JP2006183133A - Method for producing steel sheet for high strength steam piping having excellent weld heat affected zone toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a steel sheet having excellent weld heat affected zone toughness which has a yield strength of ≥550 MPa in a medium temperature region, and is suitable for a high strength welded steel pipe for steam piping. <P>SOLUTION: A steel having a composition comprising, by mass, 0.05 to 0.09% C, 0.05 to 0.20% Si, 1.5 to 2.0% Mn, ≤0.020% P, ≤0.002% S, 0.05 to 0.3% Mo, 0.005 to 0.05% Nb, 0.005 to 0.02% Ti and 0.01 to 0.04% Al, satisfying Ti/N of 2.0 to 4.0, and, if required, comprising one or more kinds selected from Cu, Ni, Cr, V, Ca and rare earth metals, and the balance Fe with inevitable impurities is heated at 1,000 to 1,200°C, is thereafter hot-rolled at a cumulative draft of ≥50% in ≤900°C and also at a rolling finishing temperature of ≤850°C, and is subsequently subjected to accelerated cooling to 400 to 550°C at a cooling rate of ≥5°C/s. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a steel sheet having a yield strength of 550 MPa or more in an intermediate temperature range, and relates to a method suitable for high strength and high toughness welded steel pipe use for steam piping.

  There are two methods for recovering oil sand from oil reservoirs buried in Canada, etc .: an open-pit method and a steam injection method in which high-temperature and high-pressure steam is inserted through a steel pipe. However, there are few areas where surface mining is applicable, and the steam injection method is applied in many areas.

  The steam fed into the oil layer by the steam injection method is in a temperature range of 300 to 350 ° C. (hereinafter referred to as an intermediate temperature range), and is fed at a high pressure of about 13 MPa. In recent years, there has been a demand for an increase in diameter and strength of a steel pipe for the purpose of improving the recovery rate of heavy oil accompanying the increase in energy demand and reducing the laying cost.

  Furthermore, when used in a cold region such as Canada, the ground part that is exposed to the outside air of −20 ° C. or lower is heated to 300 ° C. or higher during operation and is cooled to −20 ° C. or lower during non-operation. Excellent toughness is required as a steel pipe characteristic in the middle temperature range.

  Conventionally, as a steam transporting steel pipe for steam injection that can withstand this medium temperature and high pressure steam, a seamless pipe corresponding to APIX 65, 70 grade shown in Patent Document 1, Patent Document 2, and Patent Document 3 is used as a conventional technique. The maximum outer diameter of the steel pipe was 16 inches.

  In the prior art, it is difficult to further increase the diameter, and the high strength of APIX80 grade has not been obtained. On the other hand, there is Patent Document 4 regarding a manufacturing technique of a high-strength welded steel pipe capable of increasing the diameter.

However, the high temperature characteristics in the middle temperature range are not described, and it is doubtful whether it can be used as a steel pipe for steam piping. As described above, it has been difficult to stably produce a high-strength welded steel pipe for steam piping having a large diameter and strength of X80 grade or more and excellent in weld heat affected zone toughness.
Japanese Patent No. 1393876 Japanese Patent No. 1930910 JP 2000-290728 A JP 2000-355729 A

  The present invention is a low cost steel sheet that is required for high-strength welded steel pipes for steam transport of API grade X80 or higher, and that provides excellent weld heat affected zone toughness at a yield strength of 550 MPa or higher (API grade X80 or higher) even in the intermediate temperature range. It is intended to provide to. The weld heat affected zone toughness is intended for welding used for seam welding of steel pipes.

  The inventors of the present invention diligently studied the characteristics in the medium temperature range of the high-strength large-diameter welded pipe, appropriately selected the component composition and production conditions, and using a steel sheet having a strength of X80 grade or higher at room temperature and medium temperature range, In addition, the present inventors have found that a high-strength welded pipe having a strength of X80 grade or higher and a weld heat-affected zone having high toughness in the middle temperature range can be produced stably.

The present invention has been made by further studying the obtained knowledge, that is, the present invention,
1. In mass%, C: 0.05 to 0.09%, Si: 0.05 to 0.20%, Mn: 1.5 to 2.0%, P: 0.020% or less, S: 0.002 % Or less, Mo: 0. 05-0.3%, Nb: 0.005-0.05%, Ti: 0.005-0.02%, Al: 0.01-0.04%, N: 0.004-0.006% And the balance is made of Fe and inevitable impurities, and the steel having Ti / N of 2.0 to 4.0 is heated to 1000 to 1200 ° C., and the cumulative rolling reduction at 900 ° C. or less is 50% or more. And, after hot rolling at a rolling end temperature of 850 ° C. or lower, accelerated cooling to 400 to 550 ° C. at a cooling rate of 5 ° C./second or higher yield strength 550 MPa or more excellent in weld heat affected zone toughness Steel plate manufacturing method.

