JP2000282169A - Steel with excellent forgeability and machinability - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide steel having good forgeability and machinability. SOLUTION: In weight%, C: 0.1 to 0.85%,
Si: 0.01 to 1.5%, Mn: 0.05 to 2.0
%, P: 0.003 to 0.2%, S: 0.003 to
0.5%, and Zr: 0.0003-0.
01%, Te: 0.003-0.005%, Ca: 0.
0002-0.005%, Mg: 0.0003-0.0
While containing one or more of the above 05%,
Al ≦ 0.01%, total-O ≦ 0.02%, to
tal-N ≦ 0.02%, with the balance being Fe and unavoidable impurities. This steel is excellent in forging workability due to spheroidized sulfide shape, and
Has good machinability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel used in automobiles and general machines, and more particularly to steel excellent in forgeability and subsequent mechanical properties and machinability.

[0002]

2. Description of the Related Art In recent years, while the strength of steel has been increasing, the workability has been reduced, and there is a growing need for steel that does not reduce the cutting efficiency. It is known that it is effective to add machinability improving elements in order to improve machinability, but those machinability improving elements reduce high-temperature ductility,
Since the anisotropy due to rolling or forging is increased, cracks are likely to occur in parts during forging or processing. For example, Pb, Bi
Is said to improve machinability and have relatively little effect on forging, but is known to reduce high-temperature ductility. This tends to cause cracking during casting or hot forging.
Although it is known that the machinability of P is also improved, it is liable to crack during casting to lower the melting point of steel, and the amount of P added is limited. In addition, P
It is small compared to b, Bi, etc. S forms an inclusion that becomes soft under a cutting environment such as MnS to improve machinability, but the MnS size is larger than particles such as Pb and tends to be a source of stress concentration. In particular, when MnS is elongated by forging or rolling, anisotropy occurs and becomes extremely weak in a specific direction.
It is also necessary to consider such anisotropy in design.
Therefore, a technique for minimizing the anisotropy of such free-cutting elements is required. It has been argued that the addition of Te eliminates anisotropy (Japanese Patent Application Laid-Open No. 55-41943), but Te tends to crack during casting, rolling and forging.

[0003] Therefore, such steel having both hot ductility and machinability has not existed so far, and further technical innovation is required.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a steel having good hot ductility, anisotropy after working and machinability, in order to meet the above-mentioned situation.

[0005]

In order to improve the machinability, the addition of a free-cutting element S is effective. Generally, steel is processed by rolling or forging, but due to plastic flow at that time,
MnS is deformed, causing anisotropy in mechanical properties. During forging, cracks caused by the anisotropy show a substantial forging limit. Therefore, in order to improve the forgeability, it is effective to make the shape of the inclusion as close to a sphere as possible and to minimize the anisotropy. Further, even if the inclusion is elongated by plastic flow, if the size of the inclusion is small, the influence of the anisotropy can be reduced. Therefore, it is desirable that MnS for improving machinability is finely dispersed and used as a steel component for maintaining its shape in a spherical shape.

[0006] The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) In weight%, C: 0.1 to 0.8
5%, Si: 0.01 to 1.5%, Mn: 0.05 to
2.0%, P: 0.003 to 0.2%, S: 0.0
And Zr: 0.0003%.
To 0.01%, Te: 0.003 to 0.005%, C
a: 0.0002 to 0.005%, Mg: 0.0003
≦ 0.005%, Al ≦ 0.01%, total-O ≦ 0.02
%, Total-N ≦ 0.02%, and the balance is Fe
A steel excellent in forgeability and machinability characterized by being composed of unavoidable impurities.

(2) C: 0.1-0.8% by weight
5%, Si: 0.01 to 1.5%, Mn: 0.05 to
2.0%, P: 0.003 to 0.2%, S: 0.0
And Zr: 0.0003%.
To 0.01%, Te: 0.003 to 0.005%, C
a: 0.0002 to 0.005%, Mg: 0.0003
≦ 0.005%, Al ≦ 0.01%, total-O ≦ 0.02
%, Total-N ≦ 0.02%, and C
r: 0.01 to 2.0%, Ni: 0.05 to 2.0%,
Mo: 0.05 to 1.0%, B: 0.0005 to 0.
005%, one or more of which contain Fe,
A steel excellent in forgeability and machinability characterized by being composed of unavoidable impurities.

