JPH05255734A - Production of martensitic stainless steel minimal in cracking sensitivity - Google Patents

Abstract

PURPOSE:To effectively perform dehydrogenation heat treatment and to reduce cracking sensitivity by subjecting a martensitic stainless steel containing specific amounts of C, Si, Mn, Cr, Ni to cooling down to a temp. not higher than the Ms point and then to holding at specific temp. for specific time. CONSTITUTION:A martensitic stainless steel which has a composition containing, by weight, <=0.10% C, <=1.0% Si, <=1.0% Mn, 13.0-20.O% Cr, and 2.0-8.0% Ni and further containing, if necessary, one or more kinds among 1.0-5.0% Cu and <=1.0% each of Mo, Ti, and Al is cooled from a temp. in the austenite temp. region down to a temp. not higher than the Ms point and then subjected to dehydrogenation heat treatment. By this dehydrogenation heat treatment, the above steel is held at a temp. in the range of 400 deg.C to the Ac1 transformation point for >=5hr. By this method, the martensitic stainless steel where hydrogen content in the steel is reduced to about <=2ppm and which is minimal in cracking sensitivity can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a martensitic stainless steel which has low susceptibility to transformation cracks and slab cracks in slabs and products and has excellent workability after solution heat treatment.

[0002]

2. Description of the Related Art Low carbon martensitic stainless steels have high yield strength, impact toughness and weldability. In addition, 17-4PH steel and 15-5PH
Martensitic precipitation-hardening stainless steel represented by steel can obtain higher yield strength by being subjected to aging heat treatment. Therefore, these steels are mainly used as high strength structural members.

However, low-carbon martensitic stainless steel is transformed into a martensitic phase without precipitation of a ferrite phase when cooled from an austenitic state in a high temperature range, and therefore has a very high cracking susceptibility due to transformation. Furthermore,
If the hydrogen content is high, the susceptibility to transformation cracking increases and delayed fracture such as in-place cracking occurs. This tendency is found in forged products and thick plate products with large product sizes. From this, it is desirable that the hydrogen content be 2 ppm or less for a thick plate having a large product size, particularly for a thick material having a plate thickness of more than 30 mm.

In the case of ordinary steel which is transformed into a ferrite phase in the cooling process from an austenite state in a high temperature range, it is transformed into a ferrite phase having a low hydrogen solid solution amount and then kept at this temperature for a long time. Dehydrogenate. However, the martensitic stainless steel that does not transform into a ferrite phase does not have an efficient dehydrogenation method by heat treatment, and the amount of hydrogen has been reduced as much as possible in the vacuum degassing process during melting.

[0005]

However, it is difficult to reduce the hydrogen content to 2 ppm or less by vacuum degassing, and it is impossible to reduce the hydrogen content to 1 ppm. Therefore, transformation cracks and placement cracks frequently occur in the subsequent manufacturing process, the product yield is significantly reduced, and the manufacturing cost is increased. In particular,
It is the current situation that wide thick plates with a thickness of 30 mm or more cannot be manufactured because severe cracks occur during the manufacturing process. Therefore, development of an efficient dehydrogenation heat treatment method for martensitic stainless steel that does not undergo ferrite transformation is desired. The present invention seeks to satisfy this need, and
It is an object of the present invention to provide a method for producing a martensitic stainless steel having a small crack susceptibility by performing a specific heat treatment on the cooled steel to a temperature below the point to perform dehydrogenation.

[0006]

Means for Solving the Problems The present inventors have noted that the amount of hydrogen solid solution of martensitic stainless steel that does not undergo ferrite transformation is extremely low in the martensitic phase,
Aiming to reduce the amount of C to the range where cracking does not occur even if heat-treated in the martensite state, and to heat-treat in the martensite state in a temperature range with good hydrogen diffusion efficiency,
The invention has been accomplished.

FIG. 1 shows that after cooling from the austenite temperature range to room temperature and carrying out martensitic transformation, it is 350 ° C. to 75 ° C.
The change of the amount of hydrogen in steel when it hold | maintained for 10 hours in the temperature range of 0 degreeC is shown. That is, as can be seen from the figure, 400 ° C to A
In the temperature range of the c ₁ transformation point, the amount of hydrogen decreases greatly,
It can be seen that it is below ppm. On the other hand, if the temperature is maintained at a temperature not lower than the Ac ₁ transformation point, reverse transformation of martensite to austenite occurs, and the amount of hydrogen in the steel that can be solid-solved increases.

