JPH1072639A - Steel material for machine structural use, excellent in machinability, cold forgeability, and hardenability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a machinability equal to or higher than that of a Pb-added free cutting steel and to improve a cold forgeability and a hardenability by using mainly graphite and cementite as C to be added to a steel of specific composition and also specifying a graphitization ratio. SOLUTION: This steel material for machine structural use has a composition consisting of, by mass, 0.1-1.5% C, 0.5-2.0% Si, 0.1-2.0% Mn, 0.005-0.1% Al, 0.0015-0.0150% N, 0.0003-0.0150% B, <=0.020% P, <=0.035% S, <=0.0030% O, and the balance Fe with inevitable impurities. Mainly graphite and cementite are used as C to be added, and a ferrite + graphite + cementite structure is formed. Further, the graphitization ratio, defined by [(the amount of graphite)/(the amount of graphite when the contained C is all graphitized)] ×100(%), is regulated to 10-80%. After this steel material is worked into a prescribed shape, induction hardening and tempering are applied to provide a prescribed strength.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for a machine structure suitable for use in machine parts such as industrial machines and automobiles, and more particularly to a steel material for a machine structure having machinability, cold forgeability and hardenability. About.

[0002]

2. Description of the Related Art Machine parts used in industrial machines and automobiles are formed by cutting steel, cold forging or a combination thereof to a predetermined shape, and then quenching and tempering to obtain the required characteristics as mechanical parts. It is manufactured by the method of securing. First, steel materials used for such mechanical parts are required to have excellent machinability and cold forgeability. As a means for improving the machinability of the steel for machine structural use, a method of adding a free-cutting element such as Pb, S, Bi, P or the like to the steel alone or in combination is generally used. In particular, Pb is widely used because it has an extremely strong effect of improving machinability. But on the other hand, Pb is also a harmful element to the human body,
Extensive exhaust equipment is required in the manufacturing process of steel products and the processing of machine parts, and there is a problem in terms of recycling steel products. On the other hand, in order to improve the cold forgeability of steel,
Since free-cutting elements such as b, S, Te, Bi, and P deteriorate ductility and toughness, it is desirable to reduce them.

In view of the above, in order to simultaneously improve machinability and cold forgeability of steel for machine structural use, it has been considered that the structure of the steel is a two-phase structure of ferrite + graphite. For example, JP-A-51-57621 discloses that Si is used in an amount of 1.9 to 3.
A free-cutting steel with a high forgeability of 0% and containing 0.20 to 0.44% of fine graphite and having excellent cold forgeability has been proposed. In addition, Japanese Unexamined Patent Publication No.
JP-A-140411 discloses a method for improving cold workability after tempering. This method uses 0.32-0.54% C
In hot-rolled or cold-rolled steel materials with adjusted Mn, Si, and Al contents, before final cold working and quenching and tempering, 620 to 68
Annealing is performed at 0 ° C for 15 hours or more, and it is almost completely graphitized.

[0004] However, a steel having a two-phase structure of ferrite and graphite is an extremely soft combination of two phases, and therefore has excellent characteristics such as low deformation resistance during cold forging, but is soft during cutting. Therefore, the surface was apt to be peeled, and the surface condition after cutting was not always excellent. Graphite is a precipitate that is much more stable than cementite, and it is very difficult to dissolve graphite C into steel even when it is heated to the austenite region. Therefore, in the case of quenching, when the structure is ferrite + graphite, sufficient strength may not be obtained as compared with the case of ferrite + pearlite structure or ferrite + spherical cementite structure. In particular, in the case of induction hardening in which rapid heating and holding for a short time were performed, the strength was insufficient after quenching due to the ferrite + graphite structure.

[0005]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, has machinability equal to or higher than that of conventional Pb-added free-cutting steel, has a small surface roughness after cutting, and An object of the present invention is to provide a steel material for a machine structure which is excellent in cold forgeability and hardenability.

