JP6607209B2 - 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 that achieves both low-temperature toughness and wear resistance at a high level. 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 plates used for such industrial machines, parts, and transportation equipment are required to have excellent wear resistance in order to improve the life.

  It is known that the wear resistance of a steel sheet can be improved by increasing the hardness. Therefore, high-hardness steel obtained by performing heat treatment such as quenching on alloy steel to which a large amount of C and alloy elements such as Mn and Cr are added has been widely used as wear-resistant steel.

On the other hand, when the hardness is increased, the toughness decreases, but in such a situation, wear-resistant steel sheets that have both high hardness and high toughness have been proposed (see Patent Documents 1 to 3).
For example, Patent Document 1 discloses that the composition system and its heat rolling / heat treatment are optimized to control the structure at the center of the plate thickness and ensure the austenite grain refinement to achieve a uniform high hardness and high toughness up to the center of the plate thickness. A thick wear-resistant steel sheet having a surface Brinell hardness HB ≧ 470, which is compatible with the above, is disclosed. In Patent Document 2, instead of lowering the carbon equivalent Ceq, the characteristic value Mr is adjusted to a predetermined value to ensure strength and wear resistance, while at the same time achieving both low-temperature weld cracking resistance and low-temperature toughness. A worn steel sheet is disclosed. Furthermore, Patent Document 3 discloses a wear-resistant steel plate having excellent wear resistance and low-temperature toughness manufactured by adjusting the component index value Ha to a predetermined value and heating the steel slab to a temperature range of 1200 ° C to 1250 ° C. Is disclosed.

Japanese Patent No. 4259145 Japanese Patent No. 3698082 Japanese Patent No. 3273404

V.K.LAKSHMANAN: METALLURGICAL TRANSACTION A, Vol.15A (1984), p.541

  By the way, the said patent documents 1-3 are all the abrasion-resistant steel plates containing Nb. Generally, when Nb is added to a wear-resistant steel plate, toughness is improved. This is an element in which Nb mainly precipitates at a relatively low temperature of approximately 1000 ° C. or less as NbC. Since this NbC acts as a pinning particle that suppresses coarsening of the crystal grain size, the crystal grain size is refined. It is thought to do. However, in the techniques described in Patent Documents 1 to 3, NbC precipitated during slab cooling after continuous casting or during steel sheet cooling after hot rolling is coarsened and sparsely dispersed, so that NbC particles exist. The pinning effect is different between the place where it is present and the place where it does not exist. Therefore, the steel sheet has a mixed grain structure in which martensite derived from large prior austenite grains (hereinafter, austenite grains are also referred to as γ grains) and martensite derived from small prior γ grains are mixed. This is a characteristic of Nb-added steel due to the remaining of undissolved NbC due to the high solution temperature of NbC and the Ostwald growth in which fine particles of NbC disappear and further coarsening of coarse particles proceeds. It is a problem.

In the Charpy impact test, cracks occur starting from the crystal structure with the lowest toughness existing in the vicinity of the notch (hereinafter also simply referred to as the structure), and the steel sheet breaks down. Among the structures with different sizes, the toughness value of the coarsest structure is dominant. Therefore, in order to further improve the low temperature toughness, it is necessary to use a sized structure having a small crystal grain size on average, not a mixed grain structure.
Furthermore, in recent years, industrial machines and transportation equipment are often used in a severer environment than before. Along with this, steel materials are also required to have better characteristics than before, and good low temperature toughness is required even at lower temperatures.

  This invention is made | formed in view of the said actual condition, and it aims at providing the wear-resistant steel plate which made low-temperature toughness and abrasion resistance compatible at a high level. Moreover, an object of this invention is to provide about the method of manufacturing the said abrasion-resistant steel plate.

As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge. First, in order to dissolve coarse NbC precipitated in a steel material such as a slab by heating before hot rolling, the melting temperature calculated by the following formula (1) shown in Non-Patent Document 1 is exceeded. Heat the steel material. In the hot rolling performed following the heating of the steel material, strong reduction is performed in a temperature range where dislocations introduced into the steel material are not consumed as a driving force for recrystallization and precipitation of NbC occurs. Under this rolling condition, NbC can be uniformly and finely dispersed using the dislocations as nuclei. Furthermore, in order to suppress the Ostwald growth of the precipitated NbC, the steel sheet after hot rolling is rapidly cooled to maintain a state in which fine NbC is densely dispersed in the steel sheet, and then a reheating quenching process is performed. From these things, it discovered that the sized old γ grain structure excellent in toughness was obtained.
T * = 7920 / (3.4-log [Nb] [C] ^0.87 ) -273 (1)

