JPH02247330A - Production of austenitic stainless steel excellent in strength at high temperature and ductility - Google Patents

Abstract

PURPOSE:To improve strength, ductility, and corrosion resistance by subjecting an austenitic stainless steel in which respective contents of C, Si, Mn, Cr, Ni, and N are specified to hot rolling under the prescribed conditions. CONSTITUTION:An austenitic stainless steel having a composition which consists of, by weight, <=0.06% C, <=1% Si, <=2% Mn, 16-20% Cr, 6-16% Ni, 0.02-0.12% N, and the balance Fe and in which the total content of C and N is regulated to 0.08-0.13% is refined. This steel is subjected to hot rolling in which more than one-half the total number of reducing passes are carried out at >=10% draft, and further, rolling finishing temp. is regulated to 900-1000 deg.C. Subsequently, cooling is applied to the resulting hot rolled steel plate at >=3 deg.C/s cooling rate from 850 deg.C down to 500 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing austenitic stainless steel having excellent high-temperature strength and ductility.

<Prior Art> Austenitic stainless steel has been widely used as various structural materials for nuclear reactors and the like because it has excellent high-temperature strength and corrosion resistance.

Conventional austenitic stainless steel has a
It is manufactured by performing so-called solid solution treatment, which involves reheating to 010 to 1150°C and rapidly cooling. By the way, this reheating solution treatment must be performed off-line, separate from hot rolling, which increases equipment costs and is unfavorable in terms of processing efficiency and energy saving. Therefore, if solution treatment can be performed online following hot rolling,
By reducing heat treatment costs, omitting equipment, and shortening processes,
It is expected that the manufacturing cost of austenitic stainless steel sheets can be significantly reduced compared to conventional methods.

Conventionally, a method for manufacturing austenitic stainless steel sheets by omitting the solution heat treatment, which requires boat-based treatment, was disclosed in Japanese Patent Application Laid-open No. 60
JP-A-26619, JP-A-62-124220, etc. have been proposed. These methods limit the ingredients and
Furthermore, by regulating the cumulative reduction rate during hot rolling, the IJ top hot water temperature, and the cooling rate after rolling within predetermined ranges, hot rolling can be carried out without having to undergo solution treatment by reheating after hot rolling. The objective is to obtain an austenitic stainless steel sheet without precipitation of CrjJ nitrides.

However, according to these methods, it is true that the hot-rolled material has room-temperature strength equal to or higher than that of reheated solution treated material,
Although it is possible to obtain an austenitic stainless steel sheet without ductility and carbonitride precipitation, there remains the problem that it is insufficient in terms of high-temperature strength and ductility when used as a heat-resistant material.

Therefore, there has been a need to establish a method for online manufacturing of auste-Jite stainless steel sheets having high-temperature properties equivalent to or higher than those of reheated solution-treated materials for various steel types.

<Processing to be Solved by the Invention> The purpose of the present invention is to solve the problems of the prior art described above, and to provide a method for producing austenitic stainless steel having high-temperature properties equal to or higher than that of reheated solution treated material. is to provide.

<Means for solving nB> The present invention has a weight ratio of C! 0.06% or less, Si:
1.0% or less, Hn: 2.0% or less, Cr: 1
6.0-20.0%, Ni: 6.0-16.0%, N
: 0.02-0.12%, C+N 0.08-0.
13% as the basic component, B: 0.0006 as necessary
~0.010% and/or Mo: 0.1~3.0%
, Ti j O, 5% or less, Nb: 0.8% or less, an austenitic stainless steel containing one or more of the following, with the remainder consisting of iron and unavoidable impurities, is fully reduced in hot rolling. Rolling is performed at a reduction rate of 10% or more in at least half of the number of passes, and the rolling finishing temperature is 900 to 1000°C, and then 50% to 850°C.
A method for producing austenitic stainless steel having excellent high-temperature strength and ductility, characterized by cooling at a cooling rate of 3°C/s or more in the temperature range up to 00°C.

Effect> The present invention consists of regulations on steel composition and each manufacturing process,
First, the reason for limiting the components in the present invention will be explained.

C is effective in stabilizing the austenite phase and increasing strength, but as the amount increases, Cr carbides are more likely to be formed and creep rupture ductility is reduced.
The content was limited to 0.06% or less.

