JP5971415B2 - Manufacturing method of martensitic stainless hot-rolled steel strip for welded steel pipe for line pipe - Google Patents

Description

  The present invention relates to a 13% Cr hot rolled martensitic stainless steel sheet, and in particular, a gas containing a corrosive gas such as carbon dioxide. The present invention relates to a method for manufacturing a martensitic stainless hot-rolled steel strip for welded steel pipes suitable for line pipes used in gas fields and oil fields.

  In recent years, in order to cope with the rise in crude oil prices and the expected depletion of oil resources in the near future, deep oil fields that have not been excluded in the past, and development once abandoned The development of highly corrosive gas fields has become active worldwide. In such oil fields and gas fields, oil country pipes (Oil Country Tubular Goods) and steel pipes for line pipes are required to have excellent corrosion resistance.

  Conventionally, as such a steel pipe having excellent corrosion resistance, a dual phase stainless steel pipe has been mainly used. In particular, welded steel pipes have mainly been used for line pipes used for overland pipelines because they do not require heavy steel pipes that are as thick as seamless steel tubes. . In the late 1990s, martensitic stainless steels with lower cost and moderate corrosion resistance were developed and used as welded steel pipes for line pipes.

  For example, Patent Document 1 discloses a martensitic stainless hot-rolled steel strip excellent in manufacturability for use in the above-described welded steel pipes. In the technique described in Patent Document 1, in mass%, C: 0.02% or less, Si: 0.1 to 0.3%, Mn: 0.1 to 0.3%, Cr: 11 to 15%, Ni: 5 to 8%, Mo: Containing 1.5 to 3%, Al: 0.10% or less, N: 0.020% or less, adjusting the heat treatment conditions, the P value which is the amount of Ni in the gamma phase or austenite phase per unit volume% By setting 0.3 to 1.0, a hot-rolled steel sheet with a small degree of fluctuation of mechanical characteristics (excellent manufacturability) and excellent quality of material stability. It is supposed to be obtained.

  Patent Document 2 describes a method for producing a martensitic stainless steel welded pipe excellent in intergranular stress corrosion cracking resistance. In the technique described in Patent Document 2, the steel strip used as the material of the welded pipe is in mass%, C: less than 0.0200%, N: less than 0.0200%, Si: 1.0% or less, Mn: 2.0% or less, Cr: A martensitic stainless steel strip having a composition containing 10 to 14%, Ni: 3 to 8%, Mo: 1 to 4%, Al: 0.10% or less, V: 0.02 to 0.10%, Ca: 0.0005 to 0.010% As a raw material, continuously formed as an open pipe, but the end faces of the open pipe are butted together and welded to form a welded pipe, and then the seam welded part is heated to a seam welded part at a temperature in the range of 550 to 700 ° C. A seam welded portion with excellent intergranular stress corrosion cracking resistance is obtained by applying post weld heat treatment.

Japanese Patent No. 3800150 (Japanese Patent Laid-Open No. 2004-91812) JP 2011-89159 A

  However, in the technique described in Patent Document 1, a slab geometries defect occurs due to a creep phenomenon during high-temperature heating during hot rolling, leaving a problem in conveyance during rolling. It was. In addition, Patent Document 2 does not mention the conditions for hot rolling, and the manufacturability of the martensitic stainless hot-rolled steel strip having the composition described in Patent Document 2 remains unknown. There was a problem that the hot-rolled steel strip was not optimized as a material.

  The object of the present invention is to provide a method for producing a martensitic stainless hot-rolled steel strip that solves the problems of the prior art and is excellent in manufacturability that is inexpensive and can be stably produced. Here, “excellent manufacturability” means that there are few obstructive factors in the manufacturing process such as slab shape defects and edge cracking as described above. Note that the “martensitic stainless hot-rolled steel strip” targeted by the present invention has a yield strength of YS: 450 MPa or more after heat treatment including quenching and tempering, It shall mean a hot-rolled steel strip having excellent sulfide stress corrosion cracking resistance (SSC resistance) and carbon dioxide-corrosion resistance. If such a “martensitic stainless hot-rolled steel strip” is used as a raw material to produce a welded steel pipe, a high-strength welded steel pipe with a yield strength YS: API X65 to X80 of 450 MPa or more is obtained.