This invention is made | formed based on the said knowledge, The summary structure is as follows.
1. % By mass
C: more than 0.23% and 0.34% or less,
Si: 0.1% to 3.0%,
Mn: 0.50% or more and 3.00% or less,
P: 0.025% or less,
S: 0.02% or less,
Cr: 0.2% to 2.0%,
Nb: 0.005% or more and 0.100% or less,
Ti: 0.005% or more and 0.100% or less,
Al: 0.001% or more and 0.100% or less,
B: 0.0005% or more and 0.0100% or less and N: 0.01% or less, the balance Fe and the inevitable impurities component composition,
The structure at a depth of 1/4 of the sheet thickness from the surface of the steel sheet is martensite with a volume ratio of 90% or more, and has an average austenite grain average diameter of 15 μm or less and a maximum diameter of 35 μm or less. ,
A wear-resistant steel sheet having a Brinell hardness of 460 to 590 HBW 10/3000 at a 1/4 depth position from the surface of the steel sheet.

2. The component composition is further mass%,
Cu: 0.01% or more and 2.00% or less,
Ni: 0.01% or more and 5.00% or less,
Mo: 0.01% to 3.00%,
V: 0.001% to 1.000%,
W: 0.01% or more and 1.50% or less,
Ca: 0.0001% or more and 0.0200% or less,
2. The wear-resistant steel sheet according to 1 above, comprising one or more selected from Mg: 0.0001% to 0.0200% and REM: 0.0005% to 0.0500%.

3. % By mass
C: more than 0.23% and 0.34% or less,
Si: 0.1% to 3.0%,
Mn: 0.50% or more and 3.00% or less,
P: 0.025% or less,
S: 0.02% or less,
Cr: 0.2% to 2.0%,
Nb: 0.005% or more and 0.100% or less,
Ti: 0.005% or more and 0.100% or less,
Al: 0.001% or more and 0.100% or less,
B: 0.0005% or more and 0.0100% or less and N: 0.01% or less, and a steel material having a component composition that is the remaining Fe and inevitable impurities is represented by a temperature T * represented by the following formula (1). Heated to 1300 ° C.
The heated steel material is hot-rolled so that the total rolling reduction is 30% or more when the temperature at the 1/4 depth position of the plate thickness from the surface of the steel material is 1000 ° C. or lower and 850 ° C. or higher. Hot rolled steel sheet,
The hot-rolled steel sheet is cooled at an average cooling rate of 1 ° C./s or more in a range where the temperature at a 1/4 depth position from the surface of the hot-rolled steel sheet is 800 ° C. or lower and 600 ° C. or higher,
Then, after reheating the hot-rolled steel sheet at a temperature of Ac ₃ or higher and 950 ° C. or lower, an average cooling is performed in the range where the temperature at the depth position of the plate thickness 1/4 from the surface of the steel sheet is 800 ° C. or lower and 300 ° C. or higher A method for producing a wear-resistant steel sheet that is cooled at a rate of 1 ° C / s or more.
T * = 7920 / (3.4-log [Nb] [C] ^0.87 ) -273 (1)

4). The steel material is further mass%,
Cu: 0.01% or more and 2.00% or less,
Ni: 0.01% or more and 5.00% or less,
Mo: 0.01% to 3.00%,
V: 0.001% to 1.000%,
W: 0.01% or more and 1.50% or less,
Ca: 0.0001% or more and 0.0200% or less,
4. The method for producing a wear-resistant steel plate according to 3 above, comprising one or two or more selected from Mg: 0.0001% to 0.0200% and REM: 0.0005% to 0.0500%.

5. Furthermore, the manufacturing method of the abrasion-resistant steel plate according to 3 or 4 above, wherein the steel plate is tempered at a temperature of 100 ° C. or higher and 350 ° C. or lower.

  According to the present invention, a finely sized structure having a small variation in crystal grain size has excellent low temperature toughness even at a low temperature of −60 ° C., and has both low temperature toughness and wear resistance at a high level. A worn steel sheet can be provided. Here, excellent low temperature toughness means that the shock absorption energy by Charpy impact test at −60 ° C. is 35 J or more, and high wear resistance means that the Brinell hardness of the steel sheet is 460 to 590 HBW 10/3000. It means that it is above.

[Ingredient composition]
Next, a method for carrying out the present invention will be specifically described. In the present invention, it is important that the wear-resistant steel plate and the steel material used for the production thereof have the above-described 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 0.34% or less 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 this invention, C content shall be more than 0.23% and 0.34% or less. In addition, the minimum of preferable C content is 0.25%. Moreover, the upper limit of preferable C content is 0.33%.

Si: 0.1% or more and 3.0% or less Si is an element effective for deoxidation, but if the Si content is less than 0.1%, a sufficient effect cannot be obtained. Si is an element that contributes to increasing the hardness of steel by solid solution strengthening. However, when the Si content exceeds 3.0%, 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 0.1 to 3.0%. The Si content is preferably 0.2 to 2.8%, but the lower limit of the Si content is preferably 0.3%, and the upper limit of the Si content is 2.2%. Is preferred.

