JP2002241890A - High toughness non-refining steel for hot forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-refining steel which has high toughness on a level which can not always stably attained by the conventional non-refining steel even if heat treatment after hot forging is obviated in a process of producing machine parts. SOLUTION: The high toughness non-refining steel has a composition containing, by mass, 0.1 to 0.6% C, 0.01 to 2.0% Si, 0.2 to 2.0% Mn, 0.005 to 0.5% S, 0.1 to 2.0% Cr, 0.005 to 0.1% Ti, 0.003 to 0.02% N, 0.0 to 0.07% Al, 0.001 to 0.1% Sn, and further containing 0.005 to 0.1% Rem (one or more kinds selected from rare earth metals such as Ce, La and Nd) and 0.0005 to 0.005% Ca, and also satisfying the relation of (Rem%+Ca%)/(S%)>=0.05, and the balance Fe with inevitable impurities. The average diameter of the equivalent circle of inclusions is <=20 μm, and the average aspect ratio (length/width) of the inclusions is <=10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-toughness non-heat-treated steel, and more particularly to a hot-forged non-heat-treated steel made of carbon steel and low alloy steel having a C content of 0.1 to 0.6% by mass. Things.

[0002]

2. Description of the Related Art Conventionally, many mechanical parts such as automobile parts are manufactured by hot forging, quenching, tempering, so-called tempering, and then machining such as cutting and grinding. It has been.

[0003] However, the demand for improvement in productivity and energy saving through process savings has led to the elimination of the above-mentioned tempering treatment, and the use of non-heat treated steel has been expanding from the viewpoint of energy saving and cost reduction.

[0004] Generally, non-heat treated steel from which tempering treatment is omitted has a problem that its use is limited to a narrow range, for example, it is used for machine parts that do not require high toughness because it has lower toughness than tempered steel. Was.

Conventionally, as this non-heat-treated steel, a microalloyed non-heat-treated steel in which elements such as V and Nb are added in small amounts to carbon steel for machine structural use has been proposed, but the structure as hot forged is significantly coarsened. Due to the ferrite-pearlite structure, the toughness was extremely low, and the application range was extremely limited.

[0006] In order to eliminate such disadvantages, a small amount of Ti
There has been proposed a method of improving the toughness by adding crystal grains to prevent coarsening of the crystal grains (for example, JP-A-56-38448).
Publication), the effect is not always stable.

Further, one or more of Zr and Ti are added in a small amount, and a cooling rate in the casting step is ensured to disperse at least one of MnS, TiN and ZrN in a fine and uniform manner. A method for producing a non-heat treated steel for hot forging has been proposed (Japanese Patent Publication No. 6-75747).

Further, charcoal such as Al, V, Nb, Ti, B, etc.
A high toughness non-heat treated steel to which a nitride-forming element is added and 0.001 to 0.04 wt% of Ca, Te, Ce and other rare earth metals, misch metal and a mixture thereof and a method of producing the same have been proposed. (JP-A-6-340)
946).

Further, Japanese Patent Publication No. 4-70385 discloses Z
A method for improving the strength and toughness of a hot forged material by adding r and B has been proposed, and a method for improving toughness by adding a rare earth element has been proposed.

[0010]

SUMMARY OF THE INVENTION According to the present invention, a high toughness of a level which cannot be attained with conventional non-heat treated steel is always obtained even when the heat treatment after hot forging is omitted in the above-mentioned mechanical parts manufacturing process. And a non-heat treated steel having the same.

[0011]

Means for Solving the Problems In the above-mentioned mechanical part manufacturing process, the present inventors have omitted the tempering treatment after hot forging.
Various studies have been made on means for realizing a non-heat treated steel having a level of high toughness that cannot always be achieved with conventional non-heat treated steel, and while properly controlling the composition of the steel, sulfides, oxides and By reducing the size of the inclusions composed of oxides and sulfides and controlling the aspect ratio (length / width) of those inclusions to a certain level or less, even if the tempering treatment is omitted after hot forging, The present inventors have found that high toughness can be stably achieved, and have found specific means for realizing them, thereby completing the present invention.

The gist of the present invention is as follows.

