JP2010202966A - Highly corrosion-resistant stainless steel having excellent tensile property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly corrosion-resistant austenitic stainless steel having excellent tensile properties and used for piping for a heat exchanger. <P>SOLUTION: The highly corrosion-resistant stainless steel having excellent tensile properties is obtained by subjecting a steel comprising, by mass, 0.025 to 0.060% C, ≤1.0% Si, ≤2.0% Mn, ≤0.035% P, ≤0.010% S, 11.0 to 13.5% Ni, 16.5 to 18.0% Cr, 2.0 to 3.0% Mo, ≤0.04% Al, 5×C to 0.50% Ti, 0.05 to 0.08% V and 0.010 to 0.025% N, and the balance Fe with inevitable impurities to hot extrusion. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a highly corrosion-resistant austenitic stainless steel having excellent tensile properties, and is particularly used for piping for heat exchangers.

  In general, high corrosion resistance austenitic stainless steels containing Mo and Ti suppress the material sensitization (deterioration of corrosion resistance) during welding due to the addition effect of Ti, and are mainly used for piping of corrosive fluids. . However, since Ti is very easy to react with N which is a solid solution strengthening element, the N content must be kept low, and the design strength (proof strength) of the material is low due to this influence. In particular, the strength of the steel pipe of this material, which is simply produced by hot extrusion, is very low.

  Until now, the following three methods have been mainly proposed as a measure for improving the strength of austenitic stainless steel without deterioration of corrosion resistance. The first is a precipitation strengthening method in which precipitation aging treatment is performed after hot working / controlled cooling below the recrystallization temperature. For example, in Japanese Patent Laid-Open No. 5-214439 (Patent Document 1), after hot working at a cumulative rolling reduction of 50% or more at 900 to 1000 ° C., it is cooled to 800 ° C. or less at 5 ° C./s or more and 750 to 800. There has been proposed a method for producing an austenitic stainless steel having excellent strength and corrosion resistance by performing a heat treatment at a temperature between 1 ° C. for 1 to 5 hours. This is because after hot working at a temperature at which strain remains and grain boundary carbides do not precipitate, cooling is performed so that grain boundary carbides do not precipitate, and fine carbides precipitate in the grains in the final heat treatment to improve corrosion resistance. This is a method of improving the strength without deteriorating.

  The second is a dislocation strengthening method in which strain is accumulated by hot working below the recrystallization temperature, and controlled cooling is performed while suppressing recrystallization and recovery of strain. For example, in JP-A-5-320756 (Patent Document 2), after heating to 1100 to 1300 ° C., forging at 800 to 1050 ° C. and a forging ratio of 1.2 or more, the average between 800 and 500 ° C. is 50 ° C./min. A method for obtaining high-strength austenitic stainless steel having excellent seawater resistance by cooling as described above has been proposed. That is, this suppresses the recovery of work strain introduced by forging below the recrystallization temperature, and cools at a cooling rate that can suppress the precipitation of carbides and nitrides, thereby maintaining the high strength of the austenitic stainless steel. This is a method for preventing deterioration of toughness and seawater resistance.

The third is a method of increasing the strength by making the crystal grains uniformly refined by uniformly precipitating fine carbides in the structure by refinement strengthening. For example, in Japanese Patent Application Laid-Open No. 64-36725 (Patent Document 3), the heating temperature of the final hot working is set to a high temperature to dissolve the elements contributing to refinement, and the final rolling is as low as 700 ° C. or less. And a method of manufacturing a γ-based stainless steel pipe having excellent corrosion resistance and mechanical properties by quenching and solid solution heat treatment. This is because, after the material added with Ti and Nb is dissolved at a high heating temperature, the work strain is accumulated by hot working, and Ti and Nb carbides are finely precipitated in the final solution heat treatment to obtain fine and uniform. This is a method for obtaining an austenitic steel pipe having a recrystallized structure.
JP-A-5-214439 JP-A-5-320756 Japanese Unexamined Patent Publication No. 64-36725

