JPH1025549A - Martensitic stainless steel excellent in hot workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic stainless steel excellent in hot workability, capable of giving superior hot workability while obviating the necessity of minimization of sulfur content. SOLUTION: This steel has a composition consisting of, by weight, <=0.05% C, <=0.5% Si, <=1.5% Mn, <=0.03% P, 0.002-0.008% S, 10-17% Cr, 4.0-7.0% Ni, <=0.1% Al, <=0.08% N, 1.0-3.0% Mo, 1.0-3.0% Cu, 0.5S to O.05% Ti, and the balance Fe with inevitable impurities and further containing, if nevessary, either or both of 0.001-0.01% Ca and 0.003-O.03% REM and also has tempered martensitic structure. By this method, a method of manufacture of a seamless martensitic stainless steel tube excellent in hot workability can be provided.

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 has been developed using AI.
Since it is excellent in strength and corrosion resistance and relatively inexpensive as represented by SI420 steel, its application as an oil country tubular good has been increasing year by year since about 1980. Recently, low C-Ni-Mo-added steel as described in JP-A-3-120337, which has better corrosion resistance than 420 steel, or Japanese Patent Application No. 6-286913 proposed by the present applicant. Steel grades such as low-C-Cu-Ni-Mo-added steels (steel grades called so-called Modified 13Cr steels) have been developed. In addition, 3-60
No. 904, etc., a 13CR martensitic stainless steel in which the contents of impurities P and S are specified to be particularly low to improve hot workability during piercing and rolling has been developed. ing.

[0003]

When such steel is made into a seamless steel pipe, seamless pipe rolling is usually performed by the Mannesmann method. However, when the above-mentioned Modified 13Cr steel has a high S content, it is difficult to form a seamless steel pipe at the time of rolling. Rolling flaws occur. Therefore, it is necessary to reduce the S content to an extremely low level in the steel making stage, which has caused a problem of high cost. Therefore, an object of the present invention is to provide a martensitic stainless steel that can obtain good hot workability without being extremely sulfurized and is excellent in hot workability.

[0004]

Means for Solving the Problems The present inventors have investigated the hot workability of various materials having different components. As a result, in the present steel type, there is S as a factor affecting hot workability. However, if Ti is added in an appropriate amount in relation to S, even if the S content is not made extremely low, good hot workability can be obtained. It has been found that workability can be obtained. In addition, they found that the low hot workability of steel with a high S content had a good correlation with the amount of solid solution S in the steel. Was obtained.

[0005] Modified 13Cr steel (low C-low N-1.5Cu-12.7Cr)
Of Ti on hot workability of (-4.5Ni-1.5Mo)
Shown in The ordinate and the abscissa respectively indicate a reduction value (Reduction Area) and a deformation temperature T1 (Deformation Temperature) when a hot tensile test is performed under the conditions shown in FIG. The higher the aperture value, the better the hot workability. From the findings so far, it has been found that if the aperture value is 75% or more, good hot workability is exhibited at that temperature. From FIG.
It is clear that good hot workability can be obtained by adding an appropriate amount of Ti even if the S content is high.

Further, as a result of a detailed investigation of the relationship between the S content and the Ti content and the hot workability, it was found that 0.002%
In steel containing up to 0.008% of S, if Ti is added at 0.5 times or more of the S content, solute S in the steel is reduced by forming sulfide of Ti, so that It has been found that interworkability can be improved. Desirably, if Ti is added 1.0 times or more of the S content, the effect of improving hot workability is further increased. However, even if added over 0.05%, the effect saturates and conversely, coarse nitrides are precipitated to lower toughness.

The present invention has been made based on the above findings, and the gist of the present invention is that, in terms of% by weight, C: 0.05% or less, Si: 0.5% or less, Mn: 1.5% P: 0.03% or less, S: 0.002 to 0.008%, Cr: 10 to 17%, Ni: 4.0 to 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%, and the balance is Fe and unavoidable impurities, with the balance being excellent in hot workability. It is a site-based stainless steel.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. 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 the like and deteriorates corrosion resistance. Further, it increases the strength and causes deterioration of stress corrosion cracking resistance required when used as an oil country tubular good. 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 precipitation of the δ phase during hot working. However, when added in excess of 1.5%, the grain boundary strength is reduced. 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 boundary to lower the grain boundary strength and deteriorates toughness, and it is desirable that the level be as low as possible. Therefore, the upper limit was made 0.03%.

