JPH0445574B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention is used for crankshafts, gears, pins, etc. of automobile parts, and has free machinability over a wide cutting speed range, and has good properties such as mechanical properties. Concerning the characteristic wide-area free-cutting steel for automobile parts. (Prior art and its problems) In Japan, a large amount of free-cutting steel with added S or Pb is used in automobile parts. Although these steels exhibit excellent machinability in low-speed cutting using high-speed steel tools, sufficient free machinability cannot be obtained in high-speed cutting with cemented carbide tools. Therefore, the industry is progressing with the development of wide-area free-cutting steels that can obtain sufficient machinability under a wide range of cutting conditions using high speed steel and cemented carbide tools, and
Proposals have been made to combine this with deoxidation. (For example, Special Publications 1971-6088, 1984-30845, 1984-
3126, 1984-16445) However, compared to basic steels, these wide-area free-cutting steels suffer from significant deterioration in mechanical properties, especially in toughness, cold workability, and anisotropy of material strength, making them difficult to use. has been restricted. Therefore, the amount of impurities is regulated, for example, the amount of S is set to 0.010% or less and the amount of O is set to 100 ppm or less (Japanese Patent Publication No. 61-10027), and the amount of O is regulated to an even lower value of 25 to 40 ppm. There are, but
(Special Publication No. 61-16337) It is extremely difficult to simultaneously satisfy the demands of customers for machinability over a wide area and material strength. On the other hand, from the perspective of basic steel, vacuum degassing and ladle refining technologies have become widespread, and it has become possible to manufacture high-purity steel (referring to low-P, S, O, Cu, Sn, and Pb materials). However, problems have arisen in which cutting is difficult in these steels, and in conjunction with the need for faster cutting conditions, there is a desire for the emergence of new wide-area free-cutting steels with good mechanical properties for automotive parts. (Object of the Invention) The present invention has been made in order to solve the problems of the prior art described above, and has mechanical properties almost equivalent to general vacuum degassed basic steel, and has particularly good toughness. The purpose is to provide a new wide-area free-cutting steel for automobile parts. (Structure of the Invention) In order to achieve this purpose, the present inventors have developed Ca-S2
High purity of original free-cutting steel (lower P, Cu, Sn)
We investigated the combination of controlling the form and amount of inclusions by lowering oxygen levels and conducted improvement research through melting experiments in a 60-ton basic electric furnace. As a result, we were able to reduce the impurity element P in steel to 0.020% or less. Cu 0.15%
Below, Sn is regulated to 0.020% or less, and Al0.005
In steel with ~0.015% and Ca15~60ppm, O content is equal to or lower than general degassing steel.
If it is 15ppm, toughness will be improved due to high purity,
In addition, oxide inclusions that have a detrimental effect on steel quality are significantly reduced, refined, and incorporated as nuclei in the sulfide inclusions that precipitate in large quantities later. It was found that it is possible to control so that over 90% of the sulfide-based inclusions are completely encapsulated. In addition, some of the MnS forming sulfide inclusions is also CaS.
It has been found that by replacing with , it becomes difficult to deform during hot working, thereby improving mechanical properties and anisotropy of material strength. In addition, the machinability itself is determined by S content of 0.030 to 0.060%, Ca content of 15 to 60 ppm, and Al content of 0.030 to 0.060%.
It was found that when the concentration is 0.005 to 0.015%, the improvement is markedly higher than that of basic steel, and this effect is particularly evident in carbon steel,
The present invention was developed because this phenomenon is noticeable in alloy steel. The first invention steel in this application has C 0.10 to 0.70
%, Si 0.30% or less, Mn 0.30-1.70%, P0.020%
Below, S 0.030-0.060%, Cu 0.15% or less, Sn
0.020% or less, Al 0.005~0.015%, Ca 15~
60ppm, the rest 0.5-15ppm consists of iron and unavoidable impurities, and has a structure in which more than 90% of oxide inclusions are wrapped in sulfide inclusions, improving machinability over a wide cutting speed range. This is a wide-area free-cutting carbon steel for automobile parts that is characterized by its excellent mechanical properties. This steel has S added up to ~0.060%, the amount of impurity elements P, Cu, and Sn is controlled, and the amount of O is set to an extremely low value, so the amount of oxide-based inclusions is significantly reduced, and their size is fine. Therefore, it acts as a nucleus for sulfide-based inclusions that will later precipitate in large quantities. Therefore, 90% of oxide inclusions
The above can be controlled so that they are completely wrapped in sulfide inclusions. In addition, some of the MnS that forms sulfide inclusions is replaced by CaS through the efficient addition of Ca, making it difficult to deform during hot working, so the mechanical properties are different from that of general vacuum decomposition. The level is equivalent to or higher than gasified basic steel. In addition, the amount of Al is regulated to prevent the formation of alumina, and the combined addition of Ca and S provides superior machinability compared to basic steel when cutting both high-speed steel and cemented carbide tools. is obtained. Therefore, it is ideal as a processing material for automobile parts such as crankshafts, gears, and pins. The second invention steel in this application is the first invention steel.
