JP2003013173A - Steel for machine structure superior in cold forgeability, and manufacuring method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for machine structure with high cleanliness and remarkably superior in cold forgeability. SOLUTION: The steel for machine structure superior in cold forgeability is characterized by including, by wt.%, 0.08-0.61% C, 0.03-0.35% Si, 0.30-1.0% Mn, 0.02-0.06% Al, 0.002-0.2% N, and the balance Fe with unavoidable impurities, and precipitating AlN in the steel so as to be 0.04-0.050%. The steel may further include one or more of Cr<=1.00%, Mo<=0.20%, and Ni<=2.00%. The manufacturing method is characterized by forcibly introducing a shell wire containing either fluoroborate of alkali metal or fluoroborate of alkaline earth metal, into a molten steel in a melting stage; and further by subjecting the material to softening heat treatment before cold forging, through holding it at a temperature of A3 point of A3 point + 50 deg.C, in a barrel-type furnace, water cooling it, and holding it in the temperature range of 650-740 deg.C for 5 hours to 10 hours, to make the hardness to be Hv 120 or lower.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for machine structure used for industrial machines, automobile parts and the like, and provides a steel material having particularly excellent cold forgeability.

[0002]

2. Description of the Related Art In the production of steel materials, the material yield is good,
Since the dimensional accuracy is good and the material cost and machining cost can be reduced, cold forging is often used.

On the other hand, the weak point of cold forging is that if the hardness of the steel material is too high, the ductility is insufficient, or the work hardening is too high, defective products such as cracks and defective shapes occur and the forging die life is long. This leads to shortening of the cost and, conversely, a cost increase.
Therefore, the steel material is softened by performing spheroidizing annealing treatment in a heat treatment furnace after rolling. However, since the spheroidizing annealing is an inefficient work that requires a long time, there is a method of improving the cooling method after hot rolling to shorten or omit the spheroidizing annealing.
No. 09236 and Japanese Patent Application Laid-Open No. 2001-11575. However, these methods do not have sufficient softness of hardness.

Japanese Unexamined Patent Publication (Kokai) No. 10-112193 discloses a method in which Si, which is strongly related to the chemical composition of steel, particularly work hardness, is reduced and Al is increased to 0.1 to 0.3%. Further, Japanese Patent No. 3154036 discloses a method in which Ti is contained, the upper limit of the content of B is set, and variations in hardenability are suppressed.

However, these methods may deviate from the components of the mechanical structural steel defined by JIS, and the mechanical properties may change due to the refining, which limits the applications.

In addition to the above, as a method for improving the deformability of cold forging steel, many methods have been filed for a method of graphitizing cementite by graphitizing heat treatment to reduce the hardness to Hv120 or less. The weak point of these methods is that, since Si is an element that has a small bonding force with carbon atoms in the steel and promotes graphitization in order to facilitate the graphitization of cementite, the standard of Si of JIS structural steel is 0. This is a point that the content is much more than 0.5% to 2.0%, far exceeding 15% to 0.35%. Further, it was also a weak point that graphite was not easily solution-treated during tempering after graphitization, and desired mechanical properties and hardness could not be obtained.

[0007]

In order to improve cold forgeability, spheroidizing annealing is carried out to reduce the hardness of steel and spheroidize cementite, but this heat treatment requires a long time, which is inefficient. Therefore, we aim to shorten this time. Further, if the Si content in the steel is reduced in order to reduce the deformation resistance, on the other hand, deoxidation failure tends to occur.
Deoxidation with Al, which has the advantage of not forming carbides, produces the harmful deoxidation product Al ₂ O ₃ . Al ₂ O ₃ needs to be removed because the increase of inclusions leads to a decrease in deformability. It is also necessary to reduce the solute N in steel as a factor that hinders the deformability as much as possible. The present invention seeks to advantageously solve these problems.

[0008]

The present invention has the following constitution. (1) C: 0.08 to 0.61% by weight, Si:
0.03-0.35%, Mn: 0.30-1.0%, A
1: 0.02-0.06%, N: 0.002-0.02
%, The balance Fe and the composition of unavoidable impurities, and 0.03 to 0.05% of AlN was precipitated in the steel, which is excellent in cold forgeability.