[0006]

As a result of various studies, the present inventors have determined that the composition of the composition is such that it accelerates the graphitization, and that the structure is made up of the ferrite and graphite having a content of 10 to 80% of the added C content. By forming a ferrite + graphite + cementite structure in which the C content of C is cementite, the cold forging property, machinability, surface roughness after cutting and hardenability of the steel material for machine structure are improved.

When layered inclusions such as pearlite and MnS are present in steel, voids are generated from the interface between these inclusions and the matrix during cold working, and the amount of strain increases. These expand the connection and lead to early destruction. However, if graphite is present in the steel, the graphite will follow the deformation of the matrix during cold forging, and the generation of voids from the graphite-matrix interface will be suppressed, increasing the amount of strain up to fracture. The cold forgeability is improved. Further, if graphite is present in the steel, the graphite acts as a lubricant during cutting and suppresses a rise in the temperature of the tool, thereby improving machinability.

However, since it becomes difficult to dissolve graphite C into steel, quenchability, especially induction hardening, is poor with graphite alone. Therefore, in the present invention, cementite is left in the structure in addition to graphite. By leaving cementite in the steel, the graphite particle size in the steel when compared with the same C content becomes fine, and as a result, the graphite particles themselves can be easily dissolved in the matrix, and from this point, the quenchability is also high. Improves.

Further, by leaving cementite partially in the steel, the overall hardness is increased and the surface roughness after cutting is also improved. The present invention has been made based on the above idea. That is, the present invention provides mass
%, C: 0.1 to 1.5%, Si: 0.5 to 2.0%, Mn: 0.1%
2.02.0%, Al: 0.005 to 0.1%, N: 0.0015 to 0.0150
%, B: 0.0003 to 0.0150%, P: 0.020% or less, S: 0.
035% or less, O: 0.0030% or less, the balance consists of Fe and unavoidable impurities, the contained C is mainly graphite and cementite, and ｛(graphite amount) / (all contained C is graphitized. It is a steel material for machine structural use excellent in machinability, cold forgeability and hardenability, characterized in that the graphitization ratio defined by (amount of graphite)｝ × 100 (%) is 10 to 80%.

[0010] In the second invention, in addition to the first invention, mass
%, Ni: 3.0% or less, Cu: 3.0% or less, Co: 3.0% or less, excellent in machinability, cold forgeability and hardenability Steel for machine structural use. The third invention is the first invention or the second invention.
In addition to the invention, in mass%, V: 0.5% or less, Nb: 0.05%
It is a steel material for machine structures excellent in machinability, cold forgeability and hardenability, characterized by containing one or more selected from the following.

According to a fourth invention, in addition to the first invention, the second invention, or the third invention, machinability, cold forgeability, and mass% are not more than 1.0%. It is a steel material for machine structure with excellent hardenability. The fifth invention is the first invention, the second invention, the third invention, or the fourth invention.
In addition to the invention, in mass%, Ti: 0.05% or less, Zr: 0.2%
REM: A steel for machine structural use excellent in machinability, cold forgeability and hardenability, characterized by containing at least one selected from the group consisting of REM: 0.2% or less.

A sixth invention is characterized in that the steel material according to any one of the first to fifth inventions is processed into a predetermined shape and then subjected to induction hardening and tempering to impart a predetermined strength. This is a method for manufacturing a mechanical structural part.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the component composition of a steel material in the present invention will be described below. C: 0.1 to 1.5% C is an essential component for forming a graphite phase. 0.1
%, It is difficult to secure a graphite phase necessary for securing machinability. On the other hand, if it is added in excess of 1.5%, the deformation resistance during hot rolling increases, and the deformability decreases. Therefore, C is set in the range of 0.1 to 1.5%.

Si: 0.5 to 2.0% Si is an element which does not form a solid solution in cementite and promotes graphitization by destabilizing cementite.
Actively added, but if less than 0.5%, the effect is not observed. However, if it exceeds 2.0%, the strength becomes too high and the ductility deteriorates. Therefore, the content of Si is set in the range of 0.5 to 2.0%. A more preferable range is from the viewpoint of promoting graphitization.
0.7-1.8%.

