JP7211530B2 - WEAR RESISTANT STEEL AND METHOD FOR MANUFACTURING WEAR RESISTANT STEEL - Google Patents

Description

TECHNICAL FIELD The present invention relates to an abrasion-resistant steel plate, in particular, a resistant steel plate with excellent wide bending workability suitable for members of industrial machinery and transportation equipment used in fields such as construction, civil engineering and mining. It relates to worn steel plates. The present invention also relates to a method for manufacturing the wear-resistant steel plate. Here, wide-width bending workability refers to bending workability in steel sheets with a width of 200 mm or more, which is a problem in actual use.

It is known that the wear resistance of steel is improved by increasing its hardness. For this reason, steel materials that have been subjected to heat treatment such as quenching to increase their hardness have been used for members that are subject to wear due to earth, sand, rocks, and the like.

For example, Patent Document 1 describes a method of manufacturing a wear-resistant thick steel plate by hot-rolling a steel material having a predetermined chemical composition into a thick steel plate and then quenching the steel. According to the method described in Cited Document 1, by controlling the contents of C, alloying elements, and N, it has a hardness of 340 HB or more as quenched and high toughness, and low-temperature crackability of the weld zone. It is said that a wear-resistant thick steel plate with improved is obtained.

Further, in Patent Document 2, steel having a predetermined chemical composition is hot-rolled at a temperature of 900 ° C. to the Ar3 transformation point with a rolling reduction of 15% or more, and then directly quenched from a temperature of the Ar3 transformation point or higher. A method for producing a wear-resistant steel plate is described. According to the method described in Cited Document 2, it is supposed that a wear-resistant steel sheet having high hardness can be easily obtained by controlling the chemical composition and quenching conditions.

In the above techniques described in Patent Documents 1 and 2, the wear resistance is improved by increasing the hardness. On the other hand, there is a growing demand for wear-resistant steel that is excellent not only in wear resistance but also in bending workability in order to apply it to members of various shapes and to reduce the number of welding points.

In response to such demand, for example, in Patent Document 3, C: 0.05 to 0.20%, Mn: 0.50 to 2.5%, and Al: 0.02 to 2.00% by weight % and the area fraction of martensite is 5% or more and 50% or less. According to Patent Document 3, the area fraction of martensite is controlled by heating hot-rolled steel to a ferrite-austenite two-phase region temperature between Ac1 and Ac3 and then quenching, thereby It is said that a wear-resistant steel with excellent workability and weldability can be obtained.

Further, in Patent Document 4, steel having a predetermined chemical composition is cooled immediately after hot rolling to the Ms point ± 25 ° C., the cooling is interrupted, and after reheating to the Ms point + 50 ° C. or higher, room temperature A method for manufacturing a wear-resistant steel plate that cools down to According to Cited Document 4, the minimum hardness in a region from the surface to a depth of 5 mm of the steel plate obtained by the manufacturing method is 40 HV or more lower than the maximum hardness in a region further inside the steel plate, and as a result, bending workability is expected to improve.

Furthermore, in Patent Document 5, a steel having a predetermined chemical composition with a DI* (hardenability index) of 60 or more is hot rolled, and then hot-rolled at an average cooling rate of 0.5 to 2 ° C./s to a temperature of 400 ° C. or less. A method of manufacturing a wear-resistant steel plate has been proposed in which the steel is cooled to a temperature range. According to Patent Document 5, in the wear-resistant steel sheet obtained by the above manufacturing method, 400 or more Ti-based carbides with an average particle size of 0.5 to 50 μm or more are precipitated/mm ² , and as a result, heat treatment is performed. It is said that a wear resistant steel having both excellent wear resistance and bending workability can be obtained without carrying out the process.

JP-A-63-169359 JP-A-64-031928 JP-A-07-090477 JP 2006-104489 A JP 2008-169443 A

As described in Patent Documents 3 to 5, conventional methods for improving the bending workability of wear-resistant steel plates suppress the hardness of the base phase (matrix) of the steel plate to ensure bending workability, It is based on the idea of improving wear resistance through structure control or carbide precipitation. Therefore, it was difficult to sufficiently improve the hardness of the matrix phase by these methods, and it was not possible to achieve both wear resistance and bending workability.

On the other hand, since the required level of wear resistance is increasing year by year, there is a demand for a technology that can achieve both wear resistance and bending workability, which are contradictory properties, at a high level.

In addition, when a wear-resistant steel plate is processed to produce a final product, such as a member for civil engineering and construction equipment, bending is often performed under conditions such that the width of the wear-resistant steel plate is 200 mm or more. Common. Generally, bending cracks are more likely to occur as the width of a steel plate increases. Therefore, in order to evaluate the bending workability of a steel plate in actual use, a steel plate with a width of 200 mm or more should be used for evaluation. However, in the prior art as described above, no consideration is given to the bending workability at sheet widths of 200 mm or more.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and to provide a wear-resistant steel sheet having both excellent wear resistance and bending workability, which contradict each other. In particular, with respect to the bending workability, it is an object of the present invention to provide a wear-resistant steel sheet that is excellent in bending workability under severe conditions of a steel sheet width of 200 mm or more (hereinafter referred to as "wide-width bending workability").

In order to achieve the above object, the present inventors studied various factors that affect the wide-width bending workability of wear-resistant steel plates, and as a result, obtained the following findings (1) to (4).

(1) The bending workability of a wear-resistant steel plate is greatly affected by the hardness and ductility of the surface layer of the wear-resistant steel plate.

(2) In particular, if a wear-resistant steel plate has a localized hardened or softened portion, strain concentrates around the softened or hardened portion and ductility decreases, resulting in a decrease in wide-width bending workability.

(3) By reducing the hardness difference in the wear-resistant steel sheet, wide-width bending workability can be improved without lowering the hardness of the matrix phase, which greatly affects wear resistance.

(4) When manufacturing a wear-resistant steel plate, quenching is performed from the austenite temperature range to reduce the difference in cooling rate in the width direction of the steel plate during quenching, thereby reducing the difference in hardness of the wear-resistant steel plate. can.

The present invention has been completed based on the above findings and further studies. The gist of the present invention is as follows.