FIG. 2 shows the change in the hydrogen content in steel when the steel is held at 500 ° C. for 2 to 20 hours after the martensitic transformation as in FIG. The amount of hydrogen in the steel decreases as the holding time increases, and it becomes 2 ppm or less after holding for 5 hours or more. 1 more
If it is held for more than 0 hours, it will be less than 1 ppm.

From the above knowledge, the present invention is extremely superior to conventional ones by heat-treating various products such as a slab obtained in a usual manufacturing process or a steel plate or a shaped steel obtained by further hot working. It was difficult to reduce the amount of hydrogen in steel to 2
It became easy to make it below ppm, and the productivity was dramatically improved.

That is, according to the present invention, C: 0.1% or less, Si: 1.0% or less, Mn: 1.0% or less, Cr:
13.0 to 20.0%, Ni: 2.0 to 8.0% and C
One or more of u, Mo, Ti and Al, Cu is 1.0 to 5.0%, and Mo, Ti and Al are 1.
A martensitic precipitation hardening stainless steel containing 0% or less, which is cooled from the austenite state to the Ms point or lower, is heated to a temperature range of 400 to Ac ₁ transformation point and is held for 5 hours or more. And As a result, the hydrogen content in steel can be reduced to 1 ppm or less, and the mechanical properties are greatly improved.

Next, the reasons for limitation of the present invention will be described below. C effectively acts as an element that suppresses the formation of δ ferrite and improves hot workability. However, if it exceeds 0.10% by weight, impact toughness is remarkably lowered and cracking occurs when the heat treatment of the present invention is applied, so 0.10% by weight was made the upper limit. Si is an element effective in improving strength. However, the upper limit is 1.0 from the viewpoint of weldability or impact toughness.
It was set to% by weight. Mn is an element effective in improving workability. However, if it is added in an amount of more than 1.0% by weight, the austenite phase is stabilized and the amount of retained austenite after the aging treatment increases, so that the strength decreases. Therefore, the upper limit is set to 1.0% by weight.

The lower limit of Cr is 13.0% by weight, which is the minimum necessary for obtaining the corrosion resistance required for stainless steel.
However, if it exceeds 20.0% by weight, the σ phase is likely to be generated, and the impact toughness is remarkably reduced, so 20.0% by weight is set as the upper limit. Ni is an essential element for forming an austenite phase at high temperature. Therefore, the lower limit is 2.0% by weight. However, if it exceeds 8.0% by weight, the austenite phase becomes stable and the Ms point becomes room temperature or lower, so that the amount of martensite produced by the solution heat treatment or the aging treatment remarkably decreases, and the required strength is obtained. I can't. Therefore, the upper limit is set to 80% by weight.

Cu is an element added as necessary.
That is, the aging treatment finely precipitates in the martensite phase to increase the strength, and in the case of adding, the minimum necessary amount was 1.0% by weight. However, if it exceeds 5.0% by weight, grain boundary embrittlement at high temperature is promoted, hot workability is impaired and hot cracking becomes sensitive, so the upper limit was made 5.0% by weight. Al, Ti, Mo
Is added, if necessary, at least one kind is added. That is, it is an element that is finely precipitated as a compound in the martensite phase and is effective for increasing the strength of the martensite phase. However, 1.
If it exceeds 0% by weight, the effect is saturated and the cost increases remarkably, so 1.0% by weight was made the upper limit.

