JP7534595B2 - Manufacturing method of wear-resistant steel - Google Patents

Description

The present invention relates to a method for manufacturing wear-resistant steel that is suitable for use as a component part of machinery that requires wear resistance, such as construction machinery and industrial machinery.

The wear resistance of machine components is strongly determined by their surface hardness, so high-hardness steel is used for components of machines that require wear resistance, such as construction machinery for civil engineering and mining, and industrial machinery. This high-hardness steel is required to have stable wear resistance and the ability to withstand long-term use. Furthermore, in recent years, there has been an increase in demand for construction machinery and industrial machinery used in cold regions, and there is a demand for steel materials with low-temperature toughness suitable for use in such cold regions.

Patent Document 1 proposes a method for manufacturing wear-resistant steel plate in which the chemical composition is controlled, the plate is heated, hot-rolled, and then reheated and accelerated cooled.

Patent Document 2 also proposes a method for producing wear-resistant thick steel plate that controls the composition and uses fine precipitates with a diameter of 50 nm or less to suppress the growth of austenite grains during manufacturing.

Furthermore, Patent Document 3 proposes a method for producing low-alloy wear-resistant steel plate by controlling the composition, heating, hot rolling, and applying accelerated cooling immediately after the hot rolling.

Patent Document 4 discloses that it is possible to manufacture a wear-resistant steel plate with excellent low-temperature toughness by controlling the composition, controlling the slab temperature and rolling reduction after heating, hot rolling, allowing it to cool, and then reheating and quenching.

JP 2012-214890 A JP 2014-194042 A Special table 2016-509631 publication International Publication No. 2019/181130

However, the steel plate manufactured by the method described in Patent Document 1 has a high C content, making it difficult to increase toughness. In addition, the method described in Patent Document 1 does not fully consider the rolling conditions during hot rolling, and therefore there is still room for improvement in terms of improving toughness. Furthermore, many of the examples in Patent Document 1 have low reheating temperatures, and therefore there are problems in terms of ensuring high hardness.

Patent Document 2 also teaches that dispersing fine precipitates in steel can suppress the growth of austenite grains during reheating through a pinning effect, thereby refining the austenite grains. However, with this method of dispersing fine precipitates in steel, slight differences in the composition system or differences in the reheating temperature can cause large variations in the dispersion state of the precipitates, making it difficult to stably refine the austenite grains and not necessarily achieving high toughness. In addition, the P content is not necessarily kept low enough, which can further reduce toughness.

Furthermore, it is difficult to increase the toughness of steel sheets manufactured by the method described in Patent Document 3 due to their high C content. In addition, the inventors' studies have revealed that cooling (quenching) immediately after hot rolling at low temperatures creates anisotropy in the steel structure, which creates the problem of reduced toughness when fracture occurs in the rolling direction.

The manufacturing method of wear-resistant steel described in Patent Document 4 can produce wear-resistant steel with a certain level of absorbed energy in a Charpy impact test or higher, but there is room for further improvement in the manufacturing method of wear-resistant steel from the perspective of obtaining a more stable and high absorbed energy in a Charpy impact test in order to produce wear-resistant steel that can be used more stably in cold regions.

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide a method for manufacturing wear-resistant steel that has higher absorbed energy in a Charpy impact test.

The gist of the present invention is as follows.
(1) In mass%,
C: 0.10-0.30%,
Si: 0.01 to 1.20%,
Mn: 0.01-2.00%,
P: less than 0.017%;
S: 0.010% or less,
Ni: 0.01 to 1.00%,
Cu: 0.01 to 0.70%,
Cr: 0.30-1.50%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.030%, and N: 0.0001 to 0.0070%
and the balance being Fe and impurities, to 1000 to 1350°C;
hot rolling the heated steel slab at 1000°C or less and over 825°C with a reduction of 20% or more, and then hot rolling at 825°C or less and 730°C or more with a reduction of 10% or more;
A method for producing a wear-resistant steel, comprising: a step of cooling a hot-rolled steel plate to 350°C or less at an average cooling rate of 5.0°C/sec or more; and a step of reheating the cooled steel plate to 860°C or more and then quenching the steel plate.
(2) The steel slab further comprises, in mass%,
Mo: 0.80% or less,
B: 0.0030% or less,
Nb: 0.001 to 0.050% or less,
V: 0.20% or less,
W: 0.50% or less; and Zr: 0.0050% or less;
(1) A method for producing an abrasion-resistant steel according to (1), characterized in that the steel has a composition containing one or more of the above.
(3) The steel slab further comprises, in mass%,
Ca: 0.0050% or less,
The method for producing an abrasion-resistant steel according to (1) or (2), characterized in that the steel has a composition containing one or more of Mg: 0.0050% or less, and REM: 0.0050% or less.

The present invention provides wear-resistant steel with excellent low-temperature toughness that can be used in cold regions.

