JP6737208B2 - Wear-resistant steel plate - Google Patents

Description

The present invention relates to a wear-resistant steel plate, and more particularly to a wear-resistant steel plate that has excellent wear resistance at high temperatures and can be suitably used for industrial machines, transportation equipment and the like. The present invention also relates to a method for manufacturing the wear resistant steel plate.

It is known that the wear resistance of steel can be improved by increasing the hardness. Therefore, high hardness steel obtained by subjecting alloy steel containing a large amount of alloy elements such as Mn, Cr and Mo to heat treatment such as quenching has been widely used as wear resistant steel.

For example, Patent Literatures 1 and 2 propose a wear-resistant steel sheet having a surface layer portion having a Brinell hardness (HB) of 360 to 490. In the wear-resistant steel plate, a high surface hardness is realized by adding a predetermined amount of alloying element and quenching to obtain a martensite-based structure as quenched.

Japanese Patent No. 4645306 Japanese Patent No. 4735191

Some wear-resistant steel plates are used under conditions in which the temperature of the steel plate surface is as high as 300 to 500°C. Therefore, in order to prolong the service life at such a high temperature, it is important to ensure not only wear resistance at room temperature but also high wear resistance at high temperature.

However, in the wear resistant steel sheets described in Patent Documents 1 and 2, the wear resistance at high temperature was not considered.

An object of the present invention is to solve the above problems and provide a wear-resistant steel plate that exhibits high wear resistance at high temperatures. Moreover, this invention aims at providing the manufacturing method of the said abrasion resistant steel plate.

In order to achieve the above object, the inventors of the present invention have earnestly studied various factors that affect the high temperature wear resistance of the wear resistant steel plate. As a result, they have found that it is possible to manufacture a wear-resistant steel plate that exhibits high wear resistance at high temperatures by subjecting a steel plate having a high carbon content to a normal quenching treatment and then tempering it under specific conditions.

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

1. In mass %,
C: 0.34 to 0.50%,
Si: 0.05-1.00%,
Mn: 0.30 to 2.00%,
P: 0.020% or less,
S: 0.020% or less,
Al: 0.04% or less,
Cr: 0.05-5.00%,
N: 0.0050% or less, and O: 0.0050% or less,
The balance has a composition of Fe and inevitable impurities,
Brinell hardness at a depth of 1 mm from the surface is 360 to 490 HBW 10/3000,
The area fraction of tempered martensite in the structure at a depth of 1 mm from the surface is 95% or more, and the tempered martensite has 8.0×10 ⁴ pieces/mm ² or more of cementite having a circle equivalent diameter of 0.02 μm or more. Abrasion resistant steel plate, including by number density.

2. The component composition is mass%,
Cu: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
V: 0.01 to 1.00%,
W: 0.01 to 1.00%, and Co: 0.01 to 1.00%
The wear resistant steel sheet according to 1 above, which further contains one or more selected from the group consisting of:

3. The component composition is mass%,
Nb: 0.005 to 0.050%,
Ti: 0.005 to 0.050%, and B: 0.0001 to 0.0100%
The wear resistant steel sheet according to 1 or 2 above, further containing 1 or 2 or more selected from the group consisting of:

4. The component composition is mass%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0080%
The wear resistant steel plate according to any one of 1 to 3 above, further containing 1 or 2 or more selected from the group consisting of:

5. In mass %,
C: 0.34 to 0.50%,
Si: 0.05-1.00%,
Mn: 0.30 to 2.00%,
P: 0.020% or less,
S: 0.020% or less,
Al: 0.04% or less,
Cr: 0.05-5.00%,
N: 0.0050% or less, and O: 0.0050% or less,
A steel material having a composition with the balance being Fe and unavoidable impurities is heated to a heating temperature,
Hot-rolled steel sheet by hot rolling the heated steel material,
The hot rolled steel sheet is subjected to either direct quenching with a quenching start temperature of Ar ₃ transformation point or higher and reheating quenching with a quenching start temperature of Ac ₃ transformation point or higher,
A method for producing a wear-resistant steel sheet, wherein the hot-rolled steel sheet after quenching is tempered under the condition that the P value defined by the following formula (1) is 1.40×10 ^{4 to} 1.80×10 ⁴ .
Note P=(T+273)×(21.3−5.8×C+log(60×t)) (1)
(However, C in the above formula (1) is the C content (mass %) in the steel sheet, T is the tempering temperature (°C), and t is the holding time (minutes) in the tempering).

