JP2011184716A - Martensitic stainless free-cutting steel bar wire having excellent forgeability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a martensitic stainless free-cutting steel which has excellent forgeability by the control of inclusions without using heavy metals (Pb, Bi, Se, Te) exerting adverse influence on the environment, thus to improve the yield of component working which has been performed only by cutting heretofore by the combination with forging. <P>SOLUTION: The martensitic stainless free-cutting steel wire rod having excellent forgeability is composed of a steel having a composition containing, by mass, ≤0.40% C, 0.1 to 2.0% Si, 0.1 to 3.0% Mn, ≤0.05% P, 0.01 to 0.20% S, 10.5 to 20.0% Cr, ≤0.250% N, 0.002 to 0.010% Al, 0.001 to 0.010% Ca, 0.001 to 0.025% O, and the balance Fe with inevitable impurities, and also, satisfying the conditions of, by mass ratio, 0.25≤Ca/Al≤2.50 and 0.10≤Ca/O≤0.30 to form composite inclusions between CaO-SiO<SB>2</SB>-Al<SB>2</SB>O<SB>3</SB>based oxides having a low softening point and (Mn, Cr)S sulfides. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a martensitic stainless steel excellent in forgeability and machinability. For example, the manufacturing of precision parts having complicated shapes that have been manufactured only by cutting until now can be performed by forging. The present invention relates to a martensitic stainless steel that can be processed with a high material yield by performing net shaping and cutting as finishing.

Since martensitic stainless steel has high strength by heat treatment (quenching) after parts processing, it is used in fields where strength is required. In general, various equipment parts such as screws and bolts are generally manufactured by being formed by forging. This forging processing method has the advantages of high efficiency and yield, but is inferior in precise processing accuracy. On the other hand, all precision parts having complicated shapes are manufactured by cutting. The cutting method allows processing into complex shapes and has the advantage of satisfying very precise dimensional accuracy, but has the disadvantage of poor material yield because it is processed from a material with a large wire diameter. There is.
Conventionally, SUS410 (11% Cr-0.1% C) is used for parts manufactured by forging, and SUS 416 (12% Cr-0.1% C-0.2%) is used for parts manufactured by cutting. S) and the like have been used.

SUS410, which has excellent forgeability, has a contradictory characteristic that machinability is poor, and SUS416, which has excellent machinability, has poor forgeability. Therefore, parts having a complicated shape are usually manufactured by cutting using steel having high machinability even if the material yield is low.
Therefore, in order to manufacture parts having complicated shapes with high yield and productivity, martensitic stainless steels having excellent cold workability and machinability as described in Patent Documents 1 and 2 below. As described in Patent Document 3 below, as steel used for parts such as cylinders and pistons used in high pressure such as fuel pump systems for automobiles, cold work, warm forgeability In addition, steel having good machinability and excellent machinability, and having a surface hardness of HV630 or higher by induction hardening has been proposed.

Patent Document 1 describes steel characterized by having both cold forgeability and machinability by adding S and Bi in a composite manner and controlling the amount thereof. However, since Bi is a rare element, there is a problem that it is difficult to supply inexpensively and stably.
The invention described in Patent Document 2 is a martensitic free-cutting steel stainless steel that does not contain harmful elements such as Pb and has excellent corrosion resistance, cold forgeability, and machinability. It is characterized by adding Sn to improve machinability and controlling O, Al, Ca, etc. to further improve machinability. The effect of the control and, as a result, on the cold forgeability has not been studied.

The invention described in Patent Document 3 is a steel used for parts such as cylinders and pistons used in high pressure such as fuel pump systems for automobiles, and has excellent cold work and warm forgeability and excellent machining. Have been proposed, and steel having a surface hardness of HV630 or higher by induction hardening has been proposed, and the composition and size of sulfides are also specified. However, it does not describe how to control the form and composition of the sulfide, and as a result, it does not describe how it affects cold working.

As described above, until now, without using heavy metals such as Pb, without increasing the cost due to the balance of components of Ca, Al, O, the form of inclusions is controlled, and both forgeability and machinability are achieved. No martensitic stainless free-cutting steel provided in a well-balanced manner has been proposed.

