JP2006131972A - Ferritic stainless steel with excellent machinability, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide ferritic stainless steel in which machinability is improved without particularly deteriorating toughness and ductility and also to provide its manufacturing method. <P>SOLUTION: The ferritic stainless steel has a composition consisting of, by mass, ≤0.15% C, ≤1.00% Si, ≤1.50% Mn, 12 to 30% Cr, ≥0.010% S and the balance Fe with inevitable impurities and further containing, if necessary, 2.0 to 6.0% Al. Moreover, the width of sulfides is made to ≤3μm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a ferritic stainless steel excellent in machinability and toughness and a method for producing the same.

Conventionally, a ferritic stainless steel having a low coefficient of thermal expansion has been used as a heat-resistant material used in a portion where heating and cooling are repeated.
By the way, this ferritic stainless steel has not only poor toughness but also poor machinability, and has problems such as low production efficiency and yield when manufacturing desired parts by cutting, and therefore high production cost. is doing.
In particular, when a thin fiber product having a thickness of about 100 μm is manufactured by cutting, these problems become serious problems.

  For example, in an automobile muffler, ferritic stainless steel is processed into a fiber and used as a sound deadening material. In this case, the smaller the diameter of the ferrite stainless steel fiber, the higher the sound deadening effect. Therefore, it is required to finely process ferritic stainless steel during processing.

For example, conventional ferritic stainless steels that do not contain Al for such applications, and Al-containing ferritic stainless steels that have improved oxidation resistance by adding Al when heat resistance is further required (ferritic series) However, if wire drawing (drawing) is used as a method for processing and manufacturing this, machinability is not particularly problematic, but in recent years this is manufactured by cutting. In this case, since conventional ferritic stainless steel has poor machinability, it cannot perform cutting well, resulting in low production efficiency. In addition, ferritic stainless steel also has poor toughness. In addition, the fiber easily breaks during processing, yield is deteriorated, and the production cost is increased.
In addition, the low toughness is not only a problem in processing, but also the characteristics as a product (part), such as being easily broken after the product is produced.

  As a means of improving the machinability problem, which is a disadvantage of such ferritic stainless steels, it is conceivable to add S, which is a free-cutting element. However, the ductility is further deteriorated, and not only the production efficiency and the yield are further deteriorated, but also the characteristics as a part are further deteriorated.

  The following Patent Document 1 discloses an invention about an Al-containing ferritic stainless steel excellent in manufacturability and high-temperature oxidation resistance, and the following Patent Document 2 describes a high Al-containing ferritic stainless steel excellent in high-temperature oxidation resistance. The technical means for improving the workability are disclosed in each of them, and these are different from the present invention in the problem solving means and are different from the present invention.

JP-A-6-172933 JP-A-6-172932

  The present invention has been made for the purpose of providing a ferritic stainless steel having improved machinability without particularly impairing toughness and a method for producing the same, against the background described above.

  Thus, claim 1 relates to a ferritic stainless steel having excellent machinability, and in terms of mass%, C: ≦ 0.15%, Si: ≦ 1.00%, Mn: ≦ 1.50%, Cr: 12-30% , S: ≧ 0.010%, remaining Fe and inevitable impurities, and the width of the sulfide is 3 μm or less.

  According to a second aspect of the present invention, in the first aspect, Mo: ≦ 3.0% is further contained by mass%.

  According to the third aspect, in mass%, C: ≦ 0.15%, Si: ≦ 1.00%, Mn: ≦ 1.50%, Cr: 12-30%, Al: 2.0-6.0%, S: ≧ 0.010%, balance Fe And the composition of inevitable impurities, and the width of the sulfide is 3 μm or less.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the composition further comprises any one or two of Zr: 0.01 to 0.3% and Y: 0.01 to 0.3% by mass%. .

  Claim 5 relates to a method for producing a ferritic stainless steel having excellent machinability. In the ferritic stainless steel according to any one of claims 1 to 4, hot rolling or forging is performed at a forging ratio of 1000 or more. Thus, the width of the sulfide is set to 3 μm or less.

Effects and effects of the invention

  As described above, in the present invention, ferritic stainless steel, and further heat resistant ferritic stainless steel whose oxidation resistance is improved by containing Al, is contained 0.010% or more of S, and the form of sulfide is 3 μm in width. The form is controlled to be as follows.