6. The component composition is mass%,
Cu: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
V: 0.01 to 1.00%,
W: 0.01 to 1.00%, and Co: 0.01 to 1.00%
6. The method for producing a wear-resistant steel sheet as described in 5 above, which further contains one or more selected from the group consisting of:

7. The component composition is mass%,
Nb: 0.005 to 0.050%,
Ti: 0.005 to 0.050%, and B: 0.0001 to 0.0100%
7. The method for producing a wear-resistant steel sheet as described in 5 or 6 above, which further contains 1 or 2 or more selected from the group consisting of:

8. The component composition is mass%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0080%
The method for producing a wear-resistant steel plate according to any one of 5 to 7 above, further containing one or more selected from the group consisting of:

According to the present invention, it is possible to obtain a wear-resistant steel plate that exhibits high wear resistance at high temperatures.

It is a schematic diagram of a wear test device used for evaluation of wear resistance.

[Ingredient composition]
Next, a method for carrying out the present invention will be specifically described. In the present invention, it is important that the wear-resistant steel plate and the steel material used for its production have the above-mentioned composition. Therefore, first, the reason why the composition of the steel in the present invention is limited as described above will be explained. In addition, "%" regarding a component composition shall mean "mass %" unless there is particular notice.

C: 0.34 to 0.50%
C is an element having the effect of increasing the hardness in the surface layer and improving the wear resistance. In order to obtain the above effect, the C content is set to 0.34% or more. From the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, the C content is preferably 0.38% or more. On the other hand, when the C content exceeds 0.50%, the hardness of the surface layer during the quenching heat treatment excessively increases, so that the heating temperature required during the tempering heat treatment rises and the heat treatment cost rises. Therefore, the C content is 0.50% or less. Further, from the viewpoint of suppressing the temperature during the tempering heat treatment, the C content is preferably 0.45% or less.

Si: 0.05-1.00%
Si is an element that acts as a deoxidizer. Further, Si has a function of forming a solid solution in steel and increasing the hardness of the matrix phase by solid solution strengthening. In order to obtain these effects, 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 ductility and toughness decrease, and the amount of inclusions increases, which causes problems. Therefore, the Si content is 1.00% or less. The Si content is preferably 0.80% or less, more preferably 0.60% or less, and further preferably 0.40% or less.

Mn: 0.30 to 2.00%
Mn is an element having the effect of increasing the hardness of the surface layer and improving the wear resistance. In order to obtain the above effect, the Mn content is set to 0.30% or more. The Mn content is preferably 0.70% or more, more preferably 0.90% or more. On the other hand, when the Mn content exceeds 2.00%, not only the weldability and toughness decrease, but also the alloy cost becomes excessively high. Therefore, the Mn content is 2.00% or less. The Mn content is preferably 1.80% or less, and more preferably 1.60% or less.

P: 0.020% or less P is an element contained as an unavoidable impurity and segregates at the grain boundaries to adversely affect the toughness of the base material and the welded portion. Therefore, it is desirable that the P content is as low as possible, but 0.020% or less is acceptable. The lower limit of P content is not particularly limited and may be 0%. However, since P is an element that is unavoidably contained in steel as an impurity, it is industrially more than 0%. You can Further, since excessive reduction leads to a high refining cost, the P content is preferably 0.001% or more.

