TWI639712B - Vostian Iron Series Stainless Steel Plate - Google Patents

Abstract

The composition of the Vostian iron-based stainless steel plate of the present invention is in mass%, and it contains C: 0.03 to 0.15%, Si: 0.20 to 2.5%, Mn: 0.2 to 4.5%, P: 0.010 to 0.030%, S : 0.0001 to 0.0010%, Cr: 20.0 to 26.0%, Ni: 10.0 to 15.0%, Cu: 0.01 to 2.0%, Mo: 0.01 to 2.0%, Co: 0.05 to 2.50%, Al: 0.01 to 0.20%, N: 0.1 ~ 0.6%, V: 0.02 ~ 0.15%, B: 0.0002 ~ 0.0050%, Nb: 0 ~ 0.10%, Ti: 0 ~ 0.10%, Y: 0 ~ 0.10%, Ca: 0 ~ 0.010%, Mg: 0 ~ 0.010%, REM: 0 ~ 0.10%, the rest is Fe and impurities, the content of Mn [Mn] (mass%), the content of S [S] (mass%) conform to [Mn] × [S] ≦ 0.0020 When the plate thickness is 0.5 mm or less, the major axis length of the crystal grains is regarded as L1, and the minor axis length of the crystal grains is regarded as L2. The value of the aspect ratio is in accordance with the relationship of L1 / L2 ≧ 1.5, at 600 ° C. The surface hardness (HV) after the temperature was maintained for 400 hours was 300 or more. This Vostian iron-based stainless steel plate has excellent corrosion resistance and heat resistance.

Description

Vostian Iron Series Stainless Steel Plate

The invention relates to a wostian iron-based stainless steel plate.

The gasket (cylinder head gasket) located between the cylinder head and the engine body of the automobile engine is made of stainless steel for springs such as SUS301 and SUS403. SUS301 is strengthened by cold rolling, and a part of it is transformed into Asada scattered iron by processing. SUS403 is strengthened by quenching and tempering to make most of it become Asada loose iron phase. The cylinder head gasket is used in an environment at a temperature of 200 ° C or lower. Therefore, the Asada loose iron phase can also exist as a stable phase in such an environment, and can exert the sealing property of the gasket.

On the other hand, the gasket used between the flanges of the exhaust system parts of the car is used to connect higher temperature parts than the engine body or the cylinder head. Therefore, the maximum temperature is heated to 500 ~ 700. ℃. The heat-resistant temperature of SUS301 and SUS403 is about 350 ° C, and the strength cannot be maintained in such a high-temperature environment. Therefore, the gaskets for exhaust system parts use the Japanese Industrial Standard JIS G 4902 (corrosion-resistant heat-resistant superalloy plate). Prescribed NCF625, NCF718, and Japanese Industrial Specifications SUH660, SUH310, etc. specified in JIS G 4312 (heat-resistant steel plate and steel strip).

NCF625 and NCF718 are reinforced alloys utilizing the precipitation of intermetallic compounds and contain 50% or more of Ni. SUH660 is a precipitation-hardened steel containing 24 to 27% Ni, and SUH310 is a Vostian iron-based heat-resistant steel containing 19 to 22% Ni. Both contain a large amount of Ni, so they are expensive. Therefore, some people have developed a steel type containing the same stabilizing element of Vostian iron, that is, N, to reduce the Ni content, and has been used.

However, in recent years, with the requirements for low fuel consumption of engines and stricter regulations on exhaust emissions, even small cars are equipped with exhaust gas recirculation systems (EGR), turbochargers, diesel carbon particulate filters (DPF), Exhaust system parts such as exhaust heat recovery device. In order to improve fuel economy, the reduction of exhaust gas is becoming popular, and it is becoming smaller and more powerful. The temperature of exhaust gas becomes higher, and the gasket has more residual stress than the conventional gasket.

In addition, because exhaust-type parts tend to be miniaturized, in order to improve the cooling efficiency, a water-cooling mechanism is also added. Therefore, the Ni-saving heat-resistant material developed in recent years can maintain its strength while being heated to a high temperature However, due to the dew condensation of the exhaust gas after the engine was stopped, an example of the occurrence of stress corrosion cracks (hereinafter referred to as SCC) unique to Vostian iron-based stainless steel occurred.

The steel sheet disclosed in Patent Document 1 raises the solid solution limit of N by increasing Mn to 1.0 to 10.0% to increase N to 0.35 ~ 0.8%, using the solid solution strengthening effect of N to improve the high temperature strength.

The Vostian iron-based stainless steel disclosed in Patent Document 2 is to increase C-Si-N and adjust C + 2N + 0.12Si + 1.4Nb to 0.45% or more. In this way, even if exposed to a high-temperature gas phase atmosphere, It also maintains excellent decay resistance and can be used as a metal gasket. C and N are solid solution strengthening elements used as the parent phase. Si and Nb can also suppress the movement of dislocations in the high-temperature gas phase atmosphere, and are elements that increase the high-temperature strength.

