JPS5935410B2 - Manufacturing method for steel materials with good cold workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a steel material containing chromium or chromium and molybdenum and having good cold workability.

Cold processing of steel materials, especially cold forging of steel bars and wire rods, is a processing technology that has developed rapidly in recent years. Accidents such as defects due to cracks and damage to tool dies are likely to occur.

For this reason, when cold forging is used to cold forge medium-carbon steel or low-alloy steel that is hard and has poor formability, annealing (hereinafter referred to as spheroidization annealing) is required to spheroidize the carbides in the steel to improve cold workability. It is necessary to do this. However, even if conventional hot-rolled steel materials with a pearlite structure, especially low-alloy hot-rolled steel materials containing chromium, which is a carbide-forming element, are directly charged into an annealing furnace and subjected to spheroidizing annealing, the carbides in the steel are difficult to spheroidize. , flaky carbides remain, and the carbides are microscopically unevenly distributed and difficult to disperse uniformly, making it difficult to sufficiently improve the desired cold workability. In addition, since the spheroidizing annealing process takes a long time and the productivity of the annealing furnace deteriorates, and decarburization progresses on the surface of the steel material, surface scale must be removed in advance.

Also, special attention must be paid to the atmospheric gas in the annealing furnace. In order to improve these points, conventionally, when spheroidizing carbides, hot-rolled steel is subjected to drawing processing, or hot-rolled steel is made into a quenched martensitic structure and then subjected to spheroidizing annealing. It has been shown that the annealing time has been shortened.

However, both require extra drawing and heat treatment steps prior to spheroidizing annealing, which increases the cost. In addition, in the case of medium-high carbon steel, which requires upper spheroidizing annealing, which is extremely difficult to harden wire rod coils that have become uniform in weight in recent years, special measures must be taken to prevent quench cracking.

Further, in the case of the above method, although the carbides are uniformly dispersed, the carbides become too fine, resulting in high material hardness, which has disadvantages in terms of material quality, such as significantly shortening tool life.

In order to eliminate these drawbacks, the present invention aims to eliminate carbon 0.20~
0.50% acid soluble AAO. OO5~0. Steel materials containing 00% i, 0.50 to 1.50% chromium, and 0.50% or less of molybdenum, with the remainder consisting of iron and impurities, were passed through a hot finishing mill. The carbides in the steel are removed by immediately cooling the steel to a temperature range of 550°C to Ms point and annealing it directly at a temperature range of 650°C to A1 transformation point without cold working or heat treatment. It uniformly spheroidizes the grains to an appropriate size in a short period of time, greatly improving the cold workability of steel materials.

The method of the present invention will be explained in detail below.

Conventionally, after passing through a finishing mill and before being wound into a coil, rapid cooling has been carried out using cooling water, blast air, hot water, etc.; The temperature range of 700°C to 850°C is aimed at preventing coarsening of crystal grains in hot-rolled steel materials and improving surface properties, and is used at most to create a fine pearlite structure.

Even if a hot-rolled steel material having a pearlite structure obtained in this way is directly subjected to a spheroidizing annealing treatment, the carbides are difficult to spheroidize, resulting in various drawbacks as described above.

It is also known that if the steel material is quickly cooled to a temperature below the Ms point after passing through a hot finishing mill, it will become a quenched martensite structure, carbides will easily become spheroidized, and the dispersibility will be good. However, since the carbides are fine, the material hardness increases and the life of the cold forging tool is significantly shortened, making it unsuitable as an industrial method.

In addition, it is very difficult to wind up a quench-hardened wire, and there is a risk of quench cracking in medium-high carbon steels that require spheroidizing annealing, which poses many problems for practical use. As a result of research, the present inventors found that the M
Hot-rolled steel material that has been rapidly cooled to a temperature range above the s point to form an intermediate stage structure is subjected to spheroidizing annealing at a temperature range of 650°C to A1 transformation point to uniformly form spheroidal carbides with an appropriate grain size. It has been found that the annealing time can be significantly shortened. However, in carbon steel for mechanical structures (SC material), the pearlite formation rate is very fast, and it is quite difficult to obtain an intermediate stage structure during the hot rolling process in a wire rod with a large diameter.

In order to eliminate the above-mentioned drawbacks, the present invention provides 0.50% chromium.
Using a steel material containing ~1.50% or 0.50 to 1.50% chromium and 0.5% or less molybdenum, immediately cooled to a temperature of 550 C-Ms point after hot finish rolling,
This made it possible to obtain an intermediate stage structure.

Chromium is an element that increases the strength of steel, and containing too much is not favorable for cold workability, but by following the present invention, it is possible to obtain very good cold workability as shown later. It is.

The present invention is to directly charge a hot-rolled steel material having the above-mentioned components and intermediate structure into an annealing furnace without drawing or heat treatment, and uniformly spheroidize the carbides in the steel in a short time. be.

FIG. 1 shows the relationship between the spheroidizing annealing time and the spheroidization rate when a conventional hot rolled steel material is used and when the present invention is used. Table 1 shows the components of the test materials, the rapid cooling temperature after hot rolling, and the annealing temperature. According to the present invention, 95% or more of the carbide has already been spheroidized by short-time spheroidizing annealing of 1 hour, which is sufficient as a material for cold working.