Mn: 0.1 to 2.0% Mn is not only effective as a deoxidizing agent for steel but also useful for hardenability, so Mn is actively added. Inhibits graphitization. 0.1%
Addition of less than 10% has no effect on deoxidation, and addition of more than 2.0% inhibits graphitization. Therefore, Mn is 0.1 to
The range was 2.0%. The preferred range is 0.1 to 1.5% from the viewpoint of promoting graphitization.

N: 0.0015 to 0.0150% N combines with Al and B to form AlN and BN, and serves as a nucleus for crystallization of graphite. Fine dispersion of AlN and BN promotes graphitization and refines graphite particles. However, when the addition is less than 0.0015%, AlN and BN are not sufficiently formed.
If added in excess of 150%, cracking of the slab is promoted during continuous casting, so N was set in the range of 0.0015 to 0.0150%.

B: 0.0003 to 0.0150% B combines with N in steel to form BN, which acts as a nucleus for crystallization of graphite, which promotes graphitization and refines graphite grains. It is an important component in the present invention. In addition, B is an element useful for enhancing the hardenability of steel and ensuring the strength after quenching. Addition of less than 0.0003% has little effect on graphitization and improvement of hardenability. However, when added in an amount exceeding 0.0150%, B forms a solid solution in cementite to stabilize cementite, and conversely inhibits graphitization. Therefore, B is set in the range of 0.0003 to 0.0150%. The preferred range of B is 0.0005 to 0.0100% from the viewpoint of graphitization and hardenability.

Al: 0.005% to 0.1% Al reacts with N in steel to form AlN, which acts as a nucleation site for graphite to promote graphitization, and is therefore positively added. However, the effect is small when added below 0.005%, and when added over 0.1%,
Since many Al-based oxides are formed in the casting process,
Was in the range of 0.005 to 0.1%. The Al-based oxide alone is not only a starting point of fatigue failure but also hard, so that the machinability is deteriorated by abrading the tool during cutting. Therefore, the content of Al is 0.005 to 0.1%
Range.

P: 0.020% or less P is an element that inhibits graphitization, embrittles the ferrite phase, and deteriorates cold forgeability. In addition, during quenching and tempering, segregation at grain boundaries lowers the grain boundary strength, reduces resistance to fatigue crack propagation, and lowers fatigue strength. Therefore, it should be reduced as much as possible, but up to 0.020% is acceptable.

S: not more than 0.035% S forms MnS in the steel, which becomes a starting point of crack generation at the time of cold forging and degrades cold forgeability and fatigue characteristics. Therefore, it should be reduced as much as possible. % Is acceptable. Further, MnS acts as a nucleus for crystallization of graphite and promotes graphitization. From the viewpoint of graphitization, it is better that S is large, but if it is too large, it is coarsened to form coarse graphite. For this reason, S is preferably 0.001 to 0.025%.

O: 0.0030% or less O forms oxide-based nonmetallic inclusions, and reduces both the cold forgeability, the machinability and the fatigue strength. Therefore, it should be reduced as much as possible. You. Although the essential component system in the present invention has been described above, the following elements can be used as needed in the present invention. The reasons for the limitations are described below.

Ni: 3.0% or less, Cu: 3.0% or less, Co: 3.
At least one selected from 0% or less Ni, Cu, and Co are all elements that promote graphitization, and also have the effect of improving hardenability, so they promote graphitization and improve hardenability. It can be improved. However, if the addition amount is less than 0.1%, the effect is small,
Even if added over 3.0%, the effect is saturated.
i, Cu, and Co are each 3.0% or less, preferably 0.1 to 3.0%.
%.