1. in % by mass,
C: 0.15 to 0.30%,
Si: 0.05 to 1.00%,
Mn: 0.50-2.00%,
P: 0.020% or less,
S: 0.010% or less,
Al: 0.01-0.06%,
Cr: 0.10 to 1.00%, and N: 0.0100% or less,
Having a component composition consisting of the balance Fe and unavoidable impurities,
The volume fraction of martensite at a depth of 1 mm from the surface is 90% or more,
The hardness at a depth of 1 mm from the surface is 420 to 560 HBW 10/3000 in Brinell hardness,
The width direction hardness difference defined as the difference between two points adjacent to each other at intervals of 10 mm in the width direction of the hardness at a depth of 1 mm from the surface is 30Hv10 or less in Vickers hardness.
Wear-resistant steel plate.

2. The component composition, in mass%,
Nb: 0.005 to 0.020%,
Ti: 0.005-0.020%, and B: 0.0003-0.0030%
1. The wear-resistant steel plate according to 1 above, further containing one or more selected from the group consisting of:

3. The component composition, in mass%,
Cu: 0.01-0.5%,
Ni: 0.01 to 3.0%,
Mo: 0.1 to 1.0%,
V: 0.01 to 0.10%,
W: 0.01-0.5%, and Co: 0.01-0.5%
3. The wear-resistant steel plate according to 1 or 2 above, further containing one or more selected from the group consisting of:

4. The component composition, in mass%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0100%, and REM: 0.0005-0.0200%
4. The wear-resistant steel plate according to any one of 1 to 3 above, further containing one or more selected from the group consisting of

5. in % by mass,
C: 0.15 to 0.30%,
Si: 0.05 to 1.00%,
Mn: 0.50-2.00%,
P: 0.020% or less,
S: 0.010% or less,
Al: 0.01-0.06%,
Cr: 0.10 to 1.00%, and N: 0.0100% or less,
Heating a steel material having a chemical composition consisting of the balance Fe and unavoidable impurities to a heating temperature of 1300° C. or less from the Ac3 transformation point or more,
Hot-rolling the heated steel material into a hot-rolled steel sheet,
A method for producing a wear-resistant steel plate, wherein the hot-rolled steel plate is quenched,
The quenching is
(a) Direct quenching for cooling the hot-rolled steel sheet from a cooling start temperature equal to or higher than the Ar3 transformation point to a cooling stop temperature equal to or lower than the Mf point, or
(b) cooling the hot-rolled steel sheet, reheating the cooled hot-rolled steel sheet to a reheating temperature of from the Ac3 transformation point to 950 ° C. or less, and reheating the hot-rolled steel sheet after reheating from the reheating temperature, It is reheating and quenching that cools to a cooling stop temperature below the Mf point,
In the cooling process of the quenching, the difference between the average cooling rate at the widthwise center position and the average cooling rate at the widthwise 1/4 position of the hot rolled steel sheet, and the average cooling rate at the widthwise center position and the widthwise 3/4 A method for producing a wear-resistant steel plate, wherein the difference in average cooling rate at each position is 5° C./s or less.

6. The cooling stop temperature in the quenching is less than (Mf point -100 ° C.),
6. The method for producing a wear-resistant steel sheet according to 5 above, wherein after the quenching, the quenched hot-rolled steel sheet is tempered at a tempering temperature of (Mf point −80° C.) or higher and (Mf point +50° C.) or lower.

7. 7. The method for producing a wear-resistant steel plate according to 6 above, wherein the tempering is performed at the tempering temperature for 60 seconds or longer.

8. 8. The method for producing a wear-resistant steel plate according to 6 or 7 above, wherein the tempering has an average temperature increase rate of 2° C./s or more.

9. The cooling stop temperature in the quenching is Mf point or less and (Mf point -100 ° C.) or more,
6. The method for producing a wear-resistant steel sheet according to 5 above, wherein after the quenching, the quenched hot-rolled steel sheet is air-cooled.

10. The component composition, in mass%,
Nb: 0.005 to 0.020%,
Ti: 0.005-0.020%, and B: 0.0003-0.0030%
10. The method for producing a wear-resistant steel plate according to any one of 5 to 9 above, further containing one or more selected from the group consisting of

11. The component composition, in mass%,
Cu: 0.01-0.5%,
Ni: 0.01 to 3.0%,
Mo: 0.1 to 1.0%,
V: 0.01 to 0.10%,
W: 0.01-0.5%, and Co: 0.01-0.5%
11. The method for producing a wear-resistant steel plate according to any one of 5 to 10 above, further containing one or more selected from the group consisting of

12. The component composition, in mass%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0100%, and REM: 0.0005-0.0200%
12. The method for producing a wear-resistant steel plate according to any one of 5 to 11 above, further containing one or more selected from the group consisting of

According to the present invention, it is possible to produce a wear-resistant steel sheet having both excellent wear resistance and wide bending workability. According to the present invention, it is possible to realize excellent wide-width bending workability without lowering the hardness that affects wear resistance, so it is possible to meet the high level of wear resistance demanded in recent years. Therefore, the wear-resistant steel plate of the present invention can be very suitably used as a material for members of industrial machinery and transportation equipment used in fields such as construction, civil engineering and mining.

A method for carrying out the present invention will be specifically described below. In addition, the following description shows examples of preferred embodiments of the present invention, and the present invention is not limited thereto.

[Component composition]
In the present invention, it is important that the wear-resistant steel plate and the steel material used for its production have the above chemical composition. Therefore, first, the reasons for limiting the chemical composition of steel in the present invention as described above will be explained. In addition, "%" regarding a component composition means "mass %" unless otherwise indicated.

C: 0.15-0.30%
C is an element that increases the hardness of the matrix phase and improves the wear resistance. In order to obtain this effect, the C content is made 0.15% or more. The C content is preferably 0.20% or more. On the other hand, when the C content exceeds 0.30%, the hardness of the matrix phase increases excessively, and the wide-width bending workability remarkably deteriorates. Therefore, the C content is made 0.30% or less. The C content is preferably 0.28% or less.