In the case of austenitic stainless steel, when S is added because it is originally excellent in toughness, even if the form of the sulfide is controlled to the above form, it greatly affects the toughness. This does not occur in particular.
In the case of such an austenitic stainless steel, the toughness is affected by the addition amount (absolute amount) of S itself.

However, in the case of ferritic stainless steel, the toughness is originally low, so that the reduction of toughness due to the addition of S can be effectively suppressed by making the presence form of sulfides the above form.
More specifically, in the ferritic stainless steel in which the sulfide is present in a form extending thinly in the forging direction by rolling with a width of 3 μm or less, the sulfide particles are fine in the cross section, and therefore the bending direction. Or ductility and toughness become favorable compared with the case where many large round sulfides exist in the tensile direction, particularly in the tensile direction.
Therefore, for example, when ferritic stainless steel is manufactured by cutting into a thin fiber shape, the work product has good bending and tough ductility in the cutting direction.

  As a result, according to the present invention, the machinability of ferritic stainless steel can be effectively increased by adding S without greatly impairing the toughness, thereby producing a fibrous or other member by cutting. In this case, the production efficiency can be increased and the yield can be increased, and the production cost can be effectively reduced.

  Next, the manufacturing method of Claim 5 of this invention manufactures the ferritic stainless steel of Claims 1-4 by performing hot rolling or forging by the forging ratio 1000 or more, By performing the intermediate rolling or forging, the existence form of the sulfide can be made into the above-mentioned form, that is, the form elongated in a width of 3 μm or less.

In the present invention, one or more free cutting elements such as Pb, Bi, Se, Te, and Ca can be added in the range of 0.01 to 0.3%, respectively, as necessary.
In addition, any one or more of crystal grain refining elements such as Nb, Ti, V, and Ta can be added in the range of 0.01 to 0.3%, respectively.
Further, any one or more of oxidation resistance improving elements such as REM and La can be added in the range of 0.1 to 0.3%.

If hot workability deteriorates due to an increase in the amount of sulfide or addition of free-cutting elements, for example, three-way roll rolling or intermediate heating may be employed during hot working of wire rods in order to add large hot working. it can.
In order to further increase the strength, one or more of solid solution strengthening elements such as Co and W can be added in the range of 0.1 to 2.0%, respectively.

Next, the reasons for limiting the chemical components of the present invention will be described in detail below.
C: ≤ 0.15%
C is an element imparting the strength of steel. On the other hand, it is precipitated as carbides and nitrides, and in order to reduce corrosion resistance and toughness, the content is made 0.15% or less.

Si: ≦ 1.00%
Si is effective in improving high temperature oxidation resistance. On the other hand, to reduce toughness, the content was made 1.00% or less.

Mn: ≦ 1.50%
Mn is effective for improving the hot workability, but is made 1.50% or less in order to deteriorate the oxidation resistance.

Cr: 12-30%
Cr is necessary to ensure oxidation resistance, but if it exceeds the upper limit, the toughness and ductility deteriorate, so the content was made 12-30%.

S: ≧ 0.010%
Since S is effective in improving machinability, it is set to 0.010% or more.

Mo: ≤3.0%
While Mo improves the corrosion resistance, the ductility decreases when the content is large, so the content was made 3.0% or less.

Al: 2.0-6.0%
Al improves the high-temperature oxidation resistance, but when the upper limit is exceeded, the effect is saturated and the toughness is deteriorated, so the content was made 2.0 to 6.0%.

Zr: 0.01-0.3%
Zr contributes to refinement of crystal grains or improvement of oxidation resistance, but the effect is saturated even if added excessively, so the content was made 0.01 to 0.3%.

Y: 0.01-0.3%
Y contributes to the improvement of high-temperature oxidation resistance, but the effect is saturated even if added in excess, so the content was made 0.01 to 0.3%.

Pb, Bi, Se, Te, Ca: 0.01 to 0.3%
Pb, Bi, Se, Te, and Ca contribute to the improvement of machinability, so that 0.01 to 0.3% can be added as necessary.

Nb, Ti, V, Ta: 0.01 to 0.3%
Since Nb, Ti, V, and Ta contribute to crystal grain refinement, 0.01 to 0.3% can be added as necessary.

