JP4792769B2 - Ferritic stainless steel with excellent corrosion resistance and ductility and method for producing the same - Google Patents

Description

The present invention is used as a material for various parts and structural parts that require excellent corrosion resistance and ductility, for example, various bolts, joints such as various sensors, home appliance parts, machine parts, and structural members. Ferritic stainless steel, especially austenitic stainless steel even if it has a relatively thick cross section such as a round bar with a diameter of 20 mm or more, a rolled plate with a thickness of 20 mm or more, a flat bar, etc. The present invention relates to a highly corrosion-resistant ferritic stainless steel capable of obtaining high performance using a solution heat treatment equipment.

Compared with austenitic stainless steel represented by SUS304, ferritic stainless steel is generally less susceptible to stress corrosion cracking and is inexpensive because it does not contain a large amount of Ni. Widely used in the field.

In such a ferritic stainless steel, the improvement in corrosion resistance is largely due to the increased amount of Cr and Mo. However, increasing the amount of Cr and Mo improves the corrosion resistance, but makes the steel material brittle and lowers the ductility.

Therefore, it is not enough to increase the amount of Cr and Mo in order to obtain both ferritic stainless steel with excellent corrosion resistance and ductility, and the amount of C and N dissolved in the steel is reduced. In addition, the reduction of ductility due to the increase in Cr and Mo is suppressed by making the crystal grains finer.

For example, in Patent Document 1, both the corrosion resistance and press formability are improved by defining an average r value and adding Ti or Nb or both at least 8 times C + N. This utilizes the action of Ti and Nb easily combining with C and N to substantially reduce C and N dissolved in the steel. As a result, the press formability is enhanced and the intergranular corrosion resistance of the weld heat affected zone is enhanced.

Also, in Patent Document 2, ferritic stainless steel usually produced by continuous casting-hot rolling-annealing-cold rolling-finish annealing is produced by continuous casting-hot finishing rolling-annealing-pickling. It is. In normal processes, processes after cold rolling play an important role in improving workability and surface properties. In order to omit the process, in Patent Document 2, C + N + O + S is limited to 0.0100% by weight or less, hot rolling conditions are specified, and annealing conditions of 600 ° C. to 1000 ° C. are specified to ensure excellent corrosion resistance. However, necessary workability is imparted.

JP 2002-363711 A JP-A-7-268485

By the way, in general general-purpose stainless steel, austenitic stainless steel represented by SUS304 is most frequently used. In order to prevent sensitization, this austenitic stainless steel is usually heated to 1010 to 1150 ° C. and then subjected to a heat treatment for water cooling. A heat treatment furnace corresponding to the heat treatment is generally widely used, and there are many excellent products such as a highly productive facility and a facility for reducing bending during water cooling.

However, if water cooling of 1010 ° C to 1150 ° C, which is a standard heat treatment of SUS304, is applied to ferritic stainless steel containing about Mo and containing about 18% Cr, the amount of dissolved C and N increases. Adversely affects the ductility of the material. Specifically, in a product having a thin cross section such as a thin plate with a thickness of several millimeters or less, the ductility decrease is small, and even if it is cooled with water from a temperature exceeding 1000 ° C., the large ductility decrease does not occur. In the case of a round bar having a thickness of 20 mm or more, a rolled plate having a thickness of 20 mm or more, a flat bar, and the like, if heat treatment is performed under such conditions that solute C and N increase, the ductility is greatly reduced.

Therefore, among ferritic stainless steels containing Mo with a Cr content of about 18%, in steel bars, rolled plates, flat bars, etc. that have a relatively thick cross-section that are expected to suffer from ductility reduction, Despite the fact that conditions including the case of rapid cooling from 1000 ° C or higher, such as rapid cooling at 1050 ° C, are specified, heat treatment is performed at 700 ° C to 900 ° C to reduce the amount of dissolved C and N, considering ductility. Is generally applied.

Also, with regard to the technique described in the above-mentioned patent document, there is no clear description in Patent Document 1 regarding the heat treatment for improving ductility, and Patent Document 2 also describes the numerical values described in the table. Based on the explanation, it is recommended that a heat treatment of about 800 ° C be used, and whether heat treatment at a temperature of 1000 ° C or higher can be used without reducing ductility has been studied at all. Absent.

