Steel sheet for surface hardening treatment and method for producing the same

Abstract

To provide a steel sheet for surface hardening treatment that exhibits superior sliding fatigue resistance.SOLUTION: A steel sheet for surface hardening treatment, having a predetermined component composition, wherein a microstructure at a position of 1 / 2 of the sheet thickness after surface hardening treatment contains 80% or more bainite in volume fraction, and the prior austenite grain size is 15 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet for surface hardening treatment and a method for producing the same. [Background technology]

[0002] Sliding components such as gears and bearings, which are used in a sliding state, require excellent wear resistance, and for this reason, steel parts used in such applications generally undergo surface hardening treatment to ensure wear resistance.

[0003] For example, Patent Document 1 proposes a technique for subjecting bearing steel parts having a predetermined chemical composition to carburizing and quenching or carbonitriding and tempering. According to Patent Document 1, by applying this technique, it is possible to obtain carburized bearing parts with excellent wear resistance, seizure resistance, and rolling fatigue life. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-97096 Summary of the Invention [Problem to be solved by the invention]

[0005] Meanwhile, in the field of vehicles such as automobiles and trains, electrification is rapidly progressing, and as a result, sliding members used in vehicles are required to be compatible with the high rotation speeds of motors.

[0006] That is, while the rotation speed of a conventional passenger car engine is less than 10,000 rpm, the rotation speed of even the most common motors used in electric vehicles is as high as several thousand rpm. Furthermore, because the output of a motor is determined by the product of torque and rotation speed, there is a demand to increase the rotation speed of motors to tens of thousands of rpm in order to further improve efficiency and reduce size.

[0007] However, the inventors' research has revealed that, in such an environment where components are used at high rotational speeds, conventional sliding components cannot sufficiently suppress sliding damage, even if they are surface-hardened. Sliding damage is damage caused by sliding fatigue due to rolling contact on the component surface, and is also known as pitting. In other words, minute cracks that appear on the component surface expand as the number of rolling contacts (number of sliding contacts) increases, resulting in large dents or holes on the component surface.

[0008] One way to suppress sliding fatigue is to use lubricating oil. However, depending on where the part is installed, the lubricating oil may not reach the part surface sufficiently. Therefore, it is necessary to improve the sliding fatigue resistance of the sliding members (parts) themselves without relying on lubricating oil.

[0009] The present invention has been made to solve the problem of sliding damage that has become apparent as motors rotate at higher speeds as described above, and an object of the present invention is to provide a steel sheet for surface hardening treatment that has excellent sliding fatigue resistance. [Means for solving the problem]

[0010] The present inventors have conducted studies to solve the above problems and have made the following findings.

[0011] When a steel sheet is subjected to a surface hardening treatment, a surface hardened layer in which carbides and nitrides are precipitated is formed on the surface of the steel sheet after the surface hardening treatment. Beneath the surface hardened treatment layer exists a diffusion layer that is affected by the diffusion of C and N. In other words, the diffusion layer is a region whose C and N contents differ from the composition of the steel sheet before the treatment. Beneath the diffusion layer exists a core portion that is not affected by the diffusion of C and N.

[0012] In conventional surface hardening treatments, steel is typically heated for carburizing or nitriding, and then quenched and tempered. As a result, the core microstructure of the steel plate after surface hardening is a tempered structure (martensite, tempered martensite) that has been hardened by quenching and tempering.

[0013] If the microstructure of the core is such a tempered structure, when a crack occurs in the surface-hardened layer, the crack easily propagates to the core, which is thought to be because the tempered structure has excellent strength but poor toughness.

[0014] Therefore, by making the microstructure of the core after surface hardening treatment a fine structure mainly composed of bainite, it is possible to prevent the propagation of cracks and improve the sliding fatigue resistance.

[0015] In order to make the microstructure of the core after surface hardening treatment a fine structure mainly composed of bainite, when manufacturing a steel plate for surface hardening treatment, it is necessary to control the chemical composition of the steel and to carry out hot rolling, annealing, and cold rolling under specific conditions.

[0016] The present invention has been completed based on the above findings, and has the following gist and configuration.

[0017] 1. By mass%, C: 0.15~0.39%, Si: 0.02 to 0.25%, Mn: 0.02 to 0.90% P: 0.0005~0.05%, S: 0.01% or less, Al: 0.005 to 0.1%, N: 0.001 to 0.015%, Cr: 0.30~4.30%, and Ni: 0.10 to 4.80%; The balance has a composition consisting of Fe and unavoidable impurities, The microstructure at the 1 / 2 position of the plate thickness after surface hardening treatment is Contains 80% or more bainite by volume fraction, and Steel plate for surface hardening treatment, with a prior austenite grain size of 15 μm or less.

