Rolling heat treatment method for dual-phase steel of low-density steel

Abstract

The invention discloses a rolling heat treatment method for dual-phase steel of low-density steel, and belongs to the technical field of ferrous metallurgy. The method comprises the following steps: a rolling process: carrying out rough rolling on a smelted cast ingot, and then carrying out finish rolling; performing heat treatment: performing simulated continuous annealing on the steel plate subjected to finish rolling to obtain the bidirectional steel plate for the automobile; the process route is as follows: ingot casting, homogenization (1200 DEG C * 3h), hot rolling (1150 DEG C to 950 DEG C, the reduction rate being 75%), cold rolling and heat treatment. The invention aims to disclose a rolling heat treatment method for dual-phase steel of low-density steel, so that the strength and toughness of the dual-phase steel are well matched, the density of the dual-phase steel is relatively low under the condition that the use performance requirement is met, and the weight of a product can be reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel metallurgy, and particularly relates to a rolling and heat treatment method for low-density steel dual-phase steel. BACKGROUND

[0002] With the development of the automobile industry, automobile lightweighting has become an important development trend. The use of low-density steel can effectively reduce the weight of the automobile itself without sacrificing the strength and rigidity of the automobile structure, which helps to achieve the goal of energy saving and emission reduction. Developing a steel grade that combines low density and high strength and toughness is an important direction of material research and development. By adding lightweighting elements such as aluminum, manganese, silicon, and carbon, and optimizing the alloy composition and heat treatment process, a new type of steel that meets the requirements can be obtained. Some methods for improving the comprehensive mechanical properties of low-density steel through rolling deformation have problems such as complex process and high production cost, which limits the use of low-density steel in lightweighting, so new methods are needed to develop low-density dual-phase steel to improve its performance.

[0003] A Chinese patent with application number 201611024622.3 discloses a high-strength Fe-Mn-Al-C low-density cast steel and a preparation method thereof. The composition is as follows in terms of weight percentage: C: 0.05-0.35%, Si: 2.0-10.0%, Mn: 5.0-25.0%, P: ≤0.02%, S: ≤0.01%, Al: 3.0-12.0%, Cr: 0.1-0.8%, Ni: 0.1-1.2%, Mo: 0.1-0.5%, Nb: 0.01-0.08%, Ti: 0.6-1.0%, Re: 0.01-0.5%, and the remainder is Fe and unavoidable impurities. The low-density steel base body has an austenite structure, high strength, good plasticity and toughness, and good corrosion resistance.

[0004] A Chinese patent with application number 202010176397.5 discloses a high-strength high-hardness low-density steel, a preparation method and application thereof. The high-strength high-hardness low-density steel is obtained by sequentially subjecting a high-strength low-density steel to nitriding and denitriding treatment. The high-strength low-density steel comprises the following components in mass percentage: C: 0.7-1.8%, Al: 8-12%, Si: 0.3-0.9%, Mn: 25-34%, Cr: 0.3-1.2%, V: 0.1-0.7%, Ti: 0.1-0.8%, Mo: 0.7-1.3%, and the balance is Fe and unavoidable impurities. The high-strength high-hardness low-density steel obtained by the method has a yield strength of 870.21-1077.36 MPa, a tensile strength of 950.35-1127.7 MPa, a hardness of 62-68 HRC, and a density of 6.63-7.19 g / cm3.

