Wear-resistant steel

Abstract

To provide HB450-grade wear-resistant steel having required strength and toughness without use of or with suppressing use of expensive elements such as Ni and Mo.SOLUTION: Wear-resistant steel has a predetermined chemical composition. In an area of 400 μm×400 μm, among crystal particles enclosed by high-angle particle boundaries of 15° or more, an average particle diameter of the ten largest particles is 40.0 μm or more; when a carbon content by mass is represented as [C], surface hardness at a position of 0.7 mm from a surface in a thickness direction is 400 HV10 or more and also 634×[C]1 / 2+140 HV10 or more. In an area of 400 μm×400 μm, the average particle diameter of the ten largest TiN particles is 7.0 μm or less and a maximum long-diameter of clustered Al2O3 is 30.0 μm or less; the number density of TiN particles having an equivalent circular diameter of 200 nm or less is 1×105 particles / mm2 or more; and a Charpy impact absorption energy at -40°C is 27 J or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to wear-resistant steels. [Background technology]

[0002] Steel components used in mining and civil engineering machinery often need to be replaced due to wear, and to extend their service life, wear-resistant steel plates with increased hardness are used in the manufacture of these components. In particular, HB450-class wear-resistant steel plates (HB410-490 on the Brinell hardness scale, Hv440-540 on the Vickers hardness scale) are often used.

[0003] Furthermore, in environments where impacts occurring during the mining and crushing of rocks and ores cause wear, hardness alone is insufficient to ensure wear resistance, and toughness evaluated by impact tests may be required. In addition, since such steel members are often used as consumables and are frequently replaced, there is a strong demand for wear-resistant steel plates that are inexpensive and easy to produce.

[0004] For example, Patent Documents 1 to 6 propose techniques for improving the toughness of wear-resistant steel and the like. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 10-204575 [Patent Document 2] Japanese Patent Application Publication No. 10-102185 [Patent Document 3] Japanese Patent Publication No. 59-129724 [Patent Document 4] Japanese Patent Application Publication No. 59-70721 [Patent Document 5] International Publication No. 2019 / 050010 [Patent Document 6] International Publication No. 2022 / 025135 Summary of the Invention [Problem to be solved by the invention]

[0006] The wear-resistant steels disclosed in Patent Documents 1 and 2 contain 1.0 mass % or more of Mo. The wear-resistant steels disclosed in Patent Documents 3 and 4 contain 2.0 mass % or more of Ni. Ni and Mo are expensive elements, and from the viewpoint of alloy costs, it is desirable to reduce their content. Patent Document 5 discloses a high-strength steel plate with improved toughness, but does not take into consideration wear resistance. The wear-resistant steel disclosed in Patent Document 6 uses inexpensive Cr to improve toughness, but remains within the range of HB400 class.

[0007] An object of the present disclosure is to provide an HB450 class wear-resistant steel that does not use or minimizes the use of expensive elements such as Ni and Mo and that has the necessary strength and toughness. [Means for solving the problem]

[0008] The above problems can be solved by the following means. <1> The chemical composition is, in mass%, C: 0.10~0.40%, Si: 0.01 to 2.50% Mn: 0.10~2.20% P:0.0400% or less, S: 0.0200% or less, Cr: over 1.50 ~ 7.50%, Al: 0.010 to 0.150%, Ti: 0.005% to 0.050%, B: 0.0002~0.0050%, N: 0.0015 to 0.0100%, and The balance is Fe and impurities. In a 400 μm × 400 μm area, among crystal grains surrounded by a high-angle grain boundary of 15° or more, the average grain size of the 10 crystal grains in descending order of grain size is 40.0 μm or less, In a 400 μm × 400 μm area, the average particle size of the 10 largest TiN particles is 7.0 μm or less, and the maximum major axis of the Al2O3 clusters is 30.0 μm or less, The number density of TiN particles with a circle equivalent diameter of 200 nm or less is 1×10 5 pieces / mm 2 That's all, When the C content in mass% is [C], the surface hardness at a position 0.7 mm from the surface in the thickness direction is 400 HV10 or more and 634 × [C] 1 / 2 +140HV10 or more, Charpy absorbed energy at -40°C is 27J or more, Wear-resistant steel. <2> The chemical composition is, in mass%, C: 0.10~0.40%, Si: 0.01 to 2.50% Mn: 0.10~2.20% P:0.0400% or less, S: 0.0200% or less, Cr: over 1.50 ~ 7.50%, Al: 0.010 to 0.150%, Ti: 0.005% to 0.050%, B: 0.0002~0.0050%, N: 0.0015 to 0.0100%, and further containing one or more elements selected from the group consisting of the following groups A to C: The balance is Fe and impurities. (Group A) Nb: 0.050% or less, Cu: 1.00% or less, Ni: less than 1.00% Mo: less than 1.00% V: 1.00% or less, and W: 1.00% or less One or more selected from the group consisting of (Group B) Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less One or more selected from the group consisting of (Group C) Sn: 0.50% or less, and Sb: 0.50% or less One or two selected from the group consisting of In a 400 μm × 400 μm area, among crystal grains surrounded by a high-angle grain boundary of 15° or more, the average grain size of the 10 crystal grains in descending order of grain size is 40.0 μm or less, In a 400 μm × 400 μm area, the average particle size of the 10 largest TiN particles is 7.0 μm or less, and the maximum major axis of the Al2O3 clusters is 30.0 μm or less, The number density of TiN particles with a circle equivalent diameter of 200 nm or less is 1×10 5 pieces / mm 2 That's all, When the C content in mass% is [C], the surface hardness at a position 0.7 mm from the surface in the thickness direction is 400 HV10 or more and 634 × [C] 1 / 2 +140HV10 or more, Charpy absorbed energy at -40°C is 27J or more, Wear-resistant steel. <3> containing the group A <2> The wear-resistant steel described in <4> containing the group B <2> The wear-resistant steel described in <5> containing the group C <2> The wear-resistant steel described in [Effects of the Invention]