Si: 0.05-1.00%
Si is an element that acts as a deoxidizing agent. In addition, Si has the effect of increasing the hardness of the matrix phase by solid-solution strengthening in steel. If the Si content is less than 0.05%, a sufficient deoxidizing effect cannot be obtained and the amount of inclusions increases, resulting in a decrease in ductility. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.10% or more, more preferably 0.20% or more. On the other hand, if the Si content exceeds 1.00%, the amount of inclusions increases and the ductility decreases, resulting in a decrease in wide-width bending workability. Therefore, the Si content is set to 1.00% or less. The Si content is preferably 0.80% or less, more preferably 0.60% or less.

Mn: 0.50-2.00%
Mn is an element that increases the hardness of the matrix phase and improves the wear resistance. If the Mn content is less than 0.50%, the hardenability is insufficient and uniform hardness cannot be obtained. Therefore, the Mn content is set to 0.50% or more. The Mn content is preferably 0.60% or more, more preferably 0.70% or more. On the other hand, if the Mn content exceeds 2.00%, the hardness becomes too high, resulting in deterioration in wide-width bending workability. Therefore, the Mn content is set to 2.00% or less. The Mn content is preferably 1.80% or less, more preferably 1.60% or less.

P: 0.020% or less P is an element contained as an unavoidable impurity, and when it segregates at grain boundaries, it has an adverse effect, such as becoming a starting point of fracture. Therefore, it is desirable to reduce the P content as much as possible, but a P content of 0.020% or less is acceptable. Although the lower limit of the P content is not particularly limited, it is difficult to reduce it to less than 0.001% in industrial-scale production, so from the viewpoint of productivity, the P content should be 0.001% or more. is preferred.

S: 0.010% or less S is an element contained as an unavoidable impurity, exists in steel as sulfide-based inclusions such as MnS, and is an element that exerts an adverse effect, such as becoming a starting point of fracture. . Therefore, it is desirable to reduce the S content as much as possible, but 0.010% or less is permissible. Although the lower limit of the S content is not particularly limited, it is difficult to reduce it to less than 0.0001% in industrial-scale production, so from the viewpoint of productivity, the S content should be 0.0001% or more. is preferred.

Al: 0.01-0.06%
Al is an element that acts as a deoxidizing agent, refines crystal grains by forming nitrides, and improves ductility. In order to obtain these effects, the Al content is made 0.01% or more. On the other hand, if the Al content exceeds 0.06%, nitrides are excessively formed, and the occurrence of surface flaws increases. On the other hand, if the Al content exceeds 0.06%, oxide inclusions increase and the ductility decreases, resulting in a decrease in wide-width bending workability. Therefore, the Al content is set to 0.06% or less. The Al content is preferably 0.05% or less, more preferably 0.04% or less.

Cr: 0.10-1.00%
Cr is an element that acts to increase the hardness of the matrix phase and improve the wear resistance. If the Cr content is less than 0.10%, the effect of improving the hardenability due to the addition of Cr cannot be obtained, and uniform hardness cannot be obtained. Therefore, the Cr content is set to 0.10% or more. The Cr content is preferably 0.20% or more, more preferably 0.25% or more. On the other hand, when the Cr content exceeds 1.00%, ductility is lowered due to formation of precipitates, and wide-width bending workability is lowered. Therefore, the Cr content is set to 1.00% or less. The Cr content is preferably 0.85% or less, more preferably 0.80% or less.

N: 0.0100% or less N is an element contained as an unavoidable impurity, and contributes to refinement of crystal grains by forming nitrides and the like. However, excessive formation of precipitates reduces ductility and wide bending workability. Therefore, the N content is made 0.0100% or less. The N content is preferably 0.0060% or less, more preferably 0.0040% or less. Although the lower limit of the N content is not particularly limited, it is difficult to reduce it to less than 0.0010% in industrial scale production, so from the viewpoint of productivity, the N content is set to 0.0010% or more. is preferred.

A wear-resistant steel plate and a steel material in one embodiment of the present invention have a chemical composition consisting of the above components and the balance of Fe and unavoidable impurities.

Further, in another embodiment of the present invention, the above component composition is optionally Nb: 0.005-0.020%, Ti: 0.005-0.020%, and B: 0.0003-0 1 or 2 or more selected from the group consisting of 0.0030%.

Nb: 0.005-0.020%
Nb is an element that increases the hardness of the matrix phase and contributes to further improvement of wear resistance. In addition, Nb forms carbonitrides and refines prior austenite grains. When Nb is added, the Nb content should be 0.005% or more, preferably 0.007% or more, in order to obtain the above effects. On the other hand, if the Nb content exceeds 0.020%, a large amount of NbC is precipitated to lower the ductility and, as a result, the wide bending workability is lowered. Therefore, when Nb is added, the Nb content should be 0.020% or less. The Nb content is preferably 0.018% or less.

Ti: 0.005-0.020%
Ti is an element that forms nitrides in steel and refines prior austenite grains to improve ductility. In addition, when both Ti and B coexist, Ti fixes N, thereby suppressing precipitation of BN, and as a result, the effect of improving the hardenability of B increases. In order to obtain these effects, when Ti is added, the Ti content is made 0.005% or more. The Ti content is preferably 0.007% or more. On the other hand, when the Ti content exceeds 0.020%, a large amount of hard TiC precipitates, deteriorating wide-width bending workability. Therefore, when Ti is contained, the Ti content shall be 0.020% or less. The Ti content is preferably 0.015% or less.

B: 0.0003 to 0.0030%
B is an element that remarkably improves hardenability even when added in a very small amount. Therefore, the addition of B promotes the formation of martensite and can further effectively improve the wear resistance. In order to obtain this effect, when B is added, the B content is made 0.0003% or more. The B content is preferably 0.0005% or more, more preferably 0.0008% or more. On the other hand, if the B content exceeds 0.0030%, there is an adverse effect that a large amount of precipitates such as borides, which are starting points of fracture, are formed. Therefore, when B is added, the B content is made 0.0030% or less. The B content is preferably 0.0015% or less.

In another embodiment of the present invention, the above component composition is optionally Cu: 0.01 to 0.5%, Ni: 0.01 to 3.0%, Mo: 0.1 to 1.0 %, V: 0.01 to 0.10%, W: 0.01 to 0.5%, and Co: 0.01 to 0.5%. be able to.

Cu: 0.01-0.5%
Cu is an element that improves hardenability, and can be optionally added to further improve hardness. When Cu is added, the content of Cu is set to 0.01% or more in order to obtain the above effect. On the other hand, if the Cu content exceeds 0.5%, surface flaws are likely to occur, and the manufacturability is lowered, and the alloy cost is increased. Therefore, when Cu is added, the Cu content is set to 0.5% or less.