S: 0.020% or less S is an element contained as an unavoidable impurity, is present in steel as a sulfide-based inclusion such as MnS, and is an element that adversely affects the occurrence of fracture. .. Therefore, it is desirable to reduce the S content as much as possible, but 0.020% or less is acceptable. The lower limit of the S content is not particularly limited and may be 0%. However, since S is an element that is unavoidably contained in steel as an impurity, it is industrially more than 0%. You can Further, since excessive reduction leads to a high refining cost, the S content is preferably 0.0005% or more.

Al: 0.04% or less Al is an element that acts as a deoxidizing agent and has an action of refining crystal grains. In order to obtain these effects, the Al content is preferably 0.01% or more. On the other hand, when the Al content exceeds 0.04%, the oxide inclusions increase and the cleanliness decreases. Therefore, the Al content is 0.04% or less. The Al content is preferably 0.03% or less, more preferably 0.02% or less.

Cr: 0.05-5.00%
Cr is an element having the effect of increasing the hardness of the surface layer and improving the wear resistance. It also has the function of improving wear resistance at high temperatures by forming precipitates. In order to obtain the above effect, the Cr content is set to 0.05% or more. The Cr content is preferably 0.20% or more, more preferably 0.25% or more. On the other hand, if the Cr content exceeds 5.00%, the weldability decreases. Therefore, the Cr content is 5.00% or less. The Cr content is preferably 1.85% or less, more preferably 1.80% or less.

N: 0.0050% or less N is an element contained as an unavoidable impurity, but the content of 0.0050% or less is acceptable. The N content is preferably 0.0040% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, since N is an element that is unavoidably contained in steel as an impurity, it is industrially more than 0%. You can

O: 0.0050% or less O is an element contained as an unavoidable impurity, but the content of 0.0050% or less is acceptable. The O content is preferably 0.0040% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0%. However, since O is an element that is unavoidably contained in steel as an impurity, it is industrially more than 0%. You can

The wear resistant steel plate and the steel material according to the embodiment of the present invention are composed of the above components, and the balance of Fe and unavoidable impurities.

The above is the basic component composition in the present invention, but for the purpose of further improving the hardenability, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Mo: 0.01 to 1 or 2 or more selected from the group consisting of 1.00%, V: 0.01 to 1.00%, W: 0.01 to 1.00%, and Co: 0.01 to 1.00%. Further, it can be optionally contained.

Cu: 0.01-2.00%
Cu is an element that has the effect of improving wear resistance at high temperatures, and can be arbitrarily added to improve wear resistance at high temperatures. When Cu is added, the Cu content is 0.01% or more in order to obtain the above effect. On the other hand, when the Cu content exceeds 2.00%, the weldability is deteriorated and the alloy cost is increased. Therefore, when Cu is added, the Cu content is 2.00% or less.

Ni: 0.01 to 2.00%
Ni is an element having an action of improving wear resistance at high temperatures, like Cu, and can be arbitrarily added to improve wear resistance at high temperatures. When Ni is added, the Ni content is 0.01% or more in order to obtain the above effect. On the other hand, when the Ni content exceeds 2.00%, the weldability is deteriorated and the alloy cost is increased. Therefore, when Ni is added, the Ni content is 2.00% or less.

Mo: 0.01-1.00%
Mo is an element having an action of improving wear resistance at high temperatures, like Cu, and can be arbitrarily added to improve wear resistance at high temperatures. When Mo is added, the Mo content is 0.01% or more in order to obtain the above effect. On the other hand, when the Mo content exceeds 1.00%, the weldability is deteriorated and the alloy cost is increased. Therefore, when Mo is added, the Mo content is 1.00% or less.

V: 0.01 to 1.00%
V is an element having the effect of improving the wear resistance at high temperatures, like Cu, and can be added arbitrarily to improve the hardness inside the steel sheet. When V is added, the V content is 0.01% or more in order to obtain the above effect. On the other hand, when the V content exceeds 1.00%, the weldability is deteriorated and the alloy cost is increased. Therefore, when V is added, the V content is 1.00% or less.