  2. Furthermore, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, V: 0.08% or less, Ca: 0.0005 to 0.004%, REM: 0.0005 The manufacturing method of the steel plate of yield strength 550 Mpa or more excellent in the weld heat affected zone toughness of 1 containing 1 type or 2 types or more among -0.01%.

  3.1 High-strength welding for steam transport with excellent weld heat affected zone toughness having a base material with a yield strength of 550 MPa or more, characterized in that the steel plate described in 3.1 or 2 is cold-formed into a tubular shape and the butt portion is welded Steel pipe manufacturing method.

  According to the present invention, it is possible to manufacture a high-strength welded steel pipe for steam transport having a large diameter, which has an yield strength of 550 MPa or more in an intermediate temperature range and excellent weld heat-affected zone toughness, which can achieve efficient operation. The original plate can be obtained stably, which is extremely useful for industry.

  The steel plate according to the present invention is manufactured by a thick plate mill or a hot rolling mill, is cold formed by UOE forming, press bend forming, roll forming, etc., and is welded and joined by a welding method such as submerged arc welding.・ Used as a steel pipe for transporting high-pressure steam.

Below, the reason for limitation of a component composition is shown.

C: 0.05-0.09%
C is an element necessary for ensuring the strength of the steel by solid solution strengthening and precipitation strengthening. In particular, the increase in the amount of solid solution C and the formation of precipitates are important for securing the strength in the intermediate temperature range. Addition of excess C exceeding 0.09% causes deterioration of the weld heat-affected zone toughness, so the upper limit of the addition amount was set to 0.09%.

  On the other hand, if it is less than 0.05%, it becomes difficult to ensure a predetermined strength at room temperature and in the middle temperature range, so the C content is set to 0.05 to 0.09%.

Si: 0.05-0.20%
Si is added for deoxidation. If it is less than 0.05%, a sufficient deoxidation effect cannot be obtained. On the other hand, if it exceeds 0.20%, the weld heat-affected zone toughness is deteriorated. Was 0.05 to 0.20%.

Mn: 1.5 to 2.0%
Mn is an element effective in improving the strength and toughness of steel. If the content is less than 1.5%, the effect is small, and if it exceeds 2.0%, the weld heat affected zone toughness deteriorates significantly. 1.5 to 2.0%.

P: 0.020% or less P is an impurity element and deteriorates toughness, so it is desirable to reduce it as much as possible. However, excessive P reduction causes an increase in manufacturing cost, so the content of P is 0.020% or less. It was.

S: 0.002% or less Since S is an impurity element and deteriorates toughness, it is desirable to reduce it as much as possible. In addition, even when Ca is added to control the form of MnS to CaS inclusions, in the case of X80 grade high strength materials, finely dispersed CaS inclusions can also cause toughness reduction. The S content was 0.002% or less.

Mo: 0.05-0.3%
Mo is one of the elements effective for increasing the strength by solid solution strengthening and precipitation strengthening of carbide, especially in the middle temperature range, but if it is less than 0.05%, the effect is small and sufficient strength is obtained. Absent. On the other hand, when the addition exceeds 0.3%, the effect is saturated and the weld heat affected zone toughness is deteriorated. Therefore, the addition amount of Mo is set to 0.05 to 0.3%.

Nb: 0.005 to 0.05%
Nb is a component necessary to refine the microstructure by imparting sufficient strength and toughness by suppressing the growth of crystal grains during slab heating and rolling. It is also a component that forms carbides and is necessary for securing strength in the middle temperature range. The effect is remarkable at 0.005% or more, and when it exceeds 0.05%, the effect is almost saturated and the weld heat affected zone toughness is deteriorated. Therefore, the Nb content is 0.005 to 0.05%. It was.

Ti: 0.005-0.02%
Ti forms TiN together with N and suppresses the growth of austenite grains in the high temperature range of the weld heat affected zone reaching 1350 ° C. or higher, and influence of welding heat in the low temperature range of −20 ° C. or lower and in the intermediate temperature range of 300 ° C. or higher. Effective for improving toughness. The effect is remarkable at 0.005% or more, and if it exceeds 0.02%, the precipitates become coarse and cause toughness deterioration. Therefore, the Ti content is set to 0.005 to 0.02%.

Al: 0.01-0.04%
Al is added as a deoxidizer and the effect is remarkable at 0.01% or more. If it exceeds 0.04%, the toughness is deteriorated, so the Al content is set to 0.01 to 0.04%. .

