JPH09125141A - Production of high strength and high corrosion resistant martensitic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high strength and high corrosion resistant martensitic stainless steel excellent in material characteristics such as strength, toughness, weldability or the like without generating flaws such as cracks on the material to be worked at the time of hot working. SOLUTION: A stainless steel contg., by weight, <=0.04% C, <=1.0% Si, <=1.0% Mn, 11 to 14% Cr, 4 to 8% Ni, 0.5 to 4% Mo, 0.0005 to 0.005% B, 0.003 to 0.1% Al, <=0.5% Cu, <=0.04% P, <=0.005% S, <=0.04% N, 0 to 0.02% Ti and 0 to 0.5% Nb, and the balance Fe with inevitable impurities is subjected to hot rolling under the conditions of <=1000 deg.C, >=40% cumulative draft and is thereafter directly quenched to produce the objective high strength and high corrosion resistant martensitic stainless steel.

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 high-strength and high-corrosion-resistant martensitic stainless steel material, more specifically, a steel material such as thick steel plate and shaped steel suitable for structural materials of ships and buildings. The method, which does not cause flaws such as cracks during hot working,
The present invention relates to a method for producing a martensitic stainless steel material having excellent material properties such as elongation, corrosion resistance, and weldability.

[0002]

2. Description of the Related Art Among martensitic stainless steels,
A material having a low C content has a good weldability and is therefore applicable to a welded structure. However, since the Cr content is lower than that of general stainless steel, it has a drawback of being slightly inferior in corrosion resistance. High-corrosion-resistant steel containing Mo has been developed for the purpose of improving the corrosion resistance of martensitic stainless steel. Mo has a function of effectively increasing corrosion resistance in a chloride environment, and is austenitic SUS304 against martensitic stainless steel.
It is also possible to impart corrosion resistance comparable to For example, JP-A-3-188240 discloses C, N, Cr.
Also disclosed is a martensitic stainless steel which has an optimized Ni content and which has Mo and V added, is excellent in corrosion resistance, erosion resistance and weldability and has high strength.

In general, martensitic stainless steel is inferior in hot workability. The reason is that the metal structure of the martensitic stainless steel at the time of hot working is an austenite phase, and depending on the contents of C, Cr, Ni, etc., a small amount of δ ferrite phase exists, and this δ ferrite phase is This is because the inter-workability is significantly reduced. When Mo is added to such martensitic stainless steel for the purpose of enhancing the corrosion resistance, hot workability becomes a problem.

Mo is an element that has a strong tendency to segregate when molten steel solidifies, and also has a function of stabilizing the ferrite phase. Therefore, Mo segregates in the central portion of the steel ingot or slab to which Mo is added, and the δ ferrite phase is likely to remain, resulting in poor hot workability. Even if the product is designed so that δ ferrite does not remain in the product, it is difficult to avoid the influence of δ ferrite on the hot workability in the process of hot working the steel ingot or slab. . In addition, since Mo has the effect of increasing the deformation resistance at high temperatures, it also adversely affects the hot workability from the viewpoint of the deformation resistance. When such a material having poor hot workability is hot worked, for example, a rolled steel sheet may have a crack in the edge and cannot be used as a product.

When the δ ferrite phase remains in the product, the strength and toughness tend to be reduced.

Further, the thickness of the present invention is 6-8.
A steel material such as a steel plate or shaped steel of about 0 mm and a martensitic stainless steel material with a low C content are directly quenched and tempered after hot working, and then a direct quenching / tempering method (DQT method) or heat treatment It is manufactured by a reheating quenching / tempering method (RQT method) in which after hot rolling, once cooled, quenching and tempering are performed. This heat treatment develops the strength of the martensitic stainless steel.
The strengthening mechanism is not precipitation strengthening that occurs during tempering,
It is said to be solid solution strengthening and strengthening by dislocations introduced by working and martensitic transformation.

In the case of the DQT method, quenching is performed in the state where the processed structure remains, so that a fine metal structure can be obtained. Therefore, in general, the DQT method can obtain a product having higher strength than the RQT method. Also, it is more advantageous than the RQT method in terms of manufacturing cost and productivity. However, the toughness and elongation tend to be inferior to those of the RQT method.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has strength, strength, etc. without causing flaws such as cracks in a work material during hot working.
It is an object of the present invention to provide a method for producing a martensitic stainless steel material having excellent material properties such as toughness, elongation, corrosion resistance and weldability.

