JP7135737B2 - Austenitic hot-rolled steel sheet, manufacturing method thereof, and wear-resistant parts - Google Patents

Description

TECHNICAL FIELD The present invention relates to an austenitic steel material, a manufacturing method thereof, and a wear-resistant part.

It is known that the wear resistance of steel materials used for machine parts, construction members, transportation equipment parts, electric/electronic equipment parts, etc. is improved by increasing the strength. For this reason, for example, steel materials that have been subjected to heat treatment such as quenching and tempering to increase their strength have been used for members that are subjected to abrasion by soil, sand, steel balls, and the like.
On the other hand, in order to apply to members of various shapes and to reduce the number of welded parts, steel materials are increasingly required to have bending workability.
However, it is known that the bending workability of steel deteriorates as the strength of the steel increases. That is, there is a trade-off relationship between the "wear resistance" and the "bending workability" of steel materials.

Another known technique for improving the wear resistance of steel is to increase the Mn content of austenitic steel. For example, Patent Document 1 proposes an austenitic steel containing 10 to 35% by mass of Mn. Further, Patent Document 2 proposes an austenitic steel containing 18 to 32% by mass of Mn.

JP-A-10-008210 JP-A-62-139855

However, although the austenitic steel materials of Patent Documents 1 and 2 have good wear resistance, their bending workability has not been studied. In fact, even austenitic steel materials with a high Mn content often do not have sufficient bending workability.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an austenitic steel material that achieves both wear resistance and bending workability, and a method for producing the same.
Another object of the present invention is to provide a wear-resistant part having excellent wear resistance that can be manufactured by bending.

As a result of intensive research conducted by the present inventors to solve the above problems, the inventors found that the composition and metallographic structure of austenitic steel materials have a significant effect on wear resistance and bending workability. The inventors have found that an austenitic steel material having both wear resistance and bending workability can be obtained by controlling the metal structure, and have completed the present invention.

That is, the present invention includes C: 0.7 to 1.6% by mass, Si: 0.8% by mass or less, Mn: 8 to 16% by mass, P: 0.025% by mass or less, and S: 0.025% by mass or less, Al: 0.025% by mass or less, one or more selected from Nb: 0.5 to 3.0% by mass and Ti: 0.3 to 1.5 % by mass and the balance has a composition consisting of Fe and unavoidable impurities, the metal structure contains austenite and hard carbides, and the hard carbides are carbides containing one or more selected from Nb and Ti, and a thickness It is an austenitic hot-rolled steel sheet with a thickness of 3.5 to 12 mm .

In addition, the present invention includes C: 0.7 to 1.6% by mass, Si: 0.8% by mass or less, Mn: 8 to 16% by mass, P: 0.025% by mass or less, and S: 0.025% by mass or less, Al: 0.025% by mass or less, one or more selected from Nb: 0.5 to 3.0% by mass and Ti: 0.3 to 1.5 % by mass A hot rolling step of hot rolling a slab having a composition with the balance consisting of Fe and unavoidable impurities at a finishing temperature of 780 to 900 ° C. and winding at a coiling temperature of 300 to 450 ° C.;
a water toughening step of heating the hot-rolled sheet obtained in the hot-rolling step and maintaining it at a temperature of 900 to 1100° C., and then cooling it to a temperature of 500° C. or less at a cooling rate of 10° C./sec or more . , a method for producing an austenitic hot-rolled steel sheet .

Furthermore, the present invention is a wear-resistant part formed from the austenitic hot-rolled steel sheet .

ADVANTAGE OF THE INVENTION According to this invention, the austenitic steel material which made wear resistance and bending workability compatible, and its manufacturing method can be provided.
Moreover, according to the present invention, it is possible to provide a wear-resistant part having excellent wear resistance that can be manufactured by bending.

Hereinafter, embodiments of the present invention will be specifically described. The present invention is not limited to the following embodiments, and modifications and improvements can be made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention. are also within the scope of the present invention.