[0018] 2. The component composition is, in mass%, Ti: 0.040% or less, B: 0.015% or less, Nb: 0.2% or less, Sb: 0.05% or less, Cu: 1.5% or less, Mo: 0.5% or less As: 0.5% or less, Sn: 0.8% or less, Ta: 0.5% or less, Ca: 0.5% or less, Mg: 0.5% or less, W: 0.5% or less, V: 0.5% or less, Pb: 0.5% or less, Zn: 0.5% or less, and REM: 1.0% or less 2. The steel sheet for surface hardening treatment according to 1 above, further comprising at least one selected from the group consisting of:

[0019] 3. A hot rolling step in which a steel slab having the chemical composition described in 1 or 2 above is hot rolled under conditions of a finish rolling delivery temperature of 800 to 980°C and a friction coefficient of 0.21 to 0.55 to obtain a hot rolled steel sheet, and then coiled at a coiling temperature of 350 to 690°C; An annealing process in which the hot-rolled steel sheet after the hot rolling process is annealed under the conditions of an annealing temperature of 660°C or more and a holding time of 5 hours or more; a cold rolling process in which the hot-rolled steel sheet after the annealing process is cold-rolled under conditions of a friction coefficient of 0.20 to 0.50 and a rolling reduction of 38% or more to prepare the hot-rolled steel sheet for surface hardening treatment; A method for manufacturing a steel plate for surface hardening treatment, comprising: [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a steel sheet for surface hardening treatment that has excellent sliding fatigue resistance. Here, the phrase "a steel sheet for surface hardening treatment has excellent sliding fatigue resistance" means that a surface hardened steel sheet obtained by subjecting the steel sheet for surface hardening treatment to a surface hardening treatment has excellent sliding fatigue resistance.

[0021] The steel plate for surface hardening treatment obtained by subjecting the steel plate for surface hardening treatment of the present invention to surface hardening treatment has excellent sliding fatigue resistance, and therefore the steel plate for surface hardening treatment of the present invention can be suitably used as a material for motors used at high rotation speeds and sliding members (e.g., gears, bearings, etc.) used in the vicinity thereof. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present invention will be described in detail. Note that the following description shows examples of preferred embodiments of the present invention, and the present invention is not limited thereto.

[0023] [Component composition] The steel sheet for surface hardening treatment according to one embodiment of the present invention has the following chemical composition. In the following description, "%" as a unit of content represents "% by mass" unless otherwise specified.

[0024] C: 0.15 to 0.39% C is an element that improves the toughness of steel sheets for surface hardening treatment and improves the sliding fatigue resistance by forming a bainite microstructure in the core of the steel sheet after surface hardening treatment. To achieve this effect, the C content is set to 0.15% or more, preferably 0.16% or more, and more preferably 0.18% or more. On the other hand, if the C content exceeds 0.39%, high-hardness carbides are formed in the steel after surface hardening treatment, making cracks more likely to propagate, resulting in a decrease in sliding fatigue resistance. Therefore, the C content is set to 0.39% or less, preferably 0.35% or less, and more preferably 0.32% or less.

[0025] Si: 0.02 to 0.25% Si is an element that improves the toughness of steel sheets for surface hardening treatment, thereby improving the sliding fatigue resistance of the steel sheets after surface hardening treatment. To achieve this effect, the Si content is set to 0.02% or more, preferably 0.04% or more, and more preferably 0.06% or more. On the other hand, if the Si content exceeds 0.25%, cracks tend to propagate more easily in the microstructure after surface hardening treatment, resulting in a decrease in sliding fatigue resistance. This is because the formation of ferrite during annealing causes carbon to concentrate, resulting in the formation of high-hardness cementite. Therefore, the Si content is set to 0.25% or less, preferably 0.22% or less, and more preferably 0.20% or less.

[0026] Mn: 0.02 to 0.90% Mn is an element that improves the toughness of bainite formed by surface hardening treatment, thereby improving sliding fatigue resistance. To achieve this effect, the Mn content is set to 0.02% or more, preferably 0.04% or more, and more preferably 0.06% or more. On the other hand, if the Mn content exceeds 0.90%, sliding fatigue resistance after surface hardening treatment decreases. This is because the formation of bainite is suppressed and brittle martensite is more likely to form. Therefore, the Mn content is set to 0.90% or less, preferably 0.85% or less, and more preferably 0.80% or less.

[0027] P: 0.0005 to 0.05% P is an element that improves the strength of bainite formed by surface hardening treatment, thereby improving sliding fatigue resistance. To achieve this effect, the P content is set to 0.0005% or more, preferably 0.0007% or more, and more preferably 0.0009% or more. On the other hand, if the P content exceeds 0.05%, sliding fatigue resistance decreases. This is because P segregates to grain boundaries in the microstructure after surface hardening treatment, causing embrittlement. Therefore, the P content is set to 0.05% or less, preferably 0.048% or less, and more preferably 0.046% or less.

[0028] S: 0.01% or less If the S content is too high, the sliding fatigue resistance decreases. This is because S forms manganese sulfides, which embrittle the steel sheet. Therefore, the S content is set to 0.01% or less, preferably 0.008% or less, and more preferably 0.006% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction leads to an increase in costs. Therefore, from the viewpoint of manufacturing costs, it is preferable that the S content be 0.0001% or more.

[0029] Al: 0.005 to 0.1% Al is an element necessary for deoxidation. If the Al content is less than 0.005%, oxides cannot be properly removed, resulting in surface defects such as scuffs during rolling. Therefore, the Al content is set to 0.005% or more. On the other hand, if the Al content exceeds 0.1%, coarse aluminum oxides are generated, which embrittles the steel sheet for surface hardening treatment. As a result, the microstructure after surface hardening treatment also becomes embrittled, reducing the sliding fatigue resistance. Therefore, the Al content is set to 0.10% or less. Preferably, it is set to 0.08% or less, and more preferably, it is set to 0.06% or less.

