JPH01168848A - Wide-area free-cutting steel for auto parts - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention has free machinability in a wide cutting speed range for use in crankshafts, gears, bottles, etc. of automobile parts, and
Furthermore, the present invention relates to a wide-area free-cutting steel that is characterized by good properties such as mechanical properties, and a method for producing the same.

(Prior art and its problems) In Japan, a large amount of free-cutting steel to which S or Pb is added 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, in the industry, proposals have been made to develop 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 to combine S addition with Ca deoxidation. ing. (For example, Special Publick 51-608
8. Showa 51-30845. Showa 54-3126゜Showa 54-
(Refer to 16445) However, compared to basic steels, these wide-area free-cutting steels suffer from significant deterioration in mechanical properties, particularly in toughness, cold workability, and anisotropy of striking force, and their use has been limited. . Therefore, the amount of impurities is regulated, for example, the amount of S is 0.010% or less and the amount of O is 100 ppm or less (Special Publication Publication No. 6l-10027).
Although it is being regulated to 5 to 40 ppm, (
(Japanese Patent Publication No. 6l-16337) It is extremely difficult to simultaneously satisfy the wide-area machinability and driving force required by customers. On the other hand, from the basic steel side, vacuum degassing and ladle refining technologies have become widespread, and high-purity steel (low P, S, 0.
Cu, Sn, and Pb materials) have become possible to manufacture, but problems have arisen in which cutting is difficult with these steels, and combined with the need for faster cutting conditions, it is difficult to produce materials with good mechanical properties. It is hoped that a new wide-area free-cutting steel will emerge.

(Objective of the Invention) The present invention has been made 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.

(Structure of the Invention) In order to achieve the above object, the present inventors have controlled the form and amount of inclusions by increasing the purity (lower P, Cu, and Sn) and lowering the oxygen content of Ca-52 element free-cutting steel. As a result of repeated improvement research through melting experiments in a 60 ton basic electric furnace, we found that the impurity elements in steel, P, should be 0.020% or less, Cu should be 0.15% or less, and Sn should be 0.020% or less. 0.
Regulated to 020% or less, and further Alo, 005 to 0.0
15%, Ca 15-60 ppm O
The amount is equal to or less than that of general degassing steel, 5 to 15
If it is ppa+, toughness will be improved due to high purity, and oxide inclusions that have a harmful effect on steel quality will be significantly reduced, and they will become finer and less likely to be present in the sulfide inclusions that precipitate in large quantities later. It has been found that it is possible to control so that 90% or more of the oxide-based inclusions are completely engulfed in the sulfide-based inclusions because they are incorporated as nuclei. In addition, some of the MnS that forms sulfide inclusions is replaced with CaS, making it difficult to deform during hot working, improving mechanical properties and anisotropy of striking force. I discovered that. In addition, the machinability itself is 0.030~0.060% S, Ca1i: 15~60pp
It has been found that if m, Al is set to 0.005 to 0.015%, it is significantly improved over basic steel, and this effect is particularly noticeable in carbon steel and alloy steel, which led to the invention.

The first invention in this application is C0.10-0.70%, S
i 0.35% or less, Mn 0.30-1.70%, P
0.020% or less, S 0.030-0.060%, C
u 0.15% or less, Sn 0.020% or less, Al
0.005-0.015%, Ca 15-60ppm,
05~15ρ4 The remainder consists of iron and unavoidable impurities, and the machinability is improved in a wide cutting speed range, with Mlll in which more than 90% of the oxide inclusions are wrapped in sulfide inclusions. It is a wide-area free-cutting carbon steel characterized by good mechanical properties. This steel has an S of ~0.060
%, regulating the amount of impurity elements such as P, Cu, and Sn, and making 0 amount an extremely low value, the amount of oxide-based inclusions is significantly reduced, and their size is finer, so they can be added in large quantities later. Acts as a nucleus for precipitated sulfide inclusions,
Therefore, it is possible to control so that 90% or more of the oxide inclusions are completely wrapped in the sulfide inclusions. In addition, some of the MnS forming sulfide inclusions is Ca
By the efficient addition of CaS, it is replaced by CaS, making it difficult to deform during hot working, so that the mechanical properties are at a level equal to or higher than that of general vacuum degassed base steel. In addition, the amount of Al can be regulated to prevent the formation of alumina, and the combined addition of Ca and S can be
Because of its improved properties, superior machinability can be obtained when cutting both high-speed steel and cemented carbide tools compared to basic steel. Therefore, it is ideal as a processing material for automobile parts such as crankshafts, gears, and pins.

