JP6493285B2 - Abrasion resistant steel sheet and method for producing the abrasion resistant steel sheet - Google Patents

Description

  The present invention relates to a wear-resistant steel sheet, and more particularly to a wear-resistant steel sheet that can achieve both delayed fracture resistance and wear resistance at a high level and at a low cost. Moreover, this invention relates to the manufacturing method of an abrasion-resistant steel plate.

  Industrial machinery, parts, and transport equipment (eg, excavators, bulldozers, hoppers, bucket conveyors, rock crushing devices) used in the fields of construction, civil engineering, mining, etc., are subject to abrasive wear and slip due to rock, sand, ore, etc. Exposed to wear, such as wear and impact wear. Therefore, the steel used for such industrial machines, parts, and transportation equipment is required to have excellent wear resistance in order to improve the life.

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

  For example, Patent Documents 1 and 2 propose wear-resistant steel plates having a surface layer portion hardness of 460 to 590 in terms of Brinell hardness (HB). In the wear-resistant steel sheet, a high surface hardness is realized by adding a predetermined amount of alloying elements and quenching into a martensite-based structure.

  Further, the wear-resistant steel plate is used in some usage environments such that a thick steel plate having a thickness of several tens of mm is worn to the vicinity of the center of the plate thickness. Therefore, in order to lengthen the life of the steel plate, it is important to ensure high hardness not only in the surface layer of the steel plate but also in the center portion of the plate thickness.

  Further, in the field of wear-resistant steel sheets, it is required to prevent delayed fracture in addition to improving wear resistance. Delayed fracture is a phenomenon in which the steel sheet suddenly breaks even though the stress applied to the steel sheet is in a state below the yield strength. This delayed fracture phenomenon is more likely to occur as the steel sheet strength is higher, and is promoted by hydrogen intrusion into the steel sheet. An example of the delayed fracture phenomenon of the wear-resistant steel sheet is cracking after gas cutting. The steel sheet becomes brittle due to hydrogen intrusion from the combustion gas during gas cutting, and cracks occur after several hours to several days after cutting due to residual stress after gas cutting. Abrasion resistant steel plates are often cut by gas because of their high hardness, and delayed fracture after gas cutting (hereinafter sometimes referred to as “gas cut cracking”) is often a problem in wear resistant steel plates.

  Therefore, Patent Documents 3 and 4 propose wear-resistant steel sheets that suppress delayed fracture caused by gas cutting or the like by controlling the component composition and the microstructure.

Japanese Patent No. 4259145 Patent No. 4645307 Patent No. 5145804 Patent No. 5145805

  However, in the wear-resistant steel sheets described in Patent Documents 1 and 2, it is necessary to add a large amount of alloy elements in order to ensure hardness. Generally, in order to reduce the alloy cost, it is effective to reduce the usage amount of Mo or Cr, which are expensive alloy elements, and increase the usage amount of Mn, which is an inexpensive alloy element. However, when the amount of Mn used in the wear-resistant steel sheets as described in Patent Documents 1 and 2 is increased, there is a problem in that the gas cut cracking resistance is lowered.

  Further, in the wear-resistant steel sheets described in Patent Documents 3 and 4, although a certain effect is seen in suppressing gas cutting cracks, it is necessary to suppress the Mn content in order to prevent delayed fracture.

  Furthermore, gas cutting cracks are more likely to occur as the hardness and plate thickness increase, and also occur from the center of the plate thickness. Therefore, it is possible to suppress the occurrence of cracks in thick steel plates with high hardness at the plate thickness center. The techniques described in 3 and 4 were insufficient.

  As described above, it is difficult for the wear-resistant steel plate to achieve both a high level of resistance to gas cutting cracking and a high wear resistance up to the center of the thickness of the thick steel plate at a low cost.

  The present invention has been made in view of the above circumstances, and is a wear-resistant steel sheet that can achieve both delayed fracture resistance and wear resistance up to the thickness center of a thick steel sheet at a high level and at a low cost. The purpose is to provide. Moreover, an object of this invention is to provide the method of manufacturing the said abrasion-resistant steel plate.

  As a result of intensive studies to solve the above problems, the inventors of the present invention have found that delayed fracture after gas cutting in a wear-resistant steel sheet originates from grain boundary fracture occurring at the prior austenite grain boundaries of the martensite structure or bainite structure. And the grain boundary fracture is (a) residual stress generated by gas cutting, (b) hydrogen embrittlement due to hydrogen entering the steel sheet from the cutting gas during gas cutting, and (c) temperature rise during gas cutting. It has been found that the influence of temper embrittlement of steel sheets due to the overlap is generated. In addition, when the plate | board thickness of a steel plate becomes thick, since the residual stress of (a) becomes large and the hardness of a steel plate becomes large, since the hydrogen embrittlement sensitivity of (b) becomes high, it becomes easy to produce a gas cutting crack.

  Furthermore, the present inventors have found that the sheet thickness center segregation part of the steel sheet in which Mn and P, which are grain boundary embrittlement elements, are concentrated, is the starting point of gas cutting cracking, and the temperature rise at the time of gas cutting As a result of further promoting the segregation of the grain boundary embrittlement elements to the prior austenite grain boundaries in the thick center segregation part, it has been clarified that the strength of the prior austenite grain boundaries is significantly reduced and gas cutting cracks occur.

  The segregation of Mn and P to the center of the plate thickness occurs during continuous casting. In continuous casting, solidification of molten steel proceeds from the surface to the inside, but since the solid solubility limit of Mn and P is larger in the liquid phase than in the solid phase, the solidified / liquid phase interface is changed from solidified steel to molten steel. Alloy elements such as Mn and P are concentrated inside. Then, at the center position of the plate thickness, which is the final solidified portion, the central segregation portion is formed by solidification of the molten steel in which the alloy element has been remarkably concentrated.

  Therefore, based on the above knowledge, as a result of further investigation on a method for preventing cracks starting from the center segregation part, the present inventors have suppressed the center segregation of Mn and P during continuous casting, and finally It has been found that by refining the prior austenite grain size in the structure of a typical steel sheet, excellent gas cutting crack resistance can be obtained even if the Mn content in the whole steel sheet is high.