Mn: 0.50% or more and 3.00% or less Mn is an element having a function of improving the hardenability of steel. By adding Mn, the hardness of the steel after quenching is further increased, and as a result, the wear resistance can be improved. If the Mn content is less than 0.50%, the above effect cannot be obtained sufficiently, so the Mn content is 0.50% or more. On the other hand, if the Mn content exceeds 3.00%, the weldability and toughness deteriorate. Therefore, the Mn content is 3.00% or less. In addition, the minimum of preferable Mn content is 0.60%, More preferably, it is 0.70%. Moreover, the upper limit of preferable Mn content is 2.70%, More preferably, it is 2.40%, More preferably, it is 2.10%.

P: 0.025% or less P is an element having a large embrittlement effect, and when contained in a large amount, it lowers the toughness of steel. Therefore, the P content is 0.025% or less. Note that the P content is more preferably 0.020% or less. On the other hand, since the smaller the amount of P, the lower the P content is not particularly limited, and it may be 0%. However, since P is an element inevitably contained in the steel as an impurity, it may be industrially more than 0%. In addition, since excessively low P leads to the increase in refining time and a cost, it is preferable to make P content 0.001% or more.

S: 0.02% or less Since S reduces the toughness of steel, the S content is set to 0.02% or less. More preferably, the S content is 0.015% or less. On the other hand, the lower the S content, the better. Therefore, the lower limit of the S content is not particularly limited, and may be 0%, but may be industrially over 0%. In addition, since excessively low S causes increase in refining time and cost, it is preferable to make S content 0.0001% or more.

Cr: 0.2% or more and 2.0% or less Cr is an element having a function of improving the hardenability of steel. By adding Cr, the hardness of the steel after quenching becomes higher, and as a result, the wear resistance of the steel sheet can be improved. In order to acquire the said effect, it is necessary to make Cr content 0.2% or more. On the other hand, if the Cr content exceeds 2.0%, the weldability decreases. Therefore, the Cr content is set to 0.2 to 2.0%. The minimum with preferable Cr content is 0.3%, More preferably, it is 0.4%. Moreover, the upper limit with preferable Cr content is 1.8%, More preferably, it is 1.6%.

Nb: 0.005% or more and 0.100% or less Nb is an element having the effect of pinning the movement of crystal grain boundaries and suppressing the growth of crystal grains by precipitating as NbC. In order to acquire the said effect, it is necessary to make Nb content 0.005% or more. On the other hand, if the Nb content exceeds 0.100%, the weldability decreases. Therefore, the Nb content is set to 0.005 to 0.100%. The minimum with preferable Nb content is 0.009%, More preferably, it is 0.011%. Moreover, the upper limit with preferable Nb content is 0.070%, More preferably, it is 0.065%.

Ti: 0.005% or more and 0.100% or less Ti is an element having an effect of suppressing the grain growth by pinning the movement of the grain boundary by being precipitated as TiN. In order to acquire the said effect, it is necessary to make Ti content 0.005% or more. On the other hand, if the Ti content exceeds 0.100%, the cleanliness of the steel decreases, and as a result, the ductility and toughness decrease. Therefore, the Ti content is set to 0.005 to 0.100%. The minimum with preferable Ti content is 0.009%, More preferably, it is 0.024%. Moreover, the upper limit with preferable Ti content is 0.060%, More preferably, it is 0.055%.

Al: 0.001% or more and 0.100% or less Al is an element that is effective as a deoxidizing agent and has an effect of reducing the austenite grain size by forming a nitride. 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 material and the steel sheet is lowered, and as a result, ductility and toughness are lowered. Therefore, the Al content is 0.001 to 0.100%. Preferably, it is 0.005 to 0.080% of range.

B: 0.0005% or more and 0.0100% or less B is an element having an effect of improving the strength of the steel sheet by improving the hardenability by adding a trace amount. In order to acquire the said effect, B content needs to be 0.0005% or more. On the other hand, if the B content exceeds 0.0100%, the weldability decreases. Therefore, the B content is set to 0.0005 to 0.0100%. In addition, the minimum with preferable B content is 0.0007%, More preferably, it is 0.0009%. Moreover, the upper limit with preferable B content is 0.0050%.

N: 0.01% or less Since N is an element that decreases ductility and toughness, the N content is set to 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, it may be industrially more 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.
The steel sheet of the present invention contains the above components, with the balance being Fe and inevitable impurities.