N: 0.004 to 0.006%
N forms TiN together with Ti and finely disperses in the high temperature region of the weld heat affected zone reaching 1350 ° C. or higher, thereby refining the old austenite grains in the weld heat affected zone, and the low temperature range at −20 ° C. or lower and This greatly contributes to the improvement of the toughness of the weld heat affected zone in the medium temperature range above 300 ° C.

  If less than 0.004%, the effect is not sufficient. If added over 0.006%, the prior austenite grains are coarsened along with the coarsening of precipitates, and the solute N increases, resulting in a weld heat affected zone. Since toughness deteriorates, the N content is set to 0.004 to 0.006%.

Ti / N: 2.0 to 4.0
By defining Ti / N within an appropriate range, TiN is finely dispersed, and refinement of prior austenite grains in the weld heat-affected zone is achieved, and a low temperature range of −20 ° C. or lower and a temperature of 300 ° C. or higher are achieved. The toughness of the weld heat affected zone in the middle temperature range is improved.

  When Ti / N is less than 2.0, the effect is not sufficient, and when it exceeds 4.0, the coarsening of the prior austenite grains accompanying the coarsening of the precipitate is caused, and the toughness of the heat affected zone is deteriorated. , Ti / N is defined to be 2.0 to 4.0.

  In the present invention, excellent characteristics can be obtained with the above component composition, but when further improving the characteristics, one or more of Cu, Ni, Cr, V, Ca, and REM are added.

Cu: 0.50% or less Cu is one of the elements effective for improving toughness and increasing strength. However, if Cu is added in excess of 0.50%, weldability is impaired. It was set to 0.50% or less.

Ni: 0.50% or less Ni is one of elements effective for improving toughness and increasing strength. However, if it exceeds 0.50%, the effect is saturated and the manufacturing cost is increased, so Ni is added. In the case, it was 0.50% or less.

Cr: 0.50% or less Cr is one of the elements effective in increasing the strength, but if added over 0.50%, the weldability is adversely affected, so when adding Cr, 0.50% It was as follows.

V: 0.08% or less V forms a composite precipitate with Ti and contributes to an increase in strength. However, if it exceeds 0.08%, the toughness of the weld heat affected zone deteriorates, so the V content is specified to be 0.08% or less.

Ca: 0.0005 to 0.0040%
Ca improves the toughness by controlling the form of sulfide inclusions, but its effect appears at 0.0005% or more, and when it exceeds 0.0030%, the effect is saturated, and conversely, the cleanliness is lowered and the toughness is reduced. When Ca is added, the content is made 0.0005 to 0.0040%.

REM: 0.0005 to 0.01%
REM is an effective element for controlling the form of sulfide in steel, and when added, it suppresses the generation of MnS harmful to toughness. The effect of S is significant at 0.0005% or more, and if added over 0.02%, the cleanliness decreases and the toughness deteriorates. Therefore, when REM is added, the content is set to 0.0005 to 0.01%. .

  Next, the reason for limiting the manufacturing method will be described.

Heating temperature: 1000-1200 ° C
In hot rolling, in order to sufficiently advance austenitization and solid solution of carbides and obtain sufficient strength at room temperature and medium temperature, the heating temperature of the steel slab needs to be 1000 ° C. or higher. On the other hand, when the heating temperature exceeds 1200 ° C., the austenite grain growth is remarkable and the toughness of the base material deteriorates. Therefore, the heating temperature is set to 1000 to 1200 ° C.

Cumulative rolling reduction at 900 ° C. or less ≧ 50% and rolling end temperature: 850 ° C. or less The austenite non-recrystallized region rises to about 900 ° C. by adding Nb. When rolling is performed cumulatively in a temperature range of 900 ° C. or lower, austenite grains are expanded to become fine grains in the plate thickness and width directions, and the dislocation density in the grains introduced by rolling increases.

  When the cumulative rolling reduction at 900 ° C. or less is 50% or more and the rolling end temperature is 850 ° C. or less, this effect is remarkably exhibited, and the strength, particularly the strength in the middle temperature range, is increased and the toughness is remarkably improved.

  When the cumulative rolling reduction at 900 ° C. or less is less than 50% or the rolling finish temperature exceeds 850 ° C., the austenite grains are not sufficiently refined and the dislocation density in the grains is small. Toughness deteriorates. Accordingly, the cumulative rolling reduction at 900 ° C. or less is 50% or more and the rolling end temperature is 850 ° C. or less.

Cooling rate of accelerated cooling: 5 ° C./s or more The steel sheet strength tends to increase as the cooling rate of accelerated cooling increases. When the cooling rate during accelerated cooling is less than 5 ° C./s, the transformation structure is transformed at a high temperature, and the recovery of dislocation proceeds during cooling, so that sufficient strength cannot be obtained at room temperature and in the middle temperature range. Accordingly, the cooling rate during accelerated cooling is set to 5 ° C./s or more.