JP 2004-256875 A JP 2002-212680 A JP 2000-282185 A

The present invention provides a martensitic stainless free-cutting steel excellent in forgeability by controlling inclusions without using heavy metals (Pb, Bi, Se, Te) that adversely affect the environment. It is an object of the present invention to improve the yield of part processing that has been performed only by cutting by combining with forging.

The present invention has been made to solve the above-mentioned problems. By controlling the composition of oxide inclusions, the composite inclusions of oxide and sulfide are formed and finely dispersed. As a result of the improvement, the inventors have found that by controlling the amounts and ratios of trace amounts of Al, Ca, and O, machinability can be ensured without degrading forgeability.
That is, the gist of the present invention is the following contents as described in the claims.
(1)% by mass, C ≦ 0.40%, Si: 0.1 to 2.0%, Mn: 0.1 to 3.0%, P ≦ 0.05%, S: 0.01% to 0.20%, Cr: 10.5 to 20.0%, N ≦ 0.250%, Al: 0.002 to 0.010%, Ca: 0.001 to 0.010%, O: 0.001 % To 0.025%, balance Fe and inevitable impurities steel, and satisfy the conditions of 0.25 ≦ Ca / Al ≦ 2.50 and 0.10 ≦ Ca / O ≦ 0.30 by mass ratio A martensite system excellent in forgeability characterized by forming a composite inclusion of a CaO—SiO ₂ —Al ₂ O ₃ type oxide having a low softening point and a sulfide of (Mn, Cr) S Stainless steel free-cutting steel wire.
(2) The martensite stainless steel free-cutting steel wire excellent in forgeability according to (1), wherein the steel contains mass% and B ≦ 0.010%.
(3) The martensitic stainless free-cutting steel having excellent forgeability as described in (1) or (2), wherein the steel contains, by mass%, Cu: 1.0 to 4.0% wire.
(4) The martensitic stainless free-cutting excellent in forgeability according to any one of (1) to (3), wherein the steel contains, by mass%, Sn ≦ 0.50%. Steel wire rod.
(5) The steel contains, by mass%, Mo ≦ 1.50%. The martensitic stainless free-cutting excellent in forgeability according to any one of (1) to (4), Steel wire rod.

The martensitic stainless free-cutting steel with excellent forgeability according to the present invention makes it possible to efficiently process parts by forging and cutting, and is industrially useful, for example, by demonstrating the cost reduction of part processing. There is a remarkable effect.

It is a figure which shows the relationship between the amount of S of the steel which performed inclusion control, and the steel which is not performed, and forgeability (limit upsetting rate). It is a figure which shows the relationship between the value of Ca / Al ratio and Ca / O ratio, and forgeability (limit upsetting rate).

Below, the limitation reason as described in (1) of this invention is demonstrated first.
C ≦ 0.40%
When C is contained in a large amount, the corrosion resistance, forgeability, and tool wear resistance deteriorate, so the upper limit was made 0.40%. Preferably it is 0.150% or less.

Si: 0.1 to 2.0%
Si acts as a deoxidizer and is an element effective for improving oxidation resistance, so it is contained in an amount of 0.1% or more. However, inclusion of more than necessary deteriorates forgeability and tool wear resistance. The upper limit was 0%. Preferably it is 0.1 to 0.4%.

Mn: 0.1 to 3.0%
Since Mn has the effect of improving machinability as MnS, it is contained in an amount of 0.1% or more. However, excessive addition reduces the corrosion resistance and toughness, so the upper limit was made 3.0%. Preferably it is 1.0 to 2.5%.

P ≦ 0.05%
When P content is large, the hot workability is lowered, so 0.05% was made the upper limit. Preferably it is 0.04% or less.

S: 0.01-0.20%
Since S is an element that improves machinability, it is contained in an amount of 0.01% or more. However, if it is contained in a large amount, inclusions centering on sulfides become coarse and forgeability deteriorates. Therefore, a small amount of Al and Ca are added (controlled) to form a composite inclusion of a low melting point oxide and sulfide, and in order to combine machinability and forgeability, the range is set to 0.01 to 0.20%. Preferably, it is 0.01% to 0.05%.