Ni: 0.01-3.0%
Ni is an element that improves hardenability and can be optionally added to further improve hardness. When Ni is added, the Ni content is made 0.01% or more in order to obtain the above effect. On the other hand, when the Ni content exceeds 3.0%, the alloy cost increases. Therefore, the Ni content is set to 3.0% or less.

Mo: 0.1-1.0%
Mo is an element that improves hardenability and can be optionally added to further improve hardness. When Mo is added, the Mo content should be 0.1% or more to obtain the effect. On the other hand, if the Mo content exceeds 1.0%, the weldability deteriorates and the alloy cost increases. Therefore, when adding Mo, Mo content shall be 1.0% or less.

V: 0.01-0.10%
V is an element that improves hardenability and can be optionally added to further improve hardness. Moreover, V is an element that contributes to the reduction of dissolved N by precipitating as VN. When V is added, the V content is made 0.01% or more in order to obtain the above effect. On the other hand, if added in excess of 0.10%, ductility is reduced due to precipitation of hard VC. Therefore, when V is added, the V content should be 0.10% or less, preferably 0.08% or less, and more preferably 0.05% or less.

W: 0.01-0.5%
W, like Mo, is an element that improves hardenability and can be optionally added. When adding W, the W content is made 0.01% or more to obtain the above effect. On the other hand, if the W content exceeds 0.5%, the alloy cost will increase. Therefore, when adding W, the W content is made 0.5% or less.

Co: 0.01-0.5%
Co is an element that improves hardenability and can be optionally added. When Co is added, the Co content should be 0.01% or more to obtain the above effects. On the other hand, if the Co content exceeds 0.5%, the alloy cost increases, so when Co is added, the Co content is made 0.5% or less.

Also, in another embodiment of the present invention, the above component composition optionally contains Ca: 0.0005-0.0050%, Mg: 0.0005-0.0100%, and REM: 0.0005-0. 1 or 2 or more selected from the group consisting of 0200%.

Ca: 0.0005-0.0050%
Ca is an element useful for controlling the morphology of sulfide inclusions, and can be optionally added. In order to exhibit the effect, addition of 0.0005% or more is required. Therefore, when Ca is added, the Ca content is made 0.0005% or more. On the other hand, if it is added in excess of 0.0050%, the amount of inclusions in the steel increases, resulting in a decrease in ductility and a decrease in wide-width bending workability. Therefore, when Ca is contained, the Ca content is set to 0.0050% or less, preferably 0.0025% or less.

Mg: 0.0005-0.0100%
Mg is an element that forms stable oxides at high temperatures, effectively suppresses coarsening of prior austenite grains, and improves ductility. In order to exhibit the effect, addition of 0.0005% or more is required. Therefore, when adding Mg, the Mg content is made 0.0005% or more. On the other hand, if it is added in excess of 0.0100%, an increase in the amount of inclusions in the steel will lead to a decrease in ductility, resulting in a decrease in wide-width bending workability. Therefore, when Mg is contained, the Mg content is set to 0.0100% or less, preferably 0.0050% or less.

REM: 0.0005-0.0200%
Like Ca, REM (rare earth metal) also has the effect of forming oxides and sulfides in steel to improve the quality of the steel. Therefore, when REM is added, the REM content is made 0.0005% or more. On the other hand, even if it is added in excess of 0.0200%, the effect is saturated. Therefore, when REM is contained, the REM content is set to 0.0200% or less, preferably 0.0100% or less.

[Microstructure]
Volume fraction of martensite: 90% or more In the present invention, the volume fraction of martensite at a depth of 1 mm from the surface of the wear-resistant steel plate is set to 90% or more. If the volume fraction of martensite is less than 90%, the hardness of the base structure of the wear-resistant steel sheet is lowered, resulting in deterioration of wear resistance. Therefore, the volume fraction of martensite is set to 90% or more. On the other hand, since the higher the volume fraction of martensite, the better, the upper limit of the volume fraction is not particularly limited, and may be 100%. The volume fraction of martensite can be measured by the method described in Examples.

If the volume fraction of martensite is 90% or more, the desired wear resistance can be obtained regardless of the structure of the remainder, so the structure of the remainder other than martensite is not particularly limited, and any structure can be used. be able to. The structure of the remainder may be, for example, one or more selected from the group consisting of ferrite, pearlite, austenite, and bainite.

[hardness]
Brinell hardness: 420-560HBW 10/3000
The wear-resistant steel sheet of the present invention, in addition to having the above chemical composition, has a Brinell hardness of 420 to 560 HBW 10/3000 at a depth of 1 mm from the surface. The reason for limiting the surface hardness will be explained below.

The wear resistance of the steel sheet can be improved by increasing the hardness of the surface layer of the steel sheet. If the hardness at a depth of 1 mm from the surface of the steel sheet is less than 420 HBW in terms of Brinell hardness, sufficient wear resistance cannot be obtained, resulting in a short service life. Therefore, the hardness at a depth of 1 mm from the surface of the steel sheet is set to 420 HBW or more, preferably 440 HBW or more in terms of Brinell hardness. On the other hand, when the hardness at a depth of 1 mm from the surface of the steel sheet exceeds 560 HBW in terms of Brinell hardness, wide-width bending workability deteriorates. Therefore, the hardness at a depth of 1 mm from the surface of the steel sheet is set to 560 HBW or less in terms of Brinell hardness. Here, the Brinell hardness is the value (HBW 10/3000) at the position of 1/4 of the plate width measured with a load of 3000 kgf using a tungsten hard ball with a diameter of 10 mm.

[Width direction hardness difference]
Width direction hardness difference: 30Hv10 or less If a wear-resistant steel plate has localized hardened or softened areas, strain concentrates around the softened or hardened areas and ductility decreases, so excellent wide-width bending workability can be obtained. can't Therefore, in the present invention, the width direction hardness difference defined as the difference between two points adjacent to each other at intervals of 10 mm in the width direction of the hardness at a depth of 1 mm from the surface of the wear-resistant steel plate is 30Hv10 or less in terms of Vickers hardness. and By setting the hardness difference within the above range, good bending properties can be obtained even when bending over a wide width. In addition, since steel sheets are usually manufactured while being moved in the longitudinal direction (rolling direction), if the uniformity is maintained in the width direction (perpendicular to the rolling direction), the longitudinal direction will also be uniform.

