GB2175825A - Method for producing an austenitic stainless steel plate - Google Patents

Abstract

A method for producing an austenitic stainless steel plate gives superior corrosion resistance and high yield strength both at ambient and higher temperatures. The method includes the step of hot rolling a steel piece previously heated to 1000 DEG C in the recrystallization temperature range or, alternatively, reducing the previously heated piece in the recrystallization temperature range and then in the non-recrystallization temperature range, so as to effect hot rolling at higher than 800 DEG C, and cooling the resulting steel piece by accelerated cooling to a temperature at or below 550 DEG C at a mean cooling rate of higher than 2 DEG C/second. The austenitic steel may comprise less than 0.1 wt. percent of carbon, less than 5 wt. percent of manganese, less than 2 wt. percent of silicon, 6 to 65 wt. percent of nickel, 10 to 30 wt. percent of chromium, and less than 1 wt. percent of aluminium. The steel may also comprise titanium, niobium, copper and molybdenum. <IMAGE>

Description

SPECIFICATION Method for producing an austenitic stainless steel plates showing high corrosion resistance and high mechanical strength at ambient and elevated temperatures Background of the Invention This invention relates to a method for producing an austenitic stainless steel plate showing high corrosion resistance and high mechanical strength at ambient and elevated temperatures.

Austenitic stainless steels, for example, austenitic stainless steel plates, are subjected to heat treatment, such as solution annealing, in order to solution anneal chromium carbides into the matrix for improving heat resistance, and thus for an objective not contemplated in the case of the carbon steel or low alloy steel. In order to achieve such object, stainless steel plates are heated in general to an elevated temperature higher than 1000"C. As a result of this high temperature heating, austenitic crystal grins will become coarse and hence the ultimate steel product has a lower mechanical strength at ambient and higher temperatures so that the product is occasionally unsuitable as structural steel plates for which a higher mechanical strength is required.

For providing a solution to this problem, there is known a method of elevating nitrogen contents in the austenitic stainless steel. However, when resorting to this prior-art method, the yield strength of the austenitic steel can be improved only by 5 to 6 kg/mm2.

On the other hand, nitrogen contents in the austenitic stainless steel affect the corrosion resistance in a number of ways that are not always desirable. For instance, when the nitrogen contents in the austenitic stainless steel are increased, the corrosion resistance of the steel material is improved, but the resistance thereof to stress corrosion cracking is lowered. While the tendency to sensitization at the grain boundary is inhibited by up to a certain amount of nitrogen in the stainless steel, it is promoted with higher than the critical nitrogen contents. Thus the method of elevating the nitrogen contents in the stainless steel material is not necessarily desirable in consideration of the overall corrosion resistance required of the general structural steel materials.

There is also known a method of rolling the austenitic stainless steels by a controlled rolling, according to which, in the course of hot-rolling the austenitic stainless steels are subjected to accumulative reduction in the range of unrecrystallizing temperature, that is, in the range outside of the recrystallization temperature, for improving its yield strength. However, this controlled rolling has the following inconveniences. Thus, even when the austenitic stainless steels are heated prior to hot rolling to a temperature high enough to effect a solution of chromium carbides, precipitation of chromium carbides due to stress-induced in the stainless steel, known as stress-induced precipitation, is likely to occur upon hot rolling, with resulting deterioration in the corrosion resistance of the stainless steel plate.This deterioration may possibly be prevented by quenching the stainless steel product after hot rolling, as described in the Japanese Patent Kokai No. 55-107729. However, this method also is not completely satisfactory since no explicit reference is made in this Patent of the strength of the ultimate steel material.

In solution annealing, it is necessary to dissolve chromium carbides and then to effect cooling at a cooling rate such that chromium crbides are not allowed to be reprecipitated in the course of cooling.

In the controlled rolling, since the rolled product is cooled by air cooling, the materials to which the above described known method may be applied are limited from the viewpoint of preventing precipitation of chromium carbides. That is, the known method may be applied to the material with carbon contents less than 0.01 percent and with plate thicknesses less than 6 mm.