In FIG. 1, the amount of Al, Ca, and O is changed so as to be a composite inclusion of a low melting point oxide and sulfide by changing the amount of S in a component system of 12% Cr-0.1% C-0.01% N. The influence of the amount of S on the limit upsetting rate of the steel of the present invention and the comparative steel controlled is shown. The method for measuring the limit upsetting rate will be described in the section of Examples. In general, it is known that if the limit upsetting ratio is 70% or more, good workability and productivity are exhibited in forging such as header processing. It can be seen from FIG. 1 that the forgeability is reduced by increasing the amount of S, which is an element improving machinability. Moreover, when inclusion control was performed on the 0.20% S amount material, a limit upsetting rate of 70% was shown. However, the material with an S amount exceeding 0.20% did not reach a limit upsetting rate of 70% or more even when inclusion control was performed. From this test result, it is necessary to limit the amount of S in order to keep the forgeability high. Therefore, the upper limit of S is set to 0.20%.

Cr: 10.5 to 20.0%
Cr is an element that dissolves in the matrix and improves corrosion resistance. If it is 10.5% or less, the corrosion resistance deteriorates, and if it is too much, scale formation is suppressed hot, causing hot rolling defects, so the upper limit was made 20.0%. Preferably it is 11.0%-15.0%.

N ≦ 0.250%
Excessive addition of N hardens the material by solid solution strengthening, and deteriorates forgeability and tool life. Therefore, the upper limit was made 0.250%. Preferably it is 0.010%-0.030%.

Al: 0.002 to 0.010%
Al acts as a deoxidizer and generates a CaO—SiO ₂ —Al ₂ O ₃ -based oxide having a low softening point, which will be described later, to form a composite inclusion with sulfide, thereby finely dispersing sulfide. Although it is an important element, when it is contained in a large amount, it exists as a hard coarse non-metal oxide, so that forgeability is lowered. Therefore, the range is made 0.002% to 0.010%. Preferably it is 0.002 to 0.008%.

Ca: 0.001 to 0.010%
Ca is an important element for producing a CaO—SiO ₂ —Al ₂ O ₃ oxide having a low softening point. Low-melting CaO—SiO ₂ —Al ₂ O ₃ -based oxides described later are produced by delicate control of deoxidizing elements such as Al and Si and O, and the sulfides are finely formed as composite inclusions with sulfides. Disperse. In order to obtain these effects, addition of at least 0.001% is necessary. However, if contained in a large amount, these effects cannot be obtained, and the manufacturability also decreases, so the upper limit was made 0.010%. Preferably it is 0.001 to 0.007%.

O: 0.001% to 0.025%
O, like Al and Ca, becomes a CaO—SiO ₂ —Al ₂ O ₃ -based oxide and is finely dispersed as a composite inclusion with sulfide, so the inclusion of O is essential. If the content is less than 0.001%, the effect is small, and if it exceeds 0.025%, hard Cr ₂ O ₃ increases and the forgeability and machinability deteriorate, so the range is 0.001% or more and 0.025%. % Or less. Preferably it is 0.005-0.020%.

0.25 ≦ Ca / Al ≦ 2.50 in mass ratio
In order to produce a CaO—SiO ₂ —Al ₂ O ₃ -based oxide and make it a sulfide and a composite inclusion of (Mn, Cr) S, it is necessary to control the Ca / Al ratio. When the Ca / Al ratio is less than 0.25, the amount of CaO is reduced, there are many SiO ₂ -Al ₂ O ₃ -based oxides, and it becomes difficult to form composite inclusions, and there are many hard oxides Therefore, forgeability and machinability deteriorate. In addition, when the Ca / Al ratio exceeds 2.50, a large amount of 2CaO · SiO ₂ is generated and it becomes difficult to become a composite inclusion, and in addition to a reduction in forgeability improvement due to refinement of the composite inclusion, Since the nozzle melts at the time of casting and a problem occurs in manufacturability, 0.25 ≦ Ca / Al ≦ 2.5 is set in mass ratio.