<Method of manufacturing wear-resistant steel>
In general, increasing the hardness of a steel material tends to decrease its toughness, and it is not easy to ensure low-temperature toughness with a high-hardness steel material such as wear-resistant steel. The present inventors have conducted extensive research to obtain a wear-resistant steel having high toughness even at low temperatures, more specifically, to obtain a wear-resistant steel having a surface layer Brinell hardness of 360 to 500 and an absorbed energy in a Charpy impact test at -40°C of 150 J or more, and have found that it is necessary to make the average prior austenite grain size at a position 1/4 of the thickness from the surface of the steel plate in the thickness direction 15 μm or less.

The inventors have investigated various manufacturing conditions necessary to refine the prior austenite grain size, and as a result have discovered that it is essentially important to increase the number of nucleation sites for the reverse transformation from bainite or martensite to austenite during reheating during quenching. This is because by significantly increasing the number of nucleation sites for austenite reverse transformation, it is possible to refine the austenite grains when the entire material has completed reverse transformation to austenite.

The inventors discovered that in order to increase the number of nucleation sites for this austenite reverse transformation, it is important to control the temperature and reduction rate during hot rolling.

In other words, the inventors have found through detailed studies that in the wear-resistant steel of the present invention, the nucleation sites of austenite during reheating during quenching are high-angle grain boundaries such as the prior austenite grain boundaries of bainite and martensite. By controlling the temperature and reduction rate during hot rolling, the austenite grain size can be refined during hot rolling, which can increase the area of high-angle grain boundaries per unit volume during reheating during quenching after hot rolling. In other words, it is possible to increase the nucleation sites of austenite reverse transformation. Furthermore, it is estimated that such control during hot rolling can increase the energy stored in the grain boundaries by applying a reduction strain to the steel, thereby promoting reverse transformation.

As described above, by controlling the temperature and reduction rate of the hot rolling process, it was possible to increase the number of nucleation sites and impart strain to the steel. However, it was not necessarily possible to stably reduce the average prior austenite grain size to 15 μm or less by only controlling the temperature and reduction rate of the hot rolling process. Therefore, it is required to increase the number of nucleation sites and impart strain to the steel by other methods, and to further refine the austenite grain size when reverse transformation is completed. As a result of further investigation aimed at further refinement of the austenite grain size, the inventors discovered that this can be achieved by controlling the temperature and reduction rate of the hot rolling process, as well as the cooling after hot rolling by rapid cooling rather than natural cooling.

The inventors have also discovered that in order to obtain wear-resistant steel with low-temperature toughness, it is not sufficient to improve toughness by merely controlling the average prior austenite grain size, and it is necessary to make the structure mainly composed of martensite and lower bainite. In addition, an appropriate combination of various alloys is also necessary to improve toughness.

In particular, when fine austenite grain size is obtained, the hardenability generally tends to decrease, and even if hardening is performed, the hardness required for wear-resistant steel may not be obtained. Therefore, the inventors have found that it is necessary to improve the hardenability by adding Cu and Ni.

The manufacturing method of the wear-resistant steel according to the present invention is described below. The manufacturing method of the wear-resistant steel according to the present invention includes a heating process for heating a steel piece, a hot rolling process for hot rolling the heated steel piece, a cooling process for cooling the hot-rolled steel sheet, and a reheating and quenching process for reheating and quenching the cooled steel sheet.

The method for producing the steel slab used to produce the wear-resistant steel of the present invention is not particularly limited. For example, the composition of the molten steel can be adjusted, and then the steel slab can be obtained by casting. From the viewpoint of productivity, the thickness of the steel slab is preferably 200 mm or more. In addition, taking into consideration reduction of segregation and homogeneity of the heating temperature before hot rolling, the thickness of the steel slab is preferably 350 mm or less. Such a steel slab can be used in the method for producing the wear-resistant steel of the present invention described below.

(Heating process)
In the heating step, a steel slab having a predetermined composition is heated at 1000 to 1350°C. First, the reason for limiting the composition of the steel slab will be explained. In this specification, "%" for the content of a component means mass%. In the description of the composition, a numerical range expressed using "to" means a range including the numerical values written before and after "to" as the lower and upper limit values, except when "more than" or "less than" is used.

(C: 0.10-0.30%)
C (carbon) is an element effective in increasing the hardness of steel, and in the present invention, in order to ensure hardness, the lower limit of the C content is set to 0.10%. The lower limit of the C content is preferably 0.11%, and a more preferable lower limit of the C content is 0.13%. On the other hand, if the C content exceeds 0.30%, the Brinell hardness, which is the target of the wear-resistant steel produced according to the present invention, is not obtained. In some cases, the range of 500 or less may not be satisfied, and the toughness may decrease, so the upper limit of the C content is set to 0.30%. In order to improve the toughness, the upper limit of the C content is set to 0.25%. It is preferable to set the content at 0.15%, and more preferable to set the content at 0.20%.

