JP3417219B2 - Martensitic stainless steel with excellent hot workability - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a martensitic stainless steel having excellent hot workability.

[0002]

2. Description of the Related Art Martensitic stainless steel is AI
As represented by SI420 steel, it is excellent in strength and corrosion resistance and relatively inexpensive, so that its application as oil country tubular goods has been increasing year after year around 1980. Recently, a low C-Ni-Mo-added steel having a corrosion resistance superior to that of 420 steel as described in JP-A-3-120337, or Japanese Patent Application No. 6-286913 proposed by the present applicant. Steel types such as low C-Cu-Ni-Mo-added steels (so-called Modified 13Cr steels) have been developed, as disclosed in the above. In addition, Japanese Patent Fairness 3-60
As described in Japanese Patent Publication No. 904, etc., 13CR type martensitic stainless steel has been developed in which the contents of impurities P and S are specified to be particularly low to improve hot workability during piercing and rolling. ing.

[0003]

When producing such a steel into a seamless steel pipe, seamless pipe rolling is usually carried out by the Mannesmann method. However, when the Modified 13Cr steel has a high S content, it is not possible to perform seamless steel pipe rolling. Rolling defects occur. Therefore, it is necessary to reduce the S content to an extremely low level in the steelmaking stage, which causes a problem of high cost. Therefore, it is an object of the present invention to provide a martensitic stainless steel that can obtain good hot workability without extremely low sulfurization and is excellent in hot workability.

[0004]

The present inventors have investigated hot workability of various materials having different components. As a result, in this steel type, S is a factor that affects the hot workability, but if Ti is added in an appropriate amount in relation to S, good hot workability is achieved without making the S content extremely low. It has been found that workability can be obtained. Further, it was found that the low hot workability in steel having a high S content has a good correlation with the amount of solute S in the steel, and if the solute S can be reduced, good hot workability can be obtained even in the steel having a high S content. It was found that

Modified 13Cr Steel (Low C-Low N-1.5Cu-12.7Cr
Fig. 1 shows the effect of Ti on the hot workability of (-4.5Ni-1.5Mo).
Shown in. The vertical axis and the horizontal axis respectively show the reduction value (Reduction Area) and the deformation temperature T1 (Deformation Temperature) when the hot tensile test is performed under the conditions shown in FIG. The higher the drawing value, the better the hot workability. From the knowledge obtained so far, it is known that if the drawing value is 75% or more, good hot workability is exhibited at that temperature. From Figure 1,
It is clear that even if the S content is high, good hot workability can be obtained by adding an appropriate amount of Ti.

Furthermore, as a result of detailed investigation of the relationship between the S content and the Ti content and the hot workability, 0.002 in% by weight was obtained.
In a steel containing 0.008% of S, if 0.5 times or more of the content of S is added to Ti, the solid solution S in the steel is reduced by forming a sulfide of Ti, so that heat It has been found that the hot workability can be improved. Desirably, if Ti is added in an amount of 1.0 times or more the content of S, the hot workability improving effect is further enhanced. However, even if added in excess of 0.05%, the effect is saturated and, conversely, coarse nitrides are precipitated and the toughness is reduced.

The present invention is constructed on the basis of the above-mentioned findings, and the gist thereof is, in weight%, C: 0.05% or less, Si: 0.5% or less, Mn: 1.5%. Hereinafter, P: 0.03% or less, S: 0.002-0.008%, Cr: 10-17%, Ni: 4.0-7.0%, Al: 0.1% or less, N: 0 0.08% or less, Mo: 1.0 to 3.0%, Cu: 1.0 to 3.0%, Ti: 0.5S to 0.05%, and, if necessary, Ca: 0 0.001 to 0.01%, REM: 0.003 to 0.03% of 1 type or 2 types, and the balance is Fe and inevitable impurities. Site-based stainless steel.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The reasons for limiting the components of the martensitic stainless steel in the present invention are as follows. C: C is an element that forms Cr carbide and deteriorates the corrosion resistance. In addition, the strength is increased and the stress corrosion cracking resistance required when used as an oil country tubular good is deteriorated. Therefore, the upper limit is set to 0.05%. The lower the lower limit, the better.

Si: Si is added as a decarburizing agent in the steel making process. If the content exceeds 0.5%, the toughness deteriorates, so the upper limit was made 0.5%.

Mn: Mn is an austenite stabilizing element and is effective in preventing rolling flaws by suppressing the precipitation of the δ phase during hot working, but if added in excess of 1.5%, the grain boundary strength decreases. As a result, the toughness deteriorates, so 1.5% was made the upper limit.

P: P is an impurity element that segregates at the grain boundaries to lower the grain boundary strength and deteriorates the toughness. It is desirable that the level be as low as possible, but in consideration of the attainable level and cost of the current refining technology. The upper limit was 0.03%.

S: S is an impurity element that deteriorates hot workability. However, if Ti, which will be described later, is added, the Ti.
A rolling defect prevention effect can be obtained with an S content of 008% or less. On the other hand, the lower limit is 0.002 to suppress the increase of refining cost.
%. If further economic effect is desired, the lower limit is preferably 0.003%, and if it is 0.005%, the desulfurization treatment in the secondary refining can be omitted or significantly reduced, and the effect is remarkably large. .

Cr: Cr is a basic element for improving corrosion resistance, and it is necessary to add 10% or more in order to obtain sufficient corrosion resistance, but it is also a ferrite stabilizing element, and if too much, the δ phase is formed during hot working. The upper limit was set to 17% because it precipitates and deteriorates the hot workability.

