JPS6157373B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a steel material that does not generate coarse austenite crystal grains even when a controlled rolled steel material such as case hardened steel is heat treated in the austenitic region, particularly in the high temperature region, for carburizing treatment. In low-alloy steels containing aluminum, niobium, vanadium, etc., in which the growth curve of austenite crystal grains shows a sudden growth type, when carburizing heat treatment is performed in the austenite region, especially in the high temperature region, coarse austenite grains are generated and the strength increases. Not only does the toughness deteriorate, but also the variation in heat treatment strain increases, so it is necessary to suppress this occurrence as much as possible. For this reason, it was necessary to avoid holding the carburizing heat treatment temperature at a high temperature in the austenite region (approximately 950° C. or higher) for a long time. It has been said that the temperature at which austenite grains coarsen in cold-worked materials decreases. However, as a result of detailed research based on experimental facts, the inventors of the present invention found that when cold plastic working is performed from 20% to 80%, the austenite grain size number according to the initial JIS is refined to about #8. The temperature at which austenite grains begin to grow to a size of about #7.0 to 7.5 decreases, but that size is maintained until a fairly high temperature is reached, at which point the austenite grains rapidly coarsen to larger grains of about #4 or less. This invention was made based on the discovery that the temperature shifts to a higher temperature side than that of a material that is not subjected to cold working. In other words, in the present invention, compared to normalized materials, carbonitride precipitation is insufficient in as-rolled materials, and coarse austenite crystal grains are often generated during heat treatment such as carburizing, which causes strain during machining of mechanical parts. Dispersion may cause trouble. In the present invention, the material is subjected to predetermined cold working before carburizing heat treatment as it is after controlled rolling (the normalizing process is omitted), and the fine carbonitrides are made to easily precipitate to form austenite crystals during carburizing heat treatment. The purpose is to control grain coarsening. When heated to the austenite region after cold working,
The initial austenite grains become extremely fine because the precipitation position of austenite grains increases due to the increase in strain energy due to processing. Since these austenite grains are considered to have excessive surface energy, austenite crystal grains begin to precipitate at a relatively low temperature. however,
Carbonitrides, which can prevent coarsening of austenite crystal grains, have a greater tendency to disperse and precipitate when cold worked, and therefore the temperature at which austenite crystals become larger grains is lower than that of cold worked materials. It is interpreted that the temperature shifts to the higher temperature side compared to the one that is not subjected to cold working. The deterioration of the mechanical properties and heat treatment strain of steel materials is caused by the generation of significantly coarsened austenite grains. It becomes a problem. The gist of this invention is to prevent the formation of coarse austenite crystal grains even during subsequent carburizing heat treatment at an austenite region temperature by controlling the degree of cold plastic working of controlled rolled steel to 20% or more and 80% or less. This was done regarding the manufacturing method of steel materials. Next, to describe the scope of application of the present invention, the steel material is a steel type in which crystal grains exhibit a rapid growth curve at an austenite region temperature. Among them are
This is a low alloy steel containing one or more rare earth elements such as aluminum, molybdenum, chromium, vanadium, niobium, and other rare earth elements, and is a steel type shown in Table 1.

