JP4319940B2 - High carbon steel plate with excellent workability, hardenability and toughness after heat treatment - Google Patents

Description

  The present invention relates to a high carbon steel sheet having excellent workability and hardenability and excellent toughness after heat treatment.

  High carbon steel sheets used for automobile parts (gears, missions) and the like are subjected to heat treatment such as quenching and tempering after punching and forming, and adjusted to a predetermined strength. Generally, a high carbon steel sheet may be subjected to punching, bending, light drawing, and mild stretch flange processing. In addition, when the part shape is complicated, it is often produced by welding several parts.

However, in recent years, parts processing steps have been omitted and integrated molding has been promoted in order to reduce part manufacturing costs, and therefore, high-carbon steel sheets as raw materials are required to have characteristics with better workability.
In addition, since the strength is tempered by heat treatment such as quenching and tempering, excellent toughness is required as a mechanical structural component together with excellent hardenability.

The technology for providing a high carbon steel sheet having excellent workability, hardenability and toughness after heat treatment is also disclosed in Patent Documents 1 to 3 shown below.
The technology of Patent Document 1 improves the workability by reducing the Mn amount to 0.2 to 0.50% and the Si amount to 0.10% or less and performing spheroidizing annealing, and the hardenability due to the low Mn reduction. The decrease is compensated by addition of Cr and B.
However, the high carbon steel sheet manufactured by this technique does not necessarily have excellent workability and hardenability. In particular, the hardenability varies greatly and stable strength adjustment cannot be performed, and it cannot be provided for important structural parts.

The technique of Patent Document 2 is a technique in which the components are adjusted with substantially the same technical idea as Patent Document 1, and the carbide spheroidization ratio, carbide particle diameter, and ferrite particle diameter are specified in order to improve workability. A heat cycle in which the annealing is gradually cooled after the temperature is raised to a temperature higher than the Ac1 point is adopted.
However, although the steel plate manufactured by this technique becomes soft and the elongation of a normal tensile test becomes high, it may not show favorable workability depending on a processing method. In addition, this steel plate also has a large variation in hardenability and cannot be adjusted for stable strength, and cannot be provided for important structural parts.

The technology of Patent Document 3 specifies the relationship between the amount of B and P, the carbide particle size, and the spheroidization rate. However, even if this technology is applied, the hardenability is unstable, and it can be applied to structural parts. I can't.
Thus, the present situation is that the steel plate manufactured by the prior art cannot achieve both excellent workability and stable hardenability.
Japanese Patent Laid-Open No. 10-147816 JP 11-256268 A JP 2000-248330 A

  The problem to be solved by the present invention is to provide a high carbon steel sheet having excellent workability and stable hardenability and having good toughness after heat treatment.

The high carbon steel sheet has a mixed structure of ferrite and pearlite or a mixed structure of ferrite and cementite. Pearlite is a structure in which ferrite and cementite are laminated in a layered form and has poor ductility. Therefore, when workability is required, pearlite is annealed and used as a mixed structure of ferrite and spherical carbide (cementite). Since this spherical carbide is hard, there is almost no deformability, strain is discontinuous around the carbide, voids are generated around the carbide, and the voids are connected to cause cracks.
Therefore, since the workability becomes better as the amount of carbide decreases, it is useful to reduce the amount of C from the viewpoint of workability.

However, C directly affects the hardenability and the maximum hardness of quenching, and if the amount of C decreases, the hardness after quenching cannot be obtained.
It is well known that B is added as a means to compensate for the decrease in hardenability of the steel sheet having a low C content, but simply adding B can provide stable hardenability as described in the prior art. Can not.
Therefore, as a result of various investigations on the hardenability of the B-added steel, the present inventors have added Sn and Te, can obtain a stable hardenability by specifying the amount of P + S, and have excellent toughness after heat treatment. It has been found that a steel plate can be obtained.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.20 to 0.45%,
Mn: 0.40 to 1.50%
P: 0.03% or less,
S: 0.02% or less,
P + S: 0.010% or more,
Cr: 0.01-0.80%
Ti: 0.005 to 0.050%,
B: 0.0003 to 0.0050%
Comprising the balance Fe and unavoidable impurities ,
Sn: 0.05% or less,
Te: 0.05% or less, and the total of Sn + Te is 0.005% or more , and it has a mixed structure of ferrite and pearlite or a mixed structure of ferrite and cementite, and hardenability High carbon steel sheet with excellent toughness after heat treatment .