The second invention of the present application adds alloying elements of 5% or less Ni, 1.50% or less Cr, and 0.5% or less Mo to the steel of the first invention, and further contains 0.50% or less Ti, and 0.5% or less Nb.
By containing one or more grain refining elements of 0.1% or less, Vo, 1% or less, and Zr 0.1% or less, mechanical properties and heat treatment properties such as carburizing, quenching and tempering are further improved. It is a wide-area free-cutting alloy steel designed to improve the quality and quality of the product.
It is ideal as a material for gears in automobile parts because it has striking force properties that are equal to or better than basic steel, and has superior high speed and cemented carbide tool life than basic steel.

The third invention in the present application is produced by melting in a basic electric furnace, C, Mn, P, S, Ni, Cr, Mo, Cu, Sn,
Ca-5 alloy wire is added to the molten steel which has been RH degassed by adjusting the alloying elements such as Al to almost the composition range of the first invention steel or the second invention steel of the present invention, and then an inert gas is added. The method for manufacturing steel according to the first or second aspect of the invention is characterized in that the molten steel is forcibly stirred.

The reason why the manufacturing method of the present invention is limited to the combination of smelting in a basic electric furnace and R) (degassing) is that high purity (low P
, Cu, Sn) can be sufficiently deoxidized by circulating the molten steel in the degassing container.

Combinations of other smelting furnaces and degassing methods (e.g. converter →
If a combination of LF furnace, acidic electric furnace, ladle degassing, DH degassing, etc.) is adopted, the Al amount will be 0.005 to 0.0.
In a range of 15%, 0 amount is an extremely low value of 5 to 15 pp+a,
Moreover, it is extremely difficult to precipitate fine oxide inclusions.

In addition, adding Ca-5 alloy wire after RH degassing and then introducing inert gas to forcibly stir the molten steel is different from other addition methods (for example, adding Ca-5 alloy wire in the ladle). This is because Ca addition efficiency (yield) is poor when adding alloy ingots by charging, forcing addition, etc., and furthermore, re-oxidation of the molten steel (increase in oxygen content) due to the molten metal surface blowing up during addition is unavoidable.

When adding Ca-5+ alloy wire while pushing it vertically toward the bottom of the ladle (addition speed ~5m/a+in)
There is almost no movement of the molten steel surface, and almost no reoxidation of the molten steel occurs. Furthermore, by introducing nitrogen gas for several minutes through the porous plug at the bottom of the ladle, it is possible to prevent air oxidation and to homogenize the Ca additive components. Capable of flotation separation.

By doing this, the amount of Ca is set to 15 to 60 ppm within the range of A1 amount o, oos to 0.015%, and oii is set to 5 to 15 ppm.
Since the oxide inclusions can be reduced to an extremely low ppm value, the oxide inclusions are significantly reduced, become fine, and incorporated into the sulfide inclusions as nuclei.
Replaced by S. Therefore, 90% or more of the oxide inclusions can be completely wrapped in the sulfide inclusions.

According to such a manufacturing method, steel for automobile parts having good mechanical properties and excellent wide-area free machinability can be obtained.

(Function) Next, the reason for limiting the composition range of the steel of the present invention as described above will be described below. In this specification, [%] is % by weight unless otherwise specified.