Si is usually added as a deoxidizing element, but addition of more than 1.0% deteriorates hot workability, so it was limited to 1.0% or less.

Mn is added to deoxidize and improve hot workability, but 2.0
Addition of more than 2.0% impairs corrosion resistance, so it was limited to 2.0% or less.

Cr is effective in improving oxidation resistance and high-temperature strength, but if it is less than 16.0%, the effect is insufficient;
．． If added in excess of 0%, δ ferrite rods would increase and hot workability would be degraded, so the content was limited to a range of 16.0 to 20.0%.

It is an austenite-forming element and is effective in improving corrosion resistance, but if it is less than 6.0%, the effect is insufficient, and if it is added in excess of 16.0%, the cost will increase. It was limited to a range of 0 to 16.0%.

N is an effective element for improving creep rupture strength, and 0.02% or more is required to exhibit this effect.
Addition of more than 0.12% deteriorates manufacturability and creep rupture ductility, so it was limited to a range of 0.02 to 0.12%.

In addition, C+N@ has a large influence on the creep rupture strength, and C+
If Nll is less than 0.08%, sufficient leap rupture strength cannot be ensured, and if it is added in excess of 0.13%, creep rupture ductility decreases, so the content is limited to a range of 0.08 to 0.13%.

Although B is not a necessary component, it is an effective element for improving creep rupture strength and ductility because it refines and stabilizes carbides, suppresses grain boundary precipitation of carbides, and strengthens grain boundaries. It is necessary to add 0.0006% or more, but since adding more than 0.01% will cause hot cracking during welding, it is limited to a range of 0.0006 to 0.01%.

No is an element that has a remarkable effect on improving high-temperature strength and corrosion resistance, but if it is less than 0.1%, it has no effect, and if it is added in excess, it reduces creep rupture ductility, so it should be added from 0.1% to
It was limited to 3.0%.

T1 is added to improve high-temperature strength, form Tl carbides, suppress the formation of C carbides, and improve intergranular corrosion.
The amount of C (%) x 5 is sufficient, and since adding too much will reduce creep rupture ductility, it is limited to 0.5% or less.

Nb improves high temperature strength as well as Nb
It is added to form carbides, suppress the generation of Cr carbides, and improve intergranular corrosion, but (NbWt to effectively combine with C (%) x 10 is 11 minutes,
Adding too much will reduce creep rupture ductility, so 0
．． It was limited to 8% or less.

Next, the reasons for limiting the manufacturing conditions will be explained.

Uniformly recrystallized with austenitic stainless steel &
It has been found that temperature and rolling reduction are important in order to obtain an n-weave. That is, in hot working and recrystallization, it is necessary to proceed with rolling by applying large reduction or cumulative reduction from the initial stage of rolling. In order to obtain uniform recrystallized grains, it is desirable that the reduction rate in each pass be large, and the reduction rate per pass should be small (less than 10%) (repeating this process will result in a mixed grain structure, so Rolling of 10% or more must be carried out on at least half of the number.

When hot rolling is carried out as described above, the final hot finish rolling must be carried out at 900 to 1000°C.

If the hot rolling temperature exceeds 1000°C, the leap rupture strength will not be sufficient (i+), while if it is less than 900°C, the elongated unrecrystallized &[lra will increase, reducing the creep rupture ductility.

Furthermore, if the cooling rate after hot working is less than 3° C./s in the temperature range from 850° C. to 500° C., carbides will precipitate at grain boundaries during cooling and corrosion resistance will deteriorate. Therefore, the cooling rate in the temperature range from 850°C to 500°C is 3°C.
/s or more.

<Example> Using 17 types of austenitic stainless steels shown in Table 1, hot working was performed under the processing conditions and cooling conditions shown in Table 2.

The tensile properties and creep properties of these steel plates at 550°C and the structure after a 10% oxalic acid etch test were investigated and are also shown in Table 2. In Table 2, sample material N112.
3. 8.10, 12.14.16.1B, 20.2
2,24°26.28.3G, 32.34 is an example of the present invention,
Nnl, 4.5, 6°11 is a comparative example, N[L7. 9.
11, 13.15.1? , 19.21, 23.25.2
7, 29, 31, 33, and 35 are conventional examples. Comparative example 1
Since C+N is less than 0.08, sufficient high temperature strength cannot be obtained.