(1) In mass%, C: 0.1 to 0.6
%, Si: 0.01 to 2.0%, Mn: 0.2 to 2.0
%, S: 0.005 to 0.5%, Cr: 0.1 to 2.0
%, Ti: 0.005 to 0.1%, N: 0.003 to
0.02%, Al: 0.07% or less (including 0%), S
n: 0.001 to 0.1%, and Rem (Ce, La,
One or more rare earth metals such as Nd): 0.005 to 0.1
%, Ca: one or two of 0.0005 to 0.005%, and satisfies the relationship of (Rem% + Ca%) / (S%) ≧ 0.05, with the balance being Fe and unavoidable impurities. A high toughness non-heat treated steel for hot forging, characterized in that the inclusion has an average equivalent circle diameter of 20 μm or less and the inclusion has an average aspect ratio (length / width) of 10 or less.

(2) Further, in mass%, Zr: 0.00
7 to 0.1%, Mg: One or two of 0.0005 to 0.01%, and (Rem% + Ca% + 2 × Zr% + Mg%) / (S%)
The high toughness non-heat treated steel for hot forging according to the above (1), which satisfies ≧ 0.05.

(3) Further, in mass%, V: 0.01 to
0.5%, Nb: 0.01 to 0.3%, and B: 0.0
The high toughness non-heat treated steel for hot forging according to the above (1) or (2), comprising at least one or more of 003 to 0.005%.

(4) Further, in mass%, Ni: 0.01
The heat according to any one of the above (1) to (3), characterized in that it contains one or two of two kinds of Mo: 2.0% and Mo: 0.01 to 1.0%. High toughness non-heat treated steel for hot forging.

(5) Further, in mass%, Pb: 0.01
-0.3%, Bi: 0.01-0.3%, and Te:
(1) to (4), containing at least one or more of three kinds of 0.0005 to 0.3%.
The high toughness non-heat treated steel for hot forging according to any one of the above.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The requirements of each claim of the present invention will be described below.

First, the reason why the steel component is defined in claim 1 of the present invention is as follows.

C is an essential component for obtaining the strength of the steel material. If it is less than 0.1%, characteristics as a component for machine structure cannot be obtained. Variations in the characteristic properties increase, and it is difficult to obtain stable quality.

Although Si is also a useful element for securing strength, it is necessary to increase the strength by at least 0.01%, and if it exceeds 2.0%, the embrittlement of the ferrite ground becomes severe. Therefore, its content should be suppressed to 2.0% or less.

Mn has a large toughening effect and is a useful element similar to Si. If it is less than 0.2%, the hot ductility is low.
Hot rolling and hot working of steel are difficult. Also, 2.0
%, The machinability, weldability and the like are reduced, so the content is limited.

S is an element for improving machinability.
If it is less than 005%, the machinability becomes low and cutting becomes difficult,
On the other hand, in non-heat-treated steel, the toughness is improved by promoting the formation of ferrite in the cooling process after hot forging.
An increase in S causes deterioration of mechanical properties and an increase in anisotropy. If it exceeds 0.5%, in addition to the deterioration of mechanical properties and the increase in anisotropy, the hot ductility also becomes extremely low.

Cr is also an effective element for increasing the strength, but at least 0.1% or more is required for increasing the strength.
If it exceeds 2.0%, the toughness is significantly reduced, so the upper limit was made 2.0%.

Ti is an element that refines the austenite structure and improves the toughness. For this purpose, at least 0.005% or more is required. On the other hand, if it exceeds 0.1%, coarse TiN or the like is formed. As a result, the upper limit was set to 0.1%.

N is an element useful for forming a nitride with a nitride-forming element such as Al or Ti to make austenite crystal grains fine. At least 0.003% or more of N is required to exert the above effect, and the above effect is saturated even if it exceeds 0.02%, so the upper limit of N is 0.02%. did.

Al is generally an element useful for deoxidation and control of crystal grains, but it is not always necessary to add Al when Si, Mn, or Ti is contained. The amount of Al added contributes to deoxidation and control of crystal grains.
If it exceeds 7%, hot workability is impaired.
07% or less (including 0%).

Sn dissolves in ferrite and contributes to the improvement of the strength. In the steel of the present invention, the primary ferrite at the time of solidification is increased, the effect of suppressing the discharge of S into the dendrite tree and the effect of Sn itself on dendrite. The concentration in the liquid phase existing between the trees contributes to the miniaturization of the sulfide crystallized in the final solidification part, and also controls the sulfide composition by adding Rem, Ca, etc., which will be described later, to perform hot rolling or hot rolling. When the aspect ratio of sulfide was reduced by preventing the stretching by hot forging, it was found to have an effect of increasing the effect. Such an effect can be obtained at least by 0.001% or more of Sn.
Is required, and when the Sn content exceeds 0.1%, the hot ductility is significantly reduced.
It was as follows. If the decrease in hot ductility due to Sn poses a problem even in the above Sn content range, it is preferable to add Ni to ensure the hot ductility.

