JPS6369914A - Production of bar and wire rod having excellent wear resistance - Google Patents

Abstract

PURPOSE:To produce bar and wire rod having excellent wear resistance and stress corrosion cracking resistance, by making the surface layer part to fine spheroidizing structure by specifying the temp. of surface layer part and the rate of reduction in area in the hot finishing roll of bar and wire rod and then executing the proper cooling. CONSTITUTION:At the time of hot finishing rolling of the bar and wire rod is cooled at the temp. range of Ac1 point -60 deg.C and is rolled at >=50% the rate of reduction in area. In this way, the above surface layer part is made to the fine spheroidizing structure having high hardness and composing of fine ferrite and spheroidal carbide. Further, the bar and wire rod after rolling is cooled at less cooling speed than lower critical cooling speed immediately after finishing of rolling. In this way, the development of martensite caused to deterioration of toughness and ductility for inner structure in the stable austenite is restrained and ferrite-perlite structure 2 is obtd. And, the bar and wire rod having excellent wear resistance and also stress corrosion cracking resistance as roll, is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing rods and wires with excellent wear resistance.

[Conventional technology]

Among rod and wire rod products such as steel wire rods and steel bars, those used for mechanical parts and structural members that require wear resistance require high surface hardness for wear resistance and at the same time ensure toughness. Due to this need, the surface layer is tempered martensite, and the interior is adjusted to a ferrite-pearlite structure, bainite structure, or a mixture of these structures, but in general, such a structure is not achieved when hot-rolled. It is difficult to obtain rods and wires with For this purpose, it is customary to perform heat treatment such as induction hardening, tempering, or carburizing and quenching and tempering before use.

However, these heat treatments require expensive heat treatment equipment and pose major problems in terms of resources, energy, and productivity.

For the purpose of improving these problems, for example or Japanese Patent Application Laid-Open No. 51-9
9619, after hot rolling, the surface layer is cooled at a cooling rate sufficient to form martensite in the surface layer, and then the surface layer is tempered by sensible heat inside the material, thereby making the surface layer a tempered martensite. Also, a method is known in which the inside has a ferrite-pearlite structure, a bainite structure, or a mixed structure thereof.

However, steel products manufactured by this method contain sulfates,
When used in environments containing nitrates or chlorides, such as in soil or concrete containing these compounds, or as structural members or mechanical parts that frequently come into contact with these compounds, they are highly susceptible to stress corrosion cracking.
This is a big problem in practice.

In general, ferrite-pearlite or spheroidized structures have better corrosion resistance than bainite or tempered martensitic structures with respect to stress corrosion cracking.

[Problem that the invention seeks to solve]

An object of the present invention is to solve the problems of the prior art described above by controlling hot rolling and subsequent cooling to make the surface layer part a spheroidized structure consisting of fine ferrite and spheroidal carbide, and
By forming the inside with a ferrite-pearlite structure, bainite structure, or a mixture thereof, it is possible to create rods and wires that have good wear resistance as hot-rolled and are also resistant to stress corrosion cracking. To provide a manufacturing method.

[Means and operations for solving the problems] The gist of the present invention is as follows.

That is, during hot finish rolling of the rod and wire rod, only the surface layer of the rod and wire rod is cooled to a temperature range of Ac, the transformation point and Ac, a temperature 60 ° C lower than the transformation point, and rolling with a cross-sectional reduction rate of 50% or more is performed. The method comprises the steps of: forming the surface layer into a spheroidized structure consisting of fine ferrite and spheroidal carbide; and immediately cooling the rolled rod and wire rod at a cooling rate lower than the lower critical cooling rate after rolling. [1] A method for producing rods and wires with excellent wear resistance.

As a result of intensive studies to achieve the above object of the present invention, the American inventors found that during hot rolling of rods and wires, the surface layer was
It has been found that the surface hardness can be significantly increased by rolling 50% or more in the temperature range between the cr transformation point and Ac, a temperature 60° C. lower than the transformation point.