In Comparative Example 4, the hot finishing temperature was less than 900°C, so the elongated unrecrystallized structure increased and the ductility was poor. In Comparative Example 5, the number of passes at a rolling reduction rate of 10%/pass or more did not reach half or more of the total number of rolling passes, so the fibers were mixed grains and had poor ductility. In Comparative Example 6, the cooling rate in the temperature range of 850 to 500°C is as slow as lo (:/S), so carbides precipitate at grain boundaries, resulting in poor corrosion resistance. Comparative Example 111IC
exceeds 0.06%, the creep rupture ductility is poor. Also, 7. is a conventional example. 9.11,13,1
5, 17.19, 21.23, 25, 27.29.31
．． No. 33.35 does not have sufficient high-temperature strength, and particularly has extremely low yield strength in a tensile test. On the other hand, Example 2 of the present invention satisfies all the conditions of the present invention. 3. 8.
10. +2.14.1G, 18,20,22.24.2
6.2B, 30, and 32.34 have high strength and high ductility at high temperatures, and also have excellent corrosion resistance.

<Effects of the Invention> As is clear from the above examples, the present invention economically produces austenitic stainless steel with excellent high-temperature strength, ductility, and corrosion resistance by limiting the chemical composition and manufacturing conditions. It can be expected to be applied to structural materials used in high-temperature inelastic areas such as nuclear reactors where high safety is required.

Claims (1)

[Claims] 1. In terms of weight ratio, C: 0.06% or less, Si: 1.0% or less, Mn: 2.0% or less, Cr: 16.0 to 20.0%.
, Ni: 6.0-16.0%, N: 0.02-0.12
%, and C+N is 0.08 to 0.13%, the balance being iron and unavoidable impurities. Austenite with excellent high-temperature strength and ductility, characterized by being subjected to rolling, with a finishing temperature of 900 to 1000°C, and then cooling in the temperature range from 850°C to 500°C at a cooling rate of 3°C/s or more. A method for manufacturing stainless steel. 2. Weight ratio: C: 0.06% or less, Si: 1.0% or less, Mn: 2.0% or less, Cr: 16.0 to 20.0%
, Ni: 6.0-16.0%, N: 0.02-0.12
%, and has C+N as a basic component of 0.08 to 0.13%, further contains B: 0.0006 to 0.01%, and the balance is iron and inevitable impurities. In hot rolling, at least half of the total number of rolling passes are rolled at a rolling reduction rate of 10% or more, and the rolling finishing temperature is set at 900 to 1000°C, and then 8
A method for producing austenitic stainless steel having excellent high-temperature strength and ductility, characterized by cooling in the temperature range from 50°C to 500°C at a cooling rate of 3°C/s or more. 3. Weight ratio: C: 0.06% or less, Si: 1.0% or less, Mn: 2.0% or less, Cr: 16.0 to 20.0%
, Ni: 6.0-16.0%, N: 0.02-0.12
%, and C+N is 0.08-0.13% as a basic component, Mo: 0.1-3.0%, Ti: 0.
5% or less, Mb: 0.8% or less, and the remainder is iron and unavoidable impurities. Reduction rate of 10 to more than half
% or more, and the rolling finishing temperature is 900 to 10
A method for producing austenitic stainless steel having excellent high-temperature strength and ductility, characterized by cooling the temperature at 00°C and after rolling at a cooling rate of 3°C/s or more in the temperature range from 850°C to 500°C. 4. Weight ratio: C: 0.06% or less, Si: 1.0% or less, Mn: 2.0% or less, Cr: 16.0 to 20.0%
, Ni: 6.0-16.0%, N: 0.02-0.12
%, C+N is 0.08 to 0.13% as the basic component,
and contains B: 0.0006 to 0.010%, further Mo: 0.1 to 3.0%, Ti: 0.5% or less, Nb:
Austenitic stainless steel containing one or more of 0.8% or less, the remainder consisting of iron and unavoidable impurities, is hot rolled at a reduction rate of 10% or more in at least half of the total number of reduction passes. Rolled with
And the rolling finishing temperature was 900-1000℃, then 8
A method for producing austenitic stainless steel having excellent high-temperature strength and ductility, characterized by cooling in the temperature range from 50°C to 500°C at a cooling rate of 3°C/s or more.