A hot base low alloy high strength steel sheet for a white body chassis and a method for producing the same

Abstract

A kind of white body chassis is used with hot base low-alloy high-strength steel continuous hot-dip galvanized steel plate and its production method, the chemical composition in steel is as follows by mass percent: C: 0.070%~0.100%, Mn: 1.20%~1.50%, S≤0.005%, P: 0.010%~0.0.020%, Si: 0.10%~0.20%, Als: 0.030%~0.060%, Nb+Ti: 0.070%~0.130%, N≤0.0050%, the rest is Fe and unavoidable impurities.The application has short technological process, high yield ratio, good strength and toughness matching by innovative component design, reducing cold rolling process, hot rolling control cooling and annealing technology below recrystallization temperature, at the same time meets the requirements of body chassis for corrosion resistance, solves many practical problems such as insufficient forming performance when using the same yield strength level cold base low-alloy high-strength steel and dual-phase steel galvanized product to match white body chassis processing, and the thickness specification of more than 2.5mm is limited.

Description

Technical Field

[0001] This invention belongs to the field of low-alloy high-strength steel for automobiles, specifically relating to a hot-dip galvanized steel sheet of hot-dip low-alloy high-strength steel for body-in-white chassis and its production method. Background Technology

[0002] With major domestic and international automakers such as Volkswagen, BMW, Mercedes-Benz, and Stellantis continuously raising their requirements for the corrosion resistance of body-in-white, such as the requirement of 10-15 years without perforation corrosion and at least 5 years without blistering or rust affecting aesthetics in visible areas like door edges and hood edges, the use of coating products as much as possible in body-in-white has become an industry consensus. Due to the complex service environment of the body-in-white chassis, chassis rust is the leading cause of recalls of passenger vehicles worldwide, with the main focus being the functional degradation and safety hazards caused by rust. This is primarily because the hot-rolled high-strength steel used in the chassis is often supplied in a hot-rolled, pickled state, which has extremely low corrosion resistance and can no longer meet the current material requirements for lightweight, safe, and highly corrosion-resistant body-in-whites. In recent years, an increasing number of galvanized cold-based ultra-high-strength steel products with a yield strength of 550MPa have been used in the stamping or roll forming of safety components such as A-pillars, B-pillars, and sill beams of car bodies. However, their alloy cost per ton of steel is relatively high, and the thickness is generally controlled below 2.0mm, failing to meet the material requirements for car body chassis processing and forming, as well as supplying materials with thicknesses exceeding 2.5mm. Similarly, while galvanized cold-based low-alloy high-strength steel products with a yield strength of 550MPa can meet the requirements for high corrosion resistance and processing of car body chassis, they suffer from inherent production defects such as a long production process, high price per ton of steel, and difficulty in achieving thicknesses exceeding 2.5mm, thus failing to meet the stringent rigidity requirements of car body chassis.

[0003] In summary, this invention, through optimized alloy design combined with controlled rolling and cooling in the hot rolling process and annealing below the recrystallization temperature in the finished product process, proposes an alloy with low cost, short process flow, and finished product with high corrosion resistance, yield strength ratio, good plasticity and toughness, and formability. It is suitable for continuous hot-dip galvanized steel sheets of hot-dip low-alloy high-strength steel for body-in-white chassis and its production method, solving many practical problems in matching cold-dip low-alloy high-strength steel and duplex steel galvanized products of the same yield strength level with body-in-white chassis processing. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide a hot-dip galvanized steel sheet of hot-dip low-alloy high-strength steel for body-in-white chassis and its production method. Through innovative composition design, reduction of cold rolling process, hot rolling controlled rolling and cooling, and annealing technology below recrystallization temperature, the process has a short process flow and the finished product has a high yield strength ratio, good plasticity and toughness, and good formability. It solves many practical problems such as insufficient formability and limited supply of thickness specifications above 2.5mm when cold-dip low-alloy high-strength steel and duplex steel galvanized products of the same yield strength level are used in the processing of body-in-white chassis.

[0005] To achieve the above objectives, the present invention employs the following technical solution: A hot-dip galvanized steel sheet for a white body chassis made of low-alloy high-strength steel has the following chemical composition by mass percentage: C: 0.070%–0.100%, Mn: 1.20%–1.50%, S≤0.005%, P: 0.010%–0.020%, Si: 0.10%–0.20%, Als: 0.030%–0.060%, Nb+Ti: 0.070%–0.130%, N≤0.0050%, with the remainder being Fe and unavoidable impurities.