For automotive parts with the addition of one or more alloying elements of Ni 5% or less, Cr 1.50% or less, Mo 0.5% or less to further improve mechanical properties and heat treatment properties such as carburizing, quenching and tempering. It is a wide-area free-cutting alloy steel, and has material properties equal to or better than basic steel, and has superior high speed and cemented carbide tool life than basic steel.
It is ideal as a material for gears in automobile parts. The third invention steel in this application is the second invention steel.
Ti 0.5% or less, Nb 0.1% or less, V 0.1% or less,
This is a wide-range free-cutting alloy steel for automotive parts that has been added with one or more types of Zr 0.1% to improve grain size characteristics and significantly improve mechanical properties and heat treatment properties, and is superior to basic steel. It has the same material strength properties as high speed steel and cemented carbide tool life, making it ideal as a material for gears that require high strength, such as in high-horsepower four-wheel drive vehicles. (Function) Next, the reason why the composition ranges of the first, second and third invention steels were limited as described above will be described below. In this specification, [%] is % by weight unless otherwise specified. C (carbon) Automotive parts such as crankshafts, various gears,
Hardened and tempered or normalized carbon steel, and carburized and carbonitrided alloy steel are used for pins. Since the core strength of carburized steel needs to be at least HRC25 or higher, the lower limit of the C content is set at 0.10%. on the other hand,
The practical upper limit of C content in steel for quenching and tempering is 0.7%.
Therefore, the upper limit of the amount of C in the steel of the present invention is set to 0.7%. Si (Silicon) Si in steel is effective in solid solution strengthening and improving temper softening resistance, improving hardenability and material strength, but it hardens ferrite and impedes machinability, so the upper limit is is set to 0.35%. Mn (manganese) Mn in steel plays a major role in adjusting the hardenability, and is particularly preferred for improving the hardenability of carbon steel because it is inexpensive. Up to 2% is sufficient for adjusting hardenability, and if more is required,
It is preferable to adjust with Cr, Ni, etc. Sideways
The upper limit of Mn is set at 1.7%. On the other hand, if it is less than 0.30%, other elements are required to ensure hardenability, so 0.3% is set as the lower limit. P (phosphorus) P in steel reduces toughness and is a harmful element. P
It is desirable that the amount be as low as possible. For toughness improvement hardening by reducing the amount of P, the amount of S is 0.030 to 0.060%,
If the amounts of Al, Ca, and O are regulated, they become noticeable starting from 0.020%, so the upper limit is set at 0.020%, also considering economic efficiency. S (Sulfur) S in steel mainly exists in the form of MnS-based sulfides, which improves the machinability of steel but reduces toughness. Therefore, the decrease in toughness due to S addition is P,
This is covered by reducing the amounts of Cu and Sn, and
By adding Ca and regulating the amount of Al and O,
Significantly reduces and refines harmful oxide inclusions,
It is incorporated as a nucleus into the sulfide-based inclusions that will precipitate in large quantities later. This results in a structure in which more than 90% of the oxide inclusions in the steel are almost completely wrapped in sulfide inclusions, and due to the addition of Ca, some of the MnS sulfides are replaced with CaS.
