JPS60197817A - Manufacture of austenitic stainless steel material having high yield strength and superior corrosion resistance - Google Patents

Abstract

PURPOSE:To manufacture a stainless steel plate having superior yield strength and corrosion resistance at a low cost by subjecting austenitic stainless steel to heat treatment, reduction and rapid cooling each under specified conditions. CONSTITUTION:Austenitic Ni-Cr stainless steel is heated to >=1,050 deg.C to solubilize Cr carbide in the matrix, and it is rolled down at >=30% total draft in the temp. range of 850-950 deg.C so as to prevent deterioration in the corrosion resistance due to the precipitation of Cr carbide caused by a strain. The resulting austenitic stainless steel is acceleratedly cooled to at least 550 deg.C at >=2 deg.C/sec average cooling rate. Even in case of austenitic stainless steel having about 0.1% C content, the yield strength and corrosion resistance are improved without adding an expensive element such as Nb.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high yield strength austenitic stainless steel material with excellent corrosion resistance.

Heat treatment of austenitic stainless steel material (hereinafter, copper plate will be explained as an example of steel material).

That is, the purpose of solution treatment or solid solution treatment is to improve corrosion resistance by dissolving Or carbide in the matrix, unlike in the case of carbon steel or low alloy steel. Heat to high temperature. This causes the austenite grains to become coarse and the yield strength to drop significantly. Therefore, austenitic stainless steel sheets are unsuitable as steel sheets for some structures that require high strength.

One solution to the above-mentioned problems is to increase the backbone content in austenitic stainless steel. However, even if this method is adopted, the yield strength will only increase by about 9/- to 5-6, and furthermore, increasing the nitrogen content in this way will reduce the stress resistance, which is one of the drawbacks of austenitic stainless steel sheets. This is not a preferred method as it further deteriorates corrosion cracking.

Another method is to increase the yield strength by subjecting the austenitic stainless steel billet to controlled rolling, that is, by applying cumulative reduction in the non-recrystallized region of the austenite during hot rolling of the austenitic stainless steel billet. There is a way. However, this method has the following problems. That is, even if an austenitic stainless steel piece is heated to a temperature at which Cr carbide is sufficiently dissolved in solid solution before hot rolling, Cr carbide precipitates due to the strain generated in the steel sheet by hot rolling (strain-induced Cr carbide (referred to as precipitation), the surface corrosion of the steel sheet deteriorates. However, in order to perform solution treatment, it is necessary to dissolve the Cr carbide and then cool it at a cooling rate that prevents the Cr carbide from precipitating again during cooling, but in controlled rolling, cooling after rolling is performed by air cooling. From the viewpoint of preventing the precipitation of Cr carbides, the materials to which the above method can be applied are limited. That is, the composition is Cjlr, o, o1%
The material is limited to the following, and the plate thickness is limited to a thin material of 6 mm or less.

Since austenitic stainless steel generally has high deformation resistance, it is difficult to apply a large reduction in one bath. Therefore, the number of baths increases especially in the case of thin materials, and Cr
It is difficult to finish at a high temperature of 850° C. or higher to prevent strain-induced precipitation of carbides.

Also, since the thermal conductivity of austenitic stainless steel is about 1/2 lower than that of carbon steel or low alloy steel, the cooling rate during air cooling after controlled rolling becomes increasingly slower (Cr
Carbides tend to precipitate.

As is clear from the above explanation, a method for manufacturing an austenitic stainless steel sheet that has high yield strength and excellent corrosion resistance has not yet been developed.

Apart from increasing the yield strength, solution annealing not only requires a huge amount of thermal energy to heat the austenitic stainless steel piece to the high temperature mentioned above, but also It is necessary to install a heating furnace equipped with hearth rolls, etc. that can withstand high temperatures, and it is not a rational method to carry out solution treatment as an independent process.

From the above-mentioned viewpoints, this invention has high yield strength,
Moreover, the present invention provides a method for manufacturing an austenitic stainless steel plate with excellent corrosion resistance at a low cost. 850-95
Apply cumulative pressure f of 30 coefficients or more in a temperature range of 0 ° C. 1st time,
The austenitic stainless steel plate thus obtained was cooled at an average cooling rate of 2°C/Sec or more. T,! ', has a special feature of accelerated cooling to a temperature of at least 550°C.

