WO2020108595A1

WO2020108595A1 - Hot-rolled high-strength steel plate with high surface quality and low yield ratio and manufacturing method therefor

Info

Publication number: WO2020108595A1
Application number: PCT/CN2019/121864
Authority: WO
Inventors: 张建苏; 柏明卓; 庞厚君; 胡兆辉; 华骏山; 郁锋; 王明
Original assignee: 宝山钢铁股份有限公司
Priority date: 2018-11-30
Filing date: 2019-11-29
Publication date: 2020-06-04
Also published as: BR112021008309A2; CN109576581A; DE112019005950T5

Abstract

A hot-rolled high-strength steel plate with high surface quality and low yield ratio and a manufacturing method therefor. The steel plate is prepared from the components in percentage by weight: C: 0.045-0.085%; Si≤0.15%, Mn: 1.0-1.5%; P≤0.05%; S≤0.001%; Al: 0.5-2.0%; N≤0.0060%; Ti≤0.03%; B≤0.0005%; and the balance of Fe and inevitable impurities, where Mn + 20C = 2.2-3.2%. The high-strength steel plate has a simple cooling process after hot rolling, uniform performance, and good plate shape. The high-strength steel plate has good cold workability and welding performance, low production cost, and good surface quality, and can satisfy the requirements of appearance and coating of an automobile chassis and automobile wheels.

Description

Hot-rolled high-strength steel plate with high surface quality and low yield ratio and manufacturing method

Technical field

The invention belongs to automobile steel, and particularly relates to a hot-rolled high-strength steel plate with high surface quality and low yield ratio and a manufacturing method, and is particularly suitable for automobile exterior parts such as car wheels.

Background technique

In recent years, due to the increasing awareness of human environmental protection, reducing car fuel consumption and reducing CO ₂ emissions has become a common global call. Among the various measures to achieve this goal, car weight reduction is a very effective s method. This global trend and the emphasis on automobile safety have brought about the demand for various advanced high-strength, new steel materials with good processability.

Hot rolled steel plate is mainly used for car chassis, wheels, suspension and its surrounding parts, and its weight accounts for more than 25% of the total weight of the car body. Due to the increase in strength, the capacity of equipment used in the manufacture of many parts is also facing increased pressure. Therefore, it is hoped that materials with low yield strength and high tensile strength can be obtained to solve the contradictions faced. At the same time, many parts used in these parts have both appearance parts such as wheels and coated parts, which require good coatability of the steel plate surface.

The traditional dual-phase steel has a structure of ferrite + martensite. It has a low yield ratio, no yield platform, high work hardening rate, high baking hardening, long fatigue life, good welding performance, etc. Excellent performance, in order to obtain the desired structure and good performance, usually add Si to the ingredients to expand the ferrite formation area. But at the same time, the addition of Si also caused the formation of sharp olivine crystals composed of FeO-Fe ₂ SiO _{4 on the} surface of the hot-rolled steel sheet. This crystal cannot be removed cleanly during descaling, leaving the surface of the steel sheet in the rolling direction One by one "red iron sheet" defects, see Figure 1. Therefore, there is an urgent need for a steel grade that not only has the advantages of low yield ratio, high work hardening rate, etc. unique to dual-phase steel, but also has high surface quality to meet the requirements of the appearance and coating of automobile chassis and automobile wheels.

The Taiwanese patent TWI300443B proposes a hot-rolled steel plate manufacturing process with a composition of 0.01-0.08% C, Si<0.9%, Mn 0.5-1.6%, A1 1.2%, Cr 0.3-1.2%, which is cooled in two stages. In the first stage, the steel plate is cooled at a speed of 2-15°C/s from 8-40s to 730°C, and then the steel plate is cooled to a temperature below 300°C at a cooling rate of 20-150°C/s.

German patent DE10327383C5 discloses a method for producing hot-rolled dual-phase steel plate, the composition of the steel is: 0.01-0.08% C, ≤0.9% Si, 0.5-1.9% Mn, ≤1.2% Al, 0.3- 1.2% Cr, the rest is Fe, finish rolling under A3 at 50-100℃, then cool to ferrite area at 30-150℃/s, and cool for 5 seconds, then cool at 30-150℃/s To 300°C.