The austenitic steel material according to the embodiment of the present invention has C: 0.7 to 1.6% by mass, Si: 0.8% by mass or less, Mn: 8 to 16% by mass, and P: 0.025% by mass. below, S: 0.025% by mass or less, Al: 0.025% by mass or less, Nb: 0.5 to 3.0% by mass, Ti: 0.3 to 1.5% by mass, and V: 0 .5 to 3.0% by mass, the balance being Fe and unavoidable impurities, and the metallographic structure includes austenite and hard carbides.
Here, the term "inevitable impurities" as used herein means components such as O and N that are difficult to remove. Unavoidable impurities are inevitably mixed in at the stage of smelting raw materials.

C is an element effective in stabilizing austenite and improving strength. If the C content is less than 0.7% by mass, austenite becomes unstable. Therefore, the C content should be 0.7% by mass or more, preferably 0.75% by mass or more, and more preferably 0.8% by mass or more. On the other hand, when the C content exceeds 1.6% by mass, carbides tend to precipitate at grain boundaries when the steel is cooled after solution treatment at a high temperature, resulting in deterioration of bending workability. Therefore, the C content should be 1.6% by mass or less, preferably 1.5% by mass or less, and more preferably 1.2% by mass or less.

Si is an effective element for deoxidizing molten steel. However, when the Si content exceeds 0.8% by mass, precipitation of carbides at grain boundaries is promoted. Therefore, the Si content should be 0.8% by mass or less, preferably 0.7% by mass or less, and more preferably 0.6% by mass or less. On the other hand, although the lower limit of the C content is not particularly limited, it is preferably 0.5% by mass.

Mn is an austenite-forming element and is also effective in stabilizing austenite. If the Mn content is less than 8% by mass, the austenite becomes unstable, and as a result, carbides are likely to precipitate at grain boundaries, resulting in a decrease in bending workability. Therefore, the Mn content should be 8% by mass or more, preferably 10% by mass or more, and more preferably 11% by mass or more. On the other hand, if the Mn content exceeds 16% by mass, problems such as deterioration in hot workability and difficulty in producing steel sheets arise. Therefore, the Mn content is 16% by mass or less, preferably 15% by mass or less, and more preferably 14% by mass or less.

P and S are elements that affect bending workability, and the smaller the content of these, the better. Both the P content and the S content, which do not significantly affect bending workability, are 0.025% by mass or less, preferably 0.023% by mass or less, and more preferably 0.020% by mass or less.

Al is an element that is effective for deoxidizing molten steel, but it is an element that affects bending workability. Therefore, the smaller the Al content, the better. The Al content that does not significantly affect bending workability is 0.025% by mass or less, preferably 0.020% by mass or less, and more preferably 0.010% by mass or less.

Nb, Ti and V are elements that precipitate hard carbides that contribute to improving wear resistance. Therefore, one or more of Nb, Ti and V must be contained.
Here, hard carbides are carbides containing one or more selected from Nb, Ti and V. Examples of hard carbides include NbC, TiC, VC, (Nb,Ti)C, (Nb,V)C, (Ti,V)C, (Nb,Ti,V)C. Specifically, when the austenitic steel according to the embodiment of the present invention does not contain Ti and V, the hard carbides are carbides mainly composed of NbC. Further, when the austenitic steel material according to the embodiment of the present invention does not contain V, the hard carbides are carbides mainly composed of (Nb, Ti)C.
The presence or absence of precipitation of hard carbides can be confirmed by using an analysis method such as EDX (Energy Dispersive X-ray Fluorescence Analysis).