Although the characteristics of the material of the present invention are not impaired even if the spheroidizing annealing is carried out for a long time, from the viewpoint of production efficiency and prevention of decarburization on the surface of the steel material, the suitable time for the spheroidizing annealing of the present invention is 1 to 5 hours.

This provides a sufficient spheroidal structure. If the spheroidizing annealing temperature exceeds the A1 transformation point temperature, the carbide will not become spheroidized unless it is slowly cooled after annealing or annealed twice at a temperature below the A1 transformation point, and the spheroidizing annealing process is performed. Production efficiency is poor because it takes a long time to process, decarburization progresses on the surface of the steel, and carbides in the steel become coarse, reducing cold workability. Further, at temperatures below 650°C, the diffusion rate of carbon in the steel becomes slow, so that the spheroidizing annealing process takes a long time.
Therefore, the range of the spheroidizing annealing temperature of the present invention is 650℃~
It was set as the A1 transformation point. According to the present invention, by short-time spheroidizing annealing, the carbides in the steel have an appropriate size that is neither too coarse nor too fine, and are uniformly distributed, resulting in excellent cold workability.

FIG. 2 shows the spheroidized annealed material according to the present invention shown in Table 1,
This figure shows the cold workability of a spheroidized annealed material using a conventional hot rolled steel material.

It is clear that the material of the present invention does not crack when cold worked at a compression rate of 84% or less, and has extremely superior cold workability compared to conventional materials.

Next, the reason for limiting the steel material components in the present invention will be described. In the first invention, if the carbon content is less than 0.20%, cold workability is originally good and spheroidizing annealing is often necessary;
．． If it exceeds 50%, the hardness is too high and it cannot be used as cold forging steel.

Acid-soluble Al was contained in an amount of 0.005 to 0.100% in order to make crystal grains fine and improve cold workability. This range is an effective range for achieving the above objective. The lower limit of chromium was set at 0.50% in order to easily obtain an intermediate stage structure during forced cooling after hot rolling, and the upper limit was set at 1.50% from the viewpoint of increasing strength.

Next, in the second invention, carbon, acid-soluble Al, and chromium are used for the same reason as in the first invention.

Molybdenum of 0.5% or less is set in an effective range not only for the purpose of obtaining an intermediate stage structure similar to that of chromium, but also for preventing a decrease in strength due to tempering and requiring high strength. Next, examples of the present invention will be described.

After hot-rolling a steel material containing the components shown in Table 2 into a wire rod with a wire diameter of 14 mmφ, the 80 is rolled up in hot water of 100°C to 100°C, rapidly cooled to the temperature shown in Table 3, and then pulled out of the hot water and released. It got cold.

The coil unit weight is 1 ton. The above hot rolled wire rods were directly charged into an annealing furnace without cold drawing or heat treatment, and spheroidizing annealing was performed in an N2 gas atmosphere under the conditions shown in Table 3.

Table 4 shows the mechanical properties and cold workability after spheroidizing annealing. It is clear that the material of the present invention has a much higher compressibility limit at which cracks occur than the conventional material described above, and has very good cold workability. In addition, surface decarburization of the material of the present invention hardly progressed. As explained above, in the present invention, a hot rolled steel material having an intermediate stage structure within the above composition range is directly charged into an annealing furnace and subjected to spheroidizing annealing at a temperature of 650°C or higher and lower than the A1 transformation point. As mentioned above, it has various advantages and is extremely useful industrially.

Advantages 1. Cold drawing and heat treatment steps before spheroidizing annealing can be omitted.

Advantages 2. Carbides in steel become spheroidized in a short time, resulting in good production efficiency.

Advantages 3. The spheroidal carbide has an appropriate size and is uniformly distributed in the steel, so it has excellent cold workability. 0 Advantages: 4. Since the steel material becomes spheroidized in a short time at low temperatures, decarburization on the surface of the steel material hardly progresses.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between spheroidization annealing time and spheroidization rate;
FIG. 2 is a diagram showing the relationship between spheroidizing annealing time and cold workability.

Claims (1)

[Claims] 1. Carbon 0.20-0.50%, chromium 0.50-1.
A steel material containing 50% and 0.005 to 0.100% of acid-soluble Al was rapidly cooled to a temperature range of 550°C to Ms point after hot finish rolling to form an intermediate stage structure.
A method for producing a steel material with good cold workability, characterized in that carbides in the steel are uniformly spheroidized by annealing at a temperature range of 0° C. to A_1 transformation point. 2 Carbon 0.20-0.50%, chromium 0.50-1.
After hot finish rolling a steel material containing 50% acid-soluble Al, 0.005 to 0.100% acid-soluble Al, and 0.5% or less molybdenum,
It is characterized by uniformly spheroidizing the carbides in the steel by using a material that has been rapidly cooled to a temperature range of 50°C to Ms point to form an intermediate stage structure and annealing it at a temperature range of 650°C to A_1 transformation point. A method for manufacturing steel materials with good cold workability.