WO2019044970A1 - 浸炭用鋼板、及び、浸炭用鋼板の製造方法 - Google Patents
浸炭用鋼板、及び、浸炭用鋼板の製造方法 Download PDFInfo
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- C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
Definitions
- the present invention relates to a steel plate for carburizing and a method of manufacturing a steel plate for carburizing.
- the "extreme deformability” is a physical property value given by the natural logarithm of the cross-sectional shrinkage at the fracture portion of the tensile test piece, and is known to have a positive correlation with the hole expansibility. From this point of view, various techniques have been proposed in recent years.
- Patent Document 1 a technique is proposed in which the structure of a hot-rolled steel sheet is composed of ferrite and pearlite, and then spheroidizing annealing is performed to spheroidize the carbide.
- Patent Document 2 after controlling the grain size of carbides, the ratio of the number of carbides in ferrite grain boundaries to the number of carbides in ferrite grains is controlled, and further, the crystal grains of ferrite as a matrix phase There has been proposed a technique for improving the impact characteristics of a member after carburizing by controlling the diameter.
- the cold workability is controlled by further controlling the aspect ratio of the ferrite after controlling the grain size and aspect ratio of the carbide and the crystal grain size of the ferrite which is the matrix. Techniques to improve have been proposed.
- Patent No. 3094856 International Publication No. 2016/190370 International Publication No. 2016/148037
- the mechanical structure parts as described above are required to be hardenable in order to increase the strength. That is, in order to form a member having a complicated shape by cold working, it is required to secure hole expandability (that is, to realize excellent ultimate deformability) while maintaining hardenability. .
- an object of the present invention is to provide a steel plate for carburizing steel exhibiting a superior ultimate deformability before carburizing and a method for producing the same. .
- the present inventors diligently studied about the method of solving the above-mentioned subject. As a result, as described in detail below, while appropriately controlling the X-ray random strength ratio of the predetermined orientation group in the ferrite crystal grains by controlling the texture of the ferrite in the hot-rolled steel plate, the hardenability is maintained With the idea that it is possible to improve the hole expansibility (that is, to impart excellent ultimate deformability), the present invention has been completed.
- the gist of the present invention completed based on this idea is as follows.
- Al 0.0002% or more and 3.0% or less
- N 0.2% or less
- the average ratio of the X-ray random strength ratio of the orientation group is 7.0 or less
- the average equivalent circle diameter of the carbide is 5.0 ⁇ m or less
- the proportion of the number of carbides having an aspect ratio of 2.0 or less is A steel sheet for carburizing steel, which is 80% or more with respect to total carbides, and the number ratio of carbides present in ferrite grains is 60% or more with respect to all carbides.
- [3] 1 part or less of Sn: 1% or less, W: 1.0% or less, Ca: 0.01% or less, REM: 0.3% or less, in mass%, in place of the remaining part of Fe
- the steel sheet on which the above is applied is defined by the following formula (1) at an average heating rate of 5.degree. C./h to 100.degree. C./h. heated to a temperature range of less than 1 point, annealing treatment for holding 10h or 100h following temperature range below the Ac 1 point
- a method of manufacturing a steel sheet for carburizing comprising the steps of: performing an annealing step of setting the average cooling rate in the temperature range from the temperature at the end of annealing to 550 ° C. to 5 ° C./h to 100 ° C./h .
- [X] represents content (unit: mass%) of the element X, and when not containing an applicable element, suppose that zero is substituted.
- the inventors of the present invention have conducted the above-mentioned tissue control, and further focus on the improvement of the hole expansibility by controlling the texture of the parent phase ferrite, and investigate in detail the effects of the texture control. And studied. As a result, it has been found that by controlling the X-ray random intensity ratio of a specific crystal orientation group, the hole expansibility is dramatically improved.
- the inventors set the average value of the X-ray random intensity ratio of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups of ferrite crystal grains to 7.0 or less. By controlling, it has been found that the hole expandability is dramatically improved. The reason why the above-mentioned X-ray random intensity ratio of crystal orientation groups is important for hole expandability is not necessarily clear, but it is presumed to be related to the susceptibility to cracking when expanding holes. Be done.
- the holes are further expanded by controlling the X-ray random strength ratio of a specific crystal orientation group in ferrite crystal grains. Succeeded in dramatically improving sex.
- the present inventors came to the idea that the X-ray random intensity ratio of a specific crystal orientation group in ferrite crystal grains can be controlled by controlling the finish rolling conditions in the hot rolling step.
- the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups are crystal grains of ferrite formed upon phase transformation from unrecrystallized austenite. Therefore, the formation of these specific crystal orientation groups can be reduced by promoting recrystallization of austenite by controlling the finish rolling conditions, and as a result, ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 in ferrite crystal grains. It has been found that it is possible to control the X-ray random intensity ratio of the ⁇ 110> orientation group to 7.0 or less.
- the improvement of the hole expansibility by controlling the X-ray random strength ratio of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups in the ferrite crystal grains to 7.0 or less is a steel plate with high hardenability.
- the hole expansibility is significantly improved. Therefore, it becomes possible to improve hole expansibility, maintaining hardenability by structure control as outlined above. Thereby, it is possible to obtain a steel plate for carburizing which has both of the hardenability and the hole expandability.
- a carburizing steel plate and a method for manufacturing the same according to an embodiment of the present invention have been completed based on the above-described findings. Below, the steel plate for carburization concerning this embodiment completed based on this knowledge, and its manufacturing method are explained in detail.
- the steel plate for carburizing according to the present embodiment has a predetermined chemical component as described in detail below.
- the average value of the X-ray random intensity ratio of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups of ferrite crystal grains is 7.0 or less
- the average circle equivalent diameter of the carbides is 5.0 ⁇ m or less
- the number ratio of the carbides having an aspect ratio of 2.0 or less is 80% or more to all the carbides, and the carbides present in the ferrite crystal grains It has a specific microstructure in which the proportion by number is 60% or more of the total carbides.
- C is an element necessary to secure the strength of the central portion of the thickness of the finally obtained carburized member.
- C is an element which is dissolved in the grain boundaries of ferrite to increase the strength of the grain boundaries and contributes to the improvement of the hole expansibility.
- the content of C is set to 0.02% or more.
- the content of C is preferably 0.05% or more.
- the C content is less than 0.30%.
- the content of C is preferably 0.20% or less. Further, in consideration of the balance between the hole expandability and the hardenability, the content of C is more preferably 0.10% or less.
- Si silicon
- Si is an element which acts to deoxidize molten steel to make the steel sound.
- the content of Si is set to 0.005% or more.
- the content of Si is preferably 0.01% or more.
- Si dissolved in carbide stabilizes the carbide, the average equivalent circle diameter of the carbide exceeds 5.0 ⁇ m, and the hole expansibility is impaired.
- the content of Si is less than 0.5%.
- the content of Si is preferably less than 0.3%.
- Mn manganese
- Mn manganese
- Mn is an element having a function of deoxidizing molten steel to make the steel sound.
- the content of Mn is 0.01% or more.
- the content of Mn is preferably 0.1% or more.
- Mn dissolved in carbide stabilizes the carbide and the average equivalent circle diameter of the carbide exceeds 5.0 ⁇ m, and the hole expansibility deteriorates.
- the content of Mn is less than 3.0.
- the content of Mn is preferably less than 2.0%, more preferably less than 1.0%.
- P phosphorus
- P is an element which segregates in grain boundaries of ferrite to deteriorate the hole expansibility.
- the content of P is preferably 0.050% or less, more preferably 0.020% or less.
- the lower limit of the content of P is not particularly limited. However, if the content of P is reduced to less than 0.0001%, the de-P cost increases significantly, which is economically disadvantageous. Therefore, on the practical steel plate, the content of P is substantially lower than 0.0001%.
- S sulfur
- S is an element that forms inclusions and degrades the hole expansibility.
- the content of S exceeds 0.1%, coarse inclusions are generated to reduce the hole expansibility. Therefore, in the steel sheet for carburizing according to the present embodiment, the content of S is 0.1% or less.
- the content of S is preferably 0.010% or less, more preferably 0.008% or less.
- the lower limit of the content of S is not particularly limited. However, if the S content is reduced to less than 0.0005%, the de-S cost increases significantly, which is economically disadvantageous. Therefore, 0.0005% of the content of S is a practical lower limit on a practical steel plate.
- Al (aluminum) is an element which acts to deoxidize the molten steel to make the steel sound. If the content of Al is less than 0.0002%, the molten steel can not be sufficiently deoxidized. Therefore, in the steel sheet for carburizing according to the present embodiment, the content of Al (more specifically, the content of sol. Al) is set to 0.0002% or more.
- the content of Al is preferably 0.0010% or more.
- the content of Al is 3.0% or less.
- the content of Al is preferably 2.5% or less, more preferably 1.0% or less, still more preferably 0.5% or less, and still more preferably 0.1% or less.
- N nitrogen
- nitrogen is an impurity element and is an element which forms a nitride to inhibit the hole expansibility.
- the content of N exceeds 0.2%, coarse nitrides are formed and the hole expansibility is significantly reduced. Therefore, in the steel sheet for carburizing according to the present embodiment, the content of N is 0.2% or less.
