JPS62202054A - Non-heattreated steel for hot forging - Google Patents

Abstract

PURPOSE:To obtain a non-heattreated steel for hot forging, particularly for hot forging of large-sized parts, combining high strength with superior toughness, by providing a composition consisting of prescribed percentage of C, Si, Mn, P, S, Cr, B, Ti, Zr, Al, and N and the balance Fe with inevitable impurities. CONSTITUTION:The non-heattreated steel for hot forging has a composition consisting of, by weight, 0.05-0.35% C, 0.02-2.0% Si, 0.1-3.0% Mn, <=0.05% P, <=0.05% S, 0.1-3.0% Cr, 0.0005-0.01% B, 0.003-0.3% Ti, 0.001-0.5% Zr, 0.001-0.1% Al, 0.001-0.02% N, and the balance Fe. The hot-forging steel of this invention is generally subjected to heating to 1,200-1,300 deg.C and to hot forging at >=1,050 deg.C finishing temp., which is allowed to stand to be cooled and is then properly machined to be formed into non-heattreated-type product. Moreover, yield strength, ductility, and toughness can be improved to a greater extent by carrying out heating once or more up to 150-650 deg.C in the course before the finished product is obtained.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to non-thermal steel for hot forging.

(Conventional technology) Even in the past, many mechanical parts such as automobile parts are formed by hot forging, then subjected to tempering treatment consisting of quenching and tempering, and then subjected to machining such as cutting and polishing. Manufactured. Such thermal refining treatment is extremely useful as a heat treatment for adjusting the mechanical properties of parts to desired values, and has traditionally been considered an essential treatment.

However, in today's situation where there is a strong demand for streamlining production lines and improving productivity,
There are many points that should be improved in the conventional manufacturing line form from the viewpoints of (1) improving productivity by preventing quench cracking during quenching, and (2) improving productivity by preventing deformation during quenching.

As a means to solve these current problems in the conventional technology at once, it may be possible to omit the above-mentioned heat treatment, and for this purpose, a precipitation strengthening element such as V is added to refine the structure and strengthen the precipitation. Various types of so-called non-thermal forging steels have been proposed that utilize the above-mentioned methods and have the required properties as-forged.

For example, in Japanese Patent Publication No. 60-45250, polygonal ferrite is formed in austenite grains by cooling the formed part at a rate of 0°7°C/sec or less over a temperature range of 1000°C to 550°C after hot forging. It is disclosed that the material is dispersed in a large amount and has a substantially fine-grained structure.

JP-A-59-100256 discloses Ti in the N range of medium carbon steel.
It is proposed that the Ti/N ratio be limited to utilize the effect of suppressing grain coarsening.

JP-A-60-103161 discloses that C: 0.05 to 0.
Cr+Mn=2.20-5.9 within the range of 15%
It is disclosed to adjust to 0.

In this way, in the past, the components and organization of the rope! 1
! By adjusting the V and N during cooling after hot forging,
Hot forged non-l using precipitation hardening of compounds such as b.
They were obtaining il-quality steel parts.

However, these conventional non-1m steel parts have inferior toughness compared to conventional hot-forged steel parts, so they have only been put into practical use in a limited number of parts where toughness is not required. Therefore, it has been impossible to put it into practical use in important parts that require high strength and toughness.

In particular, for large hot forged parts, it is necessary to heat the steel material to 1200°C or higher to reduce the load during processing.
b. Even if grain-refining elements such as Ti are added to make the ML weave finer, most of the compounds of these elements decompose into solid solution during high-temperature heating prior to forging, and their grain-refining effect is impaired. will also disappear.

For this reason, in order to take advantage of grain refinement by refining elements, it is necessary to devise heat treatment after vigorous hot forging, and in the end, achieving high strength and high toughness is expensive and cannot be achieved with conventional technology. It was extremely difficult.

(Problems to be Solved by the Invention) Thus, an object of the present invention is to provide a non-tempered steel for hot forging, particularly for hot forging of large parts, which eliminates the drawbacks of the prior art as described above. That's true.

Another object of the present invention is to provide a high tensile strength with a tensile strength of 80 kgf/mm" or more, preferably 90 kgf/am" or more.
iz. It is an object of the present invention to provide a non-tempered steel for hot forging which has an excellent toughness of 5 kg-m7 cm" or more.

(Means for solving the problem) In order to achieve the above object, the present inventors conducted various studies and found that there was a solution from a completely different perspective from the conventional method, and completed the present invention. I let it happen.

