JPWO2018235342A1 - steel sheet - Google Patents

Abstract

The steel plate according to one aspect of the present invention has a predetermined chemical composition, and the index Q determined by the following equation (1) is 0.00 or more, and the carbon equivalent Ceq (%) determined by the following equation (2) Is less than 0.800%, and the ratio of the difference between the surface layer hardness and the thickness central portion hardness to the surface layer hardness at room temperature is 15.0% or less and the surface layer hardness at room temperature is 400 or more in Vickers hardness And the plate thickness is 40 mm or more. Q = 0.18-1.3 (log T) + 0.75 (2.7 x [C] + [Mn] + 0.45 x [Ni] + 0.8 x [Cr] + 2 x [Mo]) ... (1) Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 (2)

Description

The present invention relates to a steel plate (wear-resistant steel plate) excellent in wear resistance.
Priority is claimed on Japanese Patent Application No. 2017-121641, filed Jun. 21, 2017, the content of which is incorporated herein by reference.

  For applications such as construction machinery and industrial machinery, there is a demand for wear-resistant steel plates that can be used for a long time even under severe wear environments, and from the viewpoint of securing wear allowances by increasing plate thickness, wear resistance Improvement is required. Generally, in order to improve the wear resistance of a steel plate, it is necessary to increase the hardness of the steel plate. In particular, in the case of a thick wear-resistant steel plate having a thickness of 40 mm or more, the hardness in the vicinity of the surface of the steel plate (hereinafter referred to as “surface layer hardness”. The surface layer portion is 1 mm to 5 mm from the surface of the steel plate in the thickness direction. Not only the hardness but also the hardness at the central part in the thickness direction where hardness is difficult to obtain (hereinafter sometimes referred to as “thickness central part hardness. The central part is the thickness from the surface of the steel plate in the thickness direction The problem is to secure ± 5 mm (total 10 mm thickness) from a position separated by a half of T (that is, T / 2) (that is, the center of the plate thickness).

  The wear-resistant steel sheet is locally exposed to temperatures higher than room temperature, and may be used in severe environments, so that the reduction in hardness is small even in a temperature range higher than room temperature (eg, a temperature range of about 150 to 300 ° C.) It may be required that (high temperature hardness is excellent). In order to ensure hardness in a temperature range higher than room temperature (hereinafter sometimes referred to as "high-temperature hardness"), steel plates having an increased content of Si have been proposed (see, for example, Patent Documents 1 to 3) ).

Japanese Patent Laid-Open No. 8-41535 Japanese Patent Laid-Open No. 2001-49387 Japanese Patent Application Laid-Open No. 2002-235144

  For example, in Patent Document 1, a steel plate containing Nb is proposed, in which the content of Si is 0.40 to 1.50 mass% (hereinafter, “mass%” is simply described as “%”). However, in Patent Document 1, the plate thickness of the steel plate is 40 mm or less, and the hardness at the central portion of the plate thickness is not described, and is not studied from the viewpoint of securing a wear margin by thickening of the steel plate.

  In Patent Document 2, assuming a severe wear environment locally exposed to a temperature higher than room temperature, in order to ensure the high temperature hardness of the steel, it contains 0.5% to 1.2% of Si. Steels have been proposed which utilize precipitation strengthening with V carbides. However, steel containing a large amount of V is likely to cause slab cracking, and there is a concern that manufacturability may be reduced.

  In patent document 3, in order to ensure the high temperature hardness of a steel plate, the steel plate containing 1.00-1.50% of Si is proposed. In Patent Document 3, securing of the plate thickness central portion hardness of the steel plate is also taken into consideration, but the difference between the surface layer hardness and the plate thickness central portion hardness (hereinafter, “the hardness difference between the surface layer portion and the plate thickness central portion”, Or it may only be called a "hardness difference."), And it is not examined in the viewpoint of securing of the wear allowance by thickening of a steel plate.

  In consideration of the use environment and use form of the wear resistant steel plate, high hardness is maintained not only at room temperature but also at a high temperature environment of about 150 to 300 ° C, and sufficient in the central portion (thickness central portion) in the thickness direction Hardness may be required. The hardness of the central portion of the plate thickness can be easily secured by the increase of the content of the alloy component, but the weldability is reduced, and therefore, it is necessary to set the upper limit of the carbon equivalent. In order to secure the hardness of the steel sheet in a high temperature environment, addition of Si exceeding 1.00% is considered effective. However, the present inventors tend to be unfavorable for the wear resistance of the steel plate that the difference between the surface layer hardness and the thickness central portion hardness becomes significantly large in a steel plate containing Si of more than 1.00%. I found that.

  So far, there has been no report on the relationship between the steel sheet containing Si of more than 1.00% and the hardness difference, and studies to reduce the hardness difference at room temperature have not been sufficiently made. In view of such circumstances, the present invention can maintain high hardness not only at room temperature but also in high temperature environments, and in particular, in a steel plate having a plate thickness of 40 mm or more, the carbon equivalent is less than 0.800% An object of the present invention is to provide a steel plate excellent in wear resistance in which the difference between the surface layer hardness and the thickness central portion hardness at room temperature is 15.0% or less of the surface layer hardness.

  A steel containing more than 1.00% to 2.00% of Si is advantageous for wear resistance in that the hardness at room temperature and high temperature can be secured. On the other hand, according to the study of the present inventors, in steel plates containing Si of more than 1.00% and having a thickness of 40 mm or more, a difference between the surface layer hardness and the thickness central portion hardness tends to easily occur at room temperature. all right. This is because the central portion in the thickness direction of the steel plate has a lower cooling rate than the surface and the surface portion, and the formation of the martensitic structure is insufficient. However, the increase of the Si content The impact is not always clear.

