GRADIENT STEEL MATERIAL HAVING SURFACE LAYER WITH FERRITE AND INNER LAYER WITH FERRITE + PEARLITE AND MANUFACTURING METHOD

Abstract

Provided are a gradient steel material having a surface layer with ferrite and an inner layer with ferrite+pearlite, and a manufacturing method, the weight percentages of the components are: C≤0.15%, Si≤1%, Mn≤1.5%, the balance of Fe and inevitable impurities, and the surface layer of the steel material is ferrite, the inner layer is ferrite+pearlite. The manufacturing method thereof includes: smelting, casting, rolling, heat treatment; wherein, in the heat treatment step, the steel material is heated above the austenitizing temperature Ac3, and hold at the temperature more than 3 min to ensure that the material is completely austenitized; subsequently, it is cooled to a temperature below Ar1 at a cooling rate lower than 0.5° C./s. The present steel material does not need to be obtained by means of the compound preparation of different materials, and is only processed and prepared by a single material, the process is short, the procedure is simple, and the cost is low.

Claims

1. A gradient steel material having a surface layer with ferrite and an inner layer with ferrite+pearlite, 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 gradient steel material is a ferrite with good plasticity, and the inner layer is a ferrite+pearlite.

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

3. A manufacturing method of a gradient steel material having a surface layer with ferrite and an inner layer with ferrite+pearlite according to claim 1, characterized by including: smelting, casting, rolling and heat treatment; wherein in the heat treatment step, a steel material is heated above the austenitizing temperature Ac3, and the temperature is kept above 3 min to ensure that the material is completely austenitized; and subsequently, it is cooled to a temperature below Ar1 at a cooling rate of less than 0.5° C./s.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a schematic diagram of microstructures of a gradient steel material of the present invention, wherein a represents ferrite, and P represents pearlite;

[0023] FIG. 2 is a schematic diagram of a heat treatment process in examples of a manufacturing method of the present invention;

[0024] FIG. 3 is a microstructure diagram of a whole cross-section of a sample in Example 1 of the present invention;

[0025] FIG. 4 is a microstructure diagram of a near-surface layer of a sample in Example 1 of the present invention; and

[0026] FIG. 5 is an enlarged microstructure diagram of an inner layer of a sample in Example 1 of the present invention.

DETAILED DESCRIPTION

[0027] The present invention will be further illustrated below in conjunction with embodiments and accompanying drawings.

[0028] The components in the examples and the comparative embodiments 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+pearlite.

[0029] 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+pearlite cannot be obtained.

[0030] Table 2 shows the heat treatment process in the examples of the manufacturing method of the present invention and the comparative examples.

[0031] FIG. 1 shows the microstructure of the gradient steel material of the present invention, the surface layer is a ferrite, and the inner layer is a mixed microstructures of ferrite and pearlite. FIG. 2 shows a heat treatment process in examples of the manufacturing method of the present invention.

[0032] FIG. 3 shows a final microstructure of Example 1. It can be found from FIG. 3 that the surface layer is a ferrite within a range of about 450 μm, and the inner layer is a mixed microstructures of ferrite and pearlite. FIG. 4 is an enlarged metallograph of the near-surface layer of the sample, and it can be seen that the surface microstructure is a pure ferrite. FIG. 5 is an enlarged metallograph of the inner layer of the sample, and it can be seen that the inner layer microstructure is a mixed microstructures of ferrite and pearlite.

TABLE-US-00001 TABLE 1 Unit: weight percentages 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 cooling Surface Inner Heating Holding cooling final layer layer temperature time speed temperature microstructure microstructure Example 1  950° C. 600 s 0.05° C./ 200° C. ferrite ferrite + s pearlite Example 2  950° C. 600 s 0.10° C./ 600° C. ferrite ferrite + s pearlite Example 3 1000° C. 300 s 0.05° C./  20° C. ferrite ferrite + s pearlite Example 4 1000° C. 300 s  0.2° C./ 200° C. ferrite ferrite + s pearlite Example 5 1000° C. 300 s  0.3° C./ 600° C. ferrite ferrite + s pearlite Comparative 1000° C. 300 s  0.5° C./ 600° C. ferrite + ferrite + Example 1 s pearlite pearlite Comparative 1000° C. 300 s 0.05° C./ 600° C. ferrite + ferrite + Example 2 s pearlite pearlite