First, we conducted various studies from the perspective of realizing a non-temperature type steel for hot forging, and the following knowledge was obtained.

In other words, austenite based on the conventional effect of carbonitride dispersion to inhibit the growth of ryaustenite grains)! The reason why methods to prevent weave coarsening cannot be fully effective is that when heated to a high temperature of 1,200 to 1,300°C as in hot forging, carbonitrides are completely decomposed and hardened into austenite. However, since it dissolves, the effect of inhibiting the growth of austenite grains is completely lost.

Therefore, in order to achieve the object of the present invention, it is necessary to use a compound that does not decompose into solid solution even under heated conditions. Such compounds include MnS, TiN, ZrN
There are nonmetallic inclusions such as , Al203.5iOz.

By the way, the conventional austenite refining compound A
The decomposition temperature of EN is 1100°C.

However, in the conventional manufacturing method, these nonmetallic inclusions are coarse and only sparsely distributed, and as they are, they are not in a state where they can effectively inhibit crystal grain growth. Furthermore, in the past, it has generally been desired to reduce the number of nonmetallic inclusions as much as possible, and there has been no idea of actively utilizing them.

After conducting various experiments, we found that by using a steelmaking raw material containing Zr, the sulfides in the steel became extremely fine, out of the nonmetallic inclusions that were previously coarse and sparsely distributed. It was found that not only the oxides in the steel became dispersed, but also the oxides in the steel became extremely finely dispersed.

Due to the effect of such Zr addition, finely dispersed sulfides,
It is thought that the presence of oxides suppresses the coarsening of austenite crystal grains during high-temperature heating before hot forging. On the other hand, these non-metallic inclusions do not decompose even at such high temperatures, so why should they be exposed to high temperatures after forging? The grain growth of austenite grains in the lJI region is also suppressed, and at the same time, a large number of finely dispersed inclusions act as transformation nuclei, so these actions combine to refine the final structure in the forged layer. As a result, the toughness of the steel improves.

Furthermore, by finely dispersing sulfides and oxides, other inclusions in the network are also finely dispersed, and the toughness of the steel is further improved.

Next, we investigated from the perspective of improving the strength and toughness of hot forged materials, and found that by adding 0.01% or less of B, it was possible to achieve 90 kgf/Imm without increasing Mn, Cr, Mo, or other alloying elements. Furthermore, the combined addition with Zr prevents the formation of coarse ferrite structures that occur at austenite grain boundaries prior to bainite transformation, improving the strength and toughness of baini) and [. The present invention was completed based on the knowledge that this is particularly noticeable in hot-forged as-built materials such as those mentioned above.

In addition, in conventional non-tempered steel, the structure is ferrite-
Since it is a pearlite structure, it was thought that when B was added to it, a bainite transformed weave would be mixed, the &lI weave would become non-uniform, and the precipitation strengthening effect of the beam compound would disappear.

Therefore, the gist of the present invention is, in weight %, C: 0.05-0.35%, Si: 0.02-2
．． 0%, Mn: 0.1-3.0%, P: 0.0
5% or less, S: 0.05% or less, Cr: 0.
1 to 3.0%, B: 0.0005 to 0.01%, Ti: 0.003 to 0.3%, Zr: 0.001 to
0.5%, #0.001-0.1%, N: 0.0
01 to 0.02%, and if desired each of the following groups (1
) or 3), (1) Cu: 0.01 to 1.0%, Ni:
0.01 to 2.0%, Mo: 0.01 to 1.0%
, V: 0.001~i, o%, and Nb: 0.00
1 to 0.30% of one or more types (2) S: 0
．． 05-0.5%, Pb: 0.005-0.5%, Ca
:0.001-0.05%, Te:O, OQ1-0.
2%, Se: 0. O1-0.5%, and Bi: 0.0
1 to 0.5% of one or more kinds, and (3) at least one rare earth element, total of 0.005 to 0.5%
It is a non-11 quality steel for hot forging, consisting of 0.5% Fe and unavoidable impurities.

As described above, the present invention is essentially characterized by the combination of 8 addition and Zr addition, and according to the present invention, the strength is 90kgf/+wm'' or more,
A non-tJJ type hot forged material with high toughness of m/cm 1 or more can be obtained.

(Function) Next, the reason why the steel composition is limited as described above in the present invention will be explained in detail.