Hereinafter, the reasons for limiting the steel composition of the present invention will be described. In the following explanation, especially the% display used in the component range means mass%.
C directly affects the hardness after quenching. If the amount of C is less than 0.20%, sufficient strength as a machine structural component cannot be obtained after quenching.
On the other hand, if it exceeds 0.45%, the amount of carbide increases, voids are generated around the carbide during processing, and the mechanism of connecting these voids breaks down and deteriorates workability. .45% specified. In addition, a preferable range is 0.20 to 0.35% for the same reason.

  Mn is known to improve hardenability. However, excessive addition of Mn promotes segregation of components, disperses carbides, and increases the dispersion in particle size to deteriorate workability. Considering the balance of sex, it was specified in the range of 0.40 to 1.5%.

Cr is added within 0.80% in order to suppress the variation in the particle size of the spheroidized carbide and improve the workability. Addition exceeding 0.80% not only increases the manufacturing cost because annealing time for spheroidization is required for a long time, but also makes the steel plate hard and deteriorates workability.
On the other hand, as means for suppressing the variation in the carbide particle size, since there exist other than Cr addition such as solidification conditions, hot rolling conditions, annealing conditions, etc., the lower limit amount of Cr amount was set to 0.01%.

B is known as an element that enhances hardenability. In the present invention, 0.0050% is added at the upper limit for this purpose. If it exceeds 0.0050%, defects occur in the continuously cast slab, the surface flaws increase in the product, and the yield decreases.
On the other hand, when the addition amount is less than 0.0003%, the effect of enhancing the hardenability cannot be exhibited, so the lower limit is specified as 0.0003%.

Ti is added in order to manifest the effect of addition of B and to increase the toughness after quenching, but addition exceeding 0.050% may deteriorate the toughness depending on manufacturing conditions, so the upper limit is set. It was specified as 0.050%.
On the other hand, since the effect of Ti described above cannot be exhibited with an addition amount of less than 0.005%, the lower limit was specified to 0.005%.

  Since P deteriorates the toughness after quenching, it is necessary to make it 0.03% or less.

Since S becomes an inclusion such as MnS and degrades workability, the addition amount needs to be 0.02% or less.
However, if the amount added is too small, the hardenability becomes unstable as will be described later, so the total amount of P + S needs to be 0.010% or more.

The amount of Sn and Te is an important constituent factor of the present invention. The experimental facts that have found that Sn and Te are useful for stabilizing hardenability will be described in detail.
C: 0.22%, Si: 0.15%, Mn: 0.89%, Cr: 0.22%, Ti: 0.015-0.021%, B: 0.0012-0.0015%, Aluminum: 0.030 to 0.035%, P: 0.014 to 0.016%, S: 0.003 to 0.005%, steel with various Sn and Te changed in a vacuum melting furnace A steel ingot was made, heated at 1250 ° C., and hot rolled to a thickness of 6.0 mm. A steel plate having a thickness of 5.0 mm was made by grinding 0.5 mm from the front and back surfaces.

  The steel sheet was annealed at a temperature of 690 ° C. for 16 to 126 hours while changing the heating atmosphere. After holding at 850 ° C. × 60 minutes, the steel plate was quenched in oil at 60 ° C., and the hardness was measured with a Bickus hardness meter. The hardness was measured at 10 points in the thickness direction, and the average value was taken as the hardness. In addition, a JIS No. 4 impact test piece was made from this steel sheet, and the impact value at −20 ° C. was measured.

FIG. 1 shows the relationship between the amount of Te + Sn and the quenching hardness when the total addition amount is 0.07% or less. As can be seen from the figure, when the total addition amount of Te + Sn is 0.005% or less, some of the quenching hardness is low, and stable quenching hardness cannot be obtained.
On the other hand, when the total addition amount of Te + Sn is 0.01% or more, a quenching hardness of Hv550 or more can be stably obtained.

FIG. 2 is a graph showing the relationship between the impact value at −20 ° C. and the added amounts of Sn and Te for those having a quenching hardness of Hv: 550 or more. In addition, the measured value is abbreviate | omitted for the thing with little Sn and Te addition amount.
The upper limit of Sn and Te is preferably 0.05% or less from the fact that the impact value decreases when the added amount of Sn and Te exceeds 0.05%.
Since stable hardenability cannot be obtained when Sn + Te is less than 0.005%, the total of Sn + Te was specified to be 0.005% or more. A preferable range is 0.010% or more of Sn + Te for the same reason.