In order to ensure the flatness of the shape required for the gasket by the corrective process, the heat-resistant Vostian iron-based stainless steel for metal gaskets disclosed in Patent Document 3 is controlled by reducing the amount of N used as a solid solution strengthening element It is 0.05% or less, and Si is controlled to be higher than 2.0% and 5.0% or less to replace the reduced N.

The cold-rolled Vosstian iron-based stainless steel disclosed in Patent Document 4 has a stable Vosstian iron phase. In order to suppress recovery and recrystallization, and as a stainless steel capable of aging hardening, it contains 7 to 25% Ni , 16 to 30% Cr, 0.1 to 0.4% N, and the spring limit value is set to 220 MPa or more.

[Prior technical literature] [Patent Literature]

[Patent Document 1] Japanese Unexamined Patent Publication No. 9-279315

[Patent Document 2] Japanese Patent Laid-Open No. 2003-82441

[Patent Document 3] Japanese Patent Laid-Open No. 2011-252208

[Patent Document 4] Japanese Patent Laid-Open No. 2012-211348

However, the conventional techniques described above have the following technical problems.

Although the invention of Patent Document 1 is a material excellent in strength, high-temperature strength, decay resistance, and high-temperature oxidation resistance, it has not been reviewed in terms of corrosion resistance, and the corrosion resistance cannot be fully obtained. Characteristics.

In addition to the deterioration resistance and hardness at high temperature, the steel type described in Patent Document 2 has not been reviewed, and sufficient corrosion resistance cannot be obtained.

Although the invention of Patent Document 3 can improve the workability and the deterioration resistance at room temperature, the corrosion resistance has not been reviewed.

The invention of Patent Document 4 reviews only the limit value and hardness of the spring when used at high temperatures, and does not sufficiently review the corrosion resistance.

In view of the above-mentioned technical problems, an object of the present invention is to provide a Vostian iron-based stainless steel plate, which is suitable as a material for a metal gasket used for connecting exhaust gas flow path parts of automobile engines and the like. It is expected to have both heat resistance and corrosion resistance that have not been adequately reviewed in conventional materials for metal gaskets, and low cost.

In order to solve the aforementioned technical problems, the present inventors have been working hard to review them. First, it is estimated that the corrosion generated in the heat-resistant sealing gasket is relatively easy to occur in the part where the exhaust system is locally cooled by a cooling device; and it is caused by the heat cycle of heating and cooling. It is produced under the condition of stress; in addition, it is mainly SCC from the corrosion morphology.

In order to reduce SCC susceptibility, it is effective to use ferrous iron-based stainless steel, but sufficient high-temperature strength cannot be obtained. In addition, to improve SCC resistance in Vostian iron-based stainless steel, although elements such as Si and Mo are effective, excessive content of Si and Mo may cause fatigue life due to the formation of the Σ phase. Worse, it is not appropriate. In addition, generally speaking, the corrosion resistance of stainless steel is expressed by the Cr + 3Mo + 16N pitting resistance index. Therefore, it is also effective to increase the content of Cr or N. However, Cr is compatible with Si and Mo. There is also a problem: due to the formation of the Σ phase, excessive N content will increase the strength at room temperature, thereby compromising manufacturability. In addition, it will be formed into a gasket shape by press forming. There is a problem in shape freezing.

Therefore, the impact of other elements is the result of investigation of each element using the SCC test that simulates the actual environment. Regarding methods that do not impair heat resistance, processability, and press formability, and improve SCC resistance, the following ideas have been obtained.

(1) In order to improve SCC resistance, it is effective to reduce the number of inclusions (MnS) that will be the starting point of SCC. Therefore, although a method of reducing Mn and S can be considered, Mn is also an element for increasing the solid solution amount of N. From the viewpoint of high-temperature strength, it is desirable to contain a certain amount of N as long as it does not cause production problems such as bubble defects during solidification and ear cracks during rolling. Therefore, instead of reducing Mn, S must be reduced to 10 ppm or less (0.0010% or less).

(2) The method of containing Co in a small amount does not promote the precipitation of the Σ phase, and is effective for corrosion resistance and high-temperature strength.

The present invention has been developed based on these ideas, and the gist of the invention is as follows.