[0009]

[Means for Solving the Problems] In order to solve the above problems, the present inventors have examined a method for producing a low carbon martensitic stainless steel containing Mo having corrosion resistance equivalent to SUS 304. It was As a result of detailed investigation of the relationship between the hot workability and the trace components of the steel ingots laboratory-made and the relationship between the hot work conditions and the strength characteristics, the following new findings were obtained.

When the C and N contents are low, B
0.0005 to 0.005% by weight (hereinafter,% of chemical composition represents% by weight), hot workability can be improved.

The strength of the steel material of the product can be improved by hot working with a cumulative rolling reduction of 40% or more in the austenite region where recrystallization does not occur and subsequent direct quenching / tempering.

When B is included in the above item, B has a function of suppressing recrystallization during hot working, so that unrecrystallization rolling from a relatively high temperature range is possible. Therefore, B
Is also effective for improving the strength of steel materials. Further, since it is not necessary to lower the finishing temperature during hot rolling for the purpose of obtaining the strength of the steel material, the precipitation of carbides is suppressed and the elongation and the toughness can be prevented from lowering.

By containing 0.005 to 0.02% of Ti, the crystal grains of the steel ingot can be made finer and the hot workability can be improved.

By containing Ti and B in combination, hot workability and toughness can be simultaneously improved.

The present invention has been completed on the basis of the above findings, and the gist thereof is "by weight, C: 0.04% or less, Si: 1.0% or less, Mn: 1.0%. Hereinafter, Cr: 11-14%, Ni: 4-8%, Mo: 0.5-4%, B: 0.0005-0.005%, Al: 0.003-0.1% Cu: 0. 5% or less, P: 0.04% or less, S: 0.005% or less, N: 0.04% or less, Ti: 0 to 0.02%, V: 0 to 0.5%, Nb: 0 to Stainless steel containing 0.5% and the balance being Fe and unavoidable impurities is hot-rolled at a cumulative rolling reduction of 40% or more at 1000 ° C or less in hot rolling, and then directly quenched to obtain high strength and high strength. Manufacturing method of corrosion resistant martensitic stainless steel. "

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The martensitic stainless steel (hereinafter simply referred to as the present stainless steel) used in the manufacturing method of the present invention and the manufacturing method of the present invention will be specifically described below.

(A) Chemical composition of the present stainless steel The C: C content is 0.04% or less in order to secure weldability and corrosion resistance. If the C content exceeds 0.04%, the weldability and corrosion resistance are impaired. Further, when the C content is less than 0.01%, the yield strength becomes too low. Therefore, in order to obtain a yield strength of 90 kgf / mm ² or more, C
The content is preferably 0.01% or more.

Si, Mn: Si and Mn are elements mainly used for deoxidizing molten steel.

However, if the Si and Mn contents exceed 1%, the workability of the present stainless steel may be impaired, so the respective contents were made 1% or less. When sufficiently deoxidized by another deoxidizing element such as Al, Si and Mn may have a content rate such that they are mixed from the raw materials and the like.

Cr: Cr is an important element for ensuring the corrosion resistance of the present stainless steel, and it is necessary to contain Cr by 11% or more. However, if the Cr content exceeds 14%, the δ ferrite phase is likely to precipitate, and the hot workability and the toughness of the product deteriorate, so the upper limit was made 14%.

Ni: Ni is in the hot working temperature range.
It is an essential element for making the metal structure of this stainless steel into an austenite phase. Therefore, 4% or more is necessary.

However, if the content exceeds 8%, the austenite phase remains at room temperature, which causes the deterioration of strength and toughness. Therefore, the upper limit of the Ni content is set to 8%.

Mo: Mo is an essential element for improving the corrosion resistance of the present stainless steel.

To improve the corrosion resistance, 0.5% or more is necessary. However, if it exceeds 4%, δ ferrite tends to precipitate, and the deformation resistance at high temperature becomes high, so that the hot workability is deteriorated and the strength and toughness of the steel material of the product are lowered. Therefore, the upper limit of the Mo content is 4%.

B: B is an important element that characterizes the present stainless steel, and is essential for improving the hot workability and the toughness of the steel material of the product. In order to exert this effect, it is necessary to contain 0.0005% or more. However, if the B content exceeds 0.005%, hot embrittlement occurs in a high temperature range of 1300 ° C or higher. Therefore, the B content in this stainless steel is 0.000.
The content was 5 to 0.005%.

