JPH0196331A - Method for softening self-hardening steel material - Google Patents

Abstract

PURPOSE:To soften a self-hardening steel material to such a degree that the material can be cold worked as rolled without carrying out softening heat treatment by reheating by hot rolling the material at a specified total draft, finishing the hot rolling at below the recrystallization temp. of the material and cooling the rolled material at a specified cooling rate. CONSTITUTION:A self-hardening steel material is rough-rolled and hot rolled at >=10% total draft and the hot rolling is finished at below the recrystallization temp. of the material. The rolled material is slowly cooled at 0.05-0.7 deg.C/sec cooling rate. A completely softened material having about <200 Vickers hardness Hv is obtd. by slow cooling after rolling without carrying out special annealing.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for softening self-hardening steel materials, particularly for reheating highly self-hardening steel materials such as martensitic stainless steel and high carbon alloy steel after rolling. This method relates to a method for softening self-hardening steel that can be cold-worked as rolled without undergoing softening heat treatment, and is used in a wide range of fields that require heat-resistant materials or self-hardening steel such as tools.

[Conventional technology]

So-called self-hardening steels, such as martensitic stainless steels, which harden at a cooling rate from high temperature to air cooling, have high hardness and lack toughness when air cooled, so cold working is not possible. The quality is extremely poor. For this purpose, various annealing treatments have been conventionally performed on such steel materials in order to soften and strengthen the steel materials.

The specific methods of these conventional annealing treatments are as follows.

(a) After heating for 1 to 2 hours just above the A1 transformation point, annealing is performed to maintain the temperature just below the A1 transformation point for a long time.

(c) As in the combination of (a) and (b), after heating to a grade above the A8 transformation point, isothermal transformation annealing is performed just below the A1 transformation point.

However, in all of the above heat treatment methods, in addition to reheating the cold material, it is necessary to hold it for a long time, so high costs are unavoidable. There are many cases where heat cracking due to thermal stress is caused, which poses a quality problem.

[Problem that the invention seeks to solve]

The purpose of the present invention is to solve the problems of the above-mentioned conventional techniques for self-hardening steel materials, omit the softening heat treatment step before cold working, and perform controlled rolling during rolling, so that even if the steel material is still rolled, it can be cooled after that. An object of the present invention is to provide an effective method for softening self-hardening steel materials that can be machined.

[Means and operations for solving the problems] The gist of the present invention is as follows.

That is, in a hot rolling method for self-hardening steel, the hot finish rolling temperature is set to be below the recrystallization temperature of the material, the total reduction amount during hot rolling is set to 10% or more in terms of reduction ratio, and cooling after rolling is performed. 07℃/sec or less, 005℃/sec
This is a method for softening self-hardening steel, which is characterized by slow cooling at the above cooling rate and omitting softening heat treatment by reheating.

In the present invention, when softening a self-hardening material, the softening process is carried out by utilizing various properties imparted during hot rolling, and the main means thereof are as follows.

(a) The material temperature imparted by high-temperature heating is utilized for subsequent slow cooling treatment.

(b) Through the plastic strain imparted by hot rolling, atomic mobility is increased and the speed of structural change during the softening process is accelerated.

(c) The finish rolling temperature is set below the recrystallization temperature of the steel material so that the structure with plastic strain does not recover or recover immediately.

The reason why the finish rolling temperature is set below the recrystallization temperature of the steel material in the present invention is to allow plastic strain introduced during processing to remain. When the finish rolling temperature exceeds the recrystallization temperature,
The deformed structure due to processing recovers, recovers, or grows into new crystal grains. This is because such a recovered structure has drastically reduced lattice defects inside the crystal, and is no different from conventional reheated and annealed materials.

On the other hand, if the deformed structure remains and there are a large number of lattice defects inside the crystal, the movement of constituent atoms becomes active and softening progresses at a much faster rate than the softening behavior of conventional annealed materials. The present invention utilizes this phenomenon.

Next, regarding the conditions for the above-mentioned hot plastic strain to remain, the present inventor has determined from many experimental results that the total reduction amount during hot rolling should be 10% or more in terms of reduction ratio. I found it. This is because if the total rolling reduction is less than 10%, the lattice defects in the crystal disappear early during slow cooling after hot rolling, and the desired effect cannot be obtained. Therefore, the number of lattice defects that effectively affect softening can be ensured only when the total rolling reduction is 10% or more.

In addition to the above-mentioned softening measures, in the present invention, in order to ensure softening, when cooling after hot rolling, 07°C/s
After e, a step of slow cooling is performed at a cooling rate of 0.05° C./sec or more.