(1) A Vosstian iron-based stainless steel plate, the composition of which is based on mass%, and contains C: 0.03 to 0.15%, Si: 0.20 to 2.5%, Mn: 0.2 to 4.5%, P: 0.010 to 0.030% S: 0.0001 to 0.0010%, Cr: 20.0 to 26.0%, Ni: 10.0 to 15.0%, Cu: 0.01 to 2.0%, Mo: 0.01 to 2.0%, Co: 0.05 to 2.50%, Al: 0.01 to 0.20%, N : 0.1 ~ 0.6%, V: 0.02 ~ 0.15%, B: 0.0002 ~ 0.0050%, Nb: 0 ~ 0.10%, Ti: 0 ~ 0.10%, Y: 0 ~ 0.10%, Ca: 0 ~ 0.010%, Mg: 0 ~ 0.010%, REM: 0 ~ 0.10%, the rest are Fe and impurities, the content of Mn [Mn] (mass%), the content of S [S] (mass%) are in accordance with [Mn] × [S] ≦ The relationship of 0.0020, the thickness of the board is less than 0.5mm, will When the major axis length of the crystal grains is regarded as L1 and the minor axis length of the crystal grains is regarded as L2, the value of the aspect ratio conforms to the relationship of L1 / L2 ≧ 1.5, and the surface hardness (HV) after maintaining at 600 ° C for 400 hours ) Is 300 or more.

(2) The Vostian iron-based stainless steel plate according to the above (1), which is composed of mass% and contains Nb: 0.01 to 0.10% and / or Ti: 0.01 to 0.10%.

(3) The Vostian iron-based stainless steel plate according to the above (1), the composition of which is in mass% and contains from Y: 0.01 to 0.10%, Ca: 0.001 to 0.010%, Mg: 0.0002 to 0.010%, and REM: One or more selected from 0.01 to 0.10%.

(4) The Vostian iron-based stainless steel plate according to the above (2), the composition of which is in mass% and contains from Y: 0.01 to 0.10%, Ca: 0.001 to 0.010%, Mg: 0.0002 to 0.010%, and REM: One or more selected from 0.01 to 0.10%.

(5) The Vostian iron-based stainless steel sheet according to any one of (1) to (3) above, which is subjected to quenched and tempered rolling at a reduction rate of 20% or more during cold rolling.

According to the present invention, the system can provide a Vosstian iron system that does not need to contain more than 20% of Ni, such as SUH310, SUH660, NCF625, and NCF718, and can have both high temperature strength and corrosion resistance as long as the content of Ni is small. Stainless steel plates are particularly suitable as heat-resistant metal gaskets for automotive exhaust systems.

1‧‧‧ stainless steel plate

2‧‧‧ opening

3‧‧‧ raised pattern

10‧‧‧ Test Strip

Fig. 1 shows a test piece used in the examples. Fig. 1 (a) is a plan view; Fig. 1 (b) is an enlarged view of the A-A 'cross section in Fig. 1 (a).

First, the chemical composition of the stainless steel plate and the preferable manufacturing method of a steel plate of this invention are demonstrated. In addition, in the following description, "%" used to indicate the content of each element means "mass%" unless otherwise stated.

<C: 0.03 ~ 0.15%>

C is an element effective in improving the stability and high-temperature strength of Vosstian iron structure. In addition, C can form carbides with Cr to suppress the growth of Vostian iron particles and moderate growth of grain boundary oxidation, and can improve the peel resistance of rust. To obtain this effect, the C content must be 0.03% or more. In addition, in order to stably suppress grain growth, the C content is preferably set to 0.10% or more. On the other hand, if the C content exceeds 0.15%, the amount of Cr carbides increases, and the chromium-deficient layer at the grain boundary increases. Even if it is a Vostian iron-based stainless steel with a high Cr content similar to the steel of the present invention, use it as an automobile Exhaust manifold components and turbine parts cannot maintain the required corrosion resistance when used. Therefore, the C content is set to 0.15% or less. From the viewpoint of corrosion resistance In consideration, the C content is preferably less than 0.12%.

<Si: 0.20% ~ 2.5%>

Si has an effect of oxidation resistance, and is particularly effective in preventing peeling of scale due to discontinuous oxidation. In an environment exceeding 1000 ° C, grain boundary oxidation may occur. In order to suppress peeling of the surface scale, the Si content must be set to 0.20% or more. Therefore, the Si content is set to be 0.20% or more. In order to improve the oxidation resistance, it is better to set the Si content above 0.50%. In addition, Si is an element for stabilizing ferrous iron, which can increase the amount of δ ferrous iron in a solidified structure, thereby causing a problem in that the hot workability during hot rolling is deteriorated. Therefore, the Si content is set to 2.5% or less. In addition, Si also promotes the formation of a Σ layer, which is worrying about embrittlement during long-term use at high temperatures. Therefore, it is preferable to set the Si content to 2.0% or less, and more preferably 1.5% or less.