Next, experimental facts that have found that the amount of P + S contributes to the stability of hardenability will be described in detail.
C: 0.22%, Si: 0.15%, Mn: 0.89%, Cr: 0.22%, Ti: 0.015-0.021%, B: 0.0012-0.0015%, Al: 0.030 to 0.035%, Sn + Te amount 0.005 to 0.015% of the composition, steel with various amounts of P and S were melted in a vacuum melting furnace to make a steel ingot, Heated at 1250 ° C. and hot rolled to a thickness of 6.0 mm. A steel plate having a thickness of 5.0 mm was made by grinding 0.5 mm from the front and back surfaces.

  The steel sheet was annealed at a temperature of 690 ° C. for 16 to 126 hours while changing the heating atmosphere. After holding at 850 ° C. for 60 minutes, the steel plate was quenched in oil at 60 ° C., and the hardness was measured with a Bickus hardness meter. The hardness was measured at 10 points in the thickness direction, and the average value was taken as the hardness.

The relationship between the obtained P + S amount and the quenching hardness is shown in FIG. As can be seen from the figure, when the P + S amount is less than 0.01%, there is a quenching hardness of Hv: 500 or less, and stable quenchability cannot be obtained. Based on this fact, the amount of P + S was specified to be 0.010% or more. Preferably, for the same reason, the amount of P + S is 0.015% or more.
On the other hand, if the amount of P + S becomes too large, the workability deteriorates, so the upper limit of P + S is 0.05%. In order to obtain good workability, the content is preferably 0.035% or less.

The present invention has excellent workability and stable hardenability by adding Te and Sn to a steel whose composition is specifically adjusted to specify the amount of P + S in order to achieve both workability improvement and hardenability. In addition, it is possible to provide a steel sheet having excellent toughness after heat treatment.
By using this high-carbon steel sheet, it is possible to obtain a high degree of processing in processing of automobile drive system machine parts, etc., making it possible to manufacture parts etc. at low cost by omitting the manufacturing process. It is a very useful invention.

The steel of this invention is in mass%,
C: 0.20 to 0.45%,
Mn: 0.40 to 1.50%
P: 0.03% or less,
S: 0.02% or less,
P + S: 0.010% or more,
Cr: 0.05 to 0.80%
Ti: 0.010 to 0.050%
B: 0.0003 to 0.0050%
In addition,
Sn: 0.05% or less,
Te: One or two of 0.05% or less and the total of Sn + Te may be 0.01% or more, and other chemical components are not particularly defined, such as Si, Al, N, etc. Even if it is made to contain in the normal range, the characteristic of this invention is not impaired. However, the following is preferable.

  Si hardens the steel sheet, impairs workability, and at the same time causes surface defects of the steel sheet, so it is preferably made 1.0% or less.

  Since adding Al excessively tends to cause surface defects of the steel sheet, it is preferable to add Al in a range of 0.08% or less.

  When N is added in a large amount, Al, Ti, B and nitride are formed and the hardenability is deteriorated.

  Further, depending on the purpose, elements such as Cu, Ni, Mo, Nb, V, Ca, and Mg may be added within a range that is usually added. These elements do not particularly affect the characteristics of the present invention. Further, elements and impurities inevitably mixed in the manufacturing process do not impair the characteristics of the present invention.

The steel whose components have been adjusted as described above is made into a slab by continuous casting or ingot-bundling rolling. The slab is hot-rolled. At this time, the slab heating temperature is preferably set to 1280 ° C. or lower in order to avoid deterioration of the surface condition due to scale generation.
The finishing temperature of hot rolling is desirably Ar3 or higher from the viewpoint of workability. About coiling temperature, it is desirable to set it as 500-650 degreeC from a viewpoint of controlling to the size and distribution of the carbide | carbonized_material which makes workability favorable.
In addition, since cooling after finish rolling has an advantageous effect on workability if the size distribution of the carbides is made uniform, it is desirable to cool by pouring water so that the pearlite undergoes isothermal transformation.

The hot-rolled steel strip produced in this manner is subjected to spheroidizing annealing or cold rolling after descaling, cold rolling and annealing after annealing or spheroidizing annealing, and supplied to the product.
When annealing a hot-rolled steel strip after pickling, it is desirable to carry out at a temperature below the Ac1 point temperature in order to ensure good workability when annealing after cold rolling.

