JP2018048374A - High carbon steel sheet member and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high carbon steel sheet member having excellent hardness and excellent hardness-toughness balance.SOLUTION: There is provided a high carbon steel sheet member containing, C:0.80 mass% to 1.10 mass%, Si:0.05 mass% to 0.40 mass%, Mn:0.05 mass% to 0.50 mass%, Cr:0.01 mass% to 0.35 mass%, P:0.03 mass% or less (including 0 mass%), S:0.03 mass% or less (including 0 mass%) and the balance Fe with inevitable impurities, a steel structure contains bainite:90 area% or more, carbide with equivalent circle diameter of 0.2 μm or more:1.0 to 4.0 area% and retained austenite:0.5 area% or more, average particle diameter of old austenite is 8 μm or less, bainite block width is 2.5 μm or less, average particle diameter of the carbide is 0.3 μm to 2 μm, an average value of KAM measured by EBSD is 0.55 to 0.80 and carbide occupancy on an old austenite grain boundary is 5% or less.SELECTED DRAWING: None

Description

  The present invention relates to a high-carbon steel plate member and a method for manufacturing the same, and more particularly to a high-carbon steel plate member useful as a member used for a blade and a tool, and a method for manufacturing the same.

  In addition to excellent hardness, members used for various blades such as circular saws, band saws and carters, mainsprings, springs, tools, etc. are required to have excellent toughness so that the cutting edges and the like do not break during use.

  For example, in Patent Document 1, among carbides existing in a metal structure, the area ratio of carbides having an equivalent circle diameter of 0.5 μm or more is set to 0.50 to 4.30%, so that the Vickers hardness is 500. It is disclosed that a carbon tool steel strip used as various spring materials and valve materials of ˜650 HV can be obtained.

  Patent Document 2 provides a high carbon steel member having excellent impact characteristics while having excellent hardness of 600 to 900 HV by defining the amount of insoluble carbide according to the amount of added C. Is disclosed.

Further, in Patent Document 3, after heating and holding at a temperature of Ac ₃ point or higher (that is, after austenitization), rapid cooling to a temperature of martensite transformation temperature (Ms point) or higher and 500 ° C. or lower is performed. A steel material excellent in hardness and fatigue characteristics is manufactured by obtaining a bainite structure by so-called austempering treatment.

WO2013 / 133295 JP 2006-63384 A JP-A-6-271930

As described above, in various applications such as blades, mainsprings, springs, and tools, it is required to have not only excellent hardness but also excellent hardness-toughness balance.
The following are specifically required for the hardness and the hardness-toughness balance.

About hardness, it is calculated | required that it is 550HV or more.
The hardness-toughness balance can be evaluated by the product of the hardness (HV) and the bending angle (°) obtained by the VDA bending test. (Hardness−170) × VDA standard bending angle (HV · Is required to be greater than 12,500.

  However, in the steel materials disclosed in Patent Documents 1 to 3, it is difficult to satisfy both of these requirements, and a steel plate member that can satisfy both of these requirements has been demanded.

  The present invention has been made to meet such demands, and one of its purposes is to provide a high-carbon steel sheet member having excellent hardness and excellent hardness-toughness balance. Another object is to provide a method for producing a high carbon steel sheet member having excellent hardness and excellent hardness-toughness balance.

Aspect 1 of the present invention
C: 0.80% by mass to 1.10% by mass,
Si: 0.05 mass% to 0.40 mass%,
Mn: 0.05% by mass to 0.50% by mass,
Cr: 0.01% by mass to 0.35% by mass,
P: 0.03 mass% or less (including 0 mass%),
S: 0.03 mass% or less (including 0 mass%),
Is a high carbon steel plate member made of Fe and inevitable impurities,
The steel structure includes bainite: 90 area% or more, carbide having an equivalent circle diameter of 0.2 μm or more: 1.0 to 4.0 area% and retained austenite: 0.5 area% or more,
The average particle size of the prior austenite is 8 μm or less,
The bainite block width is 2.5 μm or less,
The carbide has an average particle size of 0.3 μm or more and 2 μm or less,
The average value of KAM measured by EBSD is 0.55 to 0.80,
This is a high carbon steel sheet member having a carbide occupancy on the prior austenite grain boundaries of 5% or less.

Aspect 2 of the present invention
The high carbon steel sheet member according to aspect 1, wherein the carbide has an average particle size of more than 1 μm and 2 μm or less.

Aspect 3 of the present invention
The high carbon steel sheet member according to aspect 1, wherein the prior austenite has an average particle diameter of 6 μm or less and the carbide has an average particle diameter of 0.3 μm or more and 1 μm or less.