As a result of various studies by the present inventors, it has been found that sulfides such as MnS drawn by hot rolling or the like not only increase the mechanical anisotropy as conventionally known, but also decrease the toughness. Has also been found to have a significant effect.

Therefore, a method for preventing a decrease in toughness due to the stretched sulfide was examined, and Rem (Ce, La, N
one or more rare earth metals such as d): 0.005 to 0.1%
And Ca: one or two of 0.0005 to 0.005%, and (Rem% + Ca%) / (S%) ≧
When contained so as to be 0.05, MnS becomes various types of Re.
At least one of m and Ca forms a solid solution of a sulfide or an inclusion in which a sulfide and an oxide are compounded, so that hot plasticity is significantly reduced as compared with MnS, and hot rolling or hot rolling is performed. The intermediate forging can very effectively prevent the inclusion from stretching like MnS, and as a result, the elongated M
It has been found that a decrease in toughness due to nS or the like can be significantly improved.

When Rem and Ca are added so as to satisfy the above conditions, substantially spherical oxides containing various Rem or Ca or both elements are formed, and such a morphological change is not caused by low-temperature toughness. It was found that it contributed to the improvement. It is preferable to add both Rem and Ca, but one of them may be added.

In order to effectively suppress stretching during hot working of sulfide by adding one or two kinds of Rem (a rare earth metal such as Ce, La, and Nd) and Ca, Rem is required to be 0.
005% or more, and Ca must be added in an amount of 0.0005% or more.
If 0.005% or more is added, sulfide clusters containing these elements are likely to appear, and the formation thereof reduces the toughness. In addition, Rem and C
Even if the addition amount of a is insufficient, stretching of the sulfide during hot working cannot be sufficiently prevented, so that (Rem% + Ca%) / (S%)
It must be contained so as to satisfy ≧ 0.05.

Further, in the non-heat treated steel for hot forging, the average value of the circle equivalent diameter of the inclusions is 20 μm or less,
The average value of the inclusion aspect ratio (length / width) is 1
If the control is made to be 0 or less, the notch effect due to inclusions such as the presence of elongated sulfide inclusions when an impact is applied is greatly reduced, and the crack propagation resistance is increased. With this effect, the decrease in toughness due to sulfide can be extremely effectively improved.

The inclusions referred to herein are oxides, sulfides, and inclusions in which oxides and sulfides are compounded. Among them, oxides and sulfides containing Rem and Ca and inclusions in which these are compounded Is also included.

In order to reduce the size of the inclusions, the components are controlled to fall within the range defined in claim 1, and
In the casting process for producing steel ingots or slabs, when inclusions such as sulfides crystallize during solidification,
It is also effective to suppress the growth of inclusions that crystallize during solidification by securing a certain cooling rate in the temperature section of 00 ° C.

The smaller the cross-sectional size of the slab or ingot, the easier it is to secure the cooling rate during the solidification, and the growth of inclusions such as MnS which crystallizes during solidification is suppressed, and the size of the inclusions is reduced. easy.

In addition, since inclusions having high plasticity at high temperatures such as sulfides are stretched by hot working as the size increases, it is not necessary to control the inclusion size to be small even in the casting step by the above-mentioned means. This is effective in suppressing the stretching in hot rolling and hot forging.

Further, when the amount of Rem or Ca added is restricted for some reason and it is not possible to sufficiently prevent stretching of sulfides or the like by hot working, Zr described in claim 2 is:
0.007 to 0.1% and Mg: 0.0005 to 0.01
% Of one or two of the two, and (R
em% + Ca% + 2 × Zr% + Mg%) / (S%) ≧
It is also effective to reduce the plasticity of the sulfide by making it contained at 0.05 so as to more reliably prevent stretching during hot working.

In order to prevent the sulfide from stretching during hot working by adding Zr and Mg, it is necessary to add 0.007% or more of Zr and 0.0005% or more of Mg. Z
When r is added in excess of 0.1%, hard oxides such as ZrO ₂ increase, which may decrease toughness.
The upper limit of the content was 0.1%. For Mg, 0.
Even if added over 01%, it is consumed for the generation of MgS in the molten steel and does not effectively act to control the form of inclusions crystallized during solidification, so the upper limit of the Mg content is 0.01%.
%.