<Mn: 0.2 ~ 4.5%>

Mn is an element used as a deoxidizer, and can expand the range of single phase of Vosstian iron, which is helpful for stabilizing the structure. In addition, Mn can help to ensure high-temperature strength by increasing the solid solution limit of N. This effect becomes clear when the Mn content is 0.2% or more. Therefore, the Mn content is set to be 0.2% or more. In addition, sulfides are formed to reduce the amount of solid solution S in the steel, and therefore have the effect of improving hot workability. Therefore, the Mn content is preferably set to 0.5% or more. On the other hand, if it is contained excessively, the corrosion resistance is deteriorated. Therefore, the Mn content is set to 4.5% or less. In addition, from the viewpoint of oxidation resistance, it is preferable to form an oxide mainly composed of Cr ₂ O ₃ , and Mn oxide is not suitable. Therefore, the Mn content is preferably set to 2.0% or less.

<P: 0.010 ~ 0.030%>

P is an element contained as an impurity in a raw material, that is, a main raw material such as molten iron and ferrochrome. It is an element harmful to hot workability. Therefore, the P content is set to 0.030% or less. However, if it is excessively reduced, it is necessary to use high-purity raw materials, which will cause an increase in cost. Therefore, the P content is set to be above 0.010%. From an economic point of view, the P content is preferably 0.020% or more.

<S: 0.0001 ~ 0.0010%>

S will form sulfide-based inclusions, resulting in deterioration of the general corrosion resistance (general corrosion or pitting corrosion) of the steel. Therefore, the lower the upper limit of the content, the better. In addition, in order to reduce the inclusion (MnS) which is the starting point of SCC occurrence, the S content is set to 0.0010% or less. In addition, the lower the S content, the better the corrosion resistance. Therefore, the S content is preferably 0.0008% or less. However, in order to reduce S, the load cost of desulfurization will increase and the manufacturing cost will increase. The S content is preferably set to be more than 0.0001%. Generally speaking, from the viewpoint of manufacturing costs, the S content is rarely adjusted within the above-mentioned range of 0.0001 to 0.0010%. However, the present invention aims to reduce inclusions (MnS), and is set to the above range, which can be said to be an extremely low S content.

<Cr: 20.0 ~ 26.0%>

Cr is an element required for ensuring oxidation resistance and corrosion resistance in the present invention. If the Cr content is less than 20.0%, these effects cannot be exhibited. On the other hand, if the Cr content is more than 26.0%, the range of the single phase of Vosstian iron will be reduced, and the thermal processability during manufacturing will be deteriorated. Therefore, the Cr content is set to 20.0 to 26.0%. From the viewpoint of oxidation resistance, the Cr content is preferably set to 24.0% or more. Increasing the Cr content causes embrittlement due to the formation of a Σ phase. Therefore, the Cr content is preferably set to 25.0% or less.

<Ni: 10.0 ~ 15.0%>

Ni is an iron phase stabilizing element, and unlike Mn, Ni is an element effective for oxidation resistance. These effects must be obtained when the Ni content is 10.0% or more. Therefore, the Ni content is set to 10.0% or more. Since Ni also has the effect of suppressing the formation of the Σ phase, it is desirable to set the Ni content to 11.0% or more. On the other hand, if it is contained excessively, the susceptibility to coagulation cracks becomes high, and the workability between heat is also deteriorated. Therefore, the Ni content is set to 15.0% or less. In addition, in order to suppress scale peeling due to discontinuous oxidation, the Ni content is preferably set to 14.0% or less.

<Cu: 0.01 ~ 2.0%>

Cu-based can be used instead of Ni as a Wastfield iron stabilizing element, and is a relatively inexpensive element. In addition, for suppressing crevice corrosion The progress of pitting and pitting is also effective. Therefore, the Cu content is preferably 0.01% or more. However, in the process of manufacturing Vostian iron-based stainless steel, Cu is often mixed from raw materials such as recycled waste materials, and in many cases, it is contained as an unavoidable impurity at a level of 0.2%. If it exceeds 2.0%, hot workability will be deteriorated. Therefore, the Cu content is set to 2.0% or less.

<Mo: 0.01 ~ 2.0%>

Mo also forms a protective rust on the surface effectively with Si or Cr. This effect must be obtained when the Mo content is 0.01%, so the Mo content is set to 0.01% or more. In addition, Mo is also an effective element for improving the corrosion resistance. Therefore, it is preferable to set the Mo content to 0.3% or more. On the other hand, Mo is also an iron stabilizing element for fertilizers. If the content of Mo is increased, the content of Ni must also be increased, so it should not be excessively contained. In addition, Mo promotes the formation of the Σ phase and causes embrittlement. Therefore, the Mo content is set to 2.0% or less. If the Mo content exceeds 0.8%, the effect of improving corrosion resistance and oxidation resistance is almost saturated. Therefore, the Mo content is preferably set to 0.8% or less.

<Co: 0.05 ~ 2.50%>

Even if Co is contained in a small amount, it is extremely effective for improving heat resistance. Therefore, the Co content is set to be 0.05% or more. However, if it is contained excessively, the hot workability is deteriorated. Therefore, the Co content is set to 2.50% or less. Co is also an element effective for corrosion resistance, so it is preferable to set the Co content to 0.10% or more. In addition, in order to suppress the shape of the Σ phase The Co content is preferably set at 2.0% or less.