When the hot-rolled steel strip is cold-rolled after pickling or spheroidizing annealing, the cold rolling rate is preferably 20% or more in order to obtain a good recrystallized structure and good workability.
On the other hand, when the cold rolling rate is high, the thickness of the hot-rolled steel strip is inevitably increased, and the dispersion of carbide tends to increase. Therefore, it is preferable to adopt a cold rolling rate of 70% or less.
The steel strip manufactured in this way is subjected to temper rolling as needed to process machine structural parts and the like.
The high-carbon steel sheet of the present invention may be a hot-rolled steel sheet or a cold-rolled steel sheet, and in any case, the effects of the features of the present invention can be obtained.

Steel having the composition shown in Table 1 was melted in a vacuum melting furnace to form a steel ingot having a thickness of 90 mm, heated and held at 1250 ° C. for 60 minutes, and then hot-rolled to produce a steel plate having a thickness of 6.0 mm. The hot rolling finishing temperature at this time was 820-860 degreeC.
After hot rolling finish, the average cooling rate is 580 to 650 ° C. at 20 ° C./second, and after holding at that temperature for 1 hour, furnace cooling is performed, and then 0.5 mm is ground from the front and back surfaces of the steel sheet to obtain a thickness of 5.0 mm. .
The Si amount was 0.10 to 0.21%, the Al amount was 0.022 to 0.035%, and the N amount was 0.0032 to 0.0038%.

The steel sheet was subjected to spheroidizing annealing at 680 to 710 ° C. and the holding time was changed to 18 to 126 hours.
From this steel plate, a test piece in which a notch of 5 mm depth of r: 10 mm was inserted from both sides in the center of the parallel part of a JIS No. 5 tensile test piece was drawn, and the drawing was measured at a pulling speed of 25 mm / min.
Note that the drawing ratio = (initial cross-sectional area−cross-sectional area at break) / initial cross-sectional area × 100. This method was adopted as an index of workability because a good correlation was found with the workability of high carbon steel sheets such as fine blanking and FCF. With this amount of C, if the aperture is 65% or more, it is acceptable.

Further, after each heating condition, after heating and holding at 850 ° C. × 60 minutes, it was quenched in oil at 60 ° C., and the cross-sectional hardness was measured at 10 points in the thickness direction with a Bickus hardness meter.
The hardenability was evaluated by the ratio of the number of Hv: 500 or more and the total number of hardness in the measured number.
A JIS No. 4 impact subsize test piece was prepared for a sample having a quenching hardness of Hv: 500 or more, and the impact value at 0 ° C. was measured. The impact value was taken as the toughness score with the average value of all tests for the same steel.

  Steel 1 is one to which Te: 0.015% and Sn: 0.002% are added. The drawing of the tensile test has an excellent workability of 71%, and all the hardened materials have Hv: 500 or more and have an excellent hardenability. Further, the impact value of the quenching material has a high value of 105 J.

  Steel 2 is an example within the scope of the present invention in which Te: 0.001% and Sn: 0.012% were added. This steel plate also has a high drawing value, which is an index of workability, of 66%, and a hardness of Hv: 500 or more is obtained for all 11 quenching hardnesses.

  Steel 3 is a comparative example in which the Ti amount is 0.001% and does not satisfy the lower limit of 0.005% of the Ti amount of the present invention. It can be seen that more than half of these steel sheets have a quenching hardness of Hv: 500 or less and do not have stable hardenability.

  Steel 4 is a comparative example in which the sum of Te + Sn is 0.003% and does not satisfy Te + Sn> 0.005% of the present invention. This steel sheet has a hardness after quenching of less than Hv: 500, indicating that it does not have stable hardenability.

Steel 5 is a comparative example in which the B amount is 0.0001% and is not more than the lower limit of the B amount. As for this steel plate, it turns out that all the steel plates have a quenching hardness Hv: 500 or less and the hardenability is poor.
Steel 6 is a comparative example outside the scope of the present invention, with P + S of 0.007%. The quenching hardness of this steel has a value of less than Hv500, indicating that the hardenability is not stable.

  Thus, it can be seen that stable hardenability cannot be obtained by simply adding B, and excellent hardenability and toughness can be obtained only by controlling the P + S amount and Se + Sn amount within a specific range.