  The steel sheet of the present invention has the above-described components as a basic composition, but is optionally Cu: 0.01% or more and 2.00% or less, Ni: 0.01% or more for the purpose of improving hardenability and weldability. 5.00% or less, Mo: 0.01% to 3.00%, V: 0.001% to 1.000%, W: 0.01% to 1.50%, Ca: 0.0001 % Or more and 0.0200% or less, Mg: 0.0001% or more and 0.0200% or less, and REM: 0.0005% or more and 0.0500% or less. Can do.

Cu: 0.01% or more and 2.00% or less Cu is an element that can improve the hardenability without greatly degrading the base material and toughness. In order to acquire the said effect, Cu content shall be 0.01% or more. On the other hand, when Cu content exceeds 2.00%, the steel plate crack resulting from the Cu concentrated layer produced | generated just under a scale will be a problem. Therefore, when adding Cu, Cu content is made 0.01 to 2.00%. The Cu content is preferably 0.05 to 1.50%.

Ni: 0.01% or more and 5.00% or less Ni is an element having an effect of improving hardenability and improving toughness. In order to acquire the said effect, Ni content shall be 0.01% or more. On the other hand, if the Ni content exceeds 5.00%, an increase in manufacturing cost becomes a problem. Therefore, when adding Ni, the Ni content is set to 0.01 to 5.00%. Note that the Ni content is preferably 0.05 to 4.50%.

Mo: 0.01% to 3.00% Mo is an element that improves the hardenability of steel. In order to acquire the said effect, Mo content shall be 0.01% or more. However, when the Mo content exceeds 3.00%, the weldability decreases. Therefore, when adding Mo, Mo content is made 0.01 to 3.00%. The Mo content is preferably 0.05 to 2.00%.

V: 0.001% or more and 1.000% or less V is an element having an effect of improving the hardenability of steel. In order to acquire the said effect, V content shall be 0.001% or more. On the other hand, if the V content exceeds 1.000%, the weldability decreases. Therefore, when V is added, the V content is set to 0.001 to 1.000%.

W: 0.01% or more and 1.50% or less W is an element having an effect of improving the hardenability of steel. In order to acquire the said effect, W content shall be 0.01% or more. On the other hand, if the W content exceeds 1.50%, the weldability decreases. Therefore, when adding W, W content shall be 0.01 to 1.50%.

Ca: 0.0001% or more and 0.0200% or less Ca is an element that improves weldability by forming an oxysulfide having high stability at high temperatures. In order to acquire the said effect, Ca content shall be 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% or more and 0.0200% or less 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% or more and 0.0500% or less 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, REM content shall be 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%.

[Organization]
In addition to having the above component composition, the wear-resistant steel sheet of the present invention has a volume ratio of a structure at a 1/4 depth position (hereinafter referred to as 1/4 position) of the thickness from the surface of the wear-resistant steel sheet. And 90% or more of martensite, the average grain size of the old γ grains is 15 μm or less, and the maximum grain size of the old γ grains is 35 μm or less. The reason for limiting the steel structure as described above will be described below.

Martensite volume ratio: 90% or more Since the quenching is insufficient, a steel material having a structure in which upper bainite is mixed in martensite has poor hardness and toughness. The phenomenon becomes remarkable when the volume ratio of martensite is less than 90%. Therefore, the volume ratio of martensite is 90% or more. The remaining structure other than martensite is not particularly limited, but ferrite, pearlite, austenite, and bainite structures 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 an environment where an abrasion-resistant steel plate is used, the hardness and toughness from the surface layer of the steel plate to the ¼ thickness position are important. Since the volume ratio of the martensite increases as it approaches the surface layer of the steel sheet, the volume ratio of the martensite was evaluated at a 1/4 position of the wear-resistant steel sheet. And as the volume fraction of martensite increases, both the hardness and toughness of the steel sheet increase. The volume ratio of the martensite can be obtained by the method described in the examples.

Average particle size of prior austenite grains: 15 μm or less The smaller the prior austenite particle size (old γ particle size) of martensite, the less likely the occurrence and propagation of brittle fracture and the higher the toughness. Therefore, the average grain size of the prior γ grains is a dominant factor for brittle crack propagation. When the average grain size of the former γ grains exceeds 15 μm, the propagation of brittle fracture cracks tends to occur and the toughness is lowered. Therefore, the average particle diameter of the old γ grains is set to 15 μm or less. On the other hand, the smaller the average particle size of the prior γ particles, the better. Therefore, the lower limit is not particularly limited, but the average particle size is usually 1 μm or more. Normally, the closer the surface layer of the steel plate is, the smaller the old γ grain size becomes, so the average grain size of the old γ grain is the equivalent circle diameter of the old γ grain at the 1/4 position of the plate thickness of the wear-resistant steel plate. The average value was used. The average particle diameter of the old γ grains can be determined by the method described in the examples described later.