Aspect 4 of the present invention
A step of spheroidizing and annealing the steel sheet having the component according to aspect 1 to obtain an annealed steel sheet having an average particle size Rc of carbide of 0.2 to 3.0 μm;
Holding the annealed steel sheet at a temperature T ₁ : 800 to 920 ° C. and a time t ₁ : 0.1 to 50 minutes so as to satisfy the following formula (1), and austenitizing:
After the austenitization, the annealed steel sheet is cooled at an average cooling rate of 50 ° C./second or more to a temperature T ₃ = 270 to 330 ° C., and the austempering temperature T ₃ and time t so as to satisfy the following equation (2): ₃ = holding at _{3 to} 15 minutes and austempering;
It is a manufacturing method of the high carbon steel plate member containing this.

15300 ≦ (T ₁ +273) × (15 + logt ₁ ) − (Rc +% C) × 400 ≦ 16800 (1)
8700 ≦ (T ₃ +273) × (15 + logt ₃ ) ≦ 9200 (2)
Where log is a common logarithm with base 10.

Aspect 5 of the present invention
It is the manufacturing method of the high carbon steel plate member of the aspect 4 whose average particle diameter Rc of the said carbide | carbonized_material is 2.0-3.0 micrometers.

Aspect 6 of the present invention
It is a manufacturing method of the high carbon steel plate member of the aspect 4 whose average particle diameter Rc of the said carbide | carbonized_material is 0.2-2.0 micrometers.

  In one embodiment of the present invention, a high carbon steel plate member having excellent hardness and excellent hardness-toughness balance can be provided, and in another embodiment, excellent hardness and excellent hardness. -The manufacturing method of the high carbon steel plate member which has toughness balance can be provided.

As a result of intensive studies, the present inventors have found that steel having a predetermined component has a steel structure (metal structure) of bainite: 90 area% or more and carbide having an equivalent circle diameter of 0.2 μm or more: 1.0 to 4.0. Area% and retained austenite: including 0.5 area% or more, old austenite average particle size: 8 μm or less, bainite block width: 2.5 μm or less, average particle size of the carbide: 0.3 μm or more, 2 μm or less, EBSD The average value of the measured KAM: 0.55 to 0.80 and the carbide occupancy ratio on the prior austenite grain boundaries: 5% or less, so that the hardness and hardness-toughness balance are both high. It has been found that a carbon steel plate member can be obtained.
Below, the detail of the high carbon steel plate which concerns on embodiment of this invention, and its manufacturing method is shown.

1. Structure parameter (1) Steel structure The steel structure of the high carbon steel sheet member of the present invention will be described in detail below.
The steel structure of the high carbon steel sheet member according to the embodiment of the present invention is as follows: bainite: 90 area% or more, carbide having an equivalent circle diameter of 0.2 μm or more: 1.0 to 4.0 area%, and retained austenite: 0.5 area. % Or more included.
In the following description of the steel structure, a mechanism that can improve various properties by having such a structure may be described. It should be noted that these are the mechanisms considered by the present inventors based on the knowledge obtained at the present time, but do not limit the technical scope of the present invention.

(1-1) Bainite: 90 area% or more By using bainite as the main component of the steel structure, it is possible to ensure the hardness necessary for the tool and the cutter, and to improve the hardness-toughness balance. Such an effect can be exhibited if the bainite fraction in the steel structure is 90 area% or more. The bainite fraction is preferably 92 area% or more and 98 area% or less, more preferably 94 area% or more and 96 area% or less.

(1-2) Carbide having an equivalent circle diameter of 0.2 μm or more: 1.0 to 4.0 area%
By securing a certain amount of undissolved carbide, coarsening of the austenite grain size during heating for austenitization can be suppressed, and the hardness-toughness balance can be improved. In addition, by securing a certain amount of undissolved carbide, the amount of dissolved C in austenite is reduced, and the toughness of bainite after heat treatment can be increased.

  In addition, the influence on the toughness of the steel plate member after the heat treatment is mainly caused by carbides having an equivalent circle diameter of 0.2 μm or more. This is because if the equivalent circle diameter of the carbide is 0.2 μm or more, there is an adverse effect that becomes a starting point of fracture when a force such as bending stress is applied from the outside. On the other hand, if the equivalent circle diameter of the carbide is less than 0.2 μm, it is unlikely to become a starting point of fracture, so the influence on toughness is negligible. Therefore, what is necessary is just to measure the quantity (area ratio) of the carbide | carbonized_material of circle equivalent diameter 0.2micrometer or more as quantitative evaluation of insoluble carbide | carbonized_material. In the present specification, the simple description of “insoluble solid carbide” means a carbide having an equivalent circle diameter of 0.2 μm or more.

If the undissolved carbide described above is 1.0 area% or more, the above-described effects can be obtained. The amount of undissolved carbide is preferably 1.3 area% or more, more preferably 1.8 area% or more.
On the other hand, when there is too much undissolved carbide amount, toughness will be reduced. Therefore, the amount of undissolved carbide is 4.0 area% or less, preferably 3.7 area% or less, more preferably 3.3 area% or less.