[0030] N: 0.001 to 0.015% N is an element that has the effect of improving the strength and toughness of steel sheets. Adding N improves toughness, and as a result, improves sliding fatigue resistance. Therefore, the N content is set to 0.001% or more, preferably 0.0012% or more, and more preferably 0.0014% or more. On the other hand, if the N content exceeds 0.015%, sliding fatigue resistance decreases. This is because N combines with Al and Ti to form precipitates on the surface of the steel sheet for surface hardening treatment. The precipitates make the surface more susceptible to cracking, resulting in embrittlement of the steel sheet for surface hardening treatment. Therefore, the N content is set to 0.015% or less, preferably 0.013% or less, and more preferably 0.011% or less.

[0031] Cr: 0.30~4.30% Cr is an element that suppresses ferrite formation during case hardening treatment and facilitates bainite transformation. Cr also has the effect of increasing the strength of steel sheets for case hardening treatment, thereby improving toughness. Therefore, adding Cr improves sliding fatigue resistance. Therefore, the Cr content is set to 0.30% or more, preferably 0.32% or more, and more preferably 0.34% or more. On the other hand, if the Cr content exceeds 4.30%, sliding fatigue resistance decreases. This is because Cr forms precipitates at grain boundaries, and these precipitates promote cracking of the steel sheet. Therefore, the Cr content is set to 4.30% or less, preferably 4.20% or less, and more preferably 4.10% or less.

[0032] Ni: 0.10 to 4.80% Like Cr, Ni is an element that inhibits ferrite formation during case hardening treatment and facilitates bainite transformation. Ni also has the effect of increasing the strength of steel sheets for case hardening treatment, thereby improving toughness. Therefore, adding Ni improves sliding fatigue resistance. Therefore, the Ni content is set to 0.10% or more, preferably 0.15% or more, and more preferably 0.20% or more. On the other hand, if the Ni content exceeds 4.80%, sliding fatigue resistance decreases. This is because niobium precipitates form at the grain boundaries of the steel sheets for case hardening treatment, promoting cracking of the steel sheets for case hardening treatment. Therefore, the Ni content is set to 4.80% or less, preferably 4.70% or less, and more preferably 4.60% or less.

[0033] In one embodiment of the present invention, the steel sheet has a composition containing the above elements, with the balance being Fe and unavoidable impurities. Incidentally, the unavoidable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are allowed to be present to a degree that does not impair the objectives of the present invention. Examples of the raw materials include iron ore, reduced iron, and scrap. Examples of the impurities include O, H, and Bi.

[0034] Furthermore, the chemical composition of the steel sheet in another embodiment of the present invention may optionally contain at least one of the elements listed below, from Ti to REM. By adding these elements, the properties of the steel sheet can be further improved. These elements can be added optionally depending on the desired properties, and therefore the lower limit of the content of each element may be 0%.

[0035] Ti: 0.040% or less, Ti is an element that inhibits grain growth during heating, and adding Ti can further improve sliding fatigue resistance. Furthermore, Ti preferentially bonds with nitrogen and oxygen, thereby inhibiting the formation of carbides and nitrides by other elements. Therefore, for example, adding Ti together with B can cause boron to remain in solid solution, further enhancing the strength-improving effect of boron. As a result, the sliding fatigue resistance of the steel sheet for surface hardening treatment can be further improved. However, if the Ti content exceeds 0.040%, the sliding fatigue resistance deteriorates. This is because coarse Ti precipitates form, embrittling the steel sheet for surface hardening treatment. Therefore, when Ti is contained, the Ti content is limited to 0.040% or less. The Ti content is preferably 0.030% or less, and more preferably 0.025% or less. On the other hand, there is no particular lower limit for the Ti content. However, when Ti is contained, in order to fully obtain the above-mentioned effects, the Ti content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.006% or more.

[0036] B: 0.015% or less B is an element that improves hardenability. Adding B further improves the strength of the steel sheet. Furthermore, by refining the crystal grains, toughness is further improved. As a result, sliding fatigue resistance is further improved. However, if the B content exceeds 0.015%, the steel sheet becomes embrittled and the sliding fatigue resistance deteriorates. This is due to the formation of boron nitrides. In other words, boron nitrides are hard, and cracks are likely to occur starting from the boron nitrides. Therefore, when B is contained, the B content is set to 0.015% or less. The B content is preferably set to 0.014% or less, and more preferably set to 0.013% or less. On the other hand, there is no particular lower limit for the B content. However, when B is added, in order to fully obtain the above-mentioned effects, the B content is preferably set to 0.0001% or more, more preferably set to 0.0002% or more, and even more preferably set to 0.0003% or more.

[0037] Nb: 0.2% or less Nb is an element that refines crystal grains, thereby further improving toughness. Adding Nb further improves sliding fatigue resistance. However, if the Nb content exceeds 0.2%, sliding fatigue resistance actually deteriorates. This is because coarse niobium precipitates are formed, promoting microcracks. Therefore, when Nb is contained, the Nb content is set to 0.2% or less. The Nb content is preferably set to 0.18% or less, and more preferably set to 0.16% or less. On the other hand, there is no particular lower limit for the Nb content. However, when Nb is contained, in order to fully obtain the above effects, the Nb content is preferably set to 0.003% or more, and more preferably set to 0.005% or more.