Maximum grain size of prior austenite grains: 35 μm or less Brittle fracture of a steel material occurs starting from a location having the lowest toughness in the structure. The larger the prior austenite particle size (old γ particle size), the worse the toughness, so the old γ particle size becomes the dominant factor for the occurrence of brittle cracks. Further, the total absorbed energy at the time of brittle fracture occurrence and propagation is the sum of the energy required for crack initiation and propagation, but usually the energy at the time of crack occurrence is larger. Therefore, in order to increase the total absorbed energy, it is important to reduce the old γ grain size and reduce the coarse old γ grain.
That is, when the maximum particle size of the old γ grains exceeds 35 μm, the toughness is remarkably deteriorated. Therefore, the maximum particle size of the old γ grains is set to 35 μm or less. In general, the closer to the surface layer of the steel sheet, the smaller the old γ grain size. Therefore, the maximum grain size of the old γ grain is the equivalent circle diameter of the old γ grain at the 1/4 position of the plate thickness of the wear-resistant steel sheet. Maximum value. The maximum diameter of the prior austenite grains can be obtained by the method described in Examples described later.

In the present invention, as described above, NbC is uniformly finely dispersed in the steel sheet, whereby the average grain size of the old γ grains is made small and a sized structure, and excellent low temperature toughness is obtained. For that purpose, it is advantageous that the number density of NbC is 5 × 10 ⁵ to 5 × 10 ⁶ / mm ² at the position of the plate thickness ¼. When the number density is less than 5 × 10 ⁵ / mm ^2, the average particle size of the old γ grains is set to 15 μm or less, or the maximum particle size of the old γ grains is set to 35 μm or less. It becomes difficult to get. On the other hand, when the number density of NbC exceeds 5 × 10 ⁶ / mm ² , the old γ grains become too fine, and the workability of the steel sheet may be remarkably deteriorated.

Furthermore, the wear-resistant steel sheet of the present invention has the following hardness.
[Brinell hardness]
Brinell hardness: 460-590 HBW 10/3000
The wear resistance of the steel sheet can be improved by increasing the hardness. As the hardness of the steel plate, the result of the hardness test at the 1/4 position of the thickness was used as a representative value, but the cooling rate at the time of quenching was faster on the surface layer side than the test position, so that the thickness of the steel layer was 1/4 position. Have the same or higher hardness.
When the hardness is less than 460 HBW in Brinell hardness, sufficient wear resistance cannot be obtained. On the other hand, if the hardness is 590 HBW or more in Brinell hardness, the bending workability deteriorates. Therefore, in the present invention, the hardness at the 1/4 position of the plate thickness is set to 460 to 590 HBW in terms of Brinell hardness. The Brinell hardness is a value measured using a hard tungsten ball having a diameter of 10 mm and a load of 3000 kgf, and is expressed as HBW10 / 3000.

Moreover, it is preferable that the abrasion-resistant steel plate of the present invention has the following impact absorption energy.
[Shock absorption energy]
In recent years, steel materials have been used in a harsher environment, and in order to exhibit good toughness even at a lower temperature than before, the impact absorption energy at −60 ° C. in the Charpy impact test is 25 J or more. It is preferable that

[Production method]
Next, the manufacturing method of the abrasion-resistant steel plate of this invention is demonstrated.
Heating temperature: T * ~ 1300 ° C
When the heating temperature in the heating process of the steel material is lower than T * ° C. represented by the following formula (1), the coarse NbC that is precipitated and grows during the casting of the steel material and does not dissolve in a solid state. It will remain as it is. Therefore, the pinning effect by NbC at the time of quenching and heating is not uniform in the steel material, resulting in a mixed grain structure. That is, since coarse γ grains grow where NbC does not exist, the martensite transformed from the γ grains also becomes coarse, and as a result, the martensite also becomes mixed grains. In steel materials manufactured from such steel materials, toughness is reduced.
On the other hand, when the heating temperature is higher than 1300 ° C., the amount of scale generation increases and the steel material yield decreases. Therefore, the said heating temperature shall be T * -1300 degreeC. The heating temperature here is the temperature at the surface of the steel material.
T * = 7920 / (3.4-log [Nb] [C] ^0.87 ) -273 (1)