In general, because of the larger deformation resistance of the austenitic stainless steel, it is difficult to achieve a larger reduction value with only one pass. Therefore, it becomes difficult to effect a high temperature finishing at higher than 800"C in order to prevent stress-induced precipitation of chromium carbides, the number of passes increasing especially in the case of the stainless steel products of reduced thicknesses.

Also, the austenitic stainless stel has a thermal conductivity about one half that of carbon steel or low alloy steel, so that the cooling rate in the course of air cooling following the controlled rolling is further lowered thus promoting precipitation of chromium carbides.

It will be seen from above that there is not as yet known a proven method for producing austenitic stainless steel plates that are superior in the yield strength and corrosion resistance.

It will be noted further that, apart from the object of providing high yield strength steel products, an abundant heat energy for heating austenitic steels to the aforementioned high temperature and a heating furnace provided with a hearth roll able to stand such elevated temperatures are required for solution annealing. Therefore, it is not reasonable to effect a solution annealing as an independent process step.

According to the method of the present invention, there may be advantageously produced austenitic stainless steels of superior corrosion resistance and yield strength without the necessity of the reheating furnace employed in the solution annealing process.

Summary of the Invention The present invention envisages to provide a method wherein the problems inherent in the above described prior art methods may be eliminated. It is therefore a principal object of the present invention to provide a method for producing an austenitic stainless steels that is superior in corrosion resistance and in yield strength at ambient and higher temperatures.

The austenitic stainless steel employed in accordance with the present invention comprises any such steel in need of solution annealing and more specifically the composition of the austenitic steel plate comprises, as basic constituents, less than 0.1 wt. percent of carbon, less than 5 wt. percent of manganese, less than 2 wt. percent of silicon, 6 to 65 wt. percent of nickel, 10 to 30 wt. percent of chromium, less than 1 wt. percent of aluminium, the balance being steel and inevitable impurities; said composition occasionally including, in addition to said basic constituents, less than 2 wt. percent of titanium, less than 2 wt. percent of niobium, less than 4 wt. percent of copper and less than 10 wt. percent of molybdenum, either singly in combination.

The present invention provides a method for producing an austenitic stainless steel plate showing a high corrosion resistance and a high mechanical strength at ambient and elevated temperatures, characterized by the steps of heating an austenitic steel plate to a temperature higher than 1000 C, hot-rolling the heated steel plate in the recrystallization range, and cooling the resulting steel plate by accelerated cooling to at least a temperature of 550"C at a mean cooling rate of higher than 2"C/second.

The present invention also provides a method for producing an austenitic stainless steel plate showing a high corrosion resistance and a high mechanical strength at ambient and elevated temperatures, characterized by the steps of heating an austenitic stainless steel plate to a temperature higher than 1000 C, reducing the heated steel piece at the recrystallization range and also at the non-recrystallization range, hot-rolling the resulting steel plate at higher than 800"C and cooling the hot-rolled steel plate by accelerated cooling to at least a temperature of 550"C at a mean cooling rate of higher than 2"C/second.

Brief Description of the Drawings Figure 1 is a diagram showing the relation between the time and the temperature in the method for producing the austenitic stainless steel plate of the present invention.

Figure 2 is a diagram similar to Fig. 1 in the conventional method for producing the austenitic stainless steel plate.

Description of the Preferred Embodiment According to the present invention, austenitic stainless steel with contents of the above elements is heated ty a temperature higher than 1000"C in order that the chromium carbides will be dissolved in the matrix to form a nearly complete solid solution. When the heating temperature of the steel is lower than 1000 C, chromium carbides are not yet sufficiently dissolved in the matrix so that it is not possible to produce the austenitic steel plate having a superior corrosion resistance.

