JP5583986B2 - Austenitic stainless free-cutting steel rod with excellent forgeability - Google Patents

Description

The present invention relates to an austenitic stainless steel excellent in forgeability and machinability. For example, the production of precision parts having a complicated shape that has been produced only by cutting until now can be carried out by forging. The present invention relates to an austenitic stainless steel that can be processed with a high material yield by performing shaping and cutting as finishing.

Austenitic stainless steel is widely used in various fields because it has excellent properties such as workability and corrosion resistance. 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, SUS304 (18% Cr-8% Ni) and SUSXM7 (17% Cr-9% Ni-3% Cu-low C, N-based) are manufactured by cutting for parts manufactured by forging. SUS303 (18% Cr-9% Ni-0.3% S) or the like has been used for parts.
SUS304 and SUSXM7, which have excellent forgeability, have the contradictory characteristics that machinability is poor, and SUS303, 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, as described in Patent Documents 1 and 2 below, austenitic stainless steel with improved cold workability and machinability. In addition, as described in Patent Documents 3 and 4 below, steels with improved machinability by changing the form of inclusions by deoxidation have also been proposed.

In Patent Document 1, the machinability is imparted by adding heavy metal Pb to Cu-based austenitic stainless steel having excellent workability and controlling the amount of oxide by adding Ca, and hot workability by adding Pb. The steel is characterized by improving the decrease in the content by addition of B. However, since Pb is toxic, its use is being limited, and nothing has been studied about the composition of Ca-based oxide inclusions described later.

In the invention described in Patent Document 2, in order to improve machinability without deteriorating the corrosion resistance, Ca, which is considered to have a small decrease in corrosion resistance, is added, and Al ₂ O ₃ which is a hard inclusion is added. It is found that machinability is improved by making the inclusions in the steel a composite of CaO · SiO ₂ and MnO · SiO ₂ and that these inclusions are cold. It has been found that it has a great effect in preventing seizure by providing a lubrication effect between the material with the die during the drawing process and rolling process. However, the influence of these inclusions on the forging process has not been studied.

In the invention described in Patent Document 3, in order to realize the machinability improvement of stainless steel without lowering the corrosion resistance, the oxide inclusions are made to be galenite (2CaO) by regulating the amount of Ca, Al, and O. -Al ₂ O ₃ · SiO ₂ ) system, which is characterized by improved machinability. However, since there is no addition of S, there is a problem in machinability, and further machinability improvement has been required to perform cutting.

The invention described in Patent Document 4 controls the amount of Al at a low concentration and in a narrow range, and also controls the amount of Ca, thereby generating a low melting point oxide containing Al, Ca, etc., and machinability. It is characterized by improving. However, since the amount of Al is a very low value, there has been a problem that manufacturing costs such as selection of raw materials and manufacturing time are increased.

  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. An austenitic stainless free-cutting steel imparted in a well-balanced manner has not been proposed.

JP 63-18039 A JP 2004-256900 A JP-A-6-145908 JP 2001-234298 A

The present invention provides an austenitic 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 to improve the yield of component processing that has been performed only by processing by combination 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 machinability can be ensured by controlling the amounts and ratios of trace amounts of Al, Ca, and O without degrading forgeability.
That is, the gist of the present invention is the following contents as described in the claims.
(1) By mass%, C ≦ 0.150%, Si: 0.1 to 2.0%, Mn 0.1 to 3.0%, P ≦ 0.05%, S: 0.01 to 0.15 %, Ni: 6.0-25.0%, Cr: 14.0-26.0%, N ≦ 0.250%, Al: 0.002-0.010%, Ca: 0.001-0. 010%, O: 0.001% to 0.025%, Cu: 1.0 to 4.0%, steel composed of the balance Fe and inevitable impurities, and 0.25 ≦ Ca / Al ≦ 2 in mass ratio .50 and 0.10 ≦ Ca / O ≦ 0.30, the CaO—SiO ₂ —Al ₂ O ₃ -based oxide having a low softening point and the sulfide of (Mn, Cr) S An austenitic stainless free-cutting steel wire excellent in forgeability characterized by forming a composite inclusion.
(2) The austenitic stainless free-cutting steel wire excellent in forgeability according to (1), wherein the steel contains mass% and B ≦ 0.010%.
( 3 ) The austenitic stainless free-cutting steel wire excellent in forgeability as described in (1) or (2) , wherein the steel contains, by mass%, Mo ≦ 1.50%.

