WO2018180643A1

WO2018180643A1 - Ferrite stainless steel having superior wear resistance at high temperature, production method for ferrite stainless steel sheet, exhaust components, high-temperature sliding components, and turbocharger components

Info

Publication number: WO2018180643A1
Application number: PCT/JP2018/010619
Authority: WO
Inventors: 濱田　純一; 睦子吉井; 俊希吉澤
Original assignee: 新日鐵住金ステンレス株式会社
Priority date: 2017-03-29
Filing date: 2018-03-16
Publication date: 2018-10-04
Also published as: MX2019011210A; JPWO2018180643A1; CN110462081B; JP6768929B2; CN110462081A

Abstract

This ferrite stainless steel having superior wear resistance at high temperature is characterized by containing, in mass%, 0.003-0.02% of C, 0.05-1.0% of Si, 0.05-1.0% of Mn, 0.01-0.05% of P, 0.0001-0.01% of S, 15-18% of Cr, 0.002-0.02% of N, 0.01-0.20% of Al, 1-3% of Cu, 1.7-3% of Mo, 0.4-0.7% of Nb, and 0.0002-0.0030% of B, the remaining portion being Fe and incidental impurities, wherein 0.06 particles/μm² or more of a Nb-containing deposit exist at a depth of 20 μm from the surface layer.

Description

Ferritic stainless steel with excellent high temperature wear resistance, manufacturing method of ferritic stainless steel sheet, exhaust parts, high temperature sliding parts, and turbocharger parts

The present invention relates to a ferritic stainless steel excellent in high-temperature wear resistance, a method for producing a ferritic stainless steel sheet, an exhaust part, a high-temperature sliding part, and a turbocharger, which are materials for heat-resistant parts that require heat resistance and workability. It relates to parts.
The present invention is particularly applicable to automobile exhaust manifolds, converters, and turbocharger parts. In particular, the present invention relates to materials suitable for internal precision parts such as nozzle mounts, nozzle plates, vanes, and back plates of turbochargers mounted on gasoline vehicles and diesel vehicles, and outer cylinders such as turbine housings.

Environment-friendly parts for automobile exhaust manifold, front pipe, center pipe, muffler and exhaust gas purification have high heat resistance such as oxidation resistance, high temperature strength, thermal fatigue characteristics, etc. in order to allow high temperature exhaust gas to flow stably. Excellent material is used. Moreover, since it is also a condensed water corrosive environment, it is also required to have excellent corrosion resistance. Stainless steel is often used for these parts from the viewpoints of stricter exhaust gas regulations, improved engine performance, and lighter body weight. Further, in recent years, exhaust gas regulations have been further strengthened, and the exhaust gas temperature flowing through the exhaust manifold directly under the engine has been on the rise due to improvements in fuel efficiency and downsizing. In addition, turbochargers and other turbochargers are often installed, and stainless steel used in exhaust manifolds and turbochargers is required to have further improved heat resistance. Regarding the rise in the exhaust gas temperature, it is expected that the exhaust gas temperature, which was conventionally about 900 ° C., will rise to about 1000 ° C.

On the other hand, the internal structure of the turbocharger is complex, and it is important to increase the supercharging efficiency and to ensure heat-resistant reliability. Heat-resistant austenitic stainless steel is mainly used. In addition to SUS310S (25% Cr-20% Ni), which is a typical heat-resistant austenitic stainless steel, Ni-based alloy, and the like, Patent Documents 1 and 2 disclose high Cr and Mo-added steels. Further, Patent Document 3 discloses an exhaust guide part of a nozzle vane type turbocharger using austenitic stainless steel to which 2 to 4% of Si is added. Patent Document 3 discloses an austenitic stainless steel component in consideration of hot workability. However, since both of them contain expensive Ni and increase in cost, the development of ferritic stainless steel not containing Ni has been expected.

Among the turbochargers, the parts to which stainless steel is mainly applied are precision parts and housings inside the nozzle vane turbocharger. Among the internal precision parts, parts called back plates and oil deflectors are located between the turbine part and compressor part and the center core, and are parts that stably rotate the turbine and compressor wheel while maintaining the sealing performance of each part. Therefore, surface smoothness is important in addition to oxidation resistance and high temperature strength. In addition, in order to adjust the flow rate and flow rate of exhaust gas, there is a nozzle component composed of precision components such as a nozzle mount, a nozzle plate, a nozzle vane, a drive ring, and a drive lever. Since these are in contact with high-temperature exhaust gas, high-temperature strength, creep characteristics, and oxidation resistance are important, and high-temperature wear resistance is important because the exhaust gas flow rate and flow rate are adjusted by opening and closing the vanes. In addition, high-temperature strength, creep, and thermal fatigue characteristics are regarded as important for turbine housings, but because they are in contact with the housing, back plate, clamps, and subsequent exhaust components in a high-temperature environment, wear resistance at high temperatures is required. The

Ferritic stainless steel is mainly used for exhaust manifolds, and Patent Documents 4 to 16 disclose technologies related to ferritic stainless steel to which Nb, Si, Cu, W, etc. are added as countermeasures for increasing the exhaust gas temperature. Has been.

