JPH0379742A - Manufacturing method of duplex stainless steel sheet with excellent pitting corrosion resistance - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of a steel sheet having a mixed structure by subjecting a duplex stainless steel with a specific structure to rolling while specifying the total rolling reduction and then to heat treatment while specifying temp. region and the total heat treatment time. CONSTITUTION:A duplex stainless steel having a structure where fine austenitic phase of <=5mum average grain size is dispersed in a ferritic matrix is used as a starting material. This steel is subjected to cold or warm rolling at >=40% total rolling reduction. The resulting rolled stock of (t)mm sheet thickness is heat-treated while regulating the total heat treatment time to >100(t)sec-<200(t)sec. In the above heat treatment, the rolled stock is passed through the temp. region of 750-950 deg.C, while being subjected to continuous temp. rise, spending a time not longer than one-third the total heat treatment time. Subsequently, soaking treatment is applied to the above stock at a temp. in a region of 980-1100 deg.C, spending a time not longer than one-sixth the total heat treatment time. Then, rapid cooling is performed at >=100 deg.C/sec cooling rate. By this method, a steel sheet reduced in the average grain size of austenitic phase dispersed in the matrix, having high pitting corrosion potential, and excellent in corrosion resistance can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is directed to a high C "2" stainless steel (hereinafter referred to as The present invention relates to a method for manufacturing duplex stainless steel (duplex stainless steel) and steel sheets thereof.

[Conventional technology]

In general, duplex stainless steel is used in chemical plants and the like because of its excellent corrosion resistance and mechanical properties, and is also standardized by JIS as 5US329J and 5O3329J2L.

In order to further improve the properties of this duplex stainless steel,
Until now, various studies have been conducted mainly from the aspect of component composition,
Duplex stainless steels shown in Japanese Patent Publication No. 51-43807, Japanese Patent Application Laid-Open No. 52-718, Japanese Patent Application Laid-open No. 117918-1980, etc. have been proposed, and the present inventor also
This is proposed in Publication No.-14099.

On the other hand, conventionally, the manufacturing method for duplex stainless steel involves the following steps: refining, continuous casting, hot rolling, heat treatment, cold rolling, and heat treatment, and the characteristics of the material have a large influence not only on its components but also on its manufacturing method. It is also well known that

[Problem to be solved by the invention]

However, conventionally, the corrosion resistance of duplex stainless steel has been evaluated based on the composition of 3! #Although many proposals have been made to improve the corrosion resistance of duplex stainless steel by controlling the structure, no proposal has been made to improve the corrosion resistance of duplex stainless steel by controlling the structure. There were limits to the improvement.

[Means to solve the problem]

Therefore, the present inventor conducted research on the structure and corrosion resistance of duplex stainless steel, and obtained the following knowledge.

(1) Conventionally, duplex stainless steel has a microstructure in which an austenite phase is dispersed in a ferrite base, and the average grain size of the austenite phase is usually about 1O to 5G-. When the average grain size is finer than this: 5 μs or less, corrosion resistance is far superior to that expected from the normal component composition. (2) Total reduction rate of 40% in cold or warm rolling. The two-phase stainless steel rolled material subjected to the above rolling is heated to 750°C to 950°C.
The temperature range of 980℃ to 1100℃ is passed through the temperature range of 1/3 or less of the total heat treatment time while continuously increasing the temperature.
We found that when soaking is carried out at a predetermined temperature within the range of °C for less than 1/6 of the total heat treatment time and then rapidly cooled, the average grain size of the austenite phase dispersed in the ferrite matrix becomes less than 5 m. I got it.

The present invention has been made based on this knowledge, and provides a duplex stainless steel with excellent corrosion resistance in which an austenite phase with an average grain size of 25 or less is dispersed in a ferrite matrix, and a total rolling reduction. = 40% or more cold-rolled or warm-rolled two-phase stainless steel rolled material is passed through a temperature range of 750 to 950°C while continuously increasing the temperature over a period of 1/3 or less of the total heat treatment time. , a method for manufacturing duplex stainless steel with excellent corrosion resistance, comprising soaking at a predetermined temperature within the range of 980°C to 1100°C for a total heat treatment time of 176 or less, and then rapidly cooling. It is.

The material used in the method of manufacturing duplex stainless steel with excellent corrosion resistance of this invention is 5US32 specified by JIS.
9J 5US329J2L, etc., although not particularly limited,
In terms of weight percent, C: 0.02% or less, Si: 2.0% or less, Mn: 3.0% or less, as described in Publication No. 099.
N1: 3~1O%, Cr: 2 (1~35%, Mo
: 0.5-6.0%, N: 0.08-0.3%, At least one of W and V: 0.03-2.0%
, B: 0.0005 to [1,01%, S: 0
．． When using a duplex stainless steel containing 00596 or less, further containing Cu: 2.0% or less as necessary, and the remainder substantially consisting of Fe, the effect of the component composition and the fine austenite phase can be They act synergistically to produce the most favorable effects.