[0038] Sb: 0.05% or less Sb is an element that suppresses the formation of nitrides on the surface of a steel sheet regardless of the annealing atmosphere, thereby further improving the sliding fatigue resistance. However, if the Sb content exceeds 0.05%, the sliding fatigue resistance actually decreases. This is because Sb segregates at grain boundaries and embrittles the steel sheet. Therefore, when Sb is contained, the Sb content is set to 0.05% or less. On the other hand, there is no particular lower limit for the Sb content. However, when Sb is added, the Sb content is preferably set to 0.0003% or more in order to fully obtain the above-mentioned effects.

[0039] Cu: 1.5% or less, Cu is an element that improves the corrosion resistance of steel sheets for surface hardening treatment. However, if the Cu content exceeds 1.5%, a Cu-rich layer forms under the scale, causing surface defects. Therefore, when Cu is contained, the Cu content is set to 1.5% or less. The Cu content is preferably set to 1.4% or less, and more preferably set to 1.3% or less. On the other hand, there is no particular lower limit for the Cu content. However, when Cu is contained, in order to fully obtain the above-mentioned effects, the Cu content is preferably set to 0.001% or more, and more preferably set to 0.002% or more.

[0040] Mo: 0.5% or less Mo is an element that further improves strength and toughness, and adding Mo further improves sliding fatigue resistance. However, if the Mo content exceeds 0.5%, sliding fatigue resistance actually decreases. This is because precipitates form at grain boundaries, promoting embrittlement of the steel sheet. Therefore, when Mo is contained, the Mo content is set to 0.5% or less. The Mo content is preferably set to 0.48% or less, and more preferably set to 0.46% or less. On the other hand, there is no particular lower limit for the Mo content. However, when Mo is contained, in order to fully obtain the above effects, the Mo content is preferably set to 0.005% or more, and more preferably set to 0.008% or more.

[0041] The following elements can be optionally added to improve the lubricity, corrosion resistance, machinability, etc. of the steel sheet. However, if the amount of these elements exceeds the upper limit, they may segregate at the grain boundaries, reducing the resistance to torsional damage.

[0042] As: 0.5% or less When As is contained, the As content is set to 0.5% or less, preferably 0.48% or less, and more preferably 0.46% or less. On the other hand, although there is no particular lower limit for the As content, it is preferably set to 0.001% or more, and more preferably 0.002% or more.

[0043] Sn: 0.8% or less When Sn is contained, the Sn content is 0.8% or less, preferably 0.78% or less, and more preferably 0.76% or less. On the other hand, although there is no particular lower limit for the Sn content, it is preferably 0.001% or more, and more preferably 0.002% or more.

[0044] Ta: 0.5% or less When Ta is contained, the Ta content is set to 0.5% or less, preferably 0.48% or less, and more preferably 0.46% or less. On the other hand, although there is no particular lower limit for the Ta content, it is preferably set to 0.001% or more, and more preferably 0.002% or more.

[0045] Ca: 0.5% or less When Ca is contained, the Ca content is set to 0.5% or less, preferably 0.48% or less, more preferably 0.46% or less. On the other hand, although there is no particular limitation on the lower limit of the Ca content, it is preferably set to 0.001% or more, and more preferably 0.002% or more.

[0046] Mg: 0.5% or less When Mg is contained, the Mg content is set to 0.5% or less, preferably 0.48% or less, more preferably 0.46% or less. On the other hand, although there is no particular lower limit for the Mg content, it is preferably set to 0.001% or more, and more preferably 0.002% or more.

[0047] W: 0.5% or less When W is contained, the W content is 0.5% or less, preferably 0.48% or less, and more preferably 0.46% or less. On the other hand, although there is no particular lower limit for the W content, it is preferably 0.001% or more, and more preferably 0.002% or more.

[0048] V: 0.5% or less When V is contained, the V content is set to 0.5% or less, preferably 0.48% or less, and more preferably 0.46% or less. On the other hand, although there is no particular lower limit for the V content, it is preferably set to 0.001% or more, and more preferably set to 0.002% or more.

[0049] Pb: 0.5% or less When Pb is contained, the Pb content is 0.5% or less, preferably 0.48% or less, and more preferably 0.46% or less. On the other hand, although there is no particular lower limit for the Pb content, it is preferably 0.001% or more, and more preferably 0.002% or more.

[0050] Zn: 0.5% or less When Zn is contained, the Zn content is set to 0.5% or less, preferably 0.48% or less, more preferably 0.46% or less. On the other hand, although there is no particular lower limit for the Zn content, it is preferably set to 0.001% or more, and more preferably 0.002% or more.

[0051] REM: 1.0% or less When REM (rare earth metal) is contained, the REM content is 1.0% or less, preferably 0.8% or less, more preferably 0.6% or less. On the other hand, although there is no particular lower limit for the REM content, it is preferably 0.001% or more, and more preferably 0.002% or more.