C (Carbon) Quenched and tempered or normalized carbon steel and carburized and carbonitrided alloy steel are used for automobile parts such as crankshafts, various gears, and pins.

Since the core strength of carburized steel requires at least HRC25 or more, the lower limit of the C content is set to 0.10%. On the other hand, since the practical upper limit of the C content of copper for quenching and tempering is 0.7%, the upper limit of the C content of the steel of the present invention is set to 0.70%.

Si (Silicon) Si in steel is effective in solid solution strengthening and improving resistance to temper softening, and improves hardenability and driving force, but since it hardens ferrite and impedes machinability, the upper limit is 0.35
%.

Mn, (Manganese) Manganese 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 to adjust the hardenability, and if more is required, it is preferable to adjust with C or Ni. Therefore, the upper limit of Mn is set at 1.7%. When the content is less than 30%, other elements are required to ensure hardenability, so the lower limit is set at 0.3%.

P (phosphorus) P in steel reduces toughness and is a harmful element. It is desirable that Pffi be as low as possible. The effect of improving toughness by reducing the amount of P is when st is 0.030 to 0.060%, and Al, C
a. When regulating the amount of 0, it becomes noticeable from 020%, so
Taking economic efficiency into consideration, the upper limit is set at 0.020%.

S (sulfur) S in steel mainly exists in the form of MnS-based sulfides.

Improves machinability of steel but reduces toughness. Therefore, by compensating for the decrease in toughness due to the addition of S by reducing the amounts of P, Cu, and Sn, and by controlling the amount of A1.0 by adding Ca, harmful oxide inclusions are significantly reduced and refined. and incorporated as nuclei into sulfide-based inclusions that 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 the sulfide inclusions,
Furthermore, by adding Ca, a part of the MnS-based sulfide is replaced with CaS, which makes it difficult to deform during hot working, thereby improving mechanical properties and, at the same time, further improving machinability. S amount is 0.
If it exceeds 0.060%, the effect of improving the mechanical properties described above will be reduced, so the upper limit is set at 0.060%.

On the other hand, if 5j1 is less than 0.030%, the effect of improving the life of a high-speed steel tool will be reduced, and the effect of the sulfide-based inclusions enclosing the oxide-based inclusions will be reduced, so the lower limit should be set at 0.030%. shall be.

Cu (Copper) Similar to P, Cu in steel reduces toughness and improves hot workability.
It is a harmful element that reduces the amount of Cu, so the amount of Cu needs to be 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) Like P, Sn in steel reduces toughness and is a harmful element. The amount of Sn should be as low as possible.

0.0 by vacuum degassing treatment of molten steel and careful selection of melted materials
A reduction of up to 20% is quite possible. Taking economic efficiency into consideration, the upper limit is set at 0.020%.

Al (Aluminum) Al in steel effectively controls oxygen levels and grain size. However, the alumina that forms in the steel inhibits the high-speed cutting performance of cemented carbide tools.
The amount of Al needs to be as low as possible. 0 amount is 5~
Al amount is 0.0 at 15ppm, Ca15-60ppm
If it exceeds 15%, the above-mentioned effect of improving high-speed machinability becomes small, so the upper limit is set at 0.15%.

Furthermore, if Alff1 becomes 0.005% or less, it becomes difficult to maintain the zero content 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 composite 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 changes from 0 to 5 to 15 ppm,
When the amount of Al is 0.005 to 0.015%, it is 15 ppm
The lower limit was set at 15 ppm. 60 again
If the amount exceeds ppm, the above-mentioned effect will be saturated, so the upper limit should be set at 6.
Set to 0ppo+.

0 (Oxygen) O in steel exists in the form of oxide inclusions and is harmful to the mechanical properties of steel, so it is necessary to keep the value as low as possible. A
l When Ca is 5 to 15 ppm at 0.015%,
The OIk that can be reduced is 5 ppm. Therefore, the lower limit of the amount of 0 is set at 5 ppm. On the other hand, if Ojl exceeds 15 ppm, the effect of improving mechanical properties is almost lost, so the upper limit is set at 15 ppm.

