JP5652844B2 - High formability carburized steel sheet - Google Patents

Description

  The present invention relates to a steel sheet that can be easily formed and does not generate an abnormal structure during carburizing.

For example, for carbon parts such as gears and shafts, medium-carbon alloy steels for machine structures defined in JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106, etc. are generally used. . Usually, steel bars are used after hot forging or cold forging, and after that, after cutting into a predetermined shape by carburizing and quenching and tempering, the mechanical properties are enhanced.
The automobile parts such as the gears and shafts are produced by cutting after forging, and thus have a problem that product yield and productivity are poor.

Therefore, gears, bearing outer rings, or pulleys, which are parts that are thinner than shafts, etc., are made of medium-thickness steel plates, processed to a desired shape by pressing, etc., and then heat treated. It is also under consideration to manufacture.
For example, Patent Document 1 proposes a steel plate for carburizing and quenching that has excellent workability and can provide a steel plate member having excellent strength and toughness after carburizing and quenching.

JP 2008-214707 A

  The steel plate proposed in Patent Document 1 is in mass%, C: 0.15 to 0.30%, Si: less than 0.15%, Mn: 0.50 to 1.5%, S: 0.02%. Hereinafter, Cr: 0.05-0.20%, Al: 0.050% or less and N: less than 0.0050%, further containing Mo: 0.30% or less as required, The content satisfies the upper and lower limits set by a predetermined relational expression including the contents of N and Ti, and the P content is set by another relational expression including the contents of B, N and Ti. The upper limit is satisfied, the balance has a chemical composition composed of Fe and impurities, and the hole expansion rate is 60% or more.

  The steel sheet has a low content of Cr and Mo, which are hardenability improving elements, in consideration of workability. For this reason, hardenability is not enough. For example, thrust bearings, bearing outer rings, clutch release parts, etc. can be used even if they are formed by pressing from a relatively thin steel plate, even if they are not very hard, but gears, bearing outer rings, pulleys, etc. In the case of using a steel plate having a thickness of 3 to 15 mm as a raw material, the desired surface hardness cannot be obtained after carburizing and quenching with the hardenability of the steel plate described in Patent Document 1. Also in the said patent document 1, the characteristic before and behind carburizing quenching is only confirmed about the steel plate about 2.5 mm thick.

  The present invention has been devised to solve such problems, and has excellent workability and carburizing and quenching even for steel plates with a thickness of 3 to 15 mm used for gears, outer rings of bearings, pulleys, etc. It aims at providing the steel plate which has the property.

In order to achieve the object, the highly workable carburizing steel sheet according to the present invention includes C: 0.10 to 0.40 mass%, Si: 0.02 to 0.40 mass%, and Mn: 1.02 to 2.0. 00 mass%, P: 0.02 mass% or less, S: 0.02 mass% or less, Cr: 0.20 to 0.70 mass%, B: 0.0003 to 0.005 mass%, Ti: 0.02 mass% The component composition including 03 to 0.20% by mass, the balance being Fe and inevitable impurities, and the constituent phase occupying 1% or more of the area ratio are only the ferrite phase and the cementite phase, and the cementite phase is 0.2 μm or more. in addition to particles with particle sizes of 392 to 1500 pieces / 10000 ^2, having a tissue Ti carbide particles having a particle size of 20~100nm are dispersed in the range of 4,000 to 20,000 pieces / 10000 ^2, 180 Hv It is characterized by exhibiting a hardness of less than
In the present specification, the particle diameter is an average of the short axis length and the long axis length.

The high formability carburized steel sheet further has a component composition including one or two of Ni: 0.20 to 2.00% by mass and Mo: 0.10 to 0.80% by mass. May be.
Further, the high workability carburized steel sheet further has a component composition including one or two of Nb: 0.02 to 0.10% by mass and V: 0.02 to 0.20% by mass, It may have a metal structure in which carbides of Ti, Nb and V having a particle diameter of 20 to 100 nm are dispersed in a range of 4000 to 20000 / 10,000 μm ² .
The total content of Ti, Nb and V is preferably 0.03 to 0.35% by mass.