[0052] [Microstructure after surface hardening treatment] The microstructure of the steel sheet for surface hardening treatment of the present invention at the half-thickness position after surface hardening treatment contains 80% or more bainite by volume fraction and the prior austenite grain size is 15 μm or less. The reason for this is explained below. In the present invention, the microstructure after surface hardening treatment is defined as the microstructure obtained after carburizing at a carburizing temperature of 930°C, a carburizing time of 1 hour, and a carbon potential of 1.1, followed by hot bath quenching at 430°C, and then cooling to room temperature.

[0053] Bainite: 80% or more As mentioned above, if the microstructure of the core of a steel sheet after surface hardening treatment is a hard tempered structure, cracks generated on the steel sheet surface will easily propagate within the tempered structure, making it impossible to obtain sufficient sliding fatigue resistance. In contrast, if the microstructure of the core is mainly bainite, toughness is improved and crack propagation is suppressed. As a result, sliding fatigue resistance is improved. Therefore, the volume fraction of bainite at the half-thickness position after surface hardening treatment is set to 80% or more. Meanwhile, the upper limit of the volume fraction of bainite is not particularly limited, and may be 100%, 98% or less, or 96% or less.

[0054] The microstructure at the half-thickness position after surface hardening treatment may contain any structure other than bainite in a total amount of 20% or less. The structure other than bainite is not particularly limited, but examples thereof include martensite, ferrite, and retained austenite.

[0055] Martensite has the same body-centered cubic (BCC) structure as bainite, and therefore has a relatively good affinity between the grain boundaries of the bainite and martensite structures, preventing embrittlement even under sliding loads. Therefore, martensite may be contained in the microstructure at a volume fraction of up to 20%. However, from the viewpoint of further improving sliding fatigue resistance, the lower the volume fraction of martensite, which is a brittle structure, the better. Therefore, the volume fraction of martensite is preferably 10% or less, and more preferably 8% or less.

[0056] Like martensite, ferrite has the same body-centered cubic (BCC) structure as bainite, and therefore does not become embrittled even when subjected to sliding loads. Therefore, ferrite may be contained in the microstructure at a volume fraction of up to 20%. However, a high ferrite volume fraction reduces strength. Therefore, from the perspective of further improving sliding fatigue resistance, the ferrite volume fraction is preferably 10% or less, more preferably 5% or less, and even more preferably 4% or less.

[0057] On the other hand, since austenite has a face-centered cubic (FCC) structure, it is desirable to reduce it as much as possible. Furthermore, if coarse cementite is generated in a steel sheet for surface hardening treatment during the hot rolling process used to manufacture the steel sheet, retained austenite with a high carbon concentration is likely to be generated in the microstructure after surface hardening treatment. This retained austenite with a high carbon concentration transforms into brittle martensite when subjected to sliding fatigue, resulting in embrittlement. Therefore, from the perspective of further improving sliding fatigue resistance, the volume fraction of retained austenite is preferably 5% or less, more preferably 3% or less, and even more preferably 0%.

[0058] In one preferred embodiment of the present invention, the microstructure of a steel plate for surface hardening treatment at a 1 / 2 position in the plate thickness direction after surface hardening treatment is, in terms of volume fraction, 80-100% bainite, 0-20% martensite, and It consists of 0-10% ferrite.

[0059] Prior austenite grain size: 15 μm or less In order to obtain the desired sliding fatigue resistance, the prior austenite grain size in the microstructure after surface hardening treatment must be 15 μm or less. This is because if the prior austenite grain size is 15 μm or less, the crystal grains become fine, resulting in improved toughness. On the other hand, there is no particular lower limit for the prior austenite grain size. However, from the viewpoint of ease of production, the prior austenite grain size is preferably 3 μm or more, and more preferably 5 μm or more.

[0060] When the microstructure at the half-thickness position after surface hardening satisfies the above conditions, excellent sliding fatigue resistance can be obtained.

[0061] Plate thickness The thickness of the steel plate for surface hardening treatment is not particularly limited and may be any thickness. However, if the plate thickness is excessively thin, it is difficult to obtain a core microstructure that is mainly bainite when surface hardening treatment is performed under standard conditions. Therefore, from the viewpoint of industrial production, the plate thickness is preferably 2.0 mm or more, and more preferably 2.2 mm or more. On the other hand, the upper limit of the plate thickness is not particularly limited, but the plate thickness is preferably 6.0 mm or less, and more preferably 5.6 mm or less.

[0062] [Manufacturing method] Next, a method for manufacturing a steel plate for surface hardening treatment according to one embodiment of the present invention will be described. The steel plate for surface hardening treatment can be manufactured by sequentially carrying out the following steps (1) to (3) on a steel slab having the above-mentioned chemical composition. (1) Hot rolling process (2) Annealing process (3) Cold rolling process

[0063] The conditions for each step are explained below. In the following explanation, the temperature control position and control method are the same as those of conventional methods used in the field. The temperature may be measured by a radiation thermometer or may be the set temperature of the heating furnace.

[0064] (Steel slab) The steel slab is not particularly limited, and any steel slab can be used as long as it has the above-mentioned composition. The method for producing the steel slab is also not particularly limited, and the steel slab can be produced by any method. For example, the composition of the steel slab may be adjusted by a blast furnace converter method or an electric furnace method. Furthermore, the casting of molten steel into a slab may be performed by a continuous casting method or by blooming. Scrap may be used as the raw material for the steel slab. Alternatively, the steel slab may be produced by mixing blast furnace pig iron and melted scrap.