As mentioned above, in the present stainless steel, the present inventors have the effect that B has a function of suppressing recrystallization during hot working, and enables non-recrystallization working from a relatively high temperature range. It was found that it has the effect of The survey will be described below.

FIG. 1 shows the hot compression processing conditions and the recrystallization rate of 50.
The results of the investigation of the relationship of% are shown. For the test material having a chemical composition within the scope of the present invention (solid line), and for the test material not containing B and having the same chemical composition (dotted line), the holding time before compression processing is plotted on the horizontal axis, and the compression processing temperature 50% on the vertical axis
The recrystallization curve is illustrated. As is clear from FIG. 1, B:
By including 0.0010% (solid line), B:
The temperature in the non-recrystallized region is at least about 50 ° C. higher than in the case of no addition (dotted line). That is, by containing B, recrystallization at the time of hot working is suppressed, and the non-recrystallization temperature range spreads to the high temperature side. Therefore, the processing start temperature at which processing is possible without causing recrystallization is , At least 5
It can be said that the temperature rises by about 0 ° C.

When B is contained, it is feared that the intergranular corrosion resistance of stainless steel may be deteriorated.
According to the test conditions specified in 0577, the pitting corrosion potential was measured and the intergranular corrosion resistance was evaluated. In the test, a test material manufactured under the conditions of the manufacturing method of the present invention was used.

FIG. 2 shows the relationship between the B content and the pitting corrosion potential. It is clear that when the B content is 0.0005% or more, the pitting potential is slightly higher than that in the case where B is not contained, and there is no problem in the pitting corrosion resistance, that is, the intergranular corrosion resistance.

Thus, B in the present stainless steel
Is extremely effective in improving the strength of the steel material obtained by the method of the present invention by combining the controlled rolling in the non-recrystallization temperature range and the direct quenching. Further, since the hot workability is improved, it is also effective for preventing the occurrence of cracks in the edges of the hot rolled steel sheet, which has been a problem in the past.

Al: Al is an element having a very high deoxidizing power of molten steel, and is usually added together with Si and Mn. In the case of this stainless steel, in order to obtain the deoxidizing effect, 0.0
03% or more is required. By deoxidation with Al,
Oxide inclusions in this stainless steel are reduced and high toughness is obtained. On the other hand, if the Al content exceeds 0.1%, the hot workability of the present stainless steel may be deteriorated. Therefore, the upper limit of the Al content is 0.1%.

Cu: Cu is an element that is added as necessary in the present stainless steel in order to increase the strength by precipitation hardening. However, the Cu content is 0.5%
When it exceeds, the hot workability is deteriorated. Therefore, C
When u was contained, it was set to 0.5% or less.

P: P is an element that is inevitably mixed from raw materials and the like, and is harmful to the toughness and corrosion resistance of the present stainless steel. Therefore, it was decided to limit the content to 0.04% or less, which is the range in which the effect does not appear.

S: S, like P, is an element that is inevitably mixed from raw materials and the like, and deteriorates the hot workability and toughness of the present stainless steel. It is preferable that the S content is low, and the S content is limited to 0.005% or less.

If the N: N content is high, the weldability and corrosion resistance of the present stainless steel tend to be impaired. Therefore, the N content is set to 0.04% or less. N is an element that is inevitably mixed in by a normal manufacturing method, and the lower limit is not particularly specified.

Ti: Ti has the function of enhancing the hot workability and toughness of the present stainless steel, and is an element added as necessary. Ti refines the crystal grains of the steel ingot or slab, and when used in combination with B, strengthens the grain boundaries, so that hot workability and toughness can be improved. In order to exert this effect, 0.005% or more is required. However, when it is excessive, coarse TiN is precipitated and the toughness is lowered. Therefore, when adding Ti, 0.005 to 0.
The range of 02% is preferable.

Nb, V: Nb and V are elements that are added alone or at the same time, if necessary, in order to improve the strength and corrosion resistance of the present stainless steel. To achieve that effect, 0.01%
The above is necessary. On the other hand, if it exceeds 0.5%, the toughness decreases. Therefore, when Nb and V are contained, the content is set to 0.01 to 0.5% in either case, and either one or both are added.

