A method for producing cold-rolled ultra-high-strength Q&P steel by pickling

Abstract

The present invention belongs to the technical field of cold-rolled ultra-high-strength steel production, and specifically relates to a cold-rolled ultra-high-strength Q&P steel acid rolling production method. The production method process includes: hot-rolled coil hood annealing and softening, hot-rolled finishing and re-coiling, and acid rolling production. The present invention reduces the difficulty of acid rolling production, and can reduce the yield strength of the hot-rolled coil by 150 to 350 MPa, the tensile strength by 200 to 450 MPa, and the elongation by 9 to 28%. The present invention enables general cold rolling production lines to have the ability to produce ultra-high-strength Q&P, and the cold-rolled ultra-high-strength Q&P steel acid-rolled coil obtained by the present invention fully meets the performance requirements after subsequent annealing or galvanizing treatment.

Description

Technical Field

[0001] The present invention belongs to the technical field of cold-rolled ultra-high-strength steel production, and in particular relates to an acid-rolling production method for cold-rolled ultra-high-strength Q&P steel. Background Art

[0002] Cold-rolled ultra-high-strength Q&P steel (quenched and partitioned steel) boasts exceptional strength and excellent ductility. Current production boasts a tensile strength exceeding 980 MPa. Finished Q&P steel contains a significant proportion of retained austenite, resulting in high elongation and excellent stamping properties. With increasing demands for lightweighting, safety, and energy conservation and emission reduction in automobiles, the demand for ultra-high-strength cold-rolled steel for automotive parts continues to grow. Q&P steel, with its excellent comprehensive properties, has become an environmentally friendly material in automotive manufacturing. It is commonly used in the manufacture of bumper beams, B-pillars, C-pillars, various safety features, reinforcements, and structural components. Cold-rolled ultra-high-strength Q&P steel utilizes significant additions of alloying elements such as Si, Mn, Mo, and Cr to enhance solid solution strengthening, precipitation strengthening, and phase transformation strengthening, ensuring the strength of the finished product. The yield strength of cold-rolled ultra-high-strength Q&P steel after hot rolling is often above 700 MPa, and the head-to-tail strength of hot-rolled strip exceeds 800 MPa, making it challenging to produce using pickling. Currently, the vast majority of steel mills have low design or actual production capacity for their existing cold-rolled pickling lines, requiring the yield strength of incoming pickling material (hot-rolled coil) to be less than 650 MPa (some even less). Furthermore, most mills lack dedicated cold-rolling lines for ultra-high-strength steel and, therefore, are unable to implement pickling for ultra-high-strength Q&P steel. A very small number of mills have cold-rolled pickling lines that can barely meet the production requirements for ultra-high-strength Q&P steel, but this presents significant production challenges. The thickness of the entire coil after pickling fluctuates significantly, making it difficult to meet the thickness accuracy of the finished product to meet order requirements. Therefore, it is crucial to provide a method to reduce the difficulty of pickling for ultra-high-strength Q&P steel.

[0003] Patent CN110616302 A provides a method for softening high-strength Q&P steel hot-rolled coils. This method has special requirements for the hot rolling production process. It is combined with a method of adding an insulation cover after the hot-rolled coil is produced to achieve the softening of the high-strength Q&P steel hot-rolled coils. It is difficult to add an insulation cover to each coil of steel after hot rolling, and the slow cooling area required for mass production is huge. This patent has a small degree of softening of the strength of the high-strength Q&P steel hot-rolled coils. The strength of the softened hot-rolled coils is still relatively high, which cannot meet the production capacity of the pickling production lines of most steel mills. In addition, this patent does not describe the technical solution for the pickling production of high-strength Q&P steel.

