Method for improving yield ratio of 780MPa-grade dual-phase galvanized steel strip

Abstract

The invention discloses a method for improving the yield ratio of a 780MPa-grade dual-phase galvanized steel strip. The method comprises the following steps: the heating temperature of a hot galvanizing process is 750-840 DEG C; the slow cooling temperature of the hot galvanizing process is 620-730 DEG C; the rapid cooling temperature of the hot galvanizing process is 430-485 DEG C; in the hot galvanizing process, the temperature of a zinc pot is 450-470 DEG C, and the galvanizing time is 5-35 seconds; the finishing elongation rate is 0 to 1.0 percent; the steel comprises the following chemical components in percentage by weight: 0.10%-0.50% of C, 0.3%-0.8% of S i, 0.5%-2.5% of Mn, less than or equal to 0.05% of P, less than or equal to 0.05% of S, 0.20%-0.50% of Cr and the balance of Fe and inevitable impurities. The invention aims to reduce the yield ratio while ensuring the strength of the steel strip, thereby improving the forming performance of the steel strip.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material processing, and particularly relates to a method for improving the yield ratio of 780MPa-grade dual-phase galvanized steel strip. BACKGROUND

[0002] Dual-phase steel is an advanced high-strength steel composed of ferrite and martensite, which has high strength, good formability and welding performance, and is widely used in the field of automobile manufacturing. However, the yield ratio (the ratio of yield strength to tensile strength) of dual-phase steel is relatively high, which limits its formability to some extent. Especially in the production process of galvanized steel strip, how to reduce the yield ratio while ensuring the strength has become a technical problem in the industry.

[0003] At present, the methods for improving the yield ratio mainly include adjusting the chemical composition and optimizing the heat treatment process, but these methods often cannot balance the strength and formability. Therefore, it is of great significance to develop a method for effectively improving the yield ratio of dual-phase galvanized steel strip. SUMMARY

[0004] The purpose of the application is to provide a method for improving the yield ratio of 780MPa-grade dual-phase galvanized steel strip, which optimizes the galvanizing process to reduce the yield ratio while ensuring the strength of the steel strip, thereby improving its formability. The mechanical properties of the steel strip meet the technical requirements: yield strength 455-487MPa, tensile strength 781-819MPa, elongation 22-24.5%, and yield ratio ≤0.60.

[0005] To solve the above technical problems, the application adopts the following technical solutions:

[0006] The method for improving the yield ratio of 780MPa-grade dual-phase galvanized steel strip of the application produces the steel strip through the steps of heating treatment, slow cooling treatment, fast cooling treatment, galvanizing treatment and flattening treatment of the steel strip with good surface quality after hot rolling and pickling; wherein:

[0007] The heating temperature of the hot galvanizing process is 750-840℃;

[0008] The slow cooling temperature of the hot galvanizing process is 620-730℃;

[0009] The fast cooling temperature of the hot galvanizing process is 430-485℃;

[0010] The temperature of entering the zinc pot of the hot galvanizing process is 450-470℃, and the galvanizing time is 5-35 seconds;

[0011] The elongation of the flattening is 0-1.0%;

[0012] The chemical composition in terms of percentage by weight is: C 0.10%-0.50%, Si 0.3%-0.8%, Mn 0.5%-2.5%, P≤0.05%, S≤0.05%, Cr 0.20%-0.50%, and the balance of Fe and inevitable impurities;

[0013] The performance of the produced steel strip meets: yield strength 455-487 MPa, tensile strength 781-819 MPa, elongation 22-24.5%, and yield strength ratio≤0.60.

[0014] Further, the steel strip is heated to 750-840℃ and kept for 10-30 minutes.

[0015] Further, the cooling rate of the slow cooling is 5-15℃ / s.

[0016] Further, the cooling rate of the slow cooling is 5℃ / s, 8℃ / s or 15℃ / s.

[0017] Further, the cooling rate of the fast cooling is 10-30℃ / s.

[0018] Further, the cooling rate of the fast cooling is 10℃ / s, 20℃ / s or 30℃ / s.

[0019] Further, the galvanizing time is 5-15 seconds.

[0020] Compared with the prior art, the beneficial technical effects of the present application are:

[0021] 1. Reducing yield strength ratio: by optimizing the galvanizing process, the present application can significantly reduce the yield strength ratio of the dual-phase galvanized steel strip, thereby improving its forming performance.

[0022] 2. Maintaining high strength: while reducing the yield strength ratio, the present application can maintain the high strength of the steel strip, meeting the requirements of the automobile, home appliance and other industries on material strength. DETAILED DESCRIPTION

[0023] The content of the present application is described in detail below through specific examples, and the examples are helpful for understanding the present application, but do not limit the content of the present application.