[0009] According to the present disclosure, there is provided an HB450 class wear-resistant steel that does not use or uses only a small amount of expensive elements such as Ni and Mo, and yet has the necessary strength and toughness. [Brief explanation of the drawings]

[0010] [Figure 1]1 is a SEM image showing an example of clustered Al2O3. DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment that is an example of the present disclosure will be described. In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values written before and after "to" as the lower and upper limits. However, when the numerical values written before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values as the lower or upper limit. The content of an element in a chemical composition may be expressed by adding "amount" to the element symbol (for example, C amount, Si amount, etc.). When only the upper limit is stated for the content of an element in the chemical composition, it means that the element does not have to be contained. The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0012] The inventors of the present disclosure have considered the above problem as follows. To improve the toughness of wear-resistant steel, it is effective to suppress the occurrence of brittle fracture. To achieve this, it is necessary to refine the structure to reduce the fracture stress, as well as to control the brittle phase, which is the fracture initiation point. Therefore, the inventors investigated the fracture initiation points of many wear-resistant steels with poor toughness, and confirmed that the fracture initiation points were mostly TiN or Al2O3. Furthermore, the inventors of the present disclosure considered that the embrittling phases found in wear-resistant steels with poor toughness were often coarse and that toughness could be improved by refining TiN and Al2O3. The inventors then investigated the relationship between the size of the embrittled phase and toughness, and attempted to clarify the size of the embrittled phase required to achieve the target toughness in HB450 grade. As a result, they discovered that the target toughness can be achieved if TiN is 7.0 μm or less and Al2O3 is in the form of clusters with a major axis of 30.0 μm or less.

[0013] The present invention was made based on the above findings. The shape of the wear-resistant steel according to the present disclosure is not particularly limited, and may be a steel plate, a steel strip, a steel section, a steel pipe, a steel bar, a steel wire, etc. Hereinafter, a wear-resistant steel plate will be mainly described as a representative example of the wear-resistant steel according to the present disclosure.

[0014] <Chemical composition> The alloying elements that make up the chemical composition of the wear-resistant steel according to the present disclosure will be described below. In the following description of the alloying elements, "%" in the content means "% by mass."

[0015] C: 0.10 to 0.40% C is an element that can improve the hardenability of steel, convert the steel structure to martensite or bainite, increase hardness, and improve wear resistance. To achieve this effect, the C content is set to 0.10% or more, preferably 0.13% or more, and more preferably 0.16%. On the other hand, excessive C content generates cementite, significantly reducing toughness. For this reason, the C content is set to 0.40% or less, preferably less than 0.34%, and more preferably 0.30% or less.

[0016] Si: 0.01 to 2.50% Si is an element necessary for deoxidation, and also has the effect of increasing steel strength through solid solution strengthening. To obtain these effects, the Si content is set to 0.01% or more, preferably 0.05% or more, and more preferably 0.08% or more. On the other hand, excessive Si content significantly reduces toughness. For this reason, the Si content is set to 2.50% or less, preferably 2.0% or less, and more preferably 1.50% or less.

[0017] Mn: 0.10 to 2.20% Mn has the effect of improving hardenability, increasing the hardness of steel, and improving wear resistance. To achieve this effect, the Mn content is set to 0.10% or more, preferably 0.30% or more, and more preferably 0.50% or more. On the other hand, if Mn is contained in excess, toughness will decrease significantly. For this reason, the Mn content is set to 2.20% or less, preferably 1.80% or less, and more preferably 1.50% or less.

[0018] P:0.0400% or less P is inevitably contained in steel, and a high P content leads to a decrease in toughness. The lower the P content, the better, and the P content is set to 0.0400% or less, preferably 0.0200% or less.