Therefore, the existing heat treatment furnace used for SUS304 austenitic stainless steel cannot often be used in processing processes such as drawing and forging that handle thick materials, and the price, corrosion resistance, stress corrosion cracking resistance, etc. It was one of the factors that hindered the use of ferritic stainless steel with excellent characteristics.

  The present invention has been made for the purpose of solving the above-mentioned problems, and it is more preferable to use SUS304 or the like for ferritic stainless steel comprising a steel bar or a rolled plate having a relatively thick cross section as described above. Compared with annealing at 700-900 ° C, which is a heat treatment condition that has been performed in the past with concern about deterioration of ductility, even if heat treatment of 1010 ° C-1150 ° C followed by water cooling is performed, similar to the stainless steel Thus, the corrosion resistance can be improved without lowering the ductility. Thereby, since the same heat treatment equipment as SUS304 can be used, more efficient production becomes possible, and a very advantageous ferritic stainless steel and a manufacturing method thereof can be provided.

In order to solve the above-mentioned problems, the invention of claim 1 of the present invention is the mass%, C: 0.0200% or less, Si: 0.70 to 1.00%, Mn: 0.50% or less, P: 0.040% or less, S : 0.025% or less, Cu: 0.60% or less, Ni: 0.60% or less, Cr: 17.50 to 20.00%, Mo: 0.50 to 1.30%, N: 0.0200% or less, Nb: 0.56% or less, O: 0.0100% or less, diffusion Reactive hydrogen: 0.30ppm or less, and (C + N) / Nb : 0.085 or less (C, N, Nb means the respective component ratios (mass%)) , the balance consists of Fe and inevitable impurities It is a ferritic stainless steel excellent in corrosion resistance and ductility, characterized by being heat treated at a high temperature (1010 to 1150 ° C.) and cooled with water.

Further, the invention according to claim 2 is a solution heat treatment (water cooling after heating at 1010 to 1150 ° C.) that is normally performed on austenitic stainless steel for the ferritic stainless steel according to claim 1. In the manufacturing method, heat treatment is performed under similar conditions.
The above-described invention has been completed by the inventors of the present invention in detail investigating the influence on ductility when the heat treatment temperature and the addition amount of various elements are changed, and obtaining the following knowledge.

 (1) Although reductive refining is naturally performed during steelmaking, the Cr oxide present in the steel is not completely reduced in the actual operation of ferritic stainless steel with a Cr content of about 18%. In many cases, the Cr oxide remains as it is. As a result, when the Cr oxide is dispersed, it causes embrittlement and becomes a major factor in reducing ductility. As a result of detailed investigations on the influence of components other than Cr on the residual amount of Cr oxide, the present inventors have found that when Si is 0.70 to 1.00% and O is limited to 0.0100% or less, Cr oxide It was found that the dispersion of steel in steel can be suppressed to a low level and ductility can be prevented from decreasing.

 (2) As the heat treatment temperature is increased, the solid solution amount of C and N increases, and the value indicating ductility such as the drawing value decreases greatly at a certain temperature. As a result of investigating the influence in detail, it was found that the influence of the amount of diffusible hydrogen is very large.

This diffusible hydrogen is contained without any particular intention by refining performed naturally during steelmaking. And as the cross-sectional dimension after processing such as hot rolling becomes larger, the hydrogen contained in the steel becomes harder to be discharged to the outside, and when the thickness, width and diameter of the material is at least 20 mm or more, the same heat treatment as SUS304 When subjected to (water cooling after heating at 1010 to 1150 ° C.), an amount of diffusible hydrogen that has a detrimental effect on ductility remains inside the material. The present inventors have strictly controlled the upper limit of the amount of diffusible hydrogen, and when the content exceeds a certain amount, the inventors have found that the boundary temperature is greatly increased by strictly regulating the upper limit by dehydrogenation treatment. It is a thing.