When the strength and toughness need to be further improved, the addition of V, Nb and B is effective.

V and Nb are both elements that form carbon and nitride to improve strength and toughness. Nb is used to suppress recrystallization of austenite during hot working and to promote the refinement of crystal grains. Also in that respect, strength toughness is improved.
When V and Nb are added in an amount of 0.01% or more, the above-mentioned improvement effect is exhibited. When the added amount of V exceeds 0.5%, the hot ductility decreases when the added amount of Nb also exceeds 0.3%. , The upper limits of the respective amounts were set to 0.5% and 0.3%.

On the other hand, B is a non-heat treated steel which has the effect of promoting the formation of ferrite and improving the hardness, but the effect appears only at 0.0003% or more. Since the ductility and toughness are also reduced, the amount of B added is limited to 0.005%.

When it is necessary to improve the hardenability and toughness, it is effective to add Ni and Mo.

Ni and Mo are effective elements for securing hardenability and improving toughness. To obtain these effects, it is necessary to add at least 0.01% or more. %, Mo is added.
The effect is saturated even if added over 1.0%, so the upper limit of Ni was set to 2.0% and the upper limit of Mo was set to 1.0%.

If there is a need to improve machinability, the addition of Pb, Bi, or Te is effective for improving machinability.

In order to obtain the effect of improving machinability, it is necessary to add 0.01% or more for Pb and Bi and 0.0005% or more for Te. Pb, Bi, T
The upper limit of the addition amount of all three elements is set to 0.3%.

[0047]

(Example 1) In this example, Table 1 was used to investigate and compare the tensile strength and low-temperature toughness of the invention steels corresponding to claims 1 and 5 and the corresponding comparative steels. A 150 kg steel ingot having the indicated chemical composition was melted in a vacuum melting furnace, and the cooling rate at which the steel ingot solidified was greatly changed. While the steel ingot of the present invention steel and some comparative steels were allowed to cool in a mold, the comparative steel No. In the smelting of steel ingots 9 and 10, the mold is placed in a heat-insulating box using heat-insulating material such as cahor, and the cooling rate is greatly reduced by heating the periphery of the mold placed in the heat-insulating box by gas heating. The inclusions crystallized during solidification such as sulfides were coarsened.

The produced steel ingot was heated to 1250 ° C., then formed into a round bar of 90φ by hot forging, and then cooled once to room temperature to obtain a sample for investigating inclusions. After that, 90φ round bar
It was again heated to 50 ° C. and hot forged to form a 50φ round bar. After the forging, the temperature was cooled from 1100 ° C. to room temperature by air cooling. The JIS No. 4 tensile test piece and JIS were cut from the 50φ round bar thus obtained by cutting.
A No. 3 impact test piece was sampled and subjected to a tensile test and a Charpy test at room temperature to evaluate strength and low-temperature toughness. Table 1 shows the results of each test. In the measurement of the diameter of inclusions and the aspect ratio (length / width) of inclusions, the presence of inclusions present in the vertical section of a 90φ round bar was photographed using an optical microscope at a magnification of 400 times for about 20 visual fields. All the inclusions in the photograph were analyzed by an image analyzer to obtain the average value of the equivalent circle diameter and the aspect ratio (length / width) of the inclusions.

[0049]

[Table 1]

As is clear from Table 1, the steels of the present invention that were hot forged and air-cooled were higher in both strength and low-temperature toughness than those that were hot-forged using comparative steels and then air-cooled. Showed a trend. Conversely, the contents of Sn, Rem, and Ca,
Even if the ratio of (Rem + Ca) / S deviates from the range specified in claim 1 of the present invention, or even if the component system satisfies the specified range, the average equivalent circle diameter and the average aspect ratio of the inclusions are out of the specified range. It can be seen that the strength and toughness are deteriorated as compared with the steel of the present invention.

In addition, the comparative steel No. which significantly reduced the cooling rate during solidification was used. No. 9 and No. 10 of the steel of the present invention. Comparing Comparative Steel Nos. 1 to 6, In No. 9, the average circle equivalent diameter and the average aspect ratio of the inclusions are No. 10, it can be seen that the average equivalent circle diameter of the inclusions is larger than that of the steel of the present invention, and as a result, the strength and the impact value are reduced. Comparative steel N
o. In No. 12, the ratio of (Rem + Ca) / S did not satisfy the range of claim 1, so that the average aspect ratio of the inclusions was large, and the strength and impact value were considerably lower than those of the invention steel.