(Si: 0.01-1.20%)
Since Si (silicon) is a deoxidizing element and contributes to improving hardness by solid solution strengthening, the lower limit of the Si content is set to 0.01% in the present invention. The lower limit of the Si content is preferably 0.10%, and more preferably 0.20%. However, if the Si content is too high, toughness and weldability deteriorate, so the upper limit of the Si content is set to 1.20%. The upper limit of the Si content is preferably 0.80%, and more preferably 0.70%.

(Mn: 0.01-2.00%)
Since Mn (manganese) contributes to increasing hardness by improving hardenability, the lower limit of the Mn content is set to 0.01% in the present invention. On the other hand, if the Mn content exceeds 2.00%, the toughness and weldability deteriorate, so the upper limit of the Mn content is set to 2. The upper limit of the Mn content is preferably 1.80%, and more preferably 1.50%.

(P: less than 0.017%)
P (phosphorus) is generally contained as an impurity, and since it segregates at grain boundaries and promotes the occurrence of brittle fracture, the P content is set to less than 0.017% in the present invention. If the P content is 0.017% or more, the toughness is significantly reduced. It is preferable that the P content is as small as possible, and although there is no lower limit, it may be, for example, more than 0% or 0.001% or more. Preferably, the P content is 0.013% or less.

(S: 0.010% or less)
S (sulfur) is generally contained as an impurity and forms sulfides such as MnS, which reduces toughness, so in the present invention, the S content is set to 0.010% or less. If it exceeds 0.010%, the toughness The S content is preferably as small as possible, and there is no lower limit, but it may be, for example, more than 0% or 0.001% or more. It is less than .007%.

(Ni: 0.01-1.00%)
Ni (nickel) contributes to increasing hardness through improvement of hardenability and also contributes to improving toughness, so it is contained in an amount of 0.01% or more. The Ni content is preferably 0.10% or more, and more preferably 0.10% or more. The Ni content is preferably 0.20% or more. Since excessive addition of Ni leads to an increase in cost, the upper limit of the Ni content is set to 1.00%. The upper limit of the Ni content is preferably 0.90%. %, more preferably 0.80%.

(Cu: 0.01-0.70%)
Cu (copper) contributes to increasing hardness by improving hardenability, so it is contained in an amount of 0.01% or more. Preferably, Cu is contained in an amount of 0.10% or more, and more preferably, 0.20% or more. However, However, excessive addition of Cu leads to a decrease in toughness and to cracking of the steel slab after casting and a decrease in weldability, so the upper limit of the Cu content is set to 0.70%. 65%, more preferably 0.60%.

(Cr: 0.30-1.50%)
Cr (chromium) contributes to increasing hardness by improving hardenability, so the Cr content is set to 0.30% or more, preferably 0.50% or more, and more preferably 0.60% or more. However, if the Cr content exceeds 1.50%, the toughness and weldability are reduced. Therefore, the upper limit of the Cr content is set to 1.50%. Preferably, the upper limit of the Cr content is set to 1.30%. %, and more preferably, the upper limit of the Cr content is 1.00%.

(Al: 0.001-0.100%)
In the present invention, Al (aluminum) is necessary as a deoxidizing element, and in order to obtain the deoxidizing effect, 0.001% or more, preferably 0.010% or more is contained. On the other hand, if Al is added excessively, Since Al oxides become coarse and become the starting points for brittle fracture, thereby reducing toughness, the upper limit of the Al content is set to 0.100%, preferably 0.090%, and more preferably 0.080%.

(Ti: 0.001-0.030%)
Ti (titanium) is an element that forms TiN and fixes N in steel. In the present invention, the lower limit of the Ti content is set to 0.001%. TiN also has a pinning effect, which fixes N in steel during hot rolling. Since Ti has the effect of refining the austenite grains before rolling, it is preferable to contain 0.005% or more of Ti, and more preferably 0.008% or more of Ti. If the Ti content exceeds 0.030%, coarse TiN is generated and the toughness is impaired, so the upper limit of the Ti content is set to 0.030%, preferably 0.020%, and more preferably 0.030%. The upper limit of the Ti content is set to 0.015%.

(N: 0.0001-0.0070%)
N (nitrogen) is an element that forms TiN and contributes to refining the structure and precipitation strengthening. Therefore, the lower limit of the N content is set to 0.0001%, preferably 0.0010%, and more preferably 0. However, if the N content is excessive, the toughness decreases, which causes surface cracks during casting and causes poor material properties due to strain aging of the manufactured steel material, so the upper limit is set to 0.0070%. Preferably, the upper limit of the N content is set to 0.0050%.