[0019] S: 0.0200% or less Like P, S is also inevitably contained in steel, and a high S content leads to a decrease in toughness. The lower the S content, the better, and the S content is set to 0.0200% or less, preferably 0.0100% or less.

[0020] Cr: More than 1.50~7.50% Cr is an element that can improve hardenability and, as a substitute for expensive elements such as Ni and Mo, can increase not only the hardness of steel but also its toughness. It also contributes to improving strength. To achieve these effects, the Cr content is set to more than 1.50%, preferably more than 2.00%, and more preferably 2.20% or more. On the other hand, excessive Cr content not only saturates the effects, but is also undesirable from the standpoint of cost. For this reason, the Cr content is set to 7.50% or less, preferably 7.00% or less, and more preferably 6.00% or less.

[0021] Al: 0.010 to 0.150% Al traps harmful N, which causes solute B to precipitate as BN precipitates, and precipitates it as AlN, resulting in improved hardenability. It also acts as a deoxidizing element. To achieve this effect, the Al content is set to 0.010% or more, preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, excessive Al content causes AlN to coarsen, resulting in a decrease in toughness. The Al content is set to 0.150% or less, preferably 0.120% or less, and more preferably 0.100% or less.

[0022] Ti: 0.005 to 0.050% Ti forms carbonitrides (TiC, TiN) with C or N and precipitates finely, suppressing the growth of steel crystal grains and improving the toughness of the steel. Furthermore, the precipitation of TiN strongly traps harmful N, which causes solute B to precipitate as BN precipitates, ensuring the solute B in the steel, ultimately improving hardenability. To achieve this effect, the Ti content is 0.005% or more, preferably 0.010% or more. On the other hand, excessive Ti content coarsens the carbonitrides and reduces toughness. The Ti content is set to 0.050% or less, preferably 0.040% or less.

[0023] B: 0.0002 to 0.0050% B dissolves in steel, improving hardenability and increasing strength. The B content is 0.0002% or more, preferably 0.0005%, and more preferably 0.0008%. On the other hand, excessive B content deteriorates weldability. Therefore, when B is added, the B content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0024] N: 0.0015 to 0.0100% N is inevitably contained in steel. Excessive N content leads to a decrease in toughness. For this reason, the N content is set to 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less. On the other hand, when Al or Ti is contained in steel, N bonds with these to form nitrides (AlN, TiN). Nitrides can refine the structure of the steel and improve the strength of the steel. The N content is set to 0.0015% or more. The N content is preferably 0.0020% or more, and more preferably 0.0025% or more.

[0025] The above-mentioned elements are elements contained in the wear-resistant steel according to the present disclosure. The balance may be Fe and impurity elements, but other elements (optional additional elements) may also be contained within a range that does not impair the effects of the wear-resistant steel according to the present disclosure. The optional additional elements will be described below.

[0026] Nb: 0.050% or less Adding Nb increases the strength of steel by forming carbides with C and precipitating them finely. Furthermore, even if Nb dissolves in steel, it improves hardenability and increases the strength of steel. To achieve this effect, the Nb content is preferably 0.005% or more, and more preferably 0.008% or more. On the other hand, excessive Nb content generates coarse NbC, reducing toughness. The Nb content is set to 0.050% or less, and more preferably 0.040% or less.

[0027] Cu:1.00% or less Adding Cu improves hardenability and thereby increases the strength of the steel. To achieve this effect, it is preferable to add Cu to a content of 0.05% or more, more preferably 0.07% or more. On the other hand, if added in excess, the effect saturates and the manufacturing cost increases significantly. For this reason, when Cu is added, the Cu content is set to 1.00% or less, preferably 0.50% or less.

[0028] Ni: Less than 1.00% Adding Ni improves hardenability and thereby increases the strength of steel. To achieve this effect, it is preferable to add Ni in an amount of 0.01% or more, more preferably 0.08% or more. On the other hand, if added in excess, the effect saturates and the manufacturing cost increases significantly. For this reason, when Ni is added, the Ni content is set to less than 1.00%, preferably 0.50% or less.

[0029] Mo: Less than 1.00% Adding Mo improves hardenability and forms fine carbonitrides, thereby increasing the high-temperature strength of the steel. To achieve this effect, it is preferable to add Mo in an amount of 0.005% or more, and more preferably 0.08% or more. On the other hand, excessive addition reduces low-temperature toughness and, since Mo is an expensive element, increases manufacturing costs. For this reason, when Mo is added, the Mo content is set to less than 1.00%, and preferably 0.50% or less.

[0030] V: 1.00% or less Adding V can increase high-temperature hardness through precipitation strengthening by forming fine carbonitrides. To achieve this effect, it is preferable to add V in an amount of 0.005% or more, and more preferably 0.05% or more. On the other hand, excessive addition of V leads to a decrease in low-temperature toughness. Therefore, when V is added, the V content is set to 1.00% or less, and preferably 0.50% or less.