  As described above, three methods have been proposed mainly for strengthening hot-worked finished steel. The first is a precipitation strengthening method as shown in Patent Document 1, which is a method of performing precipitation aging treatment after hot working / controlled cooling below the recrystallization temperature. The second is a dislocation strengthening method as in Patent Document 2, which is a method of controlling cooling so as to accumulate post-hot working strain below the recrystallization temperature and prevent precipitation of grain boundary carbides that degrade corrosion resistance. . The third is the refinement strengthening method as in Patent Document 3, in which the element that contributes to refinement is dissolved by increasing the heating temperature of the final hot working, and the working strain is accumulated by hot working, This is a method of refining and strengthening the structure by the final solution heat treatment. However, in the hot extrusion method, since the material is basically processed at the recrystallization temperature or higher, the strengthening method proposed in the past cannot be applied, and it is difficult to strengthen the material.

  In order to solve the above-mentioned problems, the inventors have intensively developed and as a result, finely dispersed Ti carbonitride (TiVCN) containing V, which is difficult to dissolve at the material temperature during hot extrusion, The present inventors have found a method capable of uniformly refining the structure during dynamic recrystallization during hot extrusion and increasing the strength. This is because in Ti-added highly corrosion-resistant austenitic stainless steel, by controlling V in a narrow range, an extruded material in which TiV carbonitride is uniformly finely dispersed can be adjusted, and hot working is performed at a temperature lower than the recrystallization temperature of the material. This is a method of strengthening only by a hot extrusion process.

  When it is desired to further increase the strength, cold working is performed, and refinement strengthening by precipitation of TiC is combined. In this case, water cooling after hot extrusion is performed at a temperature higher than 40 ° C. with respect to the final solution heat treatment temperature, and the supersaturated TiC is converted into grain boundaries and subgrains during the solution heat treatment after cold working. It is to be refined by precipitation.

The gist of the invention is that
(1) By mass%, C: 0.025 to 0.060%, Si: ≦ 1.0%, Mn: ≦ 2.0%, P: ≦ 0.035%, S: ≦ 0.010%, Ni: 11.0 to 13.5%, Cr: 16.5 to 18.0%, Mo: 2.0 to 3.0%, Al: ≦ 0.04%, Ti: 5 × C to 0.50 %, V: 0.05 to 0.08%, N: 0.010 to 0.025%, and steel consisting of the remainder Fe and inevitable impurities is obtained by hot extrusion. Excellent corrosion resistance stainless steel.

(2) High corrosion resistance stainless steel having excellent tensile properties, containing B: 5 to 20 ppm in addition to the steel described in (1).
(3) When the steel according to (1) or (2) is hot-extruded with the steel according to claim 1 or 2, the water cooling start temperature after extrusion is higher by 40 ° C. or more than the final solution heat treatment temperature. It is in the manufacturing method of the high corrosion resistance stainless steel excellent in the tensile characteristic characterized by setting it as temperature.

  As described above, the present invention can achieve high strength without deteriorating excellent corrosion resistance only by hot extrusion, and as a result, has an extremely excellent effect of reducing the amount of steel used.

Hereinafter, the reasons for limiting the component composition according to the present invention will be described.
C: 0.025 to 0.060%
C is an austenite-forming element, and is an element necessary for maintaining austenite and ensuring strength. However, if it is less than 0.025%, its effect is not sufficient, and if it exceeds 0.060%, it is not yet effective. Since the solid solution TiC increases and the workability deteriorates, the range is set to 0.025 to 0.060%.

Si: ≦ 1.0%
Si is an element necessary for deoxidation. However, if it exceeds 1.0%, the workability deteriorates, so the upper limit was made 1.0%.
Mn: ≦ 2.0%
Mn is an element necessary for deoxidation like Si. However, if it exceeds 2.0%, the workability deteriorates, so the upper limit was made 2.0%.