[0024] Example 1

[0025] 1. Select a dual-phase steel strip with required chemical composition as the base material, C: 0.114%, Si: 0.33%, Mn: 2.03%, P: 0.008%, S: 0.004%, Cr: 0.31%, and the balance of Fe and inevitable impurities.

[0026] 2. Heat the steel strip to 750℃ and keep for 30 minutes.

[0027] 3. slow cooling at a cooling rate of 5°C / s to 630°C.

[0028] 4. fast cooling at a cooling rate of 10°C / s to 430°C.

[0029] 5. dipping the steel strip into a zinc bath at 460°C for 5 seconds and then cooling at a cooling rate of 3°C / s to room temperature.

[0030] 6. skin pass treatment of the steel strip with a skin pass elongation of 0.10%.

[0031] Example 2

[0032] 1. A duplex steel strip with the required chemical composition was selected as the base material, C: 0.114%, Si: 0.33%, Mn: 2.03%, P: 0.008%, S: 0.004%, Cr: 0.31%, the balance being Fe and unavoidable impurities.

[0033] 2. The steel strip was heated to 800°C for 10 minutes.

[0034] 3. slow cooling at a cooling rate of 8°C / s to 680°C.

[0035] 4. fast cooling at a cooling rate of 20°C / s to 450°C.

[0036] 5. dipping the steel strip into a zinc bath at 450°C for 15 seconds and then cooling at a cooling rate of 5°C / s to room temperature.

[0037] 6. skin pass treatment of the steel strip with a skin pass elongation of 0.40%.

[0038] Example 3

[0039] 1. A duplex steel strip with the required chemical composition was selected as the base material, C: 0.114%, Si: 0.33%, Mn: 2.03%, P: 0.008%, S: 0.004%, Cr: 0.31%, the balance being Fe and unavoidable impurities.

[0040] 2. The steel strip was heated to 840°C for 10 minutes.

[0041] 3. slow cooling at a cooling rate of 15°C / s to 730°C.

[0042] 4. fast cooling at a cooling rate of 30°C / s to 485°C.

[0043] 5. dipping the steel strip into a zinc bath at 460°C for 10 seconds and then cooling at a cooling rate of 8°C / s to room temperature.

[0044] 6. skin pass treatment of the steel strip with a skin pass elongation of 0.70%.

[0045] The mechanical property of the steel strip of the embodiment of the present application is tested, and the test results are shown in the following table:

[0046]

[0047] The above-described embodiments are only to describe the preferred modes of the present application, and do not 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 for improving the yield strength ratio of 780MPa grade duplex galvanized steel strip, characterized in that: The steel strip with good surface quality after hot rolling and pickling is produced by heat treatment, slow cooling treatment, rapid cooling treatment, galvanizing treatment, and leveling treatment; wherein: The heating temperature for hot-dip galvanizing is 750-840℃; The slow cooling temperature for hot-dip galvanizing is 620-730℃; The rapid cooling temperature for hot-dip galvanizing is 430-485℃; The hot-dip galvanizing process involves immersing the food in a zinc bath at a temperature of 450-470℃ for 5-35 seconds. The finishing elongation rate is 0-1.0%; Its chemical composition by weight percentage is: C 0.10%-0.50%, Si 0.3%-0.8%, Mn 0.5%-2.5%, P≤0.05%, S≤0.05%, Cr 0.20%-0.50%, with the balance being Fe and unavoidable impurities; The produced steel strip has the following properties: yield strength 455-487MPa, tensile strength 781-819MPa, elongation 22-24.5%, and yield strength ratio ≤0.

60.

2. The method for improving the yield strength ratio of 780MPa grade duplex galvanized steel strip according to claim 1, characterized in that: The steel strip is heated to 750-840℃ and then held at that temperature for 10-30 minutes.

3. The method for improving the yield strength ratio of 780MPa grade duplex galvanized steel strip according to claim 1, characterized in that: The slow cooling rate is 5-15℃ / s.

4. The method for improving the yield strength ratio of 780MPa grade duplex galvanized steel strip according to claim 3, characterized in that: The slow cooling rate is 5℃ / s, 8℃ / s, or 15℃ / s.

5. The method for improving the yield strength ratio of 780MPa grade duplex galvanized steel strip according to claim 1, characterized in that: The rapid cooling rate is 10-30℃ / s.

6. The method for improving the yield strength ratio of 780MPa grade duplex galvanized steel strip according to claim 5, characterized in that: The rapid cooling rate is 10℃ / s, 20℃ / s, or 30℃ / s.

7. The method for improving the yield strength ratio of 780MPa grade duplex galvanized steel strip according to claim 1, characterized in that: The galvanizing time is 5-15 seconds.

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