Cooling stop temperature for accelerated cooling: 400-550 ° C
The strength of the steel sheet tends to increase as the cooling stop temperature for accelerated cooling decreases. However, when the cooling stop temperature for accelerated cooling exceeds 550 ° C, the growth of carbides is promoted and the amount of solute carbon is reduced. The strength, particularly sufficient strength in the middle temperature range cannot be obtained.

  On the other hand, when the cooling stop temperature is less than 400 ° C., the precipitation of the low temperature transformation product becomes remarkable and the toughness is deteriorated, and the strength in the middle temperature region is remarkably lowered due to the decomposition of the low temperature transformation product in the middle temperature region. Therefore, the cooling stop temperature for accelerated cooling is set to 400 to 550 ° C.

  In addition, although it does not specifically limit about the steel making method of steel, From a viewpoint of economical efficiency, it is desirable to cast the steel piece by the steelmaking process by a converter method, and the continuous casting process. The method of forming the steel pipe is preferably formed in the cold, and is formed by UOE forming, press bend forming, roll forming, or the like, and welded and joined by submerged arc welding or the like to produce a welded steel pipe. The heat treatment after the production of the steel pipe may be performed according to the desired characteristics and is not particularly defined.

  Using steels A to O having chemical components shown in Table 1, a steel plate (plate thickness of 15 to 25 mm) produced under the manufacturing conditions shown in Table 2 was subjected to seam welding after cold forming, and an outer diameter of 610 mm × tube thickness of 15 A steel pipe of ˜25 mm was produced.

  As steel sheet characteristics, tensile specimens were collected in a direction perpendicular to the rolling direction of the steel sheet, and the yield strength (unit MPa) at room temperature and 350 ° C. was determined. The API rectangular test was used for the tensile test at room temperature, and a round bar test piece having a diameter of 8.75 mm was used at 350 ° C., and the yield strength (unit MPa) at room temperature and 350 ° C. was 550 MPa or more.

  The weld heat-affected zone toughness of the steel pipe was evaluated by obtaining Charpy absorbed energy (J) by a Charpy impact test. The Charpy impact test specimens were 2 mm V notch full-size specimens, and three samples were taken from the center of the tube thickness with the longitudinal direction being the circumferential direction so that the notch position became the weld heat affected zone. The Charpy impact test was performed at a test temperature of -30 ° C and 350 ° C, and the average value of the three samples was evaluated.

  Table 2 shows the manufacturing conditions of the steel sheet and the obtained characteristics. The present invention steels (Nos. 1 to 12) that are within the scope of the present invention both in terms of chemical composition and steel plate production conditions have a yield strength (unit MPa) of 550 MPa or more at room temperature and 350 ° C. of the steel plate and steel pipe, and are good. The weld heat affected zone toughness is obtained.

  On the other hand, comparative steels (No. 13-17, No. 19-25) whose chemical composition or steel plate manufacturing conditions are outside the scope of the present invention have the strength at room temperature or 350 ° C. and / or the weld heat affected zone toughness. It was inferior to steel.

  In addition, the comparative steel 18 subjected to heat treatment (QT treatment) after pipe making was inferior in strength at 350 ° C. to the steel of the present invention.

Claims (3)

  In mass%, C: 0.05 to 0.09%, Si: 0.05 to 0.20%, Mn: 1.5 to 2.0%, P: 0.020% or less, S: 0.002 % Or less, Mo: 0. 05-0.3%, Nb: 0.005-0.05%, Ti: 0.005-0.02%, Al: 0.01-0.04%, N: 0.004-0.006% The balance is made of Fe and inevitable impurities, and the steel with Ti / N of 2.0 to 4.0 is heated to 1000 to 1200 ° C., and the cumulative rolling reduction at 900 ° C. or less is 50% or more, In addition, after hot rolling at a rolling end temperature of 850 ° C. or less, accelerated cooling to 400 to 550 ° C. at a cooling rate of 5 ° C./second or more is performed. A method of manufacturing a steel sheet.   Furthermore, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, V: 0.08% or less, Ca: 0.0005 to 0.004%, REM: 0.0005 The manufacturing method of the steel plate of yield strength 550 Mpa or more excellent in the weld heat affected zone toughness of Claim 1 containing 1 type or 2 types or more among -0.01%.   3. High strength welding for steam transport excellent in weld heat affected zone toughness having a base material with a yield strength of 550 MPa or more, wherein the steel sheet according to claim 1 or 2 is cold-formed into a tubular shape and its butt portion is welded. Steel pipe manufacturing method.