(2) In the above (1), further weight%
, Cr ≦ 1.00%, Mo ≦ 0.20%, Ni ≦ 2.
A steel for machine structural use which is excellent in cold forgeability and is characterized by containing one or more of 00%.

(3) C: 0.08 to 0.61 by weight%
%, Si: 0.03 to 0.35%, Mn: 0.30
1.0%, Al: 0.02-0.06%, N: 0.00
2 to 0.02%, or Cr ≦ 1.0
1 of 0%, Mo ≦ 0.20%, Ni ≦ 2.00%
Or two or more kinds are contained, and AlN in the steel is 0.03 to
In the production of steel in which 0.05% is precipitated, it is necessary to forcibly introduce a steel skin wire containing at least one of alkali metal borofluoride and alkaline earth metal borofluoride into molten steel at the molten steel stage. A method for manufacturing a steel for machine structural use, which is characterized by excellent cold forgeability.

(4) The method for producing a steel for machine structural use having excellent cold forgeability according to the above (3), characterized in that Zr is added in an amount necessary to fix an excessive amount of N.

(5) The softening heat treatment before subjecting the steel material being manufactured to cold forging is maintained at A3 point to A3 point + 50 ° C using a barrel furnace, followed by water cooling, and then in a temperature range of 650 ° C to 740 ° C. The method for producing a steel for machine structural use having excellent cold forgeability according to (3) or (4), characterized in that the hardness is maintained at Hv120 or less for 5 to 10 hours.

That is, the present inventor added Al in order to obtain desired mechanical properties and hardness without increasing the amount of Si, which is an element that promotes graphitization, and the Al was removed. Specified in the steel, considering that it is possible to prevent Al ₂ O ₃ as an acid product from forming and reduce the deformability, while reducing the solute N in steel as a factor that inhibits the deformability as much as possible. It has been found that the ferrite grains become extremely fine when a certain amount of AlN is contained.

Therefore, in order to make the amount of N in the steel sufficiently fixed by Al and precipitating as AlN at room temperature, the amounts of Al and N are added / reduced at the molten steel stage. The proper range of AlN that precipitates at room temperature is 0.04
It is 0.03% to 0.05% centered on 5%.

When there is an excessive amount of N which is not fixed by Al, a small amount of Zr is added to the molten steel and fixed as ZrN (see claim 4). However, it is desirable that finally Zr does not remain in the steel.

By the above, ferrite crystal grain number 10
~ 12 steels could be obtained. This is the invention of claim 1.

According to a second aspect of the present invention, the basic composition of the steel according to the first aspect is extended to SCr, SCM and SNCM structural alloy steels according to the JIS standard.

A third aspect of the present invention is a method for producing the steel according to the first and second aspects. That, Al ₂ as described above, but the deformation resistance Si content in order to reduce is reduced, although solved by using the deoxidizing failure problems caused to the Al as a protagonist of deacidification, by-produced during the O ₃ has a harmful effect on the cold forgeability and therefore must be removed as much as possible.

Therefore, the iron-clad wire containing the alkali metal borofluoride and the alkaline earth metal borofluoride is forcibly supplied into the molten steel covered with the slag by using an electric wire feeder. As a result, the melting points of Al ₂ O ₃ and ZrN existing in the molten steel are lowered, and they are aggregated and float-separated.

The iron-clad wire is what is called a cored wire or composite wire in the welding industry. When an iron hoop having a thickness of 0.1 to 0.2 mm is formed into a hollow wire by a roll forming method, a powdery boron wire is used. It is manufactured by encapsulating a fluorinated compound.

The borofluoride compound is included in an amount of 100 g per 1 m of the wire having an outer diameter of 9 mm. The iron skin wire is forcibly introduced into the molten steel covered with the slag at a speed of 50 m to 200 m per minute by an electric feeder.

The iron shell of the wire allows the inclusions to be introduced into the molten steel without contacting the slag, transporting the inclusions to the molten steel at the bottom of the ladle and increasing the chances of contact with the molten steel. , Cause an efficient chemical reaction.

