JPH09268323A - Hot rolled austenitic stainless steel plate excellent in recrystallization softening characteristic, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot rolled plate of austenitic stainless steel excellent in recrystallization softening characteristic by specifying hot rolling finishing temp., cooling velocity after hot rolling, and coiling temp., respectively, and also regulating the unrecrystallization ratio of austenite to a specific value or above. SOLUTION: At the time of producing a hot rolled steel plate by hot-rolling a slab, hot rolling is finished at <=950 deg.C and the resultant hot rolled plate is cooled immediately at >=5 deg.C/sec cooling rate and coiled at <=750 deg.C, by which the unrecrystallization ratio of austenite is regulated to >=70%. By this method, hot rolled plate annealing can be performed at a low temp. in a short time and the hot rolled austenitic stainless steel plate, sufficiently softened, can be obtained even if plate passing velocity is increased at the time of annealing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel hot rolled sheet which can be softened by annealing at a low temperature for a short time and a method for producing the same.

[0002]

2. Description of the Related Art Austenitic stainless steel sheets are obtained by continuously annealing and pickling hot-rolled steel sheets produced from slabs by hot rolling in a continuous annealing / pickling line equipped with an annealing furnace and a pickling tank. The final product is manufactured by cold rolling and finish annealing. In hot rolling, usually 98
The hot rolling end temperature is set to 0 ° C. or higher, and after hot rolling, air cooling is performed. Therefore, the unrecrystallized rate becomes extremely small in the process of cooling to a room temperature by cooling at a relatively slow cooling rate. In the hot-rolled sheet annealing step, the hot-rolled sheet is softened by releasing the strain accumulated in the hot-working and coarsening the recrystallized grains of the austenite structure. Solid solution precipitates form a solid solution. The softening ensures cold workability in the subsequent cold rolling process. On the other hand, the solid solution of the precipitate suppresses the roughening of the surface during pickling due to the decrease in corrosion resistance due to the residue of the precipitate at the grain boundaries. Regarding this hot rolled sheet annealing, it has been proposed to omit the process itself in order to improve productivity. For example, in Japanese Examined Patent Publication No. 1-28813, carbide is as hot-rolled by hot-rolling under the conditions of a rolling end temperature of 850 ° C. or more, a cooling rate of 10 ° C./sec or more, and a winding temperature of 500 ° C. or less. A method for obtaining an austenitic stainless steel that has almost no grain boundary precipitation and is not sensitized has been introduced.

[0003]

However, in the conventional method of omitting the hot-rolled sheet annealing, the descaling property in the pickling step is poor and the material anisotropy of the final product becomes large. The present invention has been devised to solve such a problem, and instead of omitting hot-rolled sheet annealing, it promotes softening due to recrystallization during annealing and improves the annealing-pickling process. The object is to obtain an austenitic stainless hot-rolled steel sheet having high anisotropy and high quality stability by improving productivity by increasing the sheet speed.

[0004]

In order to achieve the object, the austenitic stainless steel hot-rolled sheet of the present invention has an unrecrystallized rate of austenite of 70% or more in a state of being cooled to room temperature after hot rolling. Is characterized by. This austenitic stainless steel hot-rolled sheet is manufactured by finishing hot rolling at 950 ° C or lower, immediately cooling the hot-rolled sheet at a cooling rate of 5 ° C / sec or higher, and winding it at a temperature of 750 ° C or lower. It

[0005]

The present inventors investigated and researched the optimum hot rolling conditions that affect the softening during annealing from various points of view. As a result, it was clarified that recrystallization is promoted in the hot rolled sheet annealing step when the austenite unrecrystallized rate after completion of hot rolling is controlled to 70% or more. And it is effective in promoting recrystallization 7
The austenite unrecrystallized rate of 0% or more indicates that hot rolling was terminated at 950 ° C or less and immediately after 5 ° C /
It was found that the hot-rolled sheet was cooled at a cooling rate of not less than 2 seconds and wound at a temperature of 750 ° C. or lower.

The hot rolling conditions will be described below. Hot rolling end temperature: 950 ° C. or less Strain due to hot rolling is introduced into the steel structure, but the introduced strain due to recrystallization that progresses during hot rolling and during cooling thereafter is released. On the other hand, in the annealing heat treatment after hot rolling, the larger the strain accumulated in the hot rolled sheet, the more the recrystallization during annealing is promoted. In the present invention, since it promotes recrystallization during hot-rolled sheet annealing, the hot-rolling step suppresses recrystallization as much as possible to accumulate hot-rolling strain in the austenite structure, and re-crystallization in hot-rolled sheet annealing. In order to accelerate the above, the hot rolling end temperature is set to 950 ° C. or lower. When the hot rolling end temperature exceeds 950 ° C, recrystallization progresses during the hot rolling and in the course of cooling thereafter, and the strain is released, and the recrystallization rate at the time of hot rolled sheet annealing is reduced accordingly, resulting As a result, a sufficient softening effect cannot be obtained.