[0031] W: 1.00% or less Adding W improves hardenability and thereby increases the strength of steel. To achieve this effect, the W content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if excessive W is added, the effect saturates and the manufacturing cost increases significantly. For this reason, the W content is set to 1.00% or less, and preferably 0.50% or less.

[0032] Ca:0.0100% or less Mg: 0.0100% or less REM: 0.0100% or less The wear-resistant steel according to the present disclosure may contain at least one of Ca, Mg, and REM. Addition of Ca, Mg, and / or REM forms oxides and sulfides, which can control the morphology and dispersion of inclusions in the steel, thereby contributing to improved toughness. To achieve this effect, the Ca content is preferably 0.0005% or more, the Mg content is 0.0005% or more, and the REM content is preferably 0.0005% or more. More preferably, the Ca content is 0.0010% or more, the Mg content is 0.0010% or more, and the REM content is 0.0010% or more. On the other hand, adding too much of these elements will actually decrease toughness. Therefore, in the case of Ca, the Ca content is set to 0.0100% or less, preferably 0.0050% or less. In the case of Mg, the Mg content is set to 0.0100% or less, preferably 0.0050% or less. In the case of REM, the REM content is set to 0.0100% or less, more preferably 0.0050% or less. The term "rare earth elements (REM)" refers to a total of 17 elements, including two elements, Sc and Y, and 15 lanthanoid elements such as La, Ce, and Nd. The REM content refers to the total content of the 17 elements.

[0033] Sn: 0.50% or less Sb: 0.50% or less The wear-resistant steel according to the present disclosure may contain one or both of Sn and Sb. Addition of Sn and / or Sb improves corrosion resistance, and is expected to extend the service life of wear-resistant steel used in wet environments. The content of each of Sn and Sb is preferably 0.05% or more. On the other hand, excessive addition of Sn and Sb significantly deteriorates toughness. The content of each of Sn and Sb is 0.50% or less, preferably 0.40% or less.

[0034] <Organization> [In a 400 μm x 400 μm area, among the crystal grains surrounded by high-angle grain boundaries of 15° or more, the average grain size of the 10 largest crystal grains is 40.0 μm or less] In a 400 μm × 400 μm region, the average of the top 10 grain sizes (the 10 largest grain sizes) surrounded by high-angle grain boundaries with a crystal orientation misorientation of 15° or more is 40.0 μm or less. In this specification, the grain size of a grain surrounded by a high-angle grain boundary with a crystal orientation misorientation of 15° or more is referred to as the "high-angle grain size." As the high-angle grain size increases, fracture becomes more likely, and as the high-angle grain size decreases, toughness improves. The high-angle grain size is evaluated as the average value of the top 10 (the 10 largest) grains within a 400 μm × 400 μm region. The average of the top 10 high-angle grain sizes is more preferably 30.0 μm or less.

[0035] In this disclosure, the high-angle grain size is measured by electron backscattered diffraction (EBSD). EBSD measurements are performed in a 400 μm × 400 μm field of view at a 0.4 μm pitch. The grain size distribution is displayed using commercially available analysis software (OIM-Analysis, manufactured by TSL). The average grain size of the top 10 grains surrounded by high-angle grain boundaries with an angle of 15° or greater is calculated. When the thickness (t) of the wear-resistant steel plate is less than 16 mm, EBSD measurements are performed at the 1 / 2t portion (at a depth of t / 2 from the surface of the steel plate). When the thickness is 16 mm or greater, EBSD measurements are performed at the 1 / 4t portion (at a depth of t / 4 from the surface of the steel plate). The specimen used for EBSD measurements has a 10 mm square observation surface, and is cut from the steel plate at a position 1 / 4 of the plate width (1 / 4 width) from the end in the width direction. The observation surface is a cross section in the L (longitudinal) direction of the sample, which is electrolytically polished.

[0036] [In a 400 μm x 400 μm area, the average particle size of the 10 largest TiN particles is 7.0 μm or less, and the maximum major axis of the Al2O3 clusters is 30.0 μm or less] The mode of brittle fracture in the wear-resistant steel plate according to the present disclosure is primarily cleavage fracture, and the larger the size of the inclusions in the steel, the more likely they are to become the origin of fracture. From the studies of the inventors of the present disclosure, the size of the inclusions required to ensure the desired toughness is that the average particle size of the 10 largest TiN particles is 7.0 μm or less, and the maximum major axis of the 10 largest Al2O3 particles is 30.0 μm or less. Preferably, the average particle size of the 10 largest TiN particles is 6.0 μm or less, and the maximum major axis of the 10 largest Al2O3 particles is 20.0 μm or less.