This invention is made | formed based on the said knowledge, The summary structure is as follows.
1. A wear-resistant steel plate,
% By mass
C: more than 0.23%, 0.34% or less,
Si: 0.01 to 1.0%,
Mn: 0.30 to 2.50%,
P: 0.020% or less,
S: 0.01% or less,
Cr: 0.01 to 2.00%
Al: 0.001 to 0.100%, and N: 0.01% or less,
It consists of the balance Fe and inevitable impurities,
DI * defined by the following formula (1) and the plate thickness t [mm] have a component composition satisfying DI * / t ≧ 1.2,
The volume ratio of martensite at a depth of 1 mm from the surface of the wear-resistant steel sheet is 90% or more, and the prior austenite grain size at the center of the thickness of the wear-resistant steel sheet is 80 μm or less,
The hardness at a depth of 1 mm from the surface of the wear-resistant steel plate is 460-590 HBW 10/3000 in terms of Brinell hardness,
The Brinell hardness at the center of the plate thickness is 75% or more of the Brinell hardness at a depth of 1 mm from the surface layer,
A wear-resistant steel plate in which the Mn concentration [Mn] (mass%) and the P concentration [P] (mass%) satisfy the following expression (2) in the center thickness segregation part.
DI * = 33.85 × (0.1 × C) ^0.5 × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × ( 2.16 × Cr + 1) × (3 × Mo + 1) × (1.75 × V + 1) × (1.5 × W + 1) (1)
(However, each element symbol in the above formula (1) represents the content (% by mass) of the element, and is 0 when the element is not added)
0.04 [Mn] + [P] <0.50 (2)

2. The component composition is further in mass%,
Cu: 0.01 to 2.0%,
Ni: 0.01 to 5.0%,
Mo: 0.01 to 3.0%,
Nb: 0.001 to 0.100%,
Ti: 0.001 to 0.050%,
B: 0.0001 to 0.0100%,
V: 0.001 to 1.00%,
W: 0.01 to 1.5%,
Ca: 0.0001 to 0.0200%,
Mg: 0.0001-0.0200%, and REM: 0.0005-0.0500%
The wear-resistant steel sheet according to 1 above, comprising one or more selected from the group consisting of:

3. The wear-resistant steel sheet according to 1 or 2 above, wherein the drawing in the tensile test after undergoing temper embrittlement and subsequent hydrogen embrittlement is 10% or more.

4). Continuous casting of molten steel into slabs,
The slab is heated to 1000 ° C to 1300 ° C,
The heated slab is subjected to hot rolling in which the rolling shape ratio is 0.7 or more and the rolling reduction is 7% or more at a temperature of the center of the plate thickness of 950 ° C. or more to obtain a hot rolled steel sheet. ,
Reheating the hot-rolled steel sheet to a reheat quenching temperature;
Quenching the reheated hot-rolled steel sheet, a method for producing a wear-resistant steel sheet,
The slab has the component composition described in 1 or 2,
In the continuous casting, on the upstream side of the final solidification position of the slab, light reduction with a rolling gradient of 0.4 mm / m or more is performed twice or more,
The reheating quenching temperature is Ac _{3 to} 1050 ° C .;
The average cooling rate between 750 to 300 ° C. in the quenching is 5000 × t ^−1.8 ° ^C./s or more, where the plate thickness of the steel plate is t [mm]. The manufacturing method of the abrasion-resistant steel plate as described in 2.

5. In the method for producing a wear-resistant steel plate according to 4 above,
Furthermore, the manufacturing method of the abrasion-resistant steel plate which tempers the said hardened hot-rolled steel plate at the tempering temperature of 100-300 degreeC.

6). Continuous casting of molten steel into slabs,
The slab is heated to 1000 ° C to 1300 ° C,
The heated slab is subjected to hot rolling in which the rolling shape ratio is 0.7 or more and the rolling reduction is 7% or more at a temperature of the center of the plate thickness of 950 ° C. or more to obtain a hot rolled steel sheet. ,
A method for producing a wear-resistant steel sheet, in which the hot-rolled steel sheet is directly quenched,
The slab has the component composition described in 1 or 2,
In the continuous casting, on the upstream side of the final solidification position of the slab, light reduction with a rolling gradient of 0.4 mm / m or more is performed twice or more,
The direct quenching temperature in the direct quenching is Ac ₃ or higher,
Any one of the above 1-3, wherein the average cooling rate between 750 and 300 ° C. in the direct quenching is 5000 × t ^−1.8 ° ^C./s or more when the thickness of the steel sheet is t [mm]. The manufacturing method of the abrasion-resistant steel plate as described in a term.

7). In the method for producing a wear-resistant steel sheet according to 6 above,
Furthermore, the manufacturing method of the abrasion-resistant steel plate which tempers the said hardened hot-rolled steel plate at the tempering temperature of 100-300 degreeC.

  According to the present invention, since excellent delayed fracture resistance can be obtained without excessively suppressing the Mn content in the entire steel sheet, it is possible to achieve both delayed fracture resistance and wear resistance in a wear-resistant steel sheet at low cost. it can. The effect of the present invention is not limited to the delayed fracture resistance after gas cutting, but is also effective for delayed fracture due to other factors.

It is a schematic diagram which shows the last solidification position in continuous casting. It is a schematic diagram which shows the continuous casting method in one Embodiment of this invention.

[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 slab used for manufacturing the steel plate have the above component composition. First, the reason why the composition of steel is limited as described above in the present invention will be described. In addition, "%" regarding a component composition shall mean "mass%" unless there is particular notice.

C: more than 0.23% and not more than 0.34% C is an essential element for increasing the hardness of the martensite base. When the C content is 0.23% or less, the amount of solid solution C in the martensite structure is reduced, so that the wear resistance is lowered. On the other hand, when the C content exceeds 0.34%, the weldability and workability deteriorate. Therefore, in the present invention, the C content is more than 0.23% and 0.34% or less. The C content is preferably 0.25 to 0.32%.

Si: 0.01 to 1.0%
Si is an element effective for deoxidation, but if the Si content is less than 0.01%, a sufficient effect cannot be obtained. Si is an element that contributes to increasing the hardness of steel by solid solution strengthening. However, if the Si content exceeds 1.0%, problems such as an increase in the amount of inclusions occur in addition to a decrease in ductility and toughness. Therefore, the Si content is set to 0.01 to 1.0%. In addition, it is preferable that Si content shall be 0.01 to 0.8%.