C: When C exceeds 0.35%, the strength of the haynite structure becomes extremely high, making cutting and straightening difficult, and the toughness significantly deteriorates. In addition, the strength and toughness largely depend on the size of the hot-forged part and the cooling rate at each location, resulting in very large variations in the mechanical properties of the hot-forged product.

Therefore, the upper limit of the C content was set to 0.35%. When the amount of C is less than 0.05%, the strength of the bainite structure is low and the required strength as a mechanical structural component cannot be obtained, so 0.05% is set as the lower limit.

Si: Si is a very effective element for ensuring strength, but
If it exceeds 2%, the ferrite base becomes brittle and the toughness deteriorates significantly, so the upper limit is set to 2%, preferably 1.5%. In addition, Si is used as an element for deoxidizing molten steel, and if the content is less than 0.02%, deoxidation will be insufficient and the composition, structure, and properties of the steel will become unstable, so the lower limit should be set to 0.02%. 0.02%, preferably 0.05%.

nn: Mn is an extremely useful element with a large toughening effect;
．． Addition of 1% or more can be effective.

If the content is less than 0.1%, hot working cracks will occur, and if it is less than 0.3%, the reinforcing effect will be insufficient and the required structure and strength will not be obtained, so the lower limit is preferably 0.1% or more. was set at 0.3% or more. If the content exceeds 3%, an abnormal coarse structure that impairs toughness is formed. Therefore, the upper limit is set to 3
% or less, preferably 2.5% or less.

P, S: Both P and S deteriorate the toughness, and if the upper limit of each limited range is exceeded, it becomes difficult to obtain toughness superior to conventional non-thermal treated steel for hot forging, so P: 0 .0
5% or less, and S: 0.05% or less. Although it is preferable that these elements be contained in trace amounts as much as possible, Si may be contained in an amount greater than the upper limit value in order to improve machinability.

Cr: Cr is effective in increasing strength, but if the amount added exceeds 3%, toughness deteriorates, so the upper limit was set at 3%. If the Cr content exceeds 1.5% when the dimensions of the hot-forged parts are small or the cooling is rapid, the toughness may decrease due to localized hardening and weave, so 1. 5%
was set as the upper limit.

B: B is an extremely effective element for improving the structure of steel and making it tougher. In the conventional hot forged product imxm, addition of B was not considered because bainite is mixed in the ferrite pearlite structure, making the structure non-uniform, and the precipitation hardening effect of ν is reduced. However, according to the present invention, contrary to such conventional understanding, since the base material originally has a haynite structure, B is actively added and utilized contrary to conventional common sense.

When other alloying elements are present in large quantities, or when the dimensions of the hot forged parts are small and the cooling rate is high, the amount of B added may be small.

If the B content exceeds 0.01%, embrittlement will occur, so
This value was set as the upper limit, and 0.0005%, at which the hardenability improvement effect of B was recognized, was set as the lower limit.

Ti: Ti is 0.003% to make the effect of B effective.
Contained above. Additionally, Ti has the effect of making the austenite grains finer and making the Mi weave finer after hot forging, but if it exceeds 0.3%, the austenite grains will become coarser and the grains will become finer when heated at high temperatures. This value was set as the upper limit because it significantly deteriorates the toughness of the steel.

Zr: Treated with an additive containing Zr to remove a very small amount of Zr.
If the content of r is limited, inclusions will be dispersed very uniformly and finely, and the toughness after hot forging will be improved. In this case, the effect of improving toughness is recognized even if the Zr content is extremely low and it is not easy to quantitatively analyze the Zr content using current analytical means, but the lower limit was set at 0.001%. When the Zr content increases, in addition to the effect of fine and uniform dispersion of inclusions, very fine Zr compounds are generated and precipitated, which further effectively refines the structure and improves toughness after hot forging. The Zr compound at this time also has the effect of promoting recrystallization of austenite crystals and suppressing subsequent coarsening of crystal grains when forging is performed at a temperature of 1100° C. or higher, for example. Furthermore, in the case of cooling after high-temperature heating and forging, if an attempt is made to generate a haynite structure from coarse austenite grains at a relatively slow cooling rate, coarse ferrite crystals tend to grow from austenite grain boundaries.

This tendency is particularly strong in B-added steel, which tends to cause deterioration of mechanical properties, but by adding Zr together with B, the formation of coarse ferrite is suppressed, and the henite in the grains is also reduced to -N fine particles. , and the bainite structure becomes extremely tough. In this case, the Zr content is 0.3%
If the content exceeds 0.3%, the toughness deteriorates, so the upper limit was set at 0.3%.