[0037] The size of the inclusions is measured using a scanning electron microscope (hereinafter referred to as "SEM"). When the thickness of the wear-resistant steel plate is less than 16 mm, the size of the inclusions is measured at the 1 / 2t section. When the thickness of the wear-resistant steel plate is 16 mm or more, the size of the inclusions is measured at the 1 / 4t section. The observation surface of the sample used to measure inclusions is the L (longitudinal) cross section of the sample, which is mirror-polished. In the field of view where inclusions are determined by SEM observation (1000x magnification), the inclusion area is distinguished by image processing and its diameter is evaluated. For TiN, the average diameter is evaluated using the circle equivalent diameter. The measurement field area for the large-angle grain size and the measurement field area for the inclusions are both "400 μm×400 μm regions," but the regions measured do not need to be the same region.

[0038] On the other hand, for Al2O3, the length of the row of particles arranged in a row is evaluated as the long diameter of the Al2O3. Figure 1 is an SEM image showing an example of clustered Al2O3. The long diameter of the clustered Al2O3 particles arranged in a row is measured, and it is acceptable if all the long diameters of the clustered Al2O3 particles observed in an area of 400 μm x 400 μm are 30.0 μm or less, i.e., the maximum long diameter is 30.0 μm or less.

[0039] [Number density of TiN with a circle equivalent diameter of 200 nm or less: 1 × 10 5 pieces / mm 2 End] In the wear-resistant steel plate according to the present disclosure, in order to suppress coarsening of the structure, it is necessary to suppress coarsening of austenite grains during heating. In order to suppress coarsening of austenite grains, the higher the number density of TiN particles having a circle equivalent diameter of 200 nm or less (hereinafter, sometimes referred to as "TiN particles of 200 nm or less" or "micro TiN"), which are effective as pinning particles, the better. When the number density of TiN particles of 200 nm or less is 1×10 5 pieces / mm 2 Preferably, the number density of minute TiN particles is 3×10 or more. 5 pieces / mm 2 In particular, the upper limit of the number density of minute TiN is not specified, but in relation to the Ti content, the upper limit of the number density of TiN of 200 nm or less is 100 × 10 5 pieces / mm 2 may be.

[0040] The number density of minute TiN particles was measured using a transmission electron microscope (hereinafter referred to as "TEM"). In measuring the number density of minute TiN particles, samples were taken from the 1 / 2t section when the thickness of the wear-resistant steel plate was less than 16 mm, and from the 1 / 4t section when the thickness of the wear-resistant steel plate was 16 mm or more. A thin film sample was then prepared by electrolytic polishing, and TiN was identified using an energy dispersive X-ray spectroscopy attached to the TEM, and the number density of the particles was evaluated. The area of the thin film sample measured using the TEM was 100 μm 2 Measurements were taken at three locations in this area (10 μm×10 μm), and the average value was taken as the number density of minute TiN particles.

[0041] <Mechanical properties> [Surface hardness: 400HV10 or more and 634×[C] 1 / 2 +140HV10 or more] In order to ensure wear resistance, the surface hardness of the wear-resistant steel plate according to the present disclosure is set to 400 HV10 or more in Vickers hardness and 634 × [C] 1 / 2+140HV10 or more. [C] is the C content in the wear-resistant steel plate. The surface hardness of the wear-resistant steel plate is preferably 400HV10 or more and 634 × [C] 1 / 2 +160HV10 or more, more preferably 400HV10 or more and 634 × [C] 1 / 2 +180HV10 or more. The higher the surface hardness, the better, and there is no upper limit. From the viewpoint of ensuring toughness, the surface hardness of the wear-resistant steel plate is 634 × [C] 1 / 2 It may be +230HV10 or less.

[0042] The surface hardness of the wear-resistant steel plate is measured at a position 0.7 mm from the surface in the thickness direction, taking into account the effects of decarburization. The Vickers hardness test is conducted in accordance with JIS Z 2244:2009, and the load is 10 kgf. The Vickers hardness is the average value of three points measured on the L (longitudinal) cross section of the sample. The surface hardness must be 400 HV10 or more in Vickers hardness and 634 × [C]. 1 / 2 If the hardness is +140HV10 or more, it can be determined that the total area ratio of ferrite and retained austenite is less than 5%. From the viewpoint of ensuring absolute wear resistance regardless of the C content, rather than relative wear resistance taking the C content into consideration, the surface hardness of the wear-resistant steel plate according to the present disclosure is preferably 410 HV10 or more, more preferably 420 HV10 or more, and most preferably 450 HV10 or more.

[0043] [Charpy absorbed energy at -40°C: 27J or more] Considering use in cold regions and high altitudes, a wear-resistant steel plate that is resistant to fracture even at low temperatures is desirable, and the evaluation temperature for Charpy absorbed energy in this disclosure is set to -40°C. The absorbed energy that can withstand impacts during processing and use is 27 J or more, and more preferably 50 J or more. Charpy tests are conducted in accordance with JIS Z 2242:2018 using test pieces with V-notches. The Charpy test pieces are taken from the 1 / 4th of the thickness direction for steel materials with a thickness of 16 mm or more, and from the 1 / 2th of the thickness direction for steel plates thinner than 16 mm. The Charpy test pieces are taken with the rolling direction as the longitudinal direction. The test specimen shape is a full-size 2mm V-notch Charpy test specimen measuring 10mm x 10mm x 55mm, with a V-notch cut parallel to the plate thickness direction. Note that, as full-size 2mm V-notch Charpy test specimens cannot be taken from steel plates with a thickness of 12mm or less, sub-size 2mm V-notch Charpy test specimens measuring 5mm x 10mm x 55mm are used. In addition, to take into account variability, the absorbed energy is evaluated as the average of three measured values.