[0005] A Chinese patent with the application number 202011573734.0 discloses a high-strength high-plasticity low-density steel and a preparation method thereof. The present application relates to a high-strength high-plasticity low-density steel and a preparation method thereof, and belongs to the technical field of alloy steel. The present application solves the problems of high density, low strength and poor plasticity of alloy steel in the prior art. The high-strength high-plasticity low-density steel has a chemical composition according to mass percentage of C: 0.3-0.5%, Mn: 16-28%, Al: 2-7%, Si: 1.2-1.4%, and the balance of Fe. The preparation method of the high-strength high-plasticity low-density steel comprises the following steps: step 1: weighing and selecting each element as a raw material according to mass percentage, and smelting by using a vacuum induction furnace to obtain a steel ingot; step 2: homogenizing treatment and forging treatment are performed on the steel ingot to obtain a rod or plate after forging treatment; and step 3: solid solution treatment and aging treatment are sequentially performed on the rod or plate after forging treatment. The strength and toughness of the alloy steel are improved, and the alloy steel has low density. SUMMARY

[0006] The purpose of the present application is to provide a method for rolling and heat treatment of low-density steel dual-phase steel, so that the strength and toughness of the dual-phase steel are well matched, the density is relatively low under the condition of meeting the performance requirements, and the weight of the product can be reduced.

[0007] To solve the above technical problems, the present application adopts the following technical scheme:

[0008] The present application provides a method for rolling and heat treatment of low-density steel dual-phase steel, which comprises the following steps:

[0009] 1) Rough rolling: heat the cast ingot to 1200±50℃ and keep for 2-5 hours, and roll into a steel plate with a certain thickness through multiple passes;

[0010] 2) Finish rolling: after the rough rolling, the steel plate is heated in a heating furnace at 900-1050℃ and kept for a certain time, then rolled to 10mm thick with a pass reduction of 3-5mm, and then rolled to 4mm thick with a pass reduction of 1-2mm;

[0011] 3) Pickling: put the steel plate into a pickling solution to remove the surface oxide scale; the pickling solution is a hydrochloric acid solution, the pickling temperature is 40-80℃, and the pickling time is determined according to the thickness of the oxide scale and the material of the steel plate, generally 10-30 minutes, until the surface iron oxide scale of the steel plate is cleaned;

[0012] 4) Cold rolling: the pickled steel plate is cold rolled in multiple passes, and the pass reduction gradually decreases; the first pass can reduce the thickness from 4mm to 3.2mm with a pass reduction of 0.8mm; the subsequent passes are sequentially reduced to 2.6mm, 2.2mm, and the last pass is reduced to 2mm;

[0013] 5) heat treatment: ① the steel strip is heated to the temperature of the ferrite and austenite two-phase critical zone at a certain heating rate, the average heating rate is 1.8-3.5℃ / s; ② the structure is fully homogenized by keeping at the critical zone temperature for a period of time, generally 85-168s; ③ slow cooling stage: the steel strip is cooled to a certain temperature range at a slow speed, the cooling speed is controlled at 10±3℃ / s, and the temperature is controlled at 650-670℃; ④ fast cooling stage: including fast cooling I and fast cooling II two stages: the temperature of fast cooling I is expected to be 470℃, and the cooling speed is 65-140℃ / s; the temperature of fast cooling II is 280-315℃, and the cooling speed is 15.0-40.0℃ / s; ⑤ overaging stage: the steel strip is heated to a temperature not higher than the martensite starting temperature for heat preservation, and tempering treatment is carried out to obtain a good organization matching of the hard phase martensite and the soft phase ferrite.

[0014] Further, the rough rolling is carried out through 9 passes to form a 20mm thick steel plate.

[0015] Further, the rough rolling specific rolling parameter requirements are shown in the following table:

[0016]

[0017]

[0018] Further, the rough rolling heats the ingot to 1200℃ and keeps for 3 hours.

[0019] Further, the low-density steel dual-phase steel has the following mass percentage chemical components: C: 0.1%, Si: 0.6%, Mn: 2.2%, P: ≤0.002%, S: ≤0.002%, Al: 0.05-4.0%, Cr: 0.4%, Nb: 0.06%, Ti: 0.02%, and the rest is Fe and inevitable impurities.

[0020] Further, the Al content is ≤1.5%.

[0021] Further, the steel plate with Al content ≤1.5% can make the strength and toughness of the dual-phase steel well matched after the rolling heat treatment, the density is relatively low under the condition of meeting the performance requirements, and the product weight can be reduced.