[0065] After the steel slab is produced, it may be cooled once before the next hot rolling step. The cooling of the steel slab can be performed by any method. For example, the steel slab may be slowly cooled or may be exposed to the open air and allowed to cool. Furthermore, the steel slab may be heated for the purpose of temperature maintenance or soaking before the start of hot rolling.

[0066] It is also possible to employ direct rolling (mini-mill rolling), in which a steel slab is produced by continuous casting and then hot-rolled without being cooled.

[0067] (1) Hot rolling process In the hot rolling process, the steel slab is hot rolled to form a hot-rolled steel sheet, which is then coiled. In the hot rolling process, the finish rolling delivery temperature, friction coefficient, and coiling temperature must satisfy the following conditions.

[0068] Finishing rolling outlet temperature: 800~980℃ If the finish rolling exit temperature (FDT) in the hot rolling process is less than 800°C, the sliding fatigue resistance after surface hardening treatment will decrease. This is because hot rolling at a temperature range lower than the austenite single-phase temperature range generates coarse ferrite on the surface of the steel slab. The generated coarse ferrite remains even after the subsequent annealing and cold rolling processes. As a result, the crystal grains on the steel sheet surface will become coarse even after surface hardening treatment, reducing toughness. Therefore, the finish rolling exit temperature is set to 800°C or higher, preferably 810°C or higher, and more preferably 820°C or higher.

[0069] On the other hand, if the finish rolling delivery temperature in hot rolling exceeds 980°C, the prior austenite grains become coarse, making it impossible to obtain the desired sliding fatigue resistance. Therefore, the finish rolling delivery temperature is set to 980°C or less, preferably 975°C or less, and more preferably 970°C or less.

[0070] Friction coefficient: 0.21~0.55 As described above, in the present invention, it is important to suppress the coarsening of prior austenite grains in order to obtain the desired sliding fatigue resistance. In order to suppress the coarsening of prior austenite grains, it is necessary to control the finish rolling delivery temperature within the above-mentioned range and to control the friction coefficient in the hot rolling process.

[0071] If the friction coefficient is lower than 0.21, the prior austenite grains become coarse and the desired sliding fatigue resistance cannot be obtained, so the friction coefficient is set to 0.21 or more, preferably 0.22 or more, and more preferably 0.23 or more.

[0072] On the other hand, if the friction coefficient is higher than 0.55, the sliding fatigue resistance will actually decrease. This is because flat ferrite is generated on the steel sheet surface during the hot rolling process. The generated flat ferrite remains even after the subsequent annealing and cold rolling processes, resulting in coarse prior austenite grains after surface hardening. Therefore, the friction coefficient is set to 0.55 or less, preferably 0.53 or less, and more preferably 0.50 or less.

[0073] Here, the friction coefficient can be determined based on the angle between the material and the roll surface. Specifically, in the present invention, the value of the friction coefficient μ calculated by the following formula is used. μ=tanθ×A In the above formula, μ is the coefficient of friction, θ is the contact angle between the tangent to the roll surface and the horizontal surface of the material when the material is in contact with the roll, and A is an equipment constant. The equipment constant A varies depending on the rolling specifications such as the rolling material, the rolling roll material, the rolling speed, the rolling roll arrangement, the lubricant, and the rolling load, and generally takes a value of around 1. Therefore, in the present invention, the equipment constant A is set to 1 for convenience.

[0074] · Winding temperature: 350~690℃ If the coiling temperature is less than 350°C, hard martensite is formed in excess. As a result, the load in the subsequent cold rolling process increases, making cracks more likely to occur. Therefore, the coiling temperature is set to 350°C or higher, preferably 355°C or higher, and more preferably 360°C or higher. On the other hand, if the coiling temperature exceeds 690°C, the microstructure after surface hardening treatment does not satisfy the above-mentioned conditions, resulting in insufficient sliding fatigue resistance. This is because coarse cementite is formed during coiling. If coarse cementite is formed, the amount of retained austenite after surface hardening treatment increases, and the volume fraction of bainite becomes insufficient accordingly. Therefore, the coiling temperature is set to 690°C or lower, preferably 685°C or lower, and more preferably 690°C or lower.

[0075] After hot rolling, the steel sheet can be cooled so that the coiling temperature satisfies the above-mentioned condition. The cooling conditions after hot rolling are not particularly limited, but if cooling is started immediately after hot rolling, the sliding fatigue resistance after surface hardening treatment may be reduced. This is because ferrite is generated from the unrecrystallized, strain-containing surface rolled structure, and prior austenite grains coarsen during heating in the surface hardening treatment. Therefore, from the viewpoint of promoting recrystallization and further improving sliding fatigue resistance, it is preferable to start cooling 0.2 seconds or more after hot rolling.

[0076] (2) Annealing process Next, the hot-rolled steel sheet after the hot rolling step is annealed under the conditions of an annealing temperature of 660° C. or higher and a holding time of 5 hours or longer (annealing step).