This work is carried out just before the molten steel is poured into the mold to separate the floating inclusions of harmful inclusions. In addition, as a secondary effect of supplying the iron-clad wire, the inorganic salt to be encapsulated contains B, which is reduced by Al in the steel, and the effective B for the hardenability is 0.0002. % To be contained in the steel and help improve the hardenability of the steel.

The invention of claim 5 defines the heat treatment to be performed before the cold forging of the steel material produced by the invention of claim 3 and hot-rolled thereafter.

The rolled steel material is heat-treated using a non-stress continuous heat treatment furnace generally called a barrel furnace. The barrel furnace is a furnace that can send out a heating atmosphere at a certain speed while rotating a steel material by a screw roller, and by adjusting the speed, a continuous cooling curve of steel can be freely selected.

In order to completely precipitate AlN, the temperature is maintained at A3 point to A3 point + 50 ° C. for about 1 hour, and then water cooling is performed to change the steel structure to bainite + partial martensite. Then, it is held at 650 ° C. to 740 ° C. for 5 hours to 10 hours in a heat treatment furnace to partially graphitize cementite and anneal it to form fine-grained ferrite.

As a result, a steel material having excellent cold forgeability is obtained. Hereinafter, the reasons for limiting the component composition of the steel material in the present invention will be described.

C: 0.08% to 0.61% S10, which is a carbon steel for machine structure defined by JIS
Limited to the carbon range of C to S58C.

Si: 0.03% to 0.35% The reduction of Si is preferably 0.15% or less in order to reduce the deformation resistance, but the upper limit is set to 0.35% according to the JIS standard.

Mn: 0.30% to 1.0% Mn should be reduced as much as possible in order to reduce deformation resistance, but it was set to the above range in consideration of deterioration of hardenability.

Al: 0.02% to 0.06% Most preferable is 0.04%, but since it is the main role of the deoxidizing element, if it is less than 0.02%, it is accompanied by reduction of Si and brohol. Cause the occurrence of. If it exceeds 0.06%, the hardenability of steel is adversely affected.

N: 0.002% to 0.02% Al added as a deoxidizer is 0.045% in the form of AlN.
By making it exist within a fixed amount range centered on
The ferrite grains become extremely fine grains, and the steel becomes extremely excellent in cold forgeability. The smaller the amount of solid solution N that does not combine with Al, the better. It is preferably 0.006% or less. In addition, Zr is used to fix excess N in the steel.
It is desirable that r is an amount that does not finally exist in the steel.

Cr: 1% or less Mo: 0.20% or less Ni: 2.00% or less Cr, Mo and Ni are all steels of the present invention for cold forging of JIS standard SCr, SCM and SNCM steels. In order to be applied, any one kind or two kinds or more is added within the range of the upper limit. Also, P ≦ 0.03%, S ≦ 0.03%
May be included.

[0034]

BEST MODE FOR CARRYING OUT THE INVENTION The steel of the present invention can be easily manufactured by ordinary steelmaking and rolling operations, and will be specifically described below with reference to Examples and Comparative Examples. Table 1 shows the chemical composition of the present invention steel and the comparative steel.

[0035]

[Table 1]

Both the inventive steel and the comparative steel were melted by a double slag method in a 15TON electric furnace. Decarburization is carried out on average 0.3% by oxygen gas blowing, slag is removed from the oxidative phase, and slag is newly produced by adding lime.
The use of i and SiMn was restricted as much as possible, and Al was used as the main deoxidizer.

After tapping the steel in a ladle, 5 kg of Fe-Zr was charged and vacuum degassing was performed for 15 minutes at a vacuum degree of 1 Torr or less.

After that, the ladle was set at the position of the ladle refining furnace, the molten steel temperature was adjusted by arc heat, and when the molten steel temperature reached 1650 ° C., the iron-clad wire (iron-thickness 0.2 mm, outer diameter 9 mm) was fed to the molten steel in the ladle using an electric wire feeder. Although 100 g of sodium borofluoride was included per 1 m of the wire, this wire was fed at a speed of 100 m / min for 1.5 minutes. By this work, trace amounts of Al ₂ O ₃ and ZrN floating in the molten steel were captured and floated, and the cleanliness of the steel was enhanced. Then, molten steel was poured into the steel ingot mold, and after solidification, it was rolled into a round bar having a diameter of 50 mm by a rolling mill.