To further increase hardness and toughness, one or more of Mo, B, Nb, V, W, Zr, Ca, Mg and REM may be added.

(Mo: 0.80% or less)
Mo (molybdenum) is an element that contributes to increasing hardness by improving hardenability. In the present invention, the Mo content may be 0%, but in order to more reliably obtain this effect, the Mo content is set to 0. The Mo content is preferably 0.01% or more, and more preferably 0.05% or more. However, if the Mo content exceeds 0.80%, the toughness and weldability are reduced. Therefore, the Mo content The upper limit of the Mo content is set to 0.80%. More preferably, the upper limit of the Mo content is set to 0.60%.

(B: 0.0030% or less)
B (boron) is an element that increases hardness by improving hardenability, and increases toughness by increasing hardenability. In the present invention, the B content may be 0%. However, in order to obtain this effect more reliably, it is preferable to add 0.0001% or more, and more preferably 0.0005% or more. On the other hand, excessive addition of B reduces toughness and weldability. To prevent this, the upper limit of the B content is set to 0.0030%, and more preferably, the upper limit of the B content is set to 0.0015%.

(Nb: 0.050% or less)
Nb (niobium) is an element that contributes to increasing hardness by improving hardenability. In the present invention, the Nb content may be 0%, but in order to more reliably obtain this effect, the Nb content is set to 0. It is preferable that Nb be contained in an amount of 0.001% or more, and more preferably 0.005% or more. On the other hand, if Nb is added excessively, toughness and weldability are reduced, so the upper limit of the Nb content is set to 0.050%. , preferably 0.040%.

(V: 0.20% or less)
V (vanadium) is an element that contributes to improving hardenability and increasing hardness through precipitation strengthening. In the present invention, the V content may be 0%, but in order to more reliably obtain this effect, The V content is preferably 0.001% or more, and more preferably 0.01% or more. On the other hand, since excessive addition of V reduces toughness and weldability, the upper limit of the V content is set at 0. 20%, preferably 0.15%.

(W: 0.50% or less)
W (tungsten) is an element that contributes to increasing hardness by improving hardenability. In the present invention, the W content may be 0%, but in order to more reliably obtain this effect, it is preferable to use a content of 0%. It is preferable that W be contained in an amount of 0.001% or more. More preferably, 0.10% or more of W is contained. However, since excessive addition of W reduces toughness and weldability, the upper limit of the W content is set at 0. 50%, preferably 0.45%.

(Zr: 0.0050% or less)
Zr (zirconium) precipitates as carbides and nitrides and contributes to precipitation strengthening of steel. In the present invention, the Zr content may be 0%, but in order to more reliably obtain this effect, the Zr content is preferably 0%. The Zr content is preferably 0.0001% or more, and more preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.0050%, the Zr carbides and nitrides will become coarse. Since the toughness may decrease, the upper limit of the Zr content is set to 0.0050%, preferably 0.0040%.

(Ca: 0.0050% or less)
Ca (calcium) is an element effective in controlling the morphology of sulfides, suppressing the formation of coarse MnS, and contributing to improving toughness. In the present invention, the Ca content may be 0%, but In order to obtain this effect more reliably, it is preferable to contain 0.0001% or more of Ca, and more preferably 0.0010% or more of Ca. On the other hand, it is preferable to contain 0.0050% or more of Ca. Since the inclusion of Ca may reduce toughness, the upper limit of the Ca content is set to 0.0050%, and a more preferable upper limit of the Ca content is 0.0030%.

(Mg: 0.0050% or less)
Mg (magnesium) contributes to improving the toughness of the base material and the weld HAZ toughness. In the present invention, the Mg content may be 0%, but in order to obtain this effect more reliably, the Mg content is set to 0.0001%. It is preferable to add 0.0010% or more of Mg. More preferably, 0.0010% or more of Mg is added. On the other hand, if the Mg content exceeds 0.0050%, the effect becomes saturated and the toughness decreases. Therefore, the upper limit of the Mg content is set to 0.0050%, preferably 0.0040%.

(REM: 0.0050% or less)
REM (rare earth elements) contribute to improving the toughness of the base material and the weld HAZ toughness. In the present invention, the REM content may be 0%, but in order to obtain this effect more reliably, the REM content is set to 0. It is preferable to contain 0.0001% or more of REM, and more preferably 0.0010% or more of REM. On the other hand, if the REM content exceeds 0.0050%, the effect becomes saturated. The upper limit of the amount is 0.0050%, preferably 0.0040%.

In the wear-resistant steel produced according to the present invention, the remainder other than the above elements consists of Fe and impurities. Here, "impurities" refers to elements that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the wear-resistant steel is industrially produced, and are not elements that are intentionally added to the wear-resistant steel produced according to the present invention. In addition, "impurities" also include elements other than those described above that are contained in the wear-resistant steel at a level that does not affect the properties of the wear-resistant steel according to the present invention.