The widthwise hardness difference can be evaluated by measuring the Vickers hardness at intervals of 10mm in the widthwise direction at a position 1mm deep from the surface of the wear-resistant steel plate and obtaining the difference in hardness between adjacent measurement points. The width direction hardness difference of 30 Hv10 or less means that the hardness difference between all adjacent two points is 30 Hv10 or less, in other words, the maximum value of the hardness difference between two adjacent points is 30 Hv10 or less. means.

Thermal cutting such as gas cutting, plasma cutting, and laser cutting is generally used for cutting wear-resistant steel plates. In a heat-cut wear-resistant steel plate, the hardness of the edge portion changes due to the influence of heat during cutting. Therefore, in measuring the hardness difference in the width direction, the heat-affected zone at the end of the wear-resistant steel plate is excluded from the measurement target. More specifically, the Vickers hardness is measured at intervals of 10 mm in the width direction in a range excluding the range of 50 mm on one side in the width direction of the wear-resistant steel plate, and the hardness difference in the width direction can be obtained.

If measurements are made at intervals of more than 10 mm, changes in hardness that cause deterioration in bendability cannot be detected. On the other hand, if the measurement interval is shortened, the hardness change detection accuracy increases, but the number of measurement points becomes enormous. Moreover, as shown in Examples described later, it was confirmed that by controlling the difference in hardness measured at intervals of 10 mm, excellent performance can actually be obtained. For the above reasons, the measurement interval is set to 10 mm.

[Thickness]
The plate thickness of the wear-resistant steel plate of the present invention is not particularly limited, and can be any plate thickness. However, since a wear-resistant steel plate having a thickness of 4 to 60 mm is particularly required to have wide bending workability, it is preferable that the thickness of the wear-resistant steel plate is 4 to 60 mm.

[Production method]
Next, a method for manufacturing a wear-resistant steel plate according to one embodiment of the present invention will be described. The wear-resistant steel plate of the present invention can be produced by heating a steel material having the chemical composition described above, hot-rolling the steel material, and then performing heat treatment including quenching under the conditions described below.

[Steel material]
Any form of material can be used as the steel material. The steel material may be, for example, a steel slab.

The method of manufacturing the steel material is not particularly limited. The smelting can be performed by any method such as a converter, an electric furnace, an induction furnace, or the like. Further, the casting is preferably performed by a continuous casting method from the viewpoint of productivity, but may be performed by an ingot casting method.

[heating]
The steel material is heated to a heating temperature prior to hot rolling. The heating may be performed after once cooling the steel material obtained by a method such as casting, or the obtained steel material may be directly heated without being cooled.

Heating temperature: Ac3 transformation point or higher and 1300° C. or lower When the heating temperature is lower than the Ac3 transformation point, the microstructure of the steel sheet after heating contains a ferrite phase, so that sufficient hardness can be obtained after quenching. Not only can it not be done, but the microstructure cannot be made uniform. Therefore, the heating temperature is set to the Ac3 transformation point or higher. On the other hand, if the heating temperature is higher than 1300° C., excessive energy is required for heating, resulting in reduced productivity. Therefore, the heating temperature is set to 1300° C. or lower, preferably 1250° C. or lower, more preferably 1200° C. or lower, and even more preferably 1150° C. or lower.

Incidentally, the Ac3 transformation point can be obtained by the following formula.
Ac3 (°C) = 912.0 - 230.5 x C + 31.6 x Si - 20.4 x Mn - 39.8 x Cu - 18.1 x Ni - 14.8 x Cr + 16.8 x Mo
(However, the element symbol in the above formula is the content of each element expressed in mass %, and the content of the element not contained is 0.)

[Hot rolling]
Next, the heated steel material is hot-rolled into a hot-rolled steel sheet. The conditions for the hot rolling are not particularly limited, and can be carried out according to a conventional method. In the present invention, since the hardness of the steel sheet is controlled in the heat treatment process after hot rolling, the hot rolling conditions are not particularly limited. However, from the viewpoint of reducing the deformation resistance of the steel material and reducing the load on the rolling mill, the rolling end temperature is preferably 750° C. or higher, more preferably 800° C. or higher, and more preferably 850° C. or higher. It is more preferable that On the other hand, from the viewpoint of preventing significant coarsening of austenite grains and the resulting decrease in ductility after heat treatment, the rolling end temperature is preferably 1000° C. or lower, more preferably 950° C. or lower.

In the present invention, the hot-rolled steel sheet is subjected to heat treatment including quenching. The heat treatment can be performed by one of the two embodiments described below. In the following description, "cooling start temperature" refers to the surface temperature of the steel sheet at the start of cooling in the cooling process of quenching. The term "cooling stop temperature" refers to the surface temperature of the steel sheet at the end of cooling in the cooling process of quenching.

In one embodiment of the present invention, the obtained hot-rolled steel sheet is quenched after the hot rolling. The quenching is performed by either (a) direct quenching (DQ) or (b) reheating quenching (RQ). The cooling method for the quenching is not particularly limited, but water cooling is preferable.

(a) Direct Quenching (DQ)
When the quenching is performed by direct quenching, the hot-rolled steel sheet after hot rolling is cooled from a cooling start temperature equal to or higher than the Ar3 transformation point to a cooling stop temperature equal to or lower than the Mf point.

Cooling start temperature: Ar3 transformation point or higher If the cooling start temperature is Ar3 transformation point or higher, quenching starts from the austenite region, and a desired martensite structure can be obtained. When the cooling start temperature is less than the Ar3 point, ferrite is generated, and the volume fraction of martensite in the finally obtained microstructure is less than 90%. If the volume fraction of martensite is less than 90%, the hardness of the steel sheet cannot be sufficiently improved, and as a result, the wear resistance of the steel sheet decreases. Furthermore, if the cooling start temperature is less than the Ar3 point, a difference in hardness occurs in the width direction, resulting in deterioration in wide-width bending workability. On the other hand, although the upper limit of the cooling start temperature is not particularly limited, it is preferably 950° C. or less.