[0022] Compared with the prior art, the beneficial technical effects of the present application are:

[0023] The present application can match the strength and toughness of the dual-phase steel well through a suitable rolling and heat treatment process. For example, the microstructure of the steel can be refined, the dislocation density can be increased, and the strength can be improved by controlling the deformation amount and temperature during rolling; and the toughness can be improved and the brittleness of the material can be reduced by tempering and other operations in the heat treatment process. The rolling and heat treatment can promote the uniform distribution of alloying elements, reduce composition segregation, and make the performance of the material more consistent at different parts. At the same time, it can also help to eliminate the residual stress generated during rolling, improve the stability and reliability of the material. Since it is a low-density dual-phase steel, its density is relatively low under the condition of meeting the performance requirements, which can reduce the weight of the product. This is of great significance to the fields such as aerospace and automobiles which have high requirements for weight reduction, and can improve transportation efficiency and reduce energy consumption. BRIEF DESCRIPTION OF DRAWINGS

[0024] The present application will be further described below in conjunction with the drawings.

[0025] Figure 1 The mechanical property diagram of case 1 is shown in the figure. DETAILED DESCRIPTION

[0026] Case 1:

[0027] 1. Composition design: the target composition is C: 0.1%, Si: 0.6%, Mn: 2.2%, P: ≤0.002%, S: ≤0.002%, Al: 0.05-4.0%, Cr: 0.4%, Nb: 0.06%, Ti: 0.02%, and Nb / Ti is added for micro-alloying and grain refinement; the rest is Fe and unavoidable impurities.

[0028] 2. Melting: the test steel is smelted by a 25kg vacuum induction furnace to ensure smelting in a vacuum state with a vacuum degree of 90pa. The processed furnace charge is placed in the smelting furnace. The power supply and vacuum pump are turned on for smelting. The melting process of the steel in the furnace is observed at all times. After about 20 minutes, the steel is melted. The ingot is cast under vacuum and cooled with the furnace before being rolled.

[0029] 3. Rough rolling: the ingot is heated to 1200℃ and kept for 3 hours, and then rolled into a 20mm thick steel plate through 9 passes. The specific rolling parameter requirements are shown in the following table.

[0030]

[0031]

[0032] 4. Finish rolling: the 20mm thick steel plate is heated to 900-1050℃ in the furnace and kept for a while, then rolled to 10mm thick with a reduction of 3-5mm per pass, and then rolled to 4mm thick with a reduction of 1-2mm per pass.

[0033] 5. Pickling: put the steel plate into pickling solution to remove the surface scale. The pickling solution is usually hydrochloric acid solution, the pickling temperature is generally 40-80℃, the pickling time is determined according to the thickness of the scale and the material of the steel plate, generally 10-30 minutes, until the surface scale of the steel plate is cleaned.

[0034] 5. Cold rolling: the steel plate after pickling is cold rolled by multiple passes, and the reduction of each pass is gradually reduced. The first pass can reduce the thickness from 4mm to about 3.2mm, and the reduction is 0.8mm; the subsequent passes are sequentially reduced to 2.6mm, 2.2mm, and the last pass is reduced to 2mm.

[0035] 4. Heat treatment: ①The steel strip is heated to the temperature of the ferrite and austenite two-phase critical zone at a certain heating rate, and the average heating rate is usually 1.8℃ / s-3.5℃ / s. ②Keep at the critical zone temperature for a period of time, generally 85s-168s, to make the structure fully uniform. ③Slow cooling stage: cool to a certain temperature range at a slow speed, the cooling speed is controlled at 10±3℃ / s, and the control temperature is 650-670℃. ④Fast cooling stage: including fast cooling I and fast cooling II two stages. The expected target temperature of fast cooling I is about 470℃, and the cooling speed is 65℃ / s-140℃ / s; the temperature of fast cooling II is 280℃-315℃, and the cooling speed is 15.0℃ / s-40.0℃ / s. The temperature rapidly decreases. ⑤Overaging stage: heat the strip to a temperature not higher than the martensite start temperature (Ms) and keep it for tempering treatment to obtain a good organization matching of hard phase martensite and soft phase ferrite.

