JPH0448052A - Method for hot-working austenitic stainless steel excellent in high temperature ductility - Google Patents

Abstract

PURPOSE:To show excellent high temp. ductility in an austenitic stainless steel and to permit its forming into a complicated shape by treating a stainless steel having specified components under prescribed process conditions. CONSTITUTION:An ingot contg., by weight, <=0.15% C, <=6.0% Si, <=10.0% Mn, 4.0 to 12.0% Ni, 10.0 to 19.0% Cr, <=0.3% N and the balance iron with inevitable impurities is subjected to hot rolling, cold rolling and annealing to prepare a steel showing an austenitic phase at an ordinary temp. after the annealing. This steel is subjected to cold rolling to form a strain induced martensitic phase of >=60%. Next, this steel is heated to the temp. range of the As point (the transformation starting temp. from the martensitic phase to the austenitic phase) +30 deg.C or above to the Af point (the transformation finishing temp. from the martensitic phase to the austenitic phase) or below as well as <=900 deg.C to form its structure into a uniform and fine dual-phase one of the martensitic phase and austenitic phase. Then the steel is subjected to hot working at >=1X10<-5>/sec to <=1X10<-1>/sec strain rate. In this way, its high temp. ductility is improved, its superplasticity is shown and its forming into a complicated shape is permitted.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention has a fine two-phase structure of an austenite phase (γ phase) and a deformation-induced martensitic phase (α' phase) during hot working, and has excellent properties. The present invention relates to a hot working method for austenitic stainless steel exhibiting high-temperature ductility.

<Conventional technology> Superplastic materials are attracting attention as one of the functional materials.
Superplastic materials have been found in various alloy systems such as A1 alloy and Ti alloy. It is said that in order to exhibit this superplasticity, it is necessary to obtain a fine and uniform structure on the micron order. Among stainless steels, some steels exhibiting a ferrite-austenite phase structure have been found to have superplasticity, and for example, Japanese Patent Publication No. 63-66364 discloses a hot working method for imparting superplasticity.
Further, Japanese Patent Publication No. 2-4656 discloses a method for manufacturing a superplastic duplex steel plate.

<Problems to be Solved by the Invention> At present, as mentioned above, ferritic-austenitic duplex steel is well known as a stainless steel exhibiting superplasticity. However, other stainless steels exhibiting superplasticity are disclosed in Japanese Patent Application Laid-Open No. 63-206454.
High N-containing austenitic stainless steels have only been discovered in the No. 1, and other stainless steel superplastic materials have not been sufficiently investigated.

<Means for solving the problem> The present inventors have studied a processing method for developing superplasticity in a system different from the above-mentioned ferritic-austenitic duplex steel and high-N content austenitic stainless steel, and
In austenitic stainless steel, a large amount of martensite phase is first generated by cold rolling, and then during or prior to hot working, the martensite phase is reversely transformed into an austenite phase to form a deformation-induced martensite phase and an austenite phase. By adjusting the processing temperature to maintain a fine two-phase structure and hot working by adjusting the processing strain rate within a specified range, high-temperature ductility is dramatically improved and superplasticity is developed. This discovery led to the present invention.

Based on the above findings, the present invention provides a hot working method that solves the conventional problems mentioned above.

<Structure of the Invention> In the present invention, in weight %, C: 0.15% or less, Si:
6.0% or less, Mn: 10.0% or less, Ni:
4.0 to 12.0%, Cr: 10.0 to 19.0%,
Contains N: 0.3% or less, or in addition to the above components, if necessary, No.: 4.0% or less, Cu:
4.0% or less, one or more of Co: 4.0% or less, and Ti, A1. Contains one or more of Nb, V, and Zr, each of which is 1% or less, with a total of 1% or less, and the remainder is Fe and unavoidable impurities, and forms an austenite phase (γ phase) at room temperature after annealing. The steel is cold-rolled to produce 60% or more of the deformation-induced martensitic phase, and then the As point (temperature at which transformation from martensitic phase to austenite phase starts) +30
By heating to a temperature range from ℃ to Af point (temperature at which the transformation from martensitic phase to austenite phase ends) and below 900 ℃, a uniform fine two-phase structure of martensite phase and austenite phase is formed, and lXl0' ″5/
Provided is a method for hot working an austenitic stainless steel having excellent high-temperature ductility, which is characterized by deforming at a strain rate of not less than sec and not more than 1X1/sec.

Furthermore, the present invention cold-rolls the above steel to generate 60% or more of the strain-induced martensitic phase, and then increases the As point to +3.
After preheating at 0°C or above and below the Af point to create a uniform fine two-phase structure of martensitic phase and austenite phase,
Heating to a temperature range from the As point to the Af point and below 900°C, I
Provided is a method for hot working an austenitic stainless steel having excellent high-temperature ductility and characterized by deforming at a strain rate of 0""/see or less.