According to the present invention, by finely setting the component composition and the metal structure, it is excellent in workability such as press working, and the balance between the surface layer part and the core part is not generated even after carburizing and quenching. It is a steel plate that exhibits excellent mechanical properties.
Therefore, according to the present invention, components having excellent characteristics such as gears, outer rings of bearings and pulleys can be provided at low cost.

As described above, the steel sheet proposed in Patent Document 1 has a low content of Cr and Mo, which are hardenability improving elements, in consideration of workability. For this reason, hardenability is not enough. Therefore, further improvements in hardenability are required for gears, outer rings of bearings, pulleys, and the like made of steel plates having a thickness of 3 to 15 mm. Of course, there should be no abnormal growth of crystal grains during the carburizing treatment and deterioration of the strength and toughness of the carburized material.
Additive elements necessary for ensuring hardenability that can be sufficiently quenched even with steel plates with a thickness of 3 to 15 mm, abnormal growth of crystal grains during carburizing treatment, and strength, toughness of carburized material, etc. are hardened after annealing. There is a tendency to increase the thickness and deteriorate the workability such as press working. Workability is one of the indispensable characteristics for manufacturing gears, outer rings of bearings, pulleys, etc., and simply adding an element or increasing the amount of the additive cannot solve the problem.

Therefore, in the present invention,
(1) In order to improve hardenability, the contents of Mn, B and Cr, and the contents of Ni and Mo to be added as necessary, were finely adjusted.
(2) Further, in order to prevent coarsening during the carburizing treatment, Ti, Nb and V are added as necessary, and the amount of Ti, Nb, V carbide formed is finely adjusted. Was finely dispersed and precipitated, and the pinning action of γ grain boundaries by this carbide was utilized.
(3) Further, in order to improve the workability, the cementite particles were dispersed with a large particle size and a wide interval.
The details will be described below.

First, the component composition of the steel sheet of the present invention will be described.
C: 0.10-0.40 mass%
C is an element effective for imparting the necessary strength and hardness to steel. If it is less than 0.1% by mass, the required mechanical properties in the core cannot be ensured. As the amount of C increases, it becomes stronger and harder. However, if it is added in an amount exceeding 0.40% by mass, it becomes too hard and press workability deteriorates, and the core toughness after carburization also decreases.

Si: 0.02-0.40 mass%
Si is an element effective for deoxidizing steel, and is an element effective for imparting necessary strength and hardenability to steel and improving temper softening resistance. If the amount is less than 0.02% by mass, a sufficient effect cannot be obtained. On the contrary, if it is contained in an amount exceeding 0.40% by mass, it becomes too hard and press workability is lowered, and toughness is also lowered.

Mn: 1.00 to 2.00% by mass
Mn is an element effective for deoxidation of steel, and is an important element for ensuring the strength and hardenability necessary for steel. In order to ensure hardenability without increasing the amount of Cr, at least 1.00% by mass is necessary. However, if it is included in an amount exceeding 2.00% by mass, it becomes too hard and press workability is lowered.

P: 0.02 mass% or less P not only lowers workability and toughness, but also reduces fatigue strength by embrittlement of crystal grain boundaries after quenching and tempering, so it is desirable to reduce it as much as possible. In the present invention, the content is 0.02% by mass or less.

S: 0.02 mass% or less S forms MnS in the steel and degrades workability. Further, it is desirable to reduce as much as possible because an abnormal structure is generated at the time of carburizing and the material is deteriorated after heat treatment. In the present invention, the content is 0.02% by mass or less.

Cr: 0.20 to 0.70% by mass
Cr is an effective element for imparting strength and hardenability to steel. In order to improve hardenability, it is necessary to add at least 0.20% by mass. In the present invention, the upper limit of the amount of Cr added is 0.70% by mass because the amount of Cr added becomes harder and the workability is lowered.

B: 0.0003-0.005 mass%
B imparts hardenability to the steel during carburizing and quenching. It is an important element in the present invention. Moreover, since it has the effect | action which improves the grain boundary strength of a carburized material, it will also improve the fatigue strength and impact strength as a carburized material. In order to bring out these effects, it is necessary to add at least 0.0003% by mass. However, the effect is saturated even if added over 0.005 mass%. Therefore, the addition amount of B is set to 0.0003 to 0.005 mass%.

Ti: 0.03 to 0.20% by mass
In order to obtain the effect of improving the hardenability of B, B must not be precipitated as BN. Since Ti combines with N in the steel to produce TiN, the precipitation of BN, that is, solid solution B, is ensured by utilizing the fixing action of solid solution N by Ti. Further, the remaining Ti that has produced TiN produces fine TiC in the steel, and this carbide prevents γ grain growth during carburizing by a pinning action. Ti is an extremely important element in the present invention in that the solid solution B is ensured and the pinning action of the formed carbide is expressed.
If it is less than 0.03 mass%, the effect is insufficient. On the other hand, when Ti is added in an amount exceeding 0.2% by mass, precipitation hardening by TiC becomes remarkable and workability is drastically lowered. Therefore, in this invention, the addition amount of Ti shall be 0.03-0.20 mass%.

Ni: 0.20 to 2.00% by mass
Ni is an element having an effect of improving hardenability and an effect of improving toughness. Therefore, it adds as needed. In order to exhibit this effect, it is necessary to add at least 0.20% by mass. When it is going to be used as a carburizing steel plate, no improvement in properties can be expected even if it is added in excess of 2.00% by mass. Further, since Ni is an expensive element, excessive addition causes a significant cost increase.

Mo: 0.10 to 0.80% by mass
Mo, like Ni, is an element having an effect of improving hardenability and an effect of improving toughness. Therefore, it adds as needed. In order to exert this effect, it is necessary to add at least 0.10% by mass. When it is going to be used as a carburizing steel plate, no improvement in properties can be expected even if it exceeds 0.80% by mass. Further, since Mo is an expensive element, excessive addition causes a significant cost increase.

Nb: 0.02 to 0.10% by mass, V: 0.02 to 0.20% by mass Nb and V, like Ti, form carbides and carbonitrides and have a hardening action. It is an element to be exhibited. Therefore, it adds as needed. By adding Nb and V in addition to Ti, the grain boundary pinning effect can be more effectively exhibited. In order to exert this effect, it is necessary to add at least 0.02% by mass of any element. However, when Nb is added in an amount exceeding 0.10% by mass, and in the case of V being added at an amount exceeding 0.20% by mass, a large amount of carbides and carbonitrides are precipitated and the annealed material becomes too hard. Sexually decreases. Therefore, it is necessary that the amount of Nb and V added is not too large. The total amount of Ti, Nb and V is preferably 0.35% by mass or less.

The dough structure of the steel sheet of the present invention is basically a softened structure obtained by an annealing process and is based on a ferrite phase. Most of C added to the steel exists as cementite and part as Ti carbide. In the component range of the present invention, the area ratio of cementite stoichiometrically exists at least about 1.5%. The cementite phase may exist as “spherical or plate-like particles” alone or as “understructure of pearlite structure or bainite structure”. In the present invention, these substructures include cementite. However, since the cementite particles as the substructure of the pearlite structure and bainite structure are extremely fine and large in number, the presence of the pearlite structure and bainite structure naturally becomes a factor in controlling the number of cementite particles described later. Is. Further, when the pearlite structure or the bainite structure is included in the dough structure, the dough structure becomes hard, so the pearlite structure and the bainite structure are also inhibiting factors in this respect. It is intended for “morphic particles”.
In the present invention, the metallographic structure is mirror-polished on a cross section parallel to the rolling direction of the hot-rolled / annealed plate, and then the surface etched with a picral solution (methyl alcohol + picric acid) is scanned with a scanning electron microscope. Observe.
Moreover, it is necessary to finely adjust the carbide precipitation state as follows.

Cementite with a particle size of 0.2 μm or more: 1500 / 10,000 μm ² or less In a carbon steel sheet, one method for improving the workability is to reduce the hardness after annealing. As one of the methods for reducing the hardness after annealing, there is a method of controlling the structure so that the particle size of the carbide is increased and the interval is increased. As a result of investigating the particle size of carbides and the interval between the carbides, the inventor has found that a structure in which no more than 1500 cementite particles having a particle size of 0.2 μm or more are dispersed per 10,000 μm ² is important. Yes, in the annealed material, if cementite grains with a particle size smaller than the specified value are dispersed more than the specified value, the annealed material becomes hard and brittle, and it is difficult to process into a desired shape. It was.
In the present invention, the cementite is dispersed in a state in which the cementite particles on the surface etched with a picral solution (methyl alcohol + picric acid) are mirror-polished on a cross section parallel to the rolling direction of the hot rolled / annealed plate. The minor axis length and the major axis length of the particles are measured, and the average value is taken as the particle diameter of the particles.

Ti, Nb, V carbide with a particle size of 20-100 nm: 4000-20000 pieces / 10000 μm ²
In the present invention, carbides of Ti and Nb and V added as needed are finely dispersed and precipitated, and the grain-graining prevention during carburizing, quenching and tempering treatment by the pinning action of γ grain boundaries by the carbides is prevented. I am trying. If the particle size of these precipitated and dispersed carbides is too small or the amount of dispersion is too smaller than the specified value, the desired pinning effect cannot be obtained. Conversely, if the particle size is too large, the number of Ti, Nb, and V carbides decreases. As a result, the old γ grain size after carburizing, quenching and tempering increases, and the toughness decreases. On the other hand, if the amount of dispersion is too large, the annealed material becomes hard and the material becomes brittle, so that the workability becomes insufficient.
In the present invention, the dispersion and precipitation state of carbides such as Ti is prepared by observing a cross section parallel to the rolling direction of the hot-rolled / annealed plate by using an extraction replica method using constant-voltage electrolytic etching. Using an electron microscope, Ti, Nb, and V carbides were observed at an acceleration voltage of 200 kV, the minor axis length and major axis length of the particles were measured, and the average value was taken as the particle diameter of the particles.

Hardness: The material steel plate is preferably soft from the carburized steel plate of less than 180 HV to a desired shape by press working or the like. If it is harder than 180 HV, the life of the mold is shortened and the productivity is lowered.
When the component composition is finely defined and the metal structure is adjusted as described above, a steel sheet having a hardness of 180 HV or less is obtained.
In the present invention, the hardness of a cross section parallel to the rolling direction of the hot rolled / annealed plate is measured with a Vickers hardness meter.

Then, the manufacturing method of the steel plate for carburizing which concerns on this invention is demonstrated easily.
The steel composed of the above-described components of the present invention is melted by a usual method such as a converter, an electric furnace, etc., and after adjusting the components, a normal casting process, if necessary, a block rolling process, Rolling and subsequent annealing are performed to obtain a hot-rolled annealed sheet.
The carbide produced at the time of casting is once dissolved in the matrix, and the carbide is uniformly and finely dispersed and precipitated at the time of hot rolling, and in combination with the subsequent treatment, the desired structure and characteristics are expressed.

The heating before hot rolling should be a temperature at which the carbide is sufficiently dissolved in the matrix. Regarding the hot rolling finishing temperature, the cooling rate after the hot rolling and the coiling temperature, so that the ferrite phase crystal grains become finer and the carbide precipitates in a predetermined size and dispersed state, It is necessary to adjust so as to suppress the formation of the bainite phase.
In the present invention, a large feature is that the dispersion form of cementite is coarsened in order to improve workability. In this sense, adjustment of the annealing conditions when annealing the hot-rolled sheet is a big point.

The annealing condition is that the steel sheet is heated and held at a temperature less than (Ac1 point-30 ° C) to less than Ac1 point for 10-30 h, and then cooled at a cooling rate of 100 ° C / h or less, or the steel sheet is Ac1 point to (Ac1). It is necessary to set it as at least one of the processes cooled at the cooling rate of 50 degrees C / h or less, after heating and holding at the temperature of a point +100 degreeC for 5 to 30 hours.
Desirably, the latter step is more effective in reducing the number of cementite having a particle size of 0.2 μm or more because solid solution of cementite in the matrix is more likely to proceed. The steel sheet was heated and held at a temperature of (Ac1 point−30 ° C.) to less than Ac1 point for 10 to 30 hours, and then heated at a temperature of Ac1 point to (Ac1 point + 100 ° C.) for 5 to 30 h without lowering the temperature. After the holding, a two-stage process of cooling at a cooling rate of 50 ° C./h or less may be performed.

  The hot-rolled annealed plate manufactured by the above method is molded into gears, outer rings of bearings, pulleys, etc. by ordinary processing means such as press processing, and subjected to normal carburizing treatment to obtain a desired carburized product. The

Continuously cast slabs of converter molten steel having the compositions shown in Tables 1 and 2 are hot-rolled under the conditions shown in Table 3 to produce hot-rolled steel strips having a thickness of about 5 mm. Annealing was performed under the conditions shown. The cooling rate after holding for 10 hours in Table 3 was carried out in the range of 10 to 40 ° C./h.
In addition, the test steel shown in Table 1 is an example of the present invention, and the test steel shown in Table 2 is a comparative example.
Moreover, Ac1 point of the steel plate for high workability carburizing according to this invention exists in the range of about 710-740 degreeC. Among the steels, Nos. 4, 18, and 21 with relatively large amounts of Mn have low Ac1 points, and 22, 28, and 32 to 34 with large amounts of Cr have high Ac1 points.

About each said test material, after carrying out mirror polishing of the cross section parallel to a rolling direction, the surface etched with the picral solution (methyl alcohol + picric acid) is observed with a scanning electron microscope, and the metal structure of each test material is shown. Judged.
Moreover, after mirror-polishing the cross section parallel to the rolling direction of each test material, the cementite particles on the surface etched with a picral solution (methyl alcohol + picric acid) were observed with an optical microscope, and cementite contained in 10,000 μm ^2. The number of was measured.

Further, observation samples of cross sections parallel to the rolling direction of each test material were prepared by an extraction replica method using constant voltage electrolytic etching, and Ti, Nb, V carbides were applied at an acceleration voltage of 200 kV using a transmission electron microscope. And the number of Ti, Nb, and V carbides contained in 50 μm ² was measured. Based on the measured number, it was converted into the number of Ti, Nb, V carbides contained in 10,000 μm ² .
Furthermore, the hardness was calculated | required with the Vickers hardness meter about the cross section parallel to the rolling direction of each test material. The load was 10 kgf and the measurement position was the thickness center.
Further, for each test material, a test piece processed to have an inner diameter of 30 mm and an outer diameter of 50 mm is subjected to burring, drawing, and hole expansion processing (hole expansion ratio: 30%), and then the workability is determined depending on the presence or absence of cracks. evaluated.
The above observation and evaluation results are shown together in Table 4.

About each said test material, the carburizing quenching tempering process was performed and the various characteristics after a process were evaluated.
In the carburizing process, the temperature was first heated to 980 ° C. and maintained at that temperature for 30 minutes as a soaking step. Subsequently, after repeating the operation of the carburizing gas supply process of holding for 10 minutes while supplying the carburizing gas while maintaining the temperature at 980 ° C. and the diffusion process of holding the carburizing gas for 40 minutes and holding for 40 minutes, 870 Hold at 20 ° C. for 20 minutes. Next, quenching was performed by oil cooling from 870 ° C. (quenching process). Then, after hold | maintaining at 160 degreeC for 120 minute (s), air cooling was performed. Propane gas was used as the carburizing gas, and the surface carbon concentration after quenching was adjusted to 0.8%.
And about the cross section parallel to the rolling direction of each carburizing quenching material, after arranging the observation surface by the same method as the above, the structure observation was performed, and the prior austenite grain size of the carburized layer was obtained according to the cutting method of JIS G 0551. .

Each carburized and quenched material was examined for durability by a rolling fatigue test.
From each of the annealed materials, rounding → roughing → carburizing quenching and tempering → precision processing was performed, and finally a test piece having a thickness of 5.0 mm, an inner diameter of 28.7 mm, and an outer diameter of 63 mm was produced. Using this test piece, durability was evaluated using a thrust tester.
Test conditions are: test temperature: 25 ° C., hard sphere (mating material): SUJ2 (φ3 / 8 in (3 pieces)), stress repetition rate: 1800 cpm, lubricant: turbine # 68, load: 3.2 kN, maximum contact stress : 4.9 GPa.
A Weibull distribution was created for each steel type, and a durability test ◯ was given for a steel type having a repetition rate of 2.0 × 10 ⁷ or more with a probability of 10%, and a steel type less than that was indicated by x.
Table 5 shows the evaluation results of each carburized and quenched material.

In the inventive examples of Nos. 1 to 13, the annealed material was low in hardness and could be pressed without problems. Moreover, there was no coarsening of the crystal grains in the carburized layer after carburizing and quenching, and sufficient durability was provided.
On the other hand, No. 14, which is a comparative example, has a low C content, so that the core hardness after carburizing and quenching and tempering is insufficient, and the durability is insufficient. In addition, No.15, No.18, No.22, and No.25 have too much added amount of C, Mn, Cr, and Ti, respectively, so that the annealing hardness becomes hard at 180 HV or more and the workability is insufficient. It became.

No. 16 was not shown in Table 4 because the amount of Si added was too large, but the material became brittle and the workability was insufficient. No. 17, No. 21, and No. 23 were also not shown in Table 4 because the amount of addition of Mn, Cr, and B was small, respectively, but the hardenability was insufficient. As a result, the core hardness after carburizing, quenching and tempering was insufficient, and the durability was insufficient.
In addition, No. 19 and No. 20 are not shown in Table 4 in the same manner because the amounts of addition of P and S are too large, respectively, but the toughness after carburizing treatment is reduced and the durability is insufficient. became.

Furthermore, in No. 26, since the total addition amount of Ti + Nb + V was too large, the annealing hardness became as hard as 180 HV or more, and the workability was insufficient.
In No. 24, since the total addition amount of Ti + Nb + V was small, the old γ grain size after carburizing, quenching and tempering became large. As a result, the toughness decreased and the durability became insufficient.
In No. 27 and No. 30, since the total addition amount of Ti + Nb + V was small, the old γ grain size after carburizing and quenching and tempering became large. In addition to the decrease in toughness due to the coarsening of the former γ grain size, the durability becomes insufficient due to insufficient hardness after carburizing quenching and tempering due to insufficient hardenability (insufficient amount of Mn, Cr or B). It was. In particular, in No. 30, although the Cr addition amount exceeded the upper limit of the present invention, both the addition amounts of Mn and B were insufficient, so that the hardenability of the material as a whole was insufficient.

No. 28 and No. 33 are not shown in Table 4 due to the addition of a large amount of Cr, but the material becomes brittle and the workability is insufficient. In addition, due to the insufficient amount of Mn and B, the hardenability becomes insufficient, and the core hardness after carburizing, quenching and tempering becomes insufficient, resulting in insufficient durability.
No. 29 and No. 32 were not shown in Table 4 due to the addition of a large amount of Cr, but also caused embrittlement of the material, resulting in insufficient workability.

No. 31 is not shown in Table 4 due to the insufficient amount of Mn, Cr and B, but the hardenability becomes insufficient, and the core hardness after carburizing, quenching and tempering becomes insufficient. Therefore, the durability was insufficient.
No. 34 became hard with an annealing hardness of 180 HV or more due to the addition of a large amount of Cr, and the workability was insufficient. In addition, since the hardenability is insufficient due to the insufficient amount of Mn added and the core hardness after carburizing, quenching and tempering is insufficient, the durability is insufficient.
In No. 35, since the addition amount of Ti and the total addition amount of Ti + Nb + V were small, the old γ grain size after carburizing and quenching and tempering became large. As a result, the toughness decreased and the durability became insufficient.

In the steel sheet of the present invention, the metal structure or the precipitation state of carbides changes depending on the manufacturing conditions, particularly the annealing conditions, and affects the workability, carburizing properties, and characteristics after carburizing and quenching of the annealed materials.
Therefore, hot-rolled annealed plates were produced under the various conditions shown in Table 6 for the steels shown in Nos. The cooling rate after annealing was implemented in the range of 10-40 degree-C / h.
Each hot-rolled annealed plate was evaluated in exactly the same way as in Example 1. The results are shown in Table 7.
Further, each of the hot-rolled annealed plates was subjected to carburizing and quenching treatment in the same manner as in Example 1, and the same evaluation as in Example 1 was performed. The results are shown in Table 8.

Examples No.1-A, No.1-C, No.1-D, No.5-A, No.5-F, No.8-A, No.8-D, No.8 In -E and No. 13-A, the hardness of the annealed material was low, and no crack was generated in the evaluation of workability, and the workability was good. Moreover, there was no coarsening of the crystal grains in the carburized layer after carburizing and quenching, and sufficient durability was provided.
On the other hand, No. 1-B as a comparative example had a low annealing temperature, so that pearlite contained in the metal structure at the time of hot rolling remained, and the number of cementite particles was remarkably increased. In No. 5-B, No. 8-B, and No. 13-B, the annealing temperature was low, so that pearlite contained in the metal structure during hot rolling remained. For this reason, the number of cementite particles was remarkably increased and the hardness was increased, so that the workability was not sufficient. In addition, No.5-C, No.8-C, and No.13-C had a short retention time during annealing, resulting in an increase in the number of cementite and insufficient workability.

In No. 5-D, since the carbide was fine and the number of carbides increased, the material became brittle and the workability was insufficient. This is because the hot rolling coiling temperature is low (that is, the steel is quenched after hot rolling finish rolling), so that the precipitation amount of Ti, Nb, V carbides during hot rolling is reduced, and these carbides are made uniform and fine by annealing. It is because it precipitated.
No. 5-E has a low holding temperature during hot rolling, and Ti, Nb, and V carbides did not dissolve, so there are many carbides larger than 100 nm, and the number of Ti, Nb, and V carbides of 20 to 100 nm is small. became. As a result, the old γ grain size after carburizing, quenching and tempering increased, the toughness decreased, and the durability became insufficient.
Furthermore, in addition to the high C content, No. 13-D has a high annealing temperature, so the number of cementite particles is increased due to the generation of regenerated pearlite, and the hardness is slightly hard. became.

Claims (4)

C: 0.10 to 0.40 mass%, Si: 0.02 to 0.40 mass%, Mn: 1.00 to 2.00 mass%, P: 0.02 mass% or less, S: 0.02 Less than mass%, Cr: 0.20 to 0.70 mass%, B: 0.0003 to 0.005 mass%, Ti: 0.03 to 0.20 mass%, the balance being Fe and inevitable impurities In addition to the component composition and the constituent phase occupying 1% or more of the area ratio being only the ferrite phase and the cementite phase, the particles having a particle size of 0.2 μm or more in the cementite phase are 392 to 1500 particles / 10000 μm ² And a steel plate for carburizing, characterized by having a structure in which Ti carbide particles having a particle size of 20 to 100 nm are dispersed in a range of 4000 to 20000 particles / 10000 μm ² and exhibiting a hardness of less than 180 HV.   Furthermore, the steel plate for high workability carburizing of Claim 1 which has a component composition containing 1 type or 2 types of Ni: 0.20-2.00 mass% and Mo: 0.10-0.80 mass%. Further, Ti, Nb having a component composition containing one or two of Nb: 0.02 to 0.10% by mass and V: 0.02 to 0.20% by mass and having a particle diameter of 20 to 100 nm, and The steel plate for high workability carburizing according to claim 1 or 2, wherein the carbide of V has a metal structure dispersed in a range of 4000-20000 pieces / 10000 µm ² .   The steel sheet for high workability carburizing according to claim 3, which is Ti + Nb + V: 0.03 to 0.35% by mass.