Patent Grant
10988825
The present disclosure relates to a martensitic stainless steel sheet excellent in strength, workability, and corrosion resistance.
Gaps between exhaust system parts of automobiles are sealed with sealing parts called gaskets for the purpose of preventing leakage of exhaust gas, cooling water, lubricating oil, and the like. Since a gasket must exhibit the sealing performance both in the case where the gap widens and in the case where the gap is narrowed due to the pressure fluctuation in the pipe or the like, a convex portion called the bead is formed in the gasket. As the bead is repeatedly compressed and relaxed during use, high strength is required. Depending on the shape of the bead, severe processing may be applied, and excellent workability is also required for the gasket material. Furthermore, since gaskets are exposed to exhaust gas, cooling water, and the like during use, corrosion resistance is also required. If the gasket material has insufficient corrosion-resistance, fracture may occur due to corrosion.
Conventionally austenitic stainless steels that have both a high strength and a high workability, such as SUS 301 (17 mass % Cr-7 mass % Ni) and SUS 304 (18 mass % Cr-8 mass % Ni), have been widely used. However, since austenitic stainless steels contain a large amount of expensive element Ni, they have a major problem in terms of material cost. Another problem is that austenitic stainless steels have high susceptibility to stress corrosion cracking.
Responding to these problems, there are proposals of martensitic stainless steels such as SUS403 (12 mass % Cr-0.13 mass % C), and stainless steels that comprise a multi-phase structure containing martensite. Both are inexpensive stainless steels because of a low content of Ni, and the strength thereof can be improved by quenching heat treatment.
For example, JP2002-38243A (PTL 1) describes a martensitic stainless steel and a martensite-ferrite dual phase stainless steel which are improved in fatigue resistance by nitriding the surface layer to form an austenite phase by quenching heat treatment in a nitrogen-containing atmosphere.
JP2005-54272A (PTL 2) describes a martensite-ferrite dual phase stainless steel which achieves both hardness and workability by quenching in a dual-phase temperature range of austenite and ferrite.
JP2002-97554A (PTL 3) describes a multi-phase stainless steel having a martensite and retained austenite phase in the surface layer and a martensite single phase in the inner layer after subjection to heat treatment in a nitrogen-containing atmosphere.
In addition, JPH3-56621A (PTL 4) describes a martensite-ferrite dual phase stainless steel improved in spring characteristics after subjection to multi-phase heat treatment followed by aging treatment.
JPH8-319519A (PTL 5) describes a martensite-ferrite dual phase stainless steel having the desired hardness by specifying the cold rolling rate.
JP2001-140041A (PTL 6) describes a stainless steel in which the surface layer is made of two phases of martensite and retained austenite.
JP2006-97050A (PTL 7) describes a stainless steel in which nitrogen is absorbed in SUS 403 or the like to precipitate a nitrogen compound in the surface layer.
JPH7-316740A (PTL 8) describes a multi-phase stainless steel in which a surface layer having a depth of at least 1 μm from the outermost surface is covered with a martensite single-phase layer.
However, all of the stainless steels of PTLs 1 to 8 are insufficient to obtain workability and strength compatibly and may not satisfy the requirement for higher strength when the thickness is reduced for weight reduction.
As described above, the martensitic stainless steel is less susceptible to stress corrosion cracking and is inexpensive as compared with austenitic stainless steel in terms of cost, however, there is room for improvement in terms of both strength and workability.
It would be helpful to provide a martensitic stainless steel sheet that can achieve both excellent strength and workability and that can provide excellent corrosion resistance.
We conducted studies on the strength and workability of martensitic stainless steel sheets and obtained the following findings.
The present disclosure is based on the above discoveries and our further studies.
Specifically, the primary features of the disclosure can be summarized as follows:
1. A martensitic stainless steel comprising a chemical composition containing (consisting of), by mass %, C: 0.030% or more and less than 0.20%, Si: 0.01% or more and 2.0% or less, Mn: 0.01% or more and 3.0% or less, P: 0.050% or less, S: 0.010% or less, Cr: 10.0% or more and 16.0% or less, Ni: 0.01% or more and 0.80% or less, Al: 0.001% or more and 0.50% or less, Zr: 0.005% or more and 0.50% or less, and N: 0.030% or more and less than 0.20%, with the balance consisting of Fe and inevitable impurities.