Mn: 0.30 to 2.50%
Mn is an element having a function of improving the hardenability of steel. By adding Mn, the hardness of the steel after quenching increases, and as a result, the wear resistance can be improved. If the Mn content is less than 0.30%, the above effect cannot be obtained sufficiently, so the Mn content is set to 0.30% or more. On the other hand, if the Mn content exceeds 2.50%, the weldability and toughness are lowered, and the delayed fracture resistance is lowered. Therefore, the Mn content is 2.50% or less. The Mn content is preferably 0.50 to 2.30%.

P: 0.020% or less P is a grain boundary embrittlement element. When P segregates at the grain boundary, the toughness of the steel is lowered and the delayed fracture resistance is lowered. Therefore, the P content is 0.020% or less. In addition, it is preferable that P content shall be 0.015% or less. On the other hand, the lower the amount of P, the better. Therefore, the lower limit of the P content is not particularly limited and may be 0%. However, since P is an element that is inevitably contained in steel as an impurity, it is industrial. May be greater than 0%. In addition, excessively low P causes an increase in refining time and an increase in cost. Therefore, the P content is preferably 0.001% or more.

S: 0.01% or less Since S decreases the toughness of steel, the S content is set to 0.01% or less. The S content is preferably 0.005% or less. On the other hand, since it is preferable that S is small, the lower limit of the S content is not particularly limited and may be 0%, but industrially it may be more than 0%. In addition, since excessively low S causes increase in refining time and cost, it is preferable that the S content is 0.0001% or more.

Cr: 0.01-2.00%
Cr is an element having a function of improving the hardenability of steel. By adding Cr, the hardness of the steel after quenching increases, and as a result, the wear resistance can be improved. In order to acquire the said effect, it is necessary to make Cr content 0.01% or more. On the other hand, if the Cr content exceeds 2.00%, the weldability decreases. Therefore, the Cr content is set to 0.01 to 2.00%. It is preferably 0.05 to 1.8%.

Al: 0.001 to 0.100%
Al is an element that is effective as a deoxidizing agent and has an effect of reducing the austenite grain size by forming nitrides. In order to acquire the said effect, it is necessary to make Al content 0.001% or more. On the other hand, when the Al content exceeds 0.100%, the cleanliness of the steel is lowered, and as a result, ductility and toughness are lowered. Therefore, the Al content is 0.001 to 0.100% or less.

N: 0.01% or less Since N is an element that decreases ductility and toughness, the N content is 0.01% or less. On the other hand, since the smaller N is, the lower limit of the N content is not particularly limited and may be 0%. However, since N is an element inevitably contained in steel as an impurity, May be greater than 0%. In addition, since excessively low N causes an increase in refining time and an increase in cost, the N content is preferably 0.0005% or more.

  In addition to the above components, the steel sheet used in the present invention comprises the remaining Fe and unavoidable impurities.

  The steel sheet of the present invention has the above-described components as a basic composition, and is optionally Cu: 0.01 to 2.0%, Ni: 0.01 to 5.0 for the purpose of improving hardenability and weldability. %, Mo: 0.01 to 3.0%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.050%, B: 0.0001 to 0.0100%, V: 0.00. 001-1.00%, W: 0.01-1.5%, Ca: 0.0001-0.0200%, Mg: 0.0001-0.0200%, and REM: 0.0005-0.0500 1 or 2 or more selected from the group consisting of% can be contained.

Cu: 0.01 to 2.0%
Cu is an element that can improve the hardenability without greatly degrading the toughness of the base material and the welded joint. In order to acquire the said effect, it is necessary to make Cu content 0.01% or more. On the other hand, if the Cu content exceeds 2.0%, there will be a problem of steel plate cracking due to the Cu concentrated layer generated immediately below the scale. Therefore, when adding Cu, Cu content is made 0.01 to 2.0%. In addition, it is preferable that Cu content shall be 0.05-1.5%.

Ni: 0.01-5.0%
Ni is an element having an effect of improving hardenability and improving toughness. In order to acquire the said effect, it is necessary to make Ni content 0.01% or more. On the other hand, if the Ni content exceeds 5.0%, an increase in manufacturing cost becomes a problem. Therefore, when adding Ni, the Ni content is set to 0.01 to 5.0%. The Ni content is preferably 0.05 to 4.5%.

Mo: 0.01 to 3.0%
Mo is an element that improves the hardenability of steel. In order to acquire the said effect, it is necessary to make Mo content 0.01% or more. However, if the Mo content exceeds 3.0%, the weldability decreases. Therefore, when adding Mo, Mo content is made 0.01 to 3.0%. The Mo content is preferably 0.05 to 2.0%.

Nb: 0.001 to 0.100%
Nb is an element having an effect of reducing the prior austenite grain size by being precipitated as carbonitride. In order to acquire the said effect, it is necessary to make Nb content 0.001% or more. On the other hand, if the Nb content exceeds 0.100%, the weldability decreases. Therefore, when adding Nb, Nb content shall be 0.001 to 0.100%.

Ti: 0.001 to 0.050%
Ti is an element that has the effect of reducing the prior austenite grain size by forming nitrides. In order to acquire the said effect, it is necessary to make Ti content 0.001% or more. On the other hand, if the Ti content exceeds 0.050%, the cleanliness of the steel decreases, and as a result, the ductility and toughness decrease. Therefore, when adding Ti, Ti content is made 0.001 to 0.050%.

B: 0.0001 to 0.0100%
B is an element that has the effect of improving the hardenability by adding a trace amount and thereby improving the strength of the steel sheet. In order to acquire the said effect, B content needs to be 0.0001% or more. On the other hand, if the B content exceeds 0.0100%, the weldability decreases and the hardenability also decreases. Therefore, when adding B, B content shall be 0.0001-0.0100%. The B content is preferably 0.0001 to 0.0050%.

V: 0.001 to 1.00%
V is an element having an effect of improving the hardenability of steel. In order to acquire the said effect, it is necessary to make V content 0.001% or more. On the other hand, if the V content exceeds 1.00%, the weldability decreases. Therefore, when V is added, the V content is set to 0.001 to 1.00%.