Especially when heating to 700-900 ° C and performing air-cooling as in the past, there is time to release diffusible hydrogen during air-cooling, so if steel containing a large amount of diffusible hydrogen is used. However, after the heat treatment, hydrogen is automatically reduced sufficiently, and the amount of C and N solid solution is reduced, so the problem of reduced ductility is hardly a problem. Even when heat treatment is performed by heating to 700 to 900 ° C. and water cooling, the amount of solid solution C + N is reduced in the same manner as described above. Because of the small size, the effect of the amount of diffusible hydrogen on the ductility was not regarded as a problem at all.

In addition, when the thickness or diameter of the material is small, it is difficult to constrain the part that receives the stress to the surrounding part, so it is easy to obtain stress relaxation due to deformation on the material surface, and the distance from the inside of the material to the surface is short. Therefore, diffusible hydrogen is easily released into the atmosphere, and in the thin plate, the influence on the ductility degradation due to the presence of diffusible hydrogen has not been a significant problem.

However, in the present invention, an austenitic stainless steel without causing a problem of reducing ductility of a steel bar having a diameter of 20 mm or more or a ferritic stainless steel having a plate thickness of 20 mm or more and containing 18% Cr and Mo. Since it aims to enable heat treatment in the same solution heat treatment equipment as steel, it is unlikely that hydrogen will be released automatically by heat treatment as in the case of heating to 700 to 900 ° C and air cooling. In addition, since the amount of dissolved C + N is increased, the ductility is greatly reduced as it is. Therefore, in the present invention, after hot rolling, when diffusible hydrogen is contained to such an extent that ductility is a concern, it is heated to a temperature of about 100 to 300 ° C. before or immediately after heat treatment, By removing the diffusible hydrogen, even when the same heat treatment as that of the austenitic stainless steel is performed, the ductility can be greatly improved.

(3) In consideration of the knowledge described in (1) and (2) above, after optimizing the components, Nb is added to further reduce C and N dissolved in the steel, When the amount of C, N, and Nb is adjusted so as to satisfy the condition of (C + N) /Nb≦0.085 (C, N, and Nb mean the respective component ratios (mass%)) , 1010 ° C It was revealed that excellent ductility can be imparted even when the heat treatment is performed under the same conditions as the solution heat treatment in austenitic stainless steel, which is heated to ˜1150 ° C. and then water-cooled.

Next, the reasons for limiting each component range of the present invention will be described.
C: 0.0200% or less
C is an inevitable element that cannot be avoided even if it is added, but it is an element that significantly deteriorates the cold workability, ductility and corrosion resistance, especially the corrosion resistance of the weld heat affected zone. It is an element that adversely affects most of the important characteristics. In particular, when C is contained in a large amount in steel, the amount of solid solution after heat treatment increases, so even if the amount of diffusible hydrogen is reduced and the amount of solid solution of C and N is reduced by adding Nb. Since it becomes difficult to ensure excellent ductility, it is essential to reduce it as much as possible, and the upper limit was set to 0.0200%.

Si: 0.70 to 1.00%
Si is an element that is indispensable as a reducing agent during the refining of stainless steel. If the amount of Si is small, even if reductive refining is sufficiently performed, Cr oxides that inhibit ductility are dispersed in the steel, and the ductility is greatly reduced. Therefore, in order to prevent the Cr oxide that causes embrittlement from being dispersed in a large amount after reductive refining, 0.70% or more of Si was added. However, if the amount added is excessively increased, the dispersion of Cr oxide can be suppressed, but the ductility is lowered, so the upper limit was made 1.00%.

Mn: 0.50% or less
Mn is an element that combines with S to form MnS in steel and has an effect of improving machinability. However, a large amount of MnS precipitation lowers the corrosion resistance and impairs the ductility. Therefore, in the present invention in which both the corrosion resistance and the ductility are emphasized, it is necessary to suppress the addition amount, and the upper limit is set to 0.50%.

P: 0.040% or less
P is an unavoidable element that is unavoidably contained in production, but is an element that significantly deteriorates ductility and also an element that easily causes weld cracking. Therefore, it is desirable to reduce as much as possible. However, if the upper limit value is set too low, manufacturing becomes difficult and causes an increase in cost. Therefore, the upper limit is set to 0.040% in consideration of this point.