W: 0.01 to 1.00%
Similar to Cu, W is an element having an effect of improving the wear resistance at high temperatures, and can be arbitrarily added to improve the wear resistance at high temperatures. When W is added, the W content is 0.01% or more in order to obtain the above effects. On the other hand, when the W content exceeds 1.00%, the weldability is deteriorated and the alloy cost is increased. Therefore, when W is added, the W content is 1.00% or less.

Co: 0.01-1.00%
Co is an element having an effect of improving the wear resistance at high temperatures, like Cu, and can be added arbitrarily to improve the hardness inside the steel sheet. When Co is added, the Co content is 0.01% or more in order to obtain the above effect. On the other hand, when the Co content exceeds 1.00%, the weldability is deteriorated and the alloy cost is increased. Therefore, when Co is added, the Co content is 1.00% or less.

Moreover, in another embodiment of this invention, the said component composition is from Nb:0.005-0.050%, Ti:0.005-0.050%, and B:0.0001-0.0100%. One or more selected from the group consisting of can be further optionally contained.

Nb: 0.005-0.050%
Nb is an element that contributes to the improvement of wear resistance at high temperatures. When Nb is added, the Nb content is 0.005% or more in order to obtain the above effect. The Nb content is preferably 0.007% or more. On the other hand, when the Nb content exceeds 0.050%, a large amount of NbC is precipitated and the workability deteriorates. Therefore, when Nb is added, the Nb content is 0.050% or less. The Nb content is preferably 0.040% or less, more preferably 0.030% or less.

Ti: 0.005-0.050%
Ti is an element that has a strong tendency to form a nitride and has an action of fixing N and reducing solid solution N. Therefore, addition of Ti can improve the toughness of the base material and the welded portion. Further, when both Ti and B are added, Ti fixes N to suppress precipitation of BN, and as a result, the hardenability improving effect of B is promoted. When Ti is added to obtain these effects, the Ti content is 0.005% or more. The Ti content is preferably 0.012% or more. On the other hand, when the Ti content exceeds 0.050%, a large amount of TiC is deposited, which lowers the workability. Therefore, when Ti is contained, the Ti content is 0.050%. The Ti content is preferably 0.040% or less, more preferably 0.030% or less.

B: 0.0001 to 0.0100%
B is an element that has the effect of significantly improving the hardenability even when added in a trace amount. Therefore, the addition of B can promote the formation of martensite during quenching and further improve the wear resistance. When B is added to obtain the above effect, the B content is 0.0001% or more. The B content is preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0100%, the weldability decreases. Therefore, when B is added, the B content is 0.0100% or less. The B content is preferably 0.0050% or less, more preferably 0.0030% or less.

Moreover, in another embodiment of this invention, the said component composition is from Ca: 0.0005 to 0.0040%, Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0080%. One or more selected from the group consisting of can be further optionally contained.

Ca: 0.0005 to 0.0050%
Ca is an element that binds to S and has an action of suppressing the formation of MnS or the like that extends long in the rolling direction. Therefore, by adding Ca, it is possible to control the morphology of the sulfide-based inclusions to have a spherical shape and improve the toughness of the welded portion. When Ca is added to obtain the above effect, the Ca content is set to 0.0005% or more. On the other hand, if the Ca content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness leads to deterioration in surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, when Ca is added, the Ca content is 0.0050% or less.

Mg: 0.0005 to 0.0050%
Like Ca, Mg is an element that binds to S and has the effect of suppressing the formation of MnS and the like that extends long in the rolling direction. Therefore, by adding Mg, it is possible to control the morphology of the sulfide-based inclusions to have a spherical shape and improve the toughness of the welded portion. When Mg is added to obtain the above effect, the Mg content is set to 0.0005% or more. On the other hand, if the Mg content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness leads to deterioration in surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, when adding Mg, the Mg content is set to 0.0050% or less.

REM: 0.0005 to 0.0080%
Similar to Ca and Mg, REM (rare earth metal) is an element that binds to S and has an action of suppressing the formation of MnS or the like that extends long in the rolling direction. Therefore, by adding REM, it is possible to control the morphology of the sulfide-based inclusions to have a spherical shape and improve the toughness of the welded portion. When REM is added to obtain the above effect, the REM content is 0.0005% or more. On the other hand, if the REM content exceeds 0.0080%, the cleanliness of the steel decreases. A decrease in cleanliness leads to deterioration in surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, when REM is added, the REM content is 0.0080% or less.