W: 0.01 to 1.5%
W is an element having an effect of improving the hardenability of steel. In order to acquire the said effect, it is necessary to make W content 0.01% or more. On the other hand, if the W content exceeds 1.5%, the weldability decreases. Therefore, when adding W, W content shall be 0.01 to 1.5%.

Ca: 0.0001 to 0.0200%
Ca is an element that improves weldability by forming an oxysulfide having high stability at high temperatures. In order to acquire the said effect, it is necessary to make Ca content 0.0001% or more. On the other hand, if the Ca content exceeds 0.0200%, the cleanliness is lowered and the toughness of the steel is impaired. Therefore, when adding Ca, the Ca content is set to 0.0001 to 0.0200%.

Mg: 0.0001 to 0.0200%
Mg is an element that improves weldability by forming an oxysulfide having high stability at high temperatures. In order to acquire the said effect, it is necessary to make Mg content 0.0001% or more. On the other hand, if the Mg content exceeds 0.0200%, the effect of adding Mg is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when adding Mg, Mg content shall be 0.0001-0.0200%.

REM: 0.0005 to 0.0500%
REM (rare earth metal) is an element that improves weldability by forming an oxysulfide having high stability at high temperatures. In order to acquire the said effect, it is necessary to make REM content 0.0005% or more. On the other hand, if the REM content exceeds 0.0500%, the effect of adding REM is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when adding REM, REM content shall be 0.0005 to 0.0500%.

DI * / t ≧ 1.2
In the present invention, it is important that the component composition of the wear-resistant steel sheet further satisfies DI * / t ≧ 1.2, where DI * defined by the following formula (1) and the thickness t [mm] are satisfied. is there. DI * is an index indicating hardenability, and the larger the value, the higher the martensite ratio even if the cooling rate during quenching is slow. In order to increase the martensite at the thickness center portion during quenching and ensure a high thickness center hardness, it is necessary to satisfy DI * / t ≧ 1.2. On the other hand, the upper limit of DI * / t is not particularly limited. However, if DI * / t is too large, weldability may be deteriorated. Therefore, DI * / t is preferably 50 or less.
DI * = 33.85 × (0.1 × C) ^0.5 × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × (2 .16 × Cr + 1) × (3 × Mo + 1) × (1.75 × V + 1) × (1.5 × W + 1) (1)
(However, each element symbol in the above formula (1) represents the content (% by mass) of the element, and is 0 when the element is not added)

  The lower limit of DI * is not particularly limited, but from the viewpoint of ensuring hardenability, DI * is preferably set to 30 or more. Similarly, the upper limit value of DI * is not particularly specified, but if DI * is too high, weldability deteriorates, so DI * is preferably 300 or less.

[Organization]
In addition to having the above component composition, the wear resistant steel sheet of the present invention has a martensite volume ratio of 90% or more at a depth of 1 mm from the surface of the wear resistant steel sheet, and the center of thickness of the wear resistant steel sheet. The austenite grain size in the part has a structure of 80 μm or less. The reason for limiting the steel structure as described above will be described below.

Martensite volume fraction: 90% or more When the martensite volume fraction is less than 90%, the hardness of the base structure of the steel sheet is lowered, so that the wear resistance is lowered. Therefore, the volume ratio of martensite is 90% or more. The remaining structure other than martensite is not particularly limited, but a ferrite, pearlite, austenite, and bainite structure may exist. On the other hand, since the higher the volume ratio of martensite, the better, the upper limit of the volume ratio is not particularly limited, and may be 100%. In addition, let the volume ratio of the said martensite be the value in the position of the depth of 1 mm from the surface of an abrasion-resistant steel plate. The volume ratio of the martensite can be measured by the method described in the examples.

Prior austenite particle size: 80 μm or less When the prior austenite particle size exceeds 80 μm, the delayed fracture resistance of the wear-resistant steel sheet decreases. This is because the amount of Mn and P per unit area of the prior austenite grain boundary increases as a result of a decrease in the area of the prior austenite grain boundary, and grain boundary embrittlement becomes remarkable. Therefore, the prior austenite grain size is 80 μm or less. On the other hand, the smaller the prior austenite particle size, the better. Therefore, the lower limit is not particularly limited, but is usually 1 μm or more. The prior austenite grain size is the equivalent circle diameter of the prior austenite grains in the center of the thickness of the wear-resistant steel plate. The prior austenite particle size can be measured by the method described in the examples.

[Center segregation]
Furthermore, in the present invention, it is important that the Mn content [Mn] (mass%) and the P content [P] (mass%) satisfy the following expression (2) in the thickness center segregation portion. is there.
0.04 [Mn] + [P] <0.50 (2)

As described above, delayed fracture after gas cutting occurs from a point where Mn and P, which are grain boundary embrittlement elements, are significantly segregated in the center thickness segregating portion. As a result of further studies, it was found that the effect of P on grain boundary embrittlement is greater than that of Mn. Therefore, by controlling the concentrations of Mn and P in the plate thickness center segregation part so as to satisfy the above formula (2), the gas cutting crack resistance can be improved. On the other hand, the lower limit of the value of (0.04 [Mn] + [P]) is not particularly limited. However, since [Mn] is usually Mn content [Mn] ₀ or more in the whole steel plate and [P] is P content [P] ₀ or more in the whole steel plate, 0.04 [Mn] ₀ + [P] ₀ ≦ 0.04 [Mn] + [P]. The concentrations [Mn] and [P] of Mn and P in the plate thickness center segregation part can be measured by the method described in the examples.

[Brinell hardness]
Brinell hardness: 460-590 HBW 10/3000
The wear resistance of the steel sheet can be improved by increasing the hardness in the surface layer portion of the steel sheet. If the hardness of the steel sheet surface layer is less than 460 HBW in Brinell hardness, sufficient wear resistance cannot be obtained. On the other hand, if the hardness of the steel sheet surface layer portion is higher than 590 HBW in terms of Brinell hardness, bending workability deteriorates. Therefore, in this invention, the hardness in a steel plate surface layer part shall be 460-590 HBW by Brinell hardness. Here, the hardness is the Brinell hardness at a position 1 mm deep from the surface of the wear-resistant steel plate.

