JPH0726151B2 - Method for manufacturing case hardening steel - Google Patents

Description

[Detailed Description of the Invention] Industrial field Gears used in automobiles, industrial machines, etc., are forged, shaped such as cutting, heat treated such as normalizing,
Manufactured by carburizing heat treatment to give strength and wear resistance to the teeth. The carburizing heat treatment is usually carried out at a temperature of 925 ° C or lower in order to avoid inconveniences such as quench cracking, increase in quench strain, decrease in fatigue strength, etc. due to coarsening of the structure during heating.
Carburizing below requires a very long time and the cost thereof is enormous. Therefore, in order to reduce the cost, there is a demand for carburizing steel (case hardening steel) that does not coarsen its structure even when subjected to carburizing heat treatment at a high temperature for a short time.

The present invention relates to a method for producing case-hardening steel having excellent fatigue properties, which does not cause mixed grains or abnormally coarse grains even when subjected to carburizing heat treatment in a high temperature range of 960 ° C. or higher.

Conventional Technology In order to shorten the time for carburizing heat treatment, it is effective to increase the diffusion rate of carbon into steel, and for that purpose it is necessary to raise the heat treatment temperature. However, on the other hand, increasing the carburizing heat treatment temperature causes coarsening of the structure,
As described above, the result is an increase in quench cracking and strain, and a decrease in fatigue strength.

As a measure against the coarsening of the structure, it has been conventionally proposed to add crystal grain refining elements such as Al, V, Nb, and Ti, and to combine the conditions of rolling and heat treatment (Japanese Patent Laid-Open No. 61-166922). Kaisho 61
However, since coarsening of crystal grains is not sufficiently prevented by these techniques, Al, N, and Ti are added, and the production is performed in a specific production process. Even at a high temperature of 1000 ° C. or higher. Method for manufacturing case-hardening steel that does not coarsen its structure (Japanese Patent Laid-Open No. 63-16281)
2) is proposed.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention In JP-A-63-162812, a case-hardening steel in which crystal grains are not coarsened even at high temperatures is disclosed, and various inconveniences associated with crystal grain coarsening have been avoided, but recently The demand for higher fatigue strength has become stronger, and a case-hardening steel having high fatigue strength is required.

Means for Solving the Problems In order to improve the fatigue strength, the present inventors have studied the case-hardening steel in which coarse precipitates do not occur, and abnormal growth does not occur in crystal grains during high-temperature carburizing heat treatment. Overlaid. As a result, it was revealed that by manufacturing the case-hardening steel by the following method, it is possible to manufacture the case-hardening steel having a fine structure and high fatigue strength even if the carburizing heat treatment is performed at 960 ° C or higher. is there. That is, the present invention is, by weight%, Al: 0.005-0.050%, N: 0.0090% or more 0.0200
%, Ti: 0.003% to less than 0.020% and Ti: 0.010% to less than 0.020% Ti / N: less than 1.0 containing carbon steel / alloy steel for machine structural use After that, when casting by continuous casting, cool it in the temperature range from the freezing point to 1000 ° C under the condition that the cooling rate is 20 ° C / min. Or more, and then immediately roll it into bar steel or wire without performing slabbing. A method for manufacturing case-hardening steel excellent in fatigue characteristics, which does not cause coarsening of crystal grains during carburizing heat treatment.

The present invention is characterized in that carbon nitrides of Al and Ti are finely dispersed by adding necessary minimum amounts of Al and Ti under specific manufacturing conditions.

According to the method of the present invention, finely dispersed Al and Ti carbonitrides effectively prevent coarsening of the structure, and also a large Al or Ti oxide or carbonitride that becomes the starting point of fatigue fracture. The fatigue strength can be further improved by suppressing the generation of

Action The reasons for limiting the scope of the claims of the present invention are as follows.

Al mainly combines with N to become AlN, which prevents coarsening of crystal grains during carburizing heat treatment. Also, Al is necessary as a deoxidizing agent. With less than 0.005% Al, it is not possible to sufficiently prevent the coarsening of the structure during the carburizing heat treatment at 960 ° C or higher. Further, if it exceeds 0.050%, the fatigue strength decreases, so Al is made 0.005 to 0.050%.