Al: 8 I2 is a very useful element as a deoxidizing element, and if the content is less than 0.001%, problems such as bubbles and surface flaws may occur. Moreover, if it exceeds 0.1%, hot working cracks are likely to occur, so the lower limit was set to 0.001% and the upper limit was set to 0.1%.

N: If N exceeds 0.02%, problems such as reducing the effect of B and causing bubbles and surface flaws in the steel occur. Solid solution N degrades toughness, so it is preferable to keep it in as small a quantity as possible. On the other hand, nitrides in steel are treated by high-temperature heating,
It has the effect of preventing coarsening of austenite grains during hot forging, and if the N content is less than 0.001%, the structure will coarsen, so this value was set as the lower limit.

Cu, NiS Mo% V, Nb: All of these elements are effective for making the structure after hot forging into a uniform and fine bainite structure and improving the strength and toughness of bainite, and at least one of these elements is effective for improving the strength and toughness of bainite. Or two or more kinds are added. In order to realize this toughening effect, C
u,, st, Mo needs to be 0.01% or more, and V
.

Since 0.001% or more of Nb is required, these were set as the lower limit values. In addition, Cu 1.0%, Ni 2.0%,
Ah.

If it exceeds 1.0%, the structure after hot forging becomes an abnormally coarse structure that greatly impairs toughness, while vl.

If the content exceeds 0%, Nb2O, or 3%, the bainite structure becomes extremely brittle and the toughness deteriorates, so these were set as the respective upper limit values.

Therefore, in the present invention, CuO, 01-1.

0%, Ni: 0.01-2.0%, Mo: 0.01-1
．． 0%, v: 0.001-i, o%, Nb: 0.001
~0.3%.

Rare earth elements: In the case of hot forging using high temperature heating, the toughness can be greatly improved by adding rare earth elements in particular.

This improvement effect is even greater in Zr-treated steel, and the effect is observed when the content exceeds 10.001%.

Even if the amount of rare earth elements added exceeds 0.5%, the improvement effect is saturated, so the upper limit was set at 0.5%.

Elements on workpiece elongation: When it is required to improve machinability, S % P
Addition of one or more types of b5Cas Tes Ses Bi is effective. S: 0.05%, Pb: 0.
005%, Ca: 0.001%, Te: 0.001%,
Since Se: 0.01% and 120.01% are the minimum contents that act effectively, these were set as the lower limit values.

S: 0.5%, pb: 0.5%, Ca: 0.05%, T
Even if the content exceeds e: 0.2%, Se: 0.5%, Bi: 0.5%, the machinability improvement effect will be saturated, and the toughness will deteriorate considerably, so hot working cracking will also occur. In order to prevent this, these values were set as upper limits.

The present invention relates to a non-thermal steel for hot forging having the above-mentioned steel composition, but in order to maximize the effect of Zr addition in the present invention, it is necessary to
It is preferable that the cooling rate is 2°C/min or more during 1000°C. In terms of finely uniform dispersion of inclusions and compounds, the higher the cooling rate, the more effective it is, but this increases the likelihood of problems such as surface cracking, so set the cooling rate as high as possible within the range that can avoid problems. This is desirable. Note that, if desired, the amount and type of nonmetallic inclusions can be adjusted in advance by, for example, adjusting the degree of deoxidation, or by other means already well known to those skilled in the art.

The hot forging steel according to the present invention thus obtained is heated to -C from 1200 to 1300°C and then heated to 10
Hot forged at a finishing temperature of 50°C or higher, left to cool,
After appropriate machining, it becomes a non-temperature product. There is no restriction on the hot forging at this time, and it may be a conventional one, and a conventional austenite i-refining treatment may also be performed after this hot forging.

Note that the yield strength and ductile toughness are further improved by heating the product to 150 to 650°C at least once during the process leading to the final product.

Next, the present invention will be explained in more detail by way of examples.

Example 1 200 kg of steel with the chemical composition shown in Table 1 was melted in a low-frequency induction furnace, and after casting, it was cut out of a mold and cooled to 1400 to 1000°C by repeated intermittent air-water spray cooling. between 5
．． Cool at 2°C/min, and cool the obtained steel ingot at 80 tars per side.
The forged square bar was used as the material for the next hot forging experiment.