[0004] Patents CN 104988391 A and CN 105734213 A, etc., all achieve the production of cold-rolled Q&P steel products through specific composition design and certain hot rolling, cold rolling, and annealing processes. However, these methods place very high demands on laminar cooling and coiling temperature in hot rolling production, making them difficult to achieve in actual production processes. Furthermore, the Q&P steel hot-rolled coils obtained by these methods have very high yield and tensile strengths. In actual pickling industrial production, the rolling force of the rolling mill is enormous, exceeding the actual production capacity of most pickling production lines. Therefore, pickling thickness reduction cannot be achieved in industrial production. Summary of the Invention

[0005] To address the aforementioned shortcomings of the existing technology, the present invention provides a method for pickling cold-rolled ultra-high-strength Q&P steel. The production process includes: hood annealing and softening of the hot-rolled coil, hot-rolled finishing and recoiling, and pickling, reducing the difficulty of pickling. This method can reduce the yield strength of the hot-rolled coil by 150-350 MPa, the tensile strength by 200-450 MPa, and the elongation by 9-28%.

[0006] To achieve the above effects, the present invention provides a cold-rolled ultra-high strength Q&P steel pickling production method, which includes the following processes: hot-rolled coil hood annealing and softening, hot-rolled finishing and re-coiling, and pickling production.

[0007] Specifically, the hood annealing process for hot-rolled coils includes preheating, heating, soaking, and cooling. The preheating stage uses a relatively low temperature, with a preheating temperature of 300-450°C and a heating time of 1-3 hours. The heating stage uses a long heating period to ensure uniform temperature between the outer and inner rings of the coil, with a heating temperature of 520-650°C and a heating time of 3-5 hours. The soaking stage also uses a long holding period to ensure uniform microstructure and properties throughout the hot-rolled coil, with a temperature of 520-680°C and a soaking time of 6-9 hours. The cooling phase is divided into three stages: Cooling Stage 1 is air cooling, which reduces the temperature by increasing the flow rate of protective gas between the inner and outer hoods of the hood annealing furnace, to a temperature of ≤400°C, for 3-5 hours. Cooling Stage 2 is water cooling, which removes heat from the furnace hood by cooling the inner hood, allowing the coil to cool, with a cooling temperature of ≤180°C. The final cooling stage, Cooling Stage 3, involves natural cooling after the annealed coil is removed from the furnace. Protective gas is introduced throughout the hood annealing process to prevent oxidation.

[0008] Specifically, during the hot rolling recoiling process, the steel coil becomes loose after hood annealing and needs to be re-coiled on the hot rolling finishing production line. To avoid the friction between the coil layers caused by the loose steel coil affecting the surface quality of the strip, the uncoiling tension is 40-60KN, the winding tension is 60-80KN, and the strip speed is ≤150m / min. The outermost ring of the hood annealed hot-rolled coil is severely oxidized, and the strip head should be cut off by 5-10m.

[0009] Specifically, pickling production includes reasonable transition rules and pickling process, details of which are as follows:

[0010] (1) Transition rules

[0011] Although the strength of Q&P steel hot-rolled coils is reduced after bell annealing, the strength is still higher than that of other ordinary cold-rolled steel grades, and the production difficulty is relatively large. In order to ensure smooth rolling production and reduce the fluctuation of the thickness of the coil, an appropriate transition rule should be adopted: the yield strength of the transition coils before and after the target Q&P steel grade should be within 280MPa lower than that of the target steel grade. That is, assuming that the yield strength of the target hot-rolled coil of cold-rolled ultra-high-strength Q&P steel is N MPa, the yield strength of the hot-rolled coils before and after the transition steel grades should be between N-280 and N MPa.

[0012] (2) Acid rolling process

[0013] After hood annealing, there is a lot of iron oxide scale on the surface of the hot-rolled coil. The pickling and straightening machine should use a larger insertion amount to increase the phosphorus breaking effect. The continuous pickling production line uses a two-bending and one-straightening machine. The bending roller insertion amount is 34 to 40 mm, and the straightening roller insertion amount ranges from 15 to 22 mm.

[0014] To ensure high surface quality after pickling, the acid concentration in No. 1 and No. 2 acid tanks is 50-90 g / L, and that in No. 3 is 160-200 g / L. The acid temperature is 75-90°C, the strip speed through the acid tanks is ≤ 150 m / min, and the pickling reduction is 56.0-70.0%. The number of acid tanks should be selected based on actual production needs.