[0006] The hot-dip galvanized steel sheet for the body-in-white chassis of this invention has a high yield strength level, requiring the simultaneous utilization of solid solution strengthening, grain refinement strengthening, second-phase precipitation strengthening, and phase transformation strengthening to enhance strength. In terms of composition design, Nb+Ti composite microalloying reinforcement is added to the conventional C-Si-Mn base. The process employs hot rolling controlled rolling and cooling, and annealing below the recrystallization temperature to ensure the material possesses a high yield strength ratio, good ductility and toughness, and good formability. The design concept of the main alloying elements is as follows: Carbon: An interstitial solid solution strengthening element, mainly used here to provide sufficient carbon for pearlite transformation and second-phase precipitation, ensuring that the required phase transformation strengthening and second-phase precipitation strengthening effects are achieved. Therefore, the carbon content of this invention is C: 0.070% to 0.100%.

[0007] Manganese: an austenitizing and solid solution strengthening element, mainly playing a role in substitution solid solution strengthening and grain refinement, while also expanding the hot rolling process window. The manganese content in this invention is Mn: 1.20%–1.50%.

[0008] Silicon: A ferritic and solid solution strengthening element, it has the effects of solid solution strengthening of the ferrite matrix and grain refinement, which is beneficial to improving strength indicators. However, excessively high Si content is detrimental to the surface quality of coatings, so the addition ratio should not be too high. Therefore, the silicon content in this invention is designed as follows: Si: 0.10%-0.20%.

[0009] Niobium + Titanium: These are fine-grained strengthening elements and strong carbide elements, mainly playing a role in second-phase precipitation strengthening and fine-grained strengthening, which is beneficial to improving strength indicators. In this invention, the niobium and titanium content is designed as: Nb + Ti: 0.070%~0.130%.

[0010] Phosphorus: A ferritic and grain-refining strengthening element, it has the effect of solid solution strengthening of ferrite matrix and grain refinement strengthening, but it has a serious tendency to segregate and a strong negative effect of reducing the toughness of steel. The addition amount is limited to 0.010%-0.020% to make full use of its metallurgical benefits.

[0011] Aluminum: A ferritic and grain-refining strengthening element. It is mainly introduced unavoidably as a deoxidizer during the steelmaking process. Its content is usually controlled at 0.030% to 0.060%, which can play the role of solid solution strengthening of ferrite matrix and grain refinement strengthening.

[0012] Furthermore, the thickness of the steel plate in this invention is 2.5mm to 6.0mm.

[0013] Furthermore, the steel plate of this invention has a yield strength of 550MPa to 650MPa, a tensile strength of ≥610MPa, and a transverse elongation A. 80 ≥13%, yield strength ratio: ≥0.85. Microstructure characteristics of the steel plate: F+P, pearlite is granular, pearlite volume fraction: 25%~30%.

[0014] A method for producing hot-dip galvanized steel sheet of low-alloy high-strength steel for body-in-white chassis includes hot rolling, pickling, continuous galvanizing and finishing processes, as detailed below: The hot rolling process includes: heating the continuous casting billet to 1250℃~1300℃, the continuous casting billet in the furnace for 190~230min, the finishing rolling start temperature of 1070℃~1100℃, the finishing rolling temperature of 880~910℃, and the laminar flow cooling adopts the process of centralized cooling in the front section to 650℃~670℃ + air cooling in the rear section to the coiling temperature of 590℃~610℃.

[0015] In this invention, the laminar cooling process of the hot rolling process adopts a front-end centralized cooling + rear-end air cooling process to ensure that the resulting pearlite lamellar spacing is small and fully exerts its phase transformation strengthening effect. At the same time, it ensures that most Nb and Ti are dissolved in ferrite and pearlite, promoting their dense precipitation in the subsequent annealing and homogenization stage, thereby improving the yield and tensile strength indicators.

[0016] In the pickling process, the free acid concentration of the acid solution is ≥45g / L, and the acid solution temperature is ≥75℃.

[0017] The continuous galvanizing process includes: a soaking zone temperature controlled at 650℃~700℃, a soaking and holding time of 80s~220s, followed by rapid cooling to 440℃~460℃, a rapid cooling rate of 5℃ / s~15℃ / s, an entry temperature of 450℃~470℃ into the zinc pot, followed by cooling to 200℃~240℃ at a post-galvanizing cooling rate of ≥5℃ / s, and then cooling to room temperature through a water quenching process.

[0018] In the finishing process, the rolling force of the finishing machine is controlled at 3500kN to 4500kN.