In order to sufficiently ensure the above effects of Nb, the Nb content is set to 0.5% by mass or more, preferably 0.7% by mass, and more preferably 1.0% by mass or more. On the other hand, if the Nb content is too high, the hard carbides become coarse, and cracks occur starting from the hard carbides during bending. Therefore, the Nb content is set to 3.0% by mass or less, preferably 2.8% by mass or less, and more preferably 2.7% by mass or less.
In order to ensure the above effects of Ti, the Ti content is set to 0.3% by mass or more, preferably 0.4% by mass or more, and more preferably 0.8% by mass or more. On the other hand, if the Ti content is too high, the hard carbides become coarse, and cracks occur starting from the hard carbides during bending. Therefore, the Ti content is set to 1.5% by mass or less, preferably 1.45% by mass or less.
In order to ensure the above effects of V, the V content is set to 0.5% by mass or more, preferably 0.7% by mass, and more preferably 1.0% by mass or more. On the other hand, if the V content is too high, the hard carbides become coarse, and cracks occur starting from the hard carbides during bending. Therefore, the V content is set to 3.0% by mass or less, preferably 2.8% by mass or less, and more preferably 2.7% by mass or less.

In the austenitic steel according to the embodiment of the present invention, in addition to the above components, one or more selected from Cr: 0.20 mass% or less, Ni: 0.25 mass% or less, and Mo: 0.01 mass% or less may further include

Cr is an element effective in stabilizing austenite and improving strength. When the Cr content exceeds 0.20% by mass, problems such as deterioration in hot workability and difficulty in manufacturing steel sheets may occur. Therefore, the Cr content is 0.20% by mass or less, preferably 0.18% by mass or less, and more preferably 0.15% by mass or less. On the other hand, although the lower limit of the Cr content is not particularly limited, it is preferably 0.1% by mass from the viewpoint of sufficiently ensuring the above effects of Cr.

Ni is an element effective in stabilizing austenite and improving toughness. However, since Ni is expensive, increasing the Ni content leads to an increase in manufacturing costs. Therefore, the Ni content should be 0.25% by mass or less, preferably 0.23% by mass or less, and more preferably 0.20% by mass or less. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of sufficiently ensuring the above effects of Ni, it is preferably 0.1% by mass or more.

Mo is an element effective in suppressing embrittlement during heat treatment, and the effect can be obtained even in a small amount. However, since Mo is expensive, increasing the Mo content leads to an increase in manufacturing costs. Therefore, the Mo content should be 0.01% by mass or less, preferably 0.009% by mass or less, and more preferably 0.008% by mass or less. On the other hand, the lower limit of the Mo content is not particularly limited, but is preferably 0.001% by mass or more from the viewpoint of sufficiently securing the above effects of Mo.

An austenitic steel material according to an embodiment of the present invention has a metallographic structure containing austenite and hard carbides. Hard carbides are dispersed in austenite and affect the wear resistance and bendability of austenitic steel. From the viewpoint of improving the wear resistance of austenitic steel, the area ratio of all hard carbides (hard carbides of all sizes) in the cross section is preferably 0.5% or more, more preferably 0.55% or more, and further Preferably, it is controlled to 0.6% or more. On the other hand, too much hard carbide dispersed in austenite may become the origin of working cracks. Therefore, from the viewpoint of ensuring the bending workability of the austenitic steel material, the area ratio of all hard carbides in the cross section is preferably 3% or less, more preferably 2.9% or less, and further preferably 2.8% or less. Control.
The area ratio of all hard carbides in the cross section can be obtained as follows. First, after polishing the cross section of the austenitic steel material, the etched observation surface is observed with a confocal laser microscope (LEXT OLS3000 manufactured by Olympus Corporation). Here, when the austenitic steel material is a steel plate, the cross section for measuring the area ratio of hard carbides is preferably a cross section (L cross section) parallel to the rolling direction and the plate thickness direction. Next, the area (mm ² ) of all the hard carbides is measured on the observation image, and the area ratio is obtained by the following formula.
Area ratio (%) of all hard carbides = area of all hard carbides (mm ² )/total observed area (mm ² ) x 100
The observation area is 128 μm×128 μm×42 visual fields. The area of hard carbide can be measured by processing the observed image with image processing software.

