JP2001131631A - Method of spheroidizing-annealing steel material in short time and steel material using this method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a short time spheroidizing-annealing method of a steel material, by which the treating time can remarkably be shortened in comparison with the conventional method and thus, this method is profitably used from the view point of the energy cost. SOLUTION: To a steel material for machine structural use containing 0.15-1.10 wt.% C, the spheroidizing-annealing treatment is applied as the follows. (1) The steel material is heated in the temperature range of (the developing temperature of austenite -50 deg.C)-(the developing temperature of austenite -5 deg.C) during the heating at <=0.01 deg.C/s heating rate. (2) After heating to the temperature range of (the forming temperature of the single phase of austenite -30 deg.C)-(the forming temperature of the single phase of austenite -5 deg.C) which is the maximum heating temperature, this steel material is immediately turned to cooling. (3) After cooling in the temperature range of (the forming temperature of ferrite +10 deg.C)-(the forming temperature of ferrite -40 deg.C) during cooling, at <=0.005 deg.C/s cooling rate, this steel material is air-cooled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for facilitating machining such as plastic working and cutting, and for performing spheroidizing treatment of carbides in steel in order to improve mechanical properties. The present invention relates to a method for annealing a steel material in a short time in a spheroidizing manner and a steel material produced by the method.

[0002]

2. Description of the Related Art Conventionally, so-called machine structural steel such as carbon steel and alloy steel and bearing steel have been widely used as materials for mechanical parts used in automobiles and industrial machines. These mechanical parts are usually manufactured, for example, in the case of a dedicated steel wire rod, by spheroidizing the raw material, cutting, performing cold forging, and then performing cold working such as cutting. Here, cold working is frequently used because it is excellent in terms of working accuracy, mass productivity and cost, and spheroidizing annealing is to reduce carbides in steel in order to improve such cold workability. This is performed for the purpose of reducing the deformation resistance by spheroidizing. The structure after the spheroidizing annealing is such that spherical carbides are dispersed in a ferrite base.

However, such a spheroidizing annealing method has conventionally been regarded as a problem in that it requires high temperature and a long heating time of about 10 to 30 hours. As a solution to this problem, for example, Japanese Patent Publication No. 6-2898 discloses a heat pattern as shown in FIGS. 2 (a) and 2 (b). And there remains a major problem in terms of energy cost and temperature control. Japanese Unexamined Patent Publication No. 4-362123 discloses a heat pattern as shown in FIG. 3. In this heat pattern, even if the above-mentioned energy cost is improved, spheroidization of carbides is performed. In that respect, problems remained.

[0004] In addition, JP-A-4-333527 discloses that
A spheroidizing annealing method comprising slow cooling after the stage of holding treatment is disclosed. However, this method also has a disadvantage that basically requires two times of long-term holding treatment, Since cementite stabilizing elements such as Mn and Cr are indispensable for forming nuclei of carbides, there is a problem in terms of alloy cost.

[0005]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and provides a short-time spheroidizing annealing of a steel material that can achieve effective carbide spheroidization in a short time. It is an object of the invention to propose a method together with the steel obtained by processing according to this method.

[0006]

Means for Solving the Problems In order to achieve the above object, the present inventors have conducted repeated studies on the conditions for spheroidizing annealing of steel materials, and have found that the time required for spheroidizing annealing is reduced. It was found that the transformation behavior of the steel material, the diffusion rate of carbon, and the number of undissolved carbides before slow cooling had a significant effect. The present invention is based on the above findings.

That is, the present invention provides a method for preparing C: 0.15 to 1.10 mass
% Of steel for machine structural use containing (%), (1) During heating, (temperature at which austenite appears -50 ° C) to (temperature at which austenite appears -5 ° C) Is heated at a rate of 0.01 ° C./s or less.
℃), then immediately cool down,
(3) During cooling (temperature at which ferrite appears + 10 ° C)
~ (Temperature at which ferrite appears -40 ℃)
This is a short-time spheroidizing annealing method for steel, which is cooled at a rate of 0.005 ° C / s or less and then air-cooled.

The present invention also relates to a steel material for machine structure containing C: 0.15 to 1.10 mass% obtained by performing a spheroidizing annealing treatment, wherein the spheroidizing annealing is (1) ) During heating, the temperature range of (temperature at which austenite appears -50 ° C) to (temperature at which austenite appears -5 ° C) is 0.01 ° C.
/ S at a rate of not more than (2) After heating to the maximum heating temperature (temperature at which austenite becomes single phase -30 ° C) ~ (temperature at which austenitic single phase becomes -5 ° C),
(3) During cooling, (temperature at which ferrite appears + 10 ° C) ~ (temperature at which ferrite appears -40
(° C) at a rate of 0.005 ° C / s or less, followed by air cooling.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the invention will be described below. By the way, the present inventors have investigated the spheroidizing annealing conditions of steel containing 0.15 to 1.10 mass% of C. When the spheroidizing annealing time is shortened, the microstructure after the spheroidizing annealing is reduced. However, although the number of rod-shaped or layered carbides increases, or the degree of spheroidization is not a problem, there are many fine carbides, so that the hardness increases compared to conventional materials and the cold workability decreases. found. Also,
Further investigations revealed that the degree of spheroidization and cold workability were strongly affected by the transformation behavior of the steel, the diffusion rate of carbon, and the number of undissolved carbides before slow cooling.

