JP4904806B2 - Manufacturing method of high-strength, high-toughness steel sheet with excellent strength and deformability in the middle temperature range - Google Patents

Description

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

  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.

    There are two methods for recovering oil sand from the oil reservoir: the open-pit method and the steam injection method in which high-temperature and high-pressure steam is inserted through steel pipes. Has been.

In the steam injection method, steam in the temperature range of 300 to 350 ° C. (hereinafter, medium temperature range) is sent into the oil reservoir at a high pressure of about 13 MPa. Conventionally, steam transport for steam injection that can withstand this medium temperature and high pressure steam is used. As the steel pipe, a seamless pipe corresponding to API X65, 70 grade shown in Patent Document 1, Patent Document 2, and Patent Document 3 was used, and the maximum diameter of the steel pipe was 16 inches.
Japanese Patent No. 1393876 Japanese Patent No. 1930910 JP 2000-290728 A

  By the way, in recent years, for the purpose of improving the recovery rate of heavy oil accompanying the increase in energy demand and reducing the laying cost, it is required to increase the diameter and strength of steel pipes. Since the steel pipe may be deformed by an external force applied by this, excellent deformability is required as a steel pipe characteristic.

  Although the demand mentioned above is the same also about the high intensity | strength welded steel pipe for steam piping, sufficient response | compatibility is difficult with the steel pipe of patent documents 1-3.

  Therefore, the present invention is required for high-strength and high-toughness welded steel pipes for steam transport of API grade X80 or higher, and has high strength and high toughness with excellent deformability of yield strength of 550 MPa or higher (API grade X80 or higher) even in the intermediate temperature range. It aims at providing a steel plate at low cost.

  The inventors of the present invention diligently studied the characteristics in the middle temperature range of the high-strength large-diameter welded pipe and obtained the following knowledge.

1. Nb _eff. Is applied to the steel with 0.025% or more, accelerated cooling after controlled rolling and subsequent reheating, thereby performing reheating in the middle of bainite transformation, in addition to strengthening by bainite transformation during accelerated cooling, Precipitation strengthening due to fine precipitates precipitated from bainite and untransformed austenite and suppression of dislocation recovery in the intermediate temperature range can suppress a decrease in strength in the intermediate temperature range.
Nb _eff. (%) = 0.002 × (1-25Ti) / (C + 0.86N) (1)
Ti, C, N: mass%
When TiN is present, it becomes difficult for Nb to dissolve, and compared to the case where Ti is not added, the dispersion and precipitation of fine Nb carbides during reheating after accelerated cooling is reduced, and it is difficult to suppress the strength reduction in the middle temperature range. However, Nb _eff. When the value is 0.025 or more, even when Ti is added, fine dispersion of fine Nb carbide during reheating is sufficiently obtained, and it is possible to suppress a decrease in strength in the intermediate temperature range.

  2. Prior to dispersion precipitation of fine carbides by heating after accelerated cooling, in order to introduce a large amount of dislocations in the intragranular structure, the cumulative reduction ratio and rolling finish temperature below 900 ° C are specified, and both rolling and accelerated cooling processes are performed. By increasing the number of dislocations in the grains, it is possible to increase the dislocations by rolling and accelerated cooling, and to suppress the recovery of dislocations in the intermediate temperature range due to fine carbides that are dispersed and precipitated by heating after accelerated cooling. Can be secured.

  3. Furthermore, the chemical components are appropriately selected, the stop temperature of accelerated cooling is set to the initial stage of bainite transformation, and at the stage where a large amount of untransformed austenite is secured, the concentration of C to untransformed austenite is promoted by reheating, When a large amount of island martensite (hereinafter referred to as MA) is precipitated in the subsequent air-cooling stage, a composite structure mainly composed of bainite containing fine precipitates and MA is obtained, and uniform elongation is an index of deformability. It is possible to improve.

4). By heating at a speed higher than the heating rate in a conventional atmospheric furnace used industrially, the growth of carbides based on Nb is suppressed, and extremely fine precipitates having a particle size of less than 10 nm are reduced to 2 ×. When 10 ³ pieces / μm ³ or more are deposited, a high-strength steel sheet having a yield strength of 550 MPa or more can be obtained.