S: 0.025% or less
S combines with Mn to form MnS, improving machinability. However, excessive precipitation of MnS lowers the corrosion resistance and impairs the ductility, so the upper limit was made 0.025%.

Cu: 0.60% or less
Cu is an element that is difficult to remove by refining. When manufacturing using scrap, a small amount is unavoidable, but it is also an element that improves corrosion resistance and ductility. Therefore, when it is desired to improve the corrosion resistance and ductility, an appropriate amount can be added as necessary. However, even if added in a large amount, an effect commensurate with the increase in cost cannot be obtained, so the upper limit was made 0.60%.

Ni: 0.60% or less
Ni is an element that improves corrosion resistance and cold workability, and is also effective for solid solution strengthening. However, a large amount of addition is disadvantageous in terms of economy, so the upper limit was made 0.60%.

Cr: 17.50-20.00%
Cr is added at 17.50% or more in order to obtain good corrosion resistance. However, increasing the amount of addition improves the corrosion resistance but decreases the ductility. Therefore, the upper limit was made 20.00%.

Mo: 0.50 to 1.30%
Mo is an element effective in improving the corrosion resistance and improving the strength by strengthening the solid solution, and it is necessary to add 0.50% or more. However, Mo is an expensive element, and a large amount of addition is disadvantageous in terms of economy and causes a decrease in ductility, so the upper limit was made 1.30%.

N: 0.0200% or less,
N is an element that is inevitably contained in production, but is an element that greatly reduces cold workability, ductility, and corrosion resistance. In particular, the corrosion resistance of the weld heat affected zone is greatly adversely affected. Therefore, as in C, in order to ensure excellent ductility and corrosion resistance after heat treatment at 1010 to 1150 ° C., it is necessary to reduce as much as possible, and the upper limit was made 0.0200% or less.

Nb: 0.56% or less (however, (C + N) / Nb is 0.085 or less)
Since Nb is an element having a strong affinity with C and N, when added, it forms a carbonitride to reduce the amount of dissolved C and N. And ductility can be improved by reduction of the solid solution C and N by Nb addition. The effect of this Nb addition can of course be obtained even if the addition amount is small, but the present invention aims to enable heat treatment under the same conditions as the austenitic stainless steel as described above, It is necessary to increase the effect of Nb addition to some extent, and (C + N) / Nb should be added so that 0.085 or less ( C, N, Nb means the respective component ratio (mass%)) . However, if the amount of Nb added is too large, the effect of reducing C and N dissolved in the solution is saturated, and cold workability and hot workability are lowered due to the influence of Nb itself, so the upper limit was made 0.56%.

O: 0.0100% or less In the present invention, Si that is an element necessary for deoxidation is added in an amount of 0.70% or more in order to ensure excellent ductility even when heat-treated at a higher temperature of 1010 to 1150 ° C. than usual. It is characterized by thorough refining to prevent the formation of Cr oxides that cause ductility reduction. In order to prevent the formation of Cr oxide, it is necessary to strictly manage the upper limit of the O content, and the upper limit is set to 0.0100%.

Diffusible hydrogen: 0.30 ppm or less Diffusible hydrogen is released when the material is heated from room temperature to 400 ° C at a heating rate of 10 ° C / min. It is a component that causes significant damage to ductility. And diffusible hydrogen will be contained in steel even if it does not intend by refining at the time of steelmaking. In addition, when using a heating furnace that uses COG gas as fuel for heating before rolling, the hydrogen concentration in stainless steel is higher than when other fuels are used, which causes a reduction in ductility. ,Caution must be taken. In addition, when a material with a high temperature after rolling is cooled by sprinkling or blowing air to the material to make it easy to load and transport, the cooling rate after rolling is fast, so that diffusible hydrogen is difficult to be released, and ductility Care must be taken because it causes a drop.