(3) The steel according to the above (1) or (2) further comprises, by weight%, V: 0.05 to 1.0.
%, Nb: 0.005 to 0.2%, Ti: 0.005 to
A steel excellent in forgeability and machinability, characterized by containing one or more of 0.1%.

(4) The steel according to any one of the above (1) to (3) further contains Bi: 0.05% by weight.
A steel excellent in forgeability and machinability, characterized by containing one or two of Pb: 0.01% to 0.5%.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the steel components of the present invention will be described.

C is an element that has a great effect on the basic strength of steel, and is 0.1 to 0.85 in order to obtain a sufficient strength.
%. If it is less than 0.1%, sufficient strength cannot be obtained, and other alloy elements must be added in a larger amount.
%, It becomes close to hypereutectoid and a large amount of hard carbide is precipitated, so that the machinability is remarkably reduced.

[0013] Si is added as a deoxidizing element, but is added to strengthen ferrite and impart tempering softening resistance. In the present invention, it is also required as a deoxidizing element.
When the content is less than 0.01%, the effect is not recognized. When the content exceeds 1.5%, the material becomes brittle and the deformation resistance at high temperature increases.

Mn is necessary not only for fixing and dispersing sulfur in steel as MnS, but also for forming a solid solution in a matrix to improve hardenability and secure strength after quenching. The lower limit is 0.05%, and if it is less than that, S becomes FeS and becomes brittle. If the amount of Mn increases, the hardness of the base increases, the cold workability decreases, and the effect on strength and hardenability is saturated.
0% was made the upper limit.

[0015] Since the hardness of the base material in steel increases in the steel, and not only the cold workability but also the hot workability and casting properties decrease, the upper limit must be made 0.2%. On the other hand, the lower limit of elements that are effective in machinability is 0.00
3%.

S bonds with Mn and exists as MnS inclusions. MnS improves machinability, but MnS
S is one of the causes of anisotropy during forging. It should be adjusted according to the degree of anisotropy and the required machinability, but at the same time, it tends to cause cracks in hot and cold forging, so the upper limit was made 0.5%. The lower limit is set to 0.003%, which is a limit at which the cost does not increase significantly at the current industrial production level.

Next, the reasons for limitation of Zr, Ca, Mg, and Te will be described.

Zr is a deoxidizing element and forms an oxide. Since the oxide is considered to be ZrO ₂ and ZrO ₂ serves as a nucleus for precipitation of MnS, the number of MnS precipitation sites is increased and MnS is uniformly dispersed. Zr also forms a solid sulfide in MnS to form a complex sulfide, thereby reducing its deformability, and has a function of suppressing the elongation of the MnS shape even in rolling or hot forging. Therefore, it is an element effective for reducing anisotropy. If the content is less than 0.0003%, the effect is not remarkable. Even if 0.01% or more is added, not only the yield is extremely deteriorated, but also the hard Z
Generates a large amount of rO ₂ , ZrS, etc., but rather degrades mechanical properties such as machinability, impact value and fatigue properties. Therefore, the component range was defined as 0.0003 to 0.01%.

Ca is a deoxidizing element, which forms a soft oxide and not only improves the machinability, but also dissolves in MnS to lower its deformability, so that MnS can be formed even by rolling or hot forging. It has the function of suppressing shape elongation. Therefore, it is an element effective for reducing anisotropy. If the content is less than 0.0002%, the effect is not remarkable. Even if 0.005% or more is added, not only the yield is extremely deteriorated, but also a large amount of hard CaO is generated, and the machinability is rather reduced. Therefore, the component range was defined as 0.0002 to 0.005%.

Mg is a deoxidizing element and forms an oxide. The oxide serves as a precipitation nucleus of MnS, and is effective in fine and uniform dispersion of MnS. Therefore, it is an element effective for reducing anisotropy. If the content is less than 0.0003%, the effect is not remarkable. Even if 0.005% or more is added, the yield is extremely deteriorated and the effect is saturated. Therefore, the component range was defined as 0.0003 to 0.005%.

Te is a machinability improving element. MnT
By generating e or coexisting with MnS, it has a function of reducing the deformability of MnS and suppressing the elongation of the MnS shape. Therefore, it is an element effective for reducing anisotropy. This effect is not observed at less than 0.0003%,
%, The effect is saturated and the high-temperature ductility is reduced, which tends to cause cracking during casting.