Before hot rolling, the steel slab having the above composition is heated to 1000-1350°C. If the heating temperature of the steel slab is less than 1000°C, the alloy elements may not be sufficiently dissolved, so the lower limit is set to 1000°C. The lower limit of the heating temperature of the steel slab is preferably 1050°C, more preferably 1100°C. On the other hand, if the heating temperature of the steel slab is higher than 1350°C, the scale on the surface of the steel slab, which is the raw material, will liquefy and interfere with production, so the upper limit is set to 1350°C. The upper limit of the heating temperature of the steel slab is preferably 1300°C, more preferably 1250°C. The heating time is not limited, but may be, for example, 30 minutes to 600 minutes.

Before this heating, heating to 1100°C or higher and 1350°C or lower may be applied in order to reduce solid solution and segregation of alloy elements.

(Hot rolling process)
In the present invention, in order to increase the density of austenite nucleation during reheating by refining and flattening prior austenite grains after hot rolling, the heated steel slab is hot rolled at a reduction of 20% or more, preferably 25% or more, more preferably 30% or more, and even more preferably 35% or more at 1000°C or less and over 825°C. If the reduction is less than 20%, prior austenite grains after hot rolling may not be sufficiently refined, resulting in a decrease in toughness. In order to prevent a decrease in hardenability due to excessive refinement of austenite grain size during reheating and quenching, the upper limit of the reduction at 1000°C or less and over 825°C is preferably 75% or less. In addition, in order to leave the reduction strain after hot rolling and cooling and increase the density of austenite nucleation during reheating, hot rolling is further performed at a reduction rate of 10% or more, preferably 15% or more, more preferably 20% or more at 825°C or less and 730°C or more (i.e., from 825°C to the temperature at the end of hot rolling). If this reduction rate is less than 10%, prior austenite grains after hot rolling may not be refined sufficiently, resulting in a decrease in toughness. In addition, in order to prevent a decrease in hardenability due to excessive refinement of austenite grains during reheating and quenching, the upper limit of the reduction rate at 825°C or less and 730°C or more is preferably 80%. Furthermore, if the rolling temperature in this hot rolling is less than 730°C, i.e., if the temperature at the end of hot rolling is less than 730°C, productivity may decrease.

(Cooling process)
Next, the hot-rolled steel sheet is cooled to 350°C or less at an average cooling rate of 5.0°C/s or more. The average cooling rate is preferably 6.0°C/s or more, more preferably 7.0°C/s or more. The cooling stop temperature is preferably 300°C or less, more preferably 250°C or less. If the average cooling rate is less than 5.0°C/s, the amount of strain in the steel becomes insufficient, so that the nucleation sites cannot be sufficiently increased. Even if the cooling stop temperature exceeds 350°C, the amount of strain is also insufficient, so that the nucleation sites cannot be sufficiently increased. If the steel sheet is cooled to 350°C or less at an average cooling rate of 5.0°C/s or more, sufficient strain can be imparted to the steel in the hot rolling process, and the nucleation sites can be increased. As a result, the austenite grains that have been reverse-transformed to austenite can be refined during reheating in the next reheating and quenching process. In the manufacturing method of the wear-resistant steel according to the present invention, an average cooling rate of 5.0° C./sec or more can typically be achieved by water cooling, but it is difficult to achieve this average cooling rate by natural cooling.

(Reheating and hardening process)
Next, the steel sheet cooled after hot rolling is reheated to a temperature of 860°C or higher, and then quenched by accelerated cooling. If the reheating temperature is less than 860°C, the solid solution of the alloy elements may be insufficient, and the austenite reverse transformation may not be completed 100%, resulting in a decrease in hardenability, so the lower limit of the reheating temperature is 860°C. The reheating temperature is preferably 880°C or higher, more preferably 900°C or higher. If the reheating temperature is too high, the toughness after quenching may decrease due to coarsening of austenite grains, so the upper limit of the reheating temperature is preferably 930°C or lower. It is preferable to perform the quenching at an average cooling rate of 5.0°C/second or higher in order to ensure hardness and toughness.

The thickness of the wear-resistant steel obtained by the manufacturing method of the wear-resistant steel according to the present invention is not particularly limited. For example, the thickness may be 15 mm or more, 20 mm or more, 30 mm or more, or 40 mm or more, and may be 100 mm or less, 90 mm or less, 80 mm or less, or 70 mm or less. According to the present invention, by controlling the temperature and reduction rate during hot rolling, increasing the average cooling rate after rolling, and further controlling the temperature of reheating and quenching, it is possible to appropriately refine the prior austenite grains and ensure sufficient quenchability regardless of the thickness of the plate. More specifically, by controlling the temperature and reduction rate during hot rolling, performing hot rolling, cooling at an average cooling rate of 5.0 ° C./sec or more, and then reheating at a temperature of 860 ° C. or more, it is possible to significantly increase the nucleation sites of austenite reverse transformation. As a result, it is possible to further refine the prior austenite grains inside the steel material after the reverse transformation to austenite is completed. This effect can be obtained regardless of the thickness of the steel plate, and can be applied even when the plate thickness is large (for example, 15 mm or more, particularly 40 mm or more), which was previously difficult to achieve. There is no need to limit the shape of the wear-resistant steel, but it can be a steel plate.