0.10 ≦ Ca / O ≦ 0.30 by weight ratio
For Ca / O, it is important to control the Ca / O ratio in order to produce CaO—SiO ₂ —Al ₂ O ₃ -based oxides and to make (Mn, Cr) S sulfide and composite inclusions. It is. When the Ca / O ratio is less than 0.10, SiO ₂ —MnO—Cr ₂ O ₃ -based oxides increase, and CaO—SiO ₂ —Al ₂ O ₃ -based oxides decrease, resulting in complex interposition. It becomes difficult to produce things and forgeability deteriorates. On the other hand, if it exceeds 0.30, the amount of MnO decreases, it becomes difficult to form composite inclusions, the forgeability deteriorates, and the nozzle melts during casting, causing problems in manufacturability. 10 ≦ Ca / O ≦ 0.30.

CaO—SiO ₂ —Al ₂ O ₃ oxide By controlling trace amounts of Al, O, and Ca, it acts as an inoculation nucleus for (Mn, Cr) S sulfide, and as a composite inclusion, sulfide is added. It has been found that a low melting point CaO—SiO ₂ —Al ₂ O ₃ -based oxide that is finely dispersed is obtained, and as a result, forgeability and machinability are improved.

FIG. 2 shows the relationship between Ca / Al and Ca / O values and forgeability (limit upsetting ratio) of 12% Cr-0.1% C-0.01% N-0.1% S steel. The forgeability varies depending on the values of Ca / Al and Ca / O, and the limit setting is 70% or more at the values of 0.25 ≦ Ca / Al ≦ 2.50 and 0.10 ≦ Ca / O ≦ 0.30. The rate of inclusion is shown.

The reason for limitation described in (2) of the present invention will be described.
B ≦ 0.010%
B is an element added to improve hot workability and softening, and a stable effect can be obtained by adding 0.002% or more. However, if added in excess, the B compound precipitates and deteriorates hot workability, so the upper limit was made 0.010%. Preferably it is 0.002 to 0.004%.

The reason for limitation described in (3) of the present invention will be described.
Cu: 1.0-4.0%
Cu is an important element for improving forgeability. In order to obtain forgeability, at least 1.0% or more is necessary. However, if the content exceeds 4.0%, the hot workability deteriorates, so the upper limit was made 4.0%. Preferably it is 2.0 to 4.0%.

The reason for limitation described in (4) of the present invention will be described.
Sn ≦ 0.50%
Sn is segregated at the grain boundaries and is effective in reducing cutting resistance due to material ductility reduction and lubricity effects during cutting, and improving machinability (surface roughness, chip disposal, tool life) as well as corrosion resistance. Coexisting with sulfides that degrade the corrosion resistance of the corrosion resistance. If the addition exceeds 0.50%, the effect is saturated, and the upper limit is made 0.50% in order to deteriorate manufacturability. Preferably, it is 0.03 to 0.30%.

The reason for limitation described in (5) of the present invention will be described.
Mo ≦ 1.50%
Mo is an element effective for improving the corrosion resistance like Cr, and a stable effect can be obtained by addition of 0.1% or more. However, since the hot workability deteriorates when added in a large amount, the upper limit was made 1.50%. Preferably it is 0.10 to 1.40%.

Examples of the present invention will be described below.
Tables 1 and 2 show the evaluation results of chemical components, forgeability, and machinability (chip treatability and tool wear resistance) of the examples of the present invention.

これら化学成分の鋼は１００ｋｇ真空溶解炉にてφ１８０ｍｍ角の鋳片に鋳込み、その後、２１ｍｍφへ熱間圧延を行い棒鋼にした。その後８５０℃で焼鈍を行い、ピーリング加工およびセンタレスかこうにより２０ｍｍφの棒鋼に仕上げた。その後、鍛造性、被削性（切屑処理性、耐工具磨耗性）を調査した。
鍛造性は圧縮試験によって得られる限界据込率によって評価した。圧縮試験は直径１１ｍｍの線材から高さ１２ｍｍ、直径８ｍｍの円柱状の試験片を作製して供試材とし、同心円状の溝をもつ拘束型ダイスでの圧縮試験により評価した。試験片の初期の高さをＨ₀、割れが発生した圧縮後の高さをＨとし以下の式で求めた値を限界据込率とした。
（1−Ｈ／Ｈ₀）×１００（％）