(B) Hot Working Conditions and Quenching Conditions In the method for producing a martensitic stainless steel material of the present invention, the present stainless steel having the above chemical composition is subjected to appropriate hot working conditions and appropriate heat treatment conditions. By combining them, it is characterized by producing a high-strength steel material with good corrosion resistance based on excellent hot workability.

In the present invention, it is essential to improve the strength of the steel material by directly quenching the steel material without causing recrystallization from the worked structure obtained by the strong reduction in the non-recrystallization temperature region. Is the condition. In the case of manufacturing a thick steel plate by the manufacturing method of the present invention, thick plate rough bar rolling is suitable, and in this rolling, the time between rolling passes is about 10 to 30 seconds in the reverse type. As is clear from FIG. 1 described above, if this time is required, 10
In rolling in a temperature range exceeding 00 ° C, the work structure recrystallizes by the next rolling pass. Therefore, in order to finish the rolling in the state where the worked structure remains, it is important to include the adjusted rolling in which rolling is performed at 1000 ° C. or less and the working strain is left. To improve the strength of the steel material of the product by entering the cooling (quenching) step with a sufficient working strain left so that the cumulative rolling reduction in the adjusted rolling at 1000 ° C. or less becomes at least 40%. Is possible.

The lower limit of the temperature of the controlled rolling is not particularly limited, but it is preferably about 800 ° C. in order to prevent the elongation and the toughness from decreasing due to the increase of precipitates. Further, the upper limit of the cumulative rolling reduction in the controlled rolling is not particularly limited, but is limited by the relationship between the hot-rolled material and the thickness or cross-sectional area of the steel material of the product.

[0041]

EXAMPLES The production method of the present invention will be specifically described based on Examples.

Table 1 shows the chemical composition of the test materials used in the test. The test material was produced by melting in a high-frequency induction vacuum melting furnace and casting into a flat steel ingot having a thickness of 48 mm and a width of 190 mm. Note that the test materials A to E are examples of the present invention, and F to I are comparative examples.

[0043]

[Table 1]

Material properties such as hot workability, product strength, and corrosion resistance of the above test materials were investigated. The test method is
It is as follows.

1. Hot workability A smooth round bar tensile test piece having a diameter of 10 mm and a length of 130 mm was sampled from a steel ingot as a test material and subjected to a hot tensile test. In the hot tensile test, first, the test piece was heated to 1200 ° C. and held for 5 minutes by the direct current heating method. Next, 100 ° C /
It was cooled to the test temperature (1000 ° C.) at a speed of minutes and fractured under the conditions of a strain rate of 1 / sec and a crosshead speed of 7 mm / sec. With respect to the test piece after breaking, the cross-sectional shrinkage rate was measured and the hot workability was evaluated.

2. Hot workability and material properties A material for hot rolling having a thickness of 85 mm, a width of 100 mm and a length of 150 mm was sampled from a steel ingot as a test material. This material 1
After heating to 200 ° C. and holding for 1 hour, rolling was started from a temperature of 1100 ° C. or higher. After that, the rolling reduction is set to be 25 to 60% from the time when the temperature of the material in the middle of rolling falls to 1000 ° C. to the end of rolling, and 6
Conditioned rolling was performed by hot rolling to a thickness of 20 mm in a single pass. After hot rolling, by cooling the rolled plate from 800 ° C or higher to 100 ° C by spray water cooling,
Quenching (direct quenching) was performed. In the quenched state,
The hot workability was evaluated by investigating the defects generated in the rolled material and measuring the maximum edge crack depth. Next, the tempered rolled material was subjected to a tempering treatment under the condition of holding at a temperature of 550 ° C. for 30 minutes. Various test pieces were sampled from the direct-quenched / tempered material and subjected to respective tests. For evaluation of toughness, a No. 4 Charpy test piece specified in JIS Z2202 was sampled from the plate width direction (notch direction is rolling direction), and impact absorption energy (unit: J) was measured at -50 ° C. 14A specified in JIS Z2201 for evaluation of properties such as tensile strength
The No. 10 tensile test piece was sampled from the width direction of the rolled material, and 0.2% proof stress and tensile strength (unit: MPa) were measured at room temperature.

3. Corrosion resistance From the above direct-quenched and tempered materials, pitting corrosion test pieces were taken for evaluation of corrosion resistance. The measurement surface of the pitting corrosion test was a surface perpendicular to the rolling direction, and the pitting corrosion potential (unit: mV) at a current of 100 μA / cm ² was measured under the test conditions specified in JIS G0577.