Steels having the compositions shown in Table 3 were melted in a converter, slabs were made by continuous casting, and heated to 1250 ° C., and then hot rolled coils having a plate thickness of 4.0 mm were made. The hot rolling finishing temperature was in the range of 820 to 840 ° C, and the winding temperature was 600 to 625 ° C.
This coil was pickled and annealed at 690 ° C. for 16 to 96 hours, and a test with r = 10 at the center of the parallel part of a JIS No. 5 tensile test piece was created from this steel sheet, and the drawing was determined after the tensile test. It was. The reason for performing this test is that there is a good correlation with workability in a wide forming method of high-carbon steel sheets.
In the case of steel of this component, if the drawing was 65% or more, it was acceptable, and the drawing value was evaluated by the average value of the steels having the same composition but with different annealing conditions.

Further, after heating at 850 to 880 ° C. for 45 to 90 minutes as quenching conditions, quenching was performed in oil at 60 ° C., and the Bickus hardness was measured. The atmosphere during heating and holding was performed with a C potential of 0.25.
A No. 4 impact sub-test specimen was made from one having a quenching hardness of Hv: 500 or more, and the impact value at 0 ° C. was measured. The impact value was also evaluated by the average value of all annealing conditions and quenching conditions. The measurement results are shown in FIG.

  Steel 8 is an example within the present invention. It can be seen that the drawing has excellent workability of 78%, the quenching hardness is 18 conditions under all Hv: 500 or more, the impact value is 115 J, and the hardenability and toughness are excellent.

  Steel 9 is a comparative example in which the amounts of Te and Sn deviate from the scope of the present invention, and the workability and toughness are good. However, the quenching hardness is less than Hv: 500 under 19 conditions and stable hardenability. It turns out that it does not have.

  Steel 10 has a Ti content, and steel 11 has a B content both of which are out of the scope of the present invention. In both cases, the quenching hardness is less than Hv: 500, and the stability of the hardenability is inferior.

Steel 12 is a comparative example having a P + S amount of 0.006%, which is out of the scope of the present invention. This steel also has a drawback in the stability of hardenability.
Steel 13 is a comparative example in which Sn is 0.065%, which is outside the upper limit of the present invention. It can be seen that this steel sheet has poor workability and toughness, with the drawing being an index of workability being 70% or less and the impact value being as low as 25J.

  Thus, it can be seen that a steel sheet having excellent workability, stable hardenability, and toughness after quenching can be provided only when all the conditions of the present invention are satisfied.

Steels having the compositions shown in Table 3 were melted in a converter and slabs were made by continuous casting. This slab is heated to 1250 ° C. to form a hot rolled coil having a thickness of 6.0 mm, pickled, spheroidized, or directly cold-rolled to a thickness of 4.0 mm, 680 ° C. × 12 to 96 hours. Annealing was performed.
From this steel sheet, workability, hardenability, and toughness after quenching were investigated in the same manner as in Example 2. The child number 2 is obtained by directly cold-rolling and annealing a hot-rolled plate, and the child number 3 is obtained by cold-rolling and annealing the hot-rolled plate after spheroidizing annealing.

Steels 8 and 2 have good workability with a drawing of 72% in Examples within the scope of the present invention, and all 18 samples have a hardness of Hv: 500 or more, and have a stable hardenability. I understand that. It can also be seen that the impact value of the quenched material has an excellent toughness of 110 J.
Steel 8.3 is an example within the present invention in which a spheroidized annealed material was cold-rolled and annealed. It can be seen that this steel also has excellent workability, stable and good hardenability, and excellent toughness after quenching.

  Steels 9 and 2 and 9 and 3 are comparative examples in which the added amounts of Sn and Te are out of the scope of the present invention. When the addition of Sn and Te is outside the range of the present invention, the hardenability is not stable, and there is about 40% of the quenching hardness with Hv: less than 500.

Steels 10 · 2, 10 · 3 are comparative examples in which the amount of Ti deviates from the scope of the present invention. It can be seen that many of these steels are hardened and have a hardness less than Hv: 500, and the hardenability is unstable.
Steels 11 and 2 and 11 and 3 are comparative examples in which the amount of B deviates from the scope of the present invention, but this steel has no quenching hardness of Hv: 500 or more and has poor hardenability.