[0044] <Manufacturing method> Next, a method for manufacturing a wear-resistant steel plate according to the present disclosure will be described using a steel plate as an example. The wear-resistant steel plate according to the present disclosure is manufactured by melting steel, adjusting the composition, and casting the resulting steel billet as a steel material. The steel material is hot-rolled and either quenched as is or air-cooled. After air-cooling, the steel is reheated and quenched. After hot-rolling and quenching, the steel may be reheated and quenched.

[0045] The method for producing the steel material used to manufacture the wear-resistant steel plate according to the present disclosure is not limited, and the steel material may be produced by a known method. For example, steel billets are produced by melting in a conventional refining process using a converter, electric furnace, or the like, and then by a known method such as continuous casting or ingot-making / blooming. The billet is preferably cooled after casting and c3 The slab is reheated to a temperature above the transformation point and hot rolled. If the slab after continuous casting is charged into a heating furnace by hot charging without being cooled to 400°C or below, the coarse austenite formed during casting may remain in the slab after heating. In order to promote the refinement of the structure of the wear-resistant steel, it is preferable that the slab after continuous casting is once cooled to 400°C or below. The heating temperature of the steel slab before hot rolling is preferably 900°C or higher, more preferably 1000°C or higher, and even more preferably 1100°C or higher. The heating temperature is preferably 1250°C or lower to suppress coarsening of crystal grains and coarsening of TiN and Al2O3. The heating temperature is more preferably 1200°C or lower, and even more preferably 1150°C or lower. In hot rolling, the surface temperature of the rolled material is A r3 It is performed in a temperature range above the transformation point. r3 The transformation point is the temperature at which the transformation from austenite to ferrite begins upon cooling, and is calculated based on the chemical composition of the steel using the formula described below.

[0046] In hot rolling, from the viewpoint of ensuring low-temperature toughness, it is preferable to roll the slab so that the grain size is smaller than the austenite grain size at the time of heating. In order to introduce dislocations into the slab by rolling while promoting recrystallization and obtaining a fine structure, hot rolling is performed with a reduction ratio of 50% or more in a temperature range of 1000°C or higher. A preferable reduction ratio is 60% or more. The reduction ratio in a temperature range of 1000°C or higher (simply referred to as "reduction ratio") is calculated from the thickness of the steel material before hot rolling and the thickness of the material to be rolled at 1000°C using the following formula: Reduction rate (%) = 100 × {(thickness of steel material) - (thickness of material being rolled at 1000°C)} / thickness of steel material

[0047] The finishing temperature of hot rolling is set to A from the viewpoint of preventing the formation of ferrite. r3 The temperature is equal to or higher than the transformation point. When the rolled material is quenched as is after the hot rolling is completed, the finishing temperature of the hot rolling is 770°C or higher. On the other hand, from the viewpoint of refining the metal structure, the finishing temperature of the hot rolling is preferably 900°C or lower, and more preferably 850°C or lower. The thickness of the rolled material after hot rolling is the thickness of the wear-resistant steel, and is, for example, 8 mm or more and 50 mm or less. If the reduction rate in hot rolling is small and the thickness is too large, the prior γ grain size will coarsen, resulting in insufficient toughness, and there will be regions with insufficient hardenability, which may result in the formation of upper bainite and insufficient toughness. If the thickness is 50 mm or less, sufficient toughness can be ensured. On the other hand, if the thickness is 8 mm or more, sufficient strength as wear-resistant steel can be ensured.

[0048] After hot rolling, the rolled material is either quenched directly or air-cooled. The air-cooled rolled material is reheated and quenched. The process in which the rolled material is quenched directly after hot rolling is called direct quenching (sometimes abbreviated as "DQ"). The process in which the rolled material is air-cooled, reheated, and quenched after hot rolling is called reheat quenching (sometimes abbreviated as "RQ"). The rolled material that has been quenched after hot rolling may be reheated and quenched. The reheating temperature is set at A from the viewpoint of preventing the formation of ferrite. c3 The reheating temperature is equal to or higher than the transformation point. The reheating temperature is preferably 800°C or higher. The upper limit of the reheating temperature is preferably 900°C or lower.