The rolling conditions are hereafter explained. According to the aforementioned second feature of the present invention, rolling must be conducted in the non-recrystallization area because such is indispensable in order to procure a superior mechanical strength. The lower limit of the rolling temperature is set to be 800"C in consideration that the rolling at a temperature lower than 800"C would degrade corrosion resistance due to stress-inducing precipitation of the chrome carbides although the rolling at lower than 800"C would be more favorable from the viewpoint of improving the mechanical strength.In the aforementioned first feature of the present invention, the rolling must be conducted in the recrystallization zone in order to cope with such situations in which an extremely high yield strength is not required on the uniform structure is required.

The cooling conditions of the rolled steel plates are now explained. When the post-rolling cooling rate is less than 2 C/second or when thecolling is terminated at a temperature higher than 500"C, chromium carbides are precipitated during cooling so that thus affecting anticorrosive properties. Therefore, according to the present invention, the hot rolled steel plate is acceleratedly cooled to a temperature of at least 550"C at a mean cooling rate not lower than 2"C/second.

The present invention will be explained by referring to a specific example thereof.

Example Table 1 shows the relative composition (in weight percent) of the steel species used in the present Example. Table 2 shows the various conditions for hot rolling and accelerated cooling, the results of the tensile tests, etching with oxalic acid and Streicher tests and the hightemperature properties of the tested steels.

The austenite stainless steel samples containing the elements shown in Table 1 (steel species A through G) were previously hot-rolled and cooled under the conditions shown in Table 2 and under the conditions of accelerated cooling as shown in Fig. 1. Some of the steel samples were air-cooled after hot rolling while some other steel samples were subjected to solution heat treatment.

Table 1 Relative Composition of the Tested Steel Species Steel Total range of non Species C Si Mn P S Mi Cr Mo Nb Ti N recrystal. temp.