The purpose of adding nickel to steel is to give it the necessary hardenability.
This is to improve mechanical properties after quenching and tempering. When hardenability and mechanical properties are required for the steel of the present invention, N
Add a large amount of i. 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 C to steel is to provide necessary hardenability and facilitate heat treatment 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 necessary hardenability and improve mechanical properties. However, the effect becomes smaller as the amount added increases. Also, M.

Since these are 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, forms fine precipitates, refines crystal grains, and improves mechanical properties. The upper limit is set at 0.5% from the viewpoint of addition effect and economical efficiency.

Like Ti, Nb refines crystal grains and increases strength at room temperature and high temperature. Since the effect of addition is greater than that of Ti, the upper limit is set at 0.1% in consideration of economic efficiency.

■(Vanadium) ■ dissolves into the steel and strengthens the ferrite, refines the 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 addition effect and economy.

Zr (Zirconium) Like Ti and Nb, Zr refines crystal grains and improves mechanical properties. The upper limit was set at 0.1 in terms of additive effect and economic efficiency.
%.

(Examples) Next, the present invention will be further explained by examples. Basic steel types include 553C, 535C, SMn443, 5AE2
330,5Cr420.5Cr440,5AE4063
, 5NC631, 5AE4620. SCM420゜SN
CM420. Twelve steel types of SNCM815 were adopted. Tested invention 11 (No, A] to A20, 8l-82)
Table 1 shows the chemical components of a total of 36 heats of steel and comparison steel (NO0 old to H14).

No., Al-A20 steel of the present invention is melted in a 60 ton basic electric furnace, and contains C, Mn, P, S, Ni, Cr, Mo,
After adjusting alloying elements such as Cu, Sn, and Al, the molten steel from which the scrap has been completely removed is poured into a ladle lined with high alumina bricks, and then subjected to R for 12 to 15 minutes.
Vacuum degassing treatment using the H method is performed to achieve an ultimate vacuum of 0.
1 torr), then the top of the ladle was sealed with a lid lined with a refractory material, and Ca-5l alloy wire with a diameter of about 911 IITl was pushed vertically into the bottom of the ladle through a guide attached to the lid to perform Ca addition. (Addition rate ~
Immediately after adding the wire, nitrogen gas was introduced into the molten steel at a pressure of ~4 kg/mm2 for 2 minutes from a porous plug attached to the bottom of the ladle to forcibly stir the molten steel, and then the molten steel was further stirred in a nitrogen gas protective atmosphere. It was poured into a 2.6 ton steel ingot at the bottom.

By adding the Ca-5L alloy wire and stirring the molten steel using nitrogen gas, the Ca addition yield can be increased by 20 to 50%.

The steels of the present invention No. 81 to B2 were melted in a 90 ton basic electric furnace, and after being tapped into a ladle refining furnace (LF furnace), C, Mn, P, S, Ni, Cr, Mo,C
After adjusting alloy elements such as u, Sn, and Al, vacuum degassing treatment is performed using the RH method for about 12 minutes (at an ultimate vacuum of 0).
．． 1 torr), then the top of the ladle was sealed with a lid lined with a refractory material, and a Ca-Si alloy wire with a diameter of about 10 mm was pushed vertically into the bottom of the ladle through a guide attached to the lid to add Ca. .

Immediately after the wire addition is completed, nitrogen gas is introduced from the porous plug at the bottom of the ladle at a pressure of ~6 kg/2 for about 2 minutes to forcibly stir the molten steel, and then bloom (cross-sectional size 37
0x470mm) was continuously cast.

Comparison steel of No. Hl-814 was melted in a 60 ton basic electric furnace and contained C, Si, Mn, P, S, old, Cr, Mo.
The molten steel, in which alloying elements such as , Cu, and the like were adjusted, was tapped into a ladle, then subjected to vacuum degassing treatment using an RH method for about 15 minutes, and poured into a 2.6 ton steel ingot.