Hot rolling: The total rolling reduction in the range of 1000 ° C. or lower and 850 ° C. or higher is 30% or higher. Usually, the preferential precipitation site of NbC in the temperature range of about 700 to 1300 ° C. where hot rolling is performed is on the γ grain boundary. It is. In such a case, the interval between the precipitated NbC particles is wide, and NbC is sparsely distributed. In order to precipitate NbC finely dispersed, it is effective to use strain-induced precipitation in which NbC is precipitated on dislocations introduced into the steel material by rolling.
When the total rolling reduction is less than 30%, the amount of dislocations forming NbC precipitation sites is small, so that the dispersed state of NbC becomes sparse. When the rolling temperature is lower than 850 ° C., most of the precipitation of NbC has already been completed, so that the amount of NbC deposited by strain-induced precipitation decreases. Therefore, the dispersion state of NbC becomes sparse, and austenite grains become a mixed grain structure. And since the finely sized γ grain structure cannot be obtained by quenching and heating after hot rolling, the toughness is lowered.
On the other hand, when rolling is performed at a temperature exceeding 1000 ° C., the introduced dislocation recovers, and the dislocation disappears before the precipitation of NbC by recrystallization. Therefore, the dislocation does not become a NbC precipitation site. Therefore, the total rolling reduction in the temperature range of 1000 ° C. or lower and 850 ° C. or higher is set to 30% or higher. Note that this temperature is the temperature at the position where the plate thickness is 1/4.
When using a reversible rolling mill, the rolling reduction here is the ratio of the exit side thickness to the entrance side thickness at each rolling pass, and when using a continuous rolling mill, The ratio of the thickness of the outlet side to the thickness of the side. The total rolling reduction is the sum of rolling reductions for each rolling pass when using a reversible rolling mill, and the sum of rolling reductions for each rolling stand when using a continuous rolling mill.
The temperature here is the temperature of the steel material surface.

Average cooling rate in the temperature range of 800 ° C. or lower and 600 ° C. or higher after hot rolling: 1 ° C./s or higher When the cooling rate of the steel plate after hot rolling is less than 1 ° C., the Ostwald growth of the precipitated NbC particles Since the fine particles disappear and the coarse particles become coarser, the dispersed state of NbC becomes sparse. As a result, since martensite becomes a mixed grain structure, low temperature toughness deteriorates. Therefore, the cooling rate was set to 1 ° C./s or more. In addition, this cooling rate is an average cooling rate between 800-600 degreeC in plate | board thickness 1/4 position. The upper limit of the cooling rate is not particularly specified, but about 200 ° C./s is sufficient.
It should be noted that the temperature at the plate thickness 1/4 position can be obtained by heat transfer calculation based on the temperature of the steel plate surface.

Reheating temperature for quenching: Ac _{3 to} 950 ° C
Quenching is performed on the steel after completion of the cooling.
When the reheating temperature for quenching is less than Ac ₃ , the structure after hot rolling remains untransformed, resulting in low hardness and toughness. On the other hand, when the reheating temperature exceeds 950 ° C., the old γ grain size becomes coarse due to the growth of γ grains, so that the toughness is also lowered. Therefore, the reheating temperature at the time of quenching was set to Ac _{3 to} 950 ° C. Incidentally, Ac ₃ was used the value represented by the following equation (2).
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] (2) Formula where [] Is the content (% by mass) of the element in the parenthesis, and is 0 (zero) when the element is not added.
In addition, let the reheating temperature here be the temperature in the steel plate surface.

Quenching cooling rate: 1 ° C./s or more When the quenching cooling rate is less than 1 ° C./s, the volume ratio of martensite becomes small, so the hardness and toughness are lowered. Therefore, the quenching cooling rate is set to 1 ° C./s or more. In addition, this cooling rate is an average cooling rate between 800-300 degreeC in plate | board thickness 1/4 position. The upper limit of the cooling rate is not particularly specified, but about 200 ° C./s is sufficient.

The quenching stop temperature of the quenching is not particularly limited, but if the cooling stop temperature is higher than 250 ° C, the volume ratio of martensite may be reduced, and the hardness of the steel sheet may be lowered. Is preferred. 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. Therefore, the cooling stop temperature is preferably 20 ° C. or higher.
The cooling stop temperature is a temperature at a position where the plate thickness is 1/4.

Furthermore, if necessary, a step of tempering at a temperature of 100 to 350 ° C. can be provided after quenching.
Tempering temperature: 100-350 ° C
When tempering is performed after the tempering, the toughness and workability of the steel sheet can be improved by setting the tempering temperature to 100 ° C. or higher. On the other hand, when the tempering temperature is higher than 350 ° C., martensite may be significantly softened. Therefore, the tempering temperature is set to 100 to 350 ° C. The tempering time can be adjusted as appropriate.
The tempering temperature is the temperature of the steel sheet surface.

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.

  Next, each treatment of heating, hot rolling, and quenching was sequentially performed on the obtained slab to obtain a steel plate. Furthermore, some steel plates were reheated for tempering after quenching. The manufacturing conditions for each step are as shown in Table 2. In addition, cooling after quenching was performed by jetting water at a large flow rate from the front and back surfaces of the steel plate while moving the steel plate. The steel plate temperature was obtained by heat transfer calculation based on the temperature of the steel plate surface. Moreover, cooling at the time of quenching of the steel sheet was performed to 250 ° C. or less.