A 0.05 0.65 1.50 0.021 0.004 8.9 18.7 0.11 - - 0.045 lower than 900 C B 0.02 0.62 1.02 0.018 0.003 10.5 18.9 0.15 - - 0.049 " " " C 0.04 0.63 0.98 0.024 0.004 11.8 16.6 2.32 - - 0.043 " " 950 c D 0.04 0.62 1.67 0.020 0.003 10.4 17.4 0.10 - 0.32 0.017 " " " E 0.04 0.65 1.63 0.021 0.003 10.9 17.8 0.10 0.73 - 0.023 " " 970 c F 0.023 0.67 0.84 0.024 0.001 33.68 19.86 7.21 0.68 - 0.017 " " 970 c G 0.021 0.50 0.40 0.020 0.002 41.23 21.1 3.20 - 0.75 0.015 " " 950 c Table 2 Steel Heating Finishing Plate S2e1es Tem2.( C) Rolling Temp.Thickness(mm) Comparative material 1 A 1150 950 20 Inventive material 2 A 1150 950 20 3 " 3 A 1150 830* 20 Comparative material 4 A 1150 750 20 Inventive material 5 A 1150 950 20 Comparative material 6 A 1150 950 20 II " 7 B 1100 950 20 Inventive material 8 B 1050 950 20 Comparative material 9 B 1100 950 20 Inventive material 10 B 1100 950 20 Comparative material 11 C 1150 1030 40 Inventive material 12 C 1150 830* 10 " " 13 C 1150 1030 40 14 14 C 1150 1060 100 Comparative material 15 D 1150 950 20 Inventive material 16 D 1150 950 20 Comparative material 17 E 1150 970 20 Inventive material 18 E 1150 970 20 Comparative material 19 F 1200 900 20 Inventive material .20 F 1200 900* 20 Comparative material 21 G 1200 900 20 Inventive material 22 G 1200 900* 20 Table 2 (Continued) post-rolling colling conditions start temp. end temp. Cooling rate ( C) ( C) @ C/sec.) Remarks Comparativ material 1 940 - 0.8 Solution anneal ing at 10500C after cooling Inventive material 2 940 500 7 3 " 3 820 500 7 Comparative material 4 740 500 7 Inventive material 5 940 500 3 Comparative material 6 940 - (air cooling) 0.8 7 " 7 940 - (air cooling) Solution anneal 0.8 ing at 1050 C after cooling Inventive material 8 940 520 7 Comparative material 9 940 600 7 Inventive material 10 940 (room temp.) 7 Comparative material 11 1020 - (air cooling) Solution anneal 0.5 ing at 10500C after cooling Inventive material 12 820 300 20 13 1020 (room temp.) 5 ri " 14 1050 (room temp.) 2 Comparative material 15 940 - (air cooling) Solution anneal- 0.8 ing at 1050 C after cooling Inventive material 16 940 500 7 Comparative material 17 940 - (air cooling) Solution anneal- 0.8 ing at 1050 C after cooling Inventive material 18 940 500 7 Comparative material 19 890 - (air cooling) Solution anneal 0.8 ing at 10500C after cooling Inventive material 20 890 500 7 Comparative material 21 890 - (air cooling) Solution anneal 0.8 ing at 1050 C after cooling nventive material 22 890 500 7 Table 2 (Continued) (1) Tens. test at room temp.Oxatic acid Striker etch. test test YS TS El kg f/mm kg f/mm % (2) (4) Comparative material 1 23.0 61.2 70 o Inventive material 2 42.5 67.9 53 o 3 " 3 56.3 72.3 50 o Comparative material 4 58.5 76.6 43 # Inventive material 5 41.5 67.3 54 o Comparative material 6 38.8 66.4 54 ss - " " 7 20.4 54.3 66 o Inventive material 8 41.3 65.3 54 o Comparative material 9 42.8 66.6 52 ss - Inventive material 10 43.3 67.5 51 o Comparative material 11 25.2 57.9 66 o Inventive material 12 60.4 .72.9 45 o 13 32.9 63.5 54 0 14 30.2 60.2 56 0 Comparative material 15 22.7 60.5 65 o Inventive material 16 41.8 66.7 52 o Comparative material 17 26.2 61.5 61 o Inventive material 18 43.8 68.9 50 o Comparative material 19 32.5 63.7 56 - o Inventive material 20 43.7 69.5 51 - o Comparative material 21 33.5 65.8 53 o Inventive material 22 45.3 71.0 50 - Rolled material in the non-recrystallization range Note (1) ASTM 9 # , (2) o, step structure At dual structure (3) 10000 hrs corrosion of tested material corrosion of solution < annealed material Table 2 (Continued) 5500C High temp tens Creep VS 2 TS 2 Rupturing temp. Kg f/mm Kg f/mm Kg f/mm2 (3) Comparative material 1 12.8 39.0 18.5 Inventive material 2 30.7 41.8 21.6 e " 3 31.9 43.1 22.4 Comarative material 4 - - - Inventive material 5 29.0 42.1 21.0 Comparative material 6 - - - " 7 11.2 37.6 17.5 Inventive material 8 29.5 40.5 20.7 Comparative material 9 - - - Inventive material 10 31.8 42.3 20.9 Comparative material 11 12.3 47.6 25.4 Inventive material 12 31.3 50.8 27.8 13 16.3 48.7 26.9 14 14.6 48.7 27.3 Comparative material 15 14.3 42.3 24.5 Inventive material 16 32.5 44.3 26.5 Comparative material 17 17.9 42.1 22.0 Inventive material 18 34.2 45.6 25.0 Comparative material 19 18.3 48.3 22.4 Inventive material 20 30.5 51.6 25.5 Comparative material 21 21.7 54.6 23.7 Inventive material 22 32.0 56.4 26.3 The oxalic acid etching test in Table 2 is based on JIS. The marks o and A in the Table denote a step structure and a dual structure, respectively. The Streicher test based on ASTM is intended for checking the degree of densitization of the high nickel austenite stainless steel. The mark o denotes the ratio corrosion of tested material equal to or less than corrosion of solution heat treatment material unity. The mark * denotes the rolled material in the non-recrystallized range.

