JPH05125437A - Method for producing grain-stabilized case hardening steel - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a method for manufacturing case hardening steel for machine parts that are processed under extremely severe conditions for grain size, such as spheroidizing annealing + cold forging. [Constitution] Weight% by weight, C: 0.08 to 0.30%,
Si: 0.05-1.0%, Mn: 0.3-2.0%,
Al: 0.015-0.050%, Nb: 0.02-
0.10%, N: 0.015 to 0.030% and N
(%) ≧ 0.52 × Al (%) + 0.15 × Nb (%)
Steel containing the balance Fe and inevitably contained impurities in a range of 0.10 ° C. is heated to a temperature of 1150 ° C. or higher, and then hot-rolled at a final temperature of 950-800 ° C., and then 0.3-0. A method for producing a grain-stabilized case hardening steel, which comprises cooling to an A ¹ transformation point or less at 05 ° C./sec.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention has a fine structure in a cooled state after hot working, and is less likely to coarsen austenite crystal grains at the time of carburizing, even after undergoing a method for manufacturing a mechanical component such as cold forging. The present invention relates to a method for producing a grain-stabilized case hardening steel.

[0002]

2. Description of the Related Art When austenite crystal grains are coarsened during carburization, heat treatment strain, hardness variation, and component strength reduction are caused. Therefore, a case hardening steel with stable austenite grain size characteristics is required. ..

Further, from the viewpoint of workability and the like, it is required that the fine particles are in a rolled state. Although application of controlled rolling or the like has been considered to these, for example, these steels have the disadvantage that they tend to coarsen during high temperature heating such as carburizing because of their fine grain size.

Further, recently, it is often formed by cold forging, and in this case, a spheroidizing heat treatment is generally used as a preheat treatment. All of these are harmful to the grain size characteristics, and it is required to stabilize the grain size of case-hardening steel more than ever before.

Conventionally, in contrast to this, for example, JP-A-56 is used.
-75551 and JP-A-59-123714 are Al-N type case-hardening steels, but they have limitations in terms of stability of austenite grain size at high temperatures. On the other hand, examples of Nb added to improve stability at higher temperatures include JP-A-49-125220 and JP-A-62-99416. However, JP-A-49-12
No. 5220 does not consider the conditions of processing and heat history for surely and efficiently refining the precipitates effective for stabilizing the crystal grains, and in the case of JP-A-62-99416, Nb Although we are discussing the results of fine precipitation of carbonitride,
The study of the quantitative relationship among Al, Nb, and N to make them more refined is insufficient, and practically all conditions have grain size characteristics such as case hardening steel for cold forging. On the other hand, in the present situation, it is difficult to suppress the coarsening of crystal grains when taking a harmful step.

[0006]

The object of the present invention is to solve the above-mentioned problems of the prior art, and particularly for spheroidizing annealing + cold forging, it is very important for grain size. It is to develop a method for manufacturing case hardening steel for machine parts that are processed under extremely severe conditions.

[0007]

In order to solve the above problems, the inventor has conducted a detailed study on the relationship between the transformation characteristics of a case-hardening steel and the austenite grain size characteristics and manufacturing conditions. By considering the amount of fine amount components such as Nb and N, hot working conditions such as rolling heating and working temperature, and the cooling rate after working, AlN which has a fine structure and is effective for stabilizing the grain size, NbCN,
Finer and more uniform precipitation and dispersion, and even when heated at high temperature for a long time such as carburizing, it is possible to disperse and precipitate so that growth coarsening does not occur easily. It has become possible to manufacture case-hardening steel materials that are difficult to change. That is, the manufacturing method of the first aspect of the present invention is as follows: (1) wt%, C: 0.08 to 0.30% Si: 0.05 to 1.0% Mn: 0.3 to 2.0 % Al: 0.015 to 0.050% Nb: 0.02 to 0.10% N: 0.015 to 0.030% and N (%) ≧ 0.52 × Al (%) + 0.15 × Nb
(%), The steel containing the balance Fe and the impurities unavoidably contained is heated to a temperature of 1150 ° C. or higher, and then hot rolled at a final temperature of 950 to 800 ° C., and then 0.3 A method for manufacturing a grain-stabilized case hardening steel, characterized in that the steel is cooled to an A ¹ transformation point or less at ˜0.05 ° C./sec. ), In wt%, C: 0.08 to 0.30% Si: 0.05 to 1.0% Mn: 0.3 to 2.0% Al: 0.015 to 0.050% Nb: 0. 02 to 0.10% N: 0.015 to 0.030% and N (%) ≧ 0.52 × Al (%) + 0.15 × Nb
(%), And further contains one or more of Cr: 0.3 to 2.0% Ni: 0.3 to 5.0% Mo: 0.05 to 1.0%. After heating the steel consisting of balance Fe and inevitably contained impurities to a temperature of 1150 ° C. or higher, hot rolling is performed at a final temperature of 950 to 800 ° C., and then 0.3 to 0.05 ° C./sec. A ¹
A method for producing a grain-stabilized case hardening steel, which is characterized by cooling to below the transformation point.