In addition, the wear resistance and workability of austenitic steel tend to be affected more easily as the hardened material becomes larger. Specifically, when the equivalent circle diameter of the hard carbides in the cross section of the austenitic steel is 0.5 μm or more, the wear resistance and workability of the austenitic steel are greatly affected. Therefore, from the viewpoint of improving the wear resistance of austenitic steel materials, the area ratio of hard carbides having an equivalent circle diameter of 0.5 μm or more in the cross section is preferably 0.5% or more, more preferably 0.55% or more, and further Preferably, it is controlled to 0.6% or more. On the other hand, from the viewpoint of improving the bending workability of austenitic steel materials, the area ratio of hard carbides having an equivalent circle diameter of 0.5 μm or more in the cross section is preferably 2.7% or less, more preferably 2.6% or less, and further Preferably, it is controlled at 2.5%.

The area ratio of hard carbides having an equivalent circle diameter of 0.5 μm or more in the cross section can be obtained by the following calculation formula after observing with a confocal laser microscope in the same manner as described above.
Area ratio (%) of hard carbides with an equivalent circle diameter of 0.5 μm or more=Area of hard carbides with an equivalent circle diameter of 0.5 μm or more (mm ² )/Total observed area (mm ² )×100
The equivalent circle diameter of a certain hard carbide is the diameter of a circle equal to the area of the hard carbide on the observed image.

The austenitic steel according to the embodiment of the present invention preferably has a cross-sectional Vickers hardness of 300 HV or less, more preferably 280 HV or less, and even more preferably 260 HV or less. When the Vickers hardness is within the above range, sufficient bending workability can be ensured. On the other hand, the lower limit of the Vickers hardness of the cross section is not particularly limited, but is preferably 200 HV, more preferably 210 HV, and even more preferably 220 HV from the viewpoint of ensuring sufficient wear resistance.
Here, the term "Vickers hardness" as used herein means that measured using a Vickers hardness tester according to JIS Z2244:2009.

The austenitic steel material according to the embodiment of the present invention can be in various forms such as steel plate and cast product, but is preferably steel plate. The thickness of the steel sheet is not particularly limited and may be appropriately adjusted depending on the application, but is typically 3.5 to 12 mm.

The austenitic steel material according to the embodiment of the present invention having the characteristics described above can be produced according to methods known in the art, except that the steel having the above composition is melted. For example, when the austenitic steel material according to the embodiment of the present invention is a steel plate, it may be manufactured by hot-rolling a cast slab having the above-described composition obtained by casting, and then subjecting it to water toughening treatment. Specifically, the austenitic steel material according to the embodiment of the present invention is produced by a hot rolling process in which a cast slab having the above composition is hot rolled at a finishing temperature of 780 to 900°C and coiled at a coiling temperature of 300 to 450°C. and a water toughening step of heating the hot rolled sheet obtained in the hot rolling step and maintaining it at a temperature of 900 to 1100 ° C., and then cooling it to a temperature of 500 ° C. or less at a cooling rate of 10 ° C./sec. It can be manufactured by a method.

Since the austenitic steel material according to the embodiment of the present invention is excellent in wear resistance and bending workability, it can be used for machine parts, construction parts, parts for transportation equipment, parts for electric/electronic equipment, and the like. In particular, the austenitic steel material according to the embodiment of the present invention undergoes deformation-induced martensitic transformation when struck by soil, sand, steel balls, or the like, and exhibits sufficient wear resistance. Therefore, this austenitic steel material is suitable for use as a material for wear-resistant parts. In addition, since this austenitic steel material is excellent in bending workability, it is suitable for use as a material for wear-resistant parts manufactured by bending. Examples of wear-resistant parts include, but are not limited to, parts of construction machinery for civil engineering, molds used for vibration molding of concrete products, lining materials (liners) for shot peening machines, and the like.

EXAMPLES The content of the present invention will be described in detail below with reference to Examples, but the present invention is not construed as being limited thereto.