On the basis of the above results, various conditions of the spheroidizing annealing were used to clarify the requirements for obtaining the same cold workability as the conventional material even if the spheroidizing annealing time was shortened. Investigating the cold workability of the obtained steel material, it was found that it was important to satisfy the following three requirements.

(1) During heating, (temperature at which austenite appears-50 ° C) ~ (temperature at which austenite appears-
5 ° C.) at a rate of 0.01 ° C./s or less. This requirement is an important requirement of the present invention. In order to shorten the heat treatment time, it is effective to increase the heating rate. However, when the heating rate is increased, the structure after spheroidizing annealing shows a lot of very fine carbides compared to the conventional one, no matter how the subsequent cooling conditions are changed. It is higher than the material, and the reduction in cold workability cannot be avoided. This is because the dispersion of fine carbides strengthens the steel. Therefore, in order to remove fine carbides from the finally obtained microstructure, it is necessary to dissolve the fine carbides in the matrix during the heating process.

[0012] Thus, when the heating rate was changed in various temperature ranges and the particle size distribution of the finally obtained carbide was investigated, it was found that it was most effective to maintain the temperature just below the temperature at which austenite appeared. . Above the temperature at which austenite appears, the diffusion rate of carbon sharply decreases and the dissolution of fine carbides into the matrix becomes very slow, so a temperature lower than the temperature at which austenite appears is desirable from the viewpoint of shortening the time. It turned out that. Here, the dissolution of the carbide is determined by the temperature range and the heating rate. From actual operation and a detailed investigation of the dissolution state of the carbide, from the temperature at which austenite appears-50 ° C) to the temperature at which austenite appears- In the optimum temperature range (5 ° C.), it is most desirable to heat at a rate of 0.01 ° C./s or less. Above this rate, the dissolution of fine carbides was insufficient. Therefore, it was limited to the above temperature range and cooling rate.

(2) After heating to the maximum heating temperature range of (austenite single phase -30 ° C.) to (austenitic single phase -5 ° C.), immediately start cooling. The reason why spheroidization is specified in the above range is that when the spheroidizing annealing time is intended to be shortened, spheroidization becomes insufficient if the temperature is too high or too low outside the above temperature range. The reason is that if the temperature is too high, most of the carbides will form a solid solution, the density of the nucleation sites of the spherical carbides will decrease, and as a result regenerated pearlite will be generated during cooling, while the temperature is too low In this case, the layered perlite remained without being dissolved. In addition, after heating up to the maximum heating temperature for the purpose of shortening the time, cooling can be immediately started without holding, and it has been found that it is not necessary to hold as in the conventional case.

(3) During cooling, a temperature range of (temperature at which ferrite appears + 10 ° C.) to (temperature at which ferrite appears −40 ° C.) is cooled at a rate of 0.005 ° C./s or less, and then air-cooled. The requirements are particularly important requirements in the present invention. That is, to promote spheroidization, during cooling,
It is necessary to deposit carbon having a solid solution difference between austenite and ferrite around the undissolved carbide. for that purpose,
It is important to select the slow cooling temperature range and cooling rate. Therefore, the degree of spheroidization was examined by changing the annealing temperature range and the cooling rate in various ways. In order to obtain a stable spheroidized carbide (the temperature at which ferrite appears + 10 ° C),
It was found that it was necessary to cool at a rate of 0.005 ° C./s or less up to the temperature at which the transformation was completed (the temperature at which ferrite appeared—40 ° C.).

The reason for setting the slow cooling start temperature to (temperature at which ferrite appears + 10 ° C.) is that by setting this start temperature, the incubation time from the point of shortening to the start of transformation is short, and the whole slow cooling is started. This is because the cooling time can be shortened. If the cooling rate is too high, or if the lower limit of the temperature range for slow cooling is higher than (temperature at which ferrite appears −40 ° C.), phase-like pearlite appears and the hardness increases, and the cold workability increases. Causes a decrease in In addition, when slow cooling is started at a temperature higher than the slow cooling start temperature (temperature at which ferrite appears + 10 ° C), the cold workability is almost the same,
The effect of shortening the spheroidizing annealing time becomes insufficient. Therefore, it was limited to the above temperature range and cooling rate.

In the temperature ranges other than those shown in the above (1) and (3), even if the heating rate and the cooling rate are greatly increased, the degree of spheroidization is not particularly affected. In order to shorten the time, it is advantageous to perform rapid heating and rapid cooling. However, increasing the heating and cooling rates not only increases the equipment cost, but also complicates the operation and causes various problems such as maintenance, so that a temperature of, for example, about 1 ° C./s or less is practical.

