JP6287972B2 - Manufacturing method of martensitic high Cr steel seamless steel pipe - Google Patents

Description

  The present invention relates to a method for producing a martensitic high Cr steel seamless steel pipe used for boiler pipes and the like, and in particular, seamlessly using a material having a Cr content of 9% by mass or more and containing an alloy element such as Mo or W. The present invention relates to a method for manufacturing a martensitic high Cr steel seamless steel pipe that prevents the appearance of rolling flaws that occur on the outer surface of the steel pipe, which is called outer surface cracking, even in the case of manufacturing a steel pipe.

  Conventionally, so-called martensitic high Cr steel seamless steel pipes containing 9% by mass or more of Cr have been adopted as heat-resistant steel pipes used for boiler tubes and the like. Furthermore, in recent years, in order to increase creep strength, the use of high Cr steel seamless steel pipes to which W is added in addition to Cr and Mo is increasing.

  Seamless steel pipes are manufactured by heating a round billet that is a raw material, piercing and rolling with an inclined piercing and rolling machine called a Mannesmann Piercer, and then drawing and rolling it with a mandrel mill or a plug mill to form a hollow shell. After heating, it is manufactured by reducing the diameter with a stretch reducer or the like and finishing it to a steel pipe with a target dimension. In general, in the case of a high Cr steel seamless steel pipe, the round billet is manufactured by heating a slab (slab or bloom) manufactured by a continuous casting method, and performing hot rolling or hot forging.

  However, high Cr alloy steels are inferior in hot workability compared to general steels, so it becomes easy to generate rolling flaws on the inner and outer surfaces of the tube during piercing and rolling, which imposes severe deformation, especially Mo and W. When a high Cr steel seamless steel pipe including the above is manufactured by the above-described manufacturing method, there is a problem that rolling flaws frequently occur on the inner and outer surfaces of the product pipe.

  For such a problem, for example, Patent Document 1 discloses, in weight%, C: 0.01 to 0.15%, Mn: 1.5% or less, P: 0.020% or less, S: 0.0050. % Or less, Si: 1% or less, Al: 0.1% or less, Cr: 8 to 11%, Mo: 0.05 to 1.5%, W: 0.05 to 4%, N: 0.01 to High Cr ferritic steel containing 0.1%, the balance being Fe and inevitable impurities, and in addition to the above components, Nb: 0.01 to 0.15%, V: 0.05 to Seam of high Cr ferritic steel containing one or more of 0.5%, B: 0.0005-0.03%, Ni: 0.01-3%, the balance being Fe and inevitable impurities When the steelless tube material is heated to 1100 ° C. or higher and 1250 ° C. or lower and pierced and then stretched by inclined rolling, the rolling start temperature The 1000 ° C. or higher, and 1200 ° C. or less, cracking due to delta ferrite, the rolling method for preventing the occurrence of scratches is disclosed by optimizing the working ratio in drawing and rolling.

Further, in Patent Document 2, a steel pipe material having a composition containing at least 9% by mass and Cr: 9% is heated to a temperature of (Ac ₄ transformation point + 10 ° C.) or more, pierced by rolling by a tilt rolling method, and stretched. There is disclosed a method for producing a high Cr steel seamless pipe, which can produce a high Cr steel seamless pipe without frequent occurrence of rolling flaws.

JP-A-8-229606 JP-A-2005-14031

  However, in the technique described in Patent Document 1 or Patent Document 2, when coarse delta ferrite is present in the material at the round billet stage, the rolling flaw generated on the outer surface of the steel pipe, called outer surface cracking, is completely removed. Could not be deterred.

  The present invention has been made in view of such problems, and contains 9% by mass or more of Cr which is hot-finished by the Mannesman-Mandrel system or Mannesman-Plug Mill system, and is an alloy such as Mo or W. It aims at providing the manufacturing method of the martensitic high Cr steel seamless steel pipe which suppressed the wrinkle (hair crack) which generate | occur | produced on the outer surface of the seamless pipe steel pipe containing an element, and was excellent in the external surface quality.

  In order to solve the above-mentioned problems, the present inventors have detailed the structure near the outer surface flaw (hair cracking) generated in the martensitic high Cr steel seamless steel pipe and the structure near the outer surface of the round billet that is the material. In addition to observing, the present inventors studied diligently about the relationship between the manufacturing process of seamless steel pipes including the manufacturing process of round billets and the occurrence of cracks. In the present invention, high Cr steel means steel containing 9% or more and 10% or less by mass of Cr.