<Al: 0.01 ~ 0.20%>

In addition to being used as a deoxidizing element, Al is also an element for improving oxidation resistance. Therefore, the Al content is set to 0.01% or more. In addition, in order to improve the deoxidation efficiency, the Al content is preferably set to be 0.05% or more. On the other hand, if it is contained excessively, nitrides are formed and the amount of solid solution N is reduced, thereby lowering the high temperature strength. Therefore, the Al content is set to 0.20% or less. If the weldability is also taken into consideration, it is preferable to set the Al content to 0.15% or less.

<N: 0.1 ~ 0.6%>

N is one of the very important elements in the present invention. The system is the same as C, in addition to improving the high-temperature strength, it can also reduce the Ni content because it can improve the stability of Vostian iron. In addition, the influence on the deterioration of the corrosion resistance due to sharpening is also smaller than that of C, so the N content may be more than the C content. In order to obtain high-temperature strength that can withstand high-temperature environments, the N content is set to above 0.10%. If the effect of reducing the Ni content is also considered, the N content is preferably set to 0.25% or more. On the other hand, if the content is too large, bubble-based defects may occur during solidification in the steel making process. Therefore, the N content is set to 0.6% or less. In addition, based on the viewpoint that pressure melting equipment must be used, and the strength at room temperature is too high, the load during cold rolling is too high, which will cause poor productivity. The N content is preferably set at 0.4% or less. It is below 0.3%.

<V: 0.02 ~ 0.15%>

V is mixed as an impurity in an alloy raw material of stainless steel, and it is difficult to remove V in a refining step. Generally, V is contained in a range of 0.02 to 0.15%. In addition, V forms fine carbonitrides and has the effect of suppressing grain growth. Therefore, V can be intentionally added according to demand. In order to stably exhibit this effect, the V content is set to 0.02% or more. In order to control the crystal grain size within a certain range, the V content is preferably 0.08% or more. On the other hand, if it is contained excessively, the precipitates may become coarse, and as a result, the toughness after quench hardening may be deteriorated. Therefore, the V content is set to 0.15% or less. In addition, considering manufacturing cost and manufacturability, it is preferable to set the V content to 0.10% or less.

<B: 0.0002 ~ 0.0050%>

B is an effective element for improving hot workability, and its effect is exhibited only when the B content is 0.0002% or more. Therefore, the B content is set to 0.0002% or more. In order to improve the thermal processability in a wider temperature range, it is appropriate to set the B content to be 0.0005% or more. On the other hand, if it is excessively contained, it may cause surface defects due to poor thermal processability. Therefore, the B content is set to 0.0050% or less. Considering the corrosion resistance, the B content is preferably set to 0.0025% or less.

<Nb: 0 ~ 0.10%>

Nb can be suppressed by the formation of carbonitrides: An element that causes sharpening and deterioration of corrosion resistance due to the precipitation of chrome carbonitrides may be contained. However, because large-sized steel-making inclusions are formed, they are likely to cause surface defects, and also cause the fatigue life of the gasket to be deteriorated. Therefore, the Nb content is set to 0.10% or less. Considering the effect of improving the high-temperature strength due to ensuring the amount of solid solution C and the amount of solid solution N, the Nb content is preferably set to 0.05% or less. In order to obtain the above-mentioned effects, the Nb content is preferably set to 0.01% or more.

<Ti: 0 ~ 0.10%>

Ti is an element which can be suppressed by the formation of carbonitrides: the sharpening and the deterioration of the corrosion resistance due to the precipitation of chromium carbonitrides in stainless steel can be contained. However, since large-sized steel-making inclusions are formed, which causes the fatigue life of the gasket to be reduced, the Ti content is set to 0.10% or less. Considering the effect of improving the high-temperature strength by ensuring the amount of solid solution C and the amount of solid solution N, the Ti content is preferably set to 0.05% or less. In order to obtain the above-mentioned effects, the Ti content is preferably set to 0.01% or more.

<Y: 0 ~ 0.10%>

In addition to the effect of improving oxidation resistance, Y is also a desulfurizing element, and therefore may be contained. However, if it is contained excessively, in addition to the problem of clogging of the molten steel injection nozzle during continuous casting, large-scale oxide-based inclusions may also be formed, resulting in fatigue resistance of the gasket. Since the working life is deteriorated, it is appropriate to set the Y content to 0.10% or less. In order to obtain the above-mentioned effects, the Y content is preferably set to 0.01% or more.