  About each of the obtained steel plates, the volume ratio of martensite and the prior γ grain size were measured by the method described below. The measurement results are shown in Table 2.

[Martensite volume fraction]
A sample was collected from the obtained steel plate in the longitudinal direction 1/4 and the width center position so that the position of the plate thickness 1/4 was the observation surface. The surface of the sample was mirror-polished and further subjected to nital corrosion, and then an area of 10 mm × 10 mm was photographed using a scanning electron microscope (SEM). By analyzing the photographed image using an image analysis device, 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 taken from the longitudinal direction 1/4 and the center in the width direction of the steel sheet so that the position of the thickness 1/4 was the observation surface. The surface of the obtained sample was mirror-polished and further corroded with picric acid, and then a range of 10 mm in the longitudinal direction of the plate was photographed at a magnification of 400 times using an optical microscope. By analyzing the photographed image using an image analyzer, the average particle size and the maximum particle size of the former γ grains were obtained. The prior austenite particle size was calculated as an equivalent circle diameter.

  Furthermore, about each of the obtained steel plate, hardness and Charpy impact absorption energy were evaluated by the method described below. The evaluation results are as shown in Table 2.

[Hardness (Brinell hardness)]
As an index of wear resistance, the hardness of the steel sheet at a 1/4 position was measured. The test piece used for the measurement was collected from each steel plate obtained as described above so that the position of the steel plate with a thickness of 1/4 was the test surface. After mirror-polishing the surface of the test piece, the Brinell hardness was measured according to JIS Z2243 (2008). For the measurement, a tungsten hard sphere having a diameter of 10 mm was used, the load was 3000 kgf, a test was performed at 10 points, and the average value was defined as Brinell hardness.
A case where the Brinell hardness was 460 to 590 HBW 10/3000 was judged to be excellent in wear resistance.

[Charpy absorbed energy]
As a Charpy impact test piece, a JISV notch test piece (JIS Z2242 (2005)) was used, where the longitudinal direction of the test piece was parallel to the longitudinal direction of the steel sheet from the longitudinal direction 1/4 of the steel sheet and the thickness 1/4 of the center of the width. It collected so that it might become. Depending on the plate thickness, sub-size test pieces having test piece widths of 2.5 mm, 5 mm, and 7.5 mm were used as appropriate. The test temperature was −60 ° C., and the other test conditions were the Charpy impact test according to JIS Z2242 (2005), and the absorbed energy at −60 ° C. was obtained. Evaluation was made by a value converted to a test piece width of 10 mm by an equation of (obtained absorbed energy) × 10 / (test piece width). Six test pieces were prepared, and the average value of the six absorbed energy (after conversion) was defined as Charpy absorbed energy. And the case where the impact absorption energy after conversion in -60 degreeC is 25J or more was considered to be excellent in low temperature toughness.

  As can be seen from the results shown in Table 2, the wear-resistant steel sheet that satisfies the conditions of the present invention has an excellent hardness of Brinell hardness of 460 HBW 10/3000 or more and an excellent low temperature toughness at an extremely low temperature of −60 ° C. It was. 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 low temperature toughness.

For example, No. with low C content. In the steel plate of 14, the hardness is inferior because the amount of dissolved C in the martensite base is reduced. No. Since the steel plate No. 15 has a high C content, the hardness is too high. No. Since the steel plate No. 16 has a low Nb content, the pinning effect of NbC is small, and the old γ grain size becomes coarse, so the low temperature toughness is low. No. Since the steel plate No. 17 has a low B content, the hardenability is insufficient. As a result, the martensite volume fraction is low and the hardness is low. No. In No. 18, the heating temperature at the time of hot rolling is lower than the Nb solid solution temperature T *, so that the coarsely dispersed NbC deposited during continuous casting and remaining loosely does not re-dissolve, and thus partially. As a result of the coarse structure of the former γ grain size, the low temperature toughness is low. No. The steel plate No. 19 has a low total rolling reduction between 1000 and 850 ° C. during hot rolling, so the strain-induced precipitation effect is small, and NbC is sparsely dispersed, resulting in a partially coarse structure of the old γ grain size. As a result, low temperature toughness is low. No. Steel plate No. 20 has a slow cooling rate after hot rolling, so that Ostwald growth of NbC occurs and NbC is sparsely dispersed, resulting in a partially coarse structure of the old γ grain size, resulting in low low temperature toughness. . No. Since the steel plate No. 21 has a quenching reheating temperature lower than Ac ₃ , the martensite volume ratio is small, and as a result, the hardness is inferior. No. Since the steel plate No. 22 has a high quenching reheating temperature, the prior austenite grain size is increased, and as a result, the low temperature toughness is inferior. No. The steel plate No. 23 has a low cooling rate after reheating during quenching, so that martensitic transformation does not occur, and as a result, the hardness is inferior. No. Since the steel plate No. 24 has a high tempering temperature, the martensite is softened, and as a result, the hardness is inferior.