The reasons for limiting the steel composition and processing conditions in the present invention will be described below.

First, the reasons for limiting the steel composition are as follows.

C is an austenite-forming element and is an effective element for obtaining an austenite phase at room temperature after annealing. Also, C is A
It acts effectively on stabilizing the reversely transformed austenite phase produced by heat treatment in the temperature range from the s point +30°C to the Af point, and also works effectively on strengthening the reversely transformed austenite phase and the martensite phase. However, since C is an austenite-stabilizing element, if it is contained in a large amount, martensite phase treatment by cold rolling becomes difficult.
Further, since Cr carbide is generated during hot working to develop superplasticity and deteriorates corrosion resistance, the upper limit is set to 0.15%.

Si is an essential element in the steel of the present invention that promotes the formation of martensitic phase during cold rolling. Furthermore, Si is an element that is effective in widening the hot working temperature at which superplasticity is developed and in strengthening the reversely transformed austenite phase and martensite phase after heat treatment. However, if it is contained in a large amount, hot workability deteriorates, so the upper limit is set to 6.0%.

Mn is an austenite-forming element and is an effective element for obtaining an austenite phase at room temperature after annealing. Also Mn
acts effectively on stabilizing the reversely transformed austenite phase produced by heat treatment in the temperature range from the As point +30°C to the Af point. However, since Mn is an austenite-stabilizing element, if it is contained in a large amount, martensite phase treatment by cold rolling becomes difficult.
The upper limit is set at 10.0% because manufacturing becomes difficult due to the formation of n-huum.

Ni is an austenite-forming element, and is an essential element to obtain an austenite phase at room temperature after annealing.
It effectively acts to stabilize the reversely transformed austenite phase produced by heat treatment in the temperature range from the As point +30°C to the Af point. If the content is less than 4.0%, this effect will be insufficient. However, since Ni is an austenite stabilizing element, if it is contained in a large amount, martensite phase treatment by cold rolling becomes difficult, and even if hot working is performed, superplasticity will not be developed, so the upper limit is set to 12. Set to 0%.

Cr is a basic component of stainless steel, and must be contained in an amount of 10.0% or more to obtain good corrosion resistance.

However, since Cr is a ferrite-forming element,
If it is contained in a large amount, a large amount of δ ferrite phase will remain at room temperature after annealing, making it impossible to obtain the desired austenite phase structure, so the upper limit is set at 19.0%.

N is an austenite-forming element similar to C.

It is an effective element for obtaining an austenite phase at room temperature after annealing. In addition, N effectively acts to stabilize the reversely transformed austenite phase produced by heat treatment in the temperature range from As point +30°C to Af point, and also acts effectively to strengthen the reversely transformed austenite phase and martensite phase. do. However, since N is an austenite stabilizing element,
If it is contained in a large amount, martensite phase treatment by cold rolling becomes difficult, and blowholes are generated during melting.
Since it becomes impossible to obtain a healthy steel ingot, the upper limit is set at 0.3%.
shall be.

Mo is an element that improves corrosion resistance and is effective in strengthening the reversely transformed austenite phase and martensite phase after heat treatment. However, since Mo is a ferrite-forming element, if it is contained in a large amount, a large amount of δ ferrite phase will be generated, making it impossible to obtain the desired austenite phase structure at room temperature after annealing, so the upper limit is set to 4.0%.

Cu is an austenite-forming element like Ni, and is an effective element for obtaining an austenite phase at room temperature after annealing. Although Cu effectively acts on stabilization, if a large amount of Cu is contained, hot workability deteriorates, so the upper limit is set to 4.0%.

Like Ni, Co is an austenite-forming element, and is an effective element for obtaining an austenite phase at room temperature after annealing, as well as a reverse-transformed austenite phase produced by heat treatment in a temperature range from As point +30°C to Af point. However, since it is expensive, if it is contained in a large amount, the product price will increase, so the upper limit is set at 4.0%.

Ti, AI, Nb, V and Zr form carbides,
It suppresses the growth of crystal grains at hot working temperatures, contributes to refinement, and develops more stable superplasticity.

It is also an effective element for suppressing the formation of Cr carbides and maintaining corrosion resistance. However, if it is contained in a large amount, δ ferrite phase will be generated and hot rolling properties will deteriorate.
The upper limit for each is 1.0%, and the total amount is 1%.
The following shall apply.

Next, processing conditions for the above steel will be explained.

In the present invention, the martensite phase is removed by cold rolling to 60%
% or more, hot working is performed in a temperature range from the AS point +30°C to the ^f point and 900°C or less.