P: ≦ 0.035%
The upper limit of P is 0.035% in order to deteriorate weldability. Preferably it is 0.030%.
S: ≦ 0.010%
Since S deteriorates corrosion resistance and workability, the upper limit was made 0.010%.

Ni: 11.0-13.5%
Ni is an element necessary for obtaining an austenite structure. However, if 11.0%, the effect is not sufficient, and if it exceeds 13.0%, the cost increases, so the range was set to 11.0 to 13.5%.
Cr: 16.5 to 18.0%
Cr is an element for obtaining corrosion resistance. However, if it is less than 16.5%, the desired corrosion resistance cannot be obtained. Moreover, since addition of a large amount requires further addition of Ni and causes a cost increase, the upper limit was made 18.0%.

Mo: 2.0-3.0%
Mo is an element for remarkably improving the corrosion resistance. However, if it is less than 2.0%, desired corrosion resistance cannot be obtained. Further, addition of a large amount deteriorates workability and further requires addition of Ni, leading to an increase in cost, so the upper limit was made 3.0%.
Al: ≦ 0.04%
Al is an element added when deoxidation is necessary. However, excessive addition causes deterioration of workability, so the upper limit was made 0.04%.

Ti: 5 × C to 0.50%
Ti generates fine Ti carbonitride containing V, and contributes to refinement and high strength of the hot extruded material. Further, it is an element that fixes free C and contributes to suppression of deterioration of corrosion resistance due to sensitization. Therefore, the lower limit is 5 × C. However, if it exceeds 0.50%, a large amount of TiC is generated and the workability is deteriorated, so the upper limit was made 0.50%.

V: 0.05-0.08%
V is an element necessary for reducing the crystal grain size of the material after extrusion and improving the strength. However, if the amount is less than 0.05%, the effect is not sufficient, and if it exceeds 0.08%, carbonitrides effective for miniaturization during hot extrusion are not formed. -0.08%.

N: 0.010 to 0.025%
N generates TiV carbonitride and is added for strength improvement by refinement. However, if it is less than 0.010%, the effect is not sufficient, and if it exceeds 0.025%, coarse nitrides are generated and hot workability deteriorates, so the range is set to 0.010 to 0. 0.025%. Preferably, the content is 0.010 to 0.020%.

B: 5 to 20 ppm
B is an element that finely precipitates at grain boundaries during the solution heat treatment and contributes to miniaturization as pinning particles. For that purpose, 5 ppm or more is required. However, if it exceeds 20 ppm, the hot workability deteriorates, so the upper limit was made 20 ppm.

Water cooling start temperature after extrusion: Final solution heat treatment temperature + 40 ° C or higher TiC is precipitated during the final solution heat treatment for further refinement strengthening, so the water cooling start temperature after extrusion is higher than the solution heat treatment temperature. Then, the precipitated elements are dissolved. Since the refinement strengthening of the cold-worked material was observed when it was higher by 40 ° C. or more than the solution heat treatment temperature, it was defined as + 40 ° C.

Hereinafter, the present invention will be specifically described with reference to examples.
Ingots were produced from steels having the composition shown in Table 1 in a 1-t vacuum induction melting furnace, and forged into φ140 round bars by heating at 1280 ° C. Then, it adjusted to the billet for extrusion through peeling and punching, and after hot heating to 1210 degreeC with the induction heating furnace, it hot-extruded to the dimension of 38.0 mm in outer diameter and 5.0 mm in thickness (extrusion ratio 33). Immediately after extrusion, water cooling was performed from a water cooling start temperature of 1050 to 1100 ° C. For the evaluation of the hot finish pipe, a steel pipe obtained by subjecting the steel pipe after extrusion / water cooling to a solution heat treatment by water cooling at 1050 ° C. for 10 minutes was used.