Abrasion-resistant steel manufactured under the above conditions through hot rolling, cooling, and reheating and quenching has excellent hardness and low-temperature toughness. Specifically, such abrasion-resistant steel has a Brinell hardness of 360 to 500 at the surface, and an absorbed energy of 150 J or more in a Charpy impact test at a position 1/4 of the way from the surface in the thickness direction at -40°C. Furthermore, the manufacturing method of the abrasion-resistant steel according to the present invention does not require advanced steelmaking technology, and can reduce the manufacturing load and shorten the construction period. Therefore, it makes a significant contribution to industry, such as improving the reliability of construction machinery without compromising economic efficiency.

[Physical properties of wear-resistant steel]
Next, the structure fraction and the average grain size of prior austenite of the wear-resistant steel manufactured by the manufacturing method of the present invention will be described. Usually, in a thick steel material (steel plate), the physical properties are often evaluated at a position of 1/4 of the thickness from the surface of the steel material in the thickness direction, where the average characteristics of the steel material in the thickness direction appear. Since the average cooling rate during reheating and quenching is faster in the surface layer of the steel plate than at a position of 1/4 of the thickness from the surface of the steel plate in the thickness direction, the total area fraction of martensite and lower bainite in the surface layer tends to be higher than their area fractions at a position of 1/4 of the thickness from the surface of the steel material in the thickness direction. Therefore, since the toughness in the surface layer tends to be higher than that at a position of 1/4 of the thickness from the surface in the thickness direction, if the toughness at a position of 1/4 of the thickness from the surface in the thickness direction is high, there is no problem in using the product with respect to the toughness in the surface layer. Similarly, the total area fraction of martensite and lower bainite in the surface layer is higher than the area fraction of martensite and lower bainite at a position ¼ of the thickness from the surface of the steel material in the thickness direction, so that the surface layer tends to have a higher hardness. Therefore, if 50% or more of the area of the structure at a position ¼ of the thickness from the surface of the steel material is one or both of martensite and lower bainite, the desired Brinell hardness of the surface layer is ensured in the wear-resistant steel produced according to the present invention.

(Tissue fraction)
In the wear-resistant steel produced by the present invention, at least 50 area %, preferably at least 60 area %, more preferably at least 70 area %, and even more preferably at least 80 area % of the structure at a position of 1/4 of the thickness from the surface in the thickness direction is one or both of martensite and lower bainite. If the total structure fraction of martensite and lower bainite is less than 50 area %, the toughness decreases. There is no particular upper limit for the total structure fraction of martensite and lower bainite, and it is sufficient that it is 100 area % or less. The structure fraction is determined by corroding a steel piece taken from a position of 1/4 of the thickness from the surface of the steel plate in the thickness direction with a nital solution and observing it with an electron microscope. Specifically, 20 x 20 straight lines are drawn vertically and horizontally at intervals of 10 μm on an image of a corroded steel piece taken with an electron microscope, and it is determined whether the structure at the lattice point is martensite, lower bainite, or upper bainite. Next, based on the result of the determination, the total area fraction (area %) of martensite and lower bainite at a position 1/4 of the thickness from the surface in the thickness direction is calculated. Here, in this specification, "upper bainite" refers to cementite present at the lath interface (between laths), and "lower bainite" refers to cementite present inside the laths. Lath refers to a metal structure generated within prior austenite grain boundaries by martensitic or bainite transformation.

(average prior austenite grain size)
The wear-resistant steel manufactured by the manufacturing method of the present invention has an average prior austenite grain size of 5 μm or more and 15 μm or less at a position of 1/4 of the thickness from the surface in the thickness direction. The upper limit of the average prior austenite grain size is preferably 14 μm, more preferably 13 μm or less. If the average prior austenite grain size exceeds 15 μm, the toughness decreases. If the average prior austenite grain size is less than 5 μm, the hardenability decreases and the strength cannot be guaranteed. The cutting method (JIS G0551:2013) is adopted to determine the average prior austenite grain size in the structure. Specifically, first, a steel piece taken from a position of 1/4 of the thickness from the surface in the thickness direction is corroded with a picric acid solution to reveal the prior austenite grain boundaries, and then photographed with an optical microscope. Next, a straight line (which may be divided into multiple parts) having a length of 2 mm to 10 mm is drawn on the captured image, and the number of grain boundaries divided by the straight line is counted to calculate the average prior austenite grain size at a position 1/4 of the thickness from the surface in the thickness direction.

