Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1998-09-04
2001-09-25
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S547000
Reexamination Certificate
active
06294031
ABSTRACT:
This application is based on Japanese Patent Applications HEI 9-240110 filed on Sep. 5, 1997 and HEI 10-214001 filed on Jul. 29, 1998, the content of which is incorporated into the present application by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a production method of a heat-treated steel member.
2. Description of Related Art
Some steel members are used in an as rolled and not heat-treated state, but other steel members requiring a high wear resistance (hardness), tensile strength and toughness are heat-treated after rolling,. Such a steel member that is heat-treated and then used is called as a heat-treated member. Typical heat-treated members include a shoe, a link, a pin and a bushing used for an endless track mounted to construction vehicles such as a power shovel and a bulldozer, and a cutting edge of a blade used for a bulldozer and a vehicle with snow plough. Further, many other structural components used in various kinds of industrial machines are made from the heat-treated members.
As illustrated in
FIG. 2
, a conventional production method of a heat-treated member includes a shaping step
11
in which a steel material is shaped into a predetermined configuration to be a shaped material, and a heat treatment step
12
in which the shaped material is heat-treated. The heat treatment step
12
involves two steps of quench-hardening and tempering, which are typically both performed (Japanese Patent Publication No. HEI 3-219043).
The reason why the heat treatment step requires two steps, i.e., quench-hardening and tempering, is as follows:
Generally, with the heat-treated member, the characteristics of wear resistance (hardness), and tensile strength and toughness are incompatible characteristics with each other. More particularly, in a case where quench-hardening only is conducted, wear resistance (hardness) and tensile strength are greatly improved, while toughness is remarkably low. In the case where the heat treatment includes quench-hardening followed by tempering, although wear resistance (hardness) and tensile strength are slightly lowered, toughness is greatly improved so that necessary wear resistance (hardness), tensile strength and toughness are ensured. In other words, in a case where both quench-hardening and tempering are not conducted, the necessary wear resistance (hardness), tensile strength and toughness are not ensured.
However, the conventional production method has the following problems:
First, since the heat treatment requires two steps, i.e., quench-hardening and tempering, the number of production steps, production time and equipments are increased, resulting in an increase in the production cost.
Second, if either quench-hardening or tempering is simply removed from the heat treatment steps, any one of the necessary wear resistance (hardness), tensile strength and toughness is not ensured.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a production method of a heat-treated member wherein the number of production steps, production time and equipments can be decreased compared with the conventional heat treatment, and the necessary wear resistance (hardness), tensile strength and toughness are also ensured.
The above-described object is achieved by the following present invention:
A production method of a heat-treated member includes the steps of: shaping a steel material of low-carbon boron steel containing about 0.05-0.30% carbon by weight into a predetermined configuration to be a shaped material; and heat-treating the shaped material, the heat-treating including quench-hardening only.
In the production method according to the present invention, a heat-treatment comprising the step of quench-hardening only is conducted on the steel material; a tempering step is removed, as compared with the conventional heat treatment involving two steps, i.e., quench-hardening and tempering. As a result, the number of production steps, and the production time corresponding to the removed tempering step are decreased. In addition, equipment for conducting tempering is not necessary. As a result, cost for producing the heat-treated member can be reduced.
In the production method according to the present invention, necessary wear resistance (hardness), tensile strength and toughness are ensured even without conducting the tempering step. The reason is as follows:
In the case of a medium-carbon (alloy) steel containing 0.30-0.50% carbon by weight, the carbon (alloy) steel has a medium-carbon martensite microstructure in a quench-hardened and not tempered state. The medium-carbon martensite microstructure has a high hardness and a high tensile strength. but has a low toughness. When tempering at a low temperature (below about 200° C.) is conducted on the quench-hardened material, the medium-carbon martensite is decomposed to a low-carbon martensite and a carbide. As a result, although the necessary hardness and tensile strength are slightly decreased, the toughness is improved, which allows the quench-hardened and tempered material to be used as a heat-treated member. On the other hand, in the case of a low-carbon (alloy) steel including 0.30)% or less carbon by weight, when tempering at a low temperature (below about 200° C.) is conducted subsequent to quench-hardening, the microstructure is of a low-carbon martensite microstructure only. As a result, the necessary hardness, tensile strength and toughness are ensured, which allows the quench-hardened and tempered material to be used as a heat-treated member.
With the low-carbon (alloy) steel, the microstructure was examined after quench-hardening and before tempering. It was found that the microstructure was of a low-carbon martensite microstructure only, which was the same microstructure as that obtained when tempering at a low temperature was conducted after quench-hardening. Further, it was also found that the hardness. tensile strength and toughness were substantially the same as those obtained when tempering, was conducted after quench-hardening. In the present invention, by using low-carbon (alloy) steel as a material, tempering at a low temperature is removed from the heat treatment process without substantially decreasing hardness, tensile strength and toughness.
The reason for selecting the carbon content of 0.05-0.30% by weight is that if the carbon content is less than 0.05% by weight, the predetermined hardness and tensile strength cannot be obtained due to the too small carbon content in the low-carbon martensite microstructure generated during quench-hardening, and that if the carbon content is greater than 0.30% by weight, the microstructure generated during quench-hardening is a medium-carbon martensite microstructure having a low toughness, which requires tempering after quench-hardening. In the range close to 0.30% carbon by weight, the microstructures obtained with the low-carbon (alloy) steel and with the medium-carbon (alloy) steel exist. In order to cause only the low-carbon martensite microstructure to be generated during quench-hardening, the carbon content is to be selected preferably 0.05-0.279% by weight, and more preferably, 0.20-0.26% by weight.
With the low-carbon steel of the present invention, 0.0001-0.0100% boron by weight is added to the low-carbon steel. One reason for the addition of boron is to ensure a hardenability and another reason is to ensure a necessary toughness in the high hardness range. With regard to the hardenability, there is a problem in that, with the low-carbon steel, it is difficult to harden the core portion of the heat-treated member. In order to improve the hardenability, 0.0001-0.0100% boron by weight is added to the low-carbon steel, and more preferably, 0.0005-0.0030% boron by weight is added thereto, thereby ensuring the necessary hardenability. Due to the addition of boron, the present invention can be applied not only to a heat-treated member in which quench-hardening is conducted to a surface portion only as in a high-frequency induction-hardening, but also to a heat-
Morishima Go
Nakajima Masahiro
Niwa Kiyokazu
Sugiyama Daigo
Takeno Hiroyuki
Pillsbury & Winthrop LLP
Topy Kogyo Kabushiki Kaisha
Yee Deborah
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