The reason why the composition range of steel and the manufacturing conditions are limited as described above in the method of the present invention will be described below.

C: C is an essential element for ensuring the hardenability and strength, and at least 0.08% is necessary for ensuring the strength in ordinary mechanical parts. However, if it exceeds 0.30%, the hardness of the core portion is excessively increased, and the toughness deteriorates and the fatigue strength decreases due to the reduction of the residual stress of compression, which is not preferable for the original purpose of case hardening steel. Therefore, the upper limit of C is 0.30%.

Si: Si is usually used as a deoxidizing material, but it has a strong ferrite strengthening effect and improves hardenability.
If it is less than 0.05%, deoxidation will be insufficient, and if it exceeds 1.0%, the ductility, toughness and workability of steel will be impaired.
Therefore, the lower limit is set to 0.05% and the upper limit is set to 1.0%.

Mn: Mn is also used as a deoxidizing material and is an essential element for ensuring hardenability and strength. 0.
If it is less than 30%, this effect is not sufficient, and 2.0%
If it exceeds, the workability and toughness will be adversely affected. Therefore, the lower limit is 0.30% and the upper limit is 2.0%.

Ni, Cr, Mo: These are generally effective elements for securing strength and toughness according to the purpose and application in low alloy case-hardening steel, and are added as necessary. In each case, in order to obtain the effect of addition sufficiently, in consideration of economical efficiency, 0.3 to 5.0%, 0.3 to 2.0%,
0.05 to 1.0%.

Al: Al is an essential element for keeping the austenite grain size fine, and since it combines with N in the steel to form AlN and finely precipitates, the so-called pin-up effect causes the grain size of the crystal grains to change. It shows the effect of preventing coarsening.
If it is less than 0.015%, the precipitation amount of AlN is insufficient and the effect of preventing crystal grains is insufficient. On the other hand, if it exceeds 0.050%, the precipitate becomes coarse and, conversely, the effect of preventing the coarsening of the crystal grain is deteriorated. Therefore, the lower limit of Al is 0.015%
And the upper limit is 0.050%.

Nb: Nb is also an element essential for preventing grain boundary migration by precipitating as NbCN in steel and preventing grain boundary migration. However, if it is less than 0.02%, the amount of precipitation is insufficient and the blocking effect is not sufficient.
If it exceeds, the blocking effect will be saturated, and it will be precipitated as stringer-like non-metallic inclusions, resulting in deterioration of toughness and workability. Therefore, the lower limit is 0.02% and the upper limit is 0.
10%.

N: N is linked with Al and Nb to form AlN,
Precipitates as NbCN and refines the austenite grain size. If it is less than 0.015%, it is difficult to secure a necessary amount of precipitates for preventing coarsening of crystal grains,
If it exceeds%, there will be a problem with the soundness of the steel material. Therefore, the lower limit is 0.015% and the upper limit is 0.030%.

N (%) ≧ 0.52 × Al (%) + 0.1
It was found that adding 5 × Nb (%): N so as to satisfy this relational expression shows a stable grain blocking effect. This is because AlN and NbCN are finely and uniformly dispersed, and even when heated at a high temperature for a long time such as carburizing.
It is considered that these fine precipitates are difficult to grow.

The above are the restrictions on the alloy components in the present invention, but the present invention requires the following requirements as manufacturing conditions in order to further stabilize the austenite grain size. That is, heating temperature: 1150 ° C. or higher The heating temperature of case-hardening steel is generally about 1050 ° C. to 1250 ° C. In the present invention, the heating temperature needs to be 1150 ° C or higher.

As the material is heated for hot rolling, AlN and NbCN in the steel form a solid solution. However, if the heating temperature is not sufficient, a part of the unmelted portion will be coarsened and will not effectively contribute to the prevention of coarsening of crystal grains. Also, in the subsequent reheating for processing into parts, since the solid solution portion is precipitated with the undissolved precipitate as the nucleus, the size of the precipitate itself becomes large, and the effect of preventing coarsening of crystal grains is Decrease. When the heating temperature is 1150 ° C. or higher, a sufficient amount of AlN and NbCN can be solid-dissolved in the initial state. Therefore, the heating temperature is set to 1150 ° C. or higher.

Rolling start and end temperature: 950 to 800 ° C. The rolling start and end temperature is generally 950 to 1150 ° C., although it largely varies depending on the heating temperature and the rolling size. In the present invention, the rolling start and finish temperature needs to be 950 to 800 ° C. After heating to the above-mentioned heating temperature and rolling termination within this temperature range, a fine structure can be obtained after cooling. However, in this case, the workability is good, but in general, there is a drawback that the crystal grains are likely to become coarse during carburization, so this is covered by other conditions.