[0015] The method provided by the present invention can reduce the difficulty of pickling production of cold-rolled ultra-high-strength Q&P steel. Reducing the strength of the hot-rolled coil will not affect the performance of the final cold-rolled or galvanized Q&P steel product, because the steel coil is reheated to the austenitic state during the annealing or galvanizing process after pickling, and the quenching and partitioning processes are completed during the annealing process to obtain martensite and retained austenite structures sufficient to ensure the performance of the finished product. The martensite and retained austenite structures are most closely related to the metallurgical composition of the steel itself and the annealing process, while the impact of reduced hot-rolled coil strength on the performance of the finished product can be ignored.

[0016] The hot-rolled coil is subjected to a specific hood annealing process to reduce its strength. The main principles are as follows: hood annealing uses a relatively low temperature annealing process, without significant phase transformation. The rolling stored energy originally generated during the hot rolling process is released during the hood annealing process, causing a small number of grains to recover and recrystallize, reducing the strength of the hot-rolled coil. The annealing process can release the stress generated by hot rolling, reducing the strength of the hot-rolled coil. During the hood annealing process, the original microstructure grains and precipitates grow, and the grain growth and fine-grain strengthening effect of the microstructure are weakened. The increased size of the precipitates reduces their pinning effect, thus reducing the strength of the hot-rolled coil.

[0017] The beneficial effects of the present invention are:

[0018] (1) The present invention uses a reasonable hot-rolled coil hood annealing process, combined with specific pickling production scheduling rules and pickling production processes, to enable hot-rolled coils that were originally unable to be pickled due to excessive strength to meet the design or actual production capacity of general cold-rolled pickling production lines. At the same time, through specific pickling transition rules and pickling production processes, the large fluctuation in coil thickness during the pickling process can be reduced, and problems such as strip breakage and abnormal fluctuation in stopping force during pickling can be avoided, further reducing the difficulty of pickling production, thereby realizing stable batch pickling production of cold-rolled ultra-high strength Q&P steel, and the resulting pickling coils have high thickness accuracy and high-quality surface.

[0019] (2) The present invention can reduce the yield strength of hot-rolled coil by 150 to 350 MPa, reduce the tensile strength by 200 to 450 MPa, and increase the elongation by 9 to 28%. The present invention can ensure that the thickness fluctuation of the cold-rolled ultra-high-strength Q&P steel strip after acid rolling is guaranteed to be within the range of ±0.015 mm, and the surface of the acid-rolled strip meets the FB order requirements.

[0020] (3) The present invention is applicable to all grades of cold-rolled Q&P steel with a hot-rolled coil yield strength ≥650 MPa, a tensile strength ≥800 MPa, a hot-rolled coil thickness of 2.2 to 5.0 mm, and an acid-rolled thickness of 0.7 to 2.2 mm. The cold-rolled ultra-high-strength Q&P steel acid-rolled coil obtained by the present invention is suitable for subsequent annealing or galvanizing, where it is reheated to the austenitizing temperature and quenched. Furthermore, the cold-rolled ultra-high-strength Q&P steel acid-rolled coil obtained by the present invention fully meets the performance requirements after subsequent annealing or galvanizing. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0022] Figure 1 This is the optical microstructure diagram of Experimental Example 1 after acid rolling in Table 3 of Example 2.

[0023] Figure 2 This is the optical microstructure diagram of Experimental Example 4 after acid rolling in Table 3 of Example 2. DETAILED DESCRIPTION

[0024] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0025] Example 1:

[0026] A method for producing cold-rolled ultra-high-strength Q&P steel by pickling is disclosed. The method comprises the following steps: hot-rolled coil hood annealing and softening, hot-rolled finishing and re-coiling, and pickling production.