Mo: 1.0% or less Mo enhances hardenability and at the same time distributes less to cementite than alloy elements such as Mn and Cr. For this reason, the hardenability of a steel material can be improved, without significantly inhibiting graphitization. Further, since the steel material to which Mo is added has a large tempering softening resistance, the hardness can be improved at the same tempering temperature, and as a result, the fatigue strength can be improved. In addition, the effect of increasing the hardenability of the steel material is great, and it is easy for the steel material to which Mo is added to have a bainite structure in a hot-rolled state. The bainite structure is advantageous for producing fine graphite, and as a result, the dissolution of graphite during quenching can be completed in a short time. The addition of Mo is used when it is necessary to further improve the fatigue strength. However, the addition of less than 0.1% has a small effect, and the addition of more than 1.0% inhibits graphitization, and reduces the cold forgeability and machinability. Reduce the nature. For this reason, Mo is set to 1.0% or less, preferably in the range of 0.1 to 1.0%. Also, from the viewpoint of cold forgeability and machinability, 0.1 to 0.8
% Is preferred.

V: at least one selected from 0.5% or less and Nb: 0.05% or less V and Nb are both carbide-forming elements and form carbonitrides to increase the strength. However, since it hardly forms a solid solution in cementite, it does not significantly inhibit graphitization. Since both V and Nb are elements that improve hardenability, they may be used when it is necessary to improve fatigue strength. In the case of V, these effects are small when the addition is less than 0.05%, while the effect is saturated when added over 0.5%.
Is added to 0.5% or less, preferably in the range of 0.05 to 0.5%. On the other hand, in the case of Nb, the effect described above is small when the addition is less than 0.005%, and the effect is saturated even when the addition exceeds 0.05%, so the addition of Nb is 0.05% or less, preferably
The range is 0.005 to 0.05%.

Ti: 0.05% or less, Zr: 0.2% or less, REM:
One or more of Ti and Zr selected from 0.2% or less together form carbonitrides, which serve as nuclei for crystallization of graphite and promote graphitization. Since these carbonitrides finely disperse the graphite particles, they may be used when it is necessary to further refine the graphite particles. Further, Ti and Zr form carbonitrides, increase the effective B at the time of quenching, and improve the hardenability. In order to exhibit such an effect, it is desirable to add 0.005% or more of both Ti and Zr. On the other hand, if Ti exceeds 0.05% and Zr exceeds 0.2%, N for forming BN becomes insufficient. As a result, the graphite grains become coarse and the graphitization time becomes extremely long. , Preferably 0.005 to 0.05% and 0.2% or less, preferably 0.005 to 0.2%.

REM combines with S to form (REM) S.
This serves as a nucleus of graphitization, which promotes graphitization and refines graphite particles. Therefore, it may be used when it is necessary to refine graphite particles and promote graphitization. However, if the content is less than 0.0005%, the effect is poor. If the content exceeds 0.2%, the effect is saturated, so the content is 0.2% or less, preferably 0.0005 to 0.2%.
%.

Graphitization rate: 10 to 80% The graphitization rate is defined as: graphitization rate = {(amount of graphite) / (amount of graphite when all contained C are graphitized)} × 100 (%). If the graphitization ratio is less than 10%, the deformation resistance during cold forging increases, and the tool life during cutting significantly decreases. If the graphitization ratio exceeds 80%, the induction hardenability after cutting deteriorates. Therefore, the graphitization rate is set in the range of 10 to 80%.

In the present invention, it is not necessary for all of the C contained to be graphitized, and some of the C is present as cementite or as a solid solution. By leaving cementite, the graphite particles become fine. In the present invention, by leaving some cementite in the steel, the overall hardness is increased, and the surface roughness after cutting is improved.