Zr and Ca generate various inclusions such as oxides, sulfides and nitrides. In such a case, the nucleation of sulfides and the reduction of deformability can be more effectively achieved by adding them in combination than in the case of adding a large amount of a single element, and spheroidization of sulfides can be promoted. That is, Zr, Te, Ca, Mg
By combining one or two or more of these, a large effect can be obtained with a minimum addition amount.

Al is a deoxidizing element and forms Al ₂ O ₃ in steel. Since Al ₂ O ₃ is hard, it causes tool damage during cutting and promotes wear. Further, when Al is added, O decreases, and ZrO ₂ is hardly generated. In addition, fine ZrO ₂
It is better not to add Al in order to uniformly disperse this. This effect is due to the addition amount of Zr, the yield, and MnS
Greatly affects the distribution and shape of the hard Al ₂ O ₃
0.01 in order to suppress ZrO ₂
%. As a result, the amount of Zr added can be greatly reduced, and the effect of Zr addition as a precipitation nucleus and the effect of compounding with MnS can be increased.

When O is present in a free state, it becomes a bubble during cooling and causes pinholes. Also, Si, A
When combined with l, Zr, Ca, Mg, etc., a hard oxide is generated, so that a restriction is required. In this steel, Zr, Ca,
The upper limit was set to 0.02% at which the fine dispersion effect of Mg was lost.

When N is solid solution N, it hardens the steel. In particular, in cutting, hardening occurs near the cutting edge due to dynamic strain aging, and the life of the tool is reduced. Also, when nitrides such as Ti, Al, and V are present, the growth of austenite grains is suppressed, so that a restriction is required. Especially in the high temperature range, Ti
Generate N and ZrN. Even when nitride is not formed, bubbles are generated during casting, which causes flaws and the like. In the present invention, the upper limit is set to 0.02% at which the adverse effect is remarkable.

Next, the reason for limiting the use of one or more of Cr, Ni, Mo, and B will be described.

Cr is an element for improving hardenability and imparting softening resistance to tempering. Therefore, it is added to steels requiring high strength. In that case, 0.01% or more of addition is required. However, when added in a large amount, Cr carbides are formed and embrittled, so the upper limit is 2.0%.

Ni is effective for strengthening ferrite, improving ductility, improving hardenability and improving corrosion resistance. If less than 0.05%, the effect is not recognized, and 2.0%
%, The effect saturates in terms of mechanical properties, so the upper limit was set.

Mo is an element that imparts temper softening resistance and improves hardenability. If the content is less than 0.05%, the effect is not recognized, and even if added over 1.0%, the effect is saturated. Therefore, the addition range is set to 0.05 to 1.0%.

B has an effect on grain boundary strengthening and hardenability when B is dissolved, and has an effect on machinability since it precipitates as BN when precipitated. These effects are 0.0005
%, It is not remarkable. Even if it is added more than 0.005%, the effect is saturated, and if too much BN is precipitated, the mechanical properties of the steel are impaired. So 0.0005-0.0
The range was 05%.

Next, the reasons for limiting the inclusion of one or more of V, Nb and Ti will be described.

V forms carbonitrides and can strengthen the steel by secondary precipitation hardening. If it is less than 0.05%, there is no effect on increasing the strength, and if it exceeds 1.0%, a large amount of carbonitride precipitates, which in turn impairs the mechanical properties.
This was the upper limit.

Nb also forms carbonitrides and can strengthen the steel by secondary precipitation hardening. If it is less than 0.005%, there is no effect on increasing the strength, and if it exceeds 0.2%, a large amount of carbonitride precipitates, which impairs the mechanical properties.

[0034] Ti also forms carbonitrides and strengthens the steel.
It is also a deoxidizing element, and can improve machinability by forming a soft oxide. If the content is less than 0.005%, the effect is not recognized, and even if it exceeds 0.1%, the effect is saturated. Further, Ti becomes a nitride even at a high temperature and suppresses the growth of austenite grains. Therefore, the upper limit is set to 0.1%.

Bi and Pb are elements effective in improving machinability. The effect is not recognized at 0.05% or less, and even if added over 0.5%, not only the machinability improving effect is saturated, but also the hot forging property is reduced, which is likely to cause flaws. Therefore, Bi 0.05-0.5%, Pb
In the range of 0.05 to 0.5%, one or two of these
Seeds were included.

[0036]

EXAMPLES The effects of the present invention will be described with reference to examples.