[0027] The hot-rolled coil annealing process of cold-rolled ultra-high strength Q&P steel includes: preheating, heating, soaking, and cooling. The preheating section temperature is 300-450°C, and the heating time is 1-3 hours; the heating section temperature is 520-650°C, and the heating time is 3-5 hours; the soaking section temperature is 520-680°C, and the holding time is 6-9 hours.

[0028] Cooling is divided into three stages: Cooling stage 1 is air cooling, that is, the temperature is reduced by increasing the flow rate of protective gas between the inner and outer covers of the bell-type annealing furnace, the temperature is ≤400℃, and the cooling time is 3 to 5 hours; Cooling stage 2 is water cooling, by cooling the inner cover of the heating furnace with water, the temperature is ≤180℃; Cooling stage 3 is natural cooling after leaving the furnace.

[0029] The uncoiling tension of hot-rolled finishing recoil is 40-60KN, the coiling tension is 60-80KN, the strip speed is ≤150m / min, and the hot-rolled coil head is cut off 5-10m.

[0030] The pickling production meets the appropriate transition rules, and the yield strength of the transition steel grade hot-rolled coils before and after the target hot-rolled coils of the cold-rolled ultra-high strength Q&P steel should be within 280MPa less than that of the target steel grade.

[0031] During the pickling production process, the bending roll insertion amount of the straightening machine is 34 to 40 mm, and the straightening roll insertion amount ranges from 15 to 22 mm.

[0032] In the pickling production process, the acid concentration of No. 1 and No. 2 acid tanks is 50-90 g / L, and the acid concentration of No. 3 acid tank is 160-200 g / L; the acid temperature is 75-90°C; the speed of the strip passing through the acid tank is ≤150 m / min; and the pickling reduction rate is 56.0-70.0%.

[0033] This method is applicable to all grades of cold-rolled Q&P steel with hot-rolled coil yield strength ≥650MPa, tensile strength ≥800MPa, hot-rolled coil thickness 2.2-5.0mm, and pickling rolled thickness 0.7-2.2mm.

[0034] This method can reduce the yield strength of the hot-rolled coil of cold-rolled ultra-high-strength Q&P steel by 150 to 350 MPa, reduce the tensile strength by 200 to 450 MPa, and increase the elongation by 9 to 28%. The thickness fluctuation of the strip after pickling is guaranteed to be within the range of ±0.015 mm.

[0035] Example 2

[0036] A method for reducing the difficulty of acid rolling production of cold-rolled ultra-high strength Q&P steel is described in detail below with reference to specific embodiments.

[0037] The Q&P hot-rolled coils were preheated, heated, soaked, and cooled, followed by bell annealing, with protective gas introduced throughout the process. Specific process parameters are shown in Table 1. After bell annealing, the hot-rolled coil strength decreased, meeting the production capacity of the pickling line. A comparison of performance before and after bell annealing is shown in Table 2:

[0038] Table 1 Hot rolled coil annealing process for different grades of cold rolled Q&P steel

[0039]

[0040]

[0041] Table 2 Performance changes of different grades of cold-rolled Q&P steel before and after hot-rolled coil annealing

[0042]

[0043] After hood annealing, the hot rolled coil is re-coiled on the hot rolling finishing production line. The uncoiling tension is 40-60KN, the coiling tension is 60-80KN, the strip speed is 150m / min, and the outermost ring of the steel coil should be cut off 5-10m of the strip head.

[0044] In the pickling production, suitable steel grades are selected for strength transition in pickling production scheduling. In the embodiment, CR550 / 980Q&P adopts CR340 / 590DP or HC340LA for transition before and after pickling. In the embodiment, CR700 / 1180Q&P adopts CR420 / 780DP or HC420LA for transition before and after pickling.

[0045] The hot-rolled coil is uncoiled on the pickling line. The strip undergoes tension and leveling to remove phosphorus, passes through the acid bath, and is continuously rolled to obtain the target thickness. After pickling, the thickness fluctuation of the strip is within ±0.015mm, and the surface of the cold-hardened coil meets the FB standard. The pickling production process is shown in Table 3 below.