  In addition, the wear-resistant steel plate of the present invention has high hardness at the center of the plate thickness as well as the surface layer. Specifically, the Brinell hardness at the center of the plate thickness is 75% or more of the Brinell hardness at a depth of 1 mm from the surface layer. The Brinell hardness at the center of the plate thickness is preferably 78% or more of the Brinell hardness at a depth of 1 mm from the surface layer, more preferably 80% or more, and still more preferably 83% or more. On the other hand, the higher the Brinell hardness at the center of the plate thickness, the better. The upper limit is not particularly limited, but it is usually 100% or less of the Brinell hardness at a depth of 1 mm from the surface layer.

  The Brinell hardness is a value (HBW 10/3000) measured with a load of 3000 kgf using a tungsten hard sphere having a diameter of 10 mm. The Brinell hardness can be measured by the method described in the examples.

[Production method]
Next, the manufacturing method of the abrasion-resistant steel plate of this invention is demonstrated. The wear-resistant steel sheet of the present invention can be produced by either a method of performing reheating quenching (RQ) after hot rolling or a method of performing direct quenching (DQ) after hot rolling.

In one embodiment of the present invention in which reheating and quenching is performed, the wear-resistant steel plate can be manufactured by sequentially performing the following steps.
(1) A continuous casting process in which molten steel is continuously cast into a slab,
(2) A heating step of heating the slab to 1000 ° C. to 1300 ° C.,
(3) A hot rolling process in which the heated slab is hot rolled to form a hot rolled steel sheet,
(4-1) A reheating process for reheating the hot-rolled steel sheet to a reheating quenching temperature, and (4-2) a quenching process for quenching the reheated hot-rolled steel sheet.

In another embodiment of the present invention in which direct quenching is performed, the wear-resistant steel sheet can be manufactured by sequentially performing the following steps.
(1) A continuous casting process in which molten steel is continuously cast into a slab,
(2) A heating step of heating the slab to 1000 ° C. to 1300 ° C.,
(3) A hot rolling process in which the heated slab is hot rolled to form a hot rolled steel sheet,
(4) A direct quenching process in which the hot-rolled steel sheet is directly quenched.

In any embodiment, the component composition of the slab is as described above. Further, in the continuous casting process, light reduction with a rolling gradient of 0.4 mm / m or more is performed twice or more upstream from the final solidification position of the slab. Furthermore, the reheating quenching temperature when performing reheating quenching is Ac _{3 to} 1050 ° C., and the direct quenching temperature when performing direct quenching is Ac ₃ or higher. In both reheating quenching and direct quenching, the average cooling rate between 750 and 300 ° C. is set to 5000 × t ^−1.8 ° ^C./s or more when the plate thickness of the steel sheet is t [mm]. . Hereinafter, the reasons for limiting each condition will be described. In addition, the temperature in the following description points out the temperature in a plate | board thickness center part unless there is particular notice. The temperature at the center of the plate thickness can be obtained by heat transfer calculation. Unless otherwise specified, the following description is common for reheating and direct quenching.

Light reduction: Light reduction with a rolling gradient of 0.4 mm / m or more is performed twice or more upstream from the final solidification position of the slab.
The center segregation of a slab produced by a continuous casting machine as shown in Fig. 1 is that the alloying elements are concentrated in the molten steel at the solid / liquid interface as solidification progresses, and the molten steel that is significantly concentrated at the final solidification position is solidified. It is formed by doing. Therefore, as shown in FIG. 2, in the continuous casting machine, on the upstream side of the final solidification position of the slab, the roll gap is gradually reduced so that the roll gap becomes narrower from the upstream side to the downstream side of the continuous casting line. Thus, the central segregation can be reduced by pouring the molten steel enriched with the alloy element to the upstream side and pressing the already solidified portion. In order to obtain the effect, at the upstream side of the final solidification position of the slab, light reduction with a rolling gradient of 0.4 mm / m or more is performed twice or more, that is, (dt _a + dt _b ) / in FIG. It is necessary to perform the reduction so that L is 0.4 mm / m or more twice or more. When the number of times of light reduction with a reduction gradient of 0.4 mm / m or more is 1 or less, the effect of pushing the molten steel in the unsolidified portion upstream becomes insufficient, and the effect of reducing segregation due to light reduction becomes insufficient. Therefore, in the above (1) continuous casting step, light reduction with a rolling gradient of 0.4 mm / m or more is performed twice or more upstream from the final solidification position of the slab. On the other hand, the upper limit of the number of times of light rolling with a rolling gradient of 0.4 mm / m or more is not particularly limited, but is preferably 30 times or less from the viewpoint of cost effectiveness of installing a light rolling roll. Moreover, the upper limit of the rolling gradient under the rolling is not particularly limited, but is preferably 10.0 mm / m or less from the viewpoint of equipment protection of the light rolling roll. The final solidification position of the slab can be detected by transmitting electromagnetic ultrasonic waves through the slab.

Heating temperature: 1000-1300 ° C
When the heating temperature in the above (2) heating step is lower than 1000 ° C., the deformation resistance in the hot rolling step increases, and thus the productivity decreases. On the other hand, when the heating temperature is higher than 1300 ° C., a scale having high adhesion is generated, and therefore, descaling failure occurs, and as a result, the surface properties of the obtained steel sheet deteriorate. Therefore, the said heating temperature shall be 1000-1300 degreeC.

Hot rolling conditions: When the temperature at the center of the plate thickness is 950 ° C or higher, the rolling shape ratio is 0.7 or more and the rolling reduction is 7% or more 3 times or more Only slab segregation is reduced by light rolling during continuous casting However, since it is impossible to obtain a segregation state having excellent delayed fracture resistance, it is also necessary to utilize the segregation mitigation effect during hot rolling. The steel is subjected to strong rolling at a high temperature of 950 ° C. or higher and a rolling reduction of 7% or more in total 3 times or more, thereby obtaining an effect of reducing segregation by introducing strain and promoting atomic diffusion by recrystallization of the austenite structure. On the other hand, if the rolling temperature is 950 ° C. or lower or the rolling reduction is 7% or more and less than 3 times, the recrystallization of the structure becomes insufficient, and the segregation reduction effect cannot be obtained. On the other hand, the upper limit of the rolling reduction is not particularly limited, but is preferably 40% or less for protecting the rolling mill. Normally, as the carbon concentration in steel increases, the temperature range between the liquidus temperature and the solidus temperature becomes wider, so the residence time in the coexisting state of the solid phase and liquid phase where segregation progresses becomes longer. The central segregation of impurity elements increases. However, by combining the light reduction and hot rolling, it is possible to reduce the center segregation until the delayed fracture resistance becomes good even when the carbon concentration is high as in the wear-resistant steel.