Since it is difficult to deform during hot working, mechanical properties are improved, and machinability is further improved at the same time. The amount of S
If it exceeds 0.060%, the above-mentioned improvement in mechanical properties will be reduced, so the upper limit is set at 0.060%. On the other hand, if the amount of S is 0.030% or less, the effect of improving the life of a high-speed steel tool will be reduced, and the effect of the sulfide-based inclusions enveloping the oxide-based inclusions will be reduced, so the lower limit is set to 0.030%. CU (Copper) A harmful element that reduces toughness and hot workability at the same time as Cu and P in steel, and the amount of Cu must be kept as low as possible. However, it is difficult to remove with electric furnace refining, and the only option is to carefully select the melted material. Therefore, considering economic efficiency, the upper limit is set at 0.15%. Sn (Tin) Sn in steel reduces toughness at the same time as P, and is a harmful element. The amount of Sn should be kept as low as possible. It is fully possible to reduce it to 0.020% by vacuum degassing treatment of molten steel and careful selection of melting materials. Considering economic efficiency, the upper limit is set at 0.020%. Al (Aluminum) Al in steel effectively controls oxygen levels and grain size. However, the amount of Al needs to be as low as possible because alumina generated in the steel inhibits the high-speed cutting performance of cemented carbide tools. O amount is 5-15ppm, Ca is 15-15ppm
If the Al amount exceeds 0.015% at 60ppm, the above-mentioned effect of improving high-speed machinability will be reduced, so the upper limit should be
It shall be 0.015%. Furthermore, when the Al amount is 0.005% or less, it becomes difficult to maintain the O amount at 15 ppm or less, so the lower limit is set to 0.005%. Ca (Calcium) Ca in steel mainly exists in the form of CaO or CaS and easily forms complex compounds with other oxides/sulfides. It is known that these composite compounds form a protective layer on the cemented carbide tool surface during high-speed cutting, extending the tool life several times that of conventional steel. This effect has an O content of 5 to 15 ppm and an Al content of 0.005 to 0.015%.
Then, it becomes noticeable from 15ppm, so the lower limit is set to 15ppm.
And so. Moreover, since the above-mentioned effect is exhausted when the amount exceeds 60 ppm, the upper limit is set to 60 ppm. O (Oxygen) O in steel exists in the form of oxide inclusions and is harmful to the mechanical properties of steel, so the value must be kept as low as possible. When Ca is 5 to 15 ppm with Al 0.015%, the amount of O that can be reduced is 5 ppm. Therefore, the lower limit of the amount of O is set at 5 ppm. On the other hand, the amount of O
If it exceeds 15 ppm, the effect of improving mechanical properties will be almost gone, so the upper limit is set at 15 ppm. Ni (nickel) The purpose of adding nickel to steel is to provide the necessary hardenability and improve mechanical properties after quenching and tempering. When hardenability and mechanical properties are required for the steel of the present invention, a large amount of Ni is added. on the other hand,
When the amount of Ni increases, residual austenite becomes excessive and the surface roughness decreases, making it difficult to meet the standard roughness required for automobile parts. Therefore, the upper limit is set at 5%. Cr (Chromium) The purpose of adding Cr to steel is to provide the necessary hardenability and facilitate heat treatment operations such as carburizing. However, if it exceeds 1.50%, double carbides are generated and carburization becomes difficult, so the upper limit is set at 1.50%. Mo (Molybdenum) The purpose of adding Mo to steel is to provide the necessary hardenability and improve mechanical properties. but,
The effect becomes smaller as the amount added increases. Further, since Mo is expensive, combined addition is preferable to single addition. Therefore, the upper limit is set at 0.5%. Ti (Titanium) Ti combines with carbon, nitrogen, etc. in steel, becomes fine precipitates, refines crystal grains, and improves mechanical properties. The upper limit has been set in terms of additive effect and economic efficiency.
Set at 0.5%. Nb (niobium) Like Ti, Nb refines crystal grains and increases strength at room and high temperatures. Since the effect of addition is greater than that of Ti, the upper limit is set at 0.1% in consideration of economic efficiency. V (vanadium) V dissolves into steel and strengthens ferrite, refines crystal grains, suppresses their growth, and improves the properties of the alloying element. The upper limit is set at 0.1% from the viewpoint of additive effect and economic efficiency. Zr (Zirconium) Like Ti and Nb, Zr refines crystal grains and improves mechanical properties. The upper limit is set at 0.1% from the viewpoint of additive effect and economic efficiency. (Examples) Next, the present invention will be further explained by examples. Basic steel types include S53C, S35C, SMn443,
SAE2330, SCr420, SCr440, SAE4063,
SNC631, SAE4620, SCM420, SNCM420,
A total of 12 types of steel similar to SNCM815 were used. The first invention steel tested (No.Al~A5, B1, 2), the second invention steel (A6, A7, A11~A13, A16), and the third invention steel (A8~A10, A14, A15, Table 1 shows the chemical components of a total of 36 heats of A17 to A20) and comparative steels (No. H1 to H14). The first, second and third invented steels of No.Al~A20 are
Smelted in a 60ton basic electric furnace, C, Mn, P, S,
After adjusting alloying elements such as Ni, Cr, Mo, Cu, Sn, and Al, the molten steel from which slag has been almost completely removed is tapped into a ladle lined with high alumina bricks, and then heated for 12 to 15 minutes using the RH method. Perform vacuum degassing treatment (achieved vacuum level of 0.1 torr), then seal the top of the ladle with a lid lined with refractory material.