This invention will be explained in detail.

The austenitic stainless steel used in this invention is any austenitic stainless steel that normally requires a generalization treatment, specifically c:o, 1% or less,
Mn: 5% or less, Si: 2% or less, Ni: 6-
50%, Cr: l O-30%, Ae: 1% or less, remaining 7'il (; Iron and Komachi impurities (weight ratio) are the basic elements, and if necessary, T1 is added to these basic elements.
: 2% or less, Nb: 2% or less, Cu: 4%
Hereinafter, Mo: Contains 10% or less of one element or two or more additional elements.

Confused with this invention, austenite, which supports the above elements,
The reason why the stainless steel piece is heated to a temperature of 1050° C. or higher is to sufficiently dissolve Cr carbide in the matrix. If the heating temperature of the steel piece is less than 1050°C, Cr carbide is not sufficiently dissolved in the matrix;
It is not possible to manufacture austenitic stainless steel sheets with excellent corrosion resistance.

Next, the reason why the heated steel billet is subjected to prefectural bond reduction of 30% or more in the temperature range of 850 to 950°C will be explained. First, if rolling is performed in a temperature range of less than 850° C., which is the lower limit of the heating temperature range, the corrosion resistance will deteriorate due to strain-induced precipitation of Cr carbides. Therefore, the lower limit of heating temperature is 850℃.
limited to. Next, the upper limit of the nathermal temperature is 850
In the rutting degree range of ~950°C, it becomes an austenite non-B crystallized area. Therefore, if cumulative reduction is applied in the non-recrystallized temperature range, the strain introduced into the steel billet will be accumulated without being released, and the deformation band density will increase. Yield strength increases.

On the other hand, in a temperature range exceeding 950° C., the austenite recrystallization region occurs.

Therefore, even if a large cumulative reduction is applied in the recrystallization temperature range, the strain introduced into the vertical piece is released by the recrystallization, and the yield strength of the steel plate does not increase. Therefore, the upper limit of the heating temperature is limited to 950°C. If the cumulative rolling reduction in the temperature range of 850 to 950° C. is less than 30%, the deformation band density will not increase sufficiently, and the yield strength will not increase as expected. Therefore,
The cumulative rolling reduction rate was limited to 30 times less than 4jν.

In this invention, the pressure F in a temperature range exceeding 950°C is optional. That is, no matter what conditions the rolling is performed, it is not necessary to apply rolling at all.

The finishing temperature of rolling may be any temperature in the mild range of 850 to 950°C, but it is better to keep it as low as possible in order to improve the yield strength of the steel plate.

Next, regarding the cooling conditions for the steel plate after rolling, if the cooling rate after rolling is less than 2°C/sec, or 550°C
If it is cooled at a cooling rate of less than 2°C/sec in a blackness range exceeding 1°C, Or carbide and σ phase will precipitate during cooling, resulting in deterioration of corrosion resistance. Therefore, in the present invention, the steel plate after rolling is cooled at an average cooling rate of 2°C/sec or more for at least 5°C.
It is acceleratedly cooled to a temperature of 50°C.

Next, embodiments of the invention will be described.

SUS 304 steel (A
.

B and C) were heat treated according to process method 3 having the temperature curves shown in FIGS. 1, 2 and 3. Process 1, which has the temperature curve shown in FIG. 1, is a process in which solution treatment is performed, and in hot rolling, rolling is performed only in the austenite recrystallization region.

Process (2), which has the temperature curve shown in Figure 2, involves rolling in the recrystallized region of austenite and further rolling in the non-recrystallized region of austenite in order to improve the yield strength, which is called controlled rolling. After rolling, it is air cooled. Process III, which has the temperature curve shown in FIG. 3, is similar to process U shown in FIG. 2, in which accelerated cooling is performed immediately after applying pressure in the austenite recrystallized region and non-recrystallized region. In addition, processes I to IUK in FIGS. 1 to 3
The austenitic stainless steel pieces before rolling were all heated at a temperature of 1200° C. or higher in order to sufficiently dissolve the Or carbide in the matrix.