In other words, the wear resistant steel plate and the steel material used for manufacturing the same according to the present invention can have the following composition.
In mass %,
C: 0.23 to 0.34%,
Si: 0.05-1.00%,
Mn: 0.30 to 2.00%,
P: 0.020% or less,
S: 0.020% or less,
Al: 0.04% or less,
Cr: 0.05-5.00%,
N: 0.0050% or less,
O: 0.0050% or less,
Optionally, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Mo: 0.01 to 1.00%, V: 0.01 to 1.00%, W:0. 1 or 2 or more selected from the group consisting of 0.01 to 1.00% and Co: 0.01 to 1.00%;
1 or 2 or more selected from the group consisting of Nb: 0.005 to 0.050%, Ti: 0.005 to 0.050%, and B: 0.0001 to 0.0100%,
1 or 2 or more selected from the group consisting of Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0080%, and the balance Composition of Fe and inevitable impurities.

[Surface hardness]
Brinell hardness: 360-490HBW 10/3000
The wear resistance of a steel sheet can be improved by increasing the hardness of the surface layer of the steel sheet. If the hardness of the surface layer of the steel sheet is less than 360 HBW in terms of Brinell hardness, sufficient wear resistance cannot be obtained. On the other hand, if the hardness of the steel sheet surface layer is higher than the Brinell hardness of 490 HBW, the workability deteriorates. Therefore, in the present invention, the Brinell hardness of the surface layer of the steel sheet is 360 to 490 HBW. In addition, the said hardness shall be Brinell hardness (henceforth "surface layer hardness") in the position of the depth of 1 mm from the surface of a wear-resistant steel plate here. The Brinell hardness is a value (HBW 10/3000) measured using a tungsten hard ball having a diameter of 10 mm and a load of 3000 Kgf. The Brinell hardness can be measured by the method described in Examples.

[Surface layer]
In order to exhibit wear resistance at high temperatures, it is necessary to temper martensite and enhance the stability of the structure during wear at high temperatures. In order to sufficiently obtain this effect, the area fraction of tempered martensite in the structure at a depth of 1 mm from the surface of the steel sheet (hereinafter, also referred to as "surface layer structure") is 95% or more, and the tempered martensite is It is necessary that the sites contain cementite having a circle equivalent diameter of 0.02 μm or more at a number density of 8.0×10 ⁴ pieces/mm ² or more. Cementite having an equivalent circle diameter of 0.02 μm or more has an effect of inhibiting grain boundary migration at high temperatures and stabilizing the structure. If the number density of cementite having a circle equivalent diameter of 0.02 μm or more (hereinafter sometimes simply referred to as “the number density of cementite”) is less than 8.0×10 ⁴ pieces/mm ² , the grain boundary migration is hindered. Is insufficient, and the stability of the structure at high temperature cannot be obtained. The number density of the cementite can be measured by the method described in Examples. The surface layer structure is allowed to include a structure other than tempered martensite as long as it has an area fraction of 5% or less.

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

[Steel material]
The method for producing the steel material is not particularly limited, but, for example, molten steel having the above-described composition can be melted by a conventional method and cast to manufacture. The melting can be performed by any method such as a converter, an electric furnace and an induction furnace. The casting is preferably performed by a continuous casting method from the viewpoint of productivity, but can also be performed by an ingot-decomposition rolling method. As the steel material, for example, a steel slab can be used.

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

The heating temperature is not particularly limited, but if the heating temperature is less than 900° C., the deformation resistance of the steel material is high, so that the load on the rolling mill in hot rolling increases and it is difficult to perform hot rolling. May be Therefore, the heating temperature is preferably 900° C. or higher, more preferably 950° C. or higher, and further preferably 1100° C. or higher. On the other hand, if the heating temperature is higher than 1250° C., the oxidation of the steel becomes remarkable and the loss due to the oxidation increases, resulting in a decrease in yield. Therefore, the heating temperature is preferably 1250°C or lower, more preferably 1200°C or lower, and further preferably 1150°C or lower.