Furthermore, the present invention aims to enable heat treatment at 1010 to 1150 ° C., which is not performed because the required ductility cannot be ensured normally, and in order to achieve this, diffusible hydrogen is set to 0.30. It is essential to limit it to below ppm. In addition, since the diffusible hydrogen amount can obtain excellent ductility if it is finally 0.30 ppm or less, if the diffusible hydrogen concentration is 0.30 ppm or more after the end of rolling, Before or immediately after heat treatment at 1010 ° C to 1150 ° C, the diffusible hydrogen concentration should be reduced to 0.30 ppm or less by heating and holding at 100 to 300 ° C for a time suitable for the size of the workpiece. is required. Thereby, even when a high temperature heating of 1010 to 1150 ° C. and a heat treatment for rapid cooling are performed, it is possible to ensure excellent ductility.

Next, the reasons for limiting the conditions other than the components will be described.
The reason why the heat treatment conditions are set to 1010 ° C. to 1150 ° C. is that the present invention changes the conditions in the heat treatment equipment used in the austenitic stainless steel such as SUS304, as has been emphasized many times. The above-mentioned conditions merely show the general conditions. Specifically, after heating to the above temperature, reaching a predetermined temperature, holding the temperature for 60 seconds or more at that temperature, by heat treatment under the condition of water cooling, the solid solution of carbonitride proceeds, Compared with the case where it heat-processes at normal 700-900 degreeC, the outstanding corrosion resistance can be ensured.

Further, it is generally known that in ferritic stainless steel, excellent ductility cannot be obtained when crystal grains become coarse. However, as proposed in the present invention, the steel with reduced diffusible hydrogen and other components optimized also has excellent ductility even with coarse grains with a grain size number of about 1-2. It can be secured. However, when the grain size becomes coarse, rough skin may occur at the time of plastic deformation. In consideration of this point, it is desirable that the crystal grain size number after the heat treatment is 4 or more. In order to make the crystal grain size number 4 or more fine, sufficient strain should be introduced into the material before the heat treatment so that recrystallization occurs during the heat treatment. Specifically, rolling conditions may be set so that a sufficient amount of unrecrystallized strain remains during hot rolling, or strain is applied by cold working such as cold drawing. May be. Thereby, the crystal grain size number after heat processing can be made into 4 or more.

As explained above, optimization of components (reduction of C and N content and addition of Nb, reduction of diffusible hydrogen) is ensured so that excellent ductility is ensured even when heat treatment is performed at a high temperature of 1010 to 1150 ° C. Therefore, excellent ductility can be ensured even when heat-treated at a high temperature.

  Next, the characteristics of the steel of the present invention will be described by examples in comparison with comparative examples and conventional examples. Table 1 shows the chemical composition of the test steel used as an example. The amount of diffusible hydrogen varies before and after the heat treatment, but the amount of diffusible hydrogen shown in Table 1 is a test piece for evaluation such as a tensile test after heat treatment under the conditions described in Table 2 described later. It is the result of preparing and measuring a test piece for analysis at the same time when making.

  Among the test materials shown in Table 1, A to K steels are steels that satisfy the conditions of the components specified in the present invention, and some components of L to S steels are out of the scope of the invention. Steel.

The specimens shown in Table 1 were prepared by melting in an electric furnace, refining with AOD, and rolling to produce a φ24 round bar. The material cooling after rolling is forced air cooling. In making this preparation, some of the specimens were adjusted so that the influence of unrecrystallized strain remained by adjusting the conditions of the final pass of rolling, so that the grain size became fine. Conversely, some of the other test materials were heated and held at 1150 ° C. before final heat treatment so that the crystal grains were intentionally coarsened. This is for accurately grasping the influence on the ductility due to the change in the particle size. About this round bar, after heat-processing as shown in Table 2, the below-mentioned evaluation was performed about ductility and corrosion resistance. In addition, among the test materials shown in Table 2, when the cooling conditions during the heat treatment are water-cooled, there is no time allowance for releasing diffusible hydrogen during the cooling, so 200 A hydrogen release treatment was performed by heating to ° C. for 2 hours. However, for some of the test pieces, in order to clearly grasp the influence of the presence of diffusible hydrogen, the heat treatment shown in Table 2 was performed without intentionally performing the hydrogen releasing treatment, and the evaluation described later was performed. In addition, when the cooling conditions during the heat treatment were air-cooled, the experiment was performed without performing the above-described hydrogen releasing treatment because there is a time allowance for releasing hydrogen during cooling.