Furthermore, in the present invention, the above-mentioned material is hot-rolled at a temperature of 1250°C or lower, and a two-phase stainless steel having a structure having an austenite phase stretched in the rolling direction in a ferrite matrix is used as a starting material. In addition, there are 3 to 2 σ phases
It is preferable to use a three-phase mixed stainless steel containing 0% by volume as the starting material. When a three-phase mixed stainless steel containing the above σ phase is used as a starting material, the σ phase is extremely hard with a Vickers hardness of 300 or more, so there is a large hardness difference between the ferrite phase and the austenite phase matrix.
Furthermore, since the slip system is different, a large amount of strain is introduced around the σ phase during subsequent rolling, and more energy can be introduced than in the starting material without the σ phase. However, this σ phase rapidly disappears due to diffusion promoted by a large amount of strain during subsequent heat treatment, and the temperature: 980-110
By soaking at 0℃, a fine two-phase structure consisting of a ferrite phase and an austenite phase is formed, so σ
No phase remains. The reason why the σ phase contained in the three-phase mixed structure is specified to be 3 to 20% by volume is that if it is less than 3% by volume, the introduction of strain due to the presence of the σ phase is not sufficient, and if it exceeds 20% by volume, the σ phase will become embrittled. This is because rolling becomes impossible. The structure of the starting material can be made into a three-phase mixed structure of ferrite phase, austenite phase, and σ phase by controlling the cooling of the hot rolled coil.

First, the starting material for the above-mentioned two-phase stainless steel having a ferrite phase and an austenite phase, or the starting material for a stainless steel having a three-phase mixed structure of a ferrite phase, an austenite phase, and a σ phase, is prepared at a rolling reduction rate of 40% or more, preferably. 60
% or more to produce a rolled material having a thickness of t+am. Warm rolling here means temperature: 50 to 35
This refers to rolling at 0°C. The reason why the above rolling reduction ratio is set to 40% or more is that if the rolling reduction ratio is less than 40%, the amount of strain introduced into the rolled material will not be sufficient and it will be impossible to obtain a fine structure in the subsequent heat treatment. It is.

The rolled material with the above thickness is then heat treated,
In the heat treatment, the rolled material is heated to a temperature of 750 to 950 from normal temperature.
a step of heating to a temperature range of °C, a step of continuously raising the temperature in the temperature range of 750 to 950 °C, a step of soaking in a temperature range of 980 to 1100 °C, and a cooling rate = 100 °C after the soaking treatment. If the total heat treatment time required for the above heat treatment is T seconds, then T is more than 100 seconds and less than 200 seconds, that is, 100t<T<200t (however, t is limited by the thickness of the rolled material). Total heat treatment time T is ioo
If the time is less than 200 seconds, recovery and recrystallization will not be sufficient, while if the time is more than 200 seconds, the crystal grains will become coarse, which is not preferable.

The temperature range of 750 to 950°C corresponds to the σ phase precipitation temperature range, and it is necessary to pass the rolled material through this temperature range while continuously increasing the temperature. It is not preferable to allow the material to pass through while being held at a certain temperature. If the rolled material is kept in the temperature range of 750 to 950°C for a long time exceeding 1/3 of the total heat treatment time T, the σ phase will precipitate, or even if the σ phase does not precipitate, it will undergo roughening as a preliminary step to the σ phase precipitation. At this time, the strain energy introduced by rolling is partially or completely released due to these effects, and the desired fine austenite phase with an average grain size of 5μ or less is formed. It becomes impossible to uniformly recrystallize the ferrite base. Therefore, the residence time in the temperature range of 750 to 950°C is 1/3 or less, preferably 1/3 of the total heat treatment time T.
It is set at 20 or more and 1/3 or less.

Furthermore, the above temperature range of 980 to 1100°C is the normal annealing temperature range for duplex stainless steel, and if soaking is performed in this temperature range for a time of 1/6 or less of the total heat treatment time T, crystallization will occur after completion of recrystallization. The heat treatment can be completed before grain growth occurs, and soaking for a time exceeding 1/8 of the total heat treatment time T is not preferable because the crystal grains will become coarser after recrystallization.

Therefore, the soaking time in the temperature range of 980 to 1100° C. is set to 1/6 or less, preferably 1/20 or more and 1/8 or less of the total heat treatment time.

After soaking in the above temperature range of 980-1100℃,
Rapidly cool at a cooling rate of 100°C/sec or more. Specifically, water cooling is preferable for this rapid cooling, and if the cooling rate is less than 100° C./sec, coarsening of crystal grains after recrystallization cannot be prevented in a short time, which is not preferable.

The average grain size of the fine austenite phase crystal grains dispersed in the ferrite base of the 2FI'l stainless steel produced in this way is 5- or less, and this duplex stainless steel is as expected from the normal composition. It has a corrosion resistance far superior to that of other materials. The reasons for this are: (1) Because it has a two-phase structure in which fine and uniform austenite phases are dispersed, the electrochemical action between the phases that is unique to two-phase stainless steel works effectively.

(2) The component balance of the ferrite phase and austenite phase is favorable for improving corrosion resistance, that is, the macroscopically averaged component segregation contributes.

Two points can be considered.