The austenitic stainless free-cutting steel excellent in forgeability according to the present invention enables efficient machining of parts by forging and cutting, and is industrially useful, for example, by showing the effect of reducing the cost of parts machining. 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 ≦ 0.150%
C is an austenite phase stabilizing element, but if contained in a large amount, corrosion resistance, forgeability, and tool wear resistance deteriorate, so the upper limit was made 0.150%. Preferably it is 0.050% or less, More preferably, it is 0.030% 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 to 0.15%
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 a sulfide, and in order to combine machinability and forgeability, the range is 0.15%. Preferably, it is 0.01% to 0.12%.

  In FIG. 1, the amount of S is changed in a component system of 18% Cr-8% Ni-0.030% C-0.020% N, and Al, Ca so as to be a composite inclusion of low melting point oxide and sulfide. The influence of the S amount on the limit upsetting rate of the steel of the present invention and the comparative steel in which the O amount is 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 0.14% S amount material, a limit upsetting rate of 70% was shown. However, the material with an S amount of 0.15% or more 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 0.15%.

Ni: 6.0 to 25.0%
Ni is an element that improves corrosion resistance and improves forgeability. Therefore, the lower limit was made 6.0%. Ni is an expensive element. Excessive addition hardens the material by solid solution strengthening, and causes deterioration of forgeability and tool life. Therefore, the upper limit was made 25.0%. From the viewpoint of forgeability, it is more preferably 8.0% to 11.0%.

Cr: 14.0 to 26.0%
Cr is an element that dissolves in the matrix and improves corrosion resistance. If the amount is less than 14.0%, the corrosion resistance is deteriorated. If the amount is too large, the austenite phase becomes unstable and the formation of hot scale is suppressed, causing hot rolling defects. Therefore, the upper limit is set to 26.0%. . Preferably, it is 17.0% to 19.0%.

N ≦ 0.250%
N is an element effective for stabilizing the austenite phase. However, excessive addition hardens the material by solid solution strengthening and degrades 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 it is less than 0.001%, the effect is small, and if it exceeds 0.025%, hard Cr2O3 increases and forgeability and machinability deteriorate, so the range is 0.001% or more and 0.025% or less. did. 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, and a large amount of SiO ₂ -Al ₂ O ₃ -based oxides are present, making it 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, the mass ratio is set to 0.25 ≦ Ca / Al ≦ 2.5.

0.10 ≦ Ca / O ≦ 0.30 in mass 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 the Ca / Al and Ca / O values of 18% Cr-8% Ni-0.03% C-0.02% N-0.1% S steel and the forgeability (limit upsetting rate). Indicates. 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 austenite phase stable element and is an important element for improving forgeability. Addition of at least 1.0% or more is necessary to obtain a more preferable forgeability to be described later, but if the content exceeds 4.0%, the hot workability deteriorates, so the upper limit is 4.0%. did. Preferably it is 2.0 to 4.0%.

The reason for limitation described in (4) 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%.
Further, as a preferred form of the present invention, in order to further improve forgeability, S: 0.05% or less, C: 0.030% or less, N: 0.030% or less are preferable, and at that time When the ratio of the number of inclusions such as sulfides and oxides having a major axis of less than 2 μm between the surface layer and a depth of 1 mm in the cross section of the test piece is 80% or more, forgeability of 80% or more is exhibited. Since stress concentrates at the depth of 1 mm from the surface layer at the time of forging, inclusions are likely to be the starting point of destruction, and therefore the size and the ratio of inclusions are important. Since the forgeability deteriorates when the ratio of the number of inclusions less than 2 μm decreases, the ratio of the inclusions less than 2 μm needs to be 80% or more in order to ensure forgeability of 80% or more.