In addition, 50000 times imaging | photography was performed for 10 visual fields with the transmission electron microscope with an analyzer in the board thickness 1/4 position of the steel plate, and the number density of the NbC particle | grain was calculated | required. As a result, it was confirmed that the number density of NbC was all within 5 × 10 ⁵ to 5 × 10 ⁶ / mm ² in all the compatible steels.

Claims (5)

% By mass
C: more than 0.23% and 0.34% or less,
Si: 0.1% to 3.0%,
Mn: 0.50% or more and 3.00% or less,
P: 0.025% or less,
S: 0.02% or less,
Cr: 0.2% to 2.0%,
Nb: 0.005% or more and 0.100% or less,
Ti: 0.005% or more and 0.100% or less,
Al: 0.001% or more and 0.100% or less,
B: 0.0005% or more and 0.0100% or less and N: 0.01% or less, the balance Fe and the inevitable impurities component composition,
The structure at a depth of 1/4 of the sheet thickness from the surface of the steel sheet is martensite with a volume ratio of 90% or more, and has an average austenite grain average diameter of 15 μm or less and a maximum diameter of 35 μm or less. ,
A wear-resistant steel sheet having a Brinell hardness of 460 to 590 HBW 10/3000 at a 1/4 depth position from the surface of the steel sheet. The component composition is further mass%,
Cu: 0.01% or more and 2.00% or less,
Ni: 0.01% or more and 5.00% or less,
Mo: 0.01% to 3.00%,
V: 0.001% to 1.000%,
W: 0.01% or more and 1.50% or less,
Ca: 0.0001% or more and 0.0200% or less,
The wear-resistant steel sheet according to claim 1, comprising one or more selected from Mg: 0.0001% to 0.0200% and REM: 0.0005% to 0.0500%. % By mass
C: more than 0.23% and 0.34% or less,
Si: 0.1% to 3.0%,
Mn: 0.50% or more and 3.00% or less,
P: 0.025% or less,
S: 0.02% or less,
Cr: 0.2% to 2.0%,
Nb: 0.005% or more and 0.100% or less,
Ti: 0.005% or more and 0.100% or less,
Al: 0.001% or more and 0.100% or less,
B: 0.0005% or more and 0.0100% or less and N: 0.01% or less, and a steel material having a component composition that is the remaining Fe and inevitable impurities is represented by a temperature T * represented by the following formula (1). Heated to not less than 1 ° C and not more than 1300 ° C,
The heated steel material is hot-rolled so that the total rolling reduction is 30% or more when the temperature at the 1/4 depth position of the plate thickness from the surface of the steel material is 1000 ° C. or lower and 850 ° C. or higher. Hot rolled steel sheet,
The hot-rolled steel sheet is cooled at an average cooling rate of 1 ° C./s or more in a range where the temperature at a 1/4 depth position from the surface of the hot-rolled steel sheet is 800 ° C. or lower and 600 ° C. or higher,
Then, after reheating the hot-rolled steel sheet at a temperature of Ac ₃ or higher and 950 ° C. or lower, the average cooling is performed in a range where the temperature at the 1/4 depth position from the surface of the steel sheet is 800 ° C. or lower and 300 ° C. or higher. Cool at a rate of 1 ° C / s or higher ,
The structure at a depth of 1/4 of the sheet thickness from the surface of the steel sheet is martensite with a volume ratio of 90% or more, and has an average austenite grain average diameter of 15 μm or less and a maximum diameter of 35 μm or less. And the hardness in the 1/4 depth position of plate | board thickness from the surface of a steel plate is a manufacturing method of the wear-resistant steel plate whose Brinell hardness is 460-590HBW10 / 3000 .
T * = 7920 / (3.4-log [Nb] [C] ^0.87 ) -273 (1) The steel material is further mass%,
Cu: 0.01% or more and 2.00% or less,
Ni: 0.01% or more and 5.00% or less,
Mo: 0.01% to 3.00%,
V: 0.001% to 1.000%,
W: 0.01% or more and 1.50% or less,
Ca: 0.0001% or more and 0.0200% or less,
The manufacturing method of the wear-resistant steel plate according to claim 3, comprising one or more selected from Mg: 0.0001% to 0.0200% and REM: 0.0005% to 0.0500%.   Furthermore, the manufacturing method of the abrasion-resistant steel plate of Claim 3 or 4 which tempers the said steel plate at the temperature of 100 to 350 degreeC.