generally around 1.5%. This invention adopts an extremely low-cost composition design, controlling the Mn content at 0.64–0.97%, preferably 0.65–0.90%. Compared to the traditional 1.5% Mn content of 590MPa grade high-strength steel, this invention can reduce the cost per ton of steel by 80–140 yuan, equivalent to a reduction of approximately 3% in finished product cost.

[0026] Sulfur (S): Sulfur is an impurity element in steel. Excessive sulfur content increases the steel's tendency for hot brittleness. Sulfur in steel often exists as manganese sulfides. These sulfide inclusions are highly detrimental to the steel's impact toughness and cause anisotropy in its properties. Therefore, the sulfur content in steel should be controlled as low as possible. Thus, the sulfur content in steel should be controlled below 0.004%.

[0027] Nitrogen (N): Nitrogen is an impurity element in steel. If the N content is too high, it will combine with Ti in the steel to form large TiN particles, thereby reducing the precipitation strengthening and grain refinement strengthening effects of Ti. Therefore, the lower the nitrogen content, the better; in actual production, it is generally controlled below 0.005%.

[0028] Nb: Nb is one of the important 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. Furthermore, the refined bainite structure can effectively suppress crack propagation and increase the expansion rate during hole expansion and flanging. However, if the Nb content exceeds 0.020%, it will lead to increased alloy costs. Therefore, the Nb content in this invention is controlled at 0.012–0.020%.

[0029] Ti: Ti is one of the key alloying elements used in this invention. Ti can play a role in grain refinement and precipitation strengthening. In this invention, 0.031% Ti is added to replace the strengthening effect of Mn. At the same time, the alloy cost of Ti is very low, achieving an extremely low-cost design. If the Ti content is too high, it will increase the number of coarse carbide and nitride inclusions, thereby affecting the overall mechanical properties. Therefore, the Ti content in this invention should be controlled between 0.031% and 0.057%, preferably between 0.035% and 0.053%.

[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. Moreover, it achieves extreme low cost by reducing Mn and Nb content, adds a small amount of relatively inexpensive Ti, and limits S and N, so that the added Ti can fully exert its precipitation strengthening and grain refinement purposes. Furthermore, since this invention adopts a low-Si composition design, high-pressure water descaling before rolling is not required. Based on the above changes in composition design, this invention designs the casting speed, billet heating process, and coiling addition of the continuous casting billet, so that the added Ti can not only compensate for the strength loss after reducing Mn and Nb through precipitation strengthening, but also compensate for the reduction in porosity after reducing Si by refining the grain through controlling the size of the precipitates.

[0031] This invention increases the billet exit temperature to 1200–1240°C and the furnace dwell time to 20–40 minutes, ensuring that the steel plate temperature meets the requirements of each stage of the rolling process. To complement the reduction of Mn and increase of Ti, and to ensure Ti precipitation and fine-grain strengthening, this invention specifically studies the billet heating temperature. The heating temperature has a significant impact on the various properties of the finished product, and a heating temperature above 1200°C must be maintained. When the heating temperature is too low and the furnace dwell time is too short, Ti in the billet cannot be completely dissolved, and during subsequent rolling and cooling, sufficient fine and dispersed precipitates cannot precipitate, failing to meet the requirements for strength and porosity. However, when the heating temperature is too high and the furnace dwell time is too long, the original austenite grains before rolling become coarse, which prevents the subsequent rolling process from completing the austenite recrystallization process. This leads to mixed grains (i.e., uneven grain size) in the final product, which seriously affects product quality.

[0032] 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 0.012–0.035%. 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 (500 - 1020 * [effective Ti]), in °C.

[0033] Compared with the prior art, the beneficial effects of the present invention are:

[0034] This invention employs a reasonable chemical composition, without adding expensive alloying elements such as Mo, V, Cu, and Ni. Furthermore, by using a composition design with low Mn, low Nb, and extremely low Si content, it meets the requirements for both extremely low cost and high surface quality. The addition of a certain amount of relatively inexpensive Ti ensures the performance requirements of the material. Based on this, the present invention optimizes the heating temperature and time of the billet and precisely matches the coiling temperature according to the effective Ti, maximizing the precipitation strengthening and fine-grain strengthening effects of Ti. This achieves high strength through various strengthening processes, as well as high elongation and high porosity through the fine-grained ferrite and bainite structure, simultaneously meeting the requirements for high strength, high elongation, and high porosity. The result is a 590MPa grade ultra-low-cost, high-surface-quality automotive chassis steel, achieving a fine-grained bainite + ferrite structure with a grain size grade ≥10. It guarantees a tensile strength ≥590MPa, a yield strength ≥440MPa, an elongation ≥24%, excellent porosity expansion performance (≥120%), and a pickling streak color difference defect degradation rate of no more than 0.2%. Compared to traditional 590MPa grade high porosity steel, this invention employs an ultra-low-cost composition design while maintaining excellent surface quality, still meeting the surface quality and high porosity expansion and flanging performance requirements of automotive chassis components, and further reducing production costs. Attached Figure Description

[0035] Figure 1 The image shows the metallographic structure (bainite + ferrite) of the pickled steel sheet for automotive chassis with an ultra-low cost and high surface quality of 590MPa grade. Detailed Implementation

[0036] 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.