[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 the hot rolling can be performed according to a conventional method. However, when the rolling temperature is less than 850° C., the deformation resistance of the steel material is high, and therefore the load on the rolling mill in the hot rolling is high. It may increase and it may become difficult to perform hot rolling. Therefore, the rolling temperature is preferably 850°C or higher, more preferably 900°C or higher. On the other hand, if the rolling temperature is higher than 950° C., the heating temperature must be increased, so that the oxidation of steel becomes remarkable, and the loss due to oxidation increases, resulting in a decrease in yield. Therefore, the upper limit of the rolling temperature is preferably 1000°C or lower, and more preferably 950°C or lower.

[Quenching]
Then, the obtained hot-rolled steel sheet is quenched from the quenching start temperature to the quenching stop temperature. The quenching may be performed by either direct quenching (DQ) or reheating quenching (RQ). The cooling method in the quenching is not particularly limited, but water cooling is preferable. Here, the "quenching start temperature" means the surface temperature of the steel sheet at the start of quenching. The "quenching start temperature" may be simply referred to as "quenching temperature". The "quenching stop temperature" is the surface temperature of the steel sheet at the end of quenching. For example, when quenching is performed by water cooling, the temperature at the start of water cooling is the "quenching start temperature" and the temperature at the end of water cooling is the "quenching stop temperature".

(Direct quenching)
When the quenching is performed by direct quenching, after the hot rolling is finished, the hot rolled steel sheet is quenched without reheating. At that time, the quenching start temperature is set to the Ar ₃ transformation point or higher. This is to obtain a martensite structure by quenching from an austenitic state. If the quenching start temperature is lower than the Ar ₃ transformation point, the quenching does not sufficiently occur, so the hardness of the steel sheet cannot be sufficiently improved, and as a result, the wear resistance of the finally obtained steel sheet decreases. On the other hand, the upper limit of the quenching start temperature in direct quenching is not particularly limited, but it is preferably 950°C or lower. The quenching stop temperature will be described later.

The Ar ₃ transformation point can be obtained, for example, by the following equation (2).
_{Ar 3 (℃) = 910-273 ×} C-74 × Mn-57 × Ni-16 × Cr-9 × Mo-5 × Cu ... (2)
(However, each element symbol in the above formula (2) is the content of each element expressed in mass %, and the content of the elements not contained is 0)

(Reheating and quenching)
When the quenching is performed by reheating and quenching, after the hot rolling is finished, the hot rolled steel sheet is reheated and then quenched. At that time, the quenching start temperature is set to the Ac ₃ transformation point or higher. This is to obtain a martensite structure by quenching from an austenitic state. If the quenching start temperature is lower than the Ac ₃ transformation point, the quenching does not sufficiently occur, so that the hardness of the steel sheet cannot be sufficiently improved, and as a result, the wear resistance of the finally obtained steel sheet decreases. On the other hand, the upper limit of the quenching start temperature in reheating and quenching is not particularly limited, but it is preferably 950°C or lower. The quenching stop temperature will be described later.

The Ac ₃ transformation point can be obtained, for example, by the following equation (3).
_{Ac 3 (℃) = 912.0-230.5 ×} C + 31.6 × Si-20.4 × Mn-39.8 × Cu-18.1 × Ni-14.8 × Cr + 16.8 × Mo ... (3 )
(However, each element symbol in the above formula (3) is the content of each element expressed in mass %, and the content of elements not contained is 0)

(Average cooling rate)
The cooling rate in the above quenching is not particularly limited, and may be any value as long as it is a cooling rate at which a martensite phase is formed. For example, the average cooling rate from the start of quenching to the stop of quenching is preferably 25 to 70°C/s, and more preferably 30 to 60°C/s. The average cooling rate is a cooling rate obtained by using the temperature of the steel sheet surface.