Further, in the present invention, by leaving cementite in addition to graphite, it is possible to improve the surface hardness and quenching depth after quenching, particularly after induction hardening. The method for producing the steel of the present invention may be an ordinary method and is not particularly limited.
Usually, it is melted in a converter or an electric furnace, and degassing such as RH degassing or refining outside the furnace may be performed as necessary. The molten steel is solidified by a continuous casting method or an ingot making method. Lumps and / or
Alternatively, it is rolled into a steel plate, a steel bar, a wire rod or the like having a predetermined size by hot rolling. After rolling, the product is subjected to a graphitization treatment to obtain a product.
The graphitization is preferably performed in a temperature range of 600 ° C. to _one point Ac in an N ₂ atmosphere or a weak reducing atmosphere containing a small amount of H ₂ or the like.

[0030]

EXAMPLES Steel having the chemical composition shown in Table 1 was converted to a bloom by converter melting and continuous casting, and a steel bar was rolled to a 52 mmφ steel bar.

[0031]

[Table 1]

Steels A to R are steels whose chemical composition is within the range of the present invention, steel S is steel B, steel T is P, steel U is Al and steel V is steel outside the range of the present invention. Further, steel W is a steel equivalent to S30C of JIS standard conventionally used for mechanical structures,
Steel X is an example of a free-cutting steel obtained by adding S, Ca and Pb which are free-cutting property improving elements to S45C equivalent steel. The steel W equivalent to S30C is a steel for cold forging because of its excellent cold forgeability, and the steel X of S45C + S-Ca-Pb free-cutting steel is highly machinable because of its excellent machinability. It is used for applications that require

These steel bars were subjected to a graphitizing annealing treatment at 700 ° C. for up to 100 hours to obtain products. For the product, the measurement of the amount of graphite and the particle size of the graphite, a machinability test, a cold forging test, and an induction hardening test were performed to confirm the performance.
The hardness was measured by Vickers hardness (10 kg load).
The test method is shown below. Measurement of Graphite Content and Graphite Particle Size For a specimen for optical microscope taken from 1/4 d part of a straight bar,
After polishing, the area ratio of graphite was measured at a magnification of × 400 by using an image analyzer with a cross section of 5 places and 10 fields of view at each place without corrosion, and the average value was taken as the amount of graphite. Graphite particle size is 10
The diameter was measured for 00 to 2000 graphite particles, and the average value thereof was used.

Machinability test The machinability test was performed by using a high-speed tool steel SKH4 and cutting the 52 mmφ test piece at a cutting speed of 80 m / min under non-lubricated conditions until the tool became uncuttable. Was evaluated as the tool life. Cold Forging Test The cold forgeability was determined by preparing a cylindrical test piece of 15 mmφ × 22.5 mml from the annealed material, performing a compression test using a 300 t press, and calculating the deformation resistance from the load during the test. Here, the deformation resistance at a height reduction rate (compression rate) of 60% is shown. The number of repetitions was set to 10, and the presence or absence of cracks on the side surface of the test piece was confirmed. The compressibility at which cracks occurred in half of the test pieces after the test was used as an index of deformability as the critical compressibility.

Tensile test of quenched and tempered material A test piece of 15 mmφ × 100 mml was collected from the material, and the specimen was heated at 900 ° C. × 30
min After heating, quench in water-soluble quenching solution, then
A quenching and tempering treatment of cooling at a temperature of 500 ° C. for 1 hour and then cooling with water was performed. From the test piece after the treatment, a tensile test piece having a parallel portion of 8 mmφ x 36 mm l was prepared, a tensile test was performed, and the yield strength (YS),
The tensile strength (TS), elongation (EL), and drawing (RA) were measured.

Induction Hardenability Test In the induction hardenability test, a test piece of 30 mmφ × 100 mml was prepared from a material and subjected to induction hardening under the conditions of a frequency of 15 kHz, an output of 114 kW and a moving speed of the test piece of 10 mm / s.
After tempering at 50 ° C. for 1 hour, the surface hardness (HRC) and the effective hardening depth were measured.