Next, the evaluation method will be described.
As already explained, whether or not excellent ductility can be ensured is a big problem whether or not reductive refining is sufficiently performed and formation of Cr oxides can be prevented. Therefore, in order to confirm the presence or absence of Cr oxide formation, the round bar after rolling was cut to an appropriate length, observed with a scanning electron microscope, and component analysis was performed as necessary in the steel. It was investigated whether Cr oxide was present.

  For corrosion resistance, a 96-hour salt spray test (based on JISC0023) using a 5% NaCl aqueous solution (35 ° C) was conducted. Those exceeding 2% or less were evaluated as ○, and those exceeding 2% were evaluated as ×.

  For ductility, JIS14A test pieces having a parallel part diameter of φ8 were prepared and evaluated by elongation and drawing obtained by a tensile test. In addition, the tensile test piece was produced by machining while cooling the material with a coolant. Thereby, although the test piece has a parallel part diameter of φ8 mm, the influence of the release of diffusible hydrogen due to the diameter reduction processing is suppressed. A tensile test was conducted within 7 days after processing so as not to be affected by the release of diffusible hydrogen after processing.

In order to judge that excellent ductility is ensured from the test results, it is necessary to be equal to or higher than the elongation and drawing obtained when heat-treated at 700 to 900 ° C as before. Specifically, it is required that the elongation is 30% or more and the drawing ratio is 70% or more. After the tensile test, the chuck part of the test piece was cut and polished, and the crystal grain size number was measured with an optical microscope. The results are shown in Table 2.

  In this example, as described above, when preparing some of the test materials, the conditions of the final rolling pass are adjusted so that the influence of strain remains after rolling. All had a grain size number of 5 or more. On the other hand, what was processed so that a crystal grain became coarse intentionally became a particle size 1-2, and was clearly coarse compared with other test materials. However, it has been found that excellent characteristics cannot be obtained for Examples that do not satisfy the conditions of the present invention regardless of whether the particle size is fine or coarse. This will be described below.

Of the comparative examples shown in Table 2, Test Nos. 27 to 32 are examples in which heat treatment was performed by heating to 1010 ° C. and water cooling as in the present invention, but some components were outside the scope of the present invention. This is a comparative example. Of these, No.27 was subjected to heat treatment at 1010 ° C and water cooling without dehydrogenation in advance, so the amount of diffusible hydrogen was as high as 0.91 ppm, and as a result, the aperture was greatly reduced. Nos. 28 to 30 have a high total content of C and N compared to the Nb content, and the amount of solid solution C and N due to the addition of Nb is insufficiently reduced. Test Nos. 31 and 32 are those in which the Si content was low and reduction refining was insufficient, so that the formation of Cr-based oxides could not be suppressed and the ductility was reduced. . FIG. 1 shows the state of Cr oxide at a site approximately 5 mm away from the fracture surface of the specimen in the tensile direction cross section of the specimen after the tensile test of Test No.32. The left and right sides in the figure are the direction of tension, and it can be seen that the Cr oxide is crushed and the crack propagates to the parent phase.

  Test Nos. 25 and 26 are comparative examples evaluated without performing dehydrogenation treatment as in test No. 27, but the effect of further increasing the heat treatment temperature compared to No. 27 was investigated. It is a thing. From these results, when the heat treatment temperature is increased to 1100 ° C and 1150 ° C, the amount of solid solution of C and N increases, so the corrosion resistance is improved, but the drawing is 35% at the heat treatment temperature of 1100 ° C, 1150 ° C. It is only 3%, and it can be seen that it decreases greatly as the heat treatment temperature increases.