[0049] The following explanations of the starting temperature, cooling rate, and stopping temperature of quenching do not differ between direct quenching and reheat quenching. The starting temperature for quenching is 770°C or higher to prevent the formation of ferrite. The cooling rate for quenching is 3°C / second or higher at the surface temperature of the material to be rolled to prevent the formation of ferrite and suppress the precipitation and growth of carbides. The faster the cooling rate, the better, but there are limits depending on the capacity of the cooling equipment, the thickness of the material to be rolled, etc., and it is usually 50°C / second or lower. The cooling rate is a value calculated using the following formula. Cooling rate = (surface temperature at the start of cooling - surface temperature at the end of cooling) / cooling time Here, the "surface temperature when cooling is stopped" is the surface temperature of the rolled material at the time when water cooling is stopped (not the reheat temperature).

[0050] The quenching stop temperature is 200°C or lower in terms of the surface temperature of the rolled material from the viewpoint of preventing the formation of ferrite and suppressing the precipitation and growth of carbides. The quenching stop temperature is preferably 100°C or lower in terms of the surface temperature of the rolled material, particularly from the viewpoint of suppressing the growth of carbides. where A c3 is the transformation start temperature when heated, calculated as follows, A r3 is the transformation start temperature during cooling calculated as follows, and is calculated using the chemical composition of the steel. A c3 (℃) = 902-255×C+19×Si-11×Mn-5×Cr+13×Mo-20×Ni+55×V A r3 (℃) = 868 - 396 × C + 24.6 × Si - 68.1 × Mn - 24.8 × Cr In the above formula, C, Si, Mn, Cr, Mo, Ni, and V represent the content of each element expressed in mass %.

[0051] <Application> The uses of the wear-resistant steel according to the present disclosure are not particularly limited, but from the viewpoint of excellent strength, toughness, and wear resistance, it is suitable as a steel material used in severe wear environments, such as mining machinery and construction machinery. The thickness of the wear-resistant steel (wear-resistant steel material) according to the present disclosure is not particularly limited, and is, for example, 8 mm or more and 50 mm or less. The thickness of the steel material may be 10 mm or more, or 16 mm or more. The thickness of the steel material may be 45 mm or less, or 40 mm or less. When the wear-resistant steel according to the present disclosure is a steel bar, the diameter may be regarded as the thickness, and Vickers hardness measurement and Charpy hardness test may be performed. [Example]

[0052] Examples of the present disclosure will be described below. However, the examples described below are merely examples of the present disclosure, and the present disclosure is not limited to the examples described below.

[0053] <Example of invention> Steel billets (steel materials) having the chemical compositions shown in Table 1 were heated, soaked, hot rolled, and quenched (DQ, RQ) under the manufacturing conditions shown in Table 2 to produce steel plates (Nos. 1 to 30) with thicknesses of 8 to 50 mm.

[0054] [Table 1]

[0055] [Table 2]

[0056] The manufactured steel sheets were subjected to microstructural observation, Vickers hardness testing, and Charpy hardness testing using the methods described above. The results are shown in Tables 3 and 4. In Table 3, the "longest diameter of Al2O3" refers to the maximum long diameter of the observed cluster-like Al2O3. In Table 4, the "lower hardness limit" refers to the value calculated for each steel sheet by "634 × [C]1 / 2 + 140HV10." The same applies to Tables 7 and 8 described below.

[0057] [Table 3]

[0058] [Table 4]

[0059] <Comparative Example> Steel plates (Nos. 101 to 114) having thicknesses of 15 to 50 mm were produced from steel billets (steel materials) having the chemical compositions shown in Table 5 by heating, soaking, hot rolling, and quenching (DQ, RQ) under the production conditions shown in Table 6. Steel plates Nos. 101 to 108 are examples of steel plates having chemical compositions different from those of the wear-resistant steel according to the present disclosure, and steel plates Nos. 109 to 114 are examples of wear-resistant steel plates produced by production methods different from those described above.

[0060] [Table 5]

[0061] [Table 6]

[0062] The produced steel sheets were subjected to microstructural observation, Vickers hardness testing, and Charpy hardness testing by the methods described above. The results are shown in Tables 7 and 8.

[0063] [Table 7]

[0064] [Table 8]

[0065] As shown in Tables 1 to 4, steel plates that satisfy the requirements of the present disclosure do not contain Ni or Mo, or the content of each is kept to less than 1.00%, and also have excellent wear resistance (surface hardness) and low-temperature toughness (absorbed energy).