<Ca: 0 ~ 0.010%>

Ca-based can be used as a desulfurizing element, and since it has the effect of reducing S in steel and improving hot workability, it can also be contained. Generally speaking, CaO is added to the slag during melting and refining, and a part of this CaO becomes Ca and is melted in the steel. In addition, it also exists in steel in the form of a composite oxide such as CaO-SiO ₂ -Al ₂ O ₃ -MgO. On the other hand, if the content is too large, relatively coarse water-soluble inclusions CaS will be precipitated, resulting in poor corrosion resistance. Therefore, it is preferable to set the Ca content to 0.010% or less. In order to obtain the effect of improving hot workability, the Ca content is preferably set to 0.001% or more.

<Mg: 0 ~ 0.010%>

Mg is contained in steel as a desulfurizing element in the same way as Ca. In general, in addition to being dissolved in slag into molten steel to achieve an equilibrium amount, it is sometimes contained in the form of MgO in the composite. In oxide. In addition, MgO in refractory sometimes melts in molten steel. On the other hand, if it is contained excessively, coarse water-soluble inclusions MgS are precipitated and the corrosion resistance is deteriorated. Therefore, it is preferable to set the Mg content to 0.010% or less. It is appropriate to set the Mg content to 0.0002% or more.

<REM: 0 ~ 0.10%>

In addition to the effect of improving oxidation resistance, REM is also a desulfurizing element, and therefore may be contained. However, if it is contained excessively, in addition to the problem of clogging of the molten steel injection nozzle during continuous casting, it may also cause the fatigue life of the gasket to deteriorate due to the formation of large oxide-based inclusions. Therefore, it is appropriate to set the REM content to 0.10% or less. In order to obtain the above-mentioned effects, the REM content is preferably set to 0.01% or more.

REM refers to the general term of Sc and lanthanoid elements, and the content of REM means: the total amount of the above-mentioned elements usually includes Y in REM, but it is recorded separately in the present invention.

In the steel sheet of the present invention, the remainder of the composition is Fe and impurities. The term "impurities" as used herein means that when steel is manufactured on an industrial scale, raw materials such as ore and recycled waste, as well as ingredients mixed in for various reasons during the manufacturing process, may be allowed to interfere with the present invention. The steel sheet of the present invention exists in a range where the adverse effect is caused.

<[Mn] × [S] ≦ 0.0020>

Mn and S form sulfide MnS, which causes the corrosion resistance of the heat-resistant gasket to be deteriorated in the use environment. In particular, SCC will cause problems. In order to suppress the occurrence of cracks, the product of the Mn content [Mn] (mass%) and S content [S] (mass%) must be controlled to conform to [Mn] × [S] ≦ 0.0020 relationship. By reducing the number of MnS that is the starting point of corrosion, the occurrence of SCC can be significantly suppressed. However, if low S is required, the load of desulfurization must be increased. In order to stabilize the Wastfield iron, more Ni must be used, resulting in increased manufacturing costs. Therefore, it is appropriate to set the lower limit to 0.0001. If workability is also taken into consideration, it is better to control it to satisfy the relationship of [Mn] × [S] ≦ 0.0015.

<Aspect ratio of crystal grains>

In the present invention, no heat treatment is performed after the cold processing. Therefore, the final crystal grains become a rolled structure. In the present invention, the major axis length of the crystal grains is regarded as L1, and the minor axis length of the crystal grains is regarded as L2, and the aspect ratio, that is, the value of L1 / L2 must be specified. In order to obtain sufficient surface hardness, Control the value of the aspect ratio to L1 / L2 ≧ 1.5. If the value of the aspect ratio L1 / L2 <1.5, as a metal gasket, in addition to its insufficient surface hardness, when exposed to a high temperature environment for a long time, specifically, 400 hours at a temperature of 600 ° C In this case, it is not possible to meet the condition that the surface hardness (HV) at the time of exposure reaches 300 or more. Aspect ratio: L1 / L2 is preferably L1 / L2 ≦ 25, and L1 / L2 ≧ 5. In addition, in order to measure the aspect ratio, the metal structure observed and recorded with an optical microscope is subjected to image analysis to perform the measurement.

<Surface hardness (HV) after holding at 600 ° C for 400 hours>

The present invention is a wostian iron-based stainless steel plate used for metal gaskets. Generally, metal gaskets are different depending on the use site. If they are used around the exhaust system parts, they will be exposed for a long time. Under the high temperature environment of 500 ~ 700 ℃. Therefore, there will be changes in use Deformation, tightness and hardness will deteriorate, which is what is called "performance degradation". In order to suppress performance degradation when used at 600 ° C, the surface hardness (HV) must be 300 or more when exposed to a temperature of 600 ° C for 400 hours. Therefore, in the examples described later, the above-mentioned use environment was simulated, and the surface hardness and the like after being held at a temperature of 600 ° C. for 400 hours were measured.