FIG. 1 illustrates a preferred heat pattern of the spheroidizing treatment according to the present invention as described above.

Next, the material to be treated in the present invention will be described. If the present invention contains 0.15 to 1.10 mass% of C, the present invention can be applied to all conventionally known steels for machine structural use. it can. Here, the reason why the amount of C is limited to the range of 0.15 to 1.10 mass% is as follows. C is a useful element for forming a solid solution to strengthen the matrix and obtaining sufficient strength and wear resistance as a mechanical part. However, if the content is less than 0.15 mass%, it is spherical from the viewpoint of cold workability. There is no need for chemical annealing. On the other hand, if it exceeds 1.10 mass%, the toughness of the base material is significantly deteriorated.

Conventionally, the addition of a cementite stabilizing element such as Mn or Cr has been indispensable in order to form a nucleus of a spheroidized carbide. Since the number can be appropriately adjusted, addition of such an element is not particularly required. Therefore, Mn and Cr are
Mn <0.25 mass% and Cr <0.1 mass% may be satisfied.

The following are examples of suitable steel types to which the present invention is applied.・ JIS G 4051 Carbon steel for machine structure, especially S45C ~ S55C
(C ≧ 0.45 wt%).・ JIS G 4105 Chrome molybdenum steel, especially SCM435 ~
SCM445 (C ≧ 0.35 wt%).・ JIS G 4805 High carbon chromium bearing steel, especially SUJ 2
(Bearing steel representative).

[0021]

EXAMPLES Steel having various component compositions shown in Table 1 was melted in a converter, made into a billet by a continuous casting method, and then rolled into a 20 mmφ bar. Next, the steel bars were subjected to spheroidizing annealing under various conditions shown in Table 2, and then each material was subjected to spheroidizing annealing.
A cold workability test specimen having a height of mmφ × 22.5 mm was cut out, and the cold workability under each condition was investigated. The obtained results are also shown in Table 2. In addition, about deformation resistance, compression ratio:
Evaluation was made based on the deformation resistance when restrained compression deformation was performed under the condition of 60%. As for the deformability, n = 5 at each compression ratio.
Was subjected to constrained compression deformation, and the lowest compression rate at which cracking was observed was evaluated as the crack limit compression rate.

[0022]

[Table 1]

[0023]

[Table 2]

As is evident from Table 2, when spheroidizing annealing was performed according to the present invention, the cold workability (deformation resistance and deformation resistance) were the same as in the prior art, although the processing time was greatly reduced. Deformability) is obtained. On the contrary,
In all of the comparative examples (Nos. 11 to 20), since the spheroidizing annealing conditions are out of the proper range of the present invention, the deformation resistance or the deformability is significantly reduced.

[0025]

As described above, according to the present invention, effective spheroidization of carbides can be achieved in a short time, and the required time and energy cost can be greatly reduced as compared with the conventional one. Useful.

[Brief description of the drawings]

FIG. 1 is a preferred heat pattern of a spheroidizing heat treatment according to the present invention.

FIG. 2 is a heat pattern disclosed in Japanese Patent Publication No. 6-2898.

FIG. 3 is a heat pattern disclosed in JP-A-4-362123.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toshiyuki Hoshino 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. 1-chome (without address) Inside Kawasaki Steel Corporation Mizushima Works

Claims (2)

[Claims] When spheroidizing annealing is performed on a steel material for a machine structure containing C: 0.15 to 1.10 mass%,
(1) During heating, (the temperature at which austenite appears -50
(° C) ~ (Temperature at which austenite appears -5 ° C) is heated at a rate of 0.01 ° C / s or less. (2) Maximum heating temperature (temperature at which austenite becomes single phase -30 ° C) ~
After heating to the temperature range (temperature at which austenite becomes a single phase -5 ° C), immediately start cooling, and (3) during cooling,
Short-time spheroidizing sintering of steel characterized by cooling at a rate of 0.005 ° C / s or less in the temperature range of (temperature at which ferrite appears + 10 ° C) to (temperature at which ferrite appears -40 ° C) Annealing method. 2. A spheroidizing annealing treatment, C:
A steel material for a mechanical structure containing 0.15 to 1.10 mass%, wherein the spheroidizing annealing is carried out in the following manner: (1) During heating, (temperature at which austenite appears-50 ° C) ~ (temperature at which austenite appears- (5 ° C) at a rate of 0.01 ° C / s or less, and (2) the maximum heating temperature (temperature at which austenitic single phase becomes -30 ° C) to (temperature at which austenitic single phase becomes -5 ° C) After heating to the temperature range, it immediately starts cooling. (3) During cooling, (temperature at which ferrite appears +10
A steel material characterized by comprising a step of cooling at a rate of 0.005 ° C / s or less in a temperature range of (° C) to (temperature at which ferrite appears -40 ° C) and then air cooling.