The test materials shown in Table 1 were melted in a 2t vacuum melting furnace, decomposed and rolled into billets, and further rolled to φ60 mm. After rolling, a hot upsetting test piece for evaluating hot workability and a cold upsetting test piece for evaluating cold work were cut out and subjected to an upsetting test. A part was heated to 1200 ° C. as a heat treatment, then allowed to cool and subjected to a cutting test.

Here, the method for analyzing Zr in steel is described in JI.
The samples were processed in the same manner as in SG 1237-1997 Appendix 3 and then measured by ICP. However, the sample used for the measurement in the examples of the present invention was 2 g / steel type, and the calibration curve in ICP was set so as to be suitable for a trace amount of Zr.

[0039]

[Table 1]

FIG. 1 shows the cutting direction of the upsetting test piece.
As shown in the cutting position 1, the hot upsetting test piece 3 and the cold upsetting test piece 4 having the notch 5 formed therein were cut out so that MnS2 in the steel was in the longitudinal direction. In the upsetting test, when a load is applied with a load 6 as shown in FIG. 2 and the test piece is deformed like the deformed test piece 7, a tensile stress is generated in the outer peripheral portion in the circumferential direction. At that time, MnS in steel is often used.
Often becomes a fracture source and cause cracks 8. The workability during forging can be evaluated by the upsetting test of the test piece cut out in this way.

As shown in FIG. 3, the hot upsetting test piece had a φ20 mm × 30 mm thermocouple attached, was heated to 1000 ° C. by high frequency, and was loaded with a load 6 within 3 seconds. Was done. The test piece deforms from 9 before deformation to 10 after deformation. Forging was performed at various strains, and the strain at which cracks occurred was measured as the critical strain. Here, the strain is a so-called nominal strain defined by the equation (1).

Ε = (H ₀ −H) / H ₀ Equation (1)

Here, FIG. 3 shows ε: strain, H ₀ : test piece height before deformation, and H: test piece height after deformation.

In order to evaluate the cold workability, a cold upsetting test was performed. The material cut out as shown in FIG.
After quenching from 0 ° C., spheroidizing annealing was performed at 700 ° C. for 12 hours. After that, φ7m with 2mm notch by machining
An mx14 mm cold upsetting test piece was prepared. The definition of strain is the same as in equation (1).

Further, the machinability was evaluated by a drilling test, and Table 2 shows the cutting conditions. Cumulative hole depth 1000m
m (the so-called VL100)
In 0), the machinability was evaluated.

[0046]

[Table 2]

Table 1 shows examples in which the workability was evaluated. Table 1 Examples 1 to 22 are S45C-based steels in which the amount of S is changed, and one of Zr, Te, Ca, and Mg is added. Examples 23 to 27 are Zr as comparative examples.
It is a steel to which none of Te, Ca and Mg is added.
Table 1 shows the critical strain in hot forging, the critical strain in cold forging, and the machinability VL1000. FIG. 4 shows the critical strain in hot forging.
As a result of these comparisons, as the amount of S added increases, the critical strain decreases, but the amount of decrease is Zr, Te, Ca, Mg.
Is small when any one of them is added, and greatly decreases when no one is added. This tendency was the same for the critical strain in cold forging.

Next, Table 3 shows the critical strain and the tool life VL10 when the addition amounts of Zr, Te, Ca, and Mg were changed.
00 is shown. FIG. 5 shows the examples of Table 3 and the examples of Table 1 in which the amount of S is almost the same as the examples.
The critical strain, the tool life VL1000, the sulfide aspect ratio, and the number of sulfides per unit area are shown when r and Te are changed. When Zr and Te were added in combination, the horizontal axis was arranged by the value of Zr + Te. With respect to the embodiment similar to FIG. 5, FIG. 6 shows the critical strain, the tool life VL1000, and the tool life when the addition amounts of Ca and Mg were changed.
The sulfide aspect ratio and the number of sulfides per unit area are shown. When compounded with Zr or Te,
Sorted by a or Mg amount. Example 41 is a comparative example in which more Al was added than specified and Zr, Te, Ca, and Mg were not added. According to this result, Zr, Te, C
When a and Mg are not added, the critical strain is small. In addition, when the amount exceeds the specified range, either the critical strain or the tool life VL1000 decreases. Despite spheroidization of sulfide, the tendency that the critical strain tends to decrease is that Zr, Te, Ca, and Mg are precipitates of MnS or MnS
It is considered to form a nitride or a single sulfide without forming a composite sulfide with the same. When Al is added, not only the critical strain but also the tool life VL1000 is extremely low as compared with the others.