[0019] Compared with existing technologies, the beneficial effects of this invention are: This invention, based on the conventional C-Si-Mn composite microalloying strengthening concept supplemented by Nb+Ti, adopts a process strategy of hot rolling controlled rolling and cooling followed by annealing below the recrystallization temperature. It simultaneously leverages four strengthening mechanisms: solid solution strengthening, grain refinement strengthening, second-phase precipitation strengthening, and phase transformation strengthening. This results in a short process flow, a high yield strength ratio, and a good strength-toughness match in the finished product. It also meets the corrosion resistance requirements of car body chassis and solves many practical problems such as insufficient formability and limited supply of thickness specifications above 2.5mm when using cold-based low-alloy high-strength steel and dual-phase galvanized steel products of the same yield strength level in the processing of car body chassis. It perfectly responds to the material selection needs of OEMs for car body chassis that require high strength and high corrosion resistance. Attached Figure Description

[0020] Figure 1 This is a microstructure diagram of the steel plate from Example 1. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are merely illustrative and are not intended to limit the present invention.

[0022] Example 1: In this embodiment, the thickness of the hot-dip galvanized steel sheet used for the body-in-white chassis is 3.5 mm. Its chemical composition and mass percentage are shown in Table 1.

[0023] The production method of the hot-dip galvanized steel sheet for the body-in-white chassis using hot-dip low-alloy high-strength steel in this embodiment includes hot rolling, pickling, continuous galvanizing, and finishing processes. The specific process steps are as follows: (1) Hot rolling process: The continuous casting billet is heated to 1270℃, the continuous casting billet is in the furnace for 200 minutes, the starting rolling temperature of the finishing rolling is 1085℃, the final rolling temperature is 900℃, the laminar flow cooling section is concentrated to 665℃, and then air-cooled to the coiling temperature, and the coiling temperature is 610℃.

[0024] (2) Pickling process: acid concentration of free acid solution: 50g / L, acid solution temperature: 85℃.

[0025] (3) Continuous galvanizing process: temperature of the soaking section: 660℃, soaking time: 120s, cooling rate of the rapid cooling section: 11℃ / s, end temperature of rapid cooling: 455℃, temperature of zinc pot: 465℃, cooling rate after galvanizing: 10℃ / s, cooling temperature after galvanizing: 210℃, and then cooled to room temperature by water quenching process.

[0026] (4) Finishing process: Finishing rolling force: 3700KN.

[0027] The mechanical properties and pearlite volume fraction of the hot-dip galvanized steel sheet used in the body-in-white chassis of this embodiment are shown in Table 2. The microstructure consists of ferrite and pearlite, with the pearlite being granular. (See attached table.) Figure 1 .

[0028] Example 2: The thickness of the hot-dip galvanized steel sheet used for the body-in-white chassis in this embodiment is 2.5mm. Its chemical composition and mass percentage are shown in Table 1.

[0029] The production method of the hot-dip galvanized steel sheet for the body-in-white chassis using hot-dip low-alloy high-strength steel in this embodiment includes hot rolling, pickling, continuous galvanizing, and finishing processes. The specific process steps are as follows: (1) Hot rolling process: The continuous casting billet is heated to 1250℃, the continuous casting billet is in the furnace for 190 minutes, the starting rolling temperature of the finishing rolling is 1070℃, the final rolling temperature is 880℃, the laminar flow cooling section is concentrated to 650℃, and then air-cooled to the coiling temperature of 590℃.

[0030] (2) Pickling process: Free acid concentration of acid solution: 45g / L, acid solution temperature: 75℃.

[0031] (3) Continuous galvanizing process: temperature of the soaking section: 650℃, soaking time: 80s, cooling rate of the rapid cooling section: 15℃ / s, end temperature of rapid cooling: 460℃, temperature of zinc pot: 470℃, cooling rate after galvanizing: 12℃ / s, cooling temperature after galvanizing: 200℃, and then cooled to room temperature by water quenching process.

[0032] (4) Finishing process: Finishing rolling force: 3500KN.

[0033] Table 2 shows the mechanical properties and pearlite volume fraction of the hot-dip galvanized steel sheet used in the body-in-white chassis of this embodiment. The microstructure and adsorption... Figure 1 Similarly, they will not be listed one by one here.

[0034] Example 3: In this embodiment, the thickness of the hot-dip galvanized steel sheet used for the body-in-white chassis is 6.0 mm. Its chemical composition and mass percentage are shown in Table 1.