Ni: Ni is effective in improving corrosion resistance and toughness. It is also an austenite stabilizing element,
Inhibits the formation of the δ phase, which leads to rolling defects. Since these effects are small when the added amount is 4% or less, the lower limit is set to 4%.
Further, even if added in excess of 7%, the effect is saturated and the A _c1 transformation point is lowered to make strength refining difficult, so the optimum addition range was set to 4 to 7%.

Al: Al is added for the purpose of deoxidation and desulfurization. A sufficient effect is obtained with addition of 0.1% or less, and toughness is lowered with addition of more than 0.1%, so the upper limit was made 0.1%.

N: N is a strong austenite stabilizing element similar to Mn and Ni and is effective in preventing rolling flaws, but it increases strength like C and is necessary when used as an oil country tubular good. This causes deterioration of stress corrosion cracking resistance.
Therefore, the upper limit is set to 0.08%. The lower the lower limit, the better.

Mo: Mo is an essential element for improving pitting corrosion resistance and sulfide stress corrosion cracking resistance. These effects have a small improvement effect when added at less than 1.0%, so the lower limit was made 1.0%. Further, since it is a strong ferrite stabilizing element and the addition of more than 3% tends to generate the δ phase, the optimum addition amount is set to 1.0 to 3.0%.

Cu: Cu is an element effective for improving corrosion resistance as well as Ni, and an austenite-stabilizing element. It suppresses the formation of the δ phase and is effective in preventing rolling flaws. Although added, if less than 1.0%, these effects cannot be sufficiently obtained, so the lower limit was made 1.0%. Further, if added in excess of 3.0%, it excessively segregates at the grain boundaries to lower the grain boundary strength, and the hot workability is significantly reduced. Therefore, the optimum addition range is 1.0 to 3.0%. did.

Ti: Addition of 0.5% by weight or more of the content of S to Ti improves the hot workability because the solid solution S in the steel is reduced by forming a sulfide of Ti. Therefore, the lower limit was made 0.5 times the amount of S added. Also, 0.
Even if added in excess of 05%, the effect is saturated, and on the contrary, coarse nitrides are precipitated and the toughness is lowered, so the upper limit is set to 0.
It was set to 05%.

Ca: Ca suppresses the deterioration of hot workability due to S, and is added as necessary.
If it is less than 1%, the effect is not exhibited, and if it is added in excess of 0.01%, the effect is saturated, so the optimum addition amount was made 0.001 to 0.01%. REM: An effective element that makes the shape of inclusions spherical and renders them harmless. If it is too small, the effect is not obtained, and if it is too large, inclusions increase and the resistance to sulfide stress corrosion cracking resistance decreases, so the optimum addition amount was made 0.003 to 0.03%.

The steel of the present invention is produced by seamless tube rolling by the so-called Mannesmann system. The rolling method of the Mannesmann method used here is to use a tubular material with a rectangular or round cross section, punch it by a press roll drilling method or a Mannesmann drilling method, then stretch it by an inclined rolling machine (elongator), and further by a plug mill or a mandrel mill. It means the process of making pipes. The rolling conditions are not particularly limited, but the heating temperature for effectively preventing rolling defects is preferably in the range of 1150 to 1280 ° C.

[0022]

[Examples] Using a bloom of 215 mm square cross section having the composition shown in Table 1 as a raw material, seamless pipe rolling was performed in the process shown in FIG. After the rolling was completed, the inside and outside surfaces of the pipe were examined by visual observation.
The results are shown in Table 2. According to the present invention (Nos. 1 to 8), it is clear that the rolling flaw occurrence frequency is extremely low. on the other hand,
In the comparative examples (Nos. 9 to 12), severe rolling defects frequently occur.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

INDUSTRIAL APPLICABILITY As described above, the present invention provides a method for producing a martensitic stainless steel pipe having excellent hot workability, which has good workability without extremely low sulfurization.

[Brief description of drawings]

[Figure 1] Modified 13Cr steel (low C-low N-1.5Cu-12.7Cr-4.5
3 is a chart showing the effect of Ti on the hot workability of Ni-1.5Mo).

FIG. 2 is a chart showing a seamless rolling process.

─────────────────────────────────────────────────── ─── Continuation of front page (72) Haruyuki Nagayoshi Inventor Haruyuki 1-1, Toibata-cho, Tobata-ku, Kitakyushu City Nippon Steel Corporation Yawata Works (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. By weight%, C: 0.05% or less, Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.002-0.008. %, Cr: 10 to 17%, Ni: 4.0 to 7.0%, Al: 0.1% or less, N: 0.08% or less, Mo: 1.0 to 3.0%, Cu: 1 Martensitic stainless steel excellent in hot workability, characterized by containing 0.0 to 3.0%, Ti: 0.5S to 0.05%, and the balance being Fe and inevitable impurities. 2. By weight%, C: 0.05% or less, Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.002-0.008. %, Cr: 10 to 17%, Ni: 4.0 to 7.0%, Al: 0.1% or less, N: 0.08% or less, Mo: 1.0 to 3.0%, Cu: 1 0.0 to 3.0%, Ti: 0.5S to 0.05%, and Ca: 0.001 to 0.01%, REM: 0.003 to 0.03%, 1 or 2 A martensitic stainless steel excellent in hot workability, characterized in that it contains seeds and the balance is Fe and inevitable impurities.