These chemical components were cast into φ180 mm square slabs in a 100 kg vacuum melting furnace, and then hot rolled to 21 mmφ to form bar steel. Thereafter, annealing was performed at 850 ° C., and finished into a 20 mmφ bar steel by peeling and centerless biting. Thereafter, the forgeability and machinability (chip disposal, tool wear resistance) were investigated.
Forgeability was evaluated by the limit upsetting rate obtained by the compression test. In the compression test, a cylindrical test piece having a height of 12 mm and a diameter of 8 mm was prepared from a wire material having a diameter of 11 mm as a test material, and evaluation was performed by a compression test with a constraining die having concentric grooves. The initial height of the test piece was H ₀ , the height after compression at which cracking occurred was H, and the value obtained by the following formula was defined as the limit upsetting rate.
_{(1-H / H 0)} × 100 (%)

The test specimen was compressed at a constant speed of 10 / s with a compression tester, the presence or absence of cracks on the side face of the specimen was visually determined, and the size was evaluated based on the limit upsetting rate.
Invention Steel No. 1 to 24 had a limit upsetting rate of 70% or more.

The cutting test uses a steel bar machined to a diameter of 20 mm as a test material, and uses a carbide tool (P20 type), cutting speed (50 to 200 m / min), cutting (0.1 to 1.0 mm), feed rate ( The outer periphery was cut at 0.01 to 0.1 mm / rev). The machinability was evaluated based on chip disposal and tool wear resistance.

In the evaluation of the chip disposability, the short-damaged and regular spiral-shaped ones were marked with ◯, and the ones that were irregularly connected for a long time were marked with ×. This is because chips that discharge short and broken chips are less likely to wrinkle the surface during cutting, but chips that are irregularly connected for a long time are wrinkled on the surface or entangled with the tool. The steel of the present invention was observed to be short and broken and regular spiral chips.

Tool wear resistance was evaluated by observing the tool when cutting about 4000 m. The case where there was no tool wear was rated as ◯, and the case where large tool wear was observed locally was marked as x. Some of the steels of the present invention have a small amount of tool wear observed, but no significant tool wear was observed.
Manufacturability was evaluated by the presence or absence of surface defects after hot rolling. No surface flaws were observed for the inventive steel.

Tensile strength was tested using JIS No. 9A (G.L100 mm) and the tensile strength was measured.
On the other hand, Comparative Example No. 25-45 were inferior in any one of forgeability, machinability (chip disposal property, tool wear resistance) productivity, and cost compared with this invention.
As can be seen from the embodiments, the advantages of the present invention are clear.

As is clear from the above examples, the present invention can provide martensitic stainless free-cutting steel with excellent forgeability according to the present invention. It is extremely useful for manufacturing with good material yield and productivity by cutting.

Claims (5)

% By mass
C ≦ 0.40%,
Si: 0.1 to 2.0%,
Mn 0.1-3.0%,
P ≦ 0.05%,
S: 0.01-0.20%
Cr: 10.5-20.0%,
N ≦ 0.250%,
Al: 0.002 to 0.010%,
Ca: 0.001 to 0.010%,
O: Steel composed of 0.001% to 0.025%, balance Fe and inevitable impurities, and 0.25 ≦ Ca / Al ≦ 2.50 and 0.10 ≦ Ca / O ≦ 0.30 by mass ratio Forgeability characterized by forming a composite inclusion of a CaO—SiO ₂ —Al ₂ O ₃ oxide having a low softening point and a sulfide of (Mn, Cr) S by satisfying the condition of Martensite stainless steel free-cutting steel wire with excellent resistance.   The martensitic stainless free-cutting steel wire rod having excellent forgeability according to claim 1, wherein the steel contains, by mass%, B ≦ 0.010%.   The martensitic stainless steel free-cutting steel wire excellent in forgeability according to claim 1 or 2, wherein the steel contains Cu: 1.0 to 4.0% by mass.   The martensitic stainless steel free-cutting steel wire excellent in forgeability according to any one of claims 1 to 3, wherein the steel contains, by mass%, Sn? 0.50%. 5. The martensitic stainless free-cutting steel wire excellent in forgeability according to claim 1, wherein the steel contains, by mass%, Mo ≦ 1.50%.

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