  As a result of further investigations by the present inventors, in steel plates having a thickness of 40 mm or more and containing Si of more than 1.00%, the difference between the surface layer hardness at room temperature and the thickness central portion hardness is small. The indicator Q to derive the The index Q is obtained by the following equation (1) in consideration of the hardenability of the alloy element and the plate thickness. However, in the following formula (1), an alloying element other than Si (C) is required to reduce the difference between the surface layer hardness and the thickness central part hardness of a steel plate containing Si of more than 1.00%. , Mn, Ni, Cr, Mo), so the amount of Si is not considered. In addition, below, the hardness in room temperature may be called "room temperature hardness." Moreover, in the following, when it only says "hardness", the hardness in room temperature is shown and room temperature shows 22 +/- 5 degreeC (17-27 degreeC).

  The steel plate according to the present invention has a thickness of 40 mm or more, and there is a concern about delayed cracking due to hydrogen if it is affected by residual stress due to welding, etc. Therefore, the carbon equivalent Ceq (%) determined by the following equation (2) Is less than 0.800%. By setting the index Q determined by the following equation (1) to be 0.00 or more, the hardness difference between the surface layer portion and the thickness center portion at room temperature becomes 15.0% or less of the surface layer hardness, and the hardness difference is small. And, the carbon equivalent is low, the plate thickness is 40 mm or more, and it is possible to obtain a steel plate excellent in wear resistance. In addition, board thickness T and content [X] of each element X are substituted to following formula (1) as a dimensionless numerical value, and the unit of the calculated | required parameter | index Q calculated | required is dimensionless. Moreover, the unit of carbon equivalent Ceq calculated | required by following formula (2) is "%".

Q = 0.18-1.3 (log T) + 0.75 (2.7 x [C] + [Mn] + 0.45 x [Ni] + 0.8 x [Cr] + 2 x [Mo]) ... (1)
Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 (2)
Here, when the index Q of the above equation (1) is calculated by substituting the numerical value of the plate thickness T (mm) and the numerical value of the content [X] in mass% of each element X, the element X is not contained Substitutes 0. The carbon equivalent Ceq (%) of the above-mentioned formula (2) is calculated by substituting the numerical value of the content [X] in mass% of each element X, and substitutes 0 when the element X is not contained.

  The present invention has been made based on such findings, and the summary thereof is as follows.

[1] The steel plate according to an aspect of the present invention is, by mass%,
C: 0.20 to 0.35%,
Si: more than 1.00% to 2.00%,
Mn: 0.60 to 2.00%,
Cr: 0.10 to 2.00%,
Mo: 0.05 to 1.00%,
Al: 0.010 to 0.100%,
N: 0.0020 to 0.0100%,
B: 0.0003 to 0.0020%,
P: 0.0200% or less,
S: less than 0.0100%,
Cu: 0 to 0.500%,
Ni: 0 to 1.00%,
Nb: 0 to 0.050%,
V: 0 to 0.120%,
Ti: 0 to 0.025%,
Ca: 0 to 0.050%,
Mg: 0 to 0.050%,
REM: 0 to 0.100%, and the balance: Fe and impurities,
The index Q calculated by the following equation (1) is 0.00 or more,
It has a chemical composition in which the carbon equivalent Ceq (%) determined by the following formula (2) is less than 0.800%,
The ratio of the difference between the surface layer hardness and the thickness central portion hardness to the surface layer hardness at room temperature is 15.0% or less, and the surface layer hardness at room temperature is 400 or more in Vickers hardness,
The thickness T is 40 mm or more.
Q = 0.18-1.3 (log T) + 0.75 (2.7 x [C] + [Mn] + 0.45 x [Ni] + 0.8 x [Cr] + 2 x [Mo]) ... (1)
Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 (2)
The index Q of the formula (1) is calculated by substituting the numerical value of the thickness T (mm) and the numerical value of the content [X] of each element X in mass%, and 0 when not containing the element X substitute. The carbon equivalent Ceq (%) of the formula (2) is calculated by substituting the numerical value of the content [X] in mass% of each element X, and substitutes 0 when the element X is not contained.
[2] In the steel plate described in the above [1], the index Q is 0.04 or more,
The ratio may be 13.0% or less.
[3] In the steel plate described in the above [1] or [2], by mass%,
Ni: You may have a chemical composition which is 0.05-1.00%.
[4] The steel plate according to any one of the above [1] to [3], in mass%,
Mn may have a chemical composition which is 0.63 to 2.00%.

  According to the above aspect of the present invention, it is possible to maintain high hardness not only at room temperature but also in high temperature environments, and in particular, in steel plates having a thickness of 40 mm or more, the carbon equivalent Ceq (%) is 0.800% It is less than this, and it is possible to provide a steel plate excellent in wear resistance in which the difference between the surface layer hardness and the thickness central portion hardness at room temperature is 15.0% or less of the surface layer hardness. Industrial contribution of the steel plate according to the present invention is extremely remarkable such that it can be used for a long time even under a severe environment where the temperature is about 150 to 300 ° C.

It is a figure explaining the temperature change of the difference of the surface hardness of a steel plate, and reference | standard hardness. It is a figure explaining the hardness distribution of the plate thickness direction of a steel plate. It is a figure explaining the relationship between the hardness difference ratio deltaHv / Hvs of a steel plate, and the index Q.