Then, a round bar having a diameter of 50 mm was charged into a barrel furnace, heated to 780 to 830 ° C. and held for 1 hour, and
It was cooled to 0 ° C with water. A round bar of 550 to 600 ° C. is charged into an annealing furnace and kept in a temperature range of 650 to 740 ° C. for 5 hours,
Annealing treatment was performed. A sample to be subjected to a cold forging test was cut out from the annealed round bar.

The cold forgeability was evaluated by the hardness after annealing, the deformation resistance in the compression test, and the crack generation rate. The compression test sample has a diameter of 8 mm and a height of 12 mm,
Deformation resistance and upsetting rate of 80% at upsetting rate of 60% based on end face restraint compression test of Japan Plastic Working Society Cold Forging Subcommittee standard
The crack generation rate was measured.

The cleanliness was measured to examine the non-metallic inclusions in these samples. The measuring method is JIS G
According to 0555, sulfide and silicate inclusions are A type,
Alumina inclusions were counted as B type, and granular oxides and carbonitrides were counted as C type.

The number of visual fields for measurement is 60, the magnification is 400 times, and the area ratio of inclusions is dA60 × 400, B + C for A system.
The system is described in the column dB + C60 × 400. The results of these tests are shown in Table-2.

[0043]

[Table 2]

As is apparent from Table 2, the hardness of the invention steel after annealing is lower than that of the comparative steel of the same kind, and the deformation resistance is 10 as well.
% Or more smaller. Inventive steel also has a low incidence of compression work cracking.

By the sodium borofluoride treatment of molten steel,
The cleanliness of the invention steels is higher than that of the comparative steels, and has a favorable effect on the compression cracking occurrence rate.

[0046]

Industrial Applicability According to the present invention, it is possible to provide a steel for machine structural use which is extremely excellent in cold forgeability and has a high degree of cleanliness, and is extremely useful when applied to industrial machinery, automobile parts and the like. .

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI Theme Coat (Reference) C22C 33/04 C22C 33/04 J 38/06 38/06 38/50 38/50

Claims (5)

[Claims] 1. C: 0.08 to 0.61% by weight,
Si: 0.03 to 0.35%, Mn: 0.30 to 1.0
%, Al: 0.02-0.06%, N: 0.002-
A steel for machine structural use, which contains 0.02% and is composed of the balance Fe and unavoidable impurities, and has 0.03 to 0.05% AlN precipitated in the steel, which is excellent in cold forgeability. . 2. The method according to claim 1, further comprising C by weight%.
r ≦ 1.00%, Mo ≦ 0.20%, Ni ≦ 2.00%
A steel for machine structural use which is excellent in cold forgeability and is characterized by containing one or more of the above. 3. C: 0.08 to 0.61% by weight,
Si: 0.03 to 0.35%, Mn: 0.30 to 1.0
%, Al: 0.02-0.06%, N: 0.002-
Contains 0.02%, or further Cr ≦ 1.00
%, Mo ≦ 0.20%, Ni ≦ 2.00%, and one or more kinds of AlN in the steel.
In the production of steel in which 0.05% is precipitated, it is necessary to forcibly introduce a steel skin wire containing at least one of alkali metal borofluoride and alkaline earth metal borofluoride into molten steel at the molten steel stage. A method for manufacturing a steel for machine structural use, which is characterized by excellent cold forgeability. 4. The method for producing a steel for machine structural use having excellent cold forgeability according to claim 3, wherein Zr is added in an amount necessary to fix an excessive amount of N. 5. The softening heat treatment before subjecting the steel material being manufactured to cold forging is performed using a barrel furnace at A3 point to A3 point + 5.
After being kept at 0 ° C. and water-cooled, it is kept at a temperature range of 650 ° C. to 740 ° C. for 5 hours to 10 hours to have a hardness of Hv120 or less, which is excellent in cold forgeability according to claim 3 or 4. Method for manufacturing machine structural steel.