[0077] Annealing temperature: 660°C or higher By performing annealing, cementite precipitated in the previous steps can be uniformly redissolved in the steel, promoting branching of the cementite. As a result, strain introduction in the subsequent cold rolling step can be uniformized, preventing the formation of a structure with locally coarse prior austenite grains. However, if the annealing temperature is lower than 660°C, the redissolution of cementite becomes non-uniform, resulting in coarse prior austenite grains in the microstructure after surface hardening treatment. Therefore, the annealing temperature is set to 660°C or higher, preferably 680°C or higher. On the other hand, the upper limit of the annealing temperature is not particularly limited, but it is typically preferably set to 780°C or lower.

[0078] Holding time: 5 hours or more In the annealing step, the annealing temperature is maintained for 5 hours or more. If the maintenance time is less than 5 hours, the redissolution of cementite becomes non-uniform, and prior austenite grains in the microstructure after the surface hardening treatment become coarse. On the other hand, although there is no upper limit on the maintenance time, since the structural change saturates after the decomposition of cementite begins, the maintenance temperature is preferably 50 hours or less in consideration of the efficiency of the manufacturing process.

[0079] The annealing can be carried out by any method, for example, a continuous annealing line (CAL), a batch annealing method, or a box annealing method.

[0080] (3) Cold rolling process In the cold rolling step, the hot-rolled steel sheet after the annealing step is cold-rolled to be used for surface hardening treatment. In the cold rolling step, the friction coefficient and rolling ratio must satisfy the following conditions.

[0081] Friction coefficient: 0.20~0.50 As described in the explanation of the hot rolling process, in the present invention, it is important to suppress the coarsening of prior austenite grains in order to obtain the desired sliding fatigue resistance. In order to suppress the coarsening of prior austenite grains, it is also necessary to control the friction coefficient in the cold rolling process.

[0082] If the friction coefficient in the cold rolling process is too low, the prior austenite grains will become coarse, making it impossible to obtain the desired sliding fatigue resistance. Therefore, the friction coefficient is set to 0.20 or more, preferably 0.22 or more, and more preferably 0.24 or more. On the other hand, if the friction coefficient is too high, the sliding fatigue resistance will decrease. This is because the structure will have localized accumulated processing strain, which will result in heterogeneous recrystallization during the annealing surface hardening treatment, resulting in a locally coarsened and softened bainite structure. Therefore, the friction coefficient is set to 0.50 or less, preferably 0.48 or less, and more preferably 0.46 or less.

[0083] Rolling ratio: 38% or more If the reduction ratio in cold rolling is less than 38%, the prior austenite grains in the microstructure after surface hardening treatment will become coarse. Therefore, the reduction ratio in cold rolling is set to 38% or more, preferably 40% or more. On the other hand, although there is no particular upper limit to the reduction ratio, from the viewpoint of manufacturing, it is generally preferably set to 75% or less, more preferably 70% or less.

[0084] Through the above steps, a steel sheet for surface hardening treatment that satisfies the above-mentioned conditions can be manufactured.

[0085] After the hot rolling, annealing, and cold rolling steps, a skin-pass rolling step may be optionally added for shape correction. After the annealing step and before the cold rolling step, pickling is preferably performed at least once.

[0086] The annealing step and cold rolling step may be repeated multiple times. The second and subsequent annealing steps and cold rolling steps may be performed under any conditions. In other words, it is sufficient that the first annealing step and cold rolling step satisfy the above-mentioned conditions.

[0087] The obtained steel sheet for surface hardening treatment can be used in any method. Typically, it is preferable to process the steel sheet for surface hardening treatment into a part shape and then perform surface hardening treatment. The processing into the part shape is not particularly limited and can be performed by any method. The processing may be, for example, at least one of punching, cutting, wire drawing, bending, and polishing. Annealing may be performed during processing to remove minor strain and soften the steel sheet.

[0088] The surface hardening treatment is also not particularly limited and can be carried out by any method. Typical surface hardening treatments include carburizing and nitriding. When carburizing is performed, the conditions are not particularly limited, and general carburizing conditions can be adopted. For example, carburizing and quenching are preferably carried out at a temperature of 900°C to 950°C and a carbon potential of 0.8 to 1.2. Quenching is preferably carried out by hot bath quenching. To obtain better properties, the hot bath quenching temperature is preferably 520°C to 400°C. Similarly, when nitriding is performed, the conditions are not particularly limited, and general nitriding conditions can be adopted. For example, it is preferable to perform pre-annealing at a temperature of 900°C to 950°C, followed by nitriding at a temperature of 500°C or less. [Example]

[0089] Steels having the chemical compositions shown in Tables 1 and 2, with the balance being Fe and unavoidable impurities, were melted in a converter and formed into steel slabs by continuous casting. The resulting steel slabs were then subjected to hot rolling, annealing, and cold rolling under the conditions shown in Tables 3 and 4, respectively, to obtain steel sheets for surface hardening treatment having a thickness of 2.0 mm.

[0090] The microstructure and sliding fatigue resistance of each of the obtained steel sheets for surface hardening treatment were evaluated after surface hardening treatment according to the following procedure. The evaluation results are shown in Tables 5 and 6.

[0091] (surface hardening treatment) First, each steel plate for surface hardening treatment was subjected to gas carburizing as a surface hardening treatment. The conditions for the gas carburizing treatment were a carburizing temperature of 930°C, a carburizing time of 1 hour, and a carbon potential of 1.1. Then, the steel plate was subjected to hot bath quenching at 430°C and allowed to cool to room temperature.