  In order to achieve the above-mentioned object, the present inventors first decided to optimize the composition of the hot-rolled steel strip for welded steel pipes based on the composition of the steel material for seamless steel pipes. Steel materials for seamless steel pipes have a composition containing a large amount of expensive Ni for the purpose of preventing the formation of delta ferrite in order to facilitate hot working at high temperatures. However, from the viewpoint of providing an inexpensive material, it is advantageous in terms of material cost in the hot-rolled steel strip for welded steel pipes to make the Ni content relatively low while avoiding the expensive Ni content.

  However, with a steel material with a low Ni content, if the heating temperature is as high as the high Ni composition, a defect of shape in the steel material due to the creep phenomenon will occur during heating, and it will be used for transportation, etc. It was discovered that there are manufacturing problems such as defects.

  Therefore, as a result of further studies, the present inventors have come up with the idea that the heating temperature T of the steel material is adjusted so as to satisfy a special relational expression related to the contents of Ni, Cr, and Mo. . As a result, it has been found that the manufacturing problems described above can be avoided, and that stable and excellent manufacturing is possible. In addition, the inventors adjusted the heating temperature T of the steel material so as to satisfy the above-described special relational expression, thereby preventing the occurrence of the above-described shape defect, the generation of δ ferrite, and the heat. It has been found that no decrease in hot workability is observed.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) When a steel material is heated and hot-rolled to be coiled into a hot-rolled steel strip, the steel material is mass%, C: less than 0.0150%, N: less than 0.0200%, Si: 0.05-1.0%, Mn: 0.1-2.0%, P: 0.03% or less, S: 0.010% or less, Al: 0.001-0.10%, Cr: 10-14%, Ni: 2.0-5.0%, Mo: 1.0-4.0 %, Ti: 0.03-0.15%, Nb: 0.10% or less, V: 0.10% or less, Zr: 0.10% or less, Hf: 0.20% or less, Ta: 0.20% or less Or it is set as the steel raw material which contains 2 or more types, and consists of remainder Fe and an unavoidable impurity, The heating temperature T of the said heating is represented by following Formula (1)
1050 ≤ T ≤ 60 (Ni-0.9Cr-1.1Mo) + 1720 (1)
(Where T: heating temperature (° C.), Ni, Cr, Mo: content of each element (mass%))
A method for producing a martensitic stainless hot-rolled steel strip for welded steel pipes for line pipes with a temperature satisfying the above.
(2) In (1), in addition to the above composition, the composition further contains, by mass%, one or more selected from Cu: 4% or less, Co: 4% or less, and W: 4% or less The manufacturing method of the martensitic stainless hot-rolled steel strip for welded steel pipes for line pipes.
(3) In (1) or (2), a method for producing a martensitic stainless hot-rolled steel strip for welded steel pipes for line pipes, wherein heat treatment comprising quenching and tempering is performed after the hot rolling.

  According to the present invention, martensite having a high yield strength of YS: 450 MPa or more, excellent resistance to sulfide stress corrosion cracking (SSC resistance) and carbon dioxide corrosion resistance, which is suitable for welded steel pipes for line pipes. Site-based stainless steel hot-rolled steel strip can be manufactured with high productivity with less troubles during manufacturing, and has a remarkable industrial effect.

  In the present invention, a steel material such as a slab is heated to a temperature T that satisfies the relational expressions related to its Ni, Cr, and Mo contents, and then hot-rolled and wound into a coil to form a hot-rolled steel strip.

After hot rolling, these hot-rolled steel sheet was cooled to room temperature, further subjected to Ac ₃ transformation point (Ac ₃ transformation point) or more quenching cooling with air or a cooling rate after reheating to a temperature, then, It is preferable to perform the tempering treatment at a temperature below the Ac ₁ transformation point.

  First, the reasons for limiting the composition of the steel material used in the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.

C: Less than 0.0150%
C is an element that dissolves in steel and contributes to an increase in steel strength. In order to obtain this effect, 0.0060% or more is preferable. However, if contained in a large amount, the weld heat affected zone HAZ is hardened, causing weld cracks or degrading the weld heat affected zone toughness. For this reason, in this invention, it is desirable to reduce as much as possible and limited to less than 0.0150%. In addition, Preferably it is 0.0100% or less.

N: Less than 0.0200%
N, like C, is an element that dissolves in steel and contributes to increasing the strength of the steel. In order to obtain this effect, 0.0060% or more is preferable. However, if a large amount is contained, the welded portion is hardened to cause a weld crack or to deteriorate the weld heat affected zone toughness. Also, N combines with Ti, Nb, Zr, V, Hf, Ta, etc. to form nitrides, thus forming carbides and suppressing formation of Cr-deficient layers Ti, Nb, Zr, V, Hf, Ta The amount will be substantially reduced. For this reason, it is desirable to reduce N as much as possible. Since the adverse effect of N described above is acceptable if it is less than 0.0200%, N is limited to less than 0.0200% in the present invention. In addition, Preferably it is 0.0100% or less.