Table 2 shows the results. The conventional steel did not become graphitized. For steel W (S30C equivalent steel)
Slow annealing spheroidizing annealing after holding at 745 ° C x 15 hours, and steel X was evaluated as it is as rolled only for machinability. In other tests, spheroidizing annealing with slow cooling after holding 745 ° C x 15 hours Processing was performed after implementation. In the column for hardness after graphitization in Table 2, N
o.39 For steel (W), the hardness after spheroidizing
For o.40 (steel X), the as-rolled hardness was shown.

[0038]

[Table 2]

[0039]

[Table 3]

From Table 2, when comparing the steel of the present invention with the conventional steel, the deformation resistance and the interfacial compression ratio during cold forging are superior to the conventional cold forged steel, steel W (S30C). Also,
The machinability is also superior to conventional steel X (S45C + S-Ca-Pb free-cutting steel). When the graphitization rate is lower than the range of the present invention (No. 1 and No. 4), the deformation resistance during cold forging is high, and the tool life during cutting is lower than that of the steel of the present invention. Conversely, those having a graphitization ratio higher than the range of the present invention (No. 3, No. 3)
In 8), the surface roughness after cutting is rough, and the properties after quenching and the induction hardening property are lower than those of the present invention. Also, the time required for graphitization is longer than the scope of the present invention. However, the deformation resistance during cold forging and the tool life during cutting are rather superior to those of the present invention, and in applications where surface roughness after cutting and properties after quenching are not required, the graphitization rate It is also possible to use steel with a high hardness.

B is steel S outside the scope of the present invention (No. 35)
As compared with steel B (No. 6) having the same C content, the graphitization treatment time required to reach the same graphitization ratio is about ten times longer. Also, in the case of steel T (No. 36) and steel U (No. 37) in which P and Al are out of the range of the present invention, the annealing time is about 3 to 3 times as compared with steel B (No. 6). It takes four times longer. Further, in steel V (No. 38) in which Si was out of the range of the present invention, no graphite was generated even when the graphitization treatment was performed under the above-described conditions.

Although Ca is not added in the present invention, when fatigue strength is not required, the addition of Ca is effective for promoting graphitization and improving machinability.

[0043]

According to the present invention, not only Pb is used but also the tool life and the surface roughness after cutting are equal to or higher than those of conventional Pb free-cutting steel, and cold forging is performed. It is possible to provide a steel material having excellent properties and properties after quenching, which greatly contributes to the manufacture of machine parts.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location C22C 38/16 C22C 38/16

Claims (6)

[Claims] 1. Mass%, C: 0.1-1.5%, Si: 0.5-2.0%, Mn: 0.1-2.0%, Al: 0.005-0.1%, N: 0.0015-0.0150%, B: 0.0003-0.0150% , P: 0.020% or less, S: 0.035% or less, O: 0.0030% or less, with the balance being Fe and unavoidable impurities, containing C being mainly graphite and cementite, and having a graphitization rate defined below. A steel material for machine structure with excellent machinability, cold forgeability and hardenability characterized by being 10 to 80%. Note Graphitization rate (%) = {(graphite amount) / (graphite amount when all contained C are graphitized)} x 100 2. The method according to claim 1, further comprising at least one selected from mass%, Ni: 3.0% or less, Cu: 3.0% or less, and Co: 3.0% or less. Steel for machine structural use with excellent machinability, cold forgeability and hardenability. 3. The machinability according to claim 1 or 2, further comprising at least one selected from mass: V: 0.5% or less and Nb: 0.05% or less. Steel for machine structural use with excellent cold forgeability and hardenability. 4. In addition to claim 1, 2 or 3, mass%
A steel for machine structural use having excellent machinability, cold forgeability and hardenability, characterized by containing Mo: 1.0% or less. 5. In addition to claim 1, 2, 3 or 4, ma
ss%, Ti: 0.05% or less, Zr: 0.2% or less, REM: 0.2% or less selected from the group consisting of: Excellent steel for machine structural use. 6. A method for manufacturing a machine structural component, comprising: processing a steel material according to any one of claims 1 to 5 into a predetermined shape, followed by induction hardening and tempering to impart a predetermined strength.