(1-3) Residual austenite: 0.5 area% or more Residual austenite can exhibit the effect of relieving stress by processing-induced transformation. In order to reliably obtain such an effect, the amount of retained austenite is 0.5 area% or more, preferably 1.0 area% or more, more preferably 2.0 area% or more.

  Note that the steel structure according to the present embodiment is basically bainite, but may contain a small amount of other structures inevitably generated such as pearlite, ferrite, and grain boundary oxide layers in addition to the structure described above. Good. That is, if bainite: 90 area% or more, carbide having an equivalent circle diameter of 0.2 μm or more: 1.0 to 4.0 area% and retained austenite: 0.5 area% or more are included, for example, Other structures such as pearlite, ferrite and grain boundary oxide layers may be included.

The amount of bainite, the amount of carbide having an equivalent circle diameter of 0.2 μm or more, and the amount of retained austenite can be measured, for example, as follows.
For example, a cross section that is parallel to the longitudinal direction of the non-test object after heat treatment and perpendicular to the plate surface is cut out, and the surface that is a 1/4 position in the thickness direction is polished, and 400 μm × 400 μm in the width direction by EBSD with a resolution of 1 nm. By measuring one visual field, the amount of bainite, the amount of carbide having an equivalent circle diameter of 0.2 μm or more, and the amount of residual austenite can be obtained. For the analysis of the SEM image, for example, Image Analysis Software Image-Pro Plus manufactured by Media Cybernetics may be used.

(2) Average particle size of prior austenite: 8 μm or less If the average particle size of prior austenite exceeds 8 μm, it becomes the starting point of fracture when a force such as bending stress is applied from the outside, so the toughness is drastically decreased. To do. Therefore, the average particle diameter of prior austenite is 8 μm or less, preferably 6 μm or less.

The average particle diameter of prior austenite can be measured, for example, as follows.
For example, with a corrosive solution in which 1% of a surfactant is mixed with a picric acid saturated aqueous solution, the observation surface is etched to reveal the prior austenite grain boundaries, and an optical microscope is used at a magnification of 100 to 400 times as appropriate. By observing and comparing with the standard diagram of JIS-G0551, the prior austenite grain size can be measured.

(3) Bainite block width: 2.5 μm or less A bainite block is a region surrounded by a boundary having an orientation difference of 15 ° or more, and the bainite block width means a minor axis of the bainite block. The bainite block width affects the toughness, and when it exceeds 2.5 μm, the toughness rapidly decreases. Therefore, the bainite block width is 2.5 μm or less, preferably 2.2 μm or less, more preferably 2.0 μm or less.

  The bainite block width can be obtained, for example, by the above-mentioned EBSD, by defining the minor axis as the block width with the grain boundary having an orientation difference of 15 degrees or more as the block size, and obtaining the average value.

(4) Average particle diameter of carbide: 0.3 μm or more and 2 μm or less The average equivalent circle diameter of carbide having an equivalent circle diameter of 0.2 μm or more (sometimes simply referred to as the average particle diameter of carbide) is from 0.3 μm. When it becomes smaller, it becomes easier to completely dissolve during quenching heating. As a result, the prior γ (austenite) particle size becomes coarser, the bainite block width becomes coarser, the amount of solid solution C in bainite increases, and toughness increases. descend. Therefore, the average particle size of the carbide is 0.3 μm or more.
On the other hand, when the average particle size of the carbide is larger than 2.0 μm, the toughness is reduced due to the large carbide itself. Therefore, the average particle size of the carbide is 2.0 μm or less.

  In a preferred embodiment, the average particle size of the carbide is greater than 1 μm and not greater than 2 μm. In normal heat treatment by furnace heating, if the carbide is too small, it is likely to be dissolved during heating, the effect of refining the prior austenite grain size will be lost, and the solid solution carbon in bainite will increase and the toughness may decrease. There is sex. On the other hand, if it exceeds 2.0 μm, it may become the starting point itself and reduce toughness. Therefore, toughness can be improved by setting the average particle size of the carbide to more than 1 μm and 2 μm or less. The average particle size of the carbide is more preferably 1.2 μm or more and 1.8 μm or less.

  In another preferred embodiment, the carbide has an average particle size of 0.3 μm or more and 1 μm or less. If the average particle size of the carbide is in such a range, the toughness can be further improved in the fine rapid heat treatment. When cooled by a rapid heat treatment, even small carbides are difficult to be completely dissolved. Therefore, by keeping the original carbide fine, the carbide after heat treatment is finely dispersed to refine the prior austenite grain size, and the carbide itself is also refined to suppress toughness reduction as much as possible. The average particle size of the carbide is more preferably 0.5 μm or more and 0.8 μm or less.

  The average particle diameter of the carbide can be obtained by, for example, the above-mentioned EBSD, using cementite having an equivalent circle diameter of 0.2 μm or more as insoluble solid carbide.

(5) Average value of KAM values measured by EBSD: 0.55 to 0.80
The KAM value (Kernel Average Misorientation value) is the average value of crystal rotation (crystal orientation difference) between the target measurement point and the surrounding measurement points. If this value is large, the strain in the crystal is large. It shows that.

  In the present invention, the azimuth difference from the measurement point is calculated at all of the eight neighboring points adjacent to the target measurement point, and the average of the eight azimuth differences is obtained as the KAM value at the measurement point. At this time, among the adjacent eight points, the points having an orientation difference of 15 ° or more were excluded from the KAM analysis. That is, among the eight adjacent points, the average of the azimuth differences at the points where the azimuth difference is less than 15 ° was obtained and used as the KAM value.

KAM values are also related to hardness and toughness. The higher the average KAM value, the more strain (dislocations) introduced by the bainite transformation remains, and since it is in a hardened state, the hardness increases but the toughness decreases. However, the balance between hardness and toughness is deteriorated. In order to obtain a good hardness-toughness balance, the KAM value measured by EBSD is 0.80 or less, preferably 0.75 or less, more preferably 0.70 or less.
On the other hand, if the average value of the KAM value is too low, it corresponds to the state where the bainite is tempered at a high temperature, and the hardness is too low to obtain the desired hardness. And the hardness-toughness balance decreases. Therefore, in order to obtain a good hardness-toughness balance, the KAM value measured by EBSD is 0.55 or more, preferably 0.60 or more, more preferably 0.65 or more.

  For the KAM value, for example, the KAM value was calculated based on the data measured by the EBSD described above.

(6) Carbide occupancy rate on prior austenite grain boundaries: 5% or less When the amount of carbides present on prior austenite grain boundaries is large, toughness decreases. Therefore, the carbide occupation rate on the prior austenite grain boundary is 5% or less, preferably 4% or less, more preferably 3% or less.

  The carbide occupancy on the prior austenite grain boundary is, for example, that the observation surface is corroded with picral, and then observed by FE-SEM at 10,000 times and five fields of view. The occupancy can be obtained by measuring the length of the portion where the cementite is present.

2. Composition Below, the composition of the high carbon steel plate member concerning the present invention is explained. First, basic elements C, Si, Al, Mn, P and S will be described, and further elements that may be selectively added will be described.
In addition, unit% display of a component composition means the mass% altogether.

(1) C: 0.80 to 1.10% by mass
C is an element necessary for improving the hardness after heat treatment. Therefore, the C content is 0.80% by mass or more, preferably 0.82% by mass or more, and more preferably 0.84% by mass or more. However, if the C content is too large, the amount of coarse undissolved carbides that adversely affects toughness and impact properties increases. Therefore, the C content is 1.10% by mass or less, preferably 1.07% by mass or less, and more preferably 1.04% by mass or less.

(2) Si: 0.05-0.40 mass%
Si is an element that acts as a deoxidizer. In order to effectively exhibit such an action, the Si content is required to be 0.05% by mass or more, preferably 0.1% by mass or more, more preferably 0.20% by mass or more. On the other hand, when there is too much Si content, there exists a possibility that workability may deteriorate. Therefore, the Si content is 0.40% by mass or less, preferably 0.30% by mass or less, more preferably 0.25% by mass or less.

(3) Mn: 0.05 to 0.50 mass%
Mn has the effect of improving solid solution strengthening and improving hardenability to facilitate the formation of bainite. In order to effectively exhibit such an effect, the Mn content is 0.05% by mass or more, preferably 0.10% by mass or more, and more preferably 0.20% by mass or more.
However, when the Mn content is excessive, the residual γ amount is increased more than necessary, and the balance between toughness and hardness may be deteriorated. Therefore, the Mn content is 0.50% by mass or less, preferably 0.45% by mass or less, more preferably 0.40% by mass or less.

(4) Cr: 0.01 to 0.35 mass%
Cr has the effect of improving hardenability and facilitating the formation of bainite. In order to effectively exhibit such an effect, the Cr content is 0.01% by mass or more, preferably 0.05% by mass or more, and more preferably 0.10% by mass or more. On the other hand, when the Cr content is excessive, the toughness may be deteriorated. Therefore, Cr content is 0.35 mass% or less, Preferably it is 0.30 mass% or less, More preferably, it is 0.25 mass% or less.

(5) P: 0.03 mass% or less (including 0 mass%)
P is unavoidably present as an impurity element, and segregates at the grain boundaries to deteriorate the bending workability (toughness). Therefore, the P content is 0.03% by mass or less, preferably 0.025% by mass or less, more preferably 0.02% by mass or less.

(6) S: 0.03 mass% or less (including 0 mass%)
S is unavoidably present as an impurity element, and forms a sulfide to deteriorate bending workability (toughness). Therefore, the S content is 0.03% by mass or less, preferably 0.025% by mass or less, more preferably 0.02% by mass or less.

(7) Balance In one preferred embodiment, the balance is iron and inevitable impurities. As inevitable impurities, mixing of trace elements (for example, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. In addition, for example, like P and S, it is usually preferable that the content is small. Therefore, although it is an unavoidable impurity, there is an element that separately defines the composition range as described above. For this reason, in this specification, the term “inevitable impurities” constituting the balance is a concept that excludes elements whose composition ranges are separately defined.

  However, it is not limited to this embodiment. As long as the characteristics of the high carbon steel sheet member of the present invention can be maintained, any other element may be further included. Other elements that can be selectively contained as described above are exemplified below.

If necessary, one or more selected from the following elements may be added.
Mo: more than 0% by mass and 0.5% by mass or less, Ni: more than 0% by mass and 0.5% by mass or less, V: more than 0% by mass and 0.35% by mass or less, Nb: more than 0% by mass and 0.30% by mass Hereinafter, Ti: more than 0% by mass and 0.20% by mass or less and N: more than 0% by mass and 0.02% by mass or less.

3. Mechanical Properties As described above, the high carbon steel plate member of the present invention has a high level of hardness and hardness-toughness balance. These characteristics of the high carbon steel sheet member of the present invention will be described in detail below.

(1) Hardness The high carbon steel plate member of the present invention has a hardness of 550 HV or higher. Thereby, sufficient hardness is securable.

  The hardness is measured, for example, by performing a Vickers hardness test in accordance with the provisions of JIS Z2244 (revised version 2009) at a 1/4 position in the thickness direction of the steel plate member, and measuring 5 points with a test load of 5 kgf. The average value can be determined.

(2) Hardness-toughness balance The high-carbon steel sheet member of the present invention has excellent hardness and at the same time excellent hardness, that is, excellent hardness-toughness balance. Therefore, when the high carbon steel plate member of the present invention is used for a blade or a tool, it is possible to suppress damage to the blade edge or the like due to use.
The hardness-toughness balance can be evaluated by the hardness (HV) obtained by the above-described hardness test and the bending angle (°) obtained by the VDA bending test.
The VDA test may be performed based on, for example, the VDA standard (VDA 238-100) defined by the German Automobile Manufacturers Association.

  The high carbon steel sheet member according to the present invention has a (hardness−170) × VDA standard bending angle (HV · °) exceeding 12500. As a result, a sufficient level of hardness-toughness balance can be provided for application to a cutter / tool. The hardness-toughness balance is preferably 13500 or more. In addition, the present inventors do not simply multiply the hardness and the bending angle based on the VDA, but by multiplying the bending angle by a value obtained by subtracting 170 from the hardness, the contribution degree of hardness and toughness is obtained. We have found from many other case studies that it is possible to properly allocate and evaluate the total characteristic balance. Therefore, in this application, the evaluation of the balance between hardness and toughness is evaluated by multiplying the value obtained by subtracting 170 from the hardness and the bending angle.

4). Manufacturing method Next, the manufacturing method of the high carbon steel plate member concerning the embodiment of the present invention is explained.

(1) Steel material for rolling The steel material for rolling (slab) which has the said component composition is produced. The slab may be prepared by any known method. The following method can be illustrated.
A steel having the above component composition is melted, and a slab is produced as a steel material for rolling by ingot forming or continuous casting. Note that a slab may be obtained by subjecting a cast material obtained by ingot-making or continuous casting to partial rolling as necessary.

(2) Hot rolling Next, it hot-rolls using the above-mentioned slab, and obtains a steel plate. Hot rolling may be performed under any known conditions. Thereafter, pickling and cold rolling (skin pass) may be further performed in accordance with necessary conditions such as surface condition and plate thickness accuracy. The following method can be illustrated.
In the hot rolling, for example, the heating temperature before hot rolling is set to 1150 to 1300 ° C. The rough rolling temperature is set to 900 to 1100 ° C. in consideration of securing the temperature in the subsequent finish rolling, and the finish rolling temperature is set to 800 ° C. or higher. You may wind up after 550 degreeC and below 650 degreeC after finish rolling.

(3) Spheroidizing annealing Next, the obtained steel plate is subjected to spheroidizing annealing to obtain a steel plate having the above component composition, and having an average particle size Rc of carbide of 0.2 to 3.0 μm. .
By using an annealed steel sheet having such an average grain size Rc carbide, it is possible to stably leave undissolved carbide in a steel sheet member obtained by austenite later.

If the average particle size of the carbide is less than 0.2 μm, the toughness is lowered because the carbide is excessively dissolved during austenitization. On the other hand, if the average particle size of the carbide exceeds 3 μm, coarse undissolved carbide tends to remain in the austenitized steel plate member, and the toughness decreases.
Therefore, in the manufacturing method according to the present embodiment, an annealed steel sheet having an average particle size Rc of carbide of 0.2 to 3.0 μm is used.

  An annealed steel sheet having an average particle diameter Rc of carbide of 0.2 to 3.0 μm can be obtained by subjecting a rolled material to spheroidizing annealing under the following conditions, for example.

  The rolled material is held at a heating temperature of 685 ° C. or more and 780 ° C. or less for 1 hour to 10 hours, and then cooled to 680 ° C. at a cooling rate of 10 ° C./hour or less. Thereby, the annealed steel plate whose average particle diameter Rc of a carbide | carbonized_material is 0.2-3.0 micrometers can be obtained.

  In a preferred embodiment, an annealed steel sheet having an average particle size Rc of carbide of 2 to 3.0 μm may be obtained. By using an annealed steel sheet having a carbide having such an average grain size Rc, in a steel sheet member obtained by austenite later, undissolved carbide can be left more stably. A bainite structure having a small diameter and a bainite block width and excellent toughness can be obtained.

  An annealed steel sheet having an average particle size Rc of carbide of 2 to 3.0 μm can be obtained by subjecting the rolled material to spheroidizing annealing, for example, under the following conditions.

The rolled material is held at a heating temperature of 745 ° C. or more and 780 ° C. or less for 1 hour to 10 hours, and then cooled to 680 ° C. at a cooling rate of 10 ° C./hour or less. Thereby, the annealing steel plate whose average particle diameter Rc of a carbide | carbonized_material is 2-3.0 micrometers can be obtained.
Further, by repeating such spheroidizing annealing twice, a spherical carbide structure having a larger average particle size can be obtained. A relatively large spheroidized carbide structure can also be obtained by pickling after hot rolling, cold rolling and then spheroidizing annealing.

  Moreover, in another preferable embodiment, you may obtain the annealed steel plate whose average particle diameter Rc of a carbide | carbonized_material is 0.2-2.0 micrometers. By using an annealed steel sheet having a carbide having such an average grain size Rc, in a steel sheet member obtained by austenite later, undissolved carbide can be left more stably. A bainite structure having a small diameter and a bainite block width and excellent toughness can be obtained.

  An annealed steel sheet having an average particle size Rc of carbide of 0.2 to 2.0 μm can be obtained by subjecting a rolled material to spheroidizing annealing under the following conditions, for example.

  The rolled material is held at a heating temperature of 685 ° C. or more and less than 745 ° C. for 1 hour to 10 hours, and then cooled to 680 ° C. at a cooling rate of 10 ° C./hour or less. Thereby, the annealed steel plate whose average particle diameter Rc of a carbide | carbonized_material is 0.2-2.0 micrometers can be obtained.

(4) Austenitization Hold the annealed steel sheet after spheroidizing annealing at austenitizing temperature T ₁ : 800 to 920 ° C. and time t ₁ : 0.1 to 50 minutes so as to satisfy the following formula (1). Austenite.

15300 ≦ (T ₁ +273) × (15 + logt ₁ ) − (Rc +% C) × 400 ≦ 16800 (1)
Where log is a common logarithm with base 10.

If the improved quenching parameter M ₁ ′ (that is, (T ₁ +273) × (15 + logt ₁ ) − (Rc +% C) × 400) in the formula (1) is less than 15300, the amount of undissolved carbide becomes too much, and toughness Decreases. Therefore, the improved quenching parameter M ₁ ′ is 15300 or more, preferably 15400 or more, and more preferably 15500 or more.
On the other hand, when the improved quenching parameter M ₁ ′ exceeds 16800, the prior austenite grain size increases and the toughness decreases. Therefore, the improved quenching parameter M ₁ ′ is 16800 or less, preferably 16400 or less, and more preferably 16000 or less.

(5) Austempering treatment After the above-mentioned austenite treatment, the austempering treatment is performed to obtain a finished product of a high carbon steel sheet member.
In the austempering process, after austenization, the austempering temperature T ₃ is cooled to 270 to 330 ° C. at an average cooling rate of 50 ° C./second or more, and at the austempering temperature T ₃ , the time t ₃ = Hold for 3-15 minutes.

8700 ≦ (T ₃ +273) × (15 + logt ₃ ) ≦ 9200 (2)
Where log is a common logarithm with base 10.

When the austempering parameter M ₃ (that is, (T ₃ +273) × (15 + logt ₃ )) in the formula (2) is less than 8700, the KAM value falls outside the definition of the present invention, and the hardness-toughness balance is lowered. Therefore, it is austempered parameter _{M 3} represents 8700 or more, preferably 8800 or more, more preferably 8900 or more.
On the other hand, when the austempered parameter M ₃ exceeds 9200, KAM value outside the provisions of the present invention, the hardness - toughness balance is decreased. Therefore, austempered parameter _{M 3} represents 9200 or more, preferably 9180 or less, more preferably 9160 or less.

By the above heat treatment, the high carbon steel sheet member according to the embodiment of the present invention can be obtained.
Note that the heat treatment temperatures T ₁ and T ₃ defined in this specification may be managed by using the furnace temperature of a heat treatment furnace that normally performs heat treatment. Further, if necessary, temperature measurement means such as a thermocouple may be arranged on the surface of the material to be heat-treated and managed by measuring the material temperature.

1. Sample Preparation Six types of steel sheets containing the chemical composition described in Table 1 were annealed and spheroidized, and using this steel sheet, a test piece No. 20 mm × 60 mm was obtained. 1-30 were produced. Specimen No. 1-30 were heat-treated under the conditions shown in Table 2, and a Vickers hardness test and a VDA bending test were performed. The VDA bending test evaluates toughness.
In Tables 1 and 2 and Table 3 described later, the underlined numerical values indicate that they are out of the scope of the embodiment of the present invention.

2. Measurement of structure parameters [Measurement of steel structure]
The amount of bainite, the amount of carbide having an equivalent circle diameter of 0.2 μm or more, and the amount of retained austenite were measured as follows.

  A cross section perpendicular to the plate surface parallel to the longitudinal direction of the bent specimen after the heat treatment was cut out, and the surface at a 1/4 position in the thickness direction was polished, and 1 μm of 400 μm × 400 μm in the width direction was obtained by EBSD with a resolution of 1 nm. By measuring the visual field, the amount of bainite, the amount of retained austenite, the average equivalent circle diameter of the retained austenite, and the amount of cementite having an equivalent circle diameter of 0.2 μm or more were determined. Image analysis software Image-Pro Plus manufactured by Media Cybernetics was used for the analysis of the SEM image.

[Average particle size of former austenite]
The average particle diameter of the prior austenite is a corrosive solution in which 1% of a surfactant is mixed with a saturated aqueous solution of picric acid. The observation surface of the test piece is etched to reveal the prior austenite grain boundary, and then an optical microscope is used. The prior austenite particle size was measured by appropriately observing at a magnification of 100 to 400 times and comparing with a standard diagram of JIS-G0551.

[Bainite block width]
The bainite block width was determined by the above-mentioned EBSD, with the grain boundaries having an orientation difference of 15 degrees or more defined as the block size and the minor axis defined as the block width, and an average value was obtained.

[Average particle size of undissolved carbide]
The average particle diameter of the insoluble carbide was determined by the above-mentioned EBSD, with cementite having an equivalent circle diameter of 0.2 μm or more as insoluble solid carbide.

[Average value of KAM measured by EBSD]
The average value of KAM measured by EBSD was calculated based on the data measured by EBSD described above.

[Carbide occupancy on former austenite grain boundaries]
Carbide occupancy on the prior austenite grain boundaries was observed by FE-SEM observation after observing the observation surface with picral. The length of the part where the existing cementite exists was measured, and the occupation ratio was obtained.

3. Measurement of mechanical properties Mechanical properties of the obtained samples were evaluated.

(Hardness)
Each sample was subjected to a Vickers hardness test in accordance with the provisions of JIS Z2244 (2009 revised edition) at a 1/4 position in the thickness direction, measured at a test load of 5 kgf, and an average value was obtained. .

(Hardness-Toughness Balance)
Toughness was evaluated by VDA bending test. Specifically, evaluation was performed under the following measurement conditions based on the VDA standard (VDA238-100) defined by the German Automobile Manufacturers Association. In the present invention, the displacement at the maximum load obtained by a bending test was converted into an angle based on the VDA, and the bending angle was obtained. The results are shown in “VDA bending angle (°)” in the table.
(Measurement condition)
Test method: roll support, punch push-in roll diameter: φ30mm
Punch shape: Tip R = 0.4mm
Distance between rolls: 3.2 mm
Pushing speed: 6mm / min
Specimen size: 60mm x 20mm
Bending direction: Rolling perpendicular direction Testing machine: Universal testing machine AG-IS 50kN manufactured by Shimadzu Corporation

  In the present invention, when the measured Vickers hardness HV is 550 or more and the (hardness−170) × VDA standard bending angle (HV · °) exceeds 12500, the hardness-toughness balance is regarded as excellent. In other cases, it was evaluated as rejected (x). The results are shown in Table 3.

4). Summary Examples Nos. 1 to 5, 9 to 11, 16 to 18, and 21 to 25, which satisfy the conditions of the present invention, have a hardness of 550 HV or higher and a hardness of 12,500 ° HV or higher (hardness— 170) x VDA standard bending angle is achieved.

In contrast, sample no. In both Nos. 6 and 7, the setting of temperature and time (austemper parameter M ₃ ) in the bainite generation heat treatment (austempering process) is inappropriate. No. 6 has a large bainite block width and a small KAM value. 7, the KAM value increased. Therefore, no. Both 6 and 7 were inferior in hardness-toughness balance.

No. In _No. 8, since the improved quenching parameter M ₁ ′ was too large, the prior austenite grain size was coarsened, and undissolved carbides were also reduced. Therefore, the value of the bending angle based on (hardness−170) × VDA did not reach the target value.

No. In both Nos. 12 and 13, the setting of the temperature and time (austemper parameter M ₃ ) in the bainite generation heat treatment (austempering process) was inappropriate, so the KAM value was small and the hardness-toughness balance was inferior.

  No. 14 and 15 are both subjected to normal quenching and tempering treatment, and the hardness is appropriate, but the steel structure is not a bainite-based steel, there are many carbides on the prior austenite grain boundaries, and the amount of retained austenite is small. Therefore, the value of the bending angle based on (hardness−170) × VDA was small, and the hardness-toughness balance was inferior.

  No. In 19 and 20, since the amount of C was out of the range of the present invention, as a result, the amount of undissolved carbide was out of the range of the present invention, and the hardness-toughness balance was inferior.

No. No. 26 had an inappropriate setting of temperature and time (austemper parameter M ₃ ) in the bainite generation heat treatment (austempering), and therefore the bainite block width was increased, the KAM value was decreased, and the hardness-toughness balance was inferior. .

No. In _No. 27, the improved quenching parameter M ₁ ′ and the austempering parameter M ₃ are inappropriate, the bainite fraction and the amount of undissolved carbide are not within the scope of the present invention, and the bainite fraction is low. Was inferior.

No. In _No. 28, the improved quenching parameter M ₁ ′ was inappropriate, the amount of undissolved carbide was reduced, the old γ grain size was increased, and the hardness-toughness balance was inferior.

No. In _No. 29, the improved quenching parameter M ₁ ′ and the austempering parameter M ₃ were inappropriate, the amount of undissolved carbide was too large, the KAM average value was small, and the hardness-toughness balance was inferior.

No. 30 is inadequate austempered parameter M _3, becomes small fraction of bainite, KAM value is increased, the hardness - was inferior toughness balance.

Claims (6)

C: 0.80% by mass to 1.10% by mass,
Si: 0.05 mass% to 0.40 mass%,
Mn: 0.05% by mass to 0.50% by mass,
Cr: 0.01% by mass to 0.35% by mass,
P: 0.03 mass% or less (including 0 mass%),
S: 0.03 mass% or less (including 0 mass%),
Is a high carbon steel plate member made of Fe and inevitable impurities,
The steel structure includes bainite: 90 area% or more, carbide having an equivalent circle diameter of 0.2 μm or more: 1.0 to 4.0 area% and retained austenite: 0.5 area% or more,
The average particle size of the prior austenite is 8 μm or less,
The bainite block width is 2.5 μm or less,
The carbide has an average particle size of 0.3 μm or more and 2 μm or less,
The average value of KAM measured by EBSD is 0.55 to 0.80,
A high-carbon steel plate member having a carbide occupancy of 5% or less on the former austenite grain boundaries.   The high carbon steel plate member according to claim 1 whose average particle size of said carbide is more than 1 micrometer and 2 micrometers or less.   The high carbon steel plate member according to claim 1, wherein an average particle size of the prior austenite is 6 µm or less, and an average particle size of the carbide is 0.3 µm or more and 1 µm or less. Spheroidizing and annealing the steel sheet having the component according to claim 1 to obtain an annealed steel sheet having an average particle size Rc of carbide of 0.2 to 3.0 μm;
Holding the annealed steel sheet at a temperature T ₁ : 800 to 920 ° C. and a time t ₁ : 0.1 to 50 minutes so as to satisfy the following formula (1), and austenitizing:
After the austenitization, the annealed steel sheet is cooled at an average cooling rate of 50 ° C./second or more to a temperature T ₃ = 270 to 330 ° C., and the austempering temperature T ₃ and time t so as to satisfy the following equation (2): ₃ = holding at _{3 to} 15 minutes and austempering;
The manufacturing method of the high carbon steel plate member containing this.

15300 ≦ (T ₁ +273) × (15 + logt ₁ ) − (Rc +% C) × 400 ≦ 16800 (1)
8700 ≦ (T ₃ +273) × (15 + logt ₃ ) ≦ 9200 (2)
Where log is a common logarithm with base 10.   The manufacturing method of the high carbon steel plate member of Claim 4 whose average particle diameter Rc of the said carbide | carbonized_material is 2.0-3.0 micrometers.   The manufacturing method of the high carbon steel plate member of Claim 4 whose average particle diameter Rc of the said carbide | carbonized_material is 0.2-2.0 micrometers.