However, even if duplex stainless steel is produced by the production method of the present invention, the average grain size of the fine austenite phase is 0.
Setting it to less than 1 μm requires extremely accurate control, and it is also extremely difficult to stably reproduce such a structure, making it unsuitable and uneconomical for factory production. Therefore, the average crystal grain size of the fine austenite phase of the duplex stainless steel with excellent corrosion resistance of the present invention is preferably 0.1 m or more and 5 m or less for practical purposes.

〔Example〕

Next, the present invention will be specifically explained based on examples.

The raw materials were melted in a 60-ton electric furnace, subjected to vacuum degassing treatment, and then continuously cast to produce test material slabs having the component compositions shown in Table 1.

The slab of the above sample material was rolled with the history shown in Table 2,
A duplex stainless steel starting material having the thickness shown in Table 2 was produced, and the proportion of the σ phase of the duplex stainless steel starting material was measured by image processing, and the results are shown in Table 2. Such a duplex stainless steel starting material is rolled under the conditions shown in Table 2 to produce a rolled material with a plate thickness of tmm, and this rolled material is heat treated under the heat treatment conditions shown in Table 2. Duplex stainless steel sheets 1 to 11 of the present invention and Comparative 2 shown in
Phase stainless steel plates 1 to 6 were manufactured.

Furthermore, a starting material was prepared as a conventional duplex stainless steel plate by subjecting the sample material A shown in Table 1 to normal hot rolling and then not subjecting it to rolling or heat treatment.

The above-mentioned duplex stainless steel sheets 1 to 11 of the present invention. The structures of comparative duplex stainless steel sheets 1 to 6 and conventional duplex stainless steel sheets were observed using a metallurgical microscope, and the average crystal grain size of the austenite phase dispersed in the ferrite base was determined to be #1 by image processing. 2 table, and then these 2
In order to evaluate the corrosion resistance of the stainless steel sheet, the pitting potential in a NaCj7 solution with a concentration of 3.5% at a temperature of 0° C. was measured, and the results are shown in Table 2.

Further, the metal structures of the present invention duplex stainless steel sheet 1 and the comparative duplex stainless steel sheet 1 shown in Table 2 were taken with a microscope at a magnification of 400 times, and these photographs are shown in FIG. 1 and FIG. 2, respectively.

From the results shown in Table 2 above and the metallurgical microstructure photographs in Figures 1 and 2, it is clear that the duplex stainless steel sheets 1 to 11 of the present invention, which were rolled and heat-treated to meet the conditions of the present invention, were all rolled and heat-treated. It can be seen that the corrosion resistance is superior to that of the conventional duplex stainless steel sheet which is not treated because the average crystal grain size of the austenite phase dispersed in the ferrite base is smaller and the pitting corrosion potential is also higher. Furthermore, even if rolled and heat treated under conditions that deviate from the conditions of this invention (in Table 2, values that deviate from the conditions of this invention are marked with *), the average grain size is 5- or less. It was also found that a fine austenite phase was not obtained, and the average grain size was:
It can be seen that Comparative duplex stainless steel sheets 1 to 6 having an austenite phase of more than 5- have relatively low pitting corrosion potential and poor corrosion resistance.

〔Effect of the invention〕

The duplex stainless steel sheet of this invention exhibits better corrosion resistance than conventional stainless steel sheets, so it can be used as a material for various plants and equipment, exhibiting excellent effects over a long period of time, and contributing greatly to the development of industry. It is.

[Brief explanation of drawings]

FIG. 1 is a photo of the metallographic microstructure of the duplex stainless steel sheet 1 of the present invention, and FIG. 2 is a photo of the metallographic microstructure of the comparative duplex stainless steel sheet 1. Applicant Nippon Yakin Kogyo Co., Ltd. Agent Kazuo Akida and one other person Figure 1

Claims (3)

[Claims] (1) In a duplex stainless steel having a structure in which a fine austenite phase is dispersed in a ferrite base, the fine austenite phase has an average grain size of 5 μm.
A duplex stainless steel with excellent corrosion resistance, characterized by the following: (2) A starting material of duplex stainless steel is cold or warm rolled at a total reduction rate of 40% or more to produce a rolled material with a plate thickness of tmm, and this rolled material with a plate thickness of tmm is rolled for 100 t seconds. A method for producing a duplex stainless steel sheet with excellent corrosion resistance, in which the heat treatment is performed in a total heat treatment time exceeding 750 to 950 °C, and in a total heat treatment time of less than 200 t seconds. A step of passing the heat treatment in a time of 1/3 or less of the above total heat treatment time in a temperature range of 980 to 1100℃
A step of soaking for a time of 6 or less, and 10
A method for producing a duplex stainless steel sheet with excellent corrosion resistance, comprising the step of rapidly cooling at a cooling rate of 0° C./second or more. (3) The starting material used in the method for manufacturing a duplex stainless steel sheet with excellent corrosion resistance according to claim (2) above is a duplex stainless steel sheet with excellent corrosion resistance, characterized in that it contains 3 to 20% by volume of σ phase. manufacturing method.