In addition, the present inventors have investigated the hardness of each surface after applying various thermal histories and maintaining it at a temperature of 600 ° C. for 400 hours, and confirmed that each surface hardness (HV) reached 300. the above. In other words, even if it is used: the material that is actually used, or the material whose previous thermal history is unknown, as long as it is exposed to a temperature of 600 ° C × 400 hours, which is supposed to be the actual use environment, the surface hardness (HV) can reach Above 300, this requirement of the present invention can be met. In addition, the surface hardness (Vickers hardness) is measured according to a method established by the Japanese Industrial Standard JIS Z 2244, a load of 4.903N (HV0.5) is measured at 5 points or more, and the average value is used as a representative value.

In addition, the performance degradation must be evaluated based on the "deformation at high temperature". Therefore, the evaluation was performed based on the amount of change in the height of the ridges after holding at 600 ° C for 400 hours. The height of the raised portion refers to the height of the arc-shaped raised portion in the cross-sectional shape, and the change in the height of the above portion after being held at 600 ° C. for 400 hours was measured.

<SCC resistance>

As described above, the SCC resistance can be improved by setting the Mn and S contents to [Mn] × [S] ≦ 0.0020. The SCC resistance was evaluated based on the results of a high-pressure hot pot test at 150 ° C for 40 hours in an aqueous 0.08% NaCl solution. The autoclave test refers to a test using a pressure-resistant container in order to obtain a high-temperature aqueous solution corrosive environment. The method of the SCC test is to adjust the solution temperature and composition according to the requirements of Japanese Industrial Standard JIS G0576.

<Manufacturing process>

In the method for manufacturing a Vosstian iron-based stainless steel according to the present invention, the step for manufacturing a steel sheet for cold temper rolling is not particularly limited. The steel melted by a conventional device (for example: electric furnace) is cast by a continuous casting machine into a steel billet with a thickness of 150 to 250 mm. After cutting and grinding the surface as appropriate, it is heated to 1200 ° C or more, A hot rolling mill performs hot rolling to produce a hot-rolled steel strip having a plate thickness of about 3 to 6 mm. The hot-rolled steel strip is annealed at a temperature of about 1100 ° C, and then pickled. Next, cold rolling and annealing were repeatedly performed to produce a thin steel plate having a thickness of 0.5 mm or less. A more preferable thickness is 0.3 mm or less. Refining and annealing can be performed: annealing and pickling refining (2B refining), or BA refining in an oxidation-free gas phase atmosphere. In addition, the refining annealing referred to here refers to the annealing step before quenching and tempering.

On the other hand, cold tempering is performed after refining and annealing. The rolling step is a quenched and tempered rolling step in order to obtain the strength (surface hardness) required as a spring material for a gasket, and to change the reduction ratio in accordance with the required strength (surface hardness). In order to obtain the required strength of the heat-resistant sealing gasket, the rolling reduction rate of the quenched and tempered roll is preferably set to 20% or more. In addition, in order to make the surface hardness (HV) after holding at 600 ° C for 400 hours meet the conditions of 300 or more, in addition to using the composition specified in the present invention, the rolling reduction rate of the quenched and tempered roll is set. Above 20% is preferable, and it is better to set the rolling reduction rate to above 30%. In addition, the upper limit of the reduction ratio is because the productivity is deteriorated as the number of temper rolling is performed more and more, so it is preferable to set it at 80% or less, and it is more preferable to set it at 60% or less.

In the present invention, the annealing step is not performed after the temper rolling, but if it is a step other than annealing, the steps after the temper rolling are not particularly limited. Sometimes, it is implemented again: forced shaping process or degreasing and washing process.

[Example]

Hereinafter, the effects of the present invention will be described by examples, but the present invention is not limited to the conditions used in the following examples.

First, steel having the composition shown in Table 1 was melted and cast into a billet having a thickness of 200 mm. After heating this blank to 1250 ° C, rough hot rolling and refined hot rolling are used to produce a hot-rolled steel sheet with a thickness of 4mm. In order to simulate coiling in the temperature range of 800 ° C, heat treatment at 800 ° C is performed. After holding for an hour, let it cool in the air but. Next, the hot-rolled sheet was annealed at 1100 ° C for 20 seconds, and then water-cooled. Then, after performing a bead blasting treatment, pickling is performed to remove rust. Cold rolling, annealing and pickling were repeated several times to produce a cold rolled steel sheet having a thickness of 0.25 to 0.5 mm. The aspect ratio of the crystal grains was measured by the aforementioned method.

As shown in FIG. 1, from each stainless steel plate 1, a test piece 10 simulated as a metal gasket was produced by press forming. The test piece 10 has a circular opening 2 with an inner diameter of 80 mm, and has a width of 2.5 mm around the opening 2. The raised portion 3 having a height of 0.25 mm and a projection arc of 2R. After the test piece 10 was held at 600 ° C for 400 hours, the surface hardness was measured. The surface hardness is measured according to a method established by the Japanese Industrial Standard JIS Z 2244, a load of 4.903N (HV0.5) is applied, and the measurement is performed at 5 or more points, and the average value is used as a representative value.