[0066] As shown in Tables 5 to 8, steel plates that do not satisfy the requirements of the present disclosure are inferior in wear resistance (surface layer hardness) and / or low-temperature toughness (absorbed energy). Steel plate No. 101 has a low C content and is unable to increase the surface hardness, resulting in poor wear resistance. Steel plate No. 102 has a high C content and high strength, but is inferior in low-temperature toughness. Steel plate No. 103 has a high Mn content, which causes the formation of MnS, which becomes the starting point for fracture, resulting in poor low-temperature toughness. The steel plate No. 104 has a low Cr content and the large-angle grains are coarse, so it has poor low-temperature toughness. Steel plate No. 105 has an excessive amount of Ti, which causes coarsening of TiN, which becomes the starting point for fracture, resulting in poor low-temperature toughness. Steel plate No. 106 has an excessive amount of N, which causes TiN to coarsen, which becomes the starting point for fracture, resulting in poor low-temperature toughness. Steel plate No. 107 lacks Ti and N, and nano-sized TiN, which acts as a pinning particle, is not sufficiently produced. As a result, the gamma grain size becomes coarse, and the large-angle grain size also becomes coarse, resulting in poor low-temperature toughness. Steel plate No. 108 has an excessive amount of Al, which causes a large amount of Al2O3 to precipitate, which becomes the starting point for fracture, resulting in poor low-temperature toughness. The steel plate No. 109 was poor in low-temperature toughness because the heating temperature of the billet was high, which caused the γ grains to become coarse and the large-angle grains to become coarse. The No. 110 steel plate has a low rolling reduction rate at 1000°C or higher, which means that the γ grains cannot be refined and the large-angle grain size becomes large, resulting in poor low-temperature toughness. The No. 111 steel plate has a high rolling finish temperature and insufficient rolling in the non-recrystallized region, which prevents grain refinement and results in a large large-angle grain size, resulting in poor low-temperature toughness. The No. 112 steel plate has a low rolling end temperature, a large reduction in the two-phase region (the temperature region from the start to the end of ferrite transformation), ferrite precipitates during rolling, and the surface hardness is low, resulting in poor wear resistance. The No. 113 steel plate has a low cooling start temperature, a large reduction in the two-phase region, ferrite precipitates during rolling, and the surface hardness is low, resulting in poor wear resistance. The No. 114 steel plate has a slow cooling rate, which causes ferrite to precipitate during cooling, resulting in a low surface hardness and poor wear resistance.

Claims

1. The chemical composition, in mass%, is C: 0.10-0.40%, Si: 0.01-2.50%, Mn: 0.10-2.20%, P: 0.0400% or less, S: 0.0200% or less, Cr: more than 1.50 to 7.50%, Al: 0.010-0.150%, Ti: 0.005% to 0.050%, B: 0.0002 to 0.0050%, N: 0.0015 to 0.0100%, and The balance is Fe and impurities. In a 400 μm × 400 μm region, among crystal grains surrounded by a high-angle grain boundary of 15° or more, the average grain size of the 10 crystal grains in descending order of grain size is 40.0 μm or less, In a 400 μm×400 μm area, the average particle size of the 10 largest TiN particles is 7.0 μm or less, and clustered Al ２ O ３ The maximum major axis is 30.0 μm or less, The number density of TiN particles having a circle equivalent diameter of 200 nm or less is 1×10 ５ pieces / mm ２ That's all, When the C content in mass% is [C], the surface hardness at a position 0.7 mm from the surface in the thickness direction is 400 HV10 or more and 634 × [C] １／２ +140HV10 or more, Charpy absorbed energy at -40°C is 27J or more, Wear-resistant steel.

2. The chemical composition, in mass%, is C: 0.10-0.40%, Si: 0.01-2.50%, Mn: 0.10-2.20%, P: 0.0400% or less, S: 0.0200% or less, Cr: more than 1.50 to 7.50%, Al: 0.010-0.150%, Ti: 0.005% to 0.050%, B: 0.0002 to 0.0050%, N: 0.0015 to 0.0100%, and further containing one or more elements selected from the group consisting of the following groups A to C: The balance is Fe and impurities. (Group A) Nb: 0.050% or less, Cu: 1.00% or less, Ni: less than 1.00% Mo: less than 1.00% V: 1.00% or less, and W: 1.00% or less One or more selected from the group consisting of (Group B) Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less One or more selected from the group consisting of (Group C) Sn: 0.50% or less, and Sb: 0.50% or less One or two selected from the group consisting of In a 400 μm × 400 μm region, among crystal grains surrounded by a high-angle grain boundary of 15° or more, the average grain size of the 10 crystal grains in descending order of grain size is 40.0 μm or less, In a 400 μm×400 μm area, the average particle size of the 10 largest TiN particles is 7.0 μm or less, and cluster-like Al ２ O ３ The maximum major axis is 30.0 μm or less, The number density of TiN particles having a circle equivalent diameter of 200 nm or less is 1×10 ５ pieces / mm ２ That's all, When the C content in mass% is [C], the surface hardness at a position 0.7 mm from the surface in the thickness direction is 400 HV10 or more and 634 × [C] １／２ +140HV10 or more, Charpy absorbed energy at -40°C is 27J or more, Wear-resistant steel.

3. The wear-resistant steel according to claim 2, containing said Group A alloy.

4. The wear-resistant steel according to claim 2, containing the B group.

5. The wear-resistant steel according to claim 2, containing the group C elements.

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