Cooling rate after hot rolling: 5 ° C./sec or more,
Winding temperature: 750 ° C. or less The strain accumulated by hot rolling is released by recrystallization that occurs during cooling of the hot rolled steel strip and after winding. In order to suppress the release of strain due to this recrystallization, it is necessary to rapidly cool the hot-rolled sheet at a rate of 5 ° C / sec or more after hot rolling and wind it at a temperature of 750 ° C or less. When the hot-rolled sheet in which the hot-rolled strain remains in the austenite structure after hot-rolling is annealed, the residual strain promotes recrystallization, and the recrystallization is completed at a lower temperature in a shorter time. In normal hot-rolled sheet annealing, Cr carbide formed by combining C in austenite with Cr precipitates at grain boundaries, and a Cr-deficient layer that causes a sensitization phenomenon is easily generated near the grain boundaries. In the present invention, the hot rolling is finished at a relatively low temperature, the rapid cooling and the low temperature winding are carried out, so that the grain boundary precipitation of Cr carbide and eventually the sensitization phenomenon are also suppressed. Further, by suppressing the formation of Cr carbide, the carbide is re-dissolved in a short time in the next annealing. Also in this respect, rapid cooling at 5 ° C./sec or more and low temperature winding at 750 ° C. or less are effective.

As described above, the hot rolling end temperature is 950 ° C. or lower,
By hot rolling at a cooling rate of 5 ° C./sec or more and a coiling temperature of 750 ° C. or less, a hot-rolled sheet can be obtained in which the unrecrystallized rate of austenite is 70% or more in a state of being cooled to room temperature. . Then, the residual strain in the unrecrystallized austenite enables recrystallization softening at a low temperature and a short time during annealing of the hot-rolled sheet, thus improving the productivity.

[0009]

EXAMPLES Austenitic stainless steels A to C having the compositions shown in Table 1 were hot-rolled under the conditions shown in Table 2.
The unrecrystallized rate of the obtained hot-rolled sheet cooled to room temperature varied as shown in Table 2 depending on the hot-rolling conditions. In the comparative example, the hot rolling finishing temperature is 980 ° C or higher, the cooling rate until winding is 4 ° C / sec or less, and the winding temperature is 780 to 800.
The conventional method of ℃ is shown.

[0010]

[0011]

Each hot rolled material was placed in a heating furnace having a set furnace temperature of 1150 ° C., and immediately after the material temperature reached various set temperatures, rapid heat treatment was performed. Next, the Vickers hardness is measured, and the temperature at which the Vickers hardness becomes 160 or less is obtained from the relationship between the reached temperature of the material and the measured value of the Vickers hardness.
This critical temperature was determined as the annealing softening temperature. Table 3 shows the annealing softening temperature under each hot rolling condition. Further, the steel strips hot-rolled by each of the method of the present invention and the comparative method were continuously annealed with the annealing softening temperature shown in Table 3 as the target material temperature. Table 4 shows the time required for annealing at this time.

[0013]

[0014]

As shown in Table 3, the hot rolled sheet obtained according to the present invention has an annealing softening time of 1000 ° C. or less, which is lower than that of the comparative method. Further, as seen in Table 4, the time required for annealing of the hot-rolled sheet obtained according to the present invention is significantly shorter than that of the hot-rolled sheet produced by the conventional method. The effect of each hot rolling condition on the annealing softening temperature is shown as the relationship between the rolling end temperature and the annealing softening temperature in FIG.
2 shows the relationship between the cooling rate after hot rolling and the annealing softening temperature, and FIG. 3 shows the relationship between the winding temperature and the annealing softening temperature. From FIGS. 1 to 3, in order to set the annealing softening temperature to 1000 ° C. or lower, the rolling end temperature is 950 ° C. or lower and the cooling rate is 5
It can be seen that a winding temperature of 750 ° C / sec or more and a winding temperature of 750 ° C or less are required.

[0016]

As described above, the hot-rolled austenitic stainless steel sheet of the present invention retains the austenite unrecrystallized rate of 70% or more after being hot-rolled and cooled to room temperature. Therefore, a large amount of strain introduced by hot rolling remains, which facilitates recrystallization during hot rolled sheet annealing. Therefore, the hot-rolled sheet can be annealed at a low temperature in a short time, and a sufficiently softened steel strip can be obtained even if the sheet passing speed is increased during the annealing. Therefore, productivity is improved,
An austenitic stainless steel hot-rolled sheet excellent in quality stability without the anisotropy of the material observed when the conventional hot-rolled sheet annealing is omitted is manufactured.

[Brief description of drawings]

Fig. 1 Effect of hot rolling finish temperature on softening temperature during hot rolling

[Fig. 2] Effect of cooling rate until coiling after hot rolling on softening temperature during annealing of hot rolled sheet

Fig. 3 Effect of coiling temperature on softening temperature during hot-rolled sheet annealing

Claims (2)

[Claims] 1. A hot-rolled austenitic stainless steel sheet having an excellent recrystallization softening property in which the unrecrystallized rate of austenite is 70% or more in a state of being cooled to room temperature after hot rolling. 2. The hot rolling is finished at 950 ° C. or lower, and the hot rolled sheet is immediately cooled at a cooling rate of 5 ° C./sec or higher to 750 ° C.
A method for producing an austenitic stainless steel hot-rolled sheet excellent in recrystallization softening property in which the unrecrystallized rate of austenite is 70% or more by winding at the following temperature.