Alternatively, after the above-mentioned cold rolling, As point +30°C or more ~ Af
Preliminary heat treatment is performed at a temperature of point A
Hot working is carried out at a temperature below the f point and below 900°C. In both cases, the processing strain rate is 1x10""/se.
e or more to I x 1/sec or less. The reasons for limiting the above processing conditions are as follows.

Steel is manufactured using the usual ingot method or continuous casting method. The obtained steel ingot is forged and hot rolled, and then cold rolling and annealing are repeated one or more times to obtain a steel with an austenitic structure in the annealed state, and then cold rolling is performed to reduce the martensitic phase to 60%. The above is produced and used as a superplastic material. A when the amount of martensitic phase is 60% or less
A stable two-phase structure of martensitic and austenite phases cannot be obtained even if heated to a temperature range from the s point +30°C to the Af point, and therefore even if hot worked in the above temperature range, or After preheat treatment in the temperature range A
It does not exhibit superplasticity even when processed in a temperature range from the S point to the Af point and below 900°C.

The higher the cold rolling rate, the greater the amount of martensite phase produced, which makes it possible to maintain a fine two-phase structure of martensite phase and austenite phase during hot working, and to obtain good superplasticity. .

However, if the cold rolling rate is too high, the rolling load will be high and production will be difficult, such as edge breakage.

It is desirable that the cold rolling rate be 90% or less.

Processing that does not cause deformation (A point +30°C or higher, Af point or lower and 900°C or lower): The feature of the present invention is that it maintains a fine two-phase structure of martensite phase and austenite phase during hot working. Expresses plasticity. Therefore, during hot working, a stable ultrafine two-phase structure of martensitic phase and austenite phase is formed, and it is necessary to maintain the working temperature. The processing temperature is 30°C above the As point to ensure sufficient transformation from martensitic phase to austenite phase.
The lower limit is a moderately high temperature. Below the As point +30°C, the amount of reversely transformed austenite phase produced is small;
When the processing temperature exceeds the Af point or 900°C, most of the martensite phase transforms into the austenite phase, resulting in reverse transformation γ.
This results in a single phase, or the residual amount of martensite phase becomes too small to maintain an appropriate two-phase structure of martensite phase and austenite phase. Therefore, the hot working temperature is set to be from the As point +30°C to the Af point and 900°C or less.

By holding the steel in the above temperature range in advance immediately before hot working, the martensite phase undergoes reverse transformation to the austenite phase, forming a fine two-phase structure. Here, the holding time is set to the processing temperature lower limit (
In the low temperature range (As point + 30℃), the time is increased to about 10 to 30 minutes, and in the high temperature area near the upper limit of processing temperature (900'C or Af point), the short time is 1 full culm degree. It becomes easier to maintain a fine two-phase structure of martensite phase and austenite phase.

Strain rate during hot working (I x 10-'/5ec-I
X10-”/5ec) : Strain rate is IX1/s
If the strain rate is larger than ec, large deformation due to superplasticity cannot be obtained, and on the other hand, if the strain rate is smaller than I
It is set at Xl0-5/sec to IX10-5/sec.

As described above, excellent high-temperature ductility is exhibited by performing the above-mentioned hot working subsequent to cold rolling.

In addition, if cold rolling is followed by preheat treatment to sufficiently transform the full tensile phase 1 to austenite phase, then hot working can be performed to transform the finer martensite phase and austenite phase at room temperature. A two-phase structure can be obtained, and high-temperature ductility can be further improved. The heat treatment temperature in this case is as follows.

Preliminary heat treatment temperature (As point +30°C or more to Af point or less) 2. Preliminary heat treatment temperature is 30°C from As point to ensure sufficient transformation from martensite phase to austenite phase.
The lower limit is ℃ higher temperature. Preheat treatment temperature is As point +3
At temperatures below 0°C, the amount of reversely transformed austenite phase produced is small; on the other hand, when the preheat treatment temperature exceeds the Af point, most of the martensite phase transforms into an austenite phase, becoming a reversely transformed γ single phase, or martensite. The residual amount of the phase becomes too small, making it impossible to maintain an appropriate two-phase structure of martensite and austenite phases. Therefore, the preliminary heat treatment temperature is set from the As point +30°C to the Af point. The time for preliminary heat treatment is the lower limit (As point + 30°C)
It is preferable to carry out the process for a long time in a low temperature range near the upper limit, and for a short time in a high temperature range near the upper limit (Af point).

Thermal processing degree of preparation (above the As point to below the Af point and below 900°C): Since a two-phase structure of fine martensite and austenite phases has already been obtained through preliminary heat treatment, the lower limit of the processing temperature is set at the As point. There is no need to raise the temperature by 30°C, and the As point is sufficient. On the other hand, if the processing temperature exceeds the Af point or 900° C., an appropriate two-phase structure of martensite and austenite phases cannot be maintained as described above.