As a manufacturing method of these steels, the minimum temperature of the steel surface in the entire hot rolling process from rough rolling to finish rolling is set to 1000 to 1200 ° C., and the area reduction rate at that time is set to 98.0% or more. It was found that inclusions finely dispersed by controlling Al, Ca, and O were more finely dispersed, and the forgeability was further improved. When the rolling minimum temperature is less than 1000 ° C., it becomes lower than the softening temperature of the Ca-based oxide, and the finely dispersed inclusions are more difficult to break, and further improvement in forgeability cannot be expected. The forgeability is not satisfied. On the other hand, when the temperature is higher than 1200 ° C., embrittlement due to grain boundary melting and embrittlement due to complete solid solution / reprecipitation of harmful elements tend to occur, resulting in a problem in manufacturability. If the area reduction is less than 98.0%, it becomes difficult for the inclusions in the steel to be broken by the processing, so that the forgeability in the preferred embodiment of the present invention is not satisfied without further improvement in forgeability.

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、最低鋼材表面温度を９９０〜１２１０℃、減面率を９７．８〜９９．８％で熱間圧延を行い、１０５０℃（オーステナイト系ステンレス鋼線で組織を安定化させる温度）で溶体化処理を施し、棒鋼サンプルを製造した。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。

These chemical components are cast into φ180 mm square slabs in a 100 kg vacuum melting furnace, and then hot rolled at a minimum steel surface temperature of 990 to 1210 ° C. and a reduction in area of 97.8 to 99.8%. A solution treatment was performed at 1050 ° C. (temperature at which the structure was stabilized with an austenitic stainless steel wire) to produce a steel bar sample. 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 26 had a limit upsetting rate of 70% or more.
The cutting test uses a steel bar hot forged to a diameter of 22 mm as a test material, using a carbide tool (P20 type), cutting speed (50 to 200 m / min), cutting (0.1 to 1.0 mm), Peripheral cutting was performed at a feed rate (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.

The size of inclusions such as sulfides and oxides and the number thereof were measured by embedding and mirror-polishing the cross section of the test piece. One pixel was 0.125 μm from the surface layer to a depth of 1 mm with an optical microscope. Using a observable camera, 250 field observations were performed at 400 times, and the lengths parallel to the rolling direction were measured for the confirmed inclusions such as sulfides and oxides, and the ratio of the number was obtained and evaluated. The minimum steel surface temperature and the area reduction rate are controlled, Ca / Al and Ca / O are within the range, and the S content is 0.05% or less, the ratio of inclusions of 2 μm or less is 80% or more. there were.

Tensile strength was tested using JIS No. 9A (G.L100 mm) and the tensile strength was measured.
On the other hand, Comparative Example No. 27 to 46 were inferior in any of forgeability, machinability (chip disposability, tool wear resistance) manufacturability and cost as compared with the present 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 austenitic stainless free-cutting steel with excellent forgeability by the present invention, and forging and cutting parts that have so far been manufactured with complicated shapes only by cutting. By processing, it is extremely useful for producing materials with high yield and productivity.

Claims (3)

% By mass
C ≦ 0.150%,
Si: 0.1 to 2.0%,
Mn 0.1-3.0%,
P ≦ 0.05%,
S: 0.01 to 0.15%,
Ni: 6.0 to 25.0%,
Cr: 14.0 to 26.0%,
N ≦ 0.250%,
Al: 0.002 to 0.010%,
Ca: 0.001 to 0.010%,
O: 0.001% to 0.025%,
Cu: 1.0 to 4.0%, a steel composed of the balance Fe and inevitable impurities, and in a mass ratio of 0.25 ≦ Ca / Al ≦ 2.50 and 0.10 ≦ Ca / O ≦ 0.30 By satisfying the conditions, the forgeability is 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. Excellent austenitic stainless free-cutting steel wire.   The austenitic stainless free-cutting steel wire rod having excellent forgeability according to claim 1, wherein the steel contains B ≦ 0.010% by mass. The austenitic stainless free-cutting steel wire excellent in forgeability according to claim 1 or 2 , wherein the steel contains, by mass%, Mo ≦ 1.50%.

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