- the content of N is preferably 0.1% or less, more preferably 0.02% or less, and still more preferably 0.01% or less.
- the lower limit of the content of N is not particularly limited. However, if the content of N is reduced to less than 0.0001%, the de-N cost increases significantly, which is economically disadvantageous. Therefore, on the practical steel plate, the content of N is substantially lower than 0.0001%.
- Cr 0.005% or more and 3.0% or less
- Cr Cr (chromium) is an element having the effect of enhancing the hardenability in the finally obtained carburized member, and in the case of a steel plate for carburizing, it contributes to the further improvement of the hole expansibility by refining the ferrite crystal grains. Is an element that Therefore, in the steel sheet for carburizing according to the present embodiment, Cr may be contained as needed. When Cr is contained, it is preferable to make the content of Cr 0.005% or more in order to obtain the effect of further improving the hole expansibility. The content of Cr is more preferably 0.010% or more.
- the content of Cr is preferably 3.0% or less in order to obtain the effect of further improving the hole expansibility.
- the content of Cr is more preferably 2.0% or less, still more preferably 1.5% or less.
- Mo mobdenum
- Mo mobdenum
- Mo is an element having the effect of enhancing the hardenability in the finally obtained carburized member, and in the steel plate for carburizing, it contributes to the further improvement of the hole expansibility by refining the ferrite crystal grains. Is an element that Therefore, in the steel sheet for carburizing according to the present embodiment, Mo may be contained as needed. When Mo is contained, it is preferable to make the content of Mo 0.005% or more in order to obtain the effect of further improving the hole expansibility.
- the content of Mo is more preferably 0.010% or more. Further, in consideration of the influence of the formation of carbides and nitrides, the content of Mo is preferably 1.0% or less in order to obtain a further improvement effect of the hole expansibility.
- the content of Mo is more preferably 0.8% or less.
- Ni 0.010% or more and 3.0% or less
- Ni Ni (Nickel) is an element having the effect of enhancing the hardenability in the finally obtained carburized member, and in the steel plate for carburizing, it contributes to the further improvement of the hole expansibility by refining the ferrite crystal grains. Is an element that Therefore, in the steel sheet for carburizing according to the present embodiment, Ni may be contained as needed. When Ni is contained, it is preferable to make the content of Ni 0.010% or more in order to obtain a further improvement effect of the hole spreading property. The content of Ni is more preferably 0.050% or more.
- the content of Ni is preferably 3.0% or less in order to obtain a further improvement effect of the hole expansibility.
- the content of Ni is more preferably 2.0% or less, still more preferably 1.0% or less, and still more preferably 0.5% or less.
- Cu (copper) is an element having the effect of enhancing the hardenability in the finally obtained carburized member, and in the steel plate for carburizing, it contributes to the further improvement of the hole expansibility by refining the ferrite crystal grains. Is an element that Therefore, in the steel sheet for carburizing according to the present embodiment, Cu may be contained as needed.
- Cu in order to acquire the further improvement effect of hole spreading property, it is preferable to make content of Cu into 0.001% or more.
- the content of Cu is more preferably 0.010% or more. Further, in consideration of the influence of Cu segregating at grain boundaries, the content of Cu is preferably 2.0% or less in order to obtain a further improvement effect of the hole expansibility.
- the content of Cu is more preferably 0.80% or less.
- Co is an element having the effect of enhancing the hardenability in the finally obtained carburized member, and in the case of a steel plate for carburizing, it contributes to the further improvement of the hole expandability by refining the ferrite crystal grains. Is an element that Therefore, in the steel sheet for carburizing according to the present embodiment, Co may be contained as needed. When Co is contained, it is preferable to make the content of Co 0.001% or more in order to obtain a further improvement effect of the hole spreading property.
- the content of Co is more preferably 0.010% or more.
- the content of Co is preferably 2.0% or less in order to obtain a further improvement effect on the hole expansibility.
- the content of Co is more preferably 0.80% or less.
- Nb (niobium) is an element which contributes to the further improvement of the hole expandability by refining the ferrite crystal grains. Therefore, in the steel plate for carburizing according to the present embodiment, Nb may be contained as needed.
- the content of Nb is preferably made 0.010% or more in order to obtain a further improvement effect of the hole expansibility.
- the content of Nb is more preferably 0.035% or more.
- the content of Nb is preferably 0.150% or less in order to obtain a further improvement effect of the hole expansibility.
- the content of Nb is more preferably 0.120% or less, still more preferably 0.100% or less.
- Ti is an element which contributes to the further improvement of the hole expandability by refining the ferrite crystal grains. Therefore, in the steel plate for carburizing according to the present embodiment, Ti may be contained as needed. When Ti is contained, it is preferable to make the content of Ti 0.010% or more in order to obtain a further improvement effect of the hole expansibility.
- the content of Ti is more preferably 0.035% or more. Further, in consideration of the influence of the formation of carbides and nitrides, the content of Ti is preferably 0.150% or less in order to obtain a further improvement effect of the hole expansibility.
- the content of Ti is more preferably 0.120% or less, still more preferably 0.100% or less, still more preferably 0.050% or less, still more preferably 0.020% or less is there.
- V vanadium
- V vanadium
- the content of V is preferably 0.0005% or more in order to obtain a further improvement effect of the hole expansibility.
- the content of V is more preferably 0.0010% or more.
- the content of V is preferably 1.0% or less in order to obtain the effect of further improving the hole expansibility.
- the content of V is more preferably 0.80% or less, still more preferably 0.10% or less, and still more preferably 0.080% or less.
- B is an element that improves the strength of the grain boundaries by segregating in the grain boundaries of ferrite and further improves the hole expansibility. Therefore, in the steel plate for carburizing according to the present embodiment, B may be contained as needed.
- the content of B is preferably set to 0.0005% or more in order to obtain a further improvement effect of the hole expansibility.
- the content of B is more preferably 0.0010% or more.
- the content of B is preferably 0.01% or less.
- the content of B is more preferably 0.0075% or less, still more preferably 0.0050% or less, and still more preferably 0.0020% or less.
- Sn (tin) is an element that acts to deoxidize the molten steel to further strengthen the steel. Therefore, in the steel sheet for carburizing according to the present embodiment, Sn may be contained with the upper limit being 1.0%, as necessary. The content of Sn is more preferably 0.5% or less.
- W (tungsten) is an element which acts to deoxidize the molten steel to further strengthen the steel. Therefore, in the steel sheet for carburizing according to the present embodiment, W may be contained with an upper limit of 1.0% as necessary. The content of W is more preferably 0.5% or less.
- Ca (calcium) is an element that acts to deoxidize the molten steel to further strengthen the steel. Therefore, in the steel sheet for carburizing according to the present embodiment, Ca may be contained, with the upper limit being 0.01%, if necessary. The content of Ca is more preferably 0.006% or less.
- REM 0.3% or less
- REM rare earth metal
- REM is an element which acts to deoxidize molten steel to further strengthen the steel. Therefore, in the steel sheet for carburizing according to the present embodiment, REM may be contained, with the upper limit being 0.3%, if necessary.
- REM is a general term for a total of 17 elements which consist of Sc (scandium), Y (yttrium), and the element of a lanthanoid series, and content of REM means the total amount of the said element.
- REM is often contained using a misch metal, but in addition to La (lanthanum) and Ce (cerium), elements of a lanthanoid series may be contained in a composite. Also in such a case, the steel plate for carburizing according to the present embodiment exhibits excellent ultimate deformability. Further, even if the metal REM such as the metal La or Ce is contained, the steel sheet for carburizing according to the present embodiment exhibits excellent ultimate deformability.
- the rest Fe and impurities
- the balance of the component composition at the center of the plate thickness is Fe and impurities.
- the impurities include elements which are mixed from steel raw materials or scraps and / or unavoidably mixed in the steel making process and which do not impair the characteristics of the steel sheet for carburizing according to the present embodiment.
- the microstructure of the steel plate for carburizing according to the present embodiment is substantially composed of ferrite and carbide. More specifically, in the microstructure of the steel sheet for carburizing according to the present embodiment, the area ratio of ferrite is, for example, in the range of 80 to 95%, and the area ratio of carbide is, for example, in the range of 5 to 20%. And the total area ratio of ferrite and carbide does not exceed 100%.
- the area ratio of ferrite and carbide as described above is measured using a sample taken with the cross section perpendicular to the width direction of the carburizing steel sheet as the observation surface.
- the length of the sample may be about 10 mm to 25 mm depending on the measuring device.
- the sample is nital etched after polishing the observation surface.
- the plate thickness 1/4 position meaning the position of 1/4 of the thickness of the steel plate in the thickness direction of the steel plate from the surface of the steel plate for carburizing
- the plate thickness 3/8 position the range of 1/2 thickness position is observed with a thermal field emission scanning electron microscope (for example, JSM-7001F manufactured by JEOL).
- the observation range of each sample is observed in 10 fields of view in a range of 2500 ⁇ m 2 , and in each field of view, the ratio of the area occupied by ferrite and carbide in the field of view is measured. Then, the average value of the proportion of the area occupied by ferrite in the entire visual field and the average value of the proportion of the area occupied by the carbide in the entire visual field are respectively the area proportion of ferrite and the area proportion of the carbide.