S: S is an impurity element that deteriorates hot workability. However, if Ti described later is added, it will be 0.1.
A rolling flaw preventing effect can be obtained with an S content of 008% or less. On the other hand, the lower limit is 0.002 in order to suppress the increase in the refining cost.
%. If further economic effect is desired, the lower limit is desirably 0.003%. If the lower limit is 0.005%, the desulfurization treatment in the secondary refining can be omitted or significantly reduced, so that the effect is extremely large. .

Cr: Cr is a basic element for improving corrosion resistance, and it is necessary to add 10% or more to obtain sufficient corrosion resistance. However, it is also a ferrite stabilizing element. The upper limit was set to 17% in order to precipitate and deteriorate the hot workability.

Ni: Ni is effective in improving corrosion resistance and toughness. It is also an austenite stabilizing element,
Suppress formation of δ phase leading to rolling flaws. 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, so that the strength refining becomes difficult. Therefore, the optimum addition range is set to 4 to 7%.

Al: Al is added for the purpose of deoxidation and desulfurization. A sufficient effect can be obtained by adding 0.1% or less, and the toughness is reduced if added over 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. However, N increases the strength similarly to C and is necessary when used as an oil country tubular good. The stress corrosion cracking resistance is deteriorated.
Therefore, the upper limit is set to 0.08%. The lower the lower limit, the better.

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

Cu: Cu, like Ni, is an element effective in improving corrosion resistance and is an austenite stabilizing element. It suppresses the formation of the δ phase and is effective in preventing rolling flaws. However, if less than 1.0%, these effects cannot be sufficiently obtained, so the lower limit is set to 1.0%. Further, if added in excess of 3.0%, excessive segregation at the grain boundaries lowers the grain boundary strength, and the hot workability is significantly reduced. Therefore, the optimum addition range is 1.0 to 3.0%. did.

Ti: If Ti is added in an amount of 0.5% or more of the content of S by weight%, sulfide of Ti is formed, so that the amount of dissolved S in the steel is reduced, so that hot workability is improved. Therefore, the lower limit was set to 0.5 times the amount of S added. Also, 0.
Even if added over 0.05%, the effect is saturated and conversely, coarse nitrides are precipitated to lower the toughness.
05%.

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

The steel of the present invention is formed by seamless tube rolling according to the so-called Mannesmann method. The rolling method of the Mannesmann method as used herein means that a pipe having a rectangular cross section or a round cross section is pierced by a press roll piercing method or a Mannesmann piercing method, and then stretched by an inclined rolling mill (elongator), and further by a plug mill or a mandrel mill. It refers to the process of pipe making. The rolling conditions are not particularly limited, but the heating temperature for effectively preventing rolling flaws is preferably in the range of 1150 to 1280 ° C.

[0022]

EXAMPLE Using a bloom having a composition shown in Table 1 and having a cross section of 215 mm square, seamless pipe rolling was performed in the process shown in FIG. After the rolling was completed, the inner and outer surfaces of the tube were examined by visual observation.
Table 2 shows the results. According to the present invention (Nos. 1 to 8), it is clear that the frequency of occurrence of rolling flaws is extremely low. on the other hand,
In the comparative examples (Nos. 9 to 12), severe rolling flaws occur frequently.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

As described above, according to the present invention, there is provided a method for producing a martensitic stainless steel seamless steel pipe which can obtain good workability even without extremely low sulfuration and is excellent in hot workability.

[Brief description of the drawings]

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

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

 ──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Haruyuki Nagayoshi 1-1-1, Hibata-cho, Tobata-ku, Kitakyushu City Inside Nippon Steel Corporation Yawata Works

Claims (2)

[Claims] 1. wt%, C: 0.05% or less, Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.002 to 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 A martensitic stainless steel excellent in hot workability, characterized by containing 0.0-3.0% and Ti: 0.5S-0.05%, the balance being Fe and unavoidable impurities. 2. In weight%, C: 0.05% or less, Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.002 to 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 0.03% to 3.0%, Ti: 0.5S to 0.05%, and one or two of Ca: 0.001 to 0.01%, REM: 0.003 to 0.03%. A martensitic stainless steel excellent in hot workability, characterized by containing a seed and the balance being Fe and unavoidable impurities.