Cooling rate after processing: 0.3 to 0.05 ° C./sec The cooling rate varies depending on the rolling size, and is generally 1.0 to 0.4 at φ20 to φ50 in a normal cooling floor.
C / sec. In the present invention, after hot working is started and finished at the above temperature, 0.3 to 0.05 ° C / se
It is necessary to cool at a cooling rate of c. Therefore, in many cases, the cooling rate exceeds the range controlled by the present invention. In this case, the cooling rate is controlled by covering the cooling floor or using a slow cooling box. The present inventors have found that it is possible to produce a steel material having a fine structure and having significantly stabilized grain size characteristics by combining the conditions described above and this cooling condition. It was

By cooling within this range,
This is because AlN and NbCN are finely precipitated and effectively act to prevent the coarsening of crystal grains. If it exceeds 0.3 ° C / sec, the amount of precipitation is small, and if it is less than 0.05 ° C / sec, Precipitated particles become coarse during cooling,
This is because both of them are not effective in preventing coarsening of crystal grains.

This tendency can be further ensured by balancing the amounts of Al, Nb, and N in proper amounts, and the finding of the relationship is the most characteristic of the steel of the present invention.

[0023]

[Examples] Table 1 shows the chemical composition of the steel of the present invention and the comparative steel, each of the operating conditions such as hot heating conditions, finishing temperature and cooling rate after working, ferrite grain size, and 70% cold working after spheroidizing heat treatment. The austenite grain size of is shown. The rolling dimension is φ30.

[0024]

[Table 1]

[0025]

[Table 2]

When the finishing temperature is 950 ° C. or lower, fine crystal grains are obtained. However, in this case, the initial grain size of austenite generally becomes fine during heating during carburization, and the crystal grains tend to become coarse. However, the austenite grain size characteristics are affected by the cooling rate after processing, and the grain size characteristics are not stable by cooling in a general cooling bed. It can be seen that the austenite grain size characteristics are significantly improved by cooling at a cooling rate of 0.05 ° C./sec. However, this effect is not recognized when the heating during hot rolling is insufficient, and is not recognized even when the component range is not within the claimed range.

As described above, according to the method of the present invention, it is revealed that a case-hardened steel having a fine structure after hot rolling and having stable austenite crystal grains even when the carburizing condition is high temperature and long time is obtained. did.

[0028]

As described above, according to the present invention, austenite grains having a fine structure in a cooled state after hot working and having a fine structure in a cold state and having undergone a method for producing a mechanical component such as cold forging can be used. It is possible to manufacture grain-stabilized case hardening steel that does not easily coarsen, and its industrial effect is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a composition range of the steel of the present invention.

Front Page Continuation (72) Inventor Isao Machida 1-4-1 Chuo, Wako-shi, Saitama Stock Company Honda Technical Research Institute (72) Inventor Takeshi Takagi 1-14-1 Chuo, Wako-shi, Saitama Stock Company Inside Honda R & D Laboratories (72) Inventor Yoshikazu Umeno 1-4-1 Chuo, Wako-shi, Saitama Stock Research Laboratories Honda R & D Labs

Claims (2)

[Claims] 1. In wt%, C: 0.08 to 0.30% Si: 0.05 to 1.0% Mn: 0.3 to 2.0% Al: 0.015 to 0.050% Nb: 0.02 to 0.10% N: 0.015 to 0.030% and N (%) ≧ 0.52 × Al (%) + 0.15 × Nb
(%), The steel containing the balance Fe and the impurities unavoidably contained is heated to a temperature of 1150 ° C. or higher, and then hot rolled at a final temperature of 950 to 800 ° C., and then 0.3 A method for producing a grain-stabilized case hardening steel, which comprises cooling to an A ¹ transformation point or less at a temperature of ˜0.05 ° C./sec. 2. In wt%, C: 0.08 to 0.30% Si: 0.05 to 1.0% Mn: 0.3 to 2.0% Al: 0.015 to 0.050% Nb: 0.02 to 0.10% N: 0.015 to 0.030% and N (%) ≧ 0.52 × Al (%) + 0.15 × Nb
(%), And further contains one or more of Cr: 0.3 to 2.0% Ni: 0.3 to 5.0% Mo: 0.05 to 1.0%. After heating the steel consisting of balance Fe and inevitably contained impurities to a temperature of 1150 ° C. or higher, hot rolling is performed at a final temperature of 950 to 800 ° C., and then 0.3 to 0.05 ° C./sec. A ¹
A method for producing a grain-stabilized case hardening steel, characterized by cooling to below the transformation point.