Steels having the compositions of steel grades A to K shown in Table 1 are melted in a vacuum melting furnace, cast slabs are hot-rolled at a finishing temperature of 830 ° C. and was wound at a winding temperature of 430°C. Next, the hot-rolled sheet was heated and held at a temperature of 1000° C., and then subjected to a water toughening treatment in which it was cooled to room temperature at a cooling rate of 50° C./sec to obtain an austenitic steel sheet.
In addition, the steel having the composition of steel type L shown in Table 1 was melted in a vacuum melting furnace, and the slab obtained by casting was hot-rolled at a finishing temperature of 850 ° C. to obtain a hot-rolled sheet with a thickness of 3.5 mm. and was wound at a winding temperature of 590°C. Next, the hot-rolled sheet was heated to 840°C and held for 20 minutes, quenched in an oil bath at 60°C, and then tempered at 300°C or 400°C for 60 minutes to obtain a tempered martensitic steel sheet. .

Each steel plate obtained above was evaluated as follows.
[Evaluation of metal structure]
After polishing a cross section (L cross section) parallel to the rolling direction and thickness direction of each steel plate obtained above, the etched observation surface was observed with a confocal laser microscope (LEXT OLS 3000 manufactured by Olympus Co., Ltd.). According to the method, the area ratio (%) of hard carbides having an equivalent circle diameter of 0.5 μm or more and the area ratio (%) of all hard carbides were determined.

[Cross-section Vickers hardness]
Using a Vickers hardness tester according to JIS Z2244:2009, the Vickers hardness at the center of a cross section (L cross section) parallel to the rolling direction and thickness direction of each steel plate was measured. For the Vickers hardness, the test load was 196 N, and the average of the measured values at five locations was used as the result.

[Bendability]
A test piece having a length of 25 mm in the rolling direction, a length of 60 mm in the width direction, and a thickness of 3.5 mm was cut out from each of the steel sheets obtained above. After the cut surface was ground and trimmed with a belt grinder (#320), a 150° bending test was performed using a universal testing machine according to JIS Z2248:2006 by the V-block method. The bending test was performed with a bending radius of 3.5 mm and a test force of 300 kN, and the surface of the test piece after the bending test was visually observed. As a result of observation, ◯ indicates that there was no crack, and x indicates that there was a crack.

[Earth and sand wear test]
A test piece having a length of 48 mm in the rolling direction, a length of 55 mm in the width direction, and a thickness of 3.5 mm is cut out from each steel plate obtained above, and an abrasion test is performed using a sediment wear tester described in JP-A-2012-210665. did The test piece was fixed in a test holder attached via a horizontal arm to a rotating shaft (strut) at an angle of 30° from the horizontal plane. At this time, the distance between the rotating shaft and the test piece was set to 140 mm. Gravel (diameter of 20 mm or less) or river sand (diameter of 1.5 mm or less) was used as the material to be worn, and an abrasion test was performed in a dry atmosphere at a rotation speed of 100 rpm. The test time for pebble gravel was 24 hours, and the test time for river sand was 100 hours. The weight of the test piece reduced by abrasion (wear loss) was obtained from the difference in weight of the test piece before and after the test. Then, the wear loss rate was calculated by the following formula.
Wear loss rate (%) = Wear loss/Weight of test piece before wear test x 100

[Continuous friction and wear test]
A test piece with a rolling direction length of 50 mm × width direction length of 25 mm × plate thickness of 3.5 mm is cut out from each steel plate obtained above, and a continuous friction and wear tester described in JP 2018-48993 A is used. A wear test was performed. Specifically, a belt-shaped friction material having a grain size of #600 (average grain size 30 μm) or #2000 (average grain size 9 μm) and a hardness of 1850HV attached to the surface of WA (alumina) abrasive grains was used as a test piece. A wear test was performed by pressing and contacting the surface (cross section in the width direction) with a load F of 35.4 N (surface pressure of 0.2 N/mm ² ) and moving the strip-shaped friction material in one direction. At this time, the moving speed (friction speed) of the friction material was set to 5 m/sec, and the moving distance (friction distance L) was set to 45 m. The volume of the material lost due to abrasion was calculated from the difference in thickness of the sample piece before and after the test, and this was defined as the abrasion loss W (mm ³ ). Then, the specific wear amount was determined by the following formula.
Specific wear amount (mm ³ /Nm) = wear loss W/(load F x friction distance L)
Table 2 shows the above evaluation results.