Further, distortion introduced into the steel sheet in the rolling process is not uniform with respect to thickness direction, is determined thickness direction of the distribution by rolling shape ratio (ld / h _m) with the following formula.
ld / h _m = {R (h _i −h ₀ )} ^1/2 / {(h _i + 2h ₀ ) / 3}
Here, each symbol of each time each rolling pass ld: projected contact arc length, h _m: average thickness, R: roll radius, h _i: thickness at entrance side, h _0: thickness at delivery side of a.
To add distortion due rolled plate thickness center in the presence of center segregation, it is necessary to rolling shape ratio represented by the following formula (ld / h _m) 0.7 or more. On the other hand, if the rolling shape ratio is less than 0.7, the strain applied to the steel sheet surface layer during rolling increases, and the strain introduced into the sheet thickness center of the steel sheet decreases, resulting in insufficient recrystallization of the structure. Therefore, the required segregation reduction effect cannot be obtained. Therefore, the rolling shape ratio is set to 0.7 or more. In order to increase the rolling shape ratio, the roll radius may be increased or the reduction amount may be increased. On the other hand, the upper limit of the rolling shape ratio is not particularly limited, but is preferably 3.5 or less in order to protect the rolling mill.

Reheating quenching temperature: Ac _{3 to} 1050 ° C
When performing reheating and quenching, if the heating temperature (reheating and quenching temperature) in the above (4-1) reheating step is lower than the Ac ₃ point, the structure after hot rolling remains untransformed to a predetermined level. A martensite-based structure can no longer be obtained, and wear resistance is reduced due to a decrease in hardness. On the other hand, when the heating temperature is higher than 1050 ° C., the austenite grains become coarse during heating, so that the prior austenite grain size after quenching becomes larger than 80 μm. Therefore, the reheating quenching temperature is set to Ac _{3 to} 1050 ° C.

Direct quenching temperature: Ac ₃ or higher When direct quenching is performed, if the quenching temperature (direct quenching temperature) in the direct quenching step (4) is lower than the Ac ₃ point, the proportion of the structure other than martensite increases, and the prescribed martensite A site-based structure can no longer be obtained, and wear resistance is reduced due to a decrease in hardness. Therefore, the direct quenching temperature is set to Ac ₃ or higher. On the other hand, the upper limit of the direct quenching temperature is not particularly limited, but is 1300 ° C. or lower because the upper limit of the heating temperature during hot rolling is 1300 ° C. Here, the “direct quenching temperature” is the surface temperature of the steel sheet at the start of quenching. The direct quenching temperature can be measured using a radiation thermometer immediately before quenching.

Average cooling rate between 750 and 300 ° C .: 5000 × t ^−1.8 ° C./s or more In both cases of reheating quenching and direct quenching, the average cooling rate between 750 and 300 ° C. in the quenching process is 5000 If it is less than ^{xt −1.8} ° C / s, the martensite ratio at the center of the thickness of the steel sheet after quenching decreases and the hardness decreases. Therefore, the average cooling rate between 750 and 300 ° C. in the quenching step is set to 5000 × t ^−1.8 ° C./s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited. However, in general equipment, when the average cooling rate exceeds 300 ° C./s, the variation in the structure in the longitudinal direction and the plate width direction of the steel plate becomes remarkably large. The average cooling rate is preferably 300 ° C./s or less.

    The cooling stop temperature in the quenching step is not particularly limited, but when the cooling stop temperature is higher than 300 ° C., the martensite structure ratio may be decreased and the hardness of the steel sheet may be decreased. . On the other hand, the lower limit of the cooling stop temperature is not particularly limited, but if the cooling is continued unnecessarily, the production efficiency is lowered, so the cooling stop temperature is preferably 50 ° C. or higher.

Furthermore, in either case of reheating quenching or direct quenching, after the quenching process,
(5) A step of tempering the quenched hot-rolled steel sheet to a temperature of 100 to 300 ° C can be provided.

Tempering temperature: 100-300 ° C
By setting the tempering temperature in the tempering step to 100 ° C. or higher, the toughness and workability of the steel sheet can be improved. On the other hand, when the tempering temperature is higher than 300 ° C., the martensite structure is remarkably softened, and as a result, the wear resistance is lowered. Therefore, tempering temperature shall be 100-300 degreeC.

  After heating to the tempering temperature, the steel sheet can be air cooled. The soaking time in the tempering step is not particularly limited, but is preferably 1 minute or longer from the viewpoint of enhancing the tempering effect. On the other hand, since holding for a long time leads to a decrease in hardness, the soaking time is preferably within 3 hours.

  Next, the present invention will be described more specifically based on examples. The following examples show preferred examples of the present invention, and the present invention is not limited to the examples.

First, the slab of the component composition shown in Table 1 was manufactured by the continuous casting method. At the time of manufacturing some slabs, in order to reduce segregation at the center of the plate thickness, light reduction with a rolling gradient of 0.4 mm / m or more was performed on the upstream side of the final solidification position of the slab. Table 2 shows the conditions under the light pressure. Incidentally, Ac ₃ temperatures shown in Table 2 is the value determined by the following equation.
Ac ₃ (° C.) = 937-5722.765 ([C] /12.01- [Ti] /47.87) +56 [Si] -19.7 [Mn] -16.3 [Cu] -26.6 [Ni] -4.9 [Cr] +38.1 [Mo] +124.8 [V] -136.3 [Ti] -19 [Nb] +3315 [B]
Here, [M] is the content (mass%) of the element M, and when the element M is not added, [M] = 0 is set.

  Next, the obtained slab was sequentially subjected to heating, hot rolling, reheating, direct quenching, or reheating quenching to obtain a steel plate. Furthermore, some steel plates were reheated for tempering after quenching. The processing conditions in each step are as shown in Table 2. In addition, cooling in quenching was performed by injecting water at a high flow rate from the front and back surfaces of the steel plate while passing the plate. The cooling rate at the time of quenching is the average cooling rate between 750 and 300 ° C. obtained by heat transfer calculation, and the cooling rate at the time of quenching was 300 ° C. or less.

