JPH01139718A - Manufacture of hot rolling high cr-high al stainless steel strip - Google Patents

Abstract

PURPOSE:To prevent cracking and to stably manufacture a hot rolled high Cr-high Al stainless steel strip by treating a hot rolled steel strip having a specified compsn. consisting of Cr, Al, Ti, Nb, Zr, a rare earth metal and Fe under specified conditions. CONSTITUTION:A hot rolled high Cr-high Al stainless steel strip consisting of 17-30wt.% Cr, 2-6wt.% Al, a prescribed amt. of one or more among Ti, Nb, Zr and a rare earth metal and the balance Fe with inevitable impurities is coiled at >=900 deg.C. The coiled strip is held at >=900 deg.C for 5-10min and immediately cooled to <=450 deg.C at >=10 deg.C/sec cooling rate. A hot rolled high Cr-high Al stainless steel strip having improved toughness and causing no cracking during uncoiling at ordinary temp., cold rolling and other working is obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing a high Cr to high Al stainless steel W4 hot rolled steel strip, and in particular to a method for producing a hot rolled steel strip with excellent toughness, and corrosion resistance. , heat resistance and oxidation resistance are required.

It is used in the field of manufacturing high Cr-high Al stainless steel.

[Conventional technology]

In recent years, advances in steel refining technology have made it easier to reduce C, N, and other impurities, so Cr: 17-30%, A
High Cr-high Aj stainless steel containing l: 2 to 6%, etc. is used not only for conventional electric and hot wire applications, but also for applications that take advantage of its corrosion resistance, heat resistance, and oxidation resistance, such as automobile exhaust gas parts. It has come to be used for heater covers of ovens and microwave ovens, stove parts, etc. However, Konera's high C/high A/ series stainless steels have conventionally been inferior in toughness, and have had the problem of cracking during rolling and cold rolling of the hot rolled steel strip after hot rolling.These problems In order to solve this problem, a technique has been proposed in JP-A No. 60-228616 in which the hot-rolled steel strip is cooled at 1-b and then coiled at 800-700°C. The impact fracture surface transition temperature (hereinafter referred to as transition temperature) of high Cr-high Aj stainless steel in the composition range targeted by the present invention is 100 ° C or higher,
If it is developed at room temperature, it will crack. In order to prevent this cracking, it is necessary to maintain the coil at a temperature of 100°C or higher during coil development and processing, and the equipment components must withstand temperatures of 100°C or higher, and a heating device for heating to 100°C or higher is also required. Compared to conventional machining, this method has the drawbacks of lower work efficiency and higher manufacturing costs.

[Problem that the invention seeks to solve]

The purpose of the present invention is to solve the above-mentioned problems of the prior art and to
Method for manufacturing r-high Cr-high Al stainless steel hot-rolled steel strip that can be stably manufactured by preventing cracking during development, cold rolling, and various processing of the r-high Aj stainless steel hot-rolled steel strip is to provide.

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

That is, it contains as basic components Cr: 17-30% A1: 2-6% in weight ratio, further contains one or more selected from Ti, Nb, Zr, and rare earth metals, and the balance is Fe. and unavoidable impurities.
A step of winding a hot-rolled steel strip of l-series stainless steel at a temperature of 900°C or higher, and a step of 5-1 at a temperature of 900°C or higher after said winding.
holding for 0 minutes, and immediately after holding at 10°C for 7se.
A method for producing a high Cr-high Al stainless steel hot-rolled steel strip, comprising the step of cooling to a temperature of 450°C or less at a cooling rate of c or more.

As a result of repeated trial and error in order to improve the toughness of high C-high Al stainless steel, the present inventors obtained the following knowledge and were able to complete the present invention based on this knowledge.

In other words, high C''-high Al stainless steel is usually annealed for a short time in a continuous annealing furnace. This is because the crystal grains in the portions that are kept at high temperatures for a long period of time become significantly coarsened, resulting in a decrease in toughness, and when the temperature is lowered, embrittlement occurs at 475° C., further decreasing the toughness.

However, although short-time annealing in a conventional continuous furnace slightly improves the decrease in toughness, it cannot sufficiently prevent cracking during coil deployment. This is due to the precipitation of carbonitrides and very brittle intermetallic compounds to the grain boundaries during cooling, and the precipitation of 475
The cause is thought to be ℃ embrittlement. The present invention has been able to eliminate these causes and improve toughness by promoting recrystallization and rapid cooling of the hot rolled steel strip.

The reason why the amounts of C and Al, which are the basic components, are limited in the present invention is as follows.

C": Cr has the effect of improving corrosion resistance and high temperature oxidation resistance, but if it is less than 17%, the effect is insufficient;
%, the effect becomes saturated and becomes uneconomical, and at the same time the toughness deteriorates. Therefore, the content of Cr is limited to a range of 17 to 30%.