Summary of the invention

The purpose of the present invention is to provide a hot-rolled high-strength steel plate with high surface quality and low yield ratio and a manufacturing method, which does not require heat treatment and can be directly produced by a general hot-rolling production line. The high-strength steel plate has a simple cooling process after hot rolling, uniform performance, good shape, good cold workability and welding performance, low production cost, and good surface quality, which can meet the requirements of the appearance and coating of automobile chassis and automobile wheels. . The "high surface quality" mentioned in the present invention means that the surface has no defects of red iron skin.

To achieve the above objectives, the technical solutions of the present invention are:

A hot-rolled high-strength steel plate with high surface quality and low yield ratio. The weight percentage of the components is: C 0.045-0.085%, Si ≤ 0.15%, Mn 1.0-1.5%, P ≤ 0.05%, S ≤ 0.001%, Al 0.5-2.0%, N≤0.0060%, Ti≤0.03%, B≤0.0005%, the rest are Fe and inevitable impurities; and, Mn+20C=2.2-3.2%.

The structure of the hot-rolled high-strength steel plate of the present invention is ferrite and martensite, wherein the volume fraction content of martensite is 15-20%, and the aspect ratio of ferrite of more than 80% in ferrite ≤1.5.

The hot-rolled high-strength steel sheet of the present invention has a tensile strength ≥590 MPa, preferably ≥600 MPa; an elongation ≥20%, preferably ≥24%; a yield ratio ≤0.6, preferably ≤0.58. In some embodiments, the tensile strength of the hot-rolled high-strength steel sheet of the present invention is in the range of 590-900 MPa.

In the design of the steel composition of the present invention:

C: It is used to form sufficient carbide strengthening phase to ensure the strength level of steel. If the C content is less than 0.045%, the strength cannot meet the requirements, and if the C content is more than 0.085%, it is detrimental to the welding performance and formability. Preferably, the C content is 0.045-0.082%.

Si: There are three roles in steel:

1) Play the role of solid solution strengthening, improve the strength of steel;

2) It can accelerate the transformation of austenite to ferrite and accelerate the transformation speed of austenite to ferrite.

3) Prevent the precipitation of carbides and avoid the appearance of pearlite phase.

As a material for automobile chassis and wheels, the conventional dual-phase steel must have sufficient strength and excellent elongation, but due to the high Si content in the conventional dual-phase steel, red iron skin defects appear on the surface. In order to avoid this defect in dual-phase steel, a low Si composition is required. In the present invention, it is required to control the Si content to 0.15% or less. In some embodiments, preferably, the Si content is 0.05-0.14%.

Mn: It is a solid solution strengthening element. If the Mn content is less than 1.0%, the strength of the steel is insufficient; if the Mn content is more than 1.5%, the plasticity of the steel is reduced. Preferably, the Mn content is 1.06-1.5%

In addition, Mn+20C=2.2-3.2%, below 2.2%, the volume fraction content of martensite is less than 15%, and above 3.2%, the volume fraction content of martensite exceeds 20%. Preferably, Mn+20C=2.3-3.1%.

P: It is an impurity element in steel, and the content should be as low as possible. Preferably, P ≤ 0.015%.

S: It is also an impurity element in steel, and the content of S in steel is usually required to be 0.001% or less.

Al: It is a deoxidizing element in steel, reducing oxide inclusions in steel and pure steel, which is beneficial to improve the formability of steel plates. At the same time, adding higher content of Al can partially replace the role of Si. Therefore, in the present invention, the Al content is 0.5-2.0%. In some embodiments, the Al content is 1.0-2.0%.

Ti: It is an element that effectively refines grains and improves strength and toughness. It exists in steel in the form of carbides and carbonitrides. However, too much elements such as Ti and carbonitrides in low-carbon steel will affect the subsequent phase transformation, so the content of alloying elements needs to be controlled at the upper limit, preferably Ti≤0.03%. In some embodiments, the steel sheet of the present invention contains 0.01-0.03% Ti.

B: The grain boundary strengthening element can effectively increase the strength of the ferrite matrix. However, if the B content is too high, it will cause hot embrittlement and affect the welding performance and hot workability of the steel. Therefore, the B content needs to be strictly controlled. The boron content in the low alloy high strength wear-resistant steel of the present invention is ≤0.0005%.