  As a result, first of all, hair cracks progress to connect delta ferrite near the outer surface precipitated along the austenite grain boundary, and it seems that the part not exposed on the surface is also caused by delta ferrite. The knowledge that cracks exist is obtained. Also, the delta ferrite near the outer surface is precipitated in the step before heating the round billet, that is, the step of heating the slab and manufacturing the round billet by hot rolling, and is coarsened in the round billet heating step before piercing rolling. The knowledge that it is promoting the generation of cracks was obtained.

Furthermore, as for the steel pipe of ASME standard T92 / P92, as a result of examining the relationship between the occurrence of hair cracking and the slab heating temperature, round billet heating temperature and heating holding time in the steel pipe manufacturing process, as shown in FIG. It has been found that when the slab heating temperature is Ac ₄ points or less, the heating temperature of the round billet is Ac ₄ points or less, and the heating and holding time is 200 minutes or less, there is no occurrence of hair cracking.

The present invention has been completed on the basis of the above-mentioned findings and comprises the following gist.
(1) From a slab having a composition containing 9% or more of Cr by mass%, from a slab heating process, a slab rolling process, a round billet heating process, a piercing rolling process, a stretching rolling process, and a reduced diameter rolling process A method for producing a seamless steel pipe, wherein the heating temperature in the slab heating step is 1160 ° C. or higher and Ac ₄ points or lower, and the heating temperature in the round billet heating step is 1180 ° C. or higher and Ac ₄ points or lower and maintained by heating. The manufacturing method of the martensitic high Cr steel seamless steel pipe characterized by making time into 100 to 200 minutes.
(2) The maximum reached temperature of the cast slab which is the material to be heated in the slab heating process and the round billet which is the material to be heated in the round billet heating process is set to Ac ₄ points or less (1) ) A method for producing a martensitic high Cr steel seamless steel pipe.

  ADVANTAGE OF THE INVENTION According to this invention, generation | occurrence | production of the outer surface flaw (hair crack) in the manufacturing process of a seamless steel pipe can be suppressed, a high Cr steel seamless steel pipe can be manufactured efficiently, and there is a remarkable industrial effect.

It is a figure which shows the relationship between round billet heating temperature, heating holding time, and hair crack generation | occurrence | production.

  In the present invention, a high Cr steel slab containing 9% or more by mass of Cr is heated and processed into a round billet by hot rolling to obtain a steel pipe material, and then the round billet is heated, pierced, rolled, and stretched. A seamless steel pipe is obtained by rolling and reduced diameter rolling.

Seamless steel pipe as an object of the present invention is a martensitic high Cr steel seamless steel pipe containing Cr 9% or more, it contained in order to improve the corrosion resistance and high-temperature creep strength Ingredient set Chengdu The reason for the limitation will be described.
Hereinafter, the content (% by mass) of each composition is simply expressed as% unless otherwise specified.

C: 0.01 to 0.15%
C is an austenite stabilizing element for martensite conversion and an important element for precipitating carbides. If it is less than 0.01%, the amount of delta ferrite is remarkably increased, and the strength and toughness are impaired, and a stable carbide is not formed. On the other hand, when it exceeds 0.15%, carbides increase and workability and weldability are impaired. Therefore, C is set to 0.01 to 0.15%.

Cr: 9 to 10%
Cr is an important element in securing heat resistance and corrosion resistance. If it is less than 9%, the oxidation resistance at high temperature becomes insufficient, and the high temperature strength also decreases. On the other hand, if it exceeds 10%, the amount of delta ferrite increases and it becomes difficult to suppress wrinkles starting from delta ferrite. Therefore, Cr is 9 to 10%.

Ni: 0.1 to 3.0%
Ni is an austenite-forming element and has the effect of suppressing the formation of delta ferrite and the effect of improving heat resistance and corrosion resistance, and is added in an amount of 0.01% or more. However, if it exceeds 3%, the creep strength on the long time side is lowered, so the upper limit of the addition amount is made 3%.

Mo: 0.1 to 1.5%
Mo contributes to improvement of strength and hardness at high temperature by solid solution strengthening and stabilization of carbide, and prevents embrittlement by tempering. If it is less than 0.1%, the effect is small, and if it exceeds 1.5%, the formation of delta ferrite is promoted, so 0.1 to 1.5%.

W: 0.1-4.0%
W contributes to solid solution strengthening and fine precipitation of carbides, and remarkably improves the creep strength on the high temperature long time side. In order to ensure the high temperature strength and hardness, addition of 0.1% or more is necessary. However, if it exceeds 4.0%, coarse delta ferrite tends to be formed, and the high temperature strength and toughness are lowered. Therefore, W is set to 0.1 to 4.0%.

Nb: 0.01 to 0.15%
Nb increases toughness and has the effect of refining the structure, so 0.01% or more is added. Even if added over 0.15%, the effect is saturated, so the upper limit is made 0.15%.

  The balance is Fe and inevitable impurities.