Al: Al has the effect of improving high-temperature oxidation resistance, but if it is less than 2%, the effect is insufficient, and if it exceeds 6%, workability deteriorates, so the Al content should be 2 to 2%. It was limited to a range of 6%.

In addition, T1, Nb, Zr, rare earth metals, etc. have the effect of preventing grain boundary precipitation of Cr carbonitride and improving corrosion resistance and #4 thermal properties, but the present invention does not particularly limit their content.

Next, the reasons for limiting the manufacturing conditions will be explained. First, in the present invention, a hot-rolled steel strip is wound up at a high temperature and self-annealed while being maintained at a high temperature. Basic experiments were conducted in which the holding temperature and holding time during self-annealing were varied, and the transition temperature after annealing was investigated. The results are shown in Fig. 1. =. Figure 1 shows the transition temperature ν with annealing temperature and holding time as axes, respectively.
Two regions were shown where Ts was 40°C or higher and below -20°C. In FIG. 1, if the holding time is less than 900° C., precipitation of carbonitrides at grain boundaries is accelerated, and even if recrystallization is performed, there is no effect of improving toughness. In addition, if the holding time is less than 5 minutes, recrystallization will be insufficient and the toughness will not be improved, while if the holding time exceeds 10 minutes, carbonitrides will gradually precipitate at the grain boundaries and the toughness will decrease. In the present invention, the hot-rolled Y4 strip is wound at a temperature of 900°C or higher, and the
The hold time was limited to ~io minutes.

Next is cooling after holding at high temperature, at 450℃ and 500℃.
Cooling was carried out by varying the cooling rate up to two cooling control end temperatures of .degree. C., and the transition temperature vTs after cooling was investigated, and the results are shown in FIG.

From Figure 2, to obtain a transition temperature of -20℃ or less, 10℃/
It is necessary to cool down to 450°C or less at a cooling rate of seconds or more. The reason for this is that if the cooling rate is slow and the cooling control end temperature is high, carbonitrides will precipitate at the grain boundaries, and the toughness will further deteriorate due to 475°C embrittlement. Therefore, in the present invention, after the high temperature is maintained, the cooling rate is immediately
Cooling was limited to temperatures below °C.

The present invention is capable of promoting recrystallization through self-annealing, lowering the transition temperature, and improving toughness by winding, holding, and cooling a high-Cr, high-Al stainless steel hot-rolled steel strip with limited components under limited conditions. can.

〔Example〕

A hot-rolled steel strip of 411II and 11 thickness having the composition shown in Table 1 was wound up, held, and cooled under the conditions shown in Table 2. The hot-rolled steel strip thus obtained was developed at room temperature and examined for the presence or absence of cracks, and the results are shown in Table 2.

In Table 2, none of the examples of the present invention that satisfy the conditions of the present invention show excellent toughness without cracking during development, but all of the comparative examples that meet the conditions of the present invention do not develop. Occasionally cracks occur.

〔Effect of the invention〕

As is clear from the above examples, the present invention involves coiling, holding and cooling a high Cr-high AN stainless steel hot-rolled steel strip with a limited component under limited conditions, and thereby reducing carbonitride grain boundaries that have an adverse effect on toughness. By suppressing precipitation of and promoting recrystallization after hot rolling while preventing 475°C embrittlement, the following effects were achieved by lowering the transition temperature and improving toughness.

(a) Conventional problems such as cracking and coil breakage during coil development and cold rolling at room temperature are prevented, and productivity and yield are improved.

(b) Compared to the conventional process of heating a hot-rolled steel strip with low toughness, the process can be performed at room temperature, reducing manufacturing costs.

[Brief explanation of the drawing]

Figure 1 shows hot rolling W! in an experiment to obtain the present invention. A region diagram showing the relationship between the holding temperature and holding time of the four zones and the transition temperature after holding. Figure 2 shows the transition of the cooling rate and cooling control end temperature after holding the hot rolled steel strip at high temperature in the experiment to obtain the present invention. FIG. 3 is a diagram showing the influence on temperature.

Claims (1)

[Claims] (1) Contains Cr: 17-30% Al: 2-6% as a basic component in terms of weight ratio, and further contains one or more selected from Ti, Nb, Zr, and rare earth metals, with the remainder being a step of winding a hot-rolled steel strip of high Cr-high Al stainless steel containing Fe and inevitable impurities at a temperature of 900° C. or higher; and a step of holding at a temperature of 900° C. or higher for 5 to 10 minutes after said winding. , Immediately after the holding, cooling to a temperature of 450°C or less at a cooling rate of 10°C/sec or more. .