Approximately 9 mm in diameter is fed through a guide attached to the lid.
−Si alloy wire perpendicular to the bottom of the ladle

【table】

[Table] Calcium was added by pressing into the table. (Addition rate ~
5m/sec) Immediately after adding the wire, nitrogen gas was introduced into the molten steel at a pressure of ~4Kg/mm ² for 2 minutes from a porous plug attached to the bottom of the ladle to forcibly stir the molten steel, and then further nitrogen gas protection was applied. under the atmosphere
It was poured into a 2.6 ton steel ingot. By adding Ca-Si alloy wire and stirring the molten steel with nitrogen gas, the Ca addition yield is 20-50%.
It can be grown. The first invention steels No. B1 to B2 were melted in a 90-ton basic electric furnace and tapped into a ladle refining furnace (LF furnace). Cr, Mo,
After adjusting alloying elements such as Cu, Sn, and Al,
Vacuum degassing treatment is carried out using the minute RH method (achieved vacuum degree of 0.1 torr), then the upper part of the ladle is sealed with a lid lined with refractory material, and a Ca-Si alloy with a diameter of approximately 10 mm is passed through a guide attached to the lid. Ca addition was performed by pushing the wire vertically into the bottom of the ladle. Immediately after adding the wire, apply nitrogen gas through the porous plug at the bottom of the ladle at a pressure of ~6Kg/ ^mm2 .
After introducing the molten steel for a few minutes and forcibly stirring it, it was continuously cast into a bloom (cross-sectional dimensions: 370 x 470 mm). Comparative steels No.H1 to H14 were melted in a 60 ton basic electric furnace and were made of C, Si, Mn, P, S, Ni, Cr, Mo, Cu.
The molten steel prepared with alloying elements such as
Next, vacuum degassing treatment was performed using the RH method for about 15 minutes, and the mixture was poured into a 2.6 ton steel ingot. The obtained steel ingots of the first, second, and third invention steels, blooms, and comparative steel ingots were rolled to a round size of 70 mm, normalized, and then subjected to various investigations. First, we checked the cleanliness of non-metallic inclusions in a 70mm round rolled material.
Measured using the point counting method specified in JIS G0555. In addition, the existence form of oxide inclusions was investigated on the microscopically inspected surface, and the ratio of 20 oxide inclusions combined with sulfide inclusions (wrapped ratio) was compared in percentage terms. The results obtained are shown in the rightmost run of Table 1. First, second and third invention steels (No.Al~A20,
B1, B2) have a smaller amount of oxide inclusions than the comparison steels (No. H1 to H14), and more than 90% of the oxide inclusions are wrapped in sulfide inclusions. I know that there is. Next, Table 2 shows the tensile strength, elongation, reduction of area, impact value, and Ono rotary bending fatigue test results of the first, second, and third invention steels and comparative steel. It can be seen that the first, second and third invention steels have better elongation, reduction of area, impact value, fatigue strength, and fatigue ratio than the comparative steel. In particular, the degree of improvement in fatigue strength is large for medium carbon steel (S53CUS1). The reason why the first, second, and third invented steels exhibit better mechanical properties than comparative steels is not fully clear, but the impurity elements (P, Cu, Sn) in the steels are kept at extremely low values, and This is thought to be because most of the harmful oxide-based inclusions were wrapped up in the sulfide-based inclusions by controlling the amounts of , Al, Ca, and O. Table 3 shows the results of a drilling test, a cutting test using a high-speed steel tool, and a cutting test using a cemented carbide tool. The drilling time and high speed tool life of the first, second and third invention steels are all better than the comparison steel.