(Brinell hardness of surface layer)
The hardness of steel is indicated by the Brinell hardness of the surface layer, and the Brinell hardness of the surface layer of the wear-resistant steel produced according to the present invention is in the range of 360 to 500. The "Brinell hardness of the surface layer" is the average value of the Brinell hardnesses measured at three points 1 mm from the surface of the steel in the thickness direction. The Brinell hardness is measured in accordance with JIS Z2243:2008, using a cemented carbide ball with a diameter of 10 mm as an indenter, with a test force of 3000 kgf (HBW10/3000). In the method for producing wear-resistant steel according to the present invention, the lower limit of the Brinell hardness is preferably 370 or more, more preferably 380 or more, even more preferably 390 or more, and most preferably 400 or more. The upper limit of the Brinell hardness may be, for example, 480, 460, or 450.

(Absorbed energy in Charpy impact test at -40°C)
The toughness of steel can be indicated by the absorbed energy in a Charpy impact test. For example, when evaluated by a Charpy impact test at -40°C, the absorbed energy of the wear-resistant steel produced by the present invention is 150 J or more, preferably 160 J or more, more preferably 170 J or more, and even more preferably 180 J or more. The Charpy impact test is performed in accordance with JIS Z2242:2005, using a Charpy test piece taken from a position 1/4 of the thickness in the thickness direction from the surface, at -40°C to evaluate low-temperature toughness.

The wear-resistant steel produced by the present invention, i.e., wear-resistant steel having the above-mentioned composition, in which 50% or more of the structure at a position 1/4 of the thickness from the surface in the thickness direction is one or both of martensite and lower bainite, and the average prior austenite grain size at a position 1/4 of the thickness from the surface in the thickness direction is 5 μm or more and 15 μm or less, has an absorbed energy of 150 J or more in a Charpy impact test at -40°C. In addition, since the structure fraction of martensite and lower bainite at a position 1/4 of the thickness is 50% or more by area, the surface layer, which has a fast average cooling rate during reheating and quenching, has a Brinell hardness of 360 to 500.

Steel having the composition shown in Table 1 was melted and then cast continuously to produce steel billets with a thickness of 240 to 300 mm. The steel was melted in a converter, and the steel was first deoxidized and alloy elements were added to adjust the composition, and vacuum degassing was performed as necessary. The steel billets thus obtained were heated, hot rolled, cooled, and then reheated and quenched to produce steel samples. The content of each element shown in Table 1 was determined by chemical analysis of samples taken from the steel after production.

The heating temperature of the steel slab during manufacturing, manufacturing conditions such as hot rolling, Brinell hardness of the surface layer of the manufactured sample, the structure fraction of martensite or lower bainite at a position 1/4 of the thickness from the surface in the thickness direction, the average prior austenite grain size at a position 1/4 of the thickness from the surface in the thickness direction, and the absorbed energy value in a Charpy impact test at -40°C at a position 1/4 of the thickness from the surface in the thickness direction are each shown in Table 1.

The martensite or lower bainite fraction of the structure at 1/4 of the thickness from the surface in the thickness direction can be determined by corroding the steel piece with a nital solution and observing it with an electron microscope, as described above. Specifically, 20 x 20 straight lines were drawn vertically and horizontally at 10 μm intervals on an image taken with an electron microscope, and it was determined whether the structure at the lattice point was martensite, lower bainite, or upper bainite, and the total fraction of martensite and lower bainite was calculated in area %. The area fraction of upper bainite was calculated assuming that cementite exists at the lath interface (between laths), and that lower bainite was calculated assuming that cementite exists inside the laths.

The average prior austenite grain size at a position 1/4 of the way from the surface in the thickness direction was calculated by corroding the steel piece with a picric acid solution to reveal the prior austenite grain boundaries, drawing a straight line 2 mm to 10 mm long (which may be divided into multiple parts) on an image taken with an optical microscope, and counting the number of grain boundaries divided by the straight line, as described above.

The Charpy impact test was performed at -40°C in accordance with JIS Z2242:2005. The Brinell hardness was measured in accordance with JIS Z2243:2008 using a cemented carbide ball with a diameter of 10 mm as an indenter at a test force of 3000 kgf (HBW10/3000).

The target values for hardness and toughness of the wear-resistant steel produced by the manufacturing method of the present invention are a Brinell hardness of the surface layer of 360 to 500 and an absorbed energy of 150 J or more in a Charpy impact test at -40°C.