The Ar3 transformation point can be obtained by the following formula.
Ar3 (°C) = 910-273 x C-74 x Mn-57 x Ni-16 x Cr-9 x Mo-5 x Cu
(However, the element symbol in the above formula is the content of each element expressed in mass %, and the content of the element not contained is 0.)

Cooling stop temperature: Mf point or lower If the cooling stop temperature is higher than the Mf point, the volume fraction of martensite cannot be sufficiently increased, and desired hardness cannot be obtained. Furthermore, if the cooling stop temperature is higher than the Mf point, a difference in hardness occurs in the width direction, resulting in deterioration in wide-width bending workability. Therefore, the cooling stop temperature is set at the Mf point or lower. From the viewpoint of increasing the volume fraction of martensite, the cooling stop temperature is preferably (Mf point −100° C.) or less, more preferably (Mf point −120° C.) or less, (Mf point −150° C.) or lower. On the other hand, the lower limit of the cooling stop temperature is not particularly limited, but since excessive cooling causes a decrease in production efficiency, the cooling stop temperature is preferably room temperature or higher.

Note that the Mf point can be obtained by the following formula.
Mf (°C) = 410.5 - 407.3 x C - 7.3 x Si - 37.8 x Mn - 20.5 x Cu - 19.5 x Ni - 19.8 x Cr - 4.5 x Mo
(However, the element symbol in the above formula is the content of each element expressed in mass %, and the content of the element not contained is 0.)

(b) Reheating and quenching (RQ)
When the quenching is performed by reheating quenching, first, the hot-rolled steel sheet after the hot rolling is cooled, and the hot-rolled steel sheet after the cooling is reheated to a reheating temperature of the Ac3 transformation point or higher and 950 ° C. or lower. . Thereafter, the hot-rolled steel sheet after reheating is cooled from the reheating temperature to a cooling stop temperature equal to or lower than the Mf point.

Reheating temperature: Ac3 transformation point or higher, 950° C. or lower By reheating the hot-rolled steel sheet to the Ac3 transformation point or higher, the microstructure can be changed to austenite, so that the martensitic structure can be obtained by subsequent quenching (cooling). If the reheating temperature is lower than the Ac3 transformation point, ferrite is generated and quenching is not sufficiently performed, so that the hardness of the steel sheet cannot be sufficiently improved, and as a result, the finally obtained steel sheet has wear resistance. decreases. Therefore, the reheating temperature is made equal to or higher than the Ac3 transformation point. On the other hand, if the reheating start temperature is higher than 950° C., the crystal grains become coarse and workability deteriorates. Therefore, the reheating temperature is set to 950° C. or lower. In order to start cooling from the reheating temperature, for example, cooling may be started immediately after the hot-rolled steel sheet comes out of the furnace used for reheating.

Cooling stop temperature: Mf point or lower If the cooling stop temperature is higher than the Mf point, the volume fraction of martensite cannot be sufficiently increased, and desired hardness cannot be obtained. Furthermore, if the cooling stop temperature is higher than the Mf point, a difference in hardness occurs in the width direction, resulting in deterioration in wide-width bending workability. Therefore, the cooling stop temperature is set at the Mf point or lower. From the viewpoint of increasing the volume fraction of martensite, the cooling stop temperature is preferably (Mf point −100° C.) or less, more preferably (Mf point −120° C.) or less, (Mf point −150° C.) or lower. On the other hand, the lower limit of the cooling stop temperature is not particularly limited, but since excessive cooling causes a decrease in production efficiency, the cooling stop temperature is preferably room temperature or higher.

(Average cooling rate during quenching)
The cooling rate in the cooling process of the quenching is not particularly limited, and any value can be set as long as the cooling rate is such that the martensite phase is formed. For example, the average cooling rate from the start of quenching to the stop of quenching is preferably 10° C./s or higher, more preferably 15° C./s or higher, and even more preferably 20° C./s or higher. On the other hand, since the higher the average cooling rate during quenching, the better in principle, the upper limit is not particularly limited. However, in order to increase the cooling rate, a cooling facility that can handle it is required, so the average cooling rate is preferably 150 ° C./s or less, more preferably 100 ° C./s or less. It is preferably 80° C./s or less, more preferably 80° C./s or less. Here, the average cooling rate refers to the average cooling rate at the surface temperature at the central position in the width direction of the steel sheet. The surface temperature can be measured using a radiation thermometer or the like.

(cooling speed difference)
In the present invention, in the cooling process of the quenching, the difference between the average cooling rate at the width direction center position and the average cooling rate at the width direction 1/4 position of the hot rolled steel sheet, and the average cooling rate at the width direction center position and the width The difference in the average cooling rates at the 3/4 positions in each direction shall be 5° C./s or less. If the difference in the average cooling rate (hereinafter sometimes referred to as “cooling rate difference”) is greater than 5° C./s, the difference in Vickers hardness between two adjacent points becomes greater than 30Hv10, and wide-width bending workability is deteriorated. do. Here, the average cooling rate refers to the average cooling rate at the surface temperature of the steel sheet. The surface temperature can be measured using a radiation thermometer or the like.

(Tempering)
In one embodiment of the present invention, the quenched hot-rolled steel sheet can optionally be further tempered. Tempering can further improve the uniformity of the hardness of the steel sheet. When tempering is performed, the cooling stop temperature in the quenching is preferably less than (Mf point -100°C). After cooling is stopped at the cooling stop temperature, the steel sheet may be heated to the tempering temperature described below.

Tempering temperature: (Mf point -80°C) or more, (Mf point +50°C) or less If the tempering temperature is less than (Mf point -80°C), the effect of tempering cannot be obtained. Therefore, when tempering is performed, the tempering temperature is set to (Mf point −80° C.) or higher, preferably (Mf point −60° C.) or higher, more preferably (Mf point −50° C.) or higher. On the other hand, if the tempering temperature is higher than (Mf point+50° C.), the decrease in surface hardness becomes significant. Therefore, when tempering is performed, the tempering temperature is set to (Mf point +50°C) or lower, preferably (Mf point +30°C) or lower, more preferably (Mf point +10°C) or lower.