(Cooling stop temperature)
The cooling stop temperature in the quenching step is not particularly limited as long as it is a temperature at which a martensite phase is formed, but when the cooling stop temperature is higher than the following, the martensite microstructure ratio decreases and the hardness of the steel sheet decreases, so that the Mf point or lower. It is preferable that On the other hand, although the lower limit of the cooling stop temperature is not particularly limited, it is preferable to set the cooling stop temperature to 50° C. or higher because unnecessarily continued cooling lowers the production efficiency.

The Mf point can be calculated by the following equation (4).
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 …(4)
(However, the element symbol in the above formula (4) is the content of each element expressed in mass %, and the content of the elements not contained is 0)

(Tempering)
After the quenching is stopped, the quenched hot rolled steel sheet is reheated to the tempering temperature. By performing the reheating, the steel plate after quenching is tempered. At that time, by performing tempering under the condition that the P value defined by the following formula (1) is 1.40×10 ^{4 to} 1.80×10 ⁴ , the hardness and the structure at a position 1 mm deep from the surface are obtained. The conditions described above may be satisfied.
P=(T+273)×(21.3−5.8×C+log(60×t)) (1)
(However, C in the above formula (1) is the C content (mass %) in the steel sheet, T is the tempering temperature (°C), and t is the holding time (minutes) in the tempering).

When the P value is less than 1.40×10 ⁴ , tempering becomes insufficient, so that one or both of the surface layer hardness and the surface layer structure do not satisfy the above conditions, and thus high temperature wear resistance cannot be obtained. On the other hand, when the P value is larger than 1.80×10 ⁴ , the surface layer hardness is greatly reduced, and high high temperature wear resistance cannot be obtained.

It should be noted that if the heating temperature T is too low, the production efficiency is lowered. Therefore, the heating temperature T is preferably set to 200° C. or higher. If the heating temperature T is too high, the heat treatment cost rises. It is preferable that the temperature is not higher than °C.

From the viewpoint of manufacturing efficiency and heat treatment cost, the holding time t is preferably 180 minutes or less, more preferably 100 minutes or less, and further preferably 60 minutes or less. On the other hand, considering the uniformity of the structure, the holding time is preferably 5 minutes or more.

The tempering can be performed by any method such as heating using a heat treatment furnace, high-frequency induction heating, and electric heating.

Next, the present invention will be described more specifically based on Examples. The following example shows a preferred example of the present invention, and the present invention is not limited to the example.

First, a steel slab having the composition shown in Table 1 was manufactured by the continuous casting method.

Next, the obtained steel slab was sequentially subjected to respective treatments of heating, hot rolling, reheating, quenching, and tempering to obtain a steel plate. Table 2 shows the processing conditions in each step.

The quenching was performed by either direct quenching or reheating quenching. When performing direct quenching, the steel sheet after hot rolling was directly subjected to quenching by water cooling. Further, when performing reheating and quenching, the steel sheet after hot rolling was air-cooled, heated to a predetermined reheating temperature, and then subjected to quenching by water cooling. The water cooling in the quenching was performed by jetting a high flow rate of water from the front and back surfaces of the steel sheet while passing the hot-rolled steel sheet. The cooling rate during quenching is the average cooling rate between 650 and 300° C. obtained by heat transfer calculation, and cooling was performed up to 300° C. or less. For comparison, some steel sheets were not tempered.

For each of the obtained steel sheets, the surface hardness, the surface structure, and the wear resistance at high temperature were evaluated by the methods described below. The evaluation results are as shown in Table 2.

[Surface hardness]
Test pieces were taken from each steel sheet so that the test surface was located at a depth of 1 mm from the surface. After the test surface of the test piece was mirror-polished, the Brinell hardness was measured according to JIS Z2243 (2008). A tungsten hard ball with a diameter of 10 mm was used for the measurement, and the load was 3000 Kgf.