- the carbides in the microstructure according to this embodiment are mainly iron-based carbides such as cementite (Fe 3 C) which is a compound of iron and carbon, and ⁇ -based carbides (Fe 2 to 3 C).
- the carbides in the microstructure are compounds obtained by replacing Fe atoms in cementite with Mn, Cr, etc. in addition to the above-described iron-based carbides, alloy carbides (M 23 C 6 , M 6 C, MC, etc.) M may include Fe and other metal elements, or metal elements other than Fe.
- Most of the carbides in the microstructure according to the present embodiment are composed of iron-based carbides.
- the number may be the total number of various carbides as described above, or only the number of iron-based carbides. May be That is, as described in detail below, various proportions of the number of carbides may be those having various carbides including iron-based carbides as a population, or only iron-based carbides as a population. May be Iron-based carbides can be identified using, for example, difraction analysis or EDS (Energy dispersive X-ray spectrometry) on a sample.
- EDS Electronic dispersive X-ray spectrometry
- the present inventors also found that the crystal orientation of ferrite also greatly affects the hole expansibility.
- deformation progresses due to the orientation rotation of the crystal grains of ferrite.
- cracks can not be generated from the grain boundaries without resisting the deformation. Therefore, it has become clear that it is possible to improve the hole expansibility by controlling the amount of crystal grains that are hard to rotate in orientation.
- the reason for limitation of the microstructure which comprises the steel plate for carburization concerning this embodiment is explained in detail.
- the average value of the X-ray random intensity ratio of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups of ferrite crystal grains is 7.0 or less.
- the average value of the X-ray random intensity ratio of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups of ferrite crystal grains is 7.0 or less, good hole expansion It became clear that you could get sex.
- the average value of the above-mentioned X-ray random strength ratio exceeds 7.0, generation of cracks is promoted at the time of hole expansion, and good hole expansibility can not be obtained.
- the average value of the X-ray random strength ratio is set to 7.0 or less.
- the average value of the X-ray random intensity ratio is preferably 5.5 or less in order to further improve the ultimate deformability.
- the lower limit of the X-ray random intensity ratio is not particularly limited, but in consideration of the current general continuous hot rolling process, 0.5 is the lower limit of the substance.
- the orientation perpendicular to the plate surface is represented by [hkl] or ⁇ hkl ⁇
- the orientation parallel to the rolling direction is represented by (uvw) or ⁇ uvw>.
- ⁇ Hkl ⁇ and ⁇ uvw> are generic terms for equivalent surfaces.
- the main orientations included in ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups of ferrite crystal grains are ⁇ 100 ⁇ ⁇ 011>, ⁇ 116 ⁇ ⁇ 110>, ⁇ 114 ⁇ ⁇ 110>, ⁇ 113 ⁇ ⁇ 110>, ⁇ 112 ⁇ ⁇ 110>, ⁇ 335 ⁇ ⁇ 110>, and ⁇ 223 ⁇ ⁇ 110>.
- a sample is cut out of a carburized steel sheet so that a cross section (plate thickness cross section) perpendicular to the surface can be observed.
- the length of the sample may be about 10 mm to 25 mm depending on the measuring device.
- the 1/4 position of the thickness of the sample is measured at a measurement interval of 0.1 ⁇ m by using Electron Back Scattering Diffraction (EBSD) to obtain crystal orientation information.
- EBSD Electron Back Scattering Diffraction
- EBSD analysis uses, for example, an apparatus configured of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC type 5 detector manufactured by TSL), and an electron beam acceleration voltage of 15 kV to 25 kV , At an analysis speed of 200 to 300 points / second.
- JSM-7001F thermal field emission scanning electron microscope
- DVC type 5 detector DVC type 5 detector manufactured by TSL
- an electron beam acceleration voltage 15 kV to 25 kV
- the carbide in the present embodiment is mainly composed of iron-based carbides such as cementite (Fe 3 C) and ⁇ -based carbides (Fe 2 to 3 C).
- Fe 3 C cementite
- ⁇ -based carbides Fe 2 to 3 C
- the lower limit of the number ratio of carbides having an aspect ratio of 2.0 or less among all carbides is set to 80%.
- the proportion of the number of carbides having an aspect ratio of 2.0 or less among all the carbides is preferably 85% or more for the purpose of further improving the hole expansibility.
- the upper limit of the proportion of the number of carbides having an aspect ratio of 2.0 or less among all the carbides is not particularly limited. However, since it is difficult to achieve 98% or more in actual operation, 98% is a practical upper limit.
- the proportion of the number of carbides present in ferrite grains among the total carbides is preferably 65% or more for the purpose of further improving the hole expansibility.
- the upper limit of the proportion of the number of carbides present in the ferrite crystal grains among all the carbides is not particularly limited. However, since it is difficult to achieve 98% or more in actual operation, 98% is a practical upper limit.
- the average equivalent circle diameter of carbides needs to be 5.0 ⁇ m or less.
- the average equivalent circle diameter of carbides exceeds 5.0 ⁇ m, cracking occurs at the time of punching, and it is not possible to obtain good hole expansibility.
- the lower limit of the average equivalent circle diameter of the carbide is not particularly limited. However, since it is difficult to set the average equivalent circle diameter of the carbides to 0.01 ⁇ m or less in actual operation, 0.01 ⁇ m is a practical lower limit.
- a sample is cut out so that a cross section (plate thickness cross section) perpendicular to the surface can be observed from the carburizing steel sheet.
- the length of the sample may be about 10 mm depending on the measuring device.
- the cross section is polished and corroded to measure the position of precipitation of carbide, aspect ratio, and average equivalent circle diameter.
- diamond powder having a particle size of 1 ⁇ m to 6 ⁇ m was dispersed in dilution liquid such as alcohol or pure water.
- Corrosion is not particularly limited as long as it is a method capable of observing the shape and precipitation position of carbides, and, for example, etching with a saturated picric acid-alcohol solution is performed as a means of corroding grain boundaries of carbides and base iron.
- the base iron may be removed by about several micrometers by only constant carbide by a constant potential electrolytic etching method using a non-aqueous solvent-based electrolyte (Fujio Kurosawa et al., Journal of the Japan Institute of Metals, 43, 1068, (1979)). You may adopt the method of leaving.
- the calculation of the aspect ratio of the carbide is performed by observing the range of 10000 ⁇ m 2 of the 1/4 thickness position of the sample using a thermal field emission scanning electron microscope (for example, JSM-7001F manufactured by JEOL).
- the major axis and the minor axis of all the carbides contained in the observed field of view are measured to calculate the aspect ratio (major axis / minor axis), and the average value is determined.
- the above observation is performed in five fields of view, and the average value of five fields of view is taken as the aspect ratio of the carbide of the sample. From the total number of carbides having an aspect ratio of 2.0 or less and the total number of carbides present in the above five fields of view, the aspect ratio of all the carbides is 2. based on the aspect ratio of the obtained carbides. The number ratio of carbides which is 0 or less is calculated.
- the confirmation of the deposition position of the carbide is performed by observing the range of 10000 ⁇ m 2 of the 1/4 thickness position of the sample using a thermal field emission scanning electron microscope (for example, JSM-7001F manufactured by JEOL).
- the precipitation position is observed for all carbides contained in the observed field of view, and the proportion of carbides precipitated in the ferrite grains is calculated among all the carbides.
- the above observation is carried out in five fields of view, and the average value of the five fields of view is the ratio of carbides formed in ferrite crystal grains among carbides (that is, the proportion of the number of carbides present in ferrite crystal grains in all carbides) I assume.
- the average equivalent circle diameter of the carbide is measured by taking a four-field view of a 600 ⁇ m 2 area at a quarter thickness of a sample using a thermal field emission scanning electron microscope (eg, JSM-7001F manufactured by JEOL) .
- the major and minor axes of the reflected carbide are measured using an image analysis software (eg, IMage-Pro Plus manufactured by Media Cybernetics).
- image analysis software eg, IMage-Pro Plus manufactured by Media Cybernetics.
- the average value of the obtained major and minor axes is used as the diameter of the carbide, and the average value of the obtained diameters is calculated for all carbides reflected in the field of view.
- the average value of the diameters of the carbides in the four fields of view obtained in this manner is further averaged by the number of fields of view to obtain an average equivalent circle diameter of the carbide.
- the thickness of the steel plate for carburizing steel according to the present embodiment is not particularly limited, but is preferably, for example, 2 mm or more. By setting the plate thickness of the steel plate for carburizing to 2 mm or more, it becomes possible to further reduce the plate thickness difference in the coil width direction. More preferably, the plate thickness of the steel plate for carburizing is 2.3 mm or more.
- the thickness of the carburized steel sheet is not particularly limited, but is preferably 6 mm or less. By setting the thickness of the steel plate for carburizing to 6 mm or less, the load at the time of press forming can be reduced, and the forming of the component can be made easier.
- the thickness of the carburizing steel plate is more preferably 5.8 mm or less.
- the carburizing steel plate according to the present embodiment has been described above in detail.