The manufacturing method of the hot-rolled high-strength steel plate with high surface quality and low yield ratio according to the present invention includes the following steps:

1) Smelt and cast into billets according to the above components;

2) After the slab is heated at 1150～1250℃, it is rolled in the austenite zone, the rolling deformation is 80-95%, and the final rolling temperature is 780～850℃;

3) After finishing rolling, the steel sheet is cooled to 600 to 750°C at a cooling rate of 50 to 150°C/s or more, and then cooled in the air at a cooling rate of 1 to 10°C/s for 2 to 10 seconds, and then again at 50 -The cooling speed of -200℃/s is cooled to 50～300℃ and coiled, then air-cooled to room temperature.

In the manufacturing process of the present invention:

When the heating temperature is lower than 1150°C, the micro-alloying element is not sufficiently dissolved, and the effect of the micro-alloying element is not fully utilized, and the strength is reduced. If the heating temperature is higher than 1250°C, the grains are likely to be coarsened, which is detrimental to improving the toughness of the steel plate.

The slab is rough-rolled in the austenite re-grain boundary area, and the austenite grains are refined by recrystallization after rolling deformation. The deformation of the steel plate is 80-95%, and the final rolling temperature is controlled at austenite. In the crystallization zone 780～850℃, through the rolling deformation of the austenite low temperature zone, the deformation zone is formed in the austenite grains and the carbon nitride of the microalloy element is induced by the strain to refine the phase transformation product of the austenite To improve the toughness of the steel plate.

After the final rolling, the steel plate is cooled to 600-750°C at a cooling rate of 50-150°C/s or more (two-phase region of ferrite + austenite), and cooled at a cooling rate of 1-10°C/s in air for 2-10 Seconds, such as 3 to 10 seconds, part of the austenite is transformed into equiaxed ferrite in this temperature range (over 80% ferrite aspect ratio ≤ 1.5), and then the steel plate is at 50-200℃/s The cooling rate is cooled to 50-300°C, preferably 70-300°C, and the material is passed through the bainite zone into the martensite zone, and finally about 15-20% of martensite is formed. In order to ensure that the surface of the steel sheet is free of red iron skin defects, it must be ensured that the Si content in the composition is less than 0.10%. At the same time, in order to make up for the problems such as the decrease in strength caused by the decrease in Si content and the difficulty of ferrite formation, 0.5-2.0% Al needs to be added to make up, which can effectively improve the surface quality of the steel plate. See Figure 2. If the coiling temperature is higher than 300℃, a large amount of bainite is likely to appear in the microstructure of the steel plate, making the strength of the high-strength steel lower than 590MPa.

The beneficial effects of the invention:

The hot-rolled high-strength steel plate of the invention adopts a relatively simple composition design, substitutes Al for Si to improve the surface quality of the steel plate, does not use the more expensive elements such as Nb, Mo, Cr, etc., and the alloy cost is low; in the production process, A relatively simple production process is adopted. After hot rolling, a segmented cooling mode is added to the conventional laminar cooling process, which is easy to produce.

The steel plate of the present invention produced according to the above composition design and process design has high strength (≥590MPa), low yield strength (yield ratio ≤0.6) and good surface quality, so that the steel plate of the present invention needs good appearance and good in manufacturing The coating performance of automobile chassis and wheel parts has unique advantages.

BRIEF DESCRIPTION

Fig. 1 is a photograph of "red iron skin" defects on the surface of the steel sheet of Comparative Example 1.

Fig. 2 is a photograph of the surface of a steel plate in Example 1 of the present invention.

Figure 3 is a metallographic photograph of Example 3 of the present invention.

detailed description

The present invention will be further described below with reference to embodiments and drawings.

Table 1 is the chemical composition of steels A to G of the present invention, and H steel is a comparative steel of JPH09263885A.