Table 2 shows the conditions for each process 1), the tensile test results and corrosion of the obtained austenitic stainless steel plates 1 to 13 (comparative steel plates) to 5 and 10 to 13, and the invention steel plates 6 to 9. Test results are also shown. The Strauss test in Table 2 evaluates the corrosion state by immersing a test piece in a mixture of boiling sulfuric acid and copper sulfate and then bending the test piece. , × indicates that intergranular corrosion occurred on the entire surface, and Δ indicates that intergranular corrosion occurred in a part. In the U-bend test, a test piece is immersed in a saline solution of 20% strength for 1000 hours, and then the test piece is bent to evaluate the state of corrosion cracking. Each indicates that a has occurred.

As is clear from Table 2, comparative steel sheet 1 has good corrosion resistance (intergranular corrosion resistance and corrosion cracking resistance), but the yield strength is 21 Kg/nrl, which is less than the value specified by JIS. It's not too expensive. Specific mild steel plate 2 has increased nitrogen content to increase yield strength, but the increase in yield strength is only about 5 Kp/mA,
1. Restraining force Iff corrosion resistance is also poor.

Comparative steel plate 5. 5 is a product made by the process H shown in Fig. 2, and the comparison rt+l: ?
Process II', which produced
It is a similar process. Comparative steel plate 3, which was not subjected to If lowering in the austenite un-crystallized region, had a low yield strength, whereas in the austenite un-recrystallized region, the
Rigid plates 4 and 5 (the yield strength has increased.However, the ratio M'j'') 4: jfj
, a- and 5 are cooled after rolling or air cooled, so
C-some of ten (lc power, kawarusu il + i4 eating habits, q#
Grain boundary genus has poor feeding properties.

The steel plates 6 to 6 of the present invention have a yield strength of 55 to 56 Kq/mA at a thickness of 20 mm compared to comparative steel plates 1 to 3, and a thickness of 50 mm.
It has a significantly higher corrosion resistance of 50 K7/- and also has excellent corrosion resistance.

Comparative steel plates 10 to 13 were manufactured by the same process as the steel plates of the present invention, but the rolling conditions or cooling conditions were outside the scope of the present invention. Comparative steel plate 10 has a yield strength comparable to that of steel plate 8 of the present invention because the cooling rate is slow compared to that of the present invention, but its intergranular corrosion resistance is not good. Although the cooling rate of the comparatively beaten steel plates 11 to 13 is within the range of the present invention, since the reduction in the non-recrystallized region of austenite was applied to a temperature of less than 850 ° C., strain-induced precipitation of Or carbides occurred. 11Ii-1 Poor intergranular corrosion.

The above explanation is for the production of austenitic stainless steel cross-sectional plates, but it goes without saying that the present invention can also be applied to the production of other preparations.

As explained above, according to the present invention, it is possible to use an austenitic stainless steel pot with excellent corrosion resistance and high strength, without requiring a heavy heating furnace unlike when applying a single solution pot. This brings about extremely useful effects such as being able to be manufactured.

[Brief explanation of drawings]

Figures 1 and 81!2 are graphs showing temperature curves in the conventional method for producing an austenitic stainless steel plate, and Figures 31 and 11 are graphs showing a temperature curve in the case of producing an austenitic stainless steel plate by the method of the present invention. It is a graph. Applicant Nippon Dokan Co., Ltd. Agent Natsuo Shioya (and 2 others) Soon 1 Figure Time Child 2 Figure 3 Time

Claims (1)

[Claims] An austenitic stainless steel piece is heated to a temperature of 1050°C or higher, and then heated to a temperature of 850 to 90°C.
A cumulative reduction of 30% or more is applied in a temperature range of 50°C, and then the austenitic stainless steel sheet thus obtained is subjected to an average cooling rate of at least 5°C/Sec.
Chin I, characterized by accelerated cooling to a temperature of 50°C
A method for producing high-yield strength austenitic stainless steel material with excellent J-erodibility.