[0035] The production method of the hot-dip galvanized steel sheet for the body-in-white chassis using hot-dip low-alloy high-strength steel in this embodiment includes hot rolling, pickling, continuous galvanizing, and finishing processes. The specific process steps are as follows: (1) Hot rolling process: The continuous casting billet is heated to 1300℃, the continuous casting billet is in the furnace for 230 minutes, the finishing rolling start temperature is 1100℃, the final rolling temperature is 910℃, the laminar flow cooling section is concentrated to 670℃, and then air-cooled to the coiling temperature of 610℃.

[0036] (2) Pickling process: Free acid concentration of acid solution: 51g / L, acid solution temperature: 83℃.

[0037] (3) Continuous galvanizing process: temperature of the soaking section: 700℃, soaking time: 220s, cooling rate of the rapid cooling section: 12℃ / s, end temperature of rapid cooling: 440℃, temperature of zinc pot: 450℃, cooling rate after galvanizing: 5℃ / s, cooling temperature after galvanizing: 240℃, and then cooled to room temperature by water quenching process.

[0038] (4) Finishing process: Finishing rolling force: 4500KN.

[0039] Table 2 shows the mechanical properties and pearlite volume fraction of the hot-dip galvanized steel sheet used in the body-in-white chassis of this embodiment. The microstructure and adsorption... Figure 1 Similarly, they will not be listed one by one here.

[0040] Example 4: In this embodiment, the thickness of the hot-dip galvanized steel sheet used for the body-in-white chassis is 4.0 mm. Its chemical composition and mass percentage are shown in Table 1.

[0041] The production method of the hot-dip galvanized steel sheet for the body-in-white chassis using hot-dip low-alloy high-strength steel in this embodiment includes hot rolling, pickling, continuous galvanizing, and finishing processes. The specific process steps are as follows: (1) Hot rolling process: The continuous casting billet is heated to 1280℃, the continuous casting billet is in the furnace for 205 minutes, the finishing rolling start temperature is 1080℃, the final rolling temperature is 900℃, the laminar flow cooling section is concentrated to 655℃, and then air-cooled to the coiling temperature of 596℃.

[0042] (2) Pickling process: Free acid concentration of acid solution: 53g / L, acid solution temperature: 82℃.

[0043] (3) Continuous galvanizing process: temperature of the soaking section: 670℃, soaking time: 140s, cooling rate of the rapid cooling section: 9℃ / s, end temperature of rapid cooling: 452℃, temperature of zinc pot: 455℃, cooling rate after galvanizing: 9℃ / s, cooling temperature after galvanizing: 215℃, and then cooled to room temperature by water quenching process.

[0044] (4) Finishing process: Finishing rolling force: 3800KN.

[0045] Table 2 shows the mechanical properties and pearlite volume fraction of the hot-dip galvanized steel sheet used in the body-in-white chassis of this embodiment. The microstructure and adsorption... Figure 1 Similarly, they will not be listed one by one here.

[0046] Example 5: In this embodiment, the thickness of the hot-dip galvanized steel sheet used for the body-in-white chassis is 4.5 mm. Its chemical composition and mass percentage are shown in Table 1.

[0047] The production method of the hot-dip galvanized steel sheet for the body-in-white chassis using hot-dip low-alloy high-strength steel in this embodiment includes hot rolling, pickling, continuous galvanizing, and finishing processes. The specific process steps are as follows: (1) Hot rolling process: The continuous casting billet is heated to 1289℃, the continuous casting billet is in the furnace for 220 minutes, the finishing rolling start temperature is 1090℃, the final rolling temperature is 895℃, the laminar flow cooling section is concentrated to 660℃, and then air-cooled to the coiling temperature of 605℃.

[0048] (2) Pickling process: Free acid concentration of acid solution: 50g / L, acid solution temperature: 86℃.

[0049] (3) Continuous galvanizing process: temperature of the soaking section: 680℃, soaking time: 160s, cooling rate of the rapid cooling section: 8℃ / s, end temperature of rapid cooling: 448℃, temperature of zinc pot: 460℃, cooling rate after galvanizing: 8℃ / s, cooling temperature after galvanizing: 220℃, and then cooled to room temperature by water quenching process.

[0050] (4) Finishing process: Finishing rolling force: 4000KN.

[0051] Table 2 shows the mechanical properties and pearlite volume fraction of the hot-dip galvanized steel sheet used in the body-in-white chassis of this embodiment. The microstructure and adsorption... Figure 1 Similarly, they will not be listed one by one here.

[0052] Example 6: In this embodiment, the thickness of the hot-dip galvanized steel sheet used for the body-in-white chassis is 5.5 mm. Its chemical composition and mass percentage are shown in Table 1.