  The relationship between the Si content of the steel plate and the temperature change of hardness will be described with reference to FIG. FIG. 1 is a view for explaining the temperature change of the difference between the surface hardness of a steel plate and a reference hardness. A steel sheet with a thickness of 40 mm with a constant C content and a varied Si content is subjected to hardening treatment, and the Vickers hardness (surface hardness) HV5 on the surface of the steel sheet from room temperature to 400 ° C. is measured. Shown in 1. The vertical axis in FIG. 1 is the difference between the Vickers hardness (surface hardness) HV5 at each temperature of each steel and the Vickers hardness (reference hardness) HV5 at room temperature of a steel plate having a Si content of 0.25%. is there. In addition, Vickers hardness HV5 cut out a sample from the position of depth 5 mm from the surface of a steel plate, made test power 49.03 N (5 kgf) according to JIS Z 2252-1991, and measured it by high temperature Vickers hardness test . The measurement of the standard hardness was conducted under the same conditions as the above-mentioned high temperature Vickers hardness test except for the control of the temperature.

  It can be seen from FIG. 1 that the room temperature hardness and the high temperature hardness increase as the Si content increases, and the hardness reduction (the difference between the surface hardness and the reference hardness) in the high temperature environment also decreases. Thus, it can be seen that the steel plate containing more than 1.00% to 2.00% of Si is excellent in wear resistance in that the hardness at room temperature and high temperature can be secured.

  Next, the hardness distribution (Vickers hardness) in the thickness direction after quenching of a steel sheet (thickness 40 mm) containing Si of more than 1.00% is shown in FIG. The Vickers hardness HV5 was measured at room temperature with a test force of 49.03 N (5 kgf) in accordance with JIS Z 2244: 2009. As shown in FIG. 2, the plate thickness central portion hardness is lower than the surface layer portion hardness. Furthermore, from the results of the Vickers hardness test, the surface layer hardness Hvs (average value of Vickers hardness measured in the range of 1 mm to 5 mm from the surface of the steel plate in the thickness direction) and thickness central portion hardness Hvc (in the thickness direction The average value of Vickers hardness measured in a range of ± 5 mm (total thickness of 10 mm) from the central portion of the steel plate was determined, and the difference (hardness difference) ΔHv between the central thickness and the surface layer hardness at room temperature was calculated. That is, ΔHv is represented by the following formula (a).

  ΔHv = Hvs−Hvc (a)

  The results of the Vickers hardness test are shown in Table 1. From Table 1, it can be seen that ΔHv increases as the Si content increases. As described above, the present inventors have found that in a thick steel plate having a large Si content, a difference between the surface layer hardness at room temperature and the thickness central portion hardness is likely to occur.

  Therefore, the present inventors examined a method of reducing the difference in hardness between the surface layer portion and the central portion of the plate thickness at room temperature of a steel plate having a plate thickness of 40 mm or more containing Si of more than 1.00%. . The present inventors repeated studies in order to reduce the difference in hardness of the steel sheet in consideration of the hardenability and thickness of the alloy elements.

In order to ensure the hardness of the steel plate, it is possible that, in hot rolling, the steel plate is reheated to a temperature of ₃ or more Ac at which transformation to austenite ends at the time of temperature rise, and then water cooling etc. is performed (quenching) It is At this time, the surface layer portion of the steel plate has a high cooling rate, and sufficient hardness can be secured. On the other hand, at the central portion of the steel plate in thickness, the cooling rate is lower than that in the surface portion, so that the formation of martensite becomes insufficient and the hardness decreases.

  As described above, the cooling rate decreases at the central portion of the steel plate thickness. Therefore, in order to secure sufficient hardness in the center of thickness of the steel sheet, it is necessary to increase the content of the alloying element to enhance the hardenability. However, when the content of alloying elements is fixed, depending on the plate thickness, the hardenability may be insufficient, the cost may increase by including unnecessary amount of alloying elements, and the weldability may be impaired, etc. Problems arise. Therefore, in order to control the content of the alloying element in an appropriate range, it is necessary to consider that the cooling rate at the central portion of the plate thickness is affected by the plate thickness.

  The present inventors have a relation between the content of the alloying element having hardenability and the thickness of the steel on the difference in hardness ratio ΔHv / Hvs of various steel materials having a thickness of 40 mm or more containing Si of more than 1.00%. The index Q shown in the following equation (1) was derived. Here, the hardness difference ratio ΔHv / Hvs (%) refers to a ratio obtained by dividing the difference between the surface layer hardness at room temperature and the thickness central portion hardness by the surface layer hardness as a percentage. The hardness difference ratio ΔHv / Hvs (%) is represented by the following formula (b). In the following formula (b), Hvs is the surface layer hardness (the average value of Vickers hardness measured in the range of 1 mm to 5 mm from the surface of the steel plate in the thickness direction), and Hvc is the thickness central portion hardness (steel plate It is an average value of Vickers hardness measured in the range of ± 5 mm (a total of 10 mm thickness) from the central portion in the thickness direction.

  ΔHv / Hvs (%) = 100 × (Hvs−Hvc) / Hvs (b)

  Heretofore, in steels containing more than 1.00% Si, it has been considered that the hardenability decreases as the cooling rate decreases. However, the inventors of the present invention have a cooling rate if the steel containing more than 1.00% of Si contains alloying elements other than Si (C, Mn, Ni, Cr, Mo) to secure hardenability. It has been found that Si contributes to the improvement of the hardenability even when the. The following formula (1) is that the inventors of the present invention need to secure hardenability by containing alloy elements (C, Mn, Ni, Cr, Mo) other than Si in order to increase the plate thickness central portion hardness. The indicator Q does not include the term of Si content.