-Temperature retention After reaching the tempering temperature, the heating should be stopped. However, in one embodiment of the present invention, after heating to the tempering temperature, it can be held at said tempering temperature for any holding time. Although the holding time is not particularly limited, it is preferably 60 seconds or longer, more preferably 5 minutes or longer, from the viewpoint of enhancing the effect of tempering. On the other hand, if the holding time is excessively long, the hardness of the steel sheet may decrease, so when the temperature is held, the holding time is preferably 60 minutes or less, more preferably 30 minutes or less, and 20 minutes. It is more preferable to:

- Rate of temperature rise The rate of temperature rise to the tempering temperature in the tempering is not particularly limited. However, from the viewpoint of productivity, the average heating rate to the tempering temperature is preferably 0.1° C./s or more, more preferably 0.5° C./s or more. Further, by setting the average temperature increase rate to 2° C./s or more, carbides can be finely precipitated, and as a result, wide-width bending workability can be further improved. Therefore, from the viewpoint of further improving wide-width bending workability, the average temperature increase rate is preferably 2° C./s or more, more preferably 10° C./s or more. On the other hand, the upper limit of the average temperature increase rate is not particularly limited, but if the temperature increase rate is excessively increased, the equipment for reheating becomes large, and energy consumption increases. Therefore, the average rate of temperature increase is preferably 30° C./s or less, more preferably 25° C./s or less.

The heating (heating) in the tempering is not particularly limited and can be performed by any method. For example, at least one method selected from the group consisting of heating using a heat treatment furnace, high-frequency induction heating, and electric heating can be used. When the temperature is maintained, the reheating and temperature maintenance are preferably performed using a heat treatment furnace. Further, when the average temperature increase rate is 2° C./s or more, it is preferable to perform heating to the tempering temperature by high-frequency induction heating or electric heating. On the other hand, when using a heat treatment furnace, it is preferable to set the average temperature increase rate to 10° C./s or less. Also, the tempering can be performed either off-line or on-line.

After heating to the tempering temperature and optionally holding the temperature, heating or holding the temperature may be stopped. A subsequent cooling method is not particularly limited, and one or both of air cooling and water cooling can be used. In one embodiment of the present invention, the steel sheet can be allowed to cool to room temperature after stopping heating or temperature maintenance.

In another embodiment of the present invention, the cooling in the quenching is interrupted in a specific temperature range and then air-cooled. As a result, the steel sheet is tempered, so that the hardness uniformity of the steel sheet can be further improved as in the case of tempering in the above embodiment. This embodiment will be described below.

Cooling stop temperature: Mf point or less, (Mf point −100 ° C.) or more As described above, if the cooling stop temperature in the quenching is higher than the Mf point, the volume fraction of martensite cannot be sufficiently increased, and the desired hardness can't get Furthermore, if the cooling stop temperature is higher than the Mf point, a difference in hardness occurs in the width direction, resulting in deterioration in wide-width bending workability. Therefore, the cooling stop temperature is set at the Mf point or lower. On the other hand, if the cooling stop temperature is less than (Mf point -100°C), the tempering effect cannot be obtained even if air cooling is performed after cooling is stopped. Therefore, in this embodiment, the cooling stop temperature is set to (Mf point -100°C) or higher. From the viewpoint of enhancing the tempering effect by air cooling, the cooling stop temperature is preferably (Mf point -80°C) or higher, more preferably (Mf point -50°C) or higher.

In this embodiment, the tempering effect can be obtained by performing air cooling after cooling is stopped at the cooling stop temperature. The air cooling can be performed under any conditions without particular limitation, but the cooling rate is preferably 1° C./s or less.

EXAMPLES In order to confirm the effects of the present invention, wear-resistant steel plates were manufactured by the procedure described below, and their properties were evaluated.

First, molten steel having the chemical composition shown in Table 1 was melted to obtain a steel slab as a steel material. The obtained steel slabs were heated to the heating temperatures shown in Table 2 and then hot rolled under the conditions shown in Table 2 to obtain hot rolled steel sheets. The obtained hot-rolled steel sheets were directly quenched or reheated and quenched under the conditions shown in Table 2 to produce wear-resistant steel sheets. In some examples, tempering was performed under the conditions shown in Table 2 after quenching. In addition, in the examples in which tempering was not performed, after the quenching was stopped, air cooling was performed at a cooling rate of 1° C./s or less.

In addition, in the "cooling rate difference" column of Table 2, the difference between the average cooling rate at the center position in the width direction of the hot-rolled steel sheet and the average cooling rate at the 1/4 position in the width direction in the cooling process of quenching, The difference between the average cooling rate at the central position and the average cooling rate at the 3/4 position in the width direction is the larger value.

Next, the volume fraction of martensite (M), the hardness, the maximum value of the hardness difference in the width direction, and the wide bending radius were evaluated for each of the obtained wear-resistant steel sheets. The evaluation method is as follows.

(Volume fraction of martensite)
A sample was taken from each steel plate so that the observation position was 1 mm deep from the surface of the steel plate. After the surface of the sample was mirror-polished and further subjected to nital corrosion, a 10 mm×10 mm range was photographed using a scanning electron microscope (SEM). The area fraction of martensite was determined by analyzing the photographed image using an image analyzer. Ten visual fields were randomly observed, and the average value of the obtained area fractions was taken as the volume fraction of martensite.

(surface hardness)
A test piece for hardness measurement was taken from the obtained wear-resistant steel plate, and the Brinell hardness was measured in accordance with JIS Z 2243 (1998). In order to eliminate the effects of scales and decarburized layers existing on the surface of the wear-resistant steel plate, the above measurements were carried out after removing a region to a depth of 1 mm from the surface of the steel plate by grinding. Therefore, the measured hardness is the hardness at a depth of 1 mm from the surface of the steel sheet. In addition, the measurement position in the board width direction was made into the 1/4 position of board width. Also, in the measurement, a hard tungsten ball with a diameter of 10 mm was used, and the load was 3000 kgf.

(Width direction hardness difference)
The Vickers hardness at a depth of 1 mm from the surface of the wear-resistant steel plate was measured at intervals of 10 mm in the plate width direction. In the above measurement, both ends of the wear-resistant steel plate and a region of 50 mm per side were excluded from the measurement range. From the obtained values, the absolute value of the difference in Vickers hardness between two adjacent points was determined, and Table 3 shows the maximum value. The test load in the Vickers hardness measurement was 10 kg.