[0053] The production method of the hot-dip galvanized steel sheet for the body-in-white chassis using hot-dip low-alloy high-strength steel in this embodiment includes hot rolling, pickling, continuous galvanizing, and finishing processes. The specific process steps are as follows: (1) Hot rolling process: The continuous casting billet is heated to 1290℃, the continuous casting billet is in the furnace for 225 minutes, the finishing rolling start temperature is 1090℃, the final rolling temperature is 904℃, the laminar flow cooling section is concentrated to 665℃, and then air-cooled to the coiling temperature of 608℃.

[0054] (2) Pickling process: acid concentration of free acid solution: 50g / L, acid solution temperature: 85℃.

[0055] (3) Continuous galvanizing process: temperature of the soaking section: 690℃, soaking time: 200s, cooling rate of the rapid cooling section: 7℃ / s, end temperature of rapid cooling: 445℃, temperature of zinc pot: 455℃, cooling rate after galvanizing: 7.0℃ / s, cooling temperature after galvanizing: 230℃, and then cooled to room temperature by water quenching process.

[0056] (4) Finishing process: Finishing rolling force: 4300KN.

[0057] Table 2 shows the mechanical properties and pearlite volume fraction of the hot-dip galvanized steel sheet used in the body-in-white chassis of this embodiment. The microstructure and adsorption... Figure 1 Similarly, they will not be listed one by one here.

[0058] Table 1. Chemical composition (wt%) of steel plates in each embodiment Table 2. Longitudinal mechanical properties of steel plates in various embodiments The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A hot-dip galvanized steel sheet for a body-in-white chassis made of hot-dip low-alloy high-strength steel, characterized in that... The chemical composition of the steel, by mass percentage, is as follows: C: 0.070%~0.100%, Mn: 1.20%~1.50%, S≤0.005%, P: 0.010%~0.020%, Si: 0.10%-0.20%, Als: 0.030%~0.060%, Nb+Ti: 0.070%~0.130%, N≤0.0050%, with the remainder being Fe and unavoidable impurities.

2. The continuously hot-dip galvanized steel sheet for a body-in-white chassis using hot-dip galvanized low-alloy high-strength steel as described in claim 1, characterized in that... The thickness of the steel plate is 2.5mm to 6.0mm.

3. The continuously hot-dip galvanized steel sheet for a body-in-white chassis using hot-dip low-alloy high-strength steel as described in claim 1, characterized in that... The steel plate has a yield strength of 550MPa to 650MPa, a tensile strength of ≥610MPa, and a transverse elongation of A. 80 ≥13%, yield strength ratio: ≥0.

85.

4. The continuously hot-dip galvanized steel sheet for a body-in-white chassis using hot-dip galvanized low-alloy high-strength steel as described in claim 1, characterized in that... The microstructure of the steel plate is characterized by F+P, with pearlite in granular form and a pearlite volume fraction of 25%–30%.

5. A method for producing a hot-dip galvanized steel sheet of hot-dip low-alloy high-strength steel for a body-in-white chassis as described in any one of claims 1-4, characterized in that, The process includes hot rolling, pickling, continuous galvanizing, and finishing. The hot rolling laminar flow cooling process involves concentrated cooling at the front to 660℃~670℃ followed by air cooling at the rear to the coiling temperature. The continuous galvanizing process includes: a soaking zone temperature controlled at 650℃~700℃, a soaking time of 80s~220s, followed by rapid cooling to 440℃~460℃, a rapid cooling rate of 5℃ / s~15℃ / s, an entry temperature of 450℃~470℃ into the zinc pot, followed by post-galvanizing cooling at a rate of ≥5℃ / s to 200℃~240℃, and finally a water quenching process to room temperature.

6. The method for producing hot-dip galvanized steel sheet of hot-dip low-alloy high-strength steel for body-in-white chassis according to claim 5, characterized in that, The hot rolling process further includes: heating the continuously cast billet to 1250℃~1300℃, keeping the billet in the furnace for 190~230min, starting the finishing rolling at 1070℃~1100℃, finishing the rolling at 880~910℃, and coiling at 590℃~610℃.

7. The method for producing hot-dip galvanized steel sheet of hot-dip low-alloy high-strength steel for body-in-white chassis according to claim 5, characterized in that, In the pickling process, the free acid concentration of the acid solution is ≥45g / L, and the acid solution temperature is ≥75℃.

8. The method for producing hot-dip galvanized steel sheet of hot-dip low-alloy high-strength steel for body-in-white chassis according to claim 5, characterized in that, In the finishing process, the rolling force of the finishing machine is controlled at 3500kN to 4500kN.

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