[0037] 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. The hole expansion rate λ= 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 hole expansion rate of the steel plate prepared by this invention can reach over 120%.

[0038] Example

[0039] The following is a detailed description of each embodiment:

[0040] Table 1 is a list of chemical components of the various embodiments and comparative examples of the present invention;

[0041] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0042] Table 3 lists the performance test results of each embodiment and comparative example of the present invention; wherein, the test standards for strength and elongation are GB / T 223, and the test standard for porosity is GB / T 24524.

[0043] The embodiments of the present invention produce thin-gauge 590MPa grade ultra-low cost high surface quality automotive chassis steel according to the following steps:

[0044] 1) Desulfurization of molten iron, desulfurization target: S≤0.001%, and the exposed surface of molten iron after slag removal is not less than 95%;

[0045] 2) Conventional converter smelting and conventional refining are used to obtain molten steel with the target composition;

[0046] 3) 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 4.0-4.6m / min;

[0047] 4) 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 1200-1240℃.

[0048] 5) Perform rolling, and control the reduction rate of the first pass to be 52-63%, the reduction rate of the second pass to be 50-60%, and the reduction rate of the last pass to be 10-16%; control the rolling speed to be 8-12 m / s; and perform medium-pressure water descaling between the first and second passes, with a descaling water pressure of 200-280 bar; control the final rolling temperature to be 860-900℃.

[0049] 6) Front-end cooling is used, with a front-end cooling rate ≥120℃ / s;

[0050] 7) Perform winding and control the winding temperature = 500 - 1020 * [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, 0.0208 is substituted into the winding temperature formula, and the winding temperature = 500 - 1020 * 0.0208 ≈ 479 °C). Specifically, as shown in Table 3, it is in the range of 465 to 490 °C.

[0051] 8) 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 produce a fine-grained bainitic + ferrite microstructure with a grain size grade ≥10. The steel plate exhibits tensile strength ≥590MPa, yield strength ≥440MPa, elongation ≥24%, and expansion rate ≥120%. This significantly improves the expansion performance compared to traditional 600MPa high-strength steels such as DP600 (with an expansion rate of only about 60%), and significantly reduces production costs and improves surface quality compared to traditional high-expansion steel FB590 (which has higher alloy costs, a striped surface morphology, and an expansion rate of about 80%). The degradation rate of pickling stripe color difference defects does not exceed 0.2%, while also ensuring mechanical properties, meeting the requirements of automotive chassis components for surface quality and high expansion and flanging performance. For example, in Comparative Examples 2 and 3, relatively high amounts of Si were added, which easily resulted in tiger-skin patterns in the hot-rolled steel plates, leading to pickling stripe color difference defects after pickling, failing to meet users' high surface quality requirements.

[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, 590MPa grade, ultra-low-cost, high-surface-quality automotive chassis steel, using a CSP thin slab continuous casting and rolling process, characterized in that... Includes the following steps: 1) After desulfurization, smelting and refining of molten iron, steel with the target composition is obtained; 2) After refining, 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 4.0-4.6 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 1210-1240℃; 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 860-900℃. 5) Cooling is performed using a front-end cooling method, with a front-end cooling rate ≥120℃ / s; 6) Perform coiling. The coiling temperature is dynamically matched with the effective Ti. The coiling temperature is controlled as 500-1020*[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 100~120m / min; The composition and weight percentage of the steel used in the automobile chassis are as follows: C 0.034~0.059%, Si 0.03~0.14%, Mn 0.64~0.97%, Nb 0.012~0.019%, Ti 0.031~0.057%, N≤0.005%, S≤0.004%, with the remainder being Fe and unavoidable impurities.

2. The production method according to claim 1, characterized in that, The weight percentage content of C is 0.038~0.055%.

3. The production method according to claim 1, characterized in that, The weight percentage content of Si is 0.06~0.11%.

4. The production method according to claim 1, characterized in that, The weight percentage content of Mn is 0.65~0.90%.

5. The production method according to claim 1, characterized in that, The weight percentage content of Ti is 0.035~0.053%.

6. The production method according to claim 1, characterized in that, The temperature of the billet entering the furnace is controlled at 835-910℃, the time in the furnace is 25-35 minutes, and the temperature at the exit of the furnace is 1210-1233℃.

7. The production method according to claim 1, characterized in that, The thickness of the steel used in the automobile chassis is 1.0mm~5.0mm, the tensile strength of the steel plate is ≥590MPa, the yield strength is ≥440MPa, the elongation is ≥24%, the ultimate hole expansion rate is ≥120%, and the degradation rate of pickling stripe color difference defects does not exceed 0.2%; the metallographic structure is bainite + ferrite structure, and the grain size grade is ≥10.

8. The production method according to claim 1, characterized in that, The steel used in the automobile chassis has a thickness of 1.0mm to 4.0mm, a tensile strength ≥615MPa, a yield strength ≥490MPa, an elongation ≥25%, a hole expansion rate ≥125%, and a pickling stripe color difference defect degradation rate not exceeding 0.2%; the metallographic structure is bainite + ferrite, and the grain size grade is ≥10.5.

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