In addition, the height change of the ridge portion was measured, and this was regarded as the deterioration of the gasket and evaluated. The change in the height of the ridge portion was 30% or less, which was regarded as acceptable. In addition, the SCC was evaluated by performing a high-temperature autoclave test at 150 ° C for 40 hours in a 0.08% strength NaCl aqueous solution. In addition, a sample having a composition different from that of the present invention was used as a comparative example, and the same evaluation was performed. The method of the SCC test is to adjust the liquid temperature and composition according to the requirements of Japanese Industrial Standard JIS G0576.

The evaluation results are shown in Table 2. In addition, the rolling reduction rate of the cold rolling (quenching and rolling) performed last is indicated as "quenching and rolling reduction rate".

Test Nos. 1 to 25 of the examples of the present invention are in accordance with the composition specified in the present invention, [Mn] × [S] ≦ 0.0020, aspect ratio L1 / L2 ≧ 1.5, and the surface after being held at 600 ° C for 400 hours The hardness (HV) is 300 or more, and the height change of the ridges conforms to the target value (30% or less), and SCC and surface defects do not occur.

On the other hand, the test Nos. 26 to 47 of the comparative example did not meet the composition stipulated by the present invention, and the surface hardness after holding at 600 ° C for 400 hours did not reach 300, SCC occurred, and ridges occurred: Either the height change is more than 30% or the surface is flawed.

In addition, Test No. 48 of the comparative example did not meet the composition stipulated by the present invention, the value of the aspect ratio was very low, and the value of the surface hardness (HV) after holding at 600 ° C for 400 hours was also very low. In addition, the height of the ridges varies by more than 30%.

Test No. 49 of the comparative example, although each element falls within the range specified in the present invention, it does not meet the relationship of [Mn] × [S] ≦ 0.0020, and the aspect ratio is also low, at 600 The value of surface hardness (HV) after holding at 400 ° C for 400 hours was also low. In addition, SCC occurred, and the height of the raised portion changed by more than 30%.

In Test No. 50 of the comparative example, although each element falls within the range specified in the present invention, it does not meet the relationship of [Mn] × [S] ≦ 0.0020, and SCC occurs.

Although Test No. 51 of the comparative example conformed to the composition specified in the present invention, the value of the aspect ratio was low, and the value of the surface hardness (HV) after holding at 600 ° C for 400 hours was also low. In addition, the raised portion is high The degree change also exceeds 30%.

Claims (5)

A type of Vostian iron-based stainless steel plate, the composition of which is based on mass%. The system contains C: 0.03 to 0.15%, Si: 0.20 to 2.5%, Mn: 0.2 to 4.5%, P: 0.010 to 0.030%, and S: 0.0001. ~ 0.0010%, Cr: 20.0 ~ 26.0%, Ni: 10.0 ~ 15.0%, Cu: 0.01 ~ 2.0%, Mo: 0.01 ~ 2.0%, Co: 0.05 ~ 2.50%, Al: 0.01 ~ 0.20%, N: 0.1 ~ 0.6%, V: 0.02 ~ 0.15%, B: 0.0002 ~ 0.0050%, Nb: 0 ~ 0.10%, Ti: 0 ~ 0.10%, Y: 0 ~ 0.10%, Ca: 0 ~ 0.010%, Mg: 0 ~ 0.010 %, REM: 0 ~ 0.10%, the rest are Fe and impurities, the content of Mn [Mn] (mass%), the content of S [S] (mass%) conform to 0.0001 ≦ [Mn] × [S] ≦ 0.0020 When the plate thickness is 0.5 mm or less, the major axis length of the crystal grains is regarded as L1, and the minor axis length of the crystal grains is regarded as L2. The value of the aspect ratio is in the relationship of 25 ≧ L1 / L2 ≧ 1.5. The surface hardness (HV) after the temperature was maintained at 600 ° C for 400 hours was 300 or more and 571 or less. The Vostian iron-based stainless steel sheet according to claim 1, which has a composition content of mass% and contains Nb: 0.01 to 0.10% and / or Ti: 0.01 to 0.10%. The Vostian iron-based stainless steel plate as described in claim 1, its composition is in mass%, and contains from Y: 0.01 to 0.10%, Ca: 0.001 to 0.010%, Mg: 0.0002 to 0.010%, and REM: 0.01 to More than one selected from 0.10%. The Vostian iron-based stainless steel plate as described in claim 2, its composition is in mass%, and contains Y: 0.01 to 0.10%, Ca: 0.001 to 0.010%, Mg: 0.0002 to 0.010%, and REM: 0.01 to More than one selected from 0.10%. The Vostian iron-based stainless steel sheet according to any one of claims 1 to 3, is subjected to quenched and tempered rolling with a reduction ratio of 20% to 80% during cold rolling.