In FIGS. 4 to 6, the subscripts in the figures are as follows:
Example No. , And the indications of the marks in the figure are common.

[0050]

[Table 3]

Table 4 shows examples in which the effects on other elements were examined. The manufacturing method and the method for evaluating hot workability and machinability are the same as the examples shown in Table 1. Examples 42 to 78
Showed the hot limit strain and machinability when various alloying elements were added. The comparative example was significantly inferior in terms of hot limit strain even though the difference in machinability was small. Examples 70 to 7
The invention example is also superior to the comparative example when the basic strength as shown in FIG. 5 is changed by the C amount. Thus, the present invention can be widely applied to the range of structural steel. In Examples 76 to 78, the total-O amount and the total
It is a comparative example in which the -N amount was outside the range of the invention. Even if Zr, Te, Ca, Mg, Al, etc. were adjusted within the specified range, if the total-O amount and the total-N amount were different, both the hot limit strain and the machinability were inferior. Thus, it can be seen that the examples included in the present invention have both good workability and machinability when compared at the same S amount.

[0052]

[Table 4]

[0053]

According to the present invention, a steel having both hot workability, mechanical properties and machinability can be provided. In particular, the technology of the present invention is not significantly affected by heat treatment and microstructure,
Since it is based on sulfide shape control, it can be applied to a wide range of steels. The working is effective not only for hot forging but also for cold forging, and is effective for a wide range of steels requiring forging workability, mechanical properties, and machinability.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a test piece cutout position and a test piece shape for forging workability (hot and cold) evaluation.

FIG. 2 is a diagram illustrating a crack generation position in an upsetting test.

FIG. 3 is a diagram illustrating the definition of strain during forging processability evaluation (upsetting test).

FIG. 4 is a graph showing the effect of the amount of S on hot forgeability in Examples of Table 1.

FIG. 5 is a view showing the influence of Zr and Te on the hot forging limit strain and the tool life VL1000 for the examples in Table 3.

FIG. 6 is a diagram showing the influence of Ca and Mg on the hot forging limit strain and the tool life VL1000 for the examples in Table 3.

[Explanation of symbols]

Reference Signs List 1 cut-out position 2 MnS 3 hot upsetting test piece 4 cold upsetting test piece 5 notch 6 load 7 deformed test piece 8 crack 9 before deformation 10 after deformation H ₀ test piece height before deformation H after deformation Specimen height

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/60 C22C 38/60 (72) Inventor Koichi Isobe 12 Nakamachi, Muroran Nippon Steel Corporation Room (72) Inventor Kenji Fukuyasu Inside Nippon Steel Corporation 2-6-3 Otemachi, Chiyoda-ku, Tokyo

Claims (4)

[Claims] (1) C: 0.1 to 0.85% by weight,
Si: 0.01 to 1.5%, Mn: 0.05 to 2.0
%, P: 0.003 to 0.2%, S: 0.003 to
0.5%, and Zr: 0.0003-0.
01%, Te: 0.003-0.005%, Ca: 0.
0002-0.005%, Mg: 0.0003-0.0
While containing one or more of the above 05%,
Al ≦ 0.01%, total-O ≦ 0.02%, to
tal-N ≦ 0.02%, the balance being Fe and inevitable impurities, characterized by excellent forgeability and machinability. 2. C: 0.1 to 0.85% by weight,
Si: 0.01 to 1.5%, Mn: 0.05 to 2.0
%, P: 0.003 to 0.2%, S: 0.003 to
0.5%, and Zr: 0.0003-0.
01%, Te: 0.003-0.005%, Ca: 0.
0002-0.005%, Mg: 0.0003-0.0
While containing one or more of the above 05%,
Al ≦ 0.01%, total-O ≦ 0.02%, to
tal-N ≦ 0.02%.
01-2.0%, Ni: 0.05-2.0%, Mo:
0.05-1.0%, B: 0.0005-0.005
% Or more, with the balance being Fe and unavoidable impurities, the steel being excellent in forgeability and machinability. 3. The steel according to claim 1 or 2,
Further, by weight%, V: 0.05 to 1.0%, Nb:
A steel having excellent forgeability and machinability, characterized by containing one or more of 0.005 to 0.2% and 0.005 to 0.1% of Ti. 4. The steel according to claim 1, further comprising Bi: 0.05 to 0.5% by weight.
%, Pb: steel having excellent forgeability and machinability, characterized by containing one or two of 0.01 to 0.5%.