GRADIENT STEEL MATERIAL HAVING HIGH-PLASTICITY SURFACE LAYER AND HIGH-STRENGTH INNER LAYER, AND MANUFACTURING METHOD

Abstract

A gradient steel material with a high plastic surface layer and a high strength inner layer, and a manufacturing method are provided. Weight percentages of the components of the gradient steel material are: C≤0.15%, Si≤1%, Mn≤1.5%, and the balance of Fe and inevitable impurities, the surface layer of the steel material being ferrite, and the inner layer being ferrite+bainite. The manufacturing method therefor comprises: smelting, casting, rolling, and a heat treatment, wherein in the heat treatment step, a steel material is heated to an austenite temperature Ac3 or more and kept at said temperature for more than 3 min; thereafter, the material is cooled to a temperature range between Ar3 and Ar1 in a two-phase zone at a cooling rate of less than 0.5° C./s, and is then cooled to room temperature at a cooling rate of greater than 5° C./s. The present steel material does not need to be obtained by means of the compound preparation of different materials as only a single material is processed. At the same time, the composition of the steel material is simple. Although the internal and external microstructures are different, the difference is a gradual process, and the strength at the interface is good.

Claims

1. A gradient steel material having a high-plasticity surface layer and a high-strength inner layer, comprising the following components in percentage by weight: 0<C≤0.15%, 0<Si≤1%, 0<Mn≤1.5%, and the balance of Fe and inevitable impurities, the surface layer of the steel material being ferrite, and the inner layer being ferrite+bainite.

2. The gradient steel material having a high-plasticity surface layer and a high-strength inner layer according to claim 1, characterized in that the surface layer of the steel material is ferrite with a carbon content not higher than 0.02 wt %; and the carbon content of the surface layer of the steel material is lower than the carbon content of the inner layer of the steel material.

3. A manufacturing method for a gradient steel material having a high-plasticity surface layer and a high-strength inner layer according to claim 1, characterized by comprising: smelting, casting, rolling and a heat treatment; wherein in the heat treatment step, a steel material is heated to an austenite temperature Ac3 or more and hold at above temperature for more than 3 min to ensure complete austenitization of the material; thereafter, the material is cooled to a temperature range between Ar3 and Ar1 in a two-phase zone at a cooling rate of less than 0.5° C./s, and then rapidly cooled to room temperature at a cooling rate of greater than 5° C./s.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a schematic diagram of microstructure of a gradient steel material of the present invention, wherein B represents bainite, and F represents ferrite;

[0022] FIG. 2 is a schematic diagram of a heat treatment process in examples of a manufacturing method of the present invention;

[0023] FIG. 3 is a microstructure diagram of an end of a sample in Example 1 of the present invention; and

[0024] FIG. 4 is a microstructure diagram of an end of a sample in Example 4 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be further illustrated below in conjunction with embodiments and accompanying drawings.

[0026] The components in the examples and the comparative examples of the present invention are shown in Table 1, and the balance of Fe and unavoidable impurities. The manufacturing method for a steel material is as follows: the steel material obtained through converter or electric furnace smelting, heating and rolling is then heat-treated to finally obtain a gradient steel material having a surface layer with ferrite and an inner layer with ferrite+bainite.

[0027] The comparative examples of the application intend to illustrate that when the content of C, Si and Mn is not within the range of 0<C≤0.15%, 0<Si≤1%, 0<Mn≤1.5% as defined in the application, or the cooling rate does not meet the definition of the application, the microstructure of the gradient steel material having the surface layer with ferrite and the inner layer with ferrite+bainite cannot be obtained.

[0028] Table 2 shows the heat treatment process in the examples of the manufacturing method of the present invention and the comparative examples.

[0029] FIG. 1 shows the microstructure of the gradient steel material of the present invention, the surface layer is ferrite, and the inner layer is mixed microstructures of ferrite and bainite.

[0030] FIG. 2 shows a heat treatment process in examples of a manufacturing method of the present invention.

[0031] It can be found from FIG. 3 that in the microstructures of the gradient steel material obtained in Example 1, the surface layer is pure ferrite within a range of about 256 μm, and the inner layer is mixed microstructures of ferrite and bainite.

[0032] FIG. 4 shows the microstructures of the gradient steel material obtained in Example 4, the surface layer is pure ferrite within a range of about 171 μm and the inner layer is mixed microstructures of ferrite and bainite.

TABLE-US-00001 TABLE 1 Unit: weight percentage C Si Mn Example 1 0.054 0.26 0.51 Example 2 0.075 0.252 0.49 Example 3 0.081 0.243 1.23 Example 4 0.105 0.356 0.56 Example 5 0.112 0.366 0.76 Comparative Example 1 0.054 0.26 0.51 Comparative Example 2 0.15 1.5 0.48

TABLE-US-00002 TABLE 2 Slow Slow Surface Heating Holding cooling cooling Rapid layer Inner layer tempe time speed final cooling microstr microstructure Example 1  950° C. 300 s 0.05° C./s 850° C. 25° C./s ferrite ferrite Example 2  950° C. 600 s  0.1° C./s 850° C. 30° C./s ferrite ferrite Example3 1000° C. 300 s 0.05° C./s 850° C. 25° C./s ferrite ferrite + bainite Example 4 1000° C. 600 s 0.05° C./s 860° C. 30° C./s ferrite ferrite + bainite Example 5 1000° C. 900 s  0.1° C./s 860° C. 40° C./s ferrite ferrite + bainite Comparative 1000° C. 900 s  0.5° C./s 860° C. 30° C./s ferrite + ferrite + bainite Example 1 bainite Comparative 1000° C. 900 s 0.05° C./s 860° C. 30° C./s ferrite + ferrite + bainite Exampl 2 bainite