(limit bending radius)
A bending test piece having a width of 200 mm and a length of 300 mm was taken from the obtained steel plate, and a bending test was performed at a bending angle of 180° in accordance with JIS Z 2248. A limit bending radius R/t was obtained from the minimum bending radius R (mm) at which cracks did not occur and the plate thickness t (mm) in the bending test.

Table 3 shows the evaluation results obtained by the above method. As can be seen from the results shown in Table 3, the wear-resistant steel plate satisfying the conditions of the present invention has a surface hardness of 420 HBW 10/3000 or more in terms of Brinell hardness, and is excellent in wear resistance. In addition, the wear-resistant steel sheet that satisfies the conditions of the present invention had a limit bending radius R/t of 5.0 or less in the bending test, and had good wide-width bending workability. Thus, the wear-resistant steel plate of the present invention had both excellent wear resistance and wide bending workability. From these results, it can be seen that the present invention can improve the wide-width bending workability without lowering the surface hardness of the wear-resistant steel plate.

Claims (12)

in % by mass,
C: 0.15 to 0.30%,
Si: 0.05 to 1.00%,
Mn: 0.50-2.00%,
P: 0.020% or less,
S: 0.010% or less,
Al: 0.01-0.06%,
Cr: 0.10 to 1.00%, and N: 0.0100% or less,
Having a component composition consisting of the balance Fe and unavoidable impurities,
The volume fraction of martensite at a depth of 1 mm from the surface is 90% or more,
The hardness at a depth of 1 mm from the surface is 420 to 560 HBW 10/3000 in Brinell hardness,
The plate width is 200 mm or more,
The width direction hardness difference defined as the difference between two points adjacent to each other at intervals of 10 mm in the width direction of the hardness at a depth of 1 mm from the surface is 30 Hv10 or less in Vickers hardness between all the two adjacent points. A wear-resistant steel plate. The component composition, in mass%,
Nb: 0.005 to 0.020%,
Ti: 0.005-0.020%, and B: 0.0003-0.0030%
Further containing 1 or 2 or more selected from the group consisting of
The wear-resistant steel plate according to claim 1 , having a plate thickness of 25 mm or more . The component composition, in mass%,
Cu: 0.01-0.5%,
Ni: 0.01 to 3.0%,
Mo: 0.1 to 1.0%,
V: 0.01 to 0.10%,
W: 0.01-0.5%, and Co: 0.01-0.5%
The wear-resistant steel plate according to claim 1 or 2, further containing one or more selected from the group consisting of: The component composition, in mass%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0100%, and REM: 0.0005-0.0200%
The wear-resistant steel plate according to any one of claims 1 to 3, further containing one or more selected from the group consisting of: in % by mass,
C: 0.15 to 0.30%,
Si: 0.05 to 1.00%,
Mn: 0.50-2.00%,
P: 0.020% or less,
S: 0.010% or less,
Al: 0.01-0.06%,
Cr: 0.10 to 1.00%, and N: 0.0100% or less,
Heating a steel material having a chemical composition consisting of the balance Fe and unavoidable impurities to a heating temperature of 1300° C. or less from the Ac3 transformation point or more,
Hot-rolling the heated steel material into a hot-rolled steel sheet,
A method for producing a wear-resistant steel plate, wherein the hot-rolled steel plate is quenched,
The quenching is
(a) Direct quenching for cooling the hot-rolled steel sheet from a cooling start temperature equal to or higher than the Ar3 transformation point to a cooling stop temperature equal to or lower than the Mf point, or
(b) cooling the hot-rolled steel sheet, reheating the cooled hot-rolled steel sheet to a reheating temperature of from the Ac3 transformation point to 950 ° C., and reheating the hot-rolled steel sheet after the reheating from the reheating temperature, It is reheating and quenching that cools to a cooling stop temperature below the Mf point,
In the cooling process of the quenching, the difference between the average cooling rate at the widthwise center position and the average cooling rate at the widthwise 1/4 position of the hot rolled steel sheet, and the average cooling rate at the widthwise center position and the widthwise 3/4 The difference in average cooling rate at each position is 5 ° C./s or less ,
The wear-resistant steel plate is
The volume fraction of martensite at a depth of 1 mm from the surface is 90% or more,
The hardness at a depth of 1 mm from the surface is 420 to 560 HBW 10/3000 in Brinell hardness,
The plate width is 200 mm or more,
The width direction hardness difference defined as the difference between two points adjacent to each other at intervals of 10 mm in the width direction of the hardness at a depth of 1 mm from the surface is 30Hv10 or less in Vickers hardness between all the two adjacent points. A method for producing a wear-resistant steel plate. The cooling stop temperature in the quenching is less than (Mf point -100 ° C.),
The method for producing a wear-resistant steel plate according to claim 5, wherein after the quenching, the quenched hot-rolled steel plate is tempered at a tempering temperature of (Mf point −80° C.) or higher and (Mf point +50° C.) or lower. 7. The method for producing a wear-resistant steel plate according to claim 6, wherein in said tempering, said tempering temperature is maintained for 60 seconds or longer. The method for producing a wear-resistant steel plate according to claim 6 or 7, wherein the tempering has an average heating rate of 2°C/s or more. The cooling stop temperature in the quenching is Mf point or less and (Mf point -100 ° C.) or more,
The method for manufacturing a wear-resistant steel plate according to claim 5, wherein after the quenching, the quenched hot-rolled steel plate is air-cooled. The component composition, in mass%,
Nb: 0.005 to 0.020%,
Ti: 0.005-0.020%, and B: 0.0003-0.0030%
The method for producing a wear-resistant steel plate according to any one of claims 5 to 9, further containing one or more selected from the group consisting of: The component composition, in mass%,
Cu: 0.01-0.5%,
Ni: 0.01 to 3.0%,
Mo: 0.1 to 1.0%,
V: 0.01 to 0.10%,
W: 0.01-0.5%, and Co: 0.01-0.5%
The method for producing a wear-resistant steel plate according to any one of claims 5 to 10, further containing one or more selected from the group consisting of: The component composition, in mass%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0100%, and REM: 0.0005-0.0200%
The method for producing a wear-resistant steel plate according to any one of claims 5 to 11, further containing one or more selected from the group consisting of: