JP2007231337A - Hot rolled steel sheet and steel component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel component using a hot rolled steel sheet having more improved fatigue strength compared with the conventional one. <P>SOLUTION: In the steel component produced using the hot rolled steel sheet, at least a part has a hardened structure by quenching, and the old austenite grain size of the hardened structure is ≤12 μm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel part manufactured using a hot-rolled steel sheet and a hot-rolled steel sheet, and having a hardened structure by at least partial quenching, particularly induction quenching. Here, examples of the steel parts include automobile gears and missions.

  High carbon steel sheets used for automobile parts (gear, mission) and the like are subjected to heat treatment such as quenching and tempering after punching and forming. Conventionally, in this application, parts manufacturers have used cast materials and steel bars and formed them into parts by cutting and hot forging. In recent years, however, to simplify the forming process. In addition, attempts have been made to use a high carbon steel plate as a raw material and to form it into a part shape by stamping or press forming.

For example, in Patent Document 1, in order to give a precision punching property to a raw steel plate, and further, when heat treatment is performed after forming, crack propagation is performed by heat treatment based on a relatively simple quenching and tempering method. C: 0.7-0.9 mass%, Si: 0.5 mass% or less, Mn: 0.2-1.2 mass%, P: 0.02 mass% or less, S: 0.01 mass% or less and Cr: steel containing 0.1 to 1.0% by mass with the balance being substantially Fe, spherical with a maximum carbide length of 5.0 μm or less, a carbide spheroidization rate of 90% or more, and a particle size of 2.0 μm or more A technique has been disclosed in which a structure is formed of carbides having two or less carbides per 100 μm ² by microscopic observation of a cross section and equiaxed ferrite. However, since the present technology employs a relatively simple quenching and tempering method, the fatigue strength is not sufficient, and further improvement of the fatigue strength has been demanded.
JP 2005-336560 A

  The present invention has been developed in view of the above-described present situation, and an object of the present invention is to provide a steel part using a thick plate and a hot-rolled steel plate that has further improved fatigue strength as compared with the conventional one.

  Now, in order to effectively improve the fatigue characteristics, the inventors have conducted intensive studies especially on the quenched structure. As a result, as a hardening treatment for finally securing strength, by adopting predetermined quenching conditions, the knowledge is obtained that the prior austenite grain size in the hardened structure is remarkably refined and excellent fatigue properties are obtained. It came to.

That is, the gist configuration of the present invention is as follows.
(I) A hot-rolled steel sheet having a hardened structure formed by quenching at least partially, and having a prior austenite grain size of 12 μm or less in the hardened structure.
Here, the hot-rolled steel sheet means a hot-rolled steel sheet including a thick steel sheet manufactured on a thick plate manufacturing line and a thin steel sheet manufactured on a hot strip mill line.

(Ii) In the above (i), the hot-rolled steel sheet contains C: 0.01 to 1.5 mass%, Si: 3.0 mass% or less, Mn: 2.0 mass% or less, and Mo: 0.05 to 1.0 mass%, and the balance A hot-rolled steel sheet having a steel composition comprising Fe and inevitable impurities.

(Iii) In the above (ii), the steel composition further includes Cr: 2.5 mass% or less, Mo: 1.0 mass% or less, Cu: 1.0 mass% or less, Ni: 2.5 mass% or less, Co: 1.0 mass% or less , V: 0.5 mass% or less, Nb: 0.5 mass% or less, Ti: 0.5 mass% or less, Zr: 0.2 mass% or less, Al: 1.0 mass% or less, B: 0.01 mass% or less, W: 1.0 mass% or less, A hot-rolled steel sheet comprising one or more selected from Sb: 0.2 mass% or less and Hf: 0.1 mass% or less.

(Iv) In the above (ii) or (iii), the steel composition further includes S: 0.1 mass% or less, Te: 0.2 mass% or less, Se: 0.2 mass% or less, Ca: 0.02 mass% or less, Mg: A hot-rolled steel sheet comprising one or more selected from 0.02 mass% or less, REM: 0.03 mass% or less, Pb: 0.6 mass% or less, and Bi: 0.3 mass% or less.

(V) a steel part manufactured using a hot-rolled steel sheet,
A steel part characterized by having a hardened structure formed by quenching at least in part and having a prior austenite grain size of 12 μm or less in the hardened structure.

(Vi) In the above (v), the hot-rolled steel sheet contains C: 0.01 to 1.5 mass%, Si: 3.0 mass% or less, Mn: 2.0 mass% or less, and Mo: 0.05 to 1.0 mass%, and the balance A steel component having a steel composition comprising Fe and inevitable impurities.

(Vii) In the above (vi), the steel composition further includes Cr: 2.5 mass% or less, Mo: 1.0 mass% or less, Cu: 1.0 mass% or less, Ni: 2.5 mass% or less, Co: 1.0 mass% or less , V: 0.5 mass% or less, Nb: 0.5 mass% or less, Ti: 0.5 mass% or less, Zr: 0.2 mass% or less, Al: 1.0 mass% or less, B: 0.01 mass% or less, W: 1.0 mass% or less, A steel part characterized by containing one or more selected from Sb: 0.2 mass% or less and Hf: 0.1 mass% or less.

(Viii) In the above (vi) or (vii), the steel composition further includes S: 0.1 mass% or less, Te: 0.2 mass% or less, Se: 0.2 mass% or less, Ca: 0.02 mass% or less, Mg: A steel part characterized by containing one or more selected from 0.02 mass% or less, REM: 0.03 mass% or less, Pb: 0.6 mass% or less, and Bi: 0.3 mass% or less.

  ADVANTAGE OF THE INVENTION According to this invention, the steel component excellent in the fatigue characteristic can be stably obtained also in the machine structural component manufactured using the hot rolled steel sheet.

Next, the present invention will be specifically described.
The steel parts of the present invention are parts for machine structures such as automobile gears, missions, etc., and have various shapes and structures. In any case, hardening is performed by quenching a part or the whole particularly requiring fatigue strength. It is important that this hardened structure has an average austenite grain size of 12 μm or less.

The following describes the research results that led to this finding.
A steel material having the composition shown in the following steel a or steel b was melted in a 150 kg vacuum melting furnace and hot-rolled to obtain a hot-rolled steel sheet.
[Steel a] C: 0.48 mass%, Si: 0.55 mass%, Mn: 0.78 mass%, P: 0.011 mass%, S: 0.019 mass%, Al: 0.024 mass%, N: 0.0043 mass%, remaining Fe and inevitable Impurities.
[Steel b] C: 0.48 mass%, Si: 0.51 mass%, Mn: 0.79 mass%, P: 0.011 mass%, S: 0.021 mass%, Al: 0.024 mass%, N: 0.0039 mass%, Mo: 0.45 mass %, Ti: 0.021 mass%, B: 0.0024 mass%, balance Fe and inevitable impurities.
The obtained hot-rolled steel sheet was formed into the following gear shape by pressing, punching and finishing.
Small gear: Outer diameter 75mm, module 2.5, number of teeth 28, standard pitch circle diameter 70mm, thickness 6mm
Large gear: outer diameter 85mm, module 2.5, number of teeth 32, standard pitch diameter 80mm, thickness 6mm
This gear was quenched under various conditions using an induction hardening apparatus with a frequency of 200 kHz, then tempered under a condition of 180 ° C. × 2 h using a heating furnace, and then subjected to gear body fatigue tests. It was. In the actual fatigue test of gears, small and large gears were meshed and rotated under the conditions of a rotational speed of 3000 rpm and a load torque of 245 N · m, and the number of torque loads until any gear was damaged was evaluated.

  Moreover, the average austenite particle size of the hardened structure by induction hardening was measured about the gear produced on the same conditions. In the observation of the hardened structure, a caustic solution mainly composed of picric acid (water: 500 g of picric acid: 50 g of picric acid dissolved in picric acid aqueous solution, sodium dodecylbenzenesulfonate: 11 g, ferrous chloride: 1 g and After etching with oxalic acid (added with 1.5 g of oxalic acid), the structure was observed using an optical microscope, and the average particle size of prior austenite grains was observed. The average austenite grain size is measured 400 times (area of 1 field: 0.25 mm x 0.225 mm) to 1000 times (area of 1 field of view: 0.10 mm x 0.09 mm) using an optical microscope. Five views were observed for each of the 1/5 position, 1/2 position, and 4/5 position, the average prior austenite grain size at each position was measured, and the maximum value was defined as the average prior austenite grain size. The hardened layer thickness was a depth region from the surface until the area ratio of the martensite structure was reduced to 98%.

  FIG. 1 shows the results of investigation on the relationship between the average grain size of prior austenite grains in the hardened structure and the number of torque loads. As shown in FIG. 1, it is recognized that the fatigue strength increases as the average particle size decreases. And it turns out that the frequency | count of torque load improves markedly that an average particle diameter is 12 micrometers or less, Preferably it is less than 10 micrometers. Factors in which the average particle size affects the number of torque loads are estimated as follows.

  Impurity elements that cause fatigue failure tend to segregate at the prior austenite grain boundaries. Therefore, as the grain size of the prior austenite grain boundary becomes finer, the segregation area increases, the impurity concentration at each segregation site decreases, and the fracture strength increases. Further, the stress concentration at the prior austenite grain boundaries due to notches or the like is dispersed as the grain size becomes finer, and the stress acting on each grain boundary is reduced, resulting in an increase in fatigue strength.

Next, suitable steel components for obtaining such a structure will be described.
C: 0.01 ~ 1.5mass%
C is an element having the greatest influence on the hardenability, and increases the hardness of the hardened hardened layer and effectively contributes to the improvement of fatigue strength. However, if the content is less than 0.01 mass%, the quench hardened layer depth must be drastically increased to ensure the required fatigue strength. Even if it uses the manufacturing method to do, since refinement | miniaturization of the hardening structure | tissue after hardening becomes difficult, 0.01 mass% or more is required. On the other hand, if the content exceeds 1.5 mass%, the grain boundary strength is lowered, and accordingly, the fatigue strength is lowered, and the moldability and the fire cracking resistance are also lowered. For this reason, C was limited to the range of 0.01 to 1.5 mass%. Preferably it is the range of 0.02-0.5 mass%.

Si: 3.0mass% or less
Si not only acts as a deoxidizing agent, but also contributes effectively to improving the strength, but if the content exceeds 3.0 mass%, machinability and formability are reduced, so the Si amount is 3.0. Less than mass% is preferable. In addition, it is preferable to set it as 0.05 mass% or more for strength improvement.

Mn: 2.0 mass% or less
Mn is a useful component for improving hardenability. If the content is less than 0.2 mass%, the effect is poor, so 0.2 mass% or more is preferable. More preferably, it is 0.3 mass% or more. On the other hand, if the amount of Mn exceeds 2.0 mass%, the retained austenite after quenching increases, and on the contrary, the surface hardness decreases, and as a result, the fatigue strength decreases. Therefore, Mn is preferably 2.0 mass% or less. It should be noted that if the content of Mn is large, the base material is hardened, which may be disadvantageous in machinability. Therefore, the content is preferably 1.2 mass% or less. More preferably, it is 1.0 mass% or less.

Mo: 0.05-1.0mass% or less
Mo is an element useful for suppressing the growth of austenite grains, and for that purpose, it must be contained at 0.05 mass% or more. If added over 1.0 mass%, the machinability deteriorates, so Mo is set to 1.0 mass% or less.

In the present invention, the following elements may be further contained as reinforcing components.
Cr: 2.5 mass% or less
Cr is effective for improving the hardenability and is a useful element for securing the hardening depth. However, if contained excessively, the carbide is stabilized to promote the formation of residual carbide, and the grain boundary strength is lowered to deteriorate the fatigue strength. Therefore, it is preferable to reduce the Cr content as much as possible, but it is acceptable up to 2.5 mass%. Preferably it is 1.5 mass% or less.

Cu: 1.0 mass% or less
Cu is effective in improving the hardenability, and also dissolves in ferrite, and this solid solution strengthening improves fatigue strength. Furthermore, by suppressing the formation of carbides, a decrease in grain boundary strength due to carbides is suppressed, and fatigue strength is improved. However, if the content exceeds 1.0 mass%, cracking occurs during hot rolling, and therefore, it is preferably 1.0 mass% or less. In addition, More preferably, it is 0.5 mass% or less. Moreover, since the effect of improving hardenability and the effect of suppressing the decrease in grain boundary strength are small when added at less than 0.03 mass%, it is desirable to add 0.03 mass% or more.

Ni: 2.5 mass% or less
Since Ni is an element that improves hardenability, Ni is used when adjusting hardenability. Moreover, it is an element which suppresses the production | generation of a carbide | carbonized_material and suppresses the fall of the grain boundary strength by a carbide | carbonized_material, and improves fatigue strength. However, Ni is an extremely expensive element, and adding more than 2.5 mass% increases the cost of the steel material. Therefore, it is preferable to add 2.5 mass% or less. In addition, since the effect of improving hardenability and the effect of suppressing the decrease in grain boundary strength are small when added in an amount of less than 0.05 mass%, it is preferably contained at 0.05 mass% or more. Furthermore, it is preferably 0.1 to 1.0 mass%.

Co: 1.0 mass% or less
Co is an element that suppresses the formation of carbides, suppresses a decrease in grain boundary strength due to carbides, and improves fatigue strength. However, Co is an extremely expensive element, and the addition of more than 1.0 mass% increases the cost of the steel material, so the addition is made 1.0 mass% or less. In addition, since addition of less than 0.01 mass% has a small effect of suppressing the decrease in grain boundary strength, it is desirable to add 0.01 mass% or more. More preferably, it is 0.02 to 0.5 mass%.

V: 0.5 mass% or less
V combines with C and N in steel and acts as a precipitation strengthening element. Moreover, it is an element which improves temper softening resistance, and improves fatigue strength by these effects. However, even if the content exceeds 0.5 mass%, the effect of addition is saturated, so it is preferable that the content be 0.5 mass% or less. In addition, since the effect of improving fatigue strength is small when added at less than 0.1 mass%, it is desirable to add at 0.01 mass% or more. More preferably, it is 0.03-0.3 mass%.

Nb: 0.5 mass% or less
Nb not only has an effect of improving hardenability, but also combines with C and N in steel and acts as a precipitation strengthening element. It is also an element that improves tempering and softening resistance, and these effects improve fatigue strength. However, since the effect is saturated even if it contains exceeding 0.1 mass%, it is preferable to make it 0.1 mass% or less. In addition, since addition effect less than 0.005 mass% has a small effect of improving precipitation strengthening action and temper softening resistance, it is desirable to add 0.005 mass% or more. More preferably, it is 0.01-0.05 mass%.

Ti: 0.5 mass% or less
Ti combines with N mixed as an unavoidable impurity to form TiN. When Ti exceeds 0.1 mass%, a large amount of TiN is formed. This causes fatigue fracture and causes a significant decrease in fatigue strength. Therefore, Ti is preferably 0.1 mass% or less. Further, when Ti is added in combination with B, it prevents B from becoming BN and the effect of improving the hardenability of B is lost, and has the effect of fully exhibiting the effect of improving the hardenability of B. In order to acquire this effect, it is preferable to contain at 0.005 mass% or more. The most preferable range is 0.01 to 0.07 mass%.

Zr: 0.2 mass% or less
Zr not only has an effect of improving hardenability, but also combines with C and N in steel and acts as a precipitation strengthening element. Moreover, it is an element which improves tempering softening resistance, and improves fatigue strength by these effects. However, since the effect is saturated even if it contains exceeding 0.2 mass%, it is preferable to make it 0.2 mass% or less. It should be noted that the addition of less than 0.005 mass% is preferable because the precipitation strengthening action and the effect of improving the temper softening resistance are small. More desirably, it is 0.01 to 0.05 mass%.

Al: 1.0 mass% or less
Al is an element effective for deoxidation, and may be contained because it is also an element useful for refining the grain size of the quenched and hardened structure by suppressing austenite grain growth during quenching heating. In order to effectively exhibit the effect, it is preferable to contain 0.005 mass% or more. However, even if it contains exceeding 1.0 mass%, the effect will be saturated, and the disadvantage which raises a component cost arises rather, and it is preferable to set it as 1.0 mass% or less. More preferably, it is the range of 0.05-0.10 mass%.

B: 0.01 mass% or less B is a useful element that not only improves fatigue properties by grain boundary strengthening but also improves strength. Preferably, it is added at 0.0003 mass% or more, but it exceeds 0.01 mass%. Even so, since the effect is saturated, it was limited to 0.01 mass% or less.

W: 1.0 mass% or less W is an element useful for suppressing the growth of austenite grains. For that purpose, W is preferably contained in an amount of 0.005 mass% or more. In order to cause deterioration of machinability, W is preferably set to 1.0 mass% or less.

Sb: 0.2 mass% or less
Sb is effective for delaying the microstructure change, and has an effect of preventing deterioration of fatigue strength, particularly rolling fatigue. Therefore, Sb may be added. However, if its content exceeds 0.2 mass%, the toughness deteriorates, so it is 0.2 mass% or less, preferably 0.01 mass% or less. In addition, in order to express the improvement effect of fatigue strength, it is preferable to set it as 0.005 mass% or more.

Hf: 0.1 mass% or less
Hf is effective for delaying the microstructure change, and is effective for preventing deterioration of fatigue strength, particularly rolling fatigue. However, even if the content exceeds 0.1 mass%, it does not contribute to the improvement of strength any more, so the content is made 0.1 mass% or less. In addition, in order to express the improvement effect of fatigue strength, it is preferable to set it as 0.02 mass% or more.

Furthermore, in the present invention, the following elements may be contained in order to improve machinability.
S: 0.1 mass% or less
S is a useful element that improves the machinability by forming MnS in steel, but if it exceeds 0.1 mass%, it segregates at the grain boundary and lowers the grain boundary strength, so S is 0.1 mass. Limited to less than%. Preferably it is 0.04 mass% or less.

Te: 0.2 mass% or less, Se: 0.2 mass% or less
Te and Se, respectively, combine with Mn to form MnTe and MnSe, which improve machinability by acting as a chip breaker. However, if the content exceeds 0.2 mass%, the effect is saturated and the component cost is increased, so that the content is 0.2 mass% or less. In order to improve machinability, it is preferable to contain 0.003 mass% or more in the case of Te and 0.003 mass% or more in the case of Se.

Ca: 0.02 mass% or less, REM: 0.03 mass% or less
Ca and REM each form a sulfide with MnS, which improves the machinability by acting as a chip breaker. However, even if Ca and REM are added in amounts exceeding 0.02 mass% and 0.03 mass%, respectively, the effect is saturated and the component cost is increased. In order to improve machinability, Ca is preferably contained in an amount of 0.0001 mass% or more, and REM is preferably contained in an amount of 0.0001 mass% or more.

Mg: 0.02 mass% or less
Mg is not only a deoxidizing element but also serves as a stress concentration source and has an effect of improving machinability, and can be added as necessary. However, if added excessively, the effect is saturated and the component cost increases, so 0.02 mass% or less was added. In order to improve machinability, Mg is preferably contained in an amount of 0.0001 mass% or more.

Pb: 0.6 mass% or less, Bi: 0.3 mass% or less
Both Pb and Bi can be added for this purpose because they improve machinability by melting, lubrication and embrittlement during cutting. However, adding Pb: 0.6 mass% and Bi: exceeding 0.3 mass% not only saturates the effect but also increases the component cost. In order to improve machinability, it is preferable to contain Pb in an amount of 0.01 mass% or more and Bi in an amount of 0.01 mass% or more.

  The balance other than the elements described above is preferably Fe and inevitable impurities. Examples of the inevitable impurities include P, O, and N, and 0.03 mass%, 0.02 mass%, and 0.02 mass% can be allowed.

Next, the manufacturing method of this invention is demonstrated.
The steel piece adjusted to the above-described predetermined component composition is made into a hot-rolled steel sheet (including a thick plate) by hot rolling (including thick plate rolling). A part or all of this hot-rolled steel sheet is quenched, or, using a hot-rolled steel sheet, formed into the shape of a part, and then at least a part of the part is quenched. At least a part of this is a portion where fatigue strength is required.

  In this series of steps, first, the cooling rate after hot rolling is set to 0.5 ° C./s or more, and the cooling is stopped at 500 ° C. or less. By controlling to such conditions, a bainite structure or a martensite structure is obtained. By making the steel structure bainite or martensite, the old austenite structure in the quenched portion can be refined to an average particle size of 12 μm or less.

  As described above, with respect to the prior austenite grain size of the hardened structure, the maximum grain size / average grain size is set to 4 or less to further improve the fatigue strength. It is good to perform hot rolling so that it may become more than%.

  Next, a case where this hot-rolled steel sheet is processed into a part shape will be described. Processing can be optimally selected according to the target part shape, such as punching, press molding, cutting, or the like. Here, the processing conditions are preferably 20% or more. It becomes possible to make a finer hardened structure by quenching the hot-rolled annealed plate on a portion subjected to cold working of 20% or more under the conditions described later. In the component composition described above, when Mo is contained, a hardened structure having a fine and small maximum particle size can be obtained without particularly processing due to the effect of suppressing the austenite grain growth of Mo, but when Mo is not contained, Mo addition is essential because austenite grains do not become fine.

After being processed into a part shape, at least a part is subjected to quenching. Here, quenching conditions are particularly important. That is, the heating temperature at the time of quenching is preferably 800 to 1050 ° C., and the heating rate at 600 to 800 ° C. is preferably 300 ° C./s.
When the heating temperature is less than 800 ° C., the austenite structure is not sufficiently generated and a hardened structure cannot be obtained. On the other hand, when the heating temperature exceeds 1050 ° C., the growth rate of austenite grains is remarkably increased and the average grain size is increased. Further, the growth of austenite grains is promoted even when the temperature rising rate at 600 to 800 ° C. is less than 300 ° C./s.
In addition, it is preferable that heating temperature shall be 800-950 degreeC, and it is preferable that the temperature increase rate of 600-800 degreeC is 700 degrees C / s or more.

  Furthermore, if the residence time of 800 ° C. or higher is increased during quenching heating, austenite grains grow and, as a result, the maximum particle size tends to be more than four times the average particle size. Therefore, the residence time of 800 ° C. or more is 5 seconds or less. It is preferable that

  A gear 1 shown in FIG. 2 was manufactured as a machine structural component of the present invention. That is, the typical gear 1 shown in FIG. 2 is formed by cutting a large number of teeth 2 on its peripheral surface. And in the gear according to this invention, as shown in FIG. 3, it has the hardened structure | tissue 4 by induction hardening in the surface layer part of many teeth 2 and the tooth bottom 3 between these teeth 2. As shown in FIG. In the illustrated example, the hardened structure 4 is formed by quenching on the surface layer portions of the teeth 2 and the bottom 3, but the hardened structure by hardening is formed on the inner peripheral surface of the shaft hole 5 into which the other drive shafts are inserted, for example It is also possible to provide it.

Steel materials having the composition shown in Table 1 were melted by a converter and made into slabs by continuous casting. The slab size was 300 × 1000 mm. This slab was hot-rolled under the conditions shown in Table 2 to obtain a thick steel plate.
From the obtained steel plate, the following gear shape was formed by punching and finish cutting.
Small gear: Outer diameter 75mm, module 2.5, number of teeth 28, standard pitch circle diameter 70mm, thickness 6mm
Large gear: outer diameter 85mm, module 2.5, number of teeth 32, standard pitch diameter 80mm, thickness 6mm
This gear was hardened under the conditions shown in Table 2 using an induction hardening device with a frequency of 200 kHz, then tempered under a condition of 180 ° C. × 2 h using a heating furnace, and then a gear body fatigue test. Went. In the gear substantial fatigue test, small and large gears were meshed and rotated under the conditions of a rotational speed of 3000 rpm and a load torque of 245 N · m, and evaluation was performed by the number of torque loads until one of the gears was damaged.
The obtained results are also shown in Table 2.

Moreover, the average old austenite particle size of the hardened structure by induction hardening was calculated | required with the method similar to the method mentioned above about the gear produced on the same conditions.
Table 2 also shows these results.

It is a figure which shows the relationship between a prior-austenite average particle diameter and the frequency | count of torque load. It is a perspective view of a gear. It is a figure which shows the hardened layer in the tooth | gear and tooth bottom of a gear.

Explanation of symbols

1 gear 2 tooth 3 tooth bottom 4 hardened layer 5 shaft hole

Claims (8)

  A hot-rolled steel sheet having a hardened structure formed by quenching at least partially, and having a prior austenite grain size of 12 μm or less in the hardened structure.   In Claim 1, the said hot-rolled steel plate contains C: 0.01-1.5mass%, Si: 3.0mass% or less, Mn: 2.0mass% or less, and Mo: 0.05-1.0mass%, and the remainder is Fe and unavoidable A hot-rolled steel sheet having a steel composition made of impurities.   3. The steel composition according to claim 2, further comprising: Cr: 2.5 mass% or less, Mo: 1.0 mass% or less, Cu: 1.0 mass% or less, Ni: 2.5 mass% or less, Co: 1.0 mass% or less, V: 0.5 mass% or less, Nb: 0.5 mass% or less, Ti: 0.5 mass% or less, Zr: 0.2 mass% or less, Al: 1.0 mass% or less, B: 0.01 mass% or less, W: 1.0 mass% or less, Sb: 0.2 mass % Or less and Hf: Hot rolled steel sheet characterized by containing one or more selected from 0.1 mass% or less.   4. The steel composition according to claim 2, further comprising: S: 0.1 mass% or less, Te: 0.2 mass% or less, Se: 0.2 mass% or less, Ca: 0.02 mass% or less, Mg: 0.02 mass% or less, REM : Hot rolled steel sheet characterized by containing one or more selected from 0.03 mass% or less, Pb: 0.6 mass% or less, and Bi: 0.3 mass% or less. Steel parts manufactured using hot-rolled steel sheets,
A steel part characterized by having a hardened structure formed by quenching at least in part and having a prior austenite grain size of 12 μm or less in the hardened structure.   6. The hot-rolled steel sheet according to claim 5, wherein C: 0.01 to 1.5 mass%, Si: 3.0 mass% or less, Mn: 2.0 mass% or less, and Mo: 0.05 to 1.0 mass%, with the balance being Fe and inevitable. A steel part having a steel composition comprising impurities.   7. The steel composition according to claim 6, further comprising: Cr: 2.5 mass% or less, Mo: 1.0 mass% or less, Cu: 1.0 mass% or less, Ni: 2.5 mass% or less, Co: 1.0 mass% or less, V: 0.5 mass% or less, Nb: 0.5 mass% or less, Ti: 0.5 mass% or less, Zr: 0.2 mass% or less, Al: 1.0 mass% or less, B: 0.01 mass% or less, W: 1.0 mass% or less, Sb: 0.2 mass Steel part characterized by containing one or more selected from% or less and Hf: 0.1 mass% or less.   The steel composition according to claim 6 or 7, further comprising: S: 0.1 mass% or less, Te: 0.2 mass% or less, Se: 0.2 mass% or less, Ca: 0.02 mass% or less, Mg: 0.02 mass% or less, REM : Steel parts characterized by containing one or more selected from 0.03 mass% or less, Pb: 0.6 mass% or less, and Bi: 0.3 mass% or less.

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