Q = 0.18-1.3 (log T) + 0.75 (2.7 x [C] + [Mn] + 0.45 x [Ni] + 0.8 x [Cr] + 2 x [Mo]) ... (1)
Here, when the index Q of the above equation (1) is calculated by substituting the numerical value of the plate thickness T (mm) and the numerical value of the content [X] in mass% of each element X, the element X is not contained Substitutes 0. That is, in the above equation (1), the index Q is calculated using the plate thickness T and the content [X] of each element as a dimensionless numerical value. In addition, log of said Formula (1) is a logarithm whose base is 10, ie, common logarithm.

  FIG. 3 shows the relationship between the hardness difference ratio ΔHv / Hvs (%) and the index Q. From FIG. 3, when setting the hardness difference ratio ΔHv / Hvs (%) to 15.0% or less of the surface layer hardness Hvs as a standard for prolonging the life of a thick steel plate, it is necessary to set Q ≧ 0.00. I found it to be. In addition, it was found that when setting the hardness difference ratio ΔHv / Hvs (%) to 13.0% or less of the surface layer hardness Hvs, it is necessary to set Q ≧ 0.04.

  Furthermore, since the steel plate according to the present embodiment has a plate thickness of 40 mm or more, there is concern about hydrogen embrittlement cracking under the influence of residual stress due to welding, so the carbon equivalent Ceq represented by the following formula (2) (%) Is less than 0.800%. In addition, since it is necessary to consider the weldability of a steel plate, following formula (2) includes the term of Si content.

Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 (2)
The carbon equivalent Ceq (%) of the above-mentioned formula (2) is calculated by substituting the numerical value of the content [X] in mass% of each element X, and substitutes 0 when the element X is not contained. The unit of carbon equivalent Ceq calculated | required by the said Formula (2) is "%".

  By setting the index Q of the above equation (1) to be 0.00 or more, the hardness difference ΔHv between the surface layer portion and the thickness central portion of the steel plate at room temperature becomes 15.0% or less of the surface layer portion hardness Hvs. A small steel plate having a carbon equivalent of less than 0.800%, a plate thickness of 40 mm or more, and excellent wear resistance can be obtained.

  Hereinafter, the steel plate according to the present embodiment will be described in detail. First, the chemical composition of the steel plate according to the present embodiment will be described. In addition, unless otherwise indicated,% regarding chemical composition means mass%.

<C: 0.20 to 0.35%>
C is an element effective for improving the hardness, and in order to secure the hardness of the steel plate, the C content is made 0.20% or more. Preferably, the C content is 0.22% or more, more preferably 0.24% or more. On the other hand, if the C content exceeds 0.35%, the susceptibility to hydrogen embrittlement increases due to the increase in hardness, and there is a concern about the occurrence of cracking due to hydrogen embrittlement, so the C content is made 0.35% or less . Preferably, the C content is 0.32% or less, more preferably 0.30% or less.

<Si: more than 1.00% to 2.00%>
Si is a deoxidizer and is also an element effective for improving the hardness of the steel sheet. In the present embodiment, Si is a very important element to maintain the hardness of the steel sheet in a high temperature environment. In order to obtain the effect of the Si content, the Si content is made more than 1.00%. Preferably, the Si content is 1.10% or more, more preferably 1.20% or more or 1.30% or more. On the other hand, if the Si content exceeds 2.00%, the toughness of the steel sheet may be inhibited, so the Si content is made 2.00% or less. Preferably, the Si content is 1.90% or less, more preferably 1.80% or less.

<Mn: 0.60 to 2.00%>
Mn is an element that enhances the hardenability and improves the hardness, and in order to ensure the hardness of the steel sheet, it is necessary to contain 0.60% or more. Preferably, the Mn content is 0.70% or more, more preferably 0.80% or more. On the other hand, when Mn is contained excessively, toughness may be reduced, and formation of cementite may be promoted, resulting in a reduction in high temperature hardness of the steel sheet. Therefore, the Mn content is 2.00% or less. Preferably, the Mn content is 1.50% or less or 1.35% or less, more preferably 1.20% or less or 1.00% or less.

<Cr: 0.10 to 2.00%>
Cr is an element that improves the hardenability and improves the toughness and hardness of the steel sheet. In order to secure the toughness and hardness of the steel plate, the Cr content is made 0.10% or more. Preferably, the Cr content is 0.50% or more, more preferably 0.80% or more. On the other hand, when the Cr content exceeds 2.00%, the toughness of the steel sheet decreases, so the Cr content is made 2.00% or less. Preferably, the Cr content is 1.70% or less, more preferably 1.50% or less.

<Mo: 0.05 to 1.00%>
Mo is also an element that improves the hardenability and improves the hardness of the steel sheet. Mo is also an element effective to maintain the hardness of the steel sheet even in a high temperature environment. Therefore, the Mo content is made 0.05% or more. Preferably, the Mo content is 0.10% or more, more preferably 0.20% or more. On the other hand, when the Mo content exceeds 1.00%, the toughness of the steel sheet decreases, so the Mo content is 1.00% or less. Preferably, the Mo content is 0.60% or less, more preferably 0.40% or less.

<Al: 0.010-0.100%>
Al is an element effective as a deoxidizer. Further, Al forms N and AlN, and the crystal grains are refined to improve the toughness of the steel plate. Therefore, the Al content is made 0.010% or more. Preferably, the Al content is 0.020% or more, more preferably 0.030% or more. On the other hand, when Al is contained excessively, the toughness of the steel sheet is lowered, so the Al content is made 0.100% or less. Preferably, the Al content is 0.080% or less, more preferably 0.070% or less.

<N: 0.0020 to 0.0100%>
N is an element which forms nitride with Al or Ti, refines crystal grains, and improves the toughness of the steel plate. Therefore, the N content is made 0.0020% or more. Preferably, the N content is 0.0030% or more, more preferably 0.0040% or more. On the other hand, when N is contained excessively, coarse nitrides are formed and the toughness of the steel sheet is reduced, so the N content is made 0.0100% or less. Preferably, the N content is 0.0080% or less, more preferably 0.0060% or less.

<B: 0.0003 to 0.0020%>
B is an element effective to significantly improve the hardenability of the steel and to particularly improve the hardness of the central portion of the steel plate. Therefore, the B content is made 0.0003% or more. Preferably, the B content is 0.0005% or more, more preferably 0.0007% or more, and still more preferably 0.0010% or more. On the other hand, when B is contained excessively, a boride is formed, the hardenability is reduced, and the hardness of the steel sheet can not be secured, so the B content is made 0.0020% or less. Preferably, the B content is 0.00118% or less, more preferably 0.0016% or less.

<P: 0.0200% or less>
P is an impurity, and the P content is limited to 0.0200% or less in order to reduce the toughness and workability of the steel sheet. Preferably, the P content is 0.0150% or less, more preferably 0.0100% or less. The lower limit of the P content is preferably 0%, but the P content may be 0.0001% or more from the viewpoint of manufacturing cost.

<S: less than 0.0100%>
Similarly to P, S is an impurity and reduces the toughness of the steel sheet, so the S content is limited to less than 0.0100%. Preferably, the S content is 0.0070% or less, more preferably 0.0050% or less, and still more preferably 0.0030% or less. Although the lower limit of the S content is preferably 0%, the S content may be 0.0001% or more from the viewpoint of manufacturing cost.

  The steel plate according to the present embodiment can selectively contain one or more of Cu, Ni, Nb, V, and Ti for the purpose of improving mechanical properties such as hardness and toughness of the steel plate. . The lower limit of the content of these components is 0%.

<Cu: 0 to 0.500%>
Cu is an element that forms fine precipitates and contributes to the improvement of the strength of the steel plate, and may contain 0.001% or more. More preferably, the Cu content is 0.050% or more, more preferably 0.100% or more. On the other hand, when Cu is excessively contained, the wear resistance of the steel sheet is deteriorated, so the upper limit of the Cu content is made 0.500% or less. More preferably, the Cu content is 0.450% or less, still more preferably 0.400% or less.

<Ni: 0 to 1.00%>
Ni is an element that enhances the hardenability of the steel and contributes to the improvement of the hardness of the steel plate, and may contain 0.05% or more. More preferably, the Ni content is 0.10% or more, still more preferably 0.20% or more. On the other hand, since Ni is an expensive alloy element, the Ni content is 1.00% or less from the viewpoint of cost. More preferably, the Ni content is 0.70% or less, more preferably 0.50% or less.

<Nb: 0 to 0.050%>
Nb is an element that contributes to the grain refinement of the crystal grains by suppressing the formation of nitride and recrystallization, and in order to improve the toughness of the steel sheet, it may contain 0.005% or more. More preferably, the Nb content is made 0.007% or more, still more preferably 0.010% or more. On the other hand, if Nb is excessively contained, the toughness of the steel sheet may be reduced, so the Nb content is made 0.050% or less. More preferably, the Nb content is 0.030% or less, still more preferably 0.020% or less.

<V: 0 to 0.120%>
V is an element that contributes to the improvement of the hardness of the steel sheet, and may contain 0.010% or more. More preferably, the V content is 0.020% or more, still more preferably 0.040% or more. On the other hand, when V is contained excessively, cracking of the slab may occur to impair the productivity, so the V content is made 0.120% or less. More preferably, the V content is 0.100% or less, still more preferably 0.070% or less.

<Ti: 0 to 0.025%>
Ti is an element that forms TiN and refines crystal grains to improve the toughness of the steel plate, and may contain 0.005% or more. More preferably, the Ti content is made 0.007% or more, still more preferably 0.010% or more. On the other hand, if Ti is excessively contained, the toughness of the steel sheet may be reduced, so the Ti content is made 0.025% or less. More preferably, the Ti content is 0.020% or less, still more preferably 0.015% or less.

  In order to control the form and the like of inclusions in the steel, one or more of Ca, Mg and REM can be selectively contained. The lower limit of the content of these components is 0%.

<Ca: 0 to 0.050%>
<Mg: 0 to 0.050%>
<REM: 0 to 0.100%>
Ca, Mg, and REM are all elements that form sulfides by being combined with S to form inclusions that are difficult to stretch by hot rolling, and mainly contribute to the improvement of the toughness of the steel sheet. On the other hand, when Ca, Mg, and REM are excessively contained, these elements may form coarse oxides with O, and the toughness of the steel sheet may be reduced. Therefore, the Ca content and the Mg content are respectively 0.050% or less, and the REM content is 0.100% or less. More preferably, the Ca content, the Mg content, and the REM content are each 0.020% or less, more preferably 0.010% or less or 0.005% or less. On the other hand, in order to obtain the effect of improving the toughness of the steel sheet, the Ca content and the Mg content are each preferably 0.0005% or more, and the REM content is preferably 0.001% or more. More preferably, the Ca content and the Mg content are each 0.0007% or more, and the REM content is 0.002% or more.
In addition, REM (rare earth metal element) means a total of 17 elements which consist of Sc, Y, and a lanthanoid. The content of REM means the total content of these 17 elements.

  The balance of the chemical composition of the steel plate according to the present embodiment is Fe and impurities. Here, the impurities are components that are mixed due to various factors of the manufacturing process, including raw materials such as ore and scrap when industrially manufacturing steel plates, and the impurities of the steel plates according to the present embodiment It means what is acceptable as long as it does not adversely affect the characteristics. However, in the steel plate according to the present embodiment, among the impurities, P and S need to specify the upper limit as described above.

  Furthermore, as impurities in steel, O, Sb, Sn and As may be mixed alone or in combination of two or more. Even if these impurities are mixed, there is no particular problem as long as the level of mixing (the range of the content) of the wear resistant steel is normal. Therefore, these contents are limited to the usual mixing levels of the following wear resistant steels. The lower limit of the content of these impurities is 0%.

<O: 0.006% or less>
Although O may be mixed in the steel as an impurity in some cases, since it is an element that forms a coarse oxide, it is preferable that the O content be as small as possible. In particular, when the O content exceeds 0.006%, coarse oxides are formed in the steel and the wear resistance of the steel sheet is deteriorated, so the O content is made 0.006% or less. Preferably, the O content is 0.005% or less, more preferably 0.004% or less.

<Sb: 0.01% or less>
Sb is an element mixed from scrap as a steel material. In particular, when Sb is excessively contained, the wear resistance of the steel plate is degraded, so the Sb content is made 0.01% or less. Preferably, the Sb content is 0.007% or less and 0.005% or less.

<Sn: 0.01% or less>
Sn, like Sb, is an element mixed from scrap as a steel material. In particular, when the Sn content is excessive, the wear resistance of the steel sheet is deteriorated, so the Sn content is made 0.01% or less. Preferably, the Sn content is set to 0.007% or less and 0.005% or less.

<As: 0.01% or less>
As Sb and Sn, As is an element mixed from scraps as a steel material. In particular, when As is excessively contained, the wear resistance of the steel plate is deteriorated, so the As content is made 0.01% or less. Preferably, the As content is set to 0.007% or less and 0.005% or less.

  The steel plate according to the present embodiment has a small difference in hardness between the surface layer portion of the steel plate and the central portion of plate thickness at room temperature, and the ratio of the hardness difference to the surface layer portion hardness is 15.0% or less. The index Q determined by is set to 0.00 or more. The index Q is calculated by substituting the numerical value of the plate thickness T (mm) and the numerical value of the content [X] at mass% of each element X as a dimensionless numerical value, and when the element X is not contained, [X] Is 0. The index Q is preferably 0.01 or more, more preferably 0.04 or more, still more preferably 0.05 or more, still more preferably, in order to reduce the difference in hardness between the surface layer portion of the steel plate and the central portion of the plate thickness. Set to 0.10 or more. The upper limit of the index Q is not particularly defined, but when the index Q is increased, the carbon equivalent Ceq (%) is also increased, so the limit is naturally limited. In order to ensure weldability by setting the carbon equivalent Ceq (%) to less than 0.800%, the index Q is preferably 1.10 or less. More preferably, the index Q is 0.80 or less or 0.50 or less, and still more preferably 0.30 or less or 0.20 or less.

  Q = 0.18-1.3 (log T) + 0.75 (2.7 x [C] + [Mn] + 0.45 x [Ni] + 0.8 x [Cr] + 2 x [Mo]) ... (1)

  The steel plate according to the present embodiment has a carbon equivalent Ceq (%) of less than 0.800% in order to suppress weld cracking and secure weldability of the steel plate. The carbon equivalent Ceq (%) is also calculated by substituting the numerical value of the content [X] of each element in mass%, and when the element X is not contained, [X] is 0. Although the lower limit of the carbon equivalent Ceq (%) is not particularly defined, when the carbon equivalent Ceq (%) is decreased, the index Q is also reduced, so the limit is naturally limited. In order to make index Q into 0.00 or more and make a hardness difference small, carbon equivalent Ceq (%) has preferred 0.507% or more. In order to enhance the wear resistance of the steel plate, the carbon equivalent Ceq (%) is more preferably 0.600% or more, and even more preferably 0.650% or more. Still more preferably, the carbon equivalent Ceq (%) is 0.700% or more. In order to improve the weldability of the steel plate, the carbon equivalent Ceq (%) may be 0.785% or less, 0.770% or less, or 0.760% or less.

  Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 (2)

  The steel plate according to the present embodiment has a small difference (hardness difference) between the surface layer hardness and the thickness central portion hardness at room temperature, and the ratio of the difference between the surface layer hardness and the thickness central portion hardness to the surface layer hardness is 15. It becomes 0% or less, and can exhibit excellent abrasion resistance over a long period of time. The hardness difference ratio ΔHv / Hvs (%) is preferably as small as possible, but it is difficult to make it less than 0% or less than 1.0%. Therefore, the lower limit may be 0% or 1.0%. The hardness difference ratio ΔHv / Hvs (%) may be 3.0% or more, considering the increase in cost associated with the increase in the content of the alloy elements. The surface layer hardness and the thickness central part hardness are Vickers hardness HV5 at room temperature, and are measured in accordance with JIS Z 2244: 2009. The surface layer hardness is an average value of Vickers hardness HV5 measured in the range of 1 mm to 5 mm from the surface in the thickness direction of the steel sheet, with a cross section parallel to the rolling direction and thickness direction of the steel sheet as a measurement surface. In the measurement of the surface layer hardness of the steel plate, the Vickers hardness at a total of 25 points at five points at least every 1 mm is measured in the relevant range. The plate thickness central portion hardness is an average value of Vickers hardness HV5 measured in the range of ± 5 mm (total 10 mm thickness) from the central portion in the plate thickness direction of the steel plate on the measurement surface. In the measurement of the central part hardness of the steel plate, the Vickers hardness at a total of 55 points at five points at least every 1 mm is measured in the above-mentioned range.

In the steel plate according to the present embodiment, the surface layer hardness Hvs at room temperature is 400 or more in Vickers hardness (HV5). If the surface layer hardness Hvs is less than 400 in Vickers hardness (HV5), the strength of the surface layer portion of the steel plate is insufficient, and therefore, it can not be used for applications such as construction machinery and industrial machinery. In order to improve the wear resistance, the surface layer hardness Hvs at room temperature may be 440 or more, 460 or more, 480 or more, or 500 or more in Vickers hardness (Hv5).
In addition, the steel plate which concerns on this embodiment has shown very high hardness from surface layer part to plate thickness center part, and tensile strength is also very high. As needed, the tensile strength (TS) at room temperature may be 1000 MPa or more, 1200 MPa or more, 1350 MPa or more, or 1500 MPa or more. The upper limit of the tensile strength is not particularly required, but may be 2300 MPa or less. As for tensile strength, a round bar test specimen is collected from a full thickness test specimen (that is, a plate-like test specimen) or a position (T / 4) which is 1⁄4 of the thickness T from the steel plate surface. : Measure according to 2011.

  The steel plate according to the present embodiment is a steel plate manufactured by hot rolling, and is a steel plate having a plate thickness of 40 mm or more, preferably 42 mm or more or 50 mm or more, more preferably 60 mm or more or 80 mm or more. The upper limit of the plate thickness is not particularly limited, and may be 150 mm depending on the application. The plate thickness may be 100 mm or less in consideration of homogenization of the characteristics in the plate thickness direction of the steel plate.

  The manufacturing method of the steel plate concerning this embodiment is explained. In this embodiment, after being produced by ordinary smelting process such as converter or electric furnace, a steel piece having the above-mentioned chemical composition is produced by a known method such as continuous casting method or ingot-slab method. There is no particular limitation.

  In the present embodiment, a steel piece obtained by casting is hot-rolled, and after being water-cooled as it is or air-cooled, it is reheated and quenched to manufacture a steel plate. However, the steel plate is to be hardened as it is and not to be subjected to heat treatment such as tempering.

After melting and casting the steel, hot rolling may be performed as it is, but the steel piece is temporarily cooled to room temperature, reheated to a temperature of Ac ₃ points or more, and hot rolling is performed. It is also good. The Ac ₃ point is the temperature at which the structure of the steel becomes austenite (the austenite transformation is completed) by raising the temperature. The heating temperature for hot rolling is preferably 900 ° C. or more, more preferably 1000 ° C. or more, in order to reduce deformation resistance. On the other hand, if the heating temperature of the hot rolling is too high, the structure becomes coarse and the low temperature toughness of the steel sheet may decrease, so 1250 ° C. or less is preferable. More preferably, the heating temperature is set to 1200 ° C. or less, still more preferably 1150 ° C. or less.

The hot rolling is preferably finished at an Ar ₃ point or higher, which is a temperature at which ferrite transformation starts due to temperature decrease. The Ac ₃ point and the Ar ₃ point can be obtained from the thermal expansion behavior at the time of heating and cooling by collecting a test piece from a steel piece. Immediately after hot rolling, quenching is performed to a temperature of 250 ° C. or less, or after hot rolling, the air-cooled steel plate is reheated to a temperature of Ac ₃ points or more and quenched to a temperature of 250 ° C. or less.

  Hereinafter, the example of the steel plate concerning the present invention is mentioned, and the present invention is explained more concretely. However, the present invention is of course not limited to the following examples, and can be implemented with appropriate modifications as long as it can be adapted to the spirit of the present invention, and any of them can be technical of the present invention. It is included in the scope.

  A steel having the chemical composition shown in Table 2 was melted and hot-rolled after casting to obtain a steel plate having a thickness shown in Table 3 and air-cooled to room temperature. Thereafter, the temperature was raised to a quenching temperature shown in Table 3, and then quenching was performed to produce a steel plate having a plate thickness of 40 mm or more. Test pieces are collected from the obtained steel plate, and the cross section parallel to the rolling direction and thickness direction of the steel plate is used as the test surface, and the Vickers hardness of the surface layer portion and the central portion of thickness is in accordance with JIS Z 2244: 2009, room temperature The test force was measured as 49.03 N (5 kgf). The Vickers hardness (surface layer hardness) Hvs of the surface layer is the Vickers hardness at a total of 25 points at 5 points every 1 mm within the range of 1 mm to 5 mm (surface layer) from the surface in the thickness direction of the steel plate These were obtained from the average value (arithmetic average). Vickers hardness (plate thickness center part hardness) Hvc of the plate thickness center part is Vickers hardness at a total of 55 points at 5 points per 1 mm within a range of ± 5 mm (10 mm in total thickness) from the center part in the plate thickness direction Were measured and obtained from their mean value (arithmetic mean). The hardness difference ratio ΔHv / Hvs (%) indicating the difference in hardness between the surface layer portion and the central portion of the steel plate at room temperature is obtained using the values of the surface layer portion hardness Hvs and the plate thickness central portion hardness Hvc thus obtained. The The hardness difference ratio ΔHv / Hvs (%) is represented by the following formula (b).