Steels 12 and 2 and 12 and 3 are comparative examples in which the amount of P + S is out of the scope of the present invention. This steel is not stable in hardenability, and there are about one-quarter of one with a quenching hardness of Hv: 500 or less.
Steels 13 and 2 and 13 and 3 are comparative examples in which the amount of Sn deviates from the upper limit of the present invention. This steel has stable quenching hardness of Hv: 500 or more, but has the objective of obtaining good workability and excellent toughness with a low drawing which is an index of workability, low impact value after quenching. Not reach.

The steel shown in Table 6 was melted in a converter and a slab was made by continuous casting. The slab was heated to 1240 ° C. and hot-rolled to 6.0 mm and 4.0 mm thickness. The hot rolling finishing temperature was 800 to 860 ° C, and the winding temperature was 580 to 650 ° C.
After pickling the steel strip, a steel sheet having a thickness of 4.0 mm was subjected to carbide spheroidizing annealing at 690 to 710 ° C. for 18 to 96 hours.
The steel strip of 6.0 mm thickness was annealed at 670 to 690 ° C. for 16 to 62 hours after cold rolling to 4.0 mm thickness, and from this steel plate, the workability was evaluated by the same method as in Example 2, and C : 0.28% is 65% or more, and C: 0.35% is 60% or more.

As for the hardenability, C: 0.28% is Hv: 550, C: 0.35% is Hv: 600 as the standard hardness in the hardenability, and a hardenability higher than this hardness is obtained and the total number of tests. The ratio was evaluated.
About the thing more than reference | standard hardness, hardness was adjusted to Hv: 440-460 by tempering, and this investigation result was shown in Table 7.
The child number 1 is obtained by spheroidizing and annealing a hot-rolled sheet, and the child number 2 is obtained by cold-rolling and annealing a hot-rolled sheet.

  Steel 14 is an example within the scope of the present invention of C: 0.28%. 14 · 1 in which the hot-rolled steel strip is spheroidized and annealed and 14 · 2 in the cold-rolled and annealed state have excellent hardenability and workability, and have good toughness after heat treatment.

  Steel 15 is a comparative example in which the amount of C is the same as steel 14 but the P + S amount is out of the scope of the present invention. In this steel, both the spheroidized annealed material and the cold rolled / annealed material have a quenching hardness of less than Hv: 550, indicating that stable hardenability cannot be obtained.

  Steel 16 is an example within the scope of the present invention of C: 0.35%. 16.1 is a spheroidized annealed hot-rolled steel strip, 16.2 is cold-rolled and annealed, and both have good workability and stable hardenability, as well as excellent toughness after heat treatment. I understand that.

  On the other hand, although the C amount is the same, 17.1 and 17.2 in which the added amount of Sn + Te is out of the scope of the present invention are not stable in hardenability, and there are some in which the quenching hardness is less than Hv: 600. .

  Steel 18 is a comparative example in which the Ti, B amount, and Sn + Te amount are outside the scope of the present invention. The hardenability is as high as C: 0.45%, so the hardenability is stable, but the drawability, which is an index of workability, is as low as 58%, and the workability is lower than that of the examples of the present invention. It turns out that it is inferior, and it turns out that the toughness after heat processing is also significantly inferior compared with the Example of this invention.

As explained in detail in the above examples, in order to achieve both improvement in workability and hardenability, by adding Te and Sn to a steel whose composition has been specifically adjusted and specifying the amount of P + S, excellent workability and stability are achieved. Thus, it is possible to provide a steel sheet having excellent hardenability and excellent toughness after heat treatment.
By using this high-carbon steel sheet, it is possible to obtain a high degree of processing in processing of automobile drive system machine parts, etc., making it possible to manufacture parts etc. at low cost by omitting the manufacturing process. It is a very useful invention.

It is a figure which shows the relationship between Sn + Te amount and quenching hardness. It is a figure which shows the relationship between Sn, Te amount, and an impact value. It is a figure which shows the relationship between the amount of P + S and quenching hardness.

Claims (1)

% By mass
C: 0.20 to 0.45%,
Mn: 0.40 to 1.50%
P: 0.03% or less,
S: 0.02% or less,
P + S: 0.010% or more,
Cr: 0.01-0.80%
Ti: 0.005 to 0.050%,
B: 0.0003 to 0.0050%
Comprising the balance Fe and unavoidable impurities ,
Sn: 0.05% or less,
Te: 0.05% or less, and the total of Sn + Te is 0.005% or more , and it has a mixed structure of ferrite and pearlite or a mixed structure of ferrite and cementite, and hardenability High carbon steel sheet with excellent toughness after heat treatment .

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