JP 2002-332862 A International Publication No. 2014/157655 Japanese Patent No. 4937277 JP 2006-37176 A International Publication No. 2003/004714 Japanese Patent No. 3468156 Japanese Patent No. 3397167 JP-A-9-279312 JP 2000-169943 A Japanese Patent Laid-Open No. 10-204590 JP 2009-215648 A JP 2009-235555 A JP 2005-206944 A JP 2008-189974 A JP 2009-120893 A JP 2009-120894 A

However, since these stainless steels are designed from the viewpoints of oxidation resistance, high temperature strength, and high temperature fatigue, they do not necessarily satisfy the above performance as a turbocharger part.
SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the known art and to provide a ferritic stainless steel excellent in high temperature wear resistance suitable for use as a turbocharger component in a high temperature sliding component, particularly an automobile exhaust component, and a method for producing the same. Is to provide.

In order to solve the above-mentioned problems, the present inventors have intensively studied and found that not only the steel components but also precipitates and hardness in the vicinity of the surface layer are particularly important for high temperature wear resistance. When subjected to friction at high temperatures, it undergoes strong processing locally in the vicinity of the surface layer, so that the structural change and hardness change of the part are important. Regarding the change in structure, precipitation of various precipitates occurs, so that high-temperature strengthening is a point. The present inventors have examined in detail the precipitation behavior and wear resistance during high-temperature sliding, and in particular, have examined whether or not ferritic stainless steel can be applied to turbocharger parts.
Specifically, the present inventors conducted detailed studies on the high-temperature wear resistance of ferritic stainless steel sheets from the viewpoints of steel composition, metal structure, and high-temperature characteristics. As a result, for example, components that are exposed to extremely harsh thermal environments, such as turbochargers, ensure heat resistance with steel components and control the metal structure near the surface layer. As a result, it has been found that a ferritic stainless steel sheet having excellent high-temperature wear resistance and a component composed thereof can be obtained.
In order to improve the wear resistance at high temperatures, it is effective that the surface layer has good oxidation characteristics and is hard, but in the present invention, the hardness when exposed to fine precipitates on the surface layer for a long time at high temperatures. It was found that the amount of wear can be significantly reduced. As a result, it is possible to provide a component with improved high-temperature wear resistance using a ferritic stainless steel plate without using an expensive austenitic stainless steel plate.

The gist of the present invention for solving the above problems is as follows.
(1) By mass%, C: 0.003-0.02%, Si: 0.05-1.0%, Mn: 0.05-1.0%, P: 0.01-0.05% , S: 0.0001 to 0.01%, Cr: 15 to 18%, N: 0.002 to 0.02%, Al: 0.01 to 0.20%, Cu: 1 to 3%, Mo: 1.7 to 3%, Nb: 0.4 to 0.7%, B: 0.0002 to 0.0030%, the balance is made of Fe and inevitable impurities, and Nb is contained at a depth of 20 μm from the surface layer A ferritic stainless steel excellent in high temperature wear resistance characterized by the presence of 0.06 precipitates / μm ² or more.
(2) The high-temperature wear resistance according to (1), wherein the cross-sectional hardness at normal temperature at a depth of 20 μm from the surface layer after aging at 850 ° C. or higher for 1 hour is HV180 or higher with a Vickers hardness of 1 kg of load. Excellent ferritic stainless steel.
(3) Further, in terms of mass%, Ti: 0.005 to 0.3%, W: 0.1 to 3.0%, V: 0.05 to 1%, Zr: 0.05 to 0.3%, Sn: 0.01 to 0.5%, Ni: 0.1 to 0.5%, Co: 0.03 to 0.3%, Mg: 0.0002 to 0.01%, Sb: 0.005 to 0.5%, REM: 0.001 to 0.2%, Ga: 0.0002 to 0.3%, Ta: 0.001 to 1.0%, Ca: 0.0002 to 0.01% Ferritic stainless steel excellent in high temperature wear resistance according to (1) or (2), characterized in that it contains seeds or two or more kinds.
(4) After high-temperature wear test in which the pin with a diameter of 4 mm is pressed with an vertical load of 0.5 N in an air atmosphere at 850 ° C., and is rotated and slid until the test length reaches 20 m at a rotation radius of 10 mm and a speed of 3.3 mm / sec. The ferritic stainless steel excellent in high-temperature wear resistance according to any one of (1) to (3), wherein the wear amount of the steel is 7 μm or less.
(5) When producing a steel plate made of the ferritic stainless steel according to any one of (1) to (4), the cold-rolled sheet annealing temperature is set to a temperature exceeding 1050 ° C. to 1120 ° C., and the cooling rate to 900 ° C. The manufacturing method of the ferritic stainless steel plate excellent in high temperature abrasion resistance characterized by making it less than 10 degrees C / sec.
(6) An exhaust part comprising the ferritic stainless steel according to any one of (1) to (4).
(7) A high-temperature sliding component comprising the ferritic stainless steel according to any one of (1) to (4).
(8) A turbocharger part comprising the ferritic stainless steel according to any one of (1) to (4).

According to the present invention, it is possible to provide a ferritic stainless steel having high-temperature wear resistance that is suitable for turbocharger parts. At the same time, it contributes to cost reduction and reduction of parts manufacturing load.