N is indispensable for forming AlN and TiN which prevent coarsening of crystal grains. N is required to be 0.0090% or more to form a sufficient amount of AlN and TiN, but the effect is saturated even if 0.0200% or more is added, so the amount of N is 0.0090 to 0.0200%.
Less than

Ti serves as TiN or TiC and prevents coarsening of crystal grains.
The coarsening temperature of crystal grains tends to increase as the amount of Ti increases, and when used at a very high carburizing heat treatment temperature, a large amount of Ti is desirable, but a large amount of Ti becomes the starting point of fatigue fracture. Since it may cause coarse Ti crystal precipitates to reduce fatigue strength and cause intergranular cracking, the upper limit of Ti is less than 0.020%. However, Ti less than 0.003% has no effect.

Ti / N is involved in the generation of coarse TiN. Ti / N must be less than 1.0 in order not to generate coarse TiN that deteriorates fatigue properties. However, Ti: less than 0.01%, N: 0.01
If less than 1.0%, coarse TiN does not occur even if Ti / N is 1.0 or more, so Ti / N is not limited if Ti: less than 0.01% and N: less than 0.01%.

The cooling rate during continuous casting is an important condition for determining the size of Ti and Al precipitates. 1000 from the freezing point
By setting the cooling rate in the temperature range of ° C to 20 ° C / min or more, precipitates are finely divided and grain coarsening during carburizing heat treatment is effectively prevented, and fatigue characteristics are adversely affected. Prevents the generation of coarse precipitates.

Furthermore, in order to keep these precipitates fine during casting until finely carburized and heat treated, and to obtain a sufficient effect of preventing coarsening of crystal grains, after casting, they must be rolled into bar steel or wire immediately without slabbing. I have to.

Steels to which the present invention can be applied are JIS G4051, G4052, G4103, G4104, G4105, and G in the Japanese Industrial Standard (JIS).
The carbon steel / alloy steel for machine structure described in 4106 contains the above-mentioned limited component amounts of Al, N, and Ti, and when it is necessary to improve the machinability, a machinability improving element The addition of S, Pb, Bi, Ca, etc. does not affect the efficacy of the present invention.

Examples Below, the cooling rate during casting, the effect of omitting slabbing,
And the effects of the amounts of Al, Ti and N are shown.

After smelting steel with the components shown in Table 1 in a converter,
A slab having a cross-sectional size of 162 × 162 mm and 350 × 560 mm was cast by a continuous casting machine. The average cooling rate up to 1000 ° C after solidification of slabs with 162 × 162mm and 350 × 560mm cross sections is 45 ° C.
/ min and 9 ° C / min.

A part of the 162 × 162 mm slab was slab-rolled to 120 × 120 mm at a temperature of 1270 ° C., and a slab of 350 × 560 mm cross-section was slab-rolled to 162 × 162 mm at a temperature of 1270 ° C. These four types of steel sheets were heated to 1100 ° C., rolled into a steel bar having a diameter of 30 mm, annealed and drawn to form a steel bar having a diameter of 20 mm, and after normalizing, cut into Ono-type rotary bending fatigue test pieces. These fatigue test pieces were tested at 1000 ° C.
Carburizing was performed and tempering was performed at 180 ° C. The above steps are shown in FIG.

Table 2 shows the crystal grain size of these test pieces, the presence / absence of coarse particles, and the results of the rotating bending fatigue test. It can be seen that the steel of the present invention has a large crystal grain size (fine crystal grains) and does not generate coarse grains. Further, the steel of the present invention shows good fatigue strength.

Effects of the Invention As described above, by using steel with the composition adjusted to a specific range as a steel bar under the above-mentioned conditions, the crystal grains do not coarsen even when carburizing heat treatment is performed in a high temperature range, and the fatigue properties An excellent case-hardening steel can be manufactured.

[Brief description of drawings]

FIG. 1 is a diagram showing a manufacturing process of a test piece in an example.

Claims (1)

[Claims] 1. By weight%, Al: 0.005 to 0.050%, N: 0.0090
% To less than 0.0200%, Ti: 0.003% to less than 0.020%, and Ti: Ti / N in the range of 0.010% to less than 0.020%
When the carbon steel / alloyed steel for machine structure with a melting point of less than 1.0 is melted by a normal method and then cast by continuous casting, a cooling rate in the temperature range of 1000 ° C from the freezing point is 20 ° C / min or more. Without cooling the slab after cooling under the conditions,
A method for producing case-hardening steel, which comprises immediately rolling into a steel bar or a wire rod.