As a comparative example, hot rolling was performed by heating to 1200 ° C., the obtained rolled plate was once cooled to room temperature, then quenched (holding at 1050 ° C. for 30 minutes and then water cooling) and tempered (550 ° C.). R for 30 minutes
The same test was performed on the test material heat-treated by the QT method.

Table 2 shows the results of these tests.

[0050]

[Table 2]

As is apparent from Table 2, the test composition Nos. In which the chemical composition of the test material, the conditions of hot rolling and the conditions of heat treatment are within the scope of the present invention. In Examples 1 to 6 of the present invention, products having high values of yield strength, tensile strength, absorbed energy and pitting corrosion potential, and having sufficient strength and material properties such as toughness and corrosion resistance were obtained. In particular, the absorbed energy of the test No. of the comparative example which was heat-treated by the RQT method, although the heat-treatment of the present invention was the DQT method. The values of 14 to 18 were obtained, and it was confirmed that the toughness comparable to that of the RQT method was obtained. In addition, the cross-sectional shrinkage rate of the tensile test at 1000 ° C. of the test piece collected from the steel ingot was 77%.
The above results are high, and the edge cracks of the rolled sheet after hot rolling are hardly observed, so that the hot workability of the test material is also good.

On the other hand, the test No. In Comparative Examples 7 to 18, at least one characteristic of strength, toughness, corrosion resistance or hot workability of the product was inferior to the inventive examples. Test No. B content is lower than the range of the present invention. In Nos. 7 and 8, the absorbed energy is low and the toughness is inferior, and the ear cracks are generated and the hot workability is poor. Test No. B content is too high. 10
Has poor toughness and hot workability.

In addition, in the test No. 1 in which the C content was too high. Sample No. 9 was inferior in toughness, corrosion resistance, and hot workability. Further, in the case where the cumulative rolling reduction at a temperature of 1050 ° C. or lower is 40%, the adjusted rolling start temperature is too high. 11 and test Nos. Having a cumulative rolling reduction of less than 40% at 1000 ° C. or less. For 12 and 13,
Since the condition of the controlled rolling is out of the range of the present invention, the material properties such as strength and toughness of the product were poor. Test No. heat-treated by the RQT method. For Test Nos. 14 to 18, test No. The yield strength and tensile strength are about 10% lower than those of Examples 1 to 5 of the present invention. This is because the comparative example does not have the effect of the present invention due to the combination of the large strain accumulated by processing in the non-recrystallized region and direct quenching.
As described above, it was confirmed that the combination of working strain and direct quenching is very effective in increasing the strength of martensitic stainless steel.

As is clear from the above examples, according to the method for producing a martensitic stainless steel containing B of the present invention, strength and toughness can be obtained without causing flaws such as cracks during hot working. It is possible to obtain a steel material that is a product having excellent material properties such as corrosion resistance. Further, the steel material obtained by the method of the present invention has a low C content, and therefore has excellent weldability.

[0055]

According to the manufacturing method of the present invention, it is possible to manufacture a high-strength and high-corrosion-resistant martensitic stainless steel material without causing flaws such as edge cracks. In addition, the combination of hot working and direct quenching can simplify the process and improve the yield, resulting in improved productivity or reduced manufacturing cost. As described above, the effect of the production method of the present invention on the commercial production of martensitic stainless steel material is extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing an effect of B on a relationship between a holding time in hot working, a working temperature, and recrystallization.

FIG. 2 shows B in martensitic stainless steel.
It is a figure which shows the relationship between a content rate and a pitting corrosion potential.

Claims (1)

[Claims] 1. By weight%, C: 0.04% or less, Si: 1.0% or less, Mn: 1.0% or less, Cr: 11 to 14%, Ni: 4 to 8%, Mo: 0. 5-4%, B: 0.0005-0.005%, Al: 0.003-0.1% Cu: 0.5% or less, P: 0.04% or less, S: 0.005% or less, N: 0.04% or less, Ti: 0 to 0.02%, V: 0 to 0.5%, Nb: 0 to 0.5%, with the balance being Fe and unavoidable impurities. A method for producing a high-strength and high-corrosion-resistant martensitic stainless steel material, which comprises performing hot working directly after hot working under conditions of a cumulative rolling reduction of 40% or more at 1000 ° C or less in hot working.