- the manufacturing method for manufacturing a steel plate for carburizing steel according to the present embodiment as described above uses (A) a steel material having a chemical composition as described above and hot-rolled steel plate according to predetermined conditions. And (B) the obtained hot-rolled steel sheet for producing a hot-rolling process or the steel sheet subjected to cold rolling after the hot-rolling process in accordance with predetermined heat treatment conditions and annealing treatment And an annealing process to apply.
- predetermined heat treatment conditions and annealing treatment And an annealing process to apply an annealing process to apply.
- the hot rolling process described in detail below is a process of manufacturing a hot-rolled steel sheet according to predetermined conditions using a steel material having a predetermined chemical composition.
- steel slabs (steel materials) to be subjected to hot rolling may be steel slabs manufactured by a conventional method, and for example, steel slabs manufactured by a general method such as continuous casting slabs, thin slab casters, etc. Can.
- such steel material is heated and subjected to hot rolling, and rolling one pass before hot finishing rolling is performed at a temperature of 900 ° C. to 980 ° C. In a rolling reduction of 15% or more and 25% or less, and then finish the hot finish rolling with a rolling reduction of 6% or more in a temperature range of 800 ° C. or more and less than 920 ° C. and wind at 700 ° C. or less By taking it, it will be a hot rolled steel sheet.
- Rolling temperature one pass before hot finishing rolling 900 ° C. or more and 980 ° C. or less, rolling reduction: 15% or more and 25% or less
- recrystallization of austenite is promoted by a rolling step one pass before hot finishing rolling to form austenite grains with few lattice defects. If the rolling temperature is less than 900 ° C., or if the rolling reduction exceeds 25%, excessive lattice defects are introduced into austenite, and recrystallization of austenite in the next finish rolling step is unnecessary.
- the average value of the X-ray random intensity ratio of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups of ferrite crystal grains can not be controlled to 7.0 or less. Also, if the rolling temperature exceeds 980 ° C., or if the rolling reduction is less than 15%, coarsening of austenite grains becomes remarkable, and as a result, recrystallization of austenite grains is inhibited in the next finish rolling process. As a result, the average value of the X-ray random intensity ratio of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups of ferrite crystal grains can not be controlled to 7.0 or less.
- the rolling temperature before one pass of hot finishing rolling is set to 900 ° C. or more and 980 ° C. or less, and the rolling reduction is set to 15% or more and 25% or less.
- the rolling temperature one pass before the hot finishing rolling is It is preferable that it is 910 degreeC or more.
- the rolling temperature one pass before the hot finishing rolling Is preferably 970 ° C. or less.
- the rolling reduction is preferably 17% or more.
- the rolling reduction should be 20% or less preferable.
- rolling temperature in hot finish rolling is preferably 810 ° C. or higher .
- the rolling temperature in hot finish rolling is set to less than 920 ° C.
- the rolling temperature in hot finish rolling is preferably less than 910 ° C.
- the upper limit of the rolling reduction is not particularly limited. However, from the viewpoint of the shape stability of the hot-rolled steel sheet, 50% is a practical upper limit.
- the microstructure of the steel sheet for carburizing has an average equivalent circle diameter of carbide of 5.0 ⁇ m or less, and the X of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation group of ferrite grain.
- the ratio of the number of carbides having an average value of the line random strength ratio of 7.0 or less and the aspect ratio of 2.0 or less among all the carbides is 80% or more, and within all the carbides of ferrite grains of ferrite It is necessary that the percentage of the number of carbides formed in at least 60%.
- the steel sheet structure (hot-rolled steel sheet structure) before being subjected to the subsequent annealing step is 10% to 80% ferrite and the area ratio Containing 10% or more and 60% or less of pearlite so that the total area ratio is 100% or less, and the balance is at least one of bainite, martensite, tempered martensite, and retained austenite.
- the subsequent annealing step more specifically, spheroidizing annealing
- the hot rolling step according to the present embodiment when the coiling temperature exceeds 700 ° C., the formation of ferrite is promoted too much to suppress the formation of pearlite, and finally, in the steel plate after the annealing step, carbide Among them, it is difficult to control the proportion of carbides having an aspect ratio of 2.0 or less to 80% or more. Therefore, in the hot rolling process according to the present embodiment, the upper limit of the winding temperature is set to 700 ° C.
- the lower limit of the winding temperature in the hot rolling process according to the present embodiment is not particularly limited. However, since it is difficult to wind up at room temperature or lower in practice, room temperature is a practical lower limit.
- the coiling temperature of the hot rolling process which concerns on this embodiment is 400 degreeC or more from a viewpoint of making the aspect ratio of the carbide after the latter annealing process smaller.
- the total number of passes of hot rolling is not particularly limited, and may be an arbitrary number of passes.
- the rolling reduction before 2 passes before the hot finishing rolling is not particularly limited, and may be set as appropriate so as to obtain a desired final thickness.
- the steel plate (hot rolled steel plate) wound up by the above hot rolling processes may be rewound, pickled, and cold-rolled.
- pickling may be performed once or may be divided and performed plural times.
- the cold rolling may be cold rolling at a normal rolling reduction (eg, 30 to 90%).
- the hot-rolled steel plate and the cold-rolled steel plate include steel plates which have been subjected to temper rolling under ordinary conditions, in addition to those which have been hot-rolled and cold-rolled.
- the hot-rolled steel plate is manufactured as described above. Further, in the annealing process as described in detail below, the manufactured hot-rolled steel sheet or the steel sheet subjected to cold rolling after the hot rolling process is subjected to a specific annealing treatment to obtain The steel plate for carburizing according to the embodiment can be obtained.
- the annealing process described in detail below conforms to the predetermined heat treatment conditions for the hot-rolled steel plate obtained by the above-mentioned hot rolling process or the steel plate subjected to cold rolling after the hot rolling process.
- Annealing process spheroidizing annealing process. By this annealing treatment, pearlite generated in the hot rolling process is spheroidized.
- the hot-rolled steel sheet obtained as described above, or the steel sheet subjected to cold rolling after the hot rolling step, in an atmosphere in which the nitrogen concentration is controlled to less than 25% by volume fraction Heat at an average heating rate of 5 ° C./h or more and 100 ° C./h or less to a temperature range of Ac 1 point or less defined by the following formula (101), 10 h or more and 100 h or less in a temperature range of Ac 1 point or less
- cooling is performed such that the average cooling rate in the temperature range from the temperature at the end of the annealing to 550 ° C. is 5 ° C./h or more and 100 ° C./h or less.
- the notation [X] represents the content (unit: mass%) of the element X, and in the case where the corresponding element is not contained, zero is substituted.
- the annealing atmosphere is an atmosphere in which the nitrogen concentration is controlled to less than 25% by volume fraction.
- the nitrogen concentration is 25% or more in volume fraction, nitrides are formed in the steel sheet, which causes deterioration of hole expandability, which is not preferable. The lower the nitrogen concentration, the better.
- controlling the nitrogen concentration to 1% or less by volume fraction is disadvantageous in terms of cost, so the volume fraction of 1% is a practical lower limit of nitrogen concentration.
- the atmosphere gas for example, at least one is appropriately selected from gases such as nitrogen and hydrogen or inert gases such as argon, and the nitrogen concentration in the heating furnace used in the annealing step becomes a desired concentration.
- gases such as nitrogen and hydrogen or inert gases such as argon
- the atmosphere gas is preferably as high as the hydrogen concentration.
- the hydrogen concentration may be 95% or more by volume fraction, and the remaining portion may be nitrogen.
- the atmosphere gas in the heating furnace used in the annealing step can be controlled, for example, by appropriately measuring the gas concentration in the heating furnace while introducing the above-described gas.
- Heating conditions to a temperature range of 1 point or less of Ac at an average heating rate of 5 ° C./h to 100 ° C./h
- the hot-rolled steel plate as described above or the steel plate subjected to cold rolling after the hot rolling process is used at an average heating rate of 5 ° C./h to 100 ° C./h. It is necessary to heat to a temperature range equal to or less than an A C1 point defined by the above equation (101).
- the average heating rate is less than 5 ° C./h, the average circle equivalent diameter of the carbide exceeds 5.0 ⁇ m, and the hole expansibility deteriorates.
- the heating temperature exceeds the point A C1 defined by the above equation (101), the proportion of the number of carbides formed in the ferrite crystal grains among all the carbides is less than 60%, which is a good hole. I can not get the flexibility.
- the lower limit of the heating temperature range is not particularly limited. However, if the temperature range of the heating temperature is less than 600 ° C., the holding time in the annealing process becomes long, and the manufacturing cost becomes disadvantageous.
- the temperature range of heating temperature into 600 degreeC or more it is preferable to make the temperature range of heating temperature into 600 degreeC or more.
- the average heating rate in the annealing step according to the present embodiment is preferably 20 ° C./h or more.
- the average heating temperature in the annealing process which concerns on this embodiment shall be 50 degrees C / h or less.
- the temperature range of the heating temperature in the annealing step according to the present embodiment is more preferably 630 ° C. or higher.
- the temperature range of the heating temperature in the annealing process which concerns on this embodiment shall be 670 degrees C or less.
- the holding time in the annealing process according to the present embodiment is preferably 20 h or more. Moreover, in order to control the state of a carbide more appropriately, it is preferable to make holding time in the annealing process which concerns on this embodiment into 80 h or less.
- the average cooling rate is an average cooling rate from the heating holding temperature (in other words, the temperature at the end of annealing) to 550 ° C. If the average cooling rate is less than 5 ° C./h, the carbides become too coarse, and the hole expansibility deteriorates.
- the average cooling rate from the heating holding temperature to 550 ° C. is preferably 20 ° C./h or more.
- the average cooling rate from a heating holding temperature to 550 degreeC shall be 50 degrees C / h or less.
- the average cooling rate in a temperature range of less than 550 ° C. is not particularly limited, and cooling may be performed to a predetermined temperature range at an arbitrary average cooling rate.
- the lower limit of the temperature at which the cooling is stopped is not particularly limited. However, since it is practically difficult to cool to room temperature or lower, room temperature is a practical lower limit.
- cold working may be applied as a post-process to the steel plate for carburizing obtained as described above.
- a carburizing heat treatment may be performed on the cold-worked steel sheet for carburizing described above, for example, in the range of 0.4 to 1.0% by mass of carbon potential.
- the conditions of the carburizing heat treatment are not particularly limited, and can be appropriately adjusted to obtain desired characteristics.
- the steel sheet for carburizing may be heated to the austenite single phase region temperature, carburized, and then cooled to room temperature as it is, or once cooled to room temperature, it may be reheated and rapidly cooled.
- tempering may be applied to all or part of the members for the purpose of adjusting the strength.
- the ratio of the number of carbides having a ratio of 2.0 or less, (3) the ratio of the number of carbides formed in ferrite grains among all the carbides, and (4) the average equivalent circle diameter of the carbides are described above. It measured by the method.
- the hole-opening test was done according to JIS Z 2256 (hole-opening test method of metal material).
- the hole expansion ratio was calculated according to the test method and calculation formula specified in JIS Z 2256 by collecting a test piece from an arbitrary position of each of the obtained steel sheets for carburizing.
- a case where the obtained hole expansion ratio is 80% or more is regarded as “ex.”
- "-" was described about what the crack generate
- an ideal critical diameter which is an index representing hardenability after carburizing.
- the ideal critical diameter D i is an index calculated from the components of the steel sheet, and can be calculated according to the following equation (201) using the method of Grossmann / Hollomon, Jaffe. The larger the value of the ideal critical diameter D i , the better the hardenability.
- the steel plate for carburizing corresponding to the example of the present invention is excellent in the hole expansion ratio defined in JIS Z 2256 (the method of testing for the expansion of holes in metal materials) is 80% or more. It became clear that it has ultimate deformability. Moreover, the ideal critical diameter described as a reference also becomes 5 or more, and it turns out that the steel plate for carburizing applicable to the Example of this invention also has the outstanding hardenability.
- the steel sheet for carburizing corresponding to the comparative example of the present invention has a hole expansion ratio of less than 80%, and it is clear that the ultimate deformability is inferior. In particular, no. In 7, 11 to 15, 74, 78, 82, and 87, it was found that the hole expansion rate could not be calculated and the processability was poor because a crack occurred during the preparation (punching) of the hole expansion test piece .
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Abstract
Description
かかる着想に基づき完成された本発明の要旨は、以下の通りである。
[2]残部のFeの一部に換えて、質量%で、Cr:0.005%以上3.0%以下、Mo:0.005%以上1.0%以下、Ni:0.010%以上3.0%以下、Cu:0.001%以上2.0%以下、Co:0.001%以上2.0%以下、Nb:0.010%以上0.150%以下、Ti:0.010%以上0.150%以下、V:0.0005%以上1.0%以下、B:0.0005%以上0.01%以下の1種又は2種以上を更に含有する、[1]に記載の浸炭用鋼板。
[3]残部のFeの一部に換えて、質量%で、Sn:1.0%以下、W:1.0%以下、Ca:0.01%以下、REM:0.3%以下の1種又は2種以上を更に含有する、[1]又は[2]に記載の浸炭用鋼板。
[4][1]~[3]の何れか1つに記載の浸炭用鋼板を製造する方法であって、[1]~[3]の何れか1つに記載の化学組成を有する鋼材を加熱し、熱間仕上圧延の1パス前の圧延を、900℃以上980℃以下の温度域で15%以上25%以下の圧下率で実施し、熱間仕上圧延を、800℃以上920℃未満の温度域で6%以上の圧下率で終了し、700℃以下の温度で巻取る熱間圧延工程と、前記熱間圧延工程により得られた鋼板、又は、前記熱間圧延工程後に冷間圧延が施された鋼板を、窒素濃度を体積分率で25%未満に制御した雰囲気にて、5℃/h以上100℃/h以下の平均加熱速度で、下記式(1)で定義されるAc1点以下の温度域まで加熱し、当該Ac1点以下の温度域で10h以上100h以下保持する焼鈍処理を施した後、焼鈍終了時の温度から550℃までの温度域における平均冷却速度を5℃/h以上100℃/h以下とする冷却を施す焼鈍工程と、を含む、浸炭用鋼板の製造方法。
本発明に係る浸炭用鋼板及びその製造方法について説明するに先立ち、上記課題を解決するために本発明者らが行った検討の内容について、以下で詳細に説明する。
かかる検討に際し、本発明者らは、まず、極限変形能と相関を有する穴広げ性を向上させるための方法について、検討を行った。
まず、本発明の実施形態に係る浸炭用鋼板について、詳細に説明する。
本実施形態に係る浸炭用鋼板は、以下で詳述するような所定の化学成分を有している。加えて、本実施形態に係る浸炭用鋼板は、フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値が、7.0以下であり、炭化物の平均円相当直径が、5.0μm以下であり、アスペクト比が2.0以下である炭化物の個数割合が、全炭化物に対して80%以上であり、フェライト結晶粒内に存在する炭化物の個数割合が、全炭化物に対して60%以上であるという、特定のミクロ組織を有している。これにより、本実施形態に係る浸炭用鋼板は、浸炭前において、より一層優れた極限変形能を示すようになる。
まず、本実施形態に係る浸炭用鋼板の有する化学成分について、詳細に説明する。なお、以下の説明において、化学成分に関する「%」は、特に断りのない限り「質量%」を意味する。
C(炭素)は、最終的に得られる浸炭部材における板厚中央部の強度を確保するために必要な元素である。また、浸炭用鋼板において、Cは、フェライトの粒界に固溶して粒界の強度を上昇させ、穴広げ性の向上に寄与する元素である。
Si(ケイ素)は、溶鋼を脱酸して鋼を健全化する作用をなす元素である。Siの含有量が0.005%未満である場合には、溶鋼を十分に脱酸することができない。そのため、本実施形態に係る浸炭用鋼板において、Siの含有量は、0.005%以上とする。Siの含有量は、好ましくは0.01%以上である。一方、Siの含有量が0.5%以上となる場合には、炭化物に固溶したSiが炭化物を安定化させて、炭化物の平均円相当直径が5.0μmを超え、穴広げ性が損なわれる。そのため、本実施形態に係る浸炭用鋼板において、Siの含有量は、0.5%未満とする。Siの含有量は、好ましくは0.3%未満である。
Mn(マンガン)は、溶鋼を脱酸して鋼を健全化する作用をなす元素である。Mnの含有量が0.01%未満である場合には、溶鋼を十分に脱酸することができない。そのため、本実施形態に係る浸炭用鋼板において、Mnの含有量は、0.01%以上とする。Mnの含有量は、好ましくは0.1%以上である。一方、Mnの含有量が3.0%以上となる場合には、炭化物に固溶したMnが炭化物を安定化させて、炭化物の平均円相当直径が5.0μmを超え、穴広げ性の劣化を招く。そのため、Mnの含有量は、3.0未満とする。Mnの含有量は、好ましくは2.0%未満であり、より好ましくは1.0%未満である。
P(リン)は、フェライトの粒界に偏析して、穴広げ性を劣化させる元素である。Pの含有量が0.1%を超える場合には、フェライトの粒界の強度が著しく低下して、穴広げ性が劣化する。そのため、本実施形態に係る浸炭用鋼板において、Pの含有量は、0.1%以下とする。Pの含有量は、好ましくは0.050%以下であり、より好ましくは0.020%以下である。なお、Pの含有量の下限は、特に限定しない。ただし、Pの含有量を0.0001%未満まで低減させると、脱Pコストが大幅に上昇して、経済的に不利になる。そのため、実用鋼板上、Pの含有量は、0.0001%が実質的な下限となる。
S(硫黄)は、介在物を形成して、穴広げ性を劣化させる元素である。Sの含有量が0.1%を超える場合には、粗大な介在物が生成して穴広げ性が低下する。そのため、本実施形態に係る浸炭用鋼板において、Sの含有量は、0.1%以下とする。Sの含有量は、好ましくは0.010%以下であり、より好ましくは0.008%以下である。なお、Sの含有量の下限は、特に限定しない。ただし、Sの含有量を0.0005%未満まで低減させると、脱Sコストが大幅に上昇し、経済的に不利になる。そのため、実用鋼板上、Sの含有量は、0.0005%が実質的な下限となる。
Al(アルミニウム)は、溶鋼を脱酸して鋼を健全化する作用をなす元素である。Alの含有量が0.0002%未満である場合には、溶鋼を十分に脱酸することができない。そのため、本実施形態に係る浸炭用鋼板において、Alの含有量(より詳細には、sol.Alの含有量)は、0.0002%以上とする。Alの含有量は、好ましくは0.0010%以上である。一方、Alの含有量が3.0%を超える場合には、粗大な酸化物が生成して穴広げ性が損なわれる。そのため、Alの含有量は、3.0%以下とする。Alの含有量は、好ましくは2.5%以下であり、より好ましくは1.0%以下であり、更に好ましくは0.5%以下であり、より一層好ましくは0.1%以下である。
N(窒素)は、不純物元素であり、窒化物を形成して穴広げ性を阻害する元素である。Nの含有量が0.2%を超える場合には、粗大な窒化物が生成して穴広げ性が著しく低下する。そのため、本実施形態に係る浸炭用鋼板において、Nの含有量は、0.2%以下とする。Nの含有量は、好ましくは0.1%以下であり、より好ましくは0.02%以下であり、更に好ましくは0.01%以下である。一方、Nの含有量の下限は、特に限定しない。ただし、Nの含有量を0.0001%未満まで低減させると、脱Nコストが大幅に上昇し、経済的に不利になる。そのため、実用鋼板上、Nの含有量は、0.0001%が実質的な下限となる。
Cr(クロム)は、最終的に得られる浸炭部材において、焼入れ性を高める効果を持つ元素であるとともに、浸炭用鋼板においては、フェライトの結晶粒を微細化して穴広げ性の更なる向上に寄与する元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、Crを含有させてもよい。Crを含有させる場合、更なる穴広げ性の向上効果を得るためには、Crの含有量を0.005%以上とすることが好ましい。Crの含有量は、より好ましくは0.010%以上である。また、炭化物や窒化物の生成の影響を考慮すると、穴広げ性の更なる向上効果を得るためには、Crの含有量は、3.0%以下とすることが好ましい。Crの含有量は、より好ましくは2.0%以下であり、更に好ましくは1.5%以下である。
Mo(モリブデン)は、最終的に得られる浸炭部材において、焼入れ性を高める効果を持つ元素であるとともに、浸炭用鋼板においては、フェライトの結晶粒を微細化して穴広げ性の更なる向上に寄与する元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、Moを含有させてもよい。Moを含有させる場合、更なる穴広げ性の向上効果を得るためには、Moの含有量を0.005%以上とすることが好ましい。Moの含有量は、より好ましくは0.010%以上である。また、炭化物や窒化物の生成の影響を考慮すると、穴広げ性の更なる向上効果を得るためには、Moの含有量は、1.0%以下とすることが好ましい。Moの含有量は、より好ましくは0.8%以下である。
Ni(ニッケル)は、最終的に得られる浸炭部材において、焼入れ性を高める効果を持つ元素であるとともに、浸炭用鋼板においては、フェライトの結晶粒を微細化して穴広げ性の更なる向上に寄与する元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、Niを含有させてもよい。Niを含有させる場合、穴広げ性の更なる向上効果を得るためには、Niの含有量を0.010%以上とすることが好ましい。Niの含有量は、より好ましくは0.050%以上である。また、Niが粒界に偏析する影響を考慮すると、穴広げ性の更なる向上効果を得るためには、Niの含有量は、3.0%以下とすることが好ましい。Niの含有量は、より好ましくは2.0%以下であり、更に好ましくは1.0%以下であり、より一層好ましくは0.5%以下である。
Cu(銅)は、最終的に得られる浸炭部材において、焼入れ性を高める効果を持つ元素であるとともに、浸炭用鋼板においては、フェライトの結晶粒を微細化して穴広げ性の更なる向上に寄与する元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、Cuを含有させてもよい。Cuを含有させる場合、穴広げ性の更なる向上効果を得るためには、Cuの含有量を0.001%以上とすることが好ましい。Cuの含有量は、より好ましくは0.010%以上である。また、Cuが粒界に偏析する影響を考慮すると、穴広げ性の更なる向上効果を得るためには、Cuの含有量は、2.0%以下とすることが好ましい。Cuの含有量は、より好ましくは0.80%以下である。
Co(コバルト)は、最終的に得られる浸炭部材において、焼入れ性を高める効果を持つ元素であるとともに、浸炭用鋼板においては、フェライトの結晶粒を微細化して穴広げ性の更なる向上に寄与する元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、Coを含有させてもよい。Coを含有させる場合、穴広げ性の更なる向上効果を得るためには、Coの含有量を0.001%以上とすることが好ましい。Coの含有量は、より好ましくは0.010%以上である。また、Coが粒界に偏析する影響を考慮すると、穴広げ性の更なる向上効果を得るためには、Coの含有量は、2.0%以下とすることが好ましい。Coの含有量は、より好ましくは0.80%以下である。
Nb(ニオブ)は、フェライトの結晶粒を微細化して穴広げ性の更なる向上に寄与する元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、Nbを含有させてもよい。Nbを含有させる場合、穴広げ性の更なる向上効果を得るためには、Nbの含有量を0.010%以上とすることが好ましい。Nbの含有量は、より好ましくは0.035%以上である。また、炭化物や窒化物の生成の影響を考慮すると、穴広げ性の更なる向上効果を得るためには、Nbの含有量は、0.150%以下とすることが好ましい。Nbの含有量は、より好ましくは0.120%以下であり、更に好ましくは0.100%以下である。
Ti(チタン)は、フェライトの結晶粒を微細化して穴広げ性の更なる向上に寄与する元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、Tiを含有させてもよい。Tiを含有させる場合、穴広げ性の更なる向上効果を得るためには、Tiの含有量を0.010%以上とすることが好ましい。Tiの含有量は、より好ましくは0.035%以上である。また、炭化物や窒化物の生成の影響を考慮すると、穴広げ性の更なる向上効果を得るためには、Tiの含有量は0.150%以下とすることが好ましい。Tiの含有量は、より好ましくは0.120%以下であり、更に好ましくは0.100%以下であり、より一層好ましくは0.050%以下であり、更に一層好ましくは0.020%以下である。
V(バナジウム)は、フェライトの結晶粒を微細化して穴広げ性の更なる向上に寄与する元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、Vを含有させてもよい。Vを含有させる場合、穴広げ性の更なる向上効果を得るためには、Vの含有量は、0.0005%以上とすることが好ましい。Vの含有量は、より好ましくは0.0010%以上である。また、炭化物や窒化物の生成の影響を考慮すると、穴広げ性の更なる向上効果を得るためには、Vの含有量は、1.0%以下とすることが好ましい。Vの含有量は、より好ましくは0.80%以下であり、更に好ましくは0.10%以下であり、より一層好ましくは0.080%以下である。
B(ホウ素)は、フェライトの粒界に偏析することで粒界の強度を向上させて、穴広げ性を更に向上させる元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、Bを含有させてもよい。Bを含有させる場合、穴広げ性の更なる向上効果を得るためには、Bの含有量は、0.0005%以上とすることが好ましい。Bの含有量は、より好ましくは0.0010%以上である。また、Bを0.01%を超えて含有させたとしても、上記のような穴広げ性の更なる向上効果は飽和するため、Bの含有量は、0.01%以下とすることが好ましい。Bの含有量は、より好ましくは0.0075%以下であり、更に好ましくは0.0050%以下であり、より一層好ましくは0.0020%以下である。
Sn(スズ)は、溶鋼を脱酸して鋼を更に健全化する作用をなす元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、1.0%を上限としてSnを含有させてもよい。Snの含有量は、より好ましくは、0.5%以下である。
W(タングステン)は、溶鋼を脱酸して鋼を更に健全化する作用をなす元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、1.0%を上限としてWを含有させてもよい。Wの含有量は、より好ましくは、0.5%以下である。
Ca(カルシウム)は、溶鋼を脱酸して鋼を更に健全化する作用をなす元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、0.01%を上限としてCaを含有させてもよい。Caの含有量は、より好ましくは0.006%以下である。
REM(希土類金属)は、溶鋼を脱酸して鋼を更に健全化する作用をなす元素である。そのため、本実施形態に係る浸炭用鋼板においては、必要に応じて、0.3%を上限としてREMを含有させてもよい。
板厚中央部の成分組成の残部は、Fe及び不純物である。不純物としては、鋼原料もしくはスクラップから、及び/又は、製鋼過程で不可避的に混入し、本実施形態に係る浸炭用鋼板の特性を阻害しない範囲で許容される元素が例示される。
次に、本実施形態に係る浸炭用鋼板を構成するミクロ組織について、詳細に説明する。
本実施形態に係る浸炭用鋼板のミクロ組織は、実質的に、フェライトと炭化物とで構成される。より詳細には、本実施形態に係る浸炭用鋼板のミクロ組織において、フェライトの面積率は、例えば80~95%の範囲内であり、炭化物の面積率は、例えば5~20%の範囲内であって、かつ、フェライトと炭化物の合計面積率が100%を超えないように構成される。
以下、本実施形態に係る浸炭用鋼板を構成するミクロ組織の限定理由について、詳細に説明する。
本発明者らによる検討の結果、フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値が7.0以下であれば、良好な穴広げ性を得ることができることが明らかとなった。上記X線ランダム強度比の平均値が7.0を超える場合には、穴広げ時に亀裂の発生が助長されて、良好な穴広げ性が得られない。従って、本実施形態に係る浸炭用鋼板では、上記X線ランダム強度比の平均値を、7.0以下とする。上記X線ランダム強度比の平均値は、極限変形能のより一層の向上のために、好ましくは5.5以下である。なお、上記X線ランダム強度比の下限は、特に限定するものではないが、現行の一般的な連続熱延工程を考慮すると、0.5が実質の下限となる。
まず、浸炭用鋼板から、その表面に垂直な断面(板厚断面)が観察できるように、サンプルを切り出す。サンプルの長さは、測定装置にもよるが、10mm~25mm程度で良い。サンプルの板厚1/4位置を、0.1μmの測定間隔で電子後方散乱回折法(Electron Back Scattering Diffraction:EBSD)を用いて測定し、結晶方位情報を得る。ここでEBSD解析は、例えば、サーマル電界放射型走査電子顕微鏡(JEOL製JSM-7001F)とEBSD検出器(TSL製DVC5型検出器)で構成された装置を用い、15kV~25kVの電子線加速電圧、200~300点/秒の解析速度で実施する。EBSD解析装置に付属のソフトウェア「OIM Analysis(登録商標)」に搭載された「TEXTURE」機能を用いて、得られた結晶方位情報から、級数展開法で計算した3次元集合組織を計算する。次に「ODF」機能を用いて、3次元集合組織のうちφ2=45゜断面における(001)[1-10]、(116)[1-10]、(114)[1-10]、(113)[1-10]、(112)[1-10]、(335)[1-10]、(223)[1-10]の強度をそのままフェライト結晶粒のX線ランダム強度比として用いればよい。{100}<011>~{223}<110>方位群の平均値とは、上記方位の相加平均である。なお、上記の全ての方位の強度を得ることができない場合には、例えば、{100}<011>、{116}<110>、{114}<110>、{112}<110>、{223}<110>の各方位の相加平均で代替してもよい。なお、結晶学では、「-1」という方位は、正式には「1」の上にアッパーバーを付して表記するが、本明細書では記載の制約上、「-1」と表記している。
先だって言及したように、本実施形態における炭化物は、セメンタイト(Fe3C)及びε系炭化物(Fe2~3C)等の鉄系炭化物により主に構成される。本発明者らによる検討の結果、全炭化物のうち、アスペクト比が2.0以下である炭化物の個数割合が80%以上であれば、良好な穴広げ性を得ることができることが明らかとなった。全炭化物のうちアスペクト比が2.0以下である炭化物の個数割合が80%未満である場合には、穴広げ時に亀裂の発生が助長されて、良好な穴広げ性を得ることができない。従って、本実施形態に係る浸炭用鋼板においては、全炭化物のうちアスペクト比が2.0以下である炭化物の個数割合の下限を、80%とする。全炭化物のうちアスペクト比が2.0以下である炭化物の個数割合は、穴広げ性の更なる向上を目的として、好ましくは85%以上である。なお、全炭化物のうちアスペクト比が2.0以下である炭化物の個数割合の上限は、特に規定するものではない。ただし、実操業において98%以上とすることは困難であるため、98%が実質的な上限となる。
本発明者らによる検討の結果、全炭化物のうちフェライトの結晶粒内に存在する炭化物の個数割合が60%以上であれば、良好な穴広げ性を得ることができることが明らかとなった。全炭化物のうちフェライトの結晶粒内に存在する炭化物の個数割合が60%未満である場合には、穴広げ時に亀裂の伸展が助長されて、良好な穴広げ性を得ることができない。従って、本実施形態に係る浸炭用鋼板においては、全炭化物のうちフェライトの結晶粒内に存在する炭化物の個数割合の下限を、60%とする。全炭化物のうちフェライトの結晶粒内に存在する炭化物の個数割合は、穴広げ性の更なる向上を目的として、好ましくは65%以上である。なお、全炭化物のうちフェライトの結晶粒内に存在する炭化物の個数割合の上限は、特に規定するものではない。ただし、実操業において98%以上とすることは困難であるため、98%が実質的な上限となる。
本実施形態に係る浸炭用鋼板のミクロ組織において、炭化物の平均円相当直径は、5.0μm以下である必要がある。炭化物の平均円相当直径が5.0μmを超える場合には、打ち抜き時に割れが発生し、良好な穴広げ性を得ることができない。炭化物の平均円相当直径が小さい程、打ち抜き時の割れは発生しにくく、炭化物の平均円相当直径は、好ましくは1.0μm以下であり、より好ましくは0.8μm以下であり、更に好ましくは0.6μm以下である。炭化物の平均円相当直径の下限は、特に規定するものではない。ただし、実操業において、炭化物の平均円相当直径を0.01μm以下とすることは困難であるため、0.01μmが実質的な下限となる。
まず、浸炭用鋼板からその表面に垂直な断面(板厚断面)が観察できるようにサンプルを切り出す。サンプルの長さは、測定装置にもよるが、10mm程度で良い。断面を研磨及び腐食して、炭化物の析出位置とアスペクト比と平均円相当直径との測定に供する。ここで、研磨は、例えば、粒度600から粒度1500の炭化珪素ペーパーを使用して測定面を研磨した後、粒径が1μmから6μmのダイヤモンドパウダーをアルコール等の希釈液や純水に分散させた液体を使用して、鏡面に仕上げれば良い。腐食は、炭化物の形状と析出位置を観察できる手法であれば、特に制限されるものではなく、例えば、炭化物と地鉄の粒界を腐食する手段として、飽和ピクリン酸-アルコール溶液によるエッチングを行っても良いし、非水溶媒系電解液による定電位電解エッチング法(黒澤文夫ら、日本金属学会誌、43、1068、(1979))等により、地鉄を数マイクロメートル程度除去して炭化物のみを残存させる方法を採用してもよい。
本実施形態に係る浸炭用鋼板の板厚については、特に限定するものではないが、例えば、2mm以上とすることが好ましい。浸炭用鋼板の板厚を2mm以上とすることで、コイル幅方向の板厚差をより小さくすることが可能となる。浸炭用鋼板の板厚は、より好ましくは、2.3mm以上である。また、浸炭用鋼板の板厚は、特に限定するものではないが、6mm以下とすることが好ましい。浸炭用鋼板の板厚を6mm以下とすることで、プレス成形時の荷重を低くして、部品への成形をより容易なものとすることができる。浸炭用鋼板の板厚は、より好ましくは5.8mm以下である。
次に、以上説明したような本実施形態に係る浸炭用鋼板を製造するための方法について、詳細に説明する。
以下、上記の熱間圧延工程、及び、焼鈍工程について、詳細に説明する。
以下で詳述する熱間圧延工程は、所定の化学組成を有する鋼材を用いて、所定の条件に則して熱間圧延鋼板を製造する工程である。
本実施形態に係る熱間圧延工程では、熱間仕上圧延の1パス前の圧延工程により、オーステナイトの再結晶を促進させて、格子欠陥が少ないオーステナイト粒を形成させる。圧延温度が900℃未満である場合、又は、圧下率が25%を超える場合には、オーステナイト中に過剰に格子欠陥が導入されてしまい、次の仕上圧延工程においてオーステナイトの再結晶を必要以上に阻害し、フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値を7.0以下に制御することができなくなる。また、圧延温度が980℃を超える場合、又は、圧下率が15%未満である場合には、オーステナイト粒の粗大化が著しくなり、結果として、次の仕上圧延工程においてオーステナイト粒の再結晶が阻害されて、フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値を7.0以下に制御することができなくなる。かかる観点より、本実施形態に係る熱間圧延工程では、熱間仕上圧延の1パス前の圧延温度を900℃以上980℃以下とし、圧下率を、15%以上25%以下とする。フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値をより適切に制御するために、熱間仕上圧延の1パス前の圧延温度は、910℃以上であることが好ましい。また、フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値をより適切に制御するために、熱間仕上圧延の1パス前の圧延温度は、970℃以下であることが好ましい。フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値をより適切に制御するために、圧下率は、17%以上であることが好ましい。また、フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値をより適切に制御するために、圧下率は、20%以下であることが好ましい。
本実施形態に係る熱間圧延工程では、熱間仕上圧延工程により、オーステナイトの再結晶を促進させる。圧延温度が800℃未満である場合、又は、圧下率が6%未満である場合には、オーステナイトの再結晶が十分に促進されずに、フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値を7.0以下に制御することができなくなる。このため、本実施形態に係る熱間仕上圧延では、圧延温度を800℃以上とし、圧下率を6%以上とする。フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比をより適切に制御するために、熱間仕上圧延における圧延温度は、好ましくは810℃以上である。一方、圧延温度が920℃以上となる場合には、オーステナイトのオーステナイト粒の粗大化が著しくなり、結果として、次工程において、フェライトの生成が阻害されてしまう。このため、本実施形態に係る熱間仕上圧延では、圧延温度を920℃未満とする。フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比をより適切に制御するために、熱間仕上圧延における圧延温度は、好ましくは910℃未満である。なお、本実施形態に係る熱間仕上圧延において、圧下率の上限は特に規定するものではない。ただし、熱間圧延鋼板の形状安定性の観点から、50%が実質的な上限となる。
先だって言及したように、浸炭用鋼板のミクロ組織は、炭化物の平均円相当直径が、5.0μm以下であり、フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値が7.0以下であり、全炭化物のうちアスペクト比が2.0以下である炭化物の個数割合が80%以上であり、かつ、全炭化物のうちフェライトの結晶粒内に形成した炭化物の個数割合が60%以上である必要がある。そのためには、後段の焼鈍工程(より詳細には、球状化焼鈍)に供される前の鋼板組織(熱間圧延鋼板組織)は、面積率で10%以上80%以下のフェライトと、面積率で10%以上60%以下のパーライトとを、面積率の合計が100%以下となるように含有し、残部は、ベイナイト、マルテンサイト、焼き戻しマルテンサイト、及び、残留オーステナイトの少なくとも何れかから構成されることが好ましい。
以下で詳述する焼鈍工程は、上記の熱間圧延工程により得られた熱間圧延鋼板、又は、熱間圧延工程後に冷間圧延が施された鋼板に対して、所定の熱処理条件に則して焼鈍処理(球状化焼鈍処理)を施す工程である。かかる焼鈍処理により、熱間圧延工程において生成したパーライトを球状化させる。
ここで、下記式(101)において、[X]との表記は、元素Xの含有量(単位:質量%)を表し、該当する元素を含有しない場合はゼロを代入するものとする。
上記のような焼鈍工程において、焼鈍雰囲気は、窒素濃度を体積分率で25%未満に制御した雰囲気とする。窒素濃度が体積分率で25%以上となる場合には、鋼板中に窒化物が形成して、穴広げ性の劣化を招くため、好ましくない。かかる窒素濃度は、低ければ低いほど望ましい。ただし、窒素濃度を体積分率で1%以下に制御することは、コスト上不利であるため、体積分率1%が窒素濃度の実質的な下限となる。
本実施形態に係る焼鈍工程では、上記のような熱間圧延鋼板、又は、熱間圧延工程後に冷間圧延が施された鋼板を、5℃/h以上100℃/h以下の平均加熱速度で、上記式(101)で定めるAC1点以下の温度域まで加熱する必要がある。平均加熱速度が5℃/h未満である場合には、炭化物の平均円相当直径が5.0μmを超えて、穴広げ性が劣化する。一方、平均加熱速度が100℃/hを超える場合には、炭化物の球状化が十分に促進されずに、全炭化物のうちアスペクト比が2.0以下である炭化物の個数割合を80%以上に制御することが困難となる。また、加熱温度が、上記式(101)で定めるAC1点を超える場合には、全炭化物のうちフェライトの結晶粒内に形成した炭化物の個数割合が60%未満となってしまい、良好な穴広げ性を得ることができない。なお、加熱温度の温度域の下限は、特に規定するものではない。ただし、加熱温度の温度域が600℃未満であると、焼鈍処理における保持時間が長くなり、製造コストが不利になる。そのため、加熱温度の温度域は、600℃以上とすることが好ましい。炭化物の状態をより適切に制御するために、本実施形態に係る焼鈍工程における平均加熱速度は、20℃/h以上とすることが好ましい。また、炭化物の状態をより適切に制御するために、本実施形態に係る焼鈍工程における平均加熱温度は、50℃/h以下とすることが好ましい。炭化物の状態をより適切に制御するために、本実施形態に係る焼鈍工程における加熱温度の温度域は、630℃以上とすることがより好ましい。また、炭化物の状態をより適切に制御するために、本実施形態に係る焼鈍工程における加熱温度の温度域は、670℃以下とすることがより好ましい。
本実施形態に係る焼鈍工程では、上記のようなAc1点以下(好ましくは、600℃以上Ac1点以下)の温度域を、10h以上100h以下保持する必要がある。保持時間が10h未満である場合には、炭化物の球状化が十分に促進されずに、全炭化物のうちアスペクト比が2.0以下である炭化物の個数割合を80%以上に制御することが困難となる。一方、保持時間が100hを超える場合には、炭化物の平均円相当直径が5.0μmを超え、穴広げ性が劣化する。炭化物の状態をより適切に制御するために、本実施形態に係る焼鈍工程における保持時間は、20h以上であることが好ましい。また、炭化物の状態をより適切に制御するために、本実施形態に係る焼鈍工程における保持時間は、80h以下とすることが好ましい。
本実施形態に係る焼鈍工程において、上記のような加熱保持後、鋼板を5℃/h以上100℃/h以下の平均冷却速度で冷却する。ここで、平均冷却速度とは、加熱保持温度(換言すれば、焼鈍終了時の温度)から550℃までの平均冷却速度である。平均冷却速度が5℃/h未満である場合には、炭化物が粗大化しすぎて、穴広げ性が劣化する。一方、平均冷却速度が100℃/hを超える場合には、炭化物の球状化が十分に促進されずに、全炭化物のうちアスペクト比が2.0以下である炭化物の個数割合を80%以上に制御することが困難となる。炭化物の状態をより適切に制御するために、加熱保持温度から550℃までの平均冷却速度は、20℃/h以上とすることが好ましい。また、炭化物の状態をより適切に制御するために、加熱保持温度から550℃までの平均冷却速度は、50℃/h以下とすることが好ましい。
以上説明したような熱間圧延工程及び焼鈍工程を実施することで、先だって説明したような、本実施形態に係る浸炭用鋼板を製造することができる。
以下の表1に示す化学組成を有する鋼材を、以下の表2に示す条件で熱間圧延(及び冷間圧延)した後、焼鈍を施して、浸炭用鋼板を得た。なお、以下の表2に示す条件で熱間圧延を行った後、大気中、55℃で105時間保持した上で、以下の表2に示す条件で焼鈍を行った。以下の表1及び表2において、下線は、本発明の範囲外であることを示す。
Claims (4)
- 質量%で、
C:0.02%以上0.30%未満
Si:0.005%以上0.5%未満
Mn:0.01%以上3.0%未満
P:0.1%以下
S:0.1%以下
sol.Al:0.0002%以上3.0%以下
N:0.2%以下
を含有し、残部が、Fe及び不純物からなり、
フェライト結晶粒の{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値が、7.0以下であり、
炭化物の平均円相当直径が、5.0μm以下であり、
アスペクト比が2.0以下である炭化物の個数割合が、全炭化物に対して80%以上であり、
フェライト結晶粒内に存在する炭化物の個数割合が、全炭化物に対して60%以上である、浸炭用鋼板。 - 残部のFeの一部に換えて、質量%で、
Cr:0.005%以上3.0%以下
Mo:0.005%以上1.0%以下
Ni:0.010%以上3.0%以下
Cu:0.001%以上2.0%以下
Co:0.001%以上2.0%以下
Nb:0.010%以上0.150%以下
Ti:0.010%以上0.150%以下
V:0.0005%以上1.0%以下
B:0.0005%以上0.01%以下
の1種又は2種以上を更に含有する、請求項1に記載の浸炭用鋼板。 - 残部のFeの一部に換えて、質量%で、
Sn:1.0%以下
W:1.0%以下
Ca:0.01%以下
REM:0.3%以下
の1種又は2種以上を更に含有する、請求項1又は2に記載の浸炭用鋼板。 - 請求項1~3の何れか1項に記載の浸炭用鋼板を製造する方法であって、
請求項1~3の何れか1項に記載の化学組成を有する鋼材を加熱し、熱間仕上圧延の1パス前の圧延を、900℃以上980℃以下の温度域で15%以上25%以下の圧下率で実施し、熱間仕上圧延を、800℃以上920℃未満の温度域で6%以上の圧下率で終了し、700℃以下の温度で巻取る熱間圧延工程と、
前記熱間圧延工程により得られた鋼板、又は、前記熱間圧延工程後に冷間圧延が施された鋼板を、窒素濃度を体積分率で25%未満に制御した雰囲気にて、5℃/h以上100℃/h以下の平均加熱速度で、下記式(1)で定義されるAc1点以下の温度域まで加熱し、当該Ac1点以下の温度域で10h以上100h以下保持する焼鈍処理を施した後、焼鈍終了時の温度から550℃までの温度域における平均冷却速度を5℃/h以上100℃/h以下とする冷却を施す焼鈍工程と、
を含む、浸炭用鋼板の製造方法。
ここで、下記式(1)において、[X]との表記は、元素Xの含有量(単位:質量%)を表し、該当する元素を含有しない場合はゼロを代入するものとする。
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US16/346,461 US10934609B2 (en) | 2017-08-31 | 2018-08-30 | Steel sheet for carburizing, and method for manufacturing steel sheet for carburizing |
CN201880004175.6A CN109983145B (zh) | 2017-08-31 | 2018-08-30 | 渗碳用钢板和渗碳用钢板的制造方法 |
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BR112019008336A BR112019008336A2 (pt) | 2017-08-31 | 2018-08-30 | chapa de aço para carburação, e método de fabricação da chapa de aço para carburação |
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WO2021230150A1 (ja) * | 2020-05-13 | 2021-11-18 | 日本製鉄株式会社 | ホットスタンプ用鋼板およびホットスタンプ成形体 |
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