[0046] Table 3 Cold rolled Q&P steel pickling production plan

[0047]

[0048]

[0049] Experimental Example 1 Optical microstructure after acid rolling Figure 1 As shown, the optical microstructure of Experimental Example 4 after acid rolling is as follows Figure 2 As shown. As shown in the figure, the original ferrite + pearlite hot-rolled structure did not undergo obvious phase transformation after hood annealing, but the ferrite (white area) and pearlite (black area) structures became coarse after annealing. At the same time, the long rolled strip formed in the hot rolling process was recrystallized and grew during the annealing process. In addition, the annealing process increases the number of precipitates (small black dots in the figure) and increases their size, and the pinning effect of the precipitates is reduced. The above reasons are the reasons for the decrease in strength of the hot-rolled coil after annealing. After the hot-rolled coil is pickled, the ferrite and pearlite structures are stretched into long strips again, but are coarse relative to the unannealed hot-rolled structure.

[0050] In the above embodiments, after austenitizing, quenching and partitioning are completed in the subsequent annealing or galvanizing production process, the microstructure and performance of the obtained finished product fully meet the requirements.

[0051] Although the present invention has been described in detail with reference to the accompanying drawings and in combination with preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any person of ordinary skill in the art who can easily conceive of changes or substitutions within the technical scope disclosed in the present invention shall be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A method for producing cold-rolled ultra-high strength Q&P steel by pickling, characterized in that: The production method comprises the following steps: hot rolling coil hood annealing and softening, hot rolling finishing and re-coiling, and pickling production; The hot-rolled coil annealing process for cold-rolled ultra-high-strength Q&P steel includes: preheating, heating, soaking, and cooling. The preheating section is at a temperature of 300-450°C for 1-3 hours; the heating section is at a temperature of 520-650°C for 3-5 hours; the soaking section is at a temperature of 520-680°C for 6-9 hours. The uncoiling tension of hot-rolled finishing recoil is 40-60KN, the coiling tension is 60-80KN, the strip speed is ≤150m / min, and the hot-rolled coil head is cut off 5-10m; The pickling production meets the appropriate transition rules. Assuming that the target yield strength of the hot-rolled coil of the cold-rolled ultra-high strength Q&P steel is NMPa, the yield strength of the hot-rolled coils of the transition steel grades should be between N-280 and N MPa. During the pickling production process, the bending roll insertion amount of the straightening machine is 34 to 40 mm, and the straightening roll insertion amount ranges from 15 to 22 mm. In the pickling production process, the acid concentration of No. 1 and No. 2 acid tanks is 50-90 g / L, and the acid concentration of No. 3 acid tank is 160-200 g / L; the acid temperature is 75-90°C; the speed of the strip passing through the acid tank is ≤150 m / min; the pickling reduction rate is 56.0-70.0%.

2. The pickling rolling production method of cold-rolled ultra-high strength Q&P steel according to claim 1, characterized in that: Cooling is divided into three stages: Cooling stage 1 is air cooling, temperature ≤400℃, cooling time 3~5h; Cooling stage 2 is water cooling, temperature ≤180℃; Cooling stage 3 is natural cooling after leaving the furnace.

3. The pickling rolling production method of cold-rolled ultra-high strength Q&P steel according to any one of claims 1 to 2, characterized in that: This method is applicable to all grades of cold-rolled Q&P steel with hot-rolled coil yield strength ≥650MPa, tensile strength ≥800MPa, hot-rolled coil thickness 2.2~5.0mm, and pickling rolled thickness 0.7~2.2mm.

4. The pickling rolling production method of cold-rolled ultra-high strength Q&P steel according to any one of claims 1 to 2, characterized in that: This method can reduce the yield strength of the hot-rolled coil of cold-rolled ultra-high-strength Q&P steel by 150-350 MPa, reduce the tensile strength by 200-450 MPa, and increase the elongation by 9-28%. The thickness fluctuation of the strip after pickling is guaranteed to be within the range of ±0.015 mm.

Images