  H Hv / Hvs (%) = 100 × (Hvs-Hvc) / Hvs · · · (b)

Moreover, the sample was cut out from the steel plate, and in accordance with JIS Z 2252-1991, a high temperature Vickers hardness test was conducted at 400 ° C. with a test force of 9.807 N (1 kgf). Thus, the high temperature hardness (HV1) of the surface layer portion of the steel plate was obtained. In addition, the measurement of the high temperature hardness of the surface layer part was performed by making conditions other than control of a temperature and a test power the same as said surface layer part Vickers-hardness test (room temperature). Furthermore, a full-size V-notch Charpy test specimen in a direction parallel to the rolling direction is cut out from a position (T / 4) one-fourth of the plate thickness T from the surface of the steel plate, in accordance with JIS Z 2242: 2005. The Charpy absorbed energy (vE ₀ ) at 0 ° C. was measured.

  The judgment criteria for each evaluation item are as follows. The surface layer portion hardness Hvs (HV5) and the plate thickness central portion hardness Hvc (HV5) were all determined to be 400 or more from the viewpoint of wear resistance and 600 or less from the viewpoint of cutting processability. The high temperature hardness (HV5) of the surface layer portion was judged to be good as 300 or more from the viewpoint of the wear resistance at high temperature. The Charpy absorbed energy at 0 ° C. was determined to be 15 J or more.

  The results are shown in Table 3. No. 1 to 18 each have a chemical composition containing index Q and carbon equivalent Ceq (%), each parameter of board thickness T is within the range of the present invention, and the hardness difference ratio ΔHv / Hvs of the surface part and the center part is also 15. It is 0% or less. Each of these steels is a steel plate excellent in surface layer hardness Hvs, thickness central portion hardness Hvc, high temperature hardness of the surface layer portion, and Charpy absorbed energy at 0 ° C.

On the other hand, the numbers in Table 3 101 to 115 are comparative examples, and the chemical composition including the Q value is outside the scope of the present invention. No. 101 to 103 are examples in which the Q value is lowered due to the thickness of the plate, and the hardness difference ratio ΔHv / Hvs (%) exceeds 15.0%.
No. 106 is an example in which the Si content is insufficient and the high temperature hardness of the surface layer portion is lowered. On the other hand, no. 107 is an example in which the Si content is large and the toughness is lowered.

No. 104, 108 and 114 are examples in which the C content, the Mn content, and the B content are insufficient, respectively, and the surface layer hardness Hvs, the thickness central portion hardness Hvc, and the high temperature hardness of the surface layer decrease.
No. 1 with insufficient Cr content. In addition to the surface layer hardness Hvs, the thickness central portion hardness Hvc, and the high temperature hardness of the surface layer, 110 is an example in which the toughness is also reduced.
No. 1 with insufficient Mo content. 112 is an example in which the plate thickness central portion hardness Hvc, the high temperature hardness of the surface layer portion, and the toughness decrease.

No. 105 is an example in which the C content is high and the surface layer hardness Hvs is excessively high.
No. 1 with a high Mn content. No. 109, which has a high Cr content No. 111 with high Mo content 113 is an example in which the toughness is reduced.
No. B in which the B content is excessive. An example 115 is an example in which the surface layer hardness Hvs, the plate thickness central portion hardness Hvc, and the high temperature hardness of the surface layer portion decrease.
In all the examples, the O content was 0.006% or less, and the Sb content, the Sn content, and the As content were all 0.01% or less.

  Thus, Comparative Example No. 1 in which any one or more of the chemical composition and the Q value is outside the scope of the present invention. The difference in hardness ΔHv / Hvs, the surface layer hardness Hvs, the plate thickness central portion Hvc, and the high temperature hardness of the surface layer, and at least one of the toughness did not reach the evaluation criteria judged to be good.

Claims (4)

In mass%,
C: 0.20 to 0.35%,
Si: more than 1.00% to 2.00%,
Mn: 0.60 to 2.00%,
Cr: 0.10 to 2.00%,
Mo: 0.05 to 1.00%,
Al: 0.010 to 0.100%,
N: 0.0020 to 0.0100%,
B: 0.0003 to 0.0020%,
P: 0.0200% or less,
S: less than 0.0100%,
Cu: 0 to 0.500%,
Ni: 0 to 1.00%,
Nb: 0 to 0.050%,
V: 0 to 0.120%,
Ti: 0 to 0.025%,
Ca: 0 to 0.050%,
Mg: 0 to 0.050%,
REM: 0 to 0.100%, and the balance: Fe and impurities,
The index Q calculated by the following equation (1) is 0.00 or more,
It has a chemical composition in which the carbon equivalent Ceq (%) determined by the following formula (2) is less than 0.800%,
The ratio of the difference between the surface layer hardness and the thickness central portion hardness to the surface layer hardness at room temperature is 15.0% or less, and the surface layer hardness at room temperature is 400 or more in Vickers hardness,
A steel plate having a thickness T of 40 mm or more.
Q = 0.18-1.3 (log T) + 0.75 (2.7 x [C] + [Mn] + 0.45 x [Ni] + 0.8 x [Cr] + 2 x [Mo]) ... (1)
Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 (2)
The index Q of the formula (1) is calculated by substituting the numerical value of the thickness T (mm) and the numerical value of the content [X] of each element X in mass%, and 0 when not containing the element X substitute. The carbon equivalent Ceq (%) of the formula (2) is calculated by substituting the numerical value of the content [X] in mass% of each element X, and substitutes 0 when the element X is not contained. Said indicator Q is 0.04 or more,
The steel plate according to claim 1, wherein the ratio is 13.0% or less. In mass%,
Ni: 0.05 to 1.00%
The steel plate according to claim 1 or 2, having a chemical composition of In mass%,
Mn: 0.63 to 2.00%
The steel plate according to any one of claims 1 to 3, which has a chemical composition of

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