  Further, Test Nos. 33 to 35 are comparative examples in which the components satisfy the conditions of the present invention but the heat treatment conditions are outside the range of the conditions of the present invention. Among them, Test Nos. 33 and 35 are low in heat treatment temperature, so that C and N are not sufficiently dissolved, and the corrosion resistance is lower than that of the present invention. Test No. 34 is water cooling. However, since it was air-cooled, Cr carbide once dissolved during the cooling was precipitated again, and the corrosion resistance was lowered. In addition, No. 34 and 35, which were air-cooled after the heat treatment, contained a large amount of diffusible hydrogen of 0.55 ppm (the same test material was used for both No. 34 and 35) before the heat treatment. However, after the heat treatment, it was lowered to a sufficiently low value as shown in Table 2 even though the dehydrogenation treatment was not performed before the heat treatment. Since this is air cooling, it can be determined that hydrogen was released when passing through a temperature range of 100 to 300 ° C. during the cooling (a temperature range where diffusible hydrogen is likely to be released). Therefore, excellent properties can be obtained with respect to ductility, but the corrosion resistance is inferior to that of the present invention, and in this case, the heat treatment equipment used for austenitic stainless steel cannot be used as it is. is doing.

  On the other hand, the test pieces of Test Nos. 1 to 24 are steels of the components that satisfy the above-described conditions of the present invention, and the Si amount is optimized so that Cr oxides are not formed, and the solids of C and N C and N are added in an amount corresponding to the amount of C and N, and the amount of diffusible hydrogen is sufficiently reduced by methods such as dehydrogenation treatment as necessary so that the amount of solution does not increase and ductility does not decrease. Therefore, excellent ductility could be ensured even when heat treatment similar to that of austenitic stainless steel was performed. In addition, since heat treatment is performed at a higher temperature than conventional 18% Cr stainless steel, C and N are more fully dissolved, so the corrosion resistance can be improved compared to the conventional steel. As described above, the present invention is a steel whose corrosion resistance is improved by increasing the amount of Cr and Mo. However, since the corrosion resistance can be further improved by the present invention, the steel that does not contain a large amount of Ni has the highest level. Corrosion resistance can be achieved with inexpensive steel.

  As described above, according to the present invention, even if Mo-containing 18% Cr stainless steel is melted by refining and the hot-worked shape is thick, the ductility is not deteriorated. Since it can be manufactured by solution heat treatment using the same heat treatment equipment as the stainless steel, it is possible to produce more efficiently and has higher corrosion resistance than the conventional Mo-containing 18% Cr stainless steel. Can be improved. Accordingly, it is possible to provide stainless steel having excellent corrosion resistance and ductility without using expensive Ni, and the effect on related industries is very large.

The figure which shows the mode of Cr oxide of the tensile test piece in which reductive refining was inadequate.

Claims (2)

In mass%, C: 0.0200% or less, Si: 0.70 to 1.00%, Mn: 0.50% or less, P: 0.040% or less, S: 0.025% or less, Cu: 0.60% or less, Ni: 0.60% or less, Cr: 17.50 ~ 20.00%, Mo: 0.50 ~ 1.30%, N: 0.0200% or less, Nb: 0.56% or less, O: 0.0100% or less, Diffusible hydrogen: 0.30ppm or less, and (C + N) / Nb: 0.085 or less ( C, N, and Nb are component ratios (% by mass)) , and the balance is steel composed of Fe and inevitable impurities. Heat treatment is performed at a high temperature (1010 to 1150 ° C) and water-cooled. Ferritic stainless steel with excellent corrosion resistance and ductility characterized by being applied . In mass%, C: 0.0200% or less, Si: 0.70 to 1.00%, Mn: 0.50% or less, P: 0.040% or less, S: 0.025% or less, Cu: 0.60% or less, Ni: 0.60% or less, Cr: 17.50 ~ 20.00%, Mo: 0.50 ~ 1.30%, N: 0.0200% or less, Nb: 0.56% or less, O: 0.0100% or less, Diffusible hydrogen: 0.30ppm or less, and (C + N) / Nb: 0.085 or less ( C, N, and Nb are component ratios (mass%) of each component) , and the remaining steel is composed of Fe and inevitable impurities, and is heated to 1010 to 1150 ° C. in the heat treatment step and water-cooled. To produce ferritic stainless steel with excellent corrosion resistance and ductility.

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