[Table] The conditions for hot rolling were determined for the following reasons. The heating furnace soaking temperature in hot rolling is lower than 950℃, which makes actual rolling operation difficult, and if it exceeds 1200℃, the austenite grains become coarse and a fine uniform structure cannot be obtained after rolling. It is desirable to heat it uniformly. In addition, if the finishing temperature of hot rolling is less than 800℃, axial ferrite increases and cold workability deteriorates, and if it exceeds 950℃, the ferrite grains become coarse and a fine uniform structure cannot be obtained. A temperature range is desirable. Also, the cooling rate after rolling is
If it is less than 0.3℃/sec, a fine uniform structure cannot be obtained;
If it exceeds °C/sec, the betonite structure will increase and cold workability will deteriorate, so it is set at 0.3 to 3.0 °C/sec. The rolling reduction ratio in hot rolling is desirably 15 or more because if it is less than 15, lattice defects and strain energy that occur during hot working will be less accumulated and fine austenite grains before transformation will not be obtained. Although the upper limit depends on equipment constraints, it cannot be particularly limited because it differs depending on the dimensions of the rolled material and the dimensions of the product. for example,
Rolled material size is 180mm□ and finished product size is 38mmΦ
The rolling ratio is 28.6% when the rolled material size is the same 180mm□ and the finished product size is 28mmΦ, and the rolling ratio is 52.6% when the rolled material size is the same 180mm.
When the finished product size is 5.5mmΦ with □, the rolling ratio is 657.7
This is because the rolling reduction ratio varies greatly depending on the rolled material size, rolled material size, and finished product size. The cold plastic working mentioned in this invention is carried out near room temperature, and if the working degree is less than 20%, the effect of suppressing the generation of coarse crystals in austenite grains at high temperatures will be significantly reduced.
% or more. In addition, the degree of processing was set to 80% or less because this is the processing limit of the material, and if it exceeds 80%, processing becomes difficult. Also,
The plastic working method includes means such as forward drawing, forward extrusion, backward extrusion, and swaging. The present invention will be explained below with reference to Examples. In this example, the steel material is made into ingots after melting in a 70-ton converter, and then bloomed into billets of 180 mm square.
Using controlled rolling to 28mmΦ and 38mmΦ,
Cold plastic working was performed on a 20mmΦ300mmH cylinder by cold swaging within a range of 70% or less. The conditions for cold plastic working and carburizing heat treatment were as follows. Carburizing heat treatment holding time 6 hours, cold working temperature 20-25℃ Degree of plastic working Ho-H/Ho×100 (%) Ho: Original height of steel material, H: Height of steel material after swaging, swaging processing Dimensions: 20mmΦ×30mmH Example 1 SCr22 38mmΦ controlled rolled steel material was subjected to cold plastic working and heat treatment under the above conditions, and the results shown in the following table were obtained.

[Table] Example 2 SCM21 28mmΦ controlled rolled steel material was subjected to cold plastic working and heat treatment under the above conditions, and the results shown in the following table were obtained.

[Table] The following table also shows the relationship between the austenite grain size number and the average crystal cross-sectional area mm ² for reference.

[Table] As is clear from the above two examples, by applying cold plastic working to a degree of plastic working of 20% or more, the temperature at which coarsened austenite grains are generated is lower than that for cases with a degree of plastic working of 0% and 10%. Compared to the above, no significant coarsening of austenite grains was observed even at 1050°C. On the other hand, in the case of 0% and 10%, significant coarsening is observed at 1050°C. As described above, the present invention allows steel materials to be subjected to cold plastic working of 20% or more and heat treated without causing coarsening of austenite crystal grains even at temperatures in the austenite region of 950° C. or higher. For this reason, this method is suitable for producing case hardened steel.
In the process of cold processing → cutting → carburizing and quenching, 950
Heat treatment can be performed even in the austenite high temperature range of ℃ or higher. For example, if you want to carburize to a depth of 1.3 mm, the conventional process that would take about 4 to 5 hours at 920°C can be shortened to about 2.5 hours by maintaining the temperature at 970°C. Furthermore, even when heat treatment is performed at a temperature below 950°C, heat treatment strain due to coarsened austenite crystal grains is also improved, so this manufacturing method has a wide range of applications. As examples of cold-hardened products, bolts and collar gears that have a rear head with a shaft reduction of 20% or more are included in the scope of application.

Claims (1)

[Claims] 1. A steel material in which the grain size growth curve of austenite crystal grains exhibits a sudden growth type using the pinning effect of fine precipitates of carbonitrides in low alloy steel is
It is soaked at ℃ and hot rolled with a rolling reduction ratio of 15 or more.
Finish rolling in the range of 800-950℃, 0.3-3.0
Cooled at a cooling rate of °C/sec to produce controlled rolled steel,
A manufacturing method for suppressing coarsening of austenite crystal grains for a controlled rolled steel material, characterized by subjecting the steel material to cold plastic working of 20% or more and 80% or less, and then carburizing heat treatment at an austenite region temperature. .