The figure which shows the relationship between the hardness after aging heat processing, and the wear depth in a high temperature abrasion test of this invention steel and a comparison steel. The figure which shows the state of the Nb containing precipitate of the surface layer vicinity of invention steel A and comparative steel B. FIG.

Hereinafter, an embodiment suitable for a ferritic stainless steel plate excellent in high temperature wear resistance according to the present invention will be described in detail.

[component]
First, the component range of steel will be described. Unless otherwise indicated,% regarding component content shows the mass%.

C is an austenite-forming element. When an austenite phase is generated during high temperature use, abnormal oxidation occurs and the high temperature wear resistance is remarkably reduced. Further, when carbide is generated in the ferrite phase, the high temperature strength is lowered, so 0.02% is made the upper limit. On the other hand, excessive reduction leads to cost increase, so 0.003% is made the lower limit. Furthermore, considering the manufacturing cost, the lower limit is preferably 0.005%. Further considering the toughness, the upper limit is preferably 0.010%.

Si may be added as a deoxidizing element, and 0.05% or more is added in order to improve scale peelability and high-temperature wear resistance by internal oxidation of Si. On the other hand, addition of more than 1.0% remarkably hardens and deteriorates workability and lowers toughness, so the upper limit is made 1.0%. Furthermore, considering the manufacturing cost, pickling property at the time of manufacturing the steel sheet, and solidification cracking property at the time of welding, the upper limit of the Si content is preferably 0.5%. The lower limit is preferably 0.1%. Furthermore, considering the workability of the steel sheet, the upper limit is preferably 0.2%.

Mn is used as a deoxidizing element and is added in an amount of 0.05% or more in order to improve scale peelability. On the other hand, it is an austenite-forming element. When an austenite phase is generated during use at a high temperature, abnormal oxidation occurs and the high-temperature wear resistance is remarkably lowered, so the upper limit is made 1.0%. Furthermore, considering the manufacturing cost and the pickling property at the time of manufacturing the steel sheet, the lower limit of the Mn content is 0.2%, and the upper limit of Mn is preferably 0.3% from the viewpoint of softening.

P is an element that promotes hot workability and solidification cracking at the time of manufacture, and its content is preferably as low as possible because it hardens. However, considering refining costs, the upper limit is 0.05% and the lower limit is 0. .01%. Furthermore, considering the manufacturing cost, the lower limit of the P content is preferably 0.02%. The upper limit is preferably 0.04%.

S is an element that degrades hot workability during production and deteriorates corrosion resistance. Further, when coarse sulfide (MnS) is formed, the cleanliness is remarkably deteriorated and the hole expandability is deteriorated, so the upper limit is made 0.01%. On the other hand, excessive reduction leads to an increase in refining costs, so the lower limit is made 0.0001%. Furthermore, considering the manufacturing cost and oxidation resistance, the lower limit of the S content is preferably 0.0005%. The upper limit is preferably 0.0050%.

Cr is an element that improves corrosion resistance and oxidation resistance and improves high-temperature wear resistance. When considering the exhaust part environment, 15% or more is necessary from the viewpoint of suppressing abnormal oxidation. On the other hand, excessive addition becomes hard and deteriorates moldability, and also leads to an increase in cost, so the upper limit was made 18%. Furthermore, considering the manufacturing cost, steel plate manufacturability and workability, the lower limit of the Cr content is desirably 16%. The upper limit is desirably 17.5%.

N is an austenite-forming element like C, and when an austenite phase is generated during high-temperature use, abnormal oxidation occurs and the high-temperature wear resistance is significantly reduced. In addition, when a large amount of nitride is generated in the ferrite phase, the high temperature strength decreases, so 0.02% is made the upper limit. On the other hand, excessive reduction leads to high cost, so 0.002% is made the lower limit. From the viewpoint of cost, the lower limit is preferably 0.003%. Furthermore, from the viewpoint of weldability and intergranular corrosion, the upper limit is desirably 0.010%.

Al is added as a deoxidizing element to improve inclusion cleanliness, and to improve high temperature wear resistance by forming an internal oxide at a high temperature, it is added in an amount of 0.01% or more. On the other hand, addition of more than 0.20% makes it harder and also lowers the pickling property, so the upper limit is made 0.20%. Furthermore, considering workability and weldability, the lower limit of the Al content is preferably 0.02%. The upper limit is desirably 0.10%.

Cu is added at 1% or more because precipitation strengthening acts in a high temperature environment and improves high temperature strength, thermal fatigue characteristics, high temperature high cycle fatigue characteristics and high temperature wear resistance. On the other hand, addition of more than 3% generates an austenite phase, and the oxidation resistance and high-temperature wear resistance are remarkably deteriorated. Furthermore, considering the creep characteristics, the lower limit is desirably 1.1% and more desirably 1.2%. Furthermore, considering the manufacturability, the upper limit is desirably 2.0%.

Mo contributes to the improvement of high temperature strength by solid solution strengthening and reacts with Nb and Fe to promote the precipitation of the Laves phase. Although this Laves phase is dissolved in the product plate stage, it precipitates when the part is used in a high temperature environment, and contributes to improvement of high temperature strength and high temperature wear resistance. Since these effects are manifested at 1.7% or more, the lower limit is set to 1.7%. On the other hand, excessive addition causes deterioration of workability and toughness, so the upper limit is made 3%. Furthermore, considering that Mo is an expensive element, the upper limit is desirably 2.8%. Considering the strengthening stability due to the precipitate and the cleanliness of inclusions, the lower limit of the Mo content is preferably 2.3%.

Nb is an element that combines with C and N to improve corrosion resistance and intergranular corrosion resistance, as well as high temperature strength. The mechanism for improving the high-temperature strength includes a solid phase strengthening Laves phase precipitation strengthening. In addition, the present inventor has found that although it is precipitated as a carbonitride or a small amount of a Laves phase at the stage of the product plate, these Nb-containing precipitates are extremely effective for enhancing the high-temperature wear resistance. This is because the Nb-containing precipitate is hard and contributes to a reduction in the wear amount of the base material by increasing the hardness in the vicinity of the sliding surface. Oxidation scale greatly affects high-temperature wear resistance, but if other elements specified in the present invention (for example, oxide-forming elements such as Cr, Si, Mn, etc.) are properly added, abnormal oxidation or excessive oxidation Increase in quantity does not occur. Therefore, the wear rate of the base material is rate-determined, and the high-temperature wear resistance is better when the hard Nb-containing precipitates are dispersed. Since these effects are manifested by addition of 0.4% or more, the lower limit was made 0.4%. On the other hand, since the workability is remarkably deteriorated by adding over 0.7%, the upper limit is made 0.7%. Furthermore, considering the high temperature strength, intergranular corrosion of the weld and the alloy cost, it is desirable that the lower limit of the Nb content is 0.5% and the upper limit is 0.6%.

B is an element that generally segregates at grain boundaries and improves secondary workability. In the present invention, it was found that the grain boundary segregation of B improves the high temperature wear resistance, so 0.0002% or more is added. This is presumably because B segregates at the grain boundary in the vicinity of the surface layer to increase the grain boundary strength and improve the wearability at high temperatures. When the grain boundary strength is weak, the grain boundary is likely to wear during friction and wear, but it is considered that the grain boundary strengthening by addition of B suppresses this. In addition, the addition of B also has the effect of finely dispersing and precipitating Nb-containing precipitates within the crystal grains, and is effective in improving wear. This is because Nb-containing precipitates are prevented from precipitating at the grain boundaries due to B grain boundary segregation, and fine precipitates are precipitated within the grains, thereby improving the high-temperature wear resistance. Based on these new findings, 0.0002% or more is added in the present invention. On the other hand, addition of over 0.0030% causes deterioration of intergranular corrosion, toughness, and fatigue properties due to boride precipitation, so the upper limit is made 0.0030%. Furthermore, considering refining costs and a decrease in ductility, the lower limit of the B content is preferably 0.0002% and the upper limit is preferably 0.0020%.

The ferritic stainless steel sheet of the present invention may further contain one or two of Ti, W, V, Zr, Sn, Ni, Co, Mg, Sb, REM, Ga, Ta, and Ca.
Ti is an element that combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, room temperature ductility, and deep drawability, and is added as necessary. Moreover, in this invention, when improving normal temperature workability by precipitation of FeTiP, since the effect expresses from 0.005% or more, the minimum was made into 0.005%. On the other hand, addition of more than 0.3% increases the amount of dissolved Ti and lowers room temperature ductility, forms coarse Ti-based precipitates, degrades high-temperature wear resistance, and at the time of hole expansion processing It becomes the starting point of cracking and deteriorates press workability. In addition, the Laves phase is excessively precipitated, so that the solid solution Nb and the solid solution Mo are insufficient, and the high temperature strength is lowered. Further, since the oxidation resistance is also deteriorated, the amount of Ti added is set to 0.3% or less. Furthermore, considering the occurrence of surface flaws and toughness, the lower limit is preferably 0.05%. The upper limit is preferably 0.2%.

W, like Mo, is an element effective as a solid solution strengthening at 950 ° C., and also generates a Laves phase (Fe ₂ W), thereby causing a precipitation strengthening action and contributing to an improvement in high temperature wear resistance. In particular, when combined with Nb or Mo, the Laves phase of Fe ₂ (Nb, Mo, W) is precipitated. However, when W is added, the coarsening of the Laves phase is suppressed and the precipitation strengthening ability is improved. Furthermore, as described above, these Laves phases tend to become fine due to the coexistence with Fe-P-based precipitates. Since this works with addition of 0.1% or more, the lower limit is made 0.1%. On the other hand, the addition of more than 3.0% increases the cost and forms a coarse Laves phase to deteriorate the high temperature wear resistance. Moreover, since normal temperature ductility falls, an upper limit was made into 3.0%. Furthermore, when manufacturability, low temperature toughness and oxidation resistance are taken into consideration, the lower limit of the W addition amount is desirably 0.2%, and the upper limit is desirably 1.5%.

V is an element that improves the corrosion resistance, and is added as necessary. Moreover, VC is formed and high temperature abrasion resistance is improved. This effect is stably manifested with addition of 0.05% or more, but if added over 1%, the precipitates become coarse and the high-temperature strength decreases, and the oxidation resistance deteriorates, so the upper limit was made 1%. . Furthermore, considering the manufacturing cost and manufacturability, the lower limit is preferably 0.08%. The upper limit is preferably 0.5%.

Zr is a carbonitride-forming element like Ti and Nb, is an element that improves corrosion resistance and deep drawability, and is added as necessary. Although these effects are manifested at 0.05% or more, the productivity was markedly deteriorated by adding over 0.3%. Furthermore, considering the cost and surface quality, the lower limit is preferably 0.05%. The upper limit is preferably 0.2%.

Sn is an element that improves the corrosion resistance, and is added as necessary to improve the high temperature strength in the middle temperature range. These effects are manifested at 0.01% or more, but if added over 0.5%, manufacturability is remarkably reduced, so 0.01 to 0.5% was set. Furthermore, considering the oxidation resistance and manufacturing cost, the lower limit is preferably 0.03%. The upper limit is preferably 0.3%.

Ni is an element that improves acid resistance and toughness, and is added as necessary. These effects are manifested at 0.1% or more, but if added over 0.5%, the cost increases, and when accompanied by the formation of austenite, the high temperature wear resistance deteriorates. %. Further, considering the manufacturability, the lower limit is preferably 0.15%. The upper limit is preferably 0.3%.

Co contributes to improving high-temperature strength, so 0.03% or more is added as necessary. Addition of over 0.3% leads to toughness deterioration, so the upper limit is made 0.3%. Furthermore, considering refining costs and manufacturability, the upper limit is preferably 0.1%.

Mg is an element that may be added as a deoxidizing element and that contributes to improving the formability by refining the slab structure. Further, the Mg oxide becomes a precipitation site for carbonitrides such as Ti (C, N) and Nb (C, N), and has an effect of finely dispersing and depositing them. This effect appears at 0.0002% or more, and contributes to toughness improvement, so the lower limit was made 0.0002%. However, excessive addition leads to deterioration of weldability and corrosion resistance, so the upper limit was made 0.01%. In consideration of refining costs, the lower limit is preferably 0.0003%. The upper limit is preferably 0.0010%.

Sb contributes to improvement of corrosion resistance and high-temperature strength, so 0.005% or more is added as necessary. Since addition of more than 0.5% may excessively cause slab cracking or ductility reduction during steel sheet production, the upper limit is made 0.5%. Furthermore, considering refining costs and manufacturability, the lower limit is preferably 0.005%. The upper limit is preferably 0.15%.

REM may be added as necessary from the viewpoint of improving toughness and oxidation resistance by refining various precipitates, and this effect is manifested at 0.001% or more, so the lower limit is 0. 0.001%. However, addition of more than 0.2% significantly deteriorates castability and lowers ductility, so the upper limit was made 0.2%. Furthermore, considering refining costs and manufacturability, 0.001 to 0.05% is desirable. REM (rare earth element) refers to a generic name of two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoid) from lanthanum (La) to lutetium (Lu) according to a general definition. It may be added alone or as a mixture.

Ga may be added at 0.3% or less for improving corrosion resistance and suppressing hydrogen embrittlement. The lower limit is made 0.0002% from the viewpoint of sulfide and hydride formation. Furthermore, 0.0020% or less is desirable from the viewpoints of manufacturability and cost and from the viewpoints of ductility and toughness.

Ta is combined with C and N to contribute to the improvement of toughness, so 0.001% or more is added as necessary. However, the addition of more than 1.0% increases the cost and remarkably deteriorates manufacturability, so the upper limit is made 1.0%. Furthermore, considering refining costs and manufacturability, the lower limit is preferably 0.005%. The upper limit is preferably 0.08%.

Ca may be added for desulfurization, and since this effect is manifested at 0.0002% or more, the lower limit was made 0.0002%. However, the addition of over 0.01% produces coarse CaS and degrades toughness and corrosion resistance, so the upper limit was made 0.01%. Furthermore, considering refining costs and manufacturability, the lower limit is preferably 0.0003%. The upper limit is preferably 0.0020%.

Regarding the other components, the balance is Fe and inevitable impurities and is not particularly specified in the present invention. In the present invention, Bi or the like is added in an amount of 0.001% or more and 0.1% or less as required. May be. Note that it is desirable to reduce general harmful elements and impurity elements such as As and Pb as much as possible.

[Cross section hardness after aging heat treatment]
In the ferritic stainless steel of the present invention, it is desirable that the cross-sectional hardness at room temperature at a depth of 20 μm from the surface layer after aging at 850 ° C. for 1 hour is HV180 or more in terms of Vickers hardness with a load of 1 kg.
By using HV180 or higher, high temperature wear resistance higher than that of general-purpose austenitic stainless steels SUS310S and SUSXM15J11 can be obtained. it can.
From the viewpoint of further reducing the amount of wear and sliding stability, the hardness after aging heat treatment is preferably HV210 or more. Furthermore, when the ferritic stainless steel of the present invention is used for a turbocharger, the hardness after aging heat treatment is preferably HV250 or less from the viewpoint of workability of the turbo parts.

[High temperature wear resistance]
The ferritic stainless steel of the present invention uses the amount of wear after a high temperature friction test as an index of high temperature wear resistance. This is because it is assumed that the ferritic stainless steel of the present invention is used under the condition of sliding at a high speed in a high temperature environment like a turbocharger part.
Specifically, in an air atmosphere at 850 ° C., a pin with a diameter of 4 mm is pressed with a vertical load of 0.5 N, and a rotary radius of 10 mm and a speed of 3.3 mm / sec are rotated and slid until the test length reaches 20 m. The amount of wear after the wear test is desirably 7 μm or less.

[Number of precipitates near the surface]
The ferritic stainless steel of the present invention is limited to the presence of 0.06 / μm ² or more of Nb-containing precipitates at a depth of 20 μm from the surface layer. The reason is as follows.
As a characteristic of a ferritic stainless steel sheet used for heat-resistant applications, what is important is high-temperature strength, but particularly in the case of turbocharger parts, high-temperature wear resistance with other parts is also extremely important. For example, in a part called a nozzle vane for controlling the flow rate and flow rate of exhaust gas, a part called a nozzle plate or nozzle mount and a part called a vane slide at high speed in a high-temperature exhaust gas environment. At this time, if the amount of wear due to sliding is remarkably large or adhesion or the like occurs, the sliding characteristics deteriorate, and the flow rate or flow rate of exhaust gas cannot be controlled.
Here, since the Nb-containing precipitate has high hardness and is stable even at a relatively high temperature range, it is considered that the wear during high-temperature sliding can be reduced by the hard Nb-containing precipitate. Therefore, in the present invention, the number density of Nb-containing precipitates is defined.
The reason why the depth from the surface layer is limited to 20 μm is that the amount of wear is taken into consideration, but considering a closer relationship with the amount of wear, the Nb-containing precipitates are 0 even at a depth of 10 μm from the surface layer. .06 / μm ² or more is desirable.

[Production method]
Next, a manufacturing method will be described. The method for producing a steel sheet of the present invention comprises a steelmaking-hot rolling-annealing / pickling process or a steelmaking-hot rolling-annealing / pickling-cold rolling-annealing / pickling process. In steelmaking, a method in which steel containing the essential components and components added as necessary is subjected to electric furnace melting or converter melting, followed by secondary refining is preferable. The molten steel is made into a slab according to a known casting method (such as continuous casting). The slab is heated to a predetermined temperature and hot-rolled to a predetermined plate thickness by continuous rolling. The hot rolling may be tandem continuous hot rolling or Steckel reverse rolling, and the manufacturing conditions may be determined according to the steel composition. The steel sheet after hot rolling is generally subjected to hot-rolled sheet annealing and pickling treatment, but hot-rolled sheet annealing may be omitted. Then, it cold-rolls to predetermined plate | board thickness, and cold-rolled sheet annealing and a pickling process are performed. Usually, the annealing temperature is 1000 ° C. or higher and 1120 ° C. or lower to obtain a recrystallized structure. In the present invention, in order to ensure the number density of Nb-containing precipitates in the vicinity of the surface layer, the cold-rolled sheet annealing temperature is increased to over 1050 ° C., and Nb is dissolved as much as possible in the heating stage, and then precipitated in the subsequent cooling process. Let In that case, the cooling rate to 900 degreeC in the cooling process after a heating is prescribed | regulated to less than 10 degreeC / sec. This deposits Nb solid-solution in the heating stage in the cooling process, and the precipitation is insufficient at 10 ° C./sec or more. On the other hand, if the cooling rate is excessively slow, the Nb-containing precipitate is excessively precipitated and coarsened, resulting in insufficient high-temperature strength. Moreover, since productivity also deteriorates remarkably, it is set to 1 ° C./sec or more. Furthermore, when considering the shape, productivity, toughness, and corrosion resistance of the steel sheet, it is preferably 3 ° C./sec or more and 9 ° C./sec or less. By carrying out a process that satisfies such conditions, it can be controlled so that 0.06 pieces / μm ² or more of Nb-containing precipitates are present at a depth of 20 μm from the surface layer.
Note that other conditions in the manufacturing process may be appropriately selected. For example, what is necessary is just to design slab thickness, hot rolling board thickness, etc. suitably. In cold rolling, roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, etc. may be appropriately selected. Intermediate annealing may be put in the middle of cold rolling, and batch annealing or continuous annealing may be used. In the pickling step, treatment with sulfuric acid or hydrochloric acid may be performed in addition to nitric acid and nitric acid electrolytic pickling. The shape and material may be adjusted by temper rolling or tension leveler after annealing and pickling of the cold-rolled sheet. Furthermore, cold rolling and cold rolled sheet annealing may be omitted if a surface satisfying the requirements defined in the present invention is obtained. In addition, for the purpose of improving press molding, a lubricating film can be applied to the product plate. After the parts are processed, a special surface treatment such as nitriding or carburizing may be performed to further improve the heat resistance. In the case of the steelmaking-hot rolling-annealing / pickling process, Nb-containing precipitates are deposited during annealing after hot rolling.

In the present invention, by ensuring the number density of Nb-containing precipitates in the vicinity of the surface layer of the stainless steel sheet product, high hardening after aging heat treatment is achieved, and excellent high-temperature wear resistance is obtained. However, it is not always necessary to ensure the number density of the Nb-containing precipitates in the state of the steel plate. For example, the number density of Nb-containing precipitates may be secured by performing a heat treatment after processing into a turbocharger part or during the processing.

EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to an Example.
(Preliminary test)
First, in order to determine the desirable hardness, high temperature wear amount, and Nb content of the present invention, a high temperature wear test was conducted as a preliminary test under the following conditions, and the hardness, high temperature wear amount, and Nb content were measured.
First, steels A, B, C, D, and E shown in Table 1 were prepared.

Steel A is the steel of the present invention. Steels B and C are typical heat-resistant ferritic stainless steels. Steel B is SUS444, and steel C is SUS430LX. Steels D and E are typical austenitic stainless steels SUS310S and SUSXM15J1.

In the high temperature wear test, a pin of the same material (tip diameter 4 mm) was pressed against a steel sample processed into a disk (diameter 29 mm) shape in an air atmosphere at 850 ° C. with a vertical load of 0.5 N, a rotating radius 10 mm, a speed of 3 Rotating and sliding at 3 mm / sec. The holding time at 850 ° C. before the test was 1 hour, and the test was terminated when the test length reached 20 m. After the high temperature wear test, the wear depth was measured at room temperature. A laser microscope was used for the measurement of the wear depth, and the average of the maximum depths measured at three locations was defined as the wear depth.

FIG. 1 shows the test results of the wear depth of three types of ferritic stainless steels (A, B, C) and the wear depth of typical general-purpose austenitic stainless steels (D, E). FIG. 1 is a graph showing the relationship with the normal temperature hardness of each steel after aging heat treatment at 850 ° C. for 1 hour. For the normal temperature hardness, an average value obtained by measuring the Vickers hardness of a cross section at a depth of 20 μm from the surface layer at a load of 1 kg at five points was used.

As shown in FIG. 1, the present invention steel A not only has higher wear resistance than other typical ferritic stainless steels, but also has less wear than austenitic stainless steels and exhibits excellent high temperature wear resistance. It was. This indicates that austenitic stainless steel, which was conventionally expensive from the viewpoint of high temperature wear resistance, can be replaced with the ferritic stainless steel of the present invention, which is extremely effective economically. It can be said to be a characteristic.

The reason for having excellent high temperature wear resistance is considered to be due to the high hardness after aging heat treatment. In general, the relationship between the hardness and the wear amount of the ferritic stainless steel and the austenitic stainless steel tends to be different, which may be influenced by the crystal structure and oxide scale. Preliminary tests revealed that the present invention steel A, which has high hardness even in ferritic stainless steel, had less wear than steels B and C, and therefore the hardness after aging heat treatment was greatly related to high temperature wear resistance. It was.

From the above results, the ferritic stainless steel of the present invention has high-temperature wear resistance (wear amount of 7 μm or less) of general-purpose austenitic stainless steels SUS310S and SUSXM15J11 if the hardness after aging heat treatment is HV180 or more. I found out that Therefore, it was found that the hardness after aging heat treatment of the ferritic stainless steel of the present invention is desirably HV180 or more, and the amount of wear after the high temperature wear test is desirably 7 μm or less. This range is the range indicated by hatching in FIG.

Next, the Nb precipitation state in the range from the surface layer before the test of Steel A and Steel B used in the high temperature sliding test shown in FIG. 1 to a depth of 20 μm was observed. Specifically, a cross section parallel to the rolling direction of the steel sheet was embedded and polished, etched with aqua regia, Nb-containing precipitates were observed with a scanning electron microscope, and the number density was calculated. Judgment of Nb-containing precipitates was performed by elemental analysis of the precipitates with an analyzer attached to the scanning electron microscope, and judged by the presence or absence of Nb concentration. Here, the Nb-containing precipitates include Nb-containing carbonitrides, Laves phases, phosphides, and those that are segregated and complex-precipitated at the interfaces such as Cu precipitates and Ti-based precipitates.

The observation results are shown in FIG. In FIG. 2, the granular white portions are Nb-containing precipitates.
As shown in FIG. 2, the number density of the Nb-containing precipitates in the comparative steel B was 0.03 / μm ² , whereas the steel A excellent in high temperature wear resistance has a number density of 0.06. / Μm ² and higher than that of Comparative Steel B. Therefore, it can be seen that the hardness after aging heat treatment can be secured and the high-temperature wear resistance can be improved by setting the number density of Nb-containing precipitates with a depth of 20 μm to 0.06 pieces / μm ² or more. It was. This is presumably because Nb-containing precipitates have high hardness and are stable even in a relatively high temperature range, and thus wear is reduced by the Nb-containing precipitates.

(Turbocharger test)
Next, steels having various component compositions and production conditions were prepared, and the relationship between the density, hardness, proof stress, and wear amount of Nb-containing precipitates was investigated. Furthermore, a turbocharger was produced from the produced steel and subjected to a test. The specific procedure is as follows.

First, the prepared steel is melted and cast into a slab, and hot rolled, hot-rolled sheet annealing / pickling, cold-rolling, final annealing / pickling are performed to obtain 4.3 mm and 2.0 mm thick product plates. Obtained. The component composition of the product plate obtained was as shown in Tables 2 and 3. The final annealing conditions are shown in Tables 4 and 5 described later.

Next, the hardness measurement after the aging heat treatment at 850 ° C. for 1 hour, the high temperature sliding test, and the number density measurement of the Nb-containing precipitates were performed on the 4.3 mm thick product plate. Moreover, the high temperature tensile test was done with respect to the 2.0 mm thick product board. In the high-temperature tensile test, tensile test pieces are prepared so that the rolling direction and the tensile direction are parallel, heated to 850 ° C. at a heating rate of 100 ° C./min, holding time of 10 min, and constant speed at a crosshead speed of 1 mm / min. A tensile test was performed to obtain a 0.2% yield strength in the rolling direction.
The high temperature sliding test was performed under the same conditions as the preliminary test, and the amount of wear was measured under the same conditions as the preliminary test after the test. A wear amount of 7 μm or less was accepted and over 7 μm was rejected. Moreover, cross-sectional hardness was measured on the same conditions as a preliminary test, and the hardness after aging passed 180 or more, and made less than 180 disqualified. Furthermore, the number density of the Nb-containing precipitates was measured under the same conditions as in the preliminary test, and 0.06 / μm ² or more was accepted and less than 0.06 / μm ² was rejected. About 0.2% yield strength of the high temperature tensile test, 40 MPa or more was passed at 850 ° C., and less than 40 MPa was rejected.

In addition, the sample material is processed into nozzle mount, nozzle plate and housing parts, mounted on a known nozzle vane turbocharger, and high temperature (850 ° C) exhaust gas is allowed to flow while repeatedly opening and closing the nozzle to improve gas flowability. Examined. At this time, the steel in which there was no problem in the gas flow was accepted, and the steel in which the gas flow failure (pressure loss 10% or more) and the nozzle opening / closing were defective was rejected.

As a result of manufacturing under the manufacturing conditions shown in Tables 4 and 5, it was confirmed that the steel of the present invention example was excellent in workability, heat resistance, and surface properties and satisfied the performance as a turbocharger part. When the steel composition, Nb-containing precipitate density, and cross-sectional hardness were outside the scope of the present invention, the processing accuracy and turbocharger performance were poor, resulting in problems. Further, even when the high-temperature strength was poor, the turbocharger performance was poor due to creep deformation.

According to the present invention, it is possible to provide a ferritic stainless steel sheet that is superior in cost to austenitic stainless steel for exhaust parts that require high temperature wear resistance. In particular, by using it as a part of a turbocharger of an automobile, it becomes possible to lead to exhaust gas regulation, weight reduction, and fuel efficiency improvement. Also, it is possible to omit parts cutting and grinding and surface processing, which greatly contributes to cost reduction. Furthermore, the present invention can be applied not only to automobiles and motorcycle exhaust parts, but also to exhaust parts used in high-temperature environments such as various boilers and fuel cell systems, and high-temperature sliding parts. It is.

Claims

In mass%, C: 0.003 to 0.02%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, P: 0.01 to 0.05%, S: 0.0001 to 0.01%, Cr: 15 to 18%, N: 0.002 to 0.02%, Al: 0.01 to 0.20%, Cu: 1 to 3%, Mo: 1.7 -3%, Nb: 0.4-0.7%, B: 0.0002-0.0030%, the balance consisting of Fe and unavoidable impurities, and Nb-containing precipitates at a depth of 20 μm from the surface layer Ferritic stainless steel excellent in high temperature wear resistance characterized by the presence of 0.06 pieces / μm 2 or more.
The high-temperature wear resistance according to claim 1, characterized in that the cross-sectional hardness at room temperature at a depth of 20 µm from the surface layer after aging at 850 ° C for 1 hour is HV180 or more at a Vickers hardness of 1 kg load. Ferritic stainless steel.
Further, by mass: Ti: 0.005 to 0.3%, W: 0.1 to 3.0%, V: 0.05 to 1%, Zr: 0.05 to 0.3%, Sn: 0 0.01 to 0.5%, Ni: 0.1 to 0.5%, Co: 0.03 to 0.3%, Mg: 0.0002 to 0.01%, Sb: 0.005 to 0.5 %, REM: 0.001-0.2%, Ga: 0.0002-0.3%, Ta: 0.001-1.0%, Ca: 0.0002-0.01% The ferritic stainless steel excellent in high temperature wear resistance according to claim 1 or 2, characterized by containing at least a seed.
Wear amount after high-temperature wear test in which a pin with a diameter of 4 mm is pressed in an air atmosphere at 850 ° C. with a vertical load of 0.5 N, and a rotary radius of 10 mm, a speed of 3.3 mm / sec, and a sliding slide until the test length reaches 20 m. The ferritic stainless steel excellent in high temperature wear resistance according to any one of claims 1 to 3, characterized in that is not more than 7 µm.
When producing a steel plate made of the ferritic stainless steel according to any one of claims 1 to 4, the cold-rolled sheet annealing temperature is set to over 1050 ° C to 1120 ° C, and the cooling rate to 900 ° C is set to 10 ° C. The manufacturing method of the ferritic stainless steel plate excellent in high temperature abrasion resistance characterized by setting it as less than degrees C / sec.
An exhaust part comprising the ferritic stainless steel according to any one of claims 1 to 4.
A high-temperature sliding part comprising the ferritic stainless steel according to any one of claims 1 to 4.
A turbocharger part comprising the ferritic stainless steel according to any one of claims 1 to 4.