This square bar with a side of 801 mm and 111 mm was heated to 1250°C, then hot-forged into a square bar with a side of 30 mm at a forging finish temperature of 1100°C, and then allowed to cool naturally.

JIS from the center of the above simulation hot forged material
No. 148 tensile test piece (parallel part diameter 10 mm) and JI
A No. S a Charpy test piece was prepared and its mechanical properties were investigated.

The obtained properties are summarized in Table 2.

As shown in Tables 1 and 2, first, steel symbol 41
is a conventional non-thermal steel for hot forging, steel symbol 42 is medium-low C
Mn-Cr-89114. With these conventional steels, it is possible to make hot forged parts with a tensile strength of 80 kgf/m+" or more, but it is possible to make hot forged parts with a tensile strength of 80 kgf/m/c or more, but
m'' or more could not be obtained, and LIE-4° was almost completely brittle fractured.

Steel code grades 1 to 5 are based on the effect of Cl, and steel code grade 1 has a low C and a tensile strength of less than 70 kgf/n+m'', so it is not suitable for the purpose.

Steel symbols 6 to 8 are based on the effect of Si content, and Si
- At 2.0%, the toughness decreases slightly, but the target value is exceeded.

Steel codes 9 to 11 are based on the effect of Mn content, and steel codes 11 have high tensile strength when hot forged and left to cool, but the yield point is relatively low and the toughness is just below the target. However, properties exceeding the target values were obtained.

Steel symbols 11m12 to 13 are based on the effect of the Cr content, and the strength is insufficient at 0.03% of steel symbol N112.

The steel symbol ll&l14 is based on the effect of Ri, which is 0.0
At 090%, the target was achieved although there was a slight tendency for the toughness to decrease.

Steel symbol fish 15 to 17 are based on the effect of Zr,
Comparing steel symbol number 42 and steel code number 15 to 16, the effect of Zr is clear, but steel symbol number 1
When it becomes 7, deterioration of toughness is observed.

Steel codes No. 18 to 20 show the effect of combined addition of Zr and B, and the strength of No. 18 without B addition is barely 80K.
Although it exceeds ``gf/+am'', uEz. is 5kg.
- In floor 19 where B was added alone and the strength has not yet reached i, the strength increased to 86.2 kgf/ms, but it is difficult to recognize any improvement in toughness.Invention steel with combined addition of Zr and B Both strength and toughness were significantly improved at 20.

Steel symbols Nos. 21 and 22 show the effect of Ti.

The rope symbols Na23-29 are CuSNi, io, V
, , is an example of a composite addition system of Nb.

Steel code 30 indicates strength and toughness when the S content is increased to improve machinability, and it can be seen that the properties are superior to conventional steels with steel code 41.42.

Steel symbol 31-32 is steel symbol 3 when PB is added.
3 to 34 are the results when Te was added, and in all cases there was little deterioration in properties.

Steel codes Nos. 35 and 36 show the effects of combined Ca-5-Te addition, steel codes No. 37 shows the effects of Ss addition, and steel codes No. 1 and 38 show the effects of Bi addition, and all of them are superior to conventional steels. It shows the characteristics of

Steel code 1IkL39-40 is steel with high S content and Ce
Compared to steel code N11L30, the toughness is improved due to the addition of rare earth element Ce.

Claims (8)