As shown in Table 2, the compositions, heating temperatures, hot rolling reductions at 1000°C or less and over 825°C, hot rolling reductions at 825°C or less and 730°C or more, average cooling rates, cooling stop temperatures, and reheating temperatures of the present invention were within the ranges of the present invention for Production Nos. 1-4, 6-8, 10-11, 14-15, 17-18, and 21-27. As a result, the martensite or lower bainite structure fraction at 1/4 of the thickness from the surface in the thickness direction was 50 area % or more, and the average prior austenite grain size at 1/4 of the thickness from the surface in the thickness direction was 15 μm or less. Therefore, the Brinell hardness of the surface layer was within the range of 360-500, which is the target value of the present invention, and the absorbed energy of the Charpy impact test at -40°C was 150 J or more, which is the target value of the present invention.

On the other hand, in Table 2, Production No. 5, No. 9, No. 12, No. 13, No. 16, No. 19-20, and No. 28-46 did not meet the above targets in either or both of the Brinell hardness of the surface layer and the absorbed energy in the Charpy impact test at -40°C.

Production No. 5 was allowed to cool naturally without water cooling after rolling, resulting in a slow average cooling rate and a large prior gamma grain size. As a result, the Charpy absorbed energy at -40°C was 98 J, which is lower than the target value of the present invention. Production No. 9 also had a slow average cooling rate after rolling, resulting in a large prior gamma grain size, resulting in a Charpy absorbed energy at -40°C of 97 J, which is lower than the target value of the present invention.

Product No. 12 is an example in which the rolling reduction rate was low at temperatures below 1000°C and above 825°C, so the average prior austenite grain size at 1/4 of the thickness from the surface exceeded 15 μm, and as a result, the absorbed energy in the Charpy impact test at -40°C did not meet the target.

Product No. 13 is an example in which the rolling reduction rate was low at 825°C or less and 730°C or more, so the average prior austenite grain size at 1/4 of the thickness from the surface exceeded 15 μm, and as a result, the absorbed energy in the Charpy impact test at -40°C did not meet the target.

Product No. 16 was an example in which the reheating temperature was less than 860°C, which resulted in reduced hardenability, a martensite or lower bainite structure fraction of less than 50% by area, and the average prior austenite grain size at 1/4 of the thickness from the surface exceeded 15 μm. As a result, the Brinell hardness of the surface layer and the absorbed energy in the Charpy impact test at -40°C did not meet the targets.

Product No. 19 is an example in which the average cooling rate after rolling was slow, so the average prior austenite grain size at 1/4 of the thickness from the surface exceeded 15 μm, and as a result, the absorbed energy in the Charpy impact test at -40°C did not meet the target.

Product No. 20 is an example in which the cooling stop temperature was high, so the average prior austenite grain size at 1/4 of the thickness from the surface exceeded 15 μm, and as a result, the absorbed energy in the Charpy impact test at -40°C did not meet the target.

Production No. 28 is an example where the C content is low and the Brinell hardness of the surface layer did not reach the target. Production No. 29 is an example where the C content is high and the Brinell hardness of the surface layer and the absorbed energy of the Charpy impact test at -40°C did not reach the target values. Production No. 30 has a high Si content, Production No. 32 has a high Mn content, Production No. 33 has a high P content, Production No. 34 has a high S content, Production No. 36 has a high Cu content, Production No. 39 has a high Cr content, Production No. 40 has a high Mo content, Production No. 41 has a high V content, Production No. 42 has a high Nb content, Production No. 43 has a high Ti content, Production No. 44 has a high Al content, Production No. 45 has a high N content, and Production No. No. 46 had a high B content, so the absorbed energy in the Charpy impact test at -40°C did not reach the target value for any of the steel samples. No. 31 had a low Mn content, No. 35 had a low Cu content, No. 37 had a low Ni content, and No. 38 had a low Cr content, so the Brinell hardness of the surface layer did not reach the target value for any of the steel samples.

Claims (1)

In mass percent,
C: 0.10-0.30%,
Si: 0.01 to 1.20%,
Mn: 0.01-2.00%,
P: less than 0.017%;
S: 0.010% or less,
Ni: 0.01 to 1.00%,
Cu: 0.01 to 0.70%,
Cr: 0.30-1.50%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.030%, and N: 0.0001 to 0.0070%
and the balance being Fe and impurities, to 1000 to 1350°C;
hot rolling the heated steel slab at 1000°C or less and over 825°C with a reduction of 20% or more, and then hot rolling at 825°C or less and 730°C or more with a reduction of 10% or more;
A method for producing an abrasion-resistant steel having a Brinell hardness of 360 to 500 in a surface layer and an absorbed energy of 150 J or more in a Charpy impact test at -40°C at a position 1/4 of the thickness from the surface in the thickness direction, the method comprising the steps of: cooling a hot-rolled steel plate to 350°C or less at an average cooling rate of 5.0°C/sec or more ; and reheating the cooled steel plate to 860°C to 930°C or less, and then quenching the steel plate at an average cooling rate of 5.0°C/sec or more .