[0092] (Microstructure after surface hardening treatment) The microstructure at the half-thickness position after surface hardening treatment was evaluated using SEM (scanning electron microscope)-EBSD (electron backscatter diffraction). The specific procedure was as follows.

[0093] First, a test piece for microstructure observation was taken from the obtained surface-hardened steel sheet so that the observation surface was the half-thickness position in the rolling direction cross section (L cross section). Next, the surface of the test piece was polished, and further, etching was performed using a 3 vol% nital solution to reveal the microstructure. Thereafter, the microstructure was observed by SEM-EBSD. SEM observation was performed at a magnification of 1500x. EBSD measurement was performed under the conditions of an accelerating voltage of 15 kV, a step interval of 0.2 μm, and a magnification of 1000x.

[0094] Based on the results of the EBSD measurement, the structure within the field of view was divided into BCC and FCC phases, and the FCC phase was determined to be retained austenite. Next, the bainite, ferrite, and martensite contained in the BCC phase were identified based on the SEM observation image. Specifically, the flat area without carbides was identified as ferrite. Of the remaining area other than ferrite, the area with a lath pattern was identified as martensite, and the area containing carbides within the flat area was identified as bainite. The area fraction of each identified structure was calculated by image analysis. The above measurements were performed in two fields of view, and the average of the obtained area fractions was used as the volume fraction of each structure. Three measurements were performed for each example, and the average of the obtained volume fractions was used.

[0095] (Prior austenite grain size) Test pieces obtained in the same manner as above were subjected to γ-etching to reveal the prior austenite grain boundaries. SEM observation was then performed at a magnification of 3000x, and the prior austenite grain size was determined from the SEM images using the line segment method. Three measurements were performed for each example, and the average value of the obtained grain sizes was used.

[0096] (Sliding fatigue resistance) A 10 mm square test piece (sliding wear evaluation sample) was taken from the carburized (surface hardening) surface-hardened steel plate, and its sliding fatigue resistance was evaluated by a ball-on-disk wear test. The ball-on-disk wear test was conducted at room temperature without lubrication. The wear load was 10 N, the rotation radius was 5 mm, the sliding speed was 5.0 cm / s, and the number of rotations was 10,000. A 6 mm diameter WC carbide ball was used as the wear ball.

[0097] After the test, the thickness cross section of the test piece was observed using a digital microscope at a set magnification of 50x to check for the presence or absence of damage such as cracks at the 1 / 4 position in the thickness direction. The sliding fatigue resistance evaluation test was carried out on n=3 samples, with damage observations being carried out at five random locations per sample. If no damage such as cracks was found in the observation results, the sliding fatigue resistance was rated as "good," and if cracks were observed, the sliding fatigue resistance was rated as "insufficient."

[0098] In Example 52, the sliding fatigue resistance was evaluated after nitriding instead of the above-mentioned carburizing treatment. In the nitriding treatment, the test piece was heated at 910°C for 1 hour, and then gas nitriding was performed at 480°C for 18 hours in an NH3 + N2 atmosphere. However, the "microstructure after surface hardening treatment" shown in Table 4 is the microstructure after the above-mentioned gas carburizing treatment, as in the other examples.

[0099] [Table 1]

[0100] [Table 2]

[0101]

Table 3

[0102]

Table 4

Claims

1. In mass%, C: 0.15-0.39%, Si: 0.02-0.25%, Mn: 0.02 to 0.90%, P: 0.0005-0.05%, S: 0.01% or less, Al: 0.005-0.1%, N: 0.001 to 0.015%, Cr: 0.30 to 4.30%, and Ni: 0.10 to 4.80%; The balance has a composition consisting of Fe and unavoidable impurities, The microstructure at the 1 / 2 position of the plate thickness after surface hardening treatment is Contains 80% or more bainite by volume fraction, and A steel plate for surface hardening treatment, having a prior austenite grain size of 15 μm or less.

2. The component composition is, in mass%, Ti: 0.040% or less, B: 0.015% or less, Nb: 0.2% or less, Sb: 0.05% or less, Cu: 1.5% or less, Mo: 0.5% or less, As: 0.5% or less, Sn: 0.8% or less, Ta: 0.5% or less, Ca: 0.5% or less, Mg: 0.5% or less, W: 0.5% or less, V: 0.5% or less, Pb: 0.5% or less, Zn: 0.5% or less, and REM: 1.0% or less The steel sheet for surface hardening treatment according to claim 1, further comprising at least one selected from the group consisting of:

3. A hot rolling process in which a steel slab having the component composition according to claim 1 or 2 is hot rolled under conditions of a finish rolling delivery temperature of 800 to 980 ° C and a friction coefficient of 0.21 to 0.55 to obtain a hot rolled steel sheet, and the hot rolled steel sheet is coiled at a coiling temperature of 350 to 690 ° C; An annealing step of annealing the hot-rolled steel sheet after the hot rolling step under conditions of an annealing temperature of 660°C or more and a holding time of 5 hours or more; a cold rolling step of cold rolling the hot-rolled steel sheet after the annealing step under conditions of a friction coefficient of 0.20 to 0.50 and a rolling reduction of 38% or more to prepare the hot-rolled steel sheet for surface hardening treatment; A method for manufacturing a steel plate for surface hardening treatment, comprising:

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