Next, the reasons for limiting the slab heating temperature, the round billet heating temperature, and the heating holding time will be described.
-Slab heating temperature: 1160 ° C or more and Ac ₄ points or less When a slab obtained by continuous casting is heated and subjected to hot rolling to form a round billet, the heating temperature is set to 1160 ° C or more and Ac ₄ points or less. When the heating temperature is less than 1160 ° C., the hot deformation resistance increases and rolling becomes difficult. Also, when heated to a temperature exceeding ₄ points of Ac, delta ferrite precipitates at the austenite grain boundaries, and the delta ferrite remains in the vicinity of the surface layer of the round billet even after hot rolling, and grows by heating the next round billet. And it becomes a starting point of the outer surface flaw (hair cracking) generated in the steel pipe after rolling. In addition, Preferably it is 1160 degreeC or more (Ac _4- point-10 degreeC) or less.
-Round billet heating temperature: 1180 ° C or more and Ac ₄ points or less The round billet obtained through the slab rolling process is subjected to hot piercing rolling and constant diameter rolling to form a hollow shell, and if necessary, the hollow blank After the tube is reheated, the round billet is heated to a temperature of 1180 ° C. or higher and Ac ₄ points or lower in order to reduce the diameter and form a steel pipe. When the heating temperature is less than 1180 ° C., deformation resistance during rolling increases and exceeds the load limit of the piercing mill, so 1180 ° C. is set as the lower limit. Further, when heated to a temperature of more than Ac ₄ points, delta ferrite is precipitated in the austenite grain boundaries, to promote the occurrence of outer surface flaws become starting points of hot tearing during piercing-rolling (hair cracks), the heating temperature of the Ac ₄ points The upper limit of. The temperature is preferably 1180 ° C. or higher (Ac ₄ points−10 ° C.).
-Heat holding time of round billet: 100 to 200 minutes The round billet heated to the heating temperature is held at the heating temperature for 100 to 200 minutes. If the heating and holding time is less than 100 minutes, sufficient temperature uniformity cannot be ensured, and uneven thickness due to thermal deviation may occur. On the other hand, when the heating and holding time exceeds 200 minutes, generation of hair cracks becomes significant. Therefore, the round billet heating and holding time is set to 100 to 200 minutes. In addition, Preferably it is 100 to 170 minutes.

In addition, when the temperature of the slab or the round billet exceeds the Ac ₄ point due to overheating or the like in the temperature raising process in the slab heating step or the round billet heating step, delta ferrite is precipitated, and the vicinity of the steel pipe surface after rolling In some cases, hair cracks may occur. Therefore, it is preferable that the maximum reached temperature of the cast slab which is a material to be heated in the slab heating process and the round billet which is a material to be heated in the round billet heating process is Ac ₄ points or less.

  The American Society of Mechanical Engineers (ASME) standard T92 / P92 steel pipe (outer diameter 168.3 mm, wall thickness 22.0 mm, length 6000 mm) under the conditions shown in Table 1 (steel pipe Nos. A, B, C, D) ( 54, 92, 83, and 41), and the occurrence of cracks on the outer surface of the steel pipe was investigated. The occurrence rate of hair cracking is a value exceeding 5% of the wall thickness (lower limit: 0.5 mm). Defects are counted as harmful defects, and (number of cracked steel pipes / total number of manufactured steel pipes) × 100 Asked.

  As shown in Table 1, the steel pipe No. which does not satisfy the production method of the present invention. In B, C, and D (all are comparative examples), the occurrence rate of hair cracking is 38.6 to 63.0%, whereas the steel pipe No. of the present invention example. In A, no cracking occurred.

Claims (2)

In mass%, C is 0.01 to 0.15%, Cr is 9 to 10%, Ni is 0.1 to 3.0%, Mo is 0.1 to 1.5%, and W is 0.1 to 0.1%. 4.0%, Nb 0.01 to 0.15% saw including a cast slab having the balance consisting of Fe and unavoidable impurities as a material, slab heating step, the slab rolling process, a round billet heating step , A piercing-rolling step, a drawing and rolling step, and a diameter-reducing rolling step for producing a seamless steel pipe, wherein the heating temperature in the slab heating step is 1160 ° C. or higher and Ac ₄ points or lower, and the round billet heating step A method for producing a martensitic high Cr steel seamless steel pipe, characterized in that the heating temperature is 1180 ° C. or higher and Ac ₄ points or lower and the heating and holding time is 100 to 200 minutes. The maximum attainment temperature of the slab which is a material to be heated in the slab heating step and the round billet which is a material to be heated in the round billet heating step is set to Ac ₄ points or less. Martensitic high Cr steel seamless steel pipe manufacturing method.

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