2. The martensitic stainless steel sheet according to 1. above, wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of Cu: 0.01% or more and 3.0% or less, Mo: 0.01% or more and 0.50% or less, and Co: 0.01% or more and 0.50% or less.
3. The martensitic stainless steel sheet according to 1. or 2. above, wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of Ti: 0.001% or more and 0.50% or less, Nb: 0.001% or more and 0.50% or less, and V: 0.001% or more and 0.50% or less.
4. The martensitic stainless steel sheet according to any one of 1. to 3. above, wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of B: 0.0002% or more and 0.0100% or less, Ca: 0.0002% or more and 0.0100% or less, and Mg: 0.0002 or more and 0.0100% or less.
5. The martensitic stainless steel sheet according to any one of 1. to 4. above, having a tensile strength of 1300 MPa or more, an elongation of 7.0% or more, and an ultimate deformability of 0.5 or more.
The present disclosure can provide a martensitic stainless steel sheet that is excellent in both strength and workability and that has excellent corrosion resistance not only when only quenching treatment is performed, but also when quenching and tempering treatment is carried out. Further, a martensitic stainless steel sheet of the present disclosure can be suitably used for gasket parts of automobiles.
The following provides details of the present disclosure.
First, the chemical composition of the stainless steel sheet according to the disclosure will be described. The % representations below indicating the chemical composition are in “mass %” unless stated otherwise.
C: 0.030% or more and less than 0.20%
Si: 0.01% or more and 2.0% or less
Mn: 0.01% or more and 3.0% or less
P: 0.050% or less
S: 0.010% or less
By simply decreasing S, the effect of improving workability, particularly ultimate deformability, is limited. Therefore, as will be described later, in addition to reducing the S content, it is important to add Zr in a certain amount and to improve the ultimate deformability by synergistic effects of these.
Cr: 10.0% or more and 16.0% or less
Ni: 0.01% or more and 0.80% or less
Al: 0.001% or more and 0.50% or less
Zr: 0.005% or more and 0.50% or less
From the viewpoint of more effectively precipitating S remaining in the steel as ZrS, it is preferable to satisfy the relation of Zr %≥3*S % for Zr and S. Here, Zr % and S % represent the content by mass % of Zr and S in the steel, respectively.
N: 0.030% or more and less than 0.20%
In addition to the basic components have been described above, the stainless steel sheet disclosed herein may optionally contain at least one of:
Cu: 0.01% or more and 3.0% or less
Mo: 0.01% or more and 0.50% or less
Co: 0.01% or more and 0.50% or less
Ti: 0.001% or more and 0.50% or less
Nb: 0.001% or more and 0.50% or less
V: 0.001% or more and 0.50% or less
B: 0.0002% or more and 0.0100% or less
Ca: 0.0002% or more and 0.0100% or less
Mg: 0.0002% or more and 0.0100% or less
The components other than the above are Fe and inevitable impurities. Specifically, the chemical composition consists of, by mass %,
C: 0.030% or more and less than 0.20%,
Si: 0.01% or more and 2.0% or less,
Mn: 0.01% or more and 3.0% or less,
P: 0.050% or less,
S: 0.010% or less,
Cr: 10.0% or more to 16.0% or less,
Ni: 0.01% or more and 0.80% or less,
Al: 0.001% or more and 0.50% or less,
Zr: 0.005% or more and 0.50% or less, and
N: 0.030% or more and less than 0.20%, and optionally at least one of:
at least one selected from the group consisting of
at least one selected from the group consisting of
at least one selected from the group consisting of
Further, in order to obtain a high-strength material of 1300 MPa or more, the martensitic stainless steel sheet of the present disclosure has a structure mainly composed of a martensite phase, specifically, a structure containing 80% or more of a martensite phase with the remainder consisting of a ferrite phase and/or a retained austenite phase. It is preferable that martensite accounts for 90% or more of the structure in volume ratio, including a martensite single phase.
The volume ratio of the martensite phase can be determined as follows: a test piece is prepared from a final cold-rolled sheet (either as quenched or quenched and tempered) and etched with aqua regia, then through cross-section observation under an optical microscope for 10 observation fields at 200 times magnification, martensite phase is distinguished from ferrite phase and retained austenite phase in accordance with the microstructure shape and etching strength, the volume ratio of the martensite phase is determined by image processing, the results are averaged, and the average is used as the volume ratio of the martensite phase.