  For evaluation of the cold finish pipe, the steel pipe after extrusion water cooling is rolled to a outer diameter of 21.3 mm and a wall thickness of 2.0 mm with a cold rolling mill (area reduction rate of 77%), and after degreasing, a final solution heat treatment ( A steel pipe cooled at 1050 ° C. for 10 minutes was used. Evaluation was performed by crystal grain size measurement (JISG 0551) and tensile test (JIS Z 2201 No. 12 test piece). The determination was made with a hot-finished steel pipe having a 0.2% proof stress of 300 MPa or more. The results are shown in Table 1. In addition, a cold-finished steel pipe of 350 MPa or more was considered good. The results are shown in Table 2.

表１に、熱間仕上材の場合の本発明例をＮｏ．１〜８に、また、比較例をＮｏ．９〜１２に示す。

Table 1 shows examples of the present invention in the case of hot finishing materials. 1 to 8 and Comparative Example No. Shown in 9-12.

  As shown in Table 1, Comparative Example No. in the case of hot finishing material. Since No. 9 has a low C content, the crystal grain size number is small and the yield strength is inferior. Comparative Example No. Since No. 10 has a low V content, the crystal grain size number is small and the yield strength is inferior. Comparative Example No. Since No. 11 has a low N content, the crystal grain size number is small and the yield strength is inferior. Comparative Example No. Since No. 12 has a low V content, the crystal grain size number is small and the yield strength is poor. On the other hand, No. 1 as an example of the present invention. Since all of 1-8 satisfy the conditions of the present invention, it can be seen that the characteristics are excellent.

表２に、冷間仕上材の場合の本発明例をＮｏ．１〜５に、また、比較例をＮｏ．６〜８に示す。

Table 2 shows examples of the present invention in the case of a cold finish. 1 to 5 and Comparative Example No. Shown in 6-8.

  Comparative Example No. Since No. 6 has a low water cooling start temperature, the crystal grain size number is small and the proof strength is low. Comparative Example No. Since No. 7 has a low water cooling start temperature, the crystal grain size number is small and the proof strength is low. Comparative Example No. Since No. 8 has a low water cooling start temperature, the crystal grain size number is small and the yield strength is low. On the other hand, No. which is an example of the present invention. Since 1-5 satisfy | fills the conditions of this invention, it turns out that the characteristic is excellent.

As described above, in Ti-added highly corrosion-resistant austenitic stainless steel, by controlling V, finely dispersed TiV carbonitride is formed, and without hot working below the recrystallization temperature of the material, High strength can be achieved only by the extrusion process. INDUSTRIAL APPLICABILITY The present invention can provide a high-strength steel pipe excellent in corrosion resistance without going through an extra hot working step, and is expected to have a great industrial effect due to a reduction in the amount of steel used.

Patent Applicant Sanyo Special Steel Co., Ltd.
Attorney: Attorney Shiina

Claims (3)

% By mass
C: 0.025 to 0.060%,
Si: ≦ 1.0%,
Mn: ≦ 2.0%,
P: ≦ 0.035%,
S: ≦ 0.010%,
Ni: 11.0-13.5%,
Cr: 16.5 to 18.0%,
Mo: 2.0-3.0%,
Al: ≦ 0.04%,
Ti: 5 × C to 0.50%,
V: 0.05-0.08%,
N: 0.010 to 0.025%
A high corrosion resistance stainless steel excellent in tensile properties, characterized in that a steel comprising the balance Fe and inevitable impurities is obtained by hot extrusion. A high corrosion resistance stainless steel excellent in tensile properties, containing B: 5 to 20 ppm in addition to the steel according to claim 1. A high corrosion resistance stainless steel excellent in tensile characteristics, characterized in that when the steel according to claim 1 or 2 is hot-extruded, the water cooling start temperature after extrusion is set to a temperature higher by 40 ° C or more than the final solution heat treatment temperature. Steel manufacturing method.