[Claims] (1) In weight%, C: 0.05-0.35%, Si: 0.02-2.0%
, Mn: 0.1-3.0%, P: 0.05% or less, S:
0.05% or less, Cr: 0.1-3.0%, B: 0.0
005-0.01%, Ti: 0.003-0.3%, Z
r: 0.001-0.5%, Al: 0.001-0.1
%, N: 0.001 to 0.02%, a non-thermal steel for hot forging consisting of the remainder Fe and unavoidable impurities. (2) In weight%, C: 0.05-0.35%, Si: 0.02-2.0%
, Mn: 0.1-3.0%, P: 0.05% or less, S:
0.05% or less, Cr: 0.1-3.0%, B: 0.0
005-0.01%, Ti: 0.003-0.3%, Z
r: 0.001-0.5%, Al: 0.001-0.1
%, N: 0.001 to 0.02%, and further contains Cu: 0.01 to 1.0%, Ni: 0.01 to 2.0%.
, Mo: 0.01-1.0%, V: 0.001-1.0
%, and Nb: 0.001 to 0.30%, and one or more of them, the balance being Fe and unavoidable impurities. (3) In weight%, C: 0.05-0.35%, Si: 0.02-2.0%
, Mn: 0.1 to 3.0%, P: 0.05% or less, Cr
:0.1~3.0%, B:0.0005~0.01%,
Ti: 0.003-0.3%, Zr: 0.001-0.
5%, Al: 0.001~0.1%, N: 0.001~
0.02%, further S: 0.05-0.5%, Pb: 0.005-0.5%
, Ca: 0.001~0.05%, Te: 0.001~
0.2%, Se: 0.01-0.5%, and Bi: 0
．． A non-thermal steel for hot forging, containing one or more of 01 to 0.5%, with the balance being Fe and unavoidable impurities. (4) In weight%, C: 0.05-0.35%, Si: 0.02-2.0%
, Mn: 0.1 to 3.0%, P: 0.05% or less, Cr
:0.1~3.0%, B:0.0005~0.01%,
Ti: 0.003-0.3%, Zr: 0.001-0.
5%, Al: 0.001~0.1%, N: 0.001~
0.02%, Cu: 0.01-1.0%, Ni: 0.01-2.0%
, Mo: 0.01-1.0%, V: 0.001-1.0
%, and Nb: 0.001 to 0.30%, and further contains S: 0.05 to 0.5%, Pb: 0.005 to 0.5%
, Ca: 0.001~0.05%, Te: 0.001~
0.2%, Se: 0.01-0.5%, and Bi:
A non-thermal steel for hot forging containing 0.01 to 0.5% of one or more of the following, with the balance being Fe and unavoidable impurities. (5) In weight%, C: 0.05-0.35%, Si: 0.02-2.0%
, Mn: 0.1-3.0%, P: 0.05% or less, S:
0.05% or less, Cr: 0.1-3.0%, B: 0.0
005-0.01%, Ti: 0.003-0.3%, Z
r: 0.001-0.5%, Al: 0.001-0.1
%, N: 0.001 to 0.02%, and further contains at least one rare earth element, a total of 0.005 to 0.5
%, with the balance consisting of Fe and unavoidable impurities. (6) In weight%, C: 0.05-0.35%, Si: 0.02-2.0%
, Mn: 0.1-3.0%, P: 0.05% or less, S:
0.05% or less, Cr: 0.1-3.0%, B: 0.0
005-0.01%, Ti: 0.003-0.3%, Z
r: 0.001-0.5%, Al: 0.001-0.1
%, N: 0.001 to 0.02%, and further contains Cu: 0.01 to 1.0%, Ni: 0.01 to 2.0%.
, Mo: 0.01-1.0%, V: 0.001-1.0
%, and Nb: 0.001 to 0.30% of one or more types, and at least one rare earth element, total of 0.005 to 0.5
%, with the balance consisting of Fe and unavoidable impurities. (7) In weight%, C: 0.05-0.35%, Si: 0.02-2.0%
, Mn: 0.1 to 3.0%, P: 0.05% or less, Cr
:0.1~3.0%, B:0.0005~0.01%,
Ti: 0.003-0.3%, Zr: 0.001-0.
5%, Al: 0.001~0.1%, N: 0.001~
0.02%, further S: 0.05-0.5%, Pb: 0.005-0.5%
, Ca: 0.001~0.05%, Te: 0.001~
0.2%, Se: 0.01-0.5%, and Bi: 0
．． 01 to 0.5% of one or more kinds, and at least one rare earth element, total of 0.005 to 0.5%
%, with the balance consisting of Fe and unavoidable impurities. (8) In weight%, C: 0.05-0.35%, Si: 0.02-2.0%
, Mn: 0.1 to 3.0%, P: 0.05% or less, Cr
:0.1~3.0%, B:0.0005~0.01%,
Ti: 0.003-0.3%, Zr: 0.001-0.
5%, Al: 0.001~0.1%, N: 0.001~
0.02%, Cu: 0.01-1.0%, Ni: 0.01-2.0%
, Mo: 0.01-1.0%, V: 0.001-1.0
%, and Nb: 0.001 to 0.30%, and further contains S: 0.05 to 0.5%, Pb: 0.005 to 0.5%
,, Ca: 0.001-0.05%, Te: 0.001
~0.2%, Se: 0.01-0.5%, and Bi:
0.01-0.5% of one or more kinds, and at least one rare earth element, total of 0.005-0.5%
%, with the balance consisting of Fe and unavoidable impurities.