In the present invention, the steel containing each of the above elements is cooled from the austenitic state to below the Ms point, and further cooled to 400 ° C.
Retaining for 5 hours or more at a temperature range of to Ac ₁ transformation point. Ms
The cooling below the point is to transform into a martensite phase having a lower solid solubility limit of hydrogen than the austenite phase. The heating temperature after this was set to 400 ° C. or higher as 40
When the temperature is lower than 0 ° C, the diffusion rate of hydrogen is very slow, and it is necessary to heat for an extremely long time. Therefore, the lower limit was set to 400 ° C. However, when heated above the Ac ₁ transformation point, the martensite phase transforms to the austenite phase and the hydrogen solid solution limit in the steel increases, so the upper limit was made the Ac ₁ transformation point. Also, the reason why the heating holding time is set to 5 hours or more is that dehydrogenation cannot be sufficiently performed in less than 5 hours, and the hydrogen content in steel does not fall below 2 ppm. Especially for precipitation hardening stainless steel, 450 ° C to 55 ° C.
It is recommended to hold at 0 ° C for 5 hours or longer. When heated above 450 ° C, precipitates are generated in the martensite phase and the retained austenite phase transforms into the martensite phase, further lowering the solid solubility limit of hydrogen in steel, and when heated above 550 ° C, the retained austenite phase is retained. This is because the phase does not transform into the martensite phase and the dehydrogenation efficiency tends to be low.

[0015]

Example A to E steels having the chemical compositions shown in Table 1 were melted. The cast steel ingot was subjected to soaking diffusion heat treatment and slab rolling, and then subjected to dehydrogenation treatment under the conditions shown in Table 2. After leaving this steel ingot for 1 week, the presence or absence of cracks was confirmed by an ultrasonic flaw detection test. Further, this steel ingot was rolled into a thick plate, subjected to solution heat treatment (1040 ° C. × 30 min), a JIS No. 4 tensile test piece was sampled, and the mechanical properties were investigated. The results are shown in Table 2.

[0016]

[Table 1]

[Table 2] The method of the present invention No. The samples 1 to 6 all had an amount of hydrogen in the steel of 2 ppm or less, and no delayed fracture was observed after hot rolling. In addition, the number of hydrogen in steel is 1ppm or less. Four
The samples of Nos. 7 to 7 show good values of about 15% in elongation at break after solution heat treatment. On the other hand, as a comparison material, No. In each of the samples Nos. 8 to 12, the hydrogen content in steel exceeded 2 ppm and delayed fracture occurred after hot rolling.

From the above examples, according to the present invention, the structure control and heat treatment conditions are closely related to the hydrogen content in the steel, and they work extremely effectively, and the martensitic stainless steel has excellent manufacturability and workability. Has become possible.

[0018]

As described above, according to the present invention, the susceptibility to cracking due to hot working and gas cutting is remarkably reduced and the productivity is dramatically improved. Therefore, the martensite precipitation hardening type stainless steel of Manufacturing yield is greatly improved. In particular, it has become possible to manufacture martensite precipitation hardening stainless steel products having a large product size such as thick plates and large hot forged products, which have been considered extremely difficult to manufacture in the past. In recent years, a demand for a large-sized stainless steel structural member having a high yield strength has been increased, but the industrial effect is extremely large because the present invention can be put to practical use.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a holding temperature and a dehydrogenation amount of martensitic stainless steel.

FIG. 2 is a diagram showing the relationship between the retention time and dehydrogenation of martensitic stainless steel at 500 ° C.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukitoshi Kaku 1-1 Hibahata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works

Claims (2)

[Claims] 1. C: 0.10% by weight or less, Si: 1.0, which is cooled from the austenite temperature range to a temperature below the Ms point.
Weight% or less, Mn: 1.0 weight% or less, Cr: 13.0
To 20.0 wt% and Ni: 2.0 to 8.0 wt% martensitic stainless steel is retained for 5 hours or more in the temperature range of 400 ° C. to Ac ₁ transformation point. Of small martensitic stainless steel. 2. C: 0.10 wt% or less, Si: 1.0, which is cooled from the austenite temperature range to a temperature below the Ms point.
Weight% or less, Mn: 1.0 weight% or less, Cr: 13.0
~ 20.0 wt%, Ni: 2.0-8.0 wt%, C
Martensite precipitation hardening type that contains one or more of u, Mo, Ti and Al and contains 1.0 to 5.0% of Cu and 1.0 wt% or less of Mo, Ti and Al, respectively. A method for producing martensitic stainless steel having low cracking susceptibility, which comprises holding the stainless steel in a temperature range of 400 ° C. to Ac ₁ transformation point for 5 hours or more.