Si: 0.05-1.0%
Si is an element that acts as a deoxidizer and contributes to an increase in strength by solid solution in steel. In order to acquire such an effect, 0.05% or more of content is required. However, Si is also a ferrite-forming element, and a large content exceeding 1.0% degrades the base metal and HAZ toughness. For this reason, Si was limited to 0.05 to 1.0%. In addition, Preferably it is 0.10 to 0.5%.

Mn: 0.1-2.0%
Mn is a solid solution that contributes to increasing the strength of the steel and is an austenite formation element that suppresses ferrite formation and improves the base metal and HAZ toughness. In order to ensure such an effect, the content of 0.1% or more is required. On the other hand, even if the content exceeds 2.0%, the effect is saturated. For this reason, Mn was limited to 0.1 to 2.0%. In addition, Preferably it is 0.3 to 1.0%.

P: 0.03% or less
P is an element that segregates at the grain boundary to lower the grain boundary strength and adversely affects the resistance to sulfide stress corrosion cracking. Therefore, it is preferable to reduce as much as possible, but 0.03% is acceptable. Therefore, P is limited to 0.03% or less. From the viewpoint of hot workability, it is preferably 0.02% or less.

S: 0.010% or less
S is an element that forms sulfides such as MnS and reduces workability. In the present invention, it is preferable to reduce as much as possible, but 0.010% is acceptable. For this reason, S was limited to 0.010% or less. In addition, Preferably it is 0.005% or less.

Al: 0.001 to 0.10%
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is necessary to contain 0.001% or more. However, the content exceeding 0.10% deteriorates toughness. For this reason, Al was limited to 0.001 to 0.10%. In addition, Preferably it is 0.010 to 0.060%.

Cr: 10-14%
Cr is a basic element for improving the corrosion resistance such as carbon dioxide gas corrosion resistance, pitting corrosion resistance, sulfide stress corrosion cracking resistance, etc. In the present invention, it is necessary to contain 10% or more. On the other hand, if the content exceeds 14%, a ferrite phase is likely to be generated, and a large amount of other alloy elements is required to stably secure a martensite structure, resulting in an increase in material cost. For this reason, in the present invention, Cr is limited to the range of 10 to 14%.

Ni: 2.0-5.0%
Ni is an element that contributes to increase in strength by solid solution, improves toughness, and improves resistance to carbon dioxide corrosion. Ni is an austenite-forming element and acts effectively to stably secure a martensite structure in a low carbon region. In order to obtain such an effect, a content of 2.0% or more is required. Preferably, it is 2.5% or more. On the other hand, if the content exceeds 5.0%, the material cost increases. For this reason, Ni was limited to the range of 2.0 to 5.0%. In addition, Preferably it is 4.5% or less. More preferably, it is less than 3%.
In addition, Ni content can be suppressed by adjusting the amount of other alloy elements and manufacturing conditions.

Mo: 1.0-4.0%
Mo is an element that improves the resistance to sulfide stress corrosion cracking and pitting corrosion. To obtain such an effect, it is necessary to contain 1.0% or more. On the other hand, if the content exceeds 4.0%, ferrite is easily generated and the effect of improving resistance to sulfide stress corrosion cracking is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Mo was limited to the range of 1.0 to 4.0%. In addition, Preferably it is 1.5 to 3.0%.

Ti: 0.03-0.15%
Ti is an element that combines with C or N to form carbides or nitrides, refines crystal grains, and improves strength and toughness. In order to obtain such an effect, the content of 0.03% or more is required. On the other hand, even if it is contained in a large amount exceeding 0.15%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Ti was limited to the range of 0.03-0.15%. In addition, Preferably it is 0.05 to 0.12%.