The above temperature range corresponds to the unstable region of austenite, and processing in this temperature range causes carbides to precipitate directly from the austenite, and ferrite precipitation progresses rapidly.As a result, the structure after rolling has spherical carbides in fine ferrite. It becomes a distributed spheroidal structure.

Such a structure has extremely high hardness due to the miniaturization of ferrite grains, and has almost the same hardness as when the structure is bainite or tempered martensite. In addition, by controlling the cooling rate after rolling, we can control the internal transformation of stable austenite during rolling.
It is possible to ensure the hardness required for mechanical parts or structural members.

Next, the reasons for limiting the constituent elements in the present invention will be explained.

In hot finish rolling, the surface layer part is Ac, the transformation point is Ac,
The reason why the temperature range is limited to 60° C. lower than the transformation point is that the target spheroidized structure cannot be obtained by rolling in a temperature range exceeding the Ac1 transformation point. Also, AC,
This is because at temperatures lower than +60° C. below the transformation point, the rolling load is high and rolling becomes difficult. In addition, in this temperature range, the reduction rate was set to 50% or more in terms of area reduction rate.
If it is less than 0%, the target spheroidized structure cannot be obtained, so
It was limited to 0% or more.

In addition, the cooling rate after rolling was limited to a cooling rate at which martensite does not occur, that is, a rate slower than the lower critical cooling rate, because if the cooling rate is too fast and martensite is generated, the toughness and ductility will be reduced.

〔Example〕

345C, which is a typical steel type of carbon steel for mechanical structures according to JIS whose chemical composition is shown in Table 1, is melted in a converter, made into a bloom by continuous casting, and then rolled into a bar through billet rolling. Steel bars of 53 mmφ were made by the conventional method and the method of the present invention under the conditions shown in Table 2.

Table 1 Table 2 When examining the cross sections of these steel bars, we found that the microstructure of the as-rolled conventional steel bars was a ferrite-pearlite structure, whereas the microstructure of the examples of the present invention had a microstructure in the surface layer as shown in Figure 1. A region of approximately 2 m became a fine spheroidized structure 1 consisting of fine ferrite and spherical carbide, and a region inside this became a ferrite-pearlite structure 2.

A part of the 53-barrel steel bar manufactured by the conventional method was heated to 860℃
After heating for 60 min, oil quenching is performed, followed by 475℃
Tempering treatment was performed for X 20 min.

The cross-sectional hardness distributions of these three steel bars of the conventional example (as rolled), the conventional example (heat treated after rolling), and the example of the present invention were investigated, and the results are shown in FIG.

From FIG. 2, it can be seen that the hardness of the surface layer portion of the example of the present invention increases to the same extent as that of the conventional example in which a heat treatment was performed after rolling while the example was rolled.

〔Effect of the invention〕

As is clear from the above embodiments, the present invention limits the temperature range and area reduction rate of the surface layer during finish rolling of rods and wires,
By limiting the cooling rate after rolling, it was possible to obtain rods and wires with excellent #4 wear resistance and stress corrosion cracking resistance as they were rolled.

[Brief explanation of the drawing]

FIG. 1 is a schematic sectional view showing the cross-sectional structure of an example of the present invention, and FIG. 2 is a diagram showing a comparison of the hardness distribution of the steel bar cross section of an example of the present invention and a conventional example.

Claims (1)

[Claims] (1) During hot finish rolling of the rod and wire rod, only the surface layer of the rod and wire rod is cooled to a temperature range between the Ac_1 transformation point and a temperature 60°C lower than the Ac_1 transformation point, and rolling is performed with a reduction in area of 50% or more. The step of forming the surface layer part into a spheroidized structure consisting of fine ferrite and spheroidal carbide, and the step of cooling the rolled rod and wire rod immediately after rolling at a cooling rate lower than the lower critical cooling rate. A method for manufacturing rods and wires with excellent wear resistance.