As shown in Table 2, test no. The austenitic steel plates of 2 to 4 and 7 to 9 (invention examples) had good bending workability, and showed little wear in both the sediment wear test and the continuous friction wear test.
On the other hand, Test No. The austenitic steel sheets of Nos. 1 and 6 (comparative examples) had too little Nb content or Ti content, so the amount of wear increased in both the sediment wear test and the continuous friction wear test.
Also, test no. The austenitic steel sheets of Nos. 5 and 10 (comparative examples) had too much Nb content or too much Ti content, and thus had insufficient bending workability. It is considered that this is because the precipitated hard carbides (NbC or TiC) were coarsened and cracks were generated starting from the hardened hardened materials.
Also, test no. Since the austenitic steel sheet of No. 11 (comparative example) did not contain an element (Nb, Ti or V) that precipitates hard carbides, the amount of wear increased in both the sediment wear test and the continuous friction wear test.
Furthermore, test no. The steel sheets of Nos. 12 and 13 (comparative examples) were strengthened by quenching and tempering, so that the bending workability was not sufficient, and the amount of wear was greater than that of the inventive examples.

As can be seen from the above results, according to the present invention, it is possible to provide an austenitic steel material having both wear resistance and bending workability, and a method for producing the same. Moreover, according to the present invention, it is possible to provide a wear-resistant part having excellent wear resistance that can be manufactured by bending.

Claims (7)

C: 0.7 to 1.6% by mass, Si: 0.8% by mass or less, Mn: 8 to 16% by mass, P: 0.025% by mass or less, S: 0.025% by mass or less And, Al: 0.025% by mass or less, Nb: 0.5 to 3.0% by mass, and Ti: 0.3 to 1.5 % by mass. and a composition consisting of unavoidable impurities, the metal structure contains austenite and hard carbide,
The hard carbide is a carbide containing one or more selected from Nb and Ti,
An austenitic hot-rolled steel sheet having a thickness of 3.5 to 12 mm . The austenitic hot-rolled steel sheet according to claim 1, further comprising one or more selected from Cr: 0.20 mass% or less, Ni: 0.25 mass% or less, and Mo: 0.01 mass% or less. 3. The austenitic hot-rolled steel sheet according to claim 1, wherein the area ratio of said hard carbides having an equivalent circle diameter of 0.5 μm or more in a cross section is 0.5 to 2.7%. The austenitic hot-rolled steel sheet according to any one of claims 1 to 3 , having a cross-sectional Vickers hardness of 300 HV or less. C: 0.7 to 1.6% by mass, Si: 0.8% by mass or less, Mn: 8 to 16% by mass, P: 0.025% by mass or less, S: 0.025% by mass or less And, Al: 0.025% by mass or less, Nb: 0.5 to 3.0% by mass, and Ti: 0.3 to 1.5 % by mass. and a hot-rolling step of hot-rolling a slab having a composition consisting of unavoidable impurities at a finishing temperature of 780 to 900 ° C. and winding it at a coiling temperature of 300 to 450 ° C.;
a water toughening step of heating the hot-rolled sheet obtained in the hot-rolling step and maintaining it at a temperature of 900 to 1100° C., and then cooling it to a temperature of 500° C. or less at a cooling rate of 10° C./sec or more . , a method for producing an austenitic hot-rolled steel sheet . 6. The austenitic heat according to claim 5 , wherein the slab further contains one or more selected from Cr: 0.20% by mass or less, Ni: 0.25% by mass or less, and Mo: 0.01% by mass or less. A method for manufacturing a rolled steel sheet . A wear-resistant part formed from the austenitic hot-rolled steel sheet according to any one of claims 1 to 4 .