Nb: 0.10% or less, V: 0.10% or less, Zr: 0.10% or less, Hf: 0.20% or less, Ta: 0.20% or less
Nb, V, Zr, Hf, and Ta are all carbide-forming elements, have a precipitation strengthening function, and are selectively used for increasing strength. Contains 2 or more. In order to obtain such an effect, it is desirable to contain Nb: 0.02% or more, V: 0.02% or more, Zr: 0.03% or more, Hf: 0.03% or more, Ta: 0.03% or more. However, when Nb: 0.10%, V: 0.10%, Zr: 0.10%, Hf: 0.20%, Ta: 0.20% and a large amount are contained, toughness and weld crack resistance (weld crack resistance) decrease. For this reason, when it contains, it limits to Nb: 0.10% or less, V: 0.10% or less, Zr: 0.10% or less, Hf: 0.20% or less, Ta: 0.20% or less, respectively.

  The above components are basic components. In addition to this basic composition, one or two elements selected from Cu: 4% or less, Co: 4% or less, and W: 4% or less are selected as the selection element. The above can be contained.

  Cu, Co, and W are all elements that improve the corrosion resistance of carbon dioxide gas, or further improve the resistance to pitting corrosion, and can be selected from one or more as required. In order to acquire such an effect, it is desirable to contain Cu: 0.20% or more, Co: 0.20% or more, and W: 0.20% or more. On the other hand, even if Cu exceeds 4%, Co: 4% and W: 4%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, it is preferable to limit to Cu: 4% or less, Co: 4% or less, and W: 4% or less, respectively.

  The balance other than the components described above consists of Fe and inevitable impurities. As an inevitable impurity, O (oxygen): 0.010% or less is acceptable.

  In the present invention, the steel material having the above composition is heated and hot-rolled and wound into a coil shape to obtain a hot-rolled steel strip.

  The method for producing the steel material used in the present invention is not particularly limited, and the molten steel having the above composition is usually converted into a converter, an electric furnace, a vacuum melting furnace, etc. And a steel material such as a slab of a predetermined size by a known method such as continuous casting and ingot-making and bloomig method. It is preferable to do.

In the heating of steel materials, the following formula (1) is a special relational expression corresponding to the content of Ni, Cr, and Mo.
1050 ≤ T ≤ 60 (Ni-0.9Cr-1.1Mo) + 1720 (1)
(Where T: heating temperature (° C.), Ni, Cr, Mo: content of each element (mass%))
The heating temperature T (° C.) satisfies This makes it possible to stably produce a martensitic stainless hot-rolled steel strip without causing troubles during hot rolling such as defective shape of the steel material, precipitation of δ ferrite, and edge cracking.

  When the heating temperature T of the steel material exceeds the upper limit of the above formula (1), a shape defect occurs in the steel material due to the creep phenomenon, a defect occurs during transportation, and δ ferrite is likely to precipitate. , Hot workability decreases. On the other hand, when the heating temperature T is less than the lower limit of the above-described equation (1), deformation resistance becomes high and stable rolling becomes difficult. In addition, Preferably, the heating temperature T is 1100-1150 degreeC.

  The steel material heated to the heating temperature T satisfying the above equation (1) is hot-rolled and then cooled to the coiling temperature, wound in a coil to form a hot-rolled steel strip, to room temperature. To be cooled. The conditions for hot rolling other than the heating temperature are not particularly limited, and any hot rolling under normal conditions can be applied.

The hot-rolled steel strip cooled to room temperature after hot rolling is heated to 750 to 1000 ° C above the Ac ₃ transformation point and then cooled to 100 ° C or below at a cooling rate above air cooling, followed by Ac _A heat treatment comprising a tempering treatment tempered at 550 to 700 ° C. below the ₁ transformation point is performed. By this heat treatment, a structure mainly composed of a tempered martensite phase can be obtained, and the above-described desired high strength can be ensured. Here, the “main organization” means a case where the phase is 70% or more in area ratio.

  Note that the steel having the above composition can be made into a martensitic structure if it is cooled at a cooling rate equal to or higher than air cooling after hot rolling, so that the quenching treatment is omitted and the steel is cooled to room temperature after hot rolling. Thereafter, direct tempering treatment may be performed.

  Hereinafter, the present invention will be described in more detail based on examples.

  Molten steel having the composition shown in Table 1 was melted in a converter and made into a steel material (slab: wall thickness 265 mm) by a continuous casting method. The obtained slab was heated to the heating temperature shown in Table 2, and then hot-rolled to obtain a hot rolled steel strip having a thickness of 4.0 to 8.0 mm. In addition, the presence or absence of occurrence of troubles during hot rolling (steel material shape failure, hot workability degradation, etc.) was investigated and the manufacturability was evaluated.