REM, La: 0.1-0.3%
REM and La are elements for improving oxidation resistance, and can be added in an amount of 0.1 to 0.3% as necessary.

CO, W: 0.1-2.0%
CO and W are solid solution strengthening elements, and can be added in an amount of 0.1 to 2.0% as necessary to increase the strength.

Next, embodiments of the present invention will be described in detail below.
Inventive Example 1 so as to obtain the forging ratio shown in Table 1 by a combination of forging and rolling after melting steels having various component compositions shown in Table 1 in an electric furnace and casting into ingots of 1 to 12 t. Is φ5.55 for Invention Example 2, φ9.05 for Invention Example 3, φ9.05 for Invention Example 4, φ8.55 for Invention Example 6, and φ8.25 for Invention Example 6, A steel bar of φ22 was manufactured (unit is mm).
Then, after annealing at 800 ° C. to 1000 ° C., each of the sulfide width, toughness (drawing), and machinability (cutting resistance) was evaluated by the following methods.

<Sulphide width>
With respect to the sulfide width, arbitrary 30 visual fields in the longitudinal section direction of the test piece were observed with an optical microscope (× 400), and the maximum width in the direction perpendicular to the rolling direction was measured for the confirmed sulfide.

<Toughness (drawing)>
The drawing was evaluated by performing a room temperature tensile test with a JIS No. 4 test piece.

<Machinability (cutting resistance)>
The machinability was evaluated by the magnitude of the main component force (N) of the cutting resistance during cutting.
The test conditions were as follows.
Test conditions Tool: Carbide Peripheral speed: 100 m / min
Cutting depth: 0.1 mm
Feed amount per rotation: 0.05mm / rev
Dry

  The results are also shown in Table 1.

  In Table 1, the machinability evaluation is x in Comparative Example 1 where the S addition amount is 0.004% (the machinability evaluation is x: 35N or more, ◯: 35-25N, ◎: 25N or less).

  Comparative Example 2 with an S addition amount of 0.241% is a machinability evaluation ◎, but the sulfide width is 5 μm, and as a result, the aperture value is 51%, which is lower than that of Comparative Example 1.

  Comparative Example 3 with an S addition amount of 0.023% is a machinability evaluation ◯, but the sulfide width is 4 μm, and as a result, the aperture value is 43%, which is lower than that of Comparative Example 1.

Comparing the aperture values of Comparative Examples 1, 2, and 3, it can be seen that Comparative Example 1 with the smallest sulfide width is the best.
Moreover, it turns out that the comparative example 1 which is a high training ratio exists in the tendency for a sulfide width to be small compared with the comparative examples 2 and 3. FIG.

In consideration of the above, Invention Examples 1 to 6 were produced and evaluated, and the width of the sulfide was 3 μm or less, possessed good toughness (drawing), and ferritic stainless steel excellent in machinability. I was able to get steel.
The aperture value was set to 60% or more.

  Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be implemented in variously modified forms without departing from the spirit of the present invention.

Claims (5)

In mass%
C: ≤0.15%
Si: ≦ 1.00%
Mn: ≦ 1.50%
Cr: 12-30%
S: ≧ 0.010%
A ferritic stainless steel having excellent machinability, characterized by having a composition of the balance Fe and inevitable impurities and having a sulfide width of 3 μm or less. In mass%
Mo: ≤3.0%
The ferritic stainless steel excellent in machinability according to claim 1, further comprising: In mass%
C: ≤0.15%
Si: ≦ 1.00%
Mn: ≦ 1.50%
Cr: 12-30%
Al: 2.0-6.0%
S: ≧ 0.010%
A ferritic stainless steel excellent in machinability, characterized by having a composition of the balance Fe and inevitable impurities, and having a sulfide width of 3 μm or less. In mass%
Zr: 0.01-0.3%
Y: 0.01 to 0.3%
The ferritic stainless steel excellent in machinability according to any one of claims 1 to 3, further comprising any one or two of the above.   The manufacturing method of the ferritic stainless steel excellent in the machinability in any one of Claims 1-4 which makes the width | variety of the said sulfide 3 micrometers or less by performing hot rolling or forging by the forge ratio 1000 or more.