Conventional reheated and annealed materials were usually slowly cooled at 15 to 30°C (i.e., 0004 to 0004°C), but in the case where plastic strain remains in the previous process according to the present invention, the temperature is reduced to 0.7°C/ sec or 2500
If the cooling rate is below 0.7° C./sec, the surface can be sufficiently softened, but if the cooling rate is faster than 0.7° C./sec, the degree of surface softening is small and it is difficult to obtain the desired effect. In addition, if the cooling rate is slower than 07°C/sec, there is no particular disadvantage in the case of the present invention, but if a cooling rate of 005°C/sec can be ensured, sufficient softening is guaranteed even with lightly rolled material, so it is practically the lower limit. 0.05℃/
sec, and the cooling rate was limited to 0.05 to 07°C/sec.

〔Example〕

Examples of the present invention and comparative examples that do not meet the limited conditions of the present invention, in which a bloom of 02% C1gCr material, which is JIS 5US420fl martensitic stainless steel, is roughly rolled and then finished into a round billet with a diameter of approximately 200 mm. Table 1 shows the results of measuring the Vickers hardness Hv of each sample material by cooling at various rolling temperatures, total rolling reduction rates, and post-rolling cooling rates.

The round billet rolling of each sample material was carried out using a four-stand hole-shaped caliber rolling mill, and the rolling temperature was 900-800℃, which is below the recrystallization temperature of the sample material, and 1100-100℃, which is above the recrystallization temperature.
Based on the two levels of 00°C, the total reduction amount during finishing with 4 stands was set to 10%, 30% and 50% in terms of outer diameter reduction rate, and after the rolling was completed, it was placed in a heat insulating device installed on the cooling bed on the exit side of the mill. and set the cooling rate to 1.0 to 0.008℃/sec.
It was cooled in 7 steps until

Vickers hardness H of round billet initial material after rough rolling
v = 550, and test materials No. 1, No. 1 and No.
5 at the same time.

Table 1 The evaluation criteria for softening in Table 1 are as follows.

○: Completely softened (Hv<200) Δ: Partially softened (200≦Hv<300) × Partially softened (300≦Hv<400) #: No softening (4
00≦Hv) As is clear from the comparative test results in Table 1 (,200m++
Sample materials No. 1 and Nl15, which were not processed at all after rough rolling into +φ round billets, were completely softened by slow cooling at 0008°C/sec, regardless of the rolling temperature; This corresponds to annealing treatment and is not practical.

Regarding the rolling temperature, test material No. below the recrystallization temperature,
As is clear from the comparison between Nos. 2 to 4 and Nos. 6 to 8, which are above the recrystallization temperature, the test materials Nos. 2 to 4 according to the present invention are significantly superior, and the total reduction rate is at least 10.
% or more, complete softening is possible in combination with the cooling rate. However, as the total reduction rate increases, the cooling rate range in which complete softening can be achieved becomes wider; for example, for sample material No.
, 4, by setting the total reduction rate to 50%,
Complete softening can be achieved over a very wide cooling rate range from 07°C/sec to 0.05°C/see.

〔Effect of the invention〕

So-called self-hardening steel materials, such as full-tensile 1~ series stainless steels, have high hardness even when air-cooled (due to poor toughness, cold workability is extremely poor if left as is, so conventionally special annealing treatment is required. However, in the present invention, the rolling temperature is lower than the recrystallization temperature, the total reduction rate during hot rolling is 10% or more, and the cooling rate after rolling is 0.7°C/sec or lower. By slow cooling to 0.15° C./sec or more, the following effects could be achieved.

(a) A completely softened material with a Vickers hardness of Hv<200 could be obtained without special annealing and as it was slowly cooled after rolling. This not only makes it possible to omit the conventional reheating annealing of the material, which not only greatly reduces the cost of the material, but also avoids quality deterioration such as heating cracks and placement cracks during reheating. , HV < 200, it is now possible to provide a material with high toughness, uniform quality, and good cold workability.

(b) The present invention is configured to effectively utilize the retained heat and plastic strain of the material after hot rolling, so it achieves the goal of energy saving and does not require reheating annealing. We were able to significantly reduce material costs.

Claims (1)

[Claims] (1) In the hot rolling method for self-hardening steel, the hot finish rolling temperature is set to be below the recrystallization temperature of the material, and the total reduction amount during hot rolling is set to 10% or more in terms of reduction ratio, and after rolling When cooling, 0.7℃/sec or less, 0.05℃/sec
A method for softening self-hardening steel, characterized by slow cooling at the above cooling rate and omitting softening heat treatment by reheating.