The obtained steel ingots of the present invention, blooms, and comparative steel ingots were rolled to a round size of 70 mm, subjected to normalizing treatment, and then subjected to various investigations.

First, the cleanliness of non-metallic inclusions of a round 70mm + rolled material was determined by J
It was measured by the point counting method specified in IS GO555. In addition, the existence form of oxide-based inclusions was investigated on the micro-tested surface, and the ratio of compounding (wrapping ratio) of 20 oxide-based inclusions with sulfide-based inclusions was compared in percentage terms. The results obtained are shown in the rightmost run of Table 1.

The invention steels (No. A1 to A20, 81.B2) are the comparative steels (No.

Compared to Hl-814), the amount of oxide inclusions is small,
Furthermore, it can be seen that more than 90% of the oxide inclusions are wrapped in sulfide inclusions.

Next, Table 2 shows the tensile strength, elongation, scratch value, impact value, and Ono rotary bending fatigue test results of the steel of the present invention and the comparative steel.

It can be seen that the steel of the present invention has better elongation, fringe value, impact value, fatigue strength, and fatigue ratio than the comparative steel.

In particular, the degree of improvement in fatigue strength is large in medium carbon steel (553CUS1).

The reason why the inventive steel exhibits better mechanical properties than the comparative steel is not fully clear, but impurity elements (P, Cu,
This is thought to be because most of the harmful oxide-based inclusions were wrapped in the sulfide-based inclusions by setting the Sn) to an extremely low value and controlling the amounts of Al, Ca, and 0.

Table 3 shows the drilling test, the cutting test using a high-speed steel tool,
and cutting test results using cemented carbide tools are shown.

The drilling time and high speed tool life of the steel of the present invention are both better than the comparative steel, and the improvement rate reaches ~144% of 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 also improved, and for example, the tool life time at a cutting speed of 70 m/win reaches up to 5 times that of comparative steel. Oxygen amount is 5-15p
The reason why machinability is improved over a wide cutting speed range even at low pm values is not fully clear, but S, Al, Caf
This is thought to be due to the control of oxide inclusions in steel so that they are wrapped in sulfide inclusions due to the regulation of e.

(Effects of the invention) As detailed above, the steel of the present invention has high tensile strength, elongation,
Mechanical properties such as reduction of area, impact value, and rotational 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.

The wide-area free-cutting steel of the present invention can be used for crankshafts of automobile parts,
When used in gears, pins, etc., it simultaneously satisfies the demands of customers for excellent machinability over a wide range of cutting speeds and good mechanical properties, significantly reducing machining costs.

Claims (3)

[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% Ca15~60ppm O5~15ppm The remainder consists of iron and unavoidable impurities, and the structure has a structure in which more than 90% of oxide inclusions are wrapped in sulfide inclusions, over a wide cutting speed range. A wide-area free-cutting carbon steel characterized by improved machinability and good mechanical 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 O5-15ppm Ni 5% or less Cr 1.50% or less Mo 0.5% or less Furthermore, Ti 0.5% or less, Nb 0.1% or less, V0.
Contains one or more of 1% or less and 0.1% or less of Zr, the remainder consists of iron and unavoidable impurities, and 90% or more of the oxide inclusions are wrapped in sulfide inclusions. A wide-range free-cutting alloy steel characterized by improved machinability over a wide cutting speed range and good mechanical properties. (3) Smelted in a basic electric furnace, C, Mn, P, S,
Ca-Si alloy wire is added to molten steel which has been RH-degassed by adjusting alloying elements such as Ni, CrMo, Cu, Sn, and Al to approximately the composition range of the first invention steel or second invention steel of the present invention. The method for producing wide-area free-cutting steel according to claim 1 or 2, characterized in that the molten steel is forcibly stirred by introducing an inert gas.