  About each of the obtained steel plates, the contents of Mn and P, the volume ratio of martensite, and the prior austenite grain size were measured in the thickness center segregation part by the method described below. Table 3 shows the measurement results.

[Contents of Mn and P in the center thickness segregation part]
In order to create a measurement sample, the central portion of the obtained steel plate in both the plate width direction and the plate thickness direction has a rectangular parallelepiped shape with a width in the plate width direction of 500 mm and a thickness in the plate thickness direction of 3 mm. Cut out as follows. The cut steel was further cut into 20 equal parts in the plate width direction to obtain 20 measurement samples having a width in the plate width direction of 25 mm. After mirror-polishing a surface (width 25 mm in the plate width direction × thickness 3 mm in the plate thickness direction) perpendicular to the rolling direction of the measurement sample, the mirror-polished surface is immediately used as a measurement surface, and an electron beam microanalyzer ( EPMA) was used for quantitative analysis.

The measurement conditions by EPMA were as follows. The maximum value of (0.04 [Mn] + [P]) in the following measurement range was taken as the value of (0.04 [Mn] + [P]) in the present invention.
(EPMA measurement conditions)
Accelerating voltage: 20 kV
Irradiation current: 0.5 μA
Integration time: 0.15 seconds,
Beam diameter: 15 μm,
Measurement range: height 3 mm × width 25 mm × 20 samples.

[Martensite volume fraction]
The wear resistance of the steel sheet is mainly determined by the hardness of the surface layer portion. Therefore, a sample was taken from the center in the width direction of each steel plate obtained as described above so that the position at a depth of 1 mm from the surface was the observation position. The surface of the sample was mirror-polished and further subjected to nital corrosion, and then a range of 10 mm × 10 mm was photographed using a scanning electron microscope (SEM). By analyzing the photographed image using an image analyzer, the area fraction of martensite was obtained, and the value was used as the volume fraction of martensite in the present invention.

[Old austenite grain size]
A sample for measuring the prior austenite grain size was collected from the center in the width direction and from the center of the plate thickness where the center segregation that becomes the starting point of gas cutting cracks was present. The surface of the obtained sample was mirror-polished and further corroded with picric acid, and then an area of 10 mm × 10 mm was photographed using an optical microscope. The austenite grain size was determined by analyzing the photographed image using an image analyzer. The prior austenite particle size was calculated as an equivalent circle diameter.

  Furthermore, each of the obtained steel plates was evaluated for hardness and delayed fracture resistance by the method described below. The evaluation results are as shown in Table 3.

[Hardness (Brinell hardness)]
As an index of wear resistance, the hardness at the surface layer portion and the thickness center portion of the steel plate was measured. The test pieces used for the measurement were collected from the respective steel plates obtained as described above so that the test surface had a depth of 1 mm from the surface of the steel plate or a center position of the plate thickness. After mirror-polishing the test surface of the test piece, Brinell hardness was measured according to JIS Z2243 (2008). For the measurement, a tungsten hard ball having a diameter of 10 mm was used, and the load was 3000 kgf.

[Delayed fracture resistance evaluation test]
When the structure mainly composed of martensite is heated to about 400 ° C., P atoms existing in the vicinity of the prior austenite grain boundary diffuse into the prior austenite grain boundary, thereby causing temper embrittlement in which the grain boundary becomes brittle. Since the central segregation portion of the steel sheet has a higher concentration of P than other portions, the temper embrittlement is most noticeable in the central segregation portion. When gas cutting a steel sheet, this temper embrittlement region inevitably occurs in the vicinity of the cut surface, and hydrogen contained in the gas used for gas cutting penetrates from the gas cut surface, resulting in hydrogen embrittlement. Also occurs. Delayed fracture after gas cutting occurs from the former austenite intergranular cracks, which are significantly embrittled by temper embrittlement and hydrogen embrittlement.

Therefore, in order to evaluate the delayed fracture resistance after temper embrittlement and hydrogen embrittlement, the test was conducted according to the following procedure. First, after heating the steel plate to 400 ° C. and air cooling to give temper embrittlement treatment, the diameter of the parallel portion is 5 mm so that the length of the test piece is parallel to the plate width direction from the center of the plate thickness at the center of the plate width. A JIS 14A round bar tensile test piece (JIS Z2241 (2014)) having a parallel part length of 30 mm was collected. Further, the round bar tensile test piece was immersed in a 10% ammonium thiocyanate aqueous solution at 25 ° C. for 72 hours to absorb hydrogen into the tensile test piece. Thereafter, in order to prevent hydrogen from escaping from the tensile test piece, the surface of the tensile test piece was galvanized with a thickness of 10 to 15 μm in a plating bath made of ZnCl ₂ and NH ₄ Cl. Using the obtained tensile test piece, a tensile test was performed at a strain rate of 1.1 × 10 ⁻⁵ / sec, and the drawing rate after fracture was measured according to JIS Z2241 (2014). The tensile test was performed 5 times each, and the average value of the drawing was used for evaluation. Moreover, using the sample which absorbed hydrogen on the same conditions as the said tensile test piece, the total hydrogen discharge | release amount at the time of heating up to 400 degreeC with a temperature rising type hydrogen analyzer is 0.8-1.1 ppm. there were.

  As can be seen from the results shown in Table 3, the wear-resistant steel plate that satisfies the conditions of the present invention has an excellent surface layer hardness of Brinell hardness of 460 HBW 10/3000 or more and a hardness at the center of the plate thickness of 75% of the surface layer hardness. It had excellent internal hardness as described above, excellent ductility of 10% or more in the tensile test after being subjected to temper embrittlement and hydrogen embrittlement treatment, that is, delayed fracture resistance. In addition, since the said aperture_diaphragm | restriction is so high that it is preferable, the upper limit is not specifically limited, Usually, it is 50% or less. On the other hand, the steel plate of the comparative example that does not satisfy the conditions of the present invention was inferior in at least one of hardness and delayed fracture resistance.