[0036] 5. Mechanical properties: the mechanical properties of the steel plate after cold rolling and annealing are detected, including tensile strength, yield strength, elongation, N value. The results show that, as shown in Table 1, the sample with Al content ≤1.5% can make the strength and toughness of the dual-phase steel well matched after rolling and heat treatment, and the density is relatively low under the condition of meeting the performance requirements, which can reduce the weight of the product. Figure 1

[0037] The above-described embodiments are only to describe the preferred modes of the present application, and not to limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements of the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.​

Claims

1. A method of hot rolling and heat treatment of a low density steel dual phase steel, characterized in that: Comprise: 1) rough rolling: the ingot is heated to 1200±50℃ for 2-5 hours, and rolled into a certain thickness of steel plate through multiple passes; 2) finish rolling: the steel plate after rough rolling is heated to 900-1050℃ in the furnace for 2-5 hours, and then rolled to 10mm thick with a reduction of 3-5mm per pass, and then rolled to 4mm thick with a reduction of 1-2mm per pass; 3) pickling: the steel plate is placed in the pickling solution to remove the surface scale; the pickling solution is hydrochloric acid solution, the pickling temperature is 40-80℃, and the pickling time is determined according to the thickness of the scale and the material of the steel plate, generally 10-30 minutes, until the surface scale of the steel plate is cleaned; 4) cold rolling: the pickled steel plate is cold rolled in multiple passes, and the reduction of each pass is gradually reduced; the first pass can reduce the thickness from 4mm to 3.2mm, with a reduction of 0.8mm; the subsequent passes are reduced to 2.6mm, 2.2mm, and the last pass is reduced to 2mm; 5) heat treatment: ① the steel strip is heated to the critical temperature of ferrite and austenite at a certain heating rate, and the average heating rate is 1.8-3.5℃ / s; ② keep the temperature in the critical zone for a period of time, generally 85-168s, to make the structure fully uniform; ③ slow cooling stage: cool to a certain temperature range at a slow speed, the cooling speed is controlled at 10±3℃ / s, and the temperature is controlled at 650-670℃; ④ fast cooling stage: including fast cooling I and fast cooling II two stages: the temperature of fast cooling I is expected to be 470℃, and the cooling speed is 65-140℃ / s; the temperature of fast cooling II is 280-315℃, and the cooling speed is 15.0-40.0℃ / s; ⑤ overaging stage: the steel strip is heated to a temperature not higher than the martensite starting temperature for aging treatment, and a good organization matching of hard phase martensite and soft phase ferrite is obtained. The rough rolling is rolled into a 20mm thick steel plate through 9 passes.

2. The method of patent claim 1, characterized in that: The specific rolling parameters of rough rolling are shown in the following table:

3. The method of patent claim 1, characterized in that: The rough rolling ingot is heated to 1200℃ for 3 hours. 。 4. The method of patent claim 1, characterized in that: The chemical composition of low-density steel dual-phase steel is: C: 0.1%, Si: 0.6%, Mn: 2.2%, P: ≤0.002%, S: ≤0.002%, Al: 0.05-4.0%, Cr: 0.4%, Nb: 0.06%, Ti: 0.02%, and the rest is Fe and unavoidable impurities.

5. The method of patent claim 1, characterized in that: The Al content is ≤1.5%.

6. The method of patenting of a low-density steel dual-phase steel according to claim 5, characterized in that: After the steel plate with Al content ≤1.5% is rolled and heat treated, the strength and toughness of the dual-phase steel can be well matched, and under the condition of meeting the performance requirements, 7. The method of patenting of a low-density steel dual-phase steel according to claim 6, characterized in that: its density is relatively low, which can reduce the weight of the product. ​

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