[Surface layer]
A test piece for microstructure observation is taken from the obtained steel sheet, polished, corroded (nital corrosive liquid), and an image of the microstructure (surface microstructure) at a position 1 mm from the surface is taken using an optical microscope (magnification: 400 times). did. Images captured in 5 fields or more were analyzed to identify each phase and calculate the area fraction (average value). A phase having an area fraction of 95% or more in the surface layer structure is shown in Table 2 as a main phase.

The circle-equivalent diameter and number density of cementite were obtained by observing a position of 1 mm from the surface with an electron microscope (magnification: 10000 times) and imaging, and calculating the circle-equivalent diameter and number density of cementite by image analysis. Imaging was performed in 5 fields or more, and the average value was adopted.

[Abrasion resistance at high temperature]
A cylindrical test piece (diameter 8 mm x length 20 mm) was sampled so that the position of 1 mm in the plate thickness direction from the surface of the obtained steel plate was the test piece surface (wear test surface), and the test piece was worn under high temperature. The test was conducted. For the wear test, a wear test apparatus schematically shown in FIG. 1 was used. Place the test piece on the disc-shaped wear material (main component: alumina) connected to the rotor in the tester while keeping the temperature of the atmosphere furnace installed in the wear tester at 400°C. The test was carried out by applying 300 N rotations at a rotor rotation speed of 60 mpm while applying a load of 98 N with a weight connected to. After the test was completed, the test piece was taken out and the mass of the test piece was measured. The amount of wear was calculated from the mass difference between the test pieces before and after the test. The wear characteristics of each steel plate at high temperature were evaluated by the wear resistance ratio=(wear amount of mild steel plate)/(wear amount of each steel plate) with the wear amount of the comparative material (mild steel plate) as a reference (=1). .. In addition, here, the case where the wear resistance ratio is 1.8 or more is regarded as "excellent in wear resistance at high temperature".

As can be seen from the results shown in Tables 1 and 2, the steel sheets satisfying the conditions of the present invention have a Brinell hardness of 360 to 490 HBW 10/3000 at a depth of 1 mm from the surface and wear resistance at high temperature. Was also excellent. On the other hand, in the steel sheets of Comparative Examples in which the tempering conditions did not satisfy the conditions of the present invention, the surface layer hardness or surface layer structure did not satisfy the conditions of the present invention, and the wear resistance at high temperatures was poor. In addition, Comparative Example No. 1 having a low C content. In No. 19, the structure did not satisfy the conditions of the present invention, and the wear resistance at high temperature was poor. Furthermore, the C content is low, and the tempering treatment is not performed. In No. 20, the structure was the as-quenched martensite structure, and although the surface layer hardness was equivalent to that of the invention example, the wear resistance at high temperatures was poor.

Claims (4)

In mass %,
C: 0.34 to 0.50%,
Si: 0.05-1.00%,
Mn: 0.30 to 2.00%,
P: 0.020% or less,
S: 0.020% or less,
Al: 0.04% or less,
Cr: 0.05-5.00%,
N: 0.0050% or less, and O: 0.0050% or less,
The balance has a composition of Fe and inevitable impurities,
Brinell hardness at a depth of 1 mm from the surface is 360 to 490 HBW 10/3000,
The area fraction of tempered martensite in the structure at a depth of 1 mm from the surface is 95% or more, and the tempered martensite has 8.0×10 ⁴ pieces/mm ² or more of cementite having a circle equivalent diameter of 0.02 μm or more. Abrasion resistant steel plate, including by number density. The component composition is mass%,
Cu: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
V: 0.01 to 1.00%,
W: 0.01 to 1.00%, and Co: 0.01 to 1.00%
The wear-resistant steel sheet according to claim 1, further comprising one or more selected from the group consisting of: The component composition is mass%,
Nb: 0.005 to 0.050%,
Ti: 0.005 to 0.050%, and B: 0.0001 to 0.0100%
The wear-resistant steel plate according to claim 1 or 2, further containing 1 or 2 or more selected from the group consisting of: The component composition is mass%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0080%
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:

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