The following describes a suitable production method for the presently disclosed martensitic stainless steel.
The martensitic stainless steel sheet of the present disclosure is produced by preparing a steel having the above chemical composition through steelmaking in a melting furnace such as a converter or an electric furnace, subjecting it to secondary refining such as ladle refining or vacuum refining, followed by either continuous casting or ingot casting and blooming to obtain a semi-finished product (slab), and subjecting the slab to hot rolling, hot band annealing, and pickling to obtain a hot-rolled and annealed sheet. Further, the method may also include cold rolling, quenching heat treatment, and other optional steps such as pickling and tempering heat treatment to obtain a cold-rolled sheet.
For example, molten steel is prepared by steelmaking in a converter or an electric furnace, secondary refining is carried out by VOD method or AOD method to obtain the above chemical composition, and a slab is formed by continuous casting. The slab thus obtained is heated to 1000° C. to 1250° C. and hot rolled into a hot-rolled sheet of a desired thickness. The hot-rolled sheet is subjected to batch annealing at a temperature of 600° C. to 800° C., and then the oxide scale is removed by shot blasting and pickling to obtain a hot-rolled and annealed sheet. This hot-rolled and annealed sheet is further cold rolled, quenched, and cooled to obtain a cold-rolled sheet. In the cold rolling, two or more cold rolling steps including intermediate annealing may be performed if necessary. The total rolling reduction in the cold rolling including one or more cold rolling steps is set to 60% or more, and preferably 80% or more. From the viewpoint of obtaining desired mechanical properties (such as strength, 0.2% proof stress, elongation, and ultimate deformability), it is preferable to perform the quenching heat treatment in a range of 900° C. to 1200° C. The range is more preferably 1000° C. or higher. The range is more preferably 1100° C. or lower. The cooling rate after the quenching heat treatment is preferably 1° C./sec or more in order to obtain a desired strength. After cooling subsequent to the quenching heat treatment, tempering heat treatment may be carried out as necessary. It is preferable to perform the tempering heat treatment in a range of 100° C. to 500° C. from the viewpoint of obtaining desired properties. The range is more preferably 200° C. or higher. The range is more preferably 300° C. or lower. Further, after the quenching heat treatment and tempering heat treatment, pickling treatment may be carried out. In addition, BA finishing may be performed without pickling by performing quenching heat treatment and tempering heat treatment in a reducing atmosphere containing hydrogen.
The cold-rolled sheet product thus produced is subjected to bending processing, bead processing, drilling processing, or the like according to the use, and formed into gasket parts or the like used as a sealing material between the engine and the exhaust system parts of the automobile. The cold-rolled sheet product may also be used for members requiring springiness. If necessary, the cold-rolled sheet product may be subjected to quenching heat treatment and tempering heat treatment after formed into parts.
30 kg steel ingots having the chemical compositions listed in Table 1 were prepared by steelmaking and casting in a vacuum melting furnace. In each case, after heating to 1200° C., hot rolling was performed to obtain a sheet bar having a thickness of 25 mm and a width of 150 mm. The sheet bar was softened by being held in the furnace at 700° C. for 10 hours. Then, the sheet bar was heated to 1100° C. and hot rolled to obtain a hot-rolled sheet having a thickness of 4 mm. Then, the hot-rolled sheet was annealed in the furnace at 700° C. for 10 hours to obtain a hot-rolled and annealed sheet. Subsequently, the hot-rolled annealed sheet was cold-rolled into a cold-rolled sheet having a thickness of 0.2 mm, subjected to quenching heat treatment at a temperature in Table 2, and then cooled. At this time, the cooling rate was set to 1° C./sec or more in each case. Further, some of the cold-rolled sheets were cooled after the quenching heat treatment, and then subjected to tempering heat treatment at the temperatures listed in Table 2.
<Microstructure Observation>
For each cold-rolled martensitic stainless steel sheet (either as-quenched or quenched and tempered), a test piece was prepared for cross-section observation, etched with aqua regia, then through cross-section observation under an optical microscope for 10 observation fields at 200 times magnification, martensite phase was distinguished from ferrite phase in accordance with the shape and etching strength, the volume ratio of the martensite phase was determined by image processing, and the results were averaged. In Steel Nos. 1 to 22 and 31 to 47 of our examples and Steel Nos. 23 to 28, 30, and 48 to 50 of comparative examples, the martensite phase accounted for 80% or more of the entire structure in volume ratio. On the other hand, in Steel No. 29 of comparative example in which the Cr content was high, the martensite phase accounted for less than 80% of the entire structure in volume ratio.
<Tensile Test>
Using the cold-rolled martensitic stainless steel sheets prepared as described above (either as-quenched or quenched and tempered), JIS No. 5 tensile test pieces whose longitudinal direction was the rolling direction were prepared, and subjected to room temperature tensile tests according to JIS Z 2241 to measure tensile strength (T.S.), 0.2% proof stress (P.S.), elongation (EL), and ultimate deformability (ε1). The original gauge distance was 50 mm and the tensile speed was 10 mm/min. Each steel was tested with N=2, and the average value was evaluated.
The elongation (EL) was calculated by the following formula by butting deeply the divided test pieces such that the axes of the test pieces were on a straight line, and measuring the final gauge distance:
EL (%)=(Lu−L0)/L0*100
where EL is the elongation (elongation after fracture), L0 is the original gauge distance, and Lu is the final gauge distance.
The sheet width W and the sheet thickness T on the fractured surface of each tensile test piece after the tensile test were measured, and the ultimate deformability ε1 was calculated by the following formula together with the sheet width W0 and the sheet thickness T0 of the tensile test piece before the tensile test:
ε1=−{ln(W/W0)+ln(T/T0)}
where ε1 is the ultimate deformability, W is the sheet width on the fractured surface of the tensile test piece after the tensile test, W0 is the sheet width of the tensile test piece before the tensile test, T is the sheet thickness on the fractured surface of the tensile test piece after the tensile test, and T0 is the sheet thickness of the tensile test piece before the tensile test.
The evaluation results are also listed in Table 2. The evaluation criteria are as follows:
Tensile strength (T.S.)
0.2% proof stress (P.S.)
Elongation (EL)
Ultimate deformability (ε1)
<Corrosion Resistance Evaluation Test>
A test piece of 60 mm wide and 80 mm long was cut out from each cold-rolled sheet prepared as described above (either as-quenched or quenched and tempered) and subjected to a corrosion resistance evaluation test following the corrosion test method for automotive materials (JASO M 609-91) as specified by the Society of Automotive Engineers of Japan. The surface of each test piece was polished with #600 emery paper. In each test piece the entire back surface and 5 mm around the front surface were covered with a seal. In the test, the corrosion area ratio of the surface was measured after 15 cycles with one cycle being 5% salt spray (2 hours), 60° C. drying (4 hours), and 50° C. wetting (2 hours). The test was performed with N=2, and the one with the larger corrosion area ratio was adapted as the evaluation of the cold-rolled sheet.
The obtained results are also listed in Table 2. The evaluation criteria are as follows:
—
0.024
0.024
0.222
0.236
17.3
0.015
0.012
0.011
—
From Table 1, it can be seen that examples Nos. 1 to 22 and 31 to 47 were all excellent in strength, 0.2% proof stress, elongation, ultimate deformability, and corrosion resistance.
On the other hand, comparative examples Nos. 23 and 50 containing no Zr (both corresponding to SUS 403) failed in terms of elongation, ultimate deformability, and corrosion resistance. Comparative example No. 24 with Cr content as low as outside the appropriate range failed in terms of corrosion resistance. Comparative example No. 25 with N content as low as outside the appropriate range and comparative example No. 26 with C content as low as outside the appropriate range failed in terms of strength and 0.2% proof stress. Comparative example No. 27 with C content as high as outside the appropriate range and comparative example No. 28 with N content as high as outside the appropriate range failed in terms of elongation, ultimate deformability, and corrosion resistance. Comparative example No. 29 with Cr content as high as outside the appropriate range and with less martensite failed in terms of strength and 0.2% proof stress. Comparative examples Nos. 30, 48, and 49 with S content as high as outside the appropriate range failed in terms of ultimate deformability and corrosion resistance.
The martensitic stainless steel sheet disclosed herein is excellent in both strength (tensile strength and 0.2% proof stress) and workability (elongation, in particular, ultimate deformability), and is therefore suitable as a gasket member. It is also suitable for use in parts requiring spring resistance.
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