From the Table 2 it is seen that, in the steel species A, the inventive steel plate samples 2, 3 and 5 are markedly superior to the solution annealed material 1 in the mechanical strength not only at room temperature but also at elevated temperature, while showing corrosion resistance comparable to the material 1. Above all, a higher strength is obtained with the steel plate sample 3 additionally subjected to reduction in the unrecrystallization range. The comparative material 4 is low in the rolling completion temperature and high in mechanical strength, however, it is somewhat inferior in corrosion resistance. The comparative material 6, aircolled after rolling, is similarly inferior in corrosion resistance.

In the steel species B, the inventive sampls 8 and 10 shows good mechanical strength as compared to the solution heat treated material 7, while also showing good corrosion resistance.

The comparative test sample 9, with a higher accelerated cooling stop temperature of 600"C, is excellent in mechanical strength, but inferior in corrosion resistance.

In the steel species C, the inventive test samples 12, 13 and 14, with the plate thickness equal to 10, 40 and 100 mm, respectively and the accelerated cooling stop temperature in the range from 300"C to room temperature is also excellent in mechanical strength and corrosion resistance as compared to the solution annealed test samples 15 and 17.

In the steel species F and G, the inventive test samples, subjected to reduction at the nonrecrystallization range, is comparable to the solution annealed test samples 19 and 21, while also showing superior mechanical strength.

Although the foregoing description has been made with reference to the preparation of the austenitic stainless steel plates, it is to be noted that the present invention is also applicable to the preparation of any other steel plates.

Claims (5)

1. A method for producing an austenite stainless steel plate showing a high corrosion resistance and a high mechanical strength at ambient and elevated temperatures, characterized by the steps of heating an austenitic stainless steel plate to a temperature higher than 1000"C; hot-rolling the heated steel plate in the recrystallization range; and cooling the resulting steel plate by accelerated cooling to at least a temperature of 550"C at a mean cooling rate of higher than 2 C/second.

2. A method according to claim 1 characterized in that the composition of the austenitic steel plate includes, as basic constituents, less than 0.1 wt. percent of carbon, less than 5 wt.

percent of manganese, less than 2 wt. percent of silicon, 6 to 65 wt. percent of nickel, 10 to 30 wt. percent of chromium, less than 1 wt. percent of aluminium, the balance being steel and inevitable impurities; said composition occasionally including, in addition to said basic constituent's less than 2 wt. percent of titanium, less than 2 wt. percent of niobium, less than 4 wt.

percent of copper and less than 10 wt. percent of molybdenum, either singly or in combination.

3. A method for producing an austenitic stainless steel plates showing a high corrosion resistance and a high mechanical strength at ambient and elevated temperatures, characterized by the steps of heating an austenitic stainless steel plate to a temperature higher than 1000"C; reducing the heated steel piece at the recrystallization range of austenite, and also at the nonrecrystallization range; hot-rolling the resulting steel plate at higher than 800"C; and cooling the hot-rolled steel plate by accelerated cooling to at least a temperature of 550"C at a mean cooling rate of higher than 2"Cc/second.

4. A method according to claim 3 characterized in that the composition of the austenitic steel plate includes, as basic constituents less than 0.1 wt. percent of carbon, less than 5 wt.

percent of manganese, less than 2 wt. percent of silicon, 6 to 65 wt. percent of nickel, 10 to 30 wt. percent of chromium, less than 1 wt. percent of aluminium, the balance being steel and inevitable impurities; said composition occasionally including, in addition to said basic constituents, less than 2 wt. percent of titanium, less than 2 wt. percent of niobium, less than 4 wt.

percent of copper and less than 10 wt. percent of molybdenum, either singly or in combination.

5. A method according to claim 1 and substantially as described herein with reference to figure 1 of the accompanying drawing.