Therefore, the hot working temperature is above the As point and below the Af point and 90°C.
It is set at 0℃ or below.

<Specific disclosure of the invention> Examples of the present invention will be shown together with comparative examples.

Steel with the composition shown in Table 1 is melted by a normal method, forged and hot rolled to a thickness of 3+un, then cold rolled and annealed, and then processed to a predetermined rolling rate to a finished plate thickness of 1m111.
The sample material was prepared by cold rolling.

This sample material was subjected to hot working under various conditions shown in Table 2, and then a hot tensile test was conducted to measure the elongation. The results are also shown in Table 2.

As is clear from the results shown in Table 2, it can be seen that the steel types and working conditions of the examples according to the present invention all exhibited extremely large elongations of 300% or more and exhibited superplasticity.

On the other hand, in Table 2, all of the comparative examples outside the scope of the present invention had an elongation of 100% or less and did not have superplasticity.

<Effects of the Invention> According to the method of the present invention, excellent high-temperature ductility is exhibited in austenitic stainless steel, a steel type that has not been considered in the past, and it is possible to form complex shapes that have been difficult in the past. It therefore has significant industrial value.

Claims (4)

[Claims] (1) In weight% C: 0.15% or less Si: 6.0% or less Mn: 10.0% or less Ni: 4.0 to 12.0% Cr: 10.0 to 19.0% N: 0 .3% or less, the balance consists of Fe and unavoidable impurities, and the steel becomes an austenite phase at room temperature after annealing. After cold rolling to generate a deformation-induced martensitic phase of 60% or more, the As point ( The martensitic phase is formed by heating to a temperature range from +30°C (temperature at which transformation starts from martensitic phase to austenite phase) to Af point (temperature at which transformation ends from martensitic phase to austenite phase) and below 900°C. A uniform fine two-phase structure of austenite and
A method for hot working an austenitic stainless steel having excellent high-temperature ductility, characterized by deforming at a strain rate of 1/sec or less. (2) C: 0.15% or less Si: 6.0% or less Mn: 10.0% or less Ni: 4.0 to 12.0% Cr: 10.0 to 19.0% N: 0 .3% or less, further Mo: 4.0% or less, Cu: 4.0%
Hereinafter, one or more of Co: 4.0% or less, and one or two or more of Ti, Al, Nb, V, and Zr each of 1% or less, and the total of these is 1% or less. After cold-rolling the steel, which consists of Fe and unavoidable impurities and becomes an austenite phase at room temperature after annealing, to produce 60% or more of the deformation-induced martensitic phase, the As point +30°C or higher to the Af point is obtained. By heating to a temperature range of 900℃ or less, a uniform fine two-phase structure of martensite phase and austenite phase is formed, and 1×10^-
A method for hot working an austenitic stainless steel having excellent high-temperature ductility, characterized by deforming at a strain rate of ^5/sec or more to 1x10^-^1/sec or less. (3) C: 0.15% or less Si: 6.0% or less Mn: 10.0% or less Ni: 4.0 to 12.0% Cr: 10.0 to 19.0% N: 0 A steel containing .3% or less, the balance consisting of Fe and unavoidable impurities, which becomes an austenite phase at room temperature after annealing, is cold rolled to produce a deformation-induced martensitic phase of 60% or more, and then heated to an As point of +30°C. A uniform fine two-phase structure of a martensite phase and an austenite phase is obtained by preliminary heat treatment at a temperature between the As point and the Af point, and then heated to a temperature range between the As point and the Af point and below 900°C. ^-^
A method for hot working an austenitic stainless steel having excellent high-temperature ductility, characterized by deforming at a strain rate of 5/sec or more to 1×10^-^1/sec or less. (4) C: 0.15% or less Si: 6.0% or less Mn: 10.0% or less Ni: 4.0 to 12.0% Cr: 10.0 to 19.0% N: 0 .3% or less, further Mo: 4.0% or less, Cu: 4.0%
Hereinafter, one or more of Co: 4.0% or less, and one or two or more of Ti, Al, Nb, V, and Zr each of 1% or less, and the total of these is 1% or less. A steel containing Fe and unavoidable impurities with the remainder being Fe and unavoidable impurities, which becomes an austenite phase at room temperature after annealing, is cold rolled to produce 60% or more of a deformation-induced martensitic phase, and then the As point is +30°C or higher and the Af point or lower. After preheating to create a uniform fine two-phase structure of martensite phase and austenite phase, it is heated to a temperature range from the As point to the Af point and below 900°C to form a 1×10^-^1
A method for hot working an austenitic stainless steel having excellent high-temperature ductility, characterized by deforming at a strain rate of 1/sec or more to 1×10^-^1/sec or less.