Table 1

In the examples and comparative examples of the present invention, the slab is heated at 1150-1250°C and then rolled in the austenite zone with a rolling deformation of 80-95%; the steel sheet after the final rolling is 50-150°C/s or more The cooling rate is cooled to 600～750℃, and then air-cooled for 3-10 seconds. The air cooling rate is controlled within the range of 1～10℃/s, and then cooled to 50～300℃ again at a cooling rate of 50-200℃/s. Take up and cool to room temperature. Table 2 shows the process control during rolling and the mechanical properties of the obtained steel plate with a thickness of 3 mm. The mechanical properties were tested according to the GB6397-86 standard. The comparative example is the dual-phase steel plate of Japanese Patent JPH09263885A.

Referring to FIG. 2, the steel plate of the present invention has a clean surface without the defects of “red iron skin” shown in FIG. 1, and the surface quality is high, which can meet the requirements of the appearance and coating of automobile chassis and automobile wheels. Referring to FIG. 3, the structure of the steel plate of Example 3 is ferrite and martensite, wherein the dark color is martensite, and the volume percentage thereof is in the range of 15-20%.

Table 2

Note: The tensile lengths of the tensile samples of the examples and the comparative examples are both 50 mm; Comparative Example 1 corresponds to Comparative Example 8 of JPH09263885A; Comparative Example 2 corresponds to Example 6 of JPH09263885A

Claims

A hot-rolled high-strength steel plate with high surface quality and low yield ratio. The weight percentage of the components is: C 0.045-0.085%, Si ≤ 0.15%, Mn 1.0-1.5%, P ≤ 0.05%, S ≤ 0.001%, Al 0.5～2.0%, N≤0.0060%; Ti≤0.03%, B≤0.0005%, the rest are Fe and inevitable impurities; and, Mn+20C=2.2-3.2%.
The hot-rolled high-strength steel sheet with high surface quality and low yield ratio according to claim 1, wherein the hot-rolled high-strength steel sheet has a tensile strength ≥590 MPa and a yield ratio ≤0.6.
The hot-rolled high-strength steel sheet with high surface quality and low yield ratio according to claim 1, wherein the hot-rolled high-strength steel sheet has a tensile strength ≥600 MPa and a yield ratio ≤0.58.
The hot-rolled high-strength steel sheet with high surface quality and low yield ratio according to claim 1, characterized in that the weight percentage of the components is: C, 0.045-0.082%; Si, 0.05-0.14%; Mn, 1.06～1.5 %; P ≤ 0.015%; S ≤ 0.001%; Al, 0.5 ~ 2.0%; N ≤ 0.0060%; Ti ≤ 0.03%, B ≤ 0.0005%, the rest are Fe and inevitable impurities; and, Mn+20C=2.3 -3.1%.
The hot-rolled high-strength steel sheet with high surface quality and low yield ratio according to any one of claims 1 to 4, wherein the structure of the hot-rolled high-strength steel sheet is ferrite and martensite, wherein The volume percentage of martensite is 15-20%, and the aspect ratio of more than 80% of ferrite in ferrite is ≤1.5.
The hot-rolled high-strength steel sheet with high surface quality and low yield ratio according to claim 1, wherein the elongation of the hot-rolled high-strength steel sheet is ≥20%.
The method for manufacturing a hot-rolled high-strength steel sheet with high surface quality and low yield ratio according to claim 1, characterized in that it includes the following steps:

1) Smelting and casting into billets according to the composition of claim 1;

2) After the slab is heated at 1150～1250℃, it is rolled in the austenite zone, the rolling deformation is 80-95%, and the final rolling temperature is 780～850℃;

3) After finishing rolling, the steel sheet is cooled to 600 to 750°C at a cooling rate of 50 to 150°C/s or more, and then cooled in the air at a cooling rate of 1 to 10°C/s for 2 to 10 seconds, and then again at 50 The cooling rate of ～200℃/s is cooled to 50～300℃ and coiled, and then air-cooled to room temperature.
The method for manufacturing a hot-rolled high-strength steel sheet with high surface quality and low yield ratio according to claim 7, wherein the weight percentage of the steel sheet is: C, 0.045-0.082%; Si, 0.05-0.14% ; Mn, 1.06 to 1.5%; P ≤ 0.015%; S ≤ 0.001%; Al, 0.5 to 2.0%; N ≤ 0.0060%; Ti ≤ 0.03%, B ≤ 0.0005%, the rest is Fe and inevitable impurities; and , Mn+20C=2.3-3.1%.