【table】

【table】

【table】

[Table] (Note)
(1) Seconds: The number of seconds required to drill a 10mm hole using a drill, SKH51, round 8mm JIS standard type, at 915rpm (23.2m/〓) and a thrust of 70Kg.
(2) ld: An index calculated by taking the reciprocal of the drilling time (seconds) and setting the comparison steel (base steel) as 100.
(3) V ₆₀ : High speed tool SKH4A (6, 6, 5, 5, 15, 15
, 0.4R), 60-minute tool life speed (m/〓) when cutting at feed rate 0.2mm/rev, depth of cut 1.5mm.
(4) V ₆₀ ratio, T ₇₀ ratio: 60-minute tool life speed (V ₆₀ ) and tool life time when cutting at 70 m/〓 using comparative steel (basic steel) HSS and cemented carbide tools
Index calculated with (T ₇₀ ) as 100.
(5) Chips: Chip disposability × Not possible △ Poor ○ Good 3
Evaluated in stages.
(6) V ₆₀ : Carbide tip STi20 (−5, −6, 5, 6, 15
, 15, 0.8R), 60-minute tool life speed (m/
〓: However, the tool life standards are K _T = 0.06mm, V _B = 0.
2mm. )
(7) _T70 : Tool life time when cutting at 70m/〓 (
minutes).
The improvement rate is ~144% compared to the comparative steel. Furthermore, the disposal of chips by the high-speed steel tool is also improved. Furthermore, the high-speed cutting characteristics of the cemented carbide tool are improved, and for example, the tool life at a cutting speed of 70 m/min is ~5 times that of comparative steel. The reason why machinability is improved over a wide range of cutting speeds even when the oxygen content is as low as 5 to 15 ppm is not fully clear, but the regulation of S, Al, and Ca content reduces oxide inclusions in steel. This is thought to be due to the control so that it is wrapped in sulfide inclusions. (Effects of the invention) As detailed above, the first, second, and third invented steels have tensile strength, elongation, area of area, impact value,
Mechanical properties such as rotary bending fatigue strength are as good as or better than conventional steels, and machinability superior to conventional steels can be obtained over a wide cutting speed range. When the wide-area free-cutting steels of the first, second, and third inventions are used in automobile parts such as crankshafts, gears, and pins, excellent machinability can be achieved over a wide cutting speed range required by customers. It simultaneously satisfies good mechanical properties and can significantly reduce processing costs.

Claims (1)

[Claims] 1 C 0.10-0.70% Si 0.35% or less Mn 0.30-1.70% P 0.020% or less S 0.030-0.060% Cu 0.15% or less Sn 0.020% or less Al 0.005-0.015% Ca 15-60ppm O 5- 15ppm The remainder consists of iron and unavoidable impurities, and the structure has a structure in which more than 90% of the oxide inclusions are wrapped in sulfide inclusions, which improves machinability over a wide cutting speed range and improves mechanical properties. A wide-area free-cutting carbon steel for automobile parts that is characterized by good properties. 2 C 0.10-0.70% Si 0.35% or less Mn 0.30-1.70% P 0.020% or less S 0.030-0.060% Cu 0.15% or less Sn 0.020% or less Al 0.005-0.015% Ca 15-60ppm O 5-15ppm Furthermore, Ni 5% or less Contains one or more of the following: Cr 1.50% or less Mo 0.5% or less, the remainder consists of iron and unavoidable impurities, and has a structure in which 90% or more of oxide inclusions are wrapped in sulfide inclusions. A wide-range free-cutting alloy steel for automotive parts that is characterized by improved machinability and good mechanical properties in the cutting speed range of . 3 C 0.10-0.70% Si 0.35% or less Mn 0.30-1.70% P 0.020% or less S 0.030-0.060% Cu 0.15% or less Sn 0.020% or less Al 0.005-0.015% Ca 15-60ppm O 5-15ppm Furthermore, Ni 5% or less Contains one or more of the following: Cr 1.50% or less Mo 0.5% or less, Ti 0.5% or less Nb 0.1% or less V 0.1% or less Zr 0.1% or less, the remainder consists of iron and inevitable impurities, with oxide-based inclusions Wide-area free cutting for automotive parts, characterized by improved machinability over a wide cutting speed range and good mechanical properties, with a structure in which more than 90% of the material is wrapped in sulfide-based inclusions. Alloy steel.