  Next, a part of the obtained hot-rolled steel strip was subjected to quenching treatment and tempering treatment under the conditions shown in Table 2. In addition, when trouble occurred during the hot rolling, the quenching process and the tempering process were not performed.

Test materials were collected from the heat-treated hot-rolled steel strip and subjected to a tensile test and a corrosion resistance test. The test conditions were as follows.
(1) Tensile test In accordance with API 5LC regulations, API specimen specimens by API standard 5CT are collected from the test materials, tensile tests are performed, and tensile properties (yield strength YS, Tensile strength TS) was determined.
(2) Corrosion resistance test Corrosion specimens 3mm thick x 30mm wide x 40mm long are made from the test material by machining, and carbon dioxide corrosion test Tensile specimens were collected from the test materials in accordance with NACE-TM0177, Method A, and subjected to a sulfide stress corrosion cracking test.

The carbon dioxide gas corrosion test is performed by immersing a corrosion test piece in a 20% by mass NaCl aqueous solution (liquid temperature: 150 ° C, CO ₂ gas partial pressure: 3.0 MPa CO ₂ gas atmosphere) held in an autoclave. The immersion period was 168 hr). The test piece after the corrosion test was weighed, and the corrosion rate was calculated from the weight loss before and after the corrosion test. Moreover, about the test piece after a corrosion test, the presence or absence of pitting corrosion (observing pitting corrosion) was observed with a 10 times magnifier (magnifying glass). Corrosion rate: 0.1 mm / y or less and no pitting corrosion were evaluated as “good” as being excellent in carbon dioxide gas corrosion resistance. Other than that, the carbon dioxide gas corrosion resistance was inferior, and was evaluated as x.

The sulfide stress corrosion cracking test was carried out using a four-point bending test method in accordance with the EFC17 regulations and applying 90% YS stress and holding it for 720 hours. The case where it did not break after 720 hours passed was evaluated as “good” because it was excellent in resistance to sulfide stress corrosion cracking, and “x” otherwise. The test solution used was adjusted to pH 3.5 by adding CH ₃ COONa to (5.0 mass% NaCl + 0.5 mass% acetic acid) aqueous solution (liquid temperature: 24 ° C). %, (10% H ₂ S + 90% CO ₂ ) was conducted in an environment where a gas was passed.

  The obtained results are shown in Table 3. In addition, about the thing which had a trouble at the time of manufacture and was not able to obtain a test material, it did not test and described in Table 3 "not measured".

本発明例はいずれも、製造時のトラブル発生は認められず、製造性に優れ、所望の高強度と、耐炭酸ガス腐食性、耐硫化物応力腐食割れ性に優れた熱延鋼帯となっている。これに対し、本発明範囲を外れる比較例は、製造性に劣るか、あるいは耐炭酸ガス腐食性、耐硫化物応力腐食割れ性が低下している。

In all of the examples of the present invention, occurrence of trouble during production was not observed, and the hot rolled steel strip was excellent in manufacturability, desired high strength, and excellent in carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance. ing. On the other hand, the comparative example which is out of the scope of the present invention is inferior in manufacturability, or has poor carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance.

Claims (3)

When heating and hot rolling a steel material to make a hot rolled steel strip,
The steel material is, in mass%, C: less than 0.0150%, N: less than 0.0200%, Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.010% or less, Al: 0.001 -0.10%, Cr: 10-14%, Ni: 2.0-5.0 %, Mo: 1.5-3.0 %, Ti: 0.05-0.12 %, Nb: 0.10% or less, V: 0.10% or less, Zr: A steel material containing one or more selected from 0.10% or less, Hf: 0.20% or less, Ta: 0.20% or less, and having a composition comprising the balance Fe and inevitable impurities,
The manufacturing method of the martensitic stainless hot-rolled steel strip for welded steel pipes for line pipes which makes the heating temperature T of the said heating the temperature which satisfies the following (1) formula.
Record
1050 ≤ T ≤ 60 (Ni-0.9Cr-1.1Mo) + 1720 (1)
Where T: heating temperature (° C),
Ni, Cr, Mo: Content of each element (mass%)   The composition according to claim 1, further comprising one or more selected from Cu: 4% or less, Co: 4% or less, and W: 4% or less in addition to the composition. Manufacturing method of martensitic stainless hot rolled steel strip for welded steel pipes for line pipes.   The manufacturing method of the martensitic stainless hot-rolled steel strip for welded steel pipes for line pipes of Claim 1 or 2 which performs the heat processing which consists of a quenching process and a tempering process after the said hot rolling.