Example 2 A steel having a chemical composition shown in Table 2 was melted by a blast furnace and a converter method, cast into a 220 mm square bloom slab, then slab-rolled and rolled into a 90 mm round bar. Molded.

Using the round bar obtained by the above method, 125
After heating to 0 ° C, hot forging and forming into a 50φ round bar,
After the forging, the air was cooled from 1100 ° C. to room temperature. In the same manner as in Example 1, a JIS No. 4 tensile test piece or a JIS No. 3 impact test piece was sampled from the material thus obtained by cutting and subjected to a tensile test and a Charpy test at room temperature. Table 2 also shows the results of each test.

[0054]

[Table 2]

Also in this test, the average value of the equivalent circle diameter of the inclusions and the aspect ratio of the inclusions were measured on a vertical section of a 90φ round bar by using the same image analyzer as in Example 1.

The invention steel corresponding to claim 1 is No. 1. 1,
No. 2, 4, 10, and 11 invention steels. 12, 1
3, 15, 21, 22, and 23 are comparative steels. In addition, the invention steel corresponding to claim 2 is No. 2. Inventive steels Nos. 8 and 9; 19 and 20 are comparative steels. No.
The steel materials of Nos. 3, 5, 6, and 9 are invention steels corresponding to claim 3, and the comparative steels are No. 3 steels. 14, 16, 17, and 20 steel materials. Further, invention steels corresponding to claim 4 and claim 5 are No. 4 respectively. 7 and no. Nos. 5 and 6 and the corresponding comparative steels were Nos. 18 and No. 16, 17
It is.

Also in the present embodiment, when hot forging is performed using the invention steel according to each claim, the tensile strength and toughness at room temperature are lower than when using the comparison steel corresponding to each invention steel. High values were obtained, and it was also confirmed in this example that the steel of the present invention was an excellent non-heat treated steel for hot forging.

Further, as can be seen from Table 2, even in the present embodiment, even if the component system does not satisfy the range of each claimed steel or the component system satisfies the specified range, the average circle of other inclusions is not satisfied. It can be seen that when the equivalent diameter and the average aspect ratio are out of the specified ranges, the strength and toughness are considerably reduced as compared with the steel of the present invention.

[0059]

As described above, in the steel of the present invention, a high strength and toughness value can be obtained without performing the tempering treatment, and by omitting the tempering treatment, the energy and the production cost can be largely reduced. Becomes

Claims (5)

[Claims] C .: 0.1 to 0.6% by mass, S
i: 0.01 to 2.0%, Mn: 0.2 to 2.0%,
S: 0.005 to 0.5%, Cr: 0.1 to 2.0%,
Ti: 0.005 to 0.1%, N: 0.003 to 0.0
2%, Al: 0.07% or less (including 0%), Sn: 0.
001-0.1%, Rem (at least one of rare earth metals such as Ce, La, Nd): 0.005-0.1%, C
a: contains 0.0005 to 0.005% of one or two kinds, satisfies the relationship of (Rem% + Ca%) / (S%) ≧ 0.05, and the balance consists of Fe and inevitable impurities; A high toughness non-heat treated steel for hot forging, characterized in that the inclusion has an average equivalent circle diameter of 20 μm or less and the average aspect ratio (length / width) of the inclusion is 10 or less. 2. In addition, Zr: 0.007% by mass%
0.1%, Mg: One or two of two types of 0.0005 to 0.01%, and (Rem% + Ca% + 2 × Zr% + Mg%) / (S%)
2. The high toughness non-heat treated steel for hot forging according to claim 1, wherein a relationship of ≧ 0.05 is satisfied. 3. Further, in mass%, V: 0.01 to 0.5.
%, Nb: 0.01 to 0.3%, and B: 0.0003
The high toughness non-heat-treated steel for hot forging according to claim 1 or 2, further comprising at least one of three kinds of -0.005%. 4. Ni: 0.01 to 2.% by mass.
The high toughness non-hot tough steel according to any one of claims 1 to 3, further comprising one or two of 0% and Mo: 0.01 to 1.0%. Tempered steel. 5. Pb: 0.01 to 0.1% by mass.
3%, Bi: 0.01 to 0.3%, and Te: 0.00
The high-toughness non-heat-treated steel for hot forging according to any one of claims 1 to 4, comprising at least one or more of the three types from 0.05 to 0.3%.