For example, no. The steel sheets Nos. 17 and 29 are not suitable for light rolling conditions, so the degree of center segregation of Mn and P, which are grain boundary embrittlement elements, is large, and the delayed fracture resistance is poor. No. with low C content In the 18 steel plate, the hardness is inferior because the amount of dissolved C in the martensite base is reduced. No. Since the steel plates 19 and 30 have a small DI * / t value, the martensite ratio at the central portion of the thickness is low even when quenching is performed by quenching, so the hardness at the central portion is inferior. No. The steel sheets Nos. 20 and 31 are insufficient in hot rolling during hot rolling, so that the degree of center segregation of Mn and P, which are grain boundary embrittlement elements, is large, and delayed fracture resistance is poor. No. Since the steel plate No. 21 has a high reheating quenching temperature, the prior austenite grain size is increased, and as a result, the delayed fracture resistance is inferior. No. Since the steel plate No. 22 has a reheating quenching temperature lower than Ac ₃ , the martensite volume fraction is low, and as a result, the hardness is inferior. No. Since the steel plate No. 23 has a low cooling rate at the time of reheating and quenching, the martensite ratio at the center portion of the plate thickness is low, so that the hardness at the center portion of the plate thickness is inferior. No. The steel plates Nos. 24 and 34 have a high tempering temperature, so softening occurs, and as a result, the hardness is inferior. No. Since the steel plate No. 32 has a direct quenching temperature lower than Ac ₃ , the martensite volume fraction is low, and as a result, the hardness is inferior. No. Since the steel plate No. 33 has a low cooling rate at the time of direct quenching, the martensite ratio at the central portion of the plate thickness is low, so the hardness of the central portion of the plate thickness is inferior.

DESCRIPTION OF SYMBOLS 1 Continuous casting machine 2 Tundish 3 Molten steel 4 Mold 5 Roll 6 Unsolidified layer 7 Slab (solidified area)
8 Final solidification position 9 Rolling roll

Claims (6)

A wear-resistant steel plate,
% By mass
C: more than 0.23%, 0.34% or less,
Si: 0.01 to 1.0%,
Mn: 0.30 to 2.50%,
P: 0.020% or less,
S: 0.01% or less,
Cr: 0.01 to 2.00%
Al: 0.001 to 0.100%, and N: 0.01% or less,
It consists of the balance Fe and inevitable impurities,
DI * defined by the following formula (1) and the plate thickness t [mm] have a component composition satisfying DI * / t ≧ 1.2,
The volume ratio of martensite at a depth of 1 mm from the surface of the wear-resistant steel sheet is 90% or more, and the prior austenite grain size at the center of the thickness of the wear-resistant steel sheet is 80 μm or less,
The hardness at a depth of 1 mm from the surface of the wear-resistant steel plate is 460-590 HBW 10/3000 in terms of Brinell hardness,
The Brinell hardness at the center of the plate thickness is 75% or more of the Brinell hardness at a depth of 1 mm from the surface layer,
A wear-resistant steel plate in which the Mn concentration [Mn] (mass%) and the P concentration [P] (mass%) satisfy the following expression (2) in the center thickness segregation part.
DI * = 33.85 × (0.1 × C) ^0.5 × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × (2.16 × Cr + 1) × (3 × Mo + 1) × (1.75 × V + 1) × (1.5 × W + 1) (1)
(However, each element symbol in the above formula (1) represents the content (% by mass) of the element, and is 0 when the element is not added)
0.04 [Mn] + [P] <0.50 (2) The component composition is further in mass%,
Cu: 0.01 to 2.0%,
Ni: 0.01 to 5.0%,
Mo: 0.01 to 3.0%,
Nb: 0.001 to 0.100%,
Ti: 0.001 to 0.050%,
B: 0.0001 to 0.0100%,
V: 0.001 to 1.00%,
W: 0.01 to 1.5%,
Ca: 0.0001 to 0.0200%,
Mg: 0.0001-0.0200%, and REM: 0.0005-0.0500%
The wear-resistant steel sheet according to claim 1, comprising one or more selected from the group consisting of: Continuous casting of molten steel into slabs,
The slab is heated to 1000 ° C to 1300 ° C,
The heated slab is subjected to hot rolling in which the rolling shape ratio is 0.7 or more and the rolling reduction is 7% or more at a temperature of the center of the plate thickness of 950 ° C. or more to obtain a hot rolled steel sheet. ,
Reheating the hot-rolled steel sheet to a reheat quenching temperature;
Quenching the reheated hot-rolled steel sheet, a method for producing a wear-resistant steel sheet,
The slab has the component composition according to claim 1 or 2,
In the continuous casting, on the upstream side of the final solidification position of the slab, light reduction with a rolling gradient of 0.4 mm / m or more is performed twice or more,
The reheating quenching temperature is Ac _{3 to} 1050 ° C .;
The average cooling rate between 750 to 300 ° C. in the quenching, when the plate thickness of the steel sheet was t [mm], is 5000 × ^{t -1.8} ℃ / s or higher, according to claim 1 or 2 Method for producing a wear-resistant steel sheet. In the manufacturing method of the abrasion-resistant steel plate according to claim 3 ,
Furthermore, the manufacturing method of the abrasion-resistant steel plate which tempers the said hardened hot-rolled steel plate at the tempering temperature of 100-300 degreeC. Continuous casting of molten steel into slabs,
The slab is heated to 1000 ° C to 1300 ° C,
The heated slab is subjected to hot rolling in which the rolling shape ratio is 0.7 or more and the rolling reduction is 7% or more at a temperature of the center of the plate thickness of 950 ° C. or more to obtain a hot rolled steel sheet. ,
A method for producing a wear-resistant steel sheet, in which the hot-rolled steel sheet is directly quenched,
The slab has the component composition according to claim 1 or 2,
In the continuous casting, on the upstream side of the final solidification position of the slab, light reduction with a rolling gradient of 0.4 mm / m or more is performed twice or more,
The direct quenching temperature in the direct quenching is Ac ₃ or higher,
The average cooling rate between 750 to 300 ° C. in the direct quenching is the plate thickness of the steel sheet when the t [mm], is 5000 × ^{t -1.8} ℃ / s or higher, to claim 1 or 2 The manufacturing method of the abrasion-resistant steel plate of description. In the manufacturing method of the abrasion-resistant steel plate according to claim 5 ,
Furthermore, the manufacturing method of the abrasion-resistant steel plate which tempers the said hardened hot-rolled steel plate at the tempering temperature of 100-300 degreeC.

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