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1% by mass, C: 0.05 to 0.08%, Si: 0.05 to 0.2%, Mn: 1.6 to 2%, P: 0.02% or less, S: 0.002% Hereinafter, Mo: 0.05 to 0.3%, Nb: 0.03 to 0.07%, Ti: 0.02% or less, Al: 0.04% or less, REM: 0.015% or less, N: Nb _eff. Containing 0.006% or less, the balance being Fe and inevitable impurities, and represented by the formula (1) _. : After heating the steel which is 0.025% or more to 1100 to 1200 ° C, the hot rolling at 900 ° C or less is 50% or more and the rolling finish temperature is 850 ° C or less, 5 ° C / second After accelerated cooling to over 550 ° C. and below 650 ° C. at the above cooling rate, immediately reheating to 610-720 ° C. at a temperature rising rate of 0.5 ° C./s or more is performed. A method for producing a high-strength, high-toughness steel sheet having excellent strength and deformability.
Nb _eff. = 0.002 × (1-25Ti) / (C + 0.86N) (1)
Ti, C, N is mass%
2 Further, in terms of mass%, 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% The manufacturing method of the high intensity | strength high toughness steel plate excellent in the intensity | strength in the intermediate temperature range of 1 and the deformability which contain 1 type or 2 types among them.
3. A high-strength, high-toughness welded steel pipe excellent in strength and deformability in an intermediate temperature range, characterized in that the steel plate obtained by the method according to 1 or 2 is cold-formed into a tubular shape and its butt portion is welded Manufacturing method.

  According to the present invention, it is possible to manufacture a high-strength, high-toughness welded steel pipe for steam transport having a large diameter and having a yield strength of 550 MPa or more and excellent deformability at room temperature and medium temperature range, which can achieve operational efficiency. The original plate can be obtained stably, which is extremely useful for industry.

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

C: 0.05-0.08%
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.

  Further, it is important for concentrating C into untransformed austenite at the time of reheating after accelerated cooling and precipitating a large amount of MA at the subsequent air cooling, thereby ensuring sufficient uniform elongation at room temperature and in the middle temperature range.

  Addition of excess C exceeding 0.08% causes deterioration of toughness and weldability, so the upper limit of the addition amount was set to 0.08%.

  On the other hand, if it is less than 0.05%, the amount of C enriched in untransformed austenite at the time of reheating after accelerated cooling is small and the amount of MA decreases, making it difficult to ensure sufficient uniform elongation at room temperature and in the middle temperature range. Therefore, the C content is set to 0.05 to 0.08%.

Si: 0.05 to 0.2%
Si is added for deoxidation, and if it is less than 0.05%, a sufficient deoxidation effect cannot be obtained. On the other hand, if it exceeds 0.2%, the toughness is deteriorated, so the Si content is set to 0.05 to 0.2%.

Mn: 1.6-2%
Mn is an element effective for improving the strength and toughness of steel. If the content is less than 1.6%, the effect is small, and if it exceeds 2%, the toughness and weldability are significantly deteriorated. ˜2%.

P: 0.02% or less P is an impurity element and significantly 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 0.02%. It was as follows.

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

Mo: 0.05-0.3%
Mo greatly contributes to an increase in strength at room temperature and medium temperature by solid solution or formation of precipitates, but if it is less than 0.05%, the effect is small and sufficient strength cannot be obtained at room temperature and medium temperature. On the other hand, if adding over 0.3%, the toughness and weldability are deteriorated, so the addition amount of Mo was made 0.05 to 0.3%.

Nb: 0.03 to 0.07%
Nb is an important element in the present invention, and is a component that forms carbides and is required for securing strength at room temperature and in the middle temperature range. It is also necessary to refine the microstructure by suppressing the growth of crystal grains during slab heating and rolling, and to impart sufficient strength and toughness. The effect is remarkable at 0.03% or more, and when it exceeds 0.07%, the effect is almost saturated and the toughness is deteriorated. Therefore, the Nb content is set to 0.005 to 0.07%.

Ti: 0.02% or less Ti forms TiN and suppresses grain growth at the time of slab heating or welding heat affected zone, brings about refinement of the microstructure and improves toughness, but 0.02% If added over the range, the toughness is deteriorated, so the Ti content is set to 0.02% or less.

Al: 0.04% or less Al is added as a deoxidizer, but if it exceeds 0.04%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is 0.04% or less. did.

REM: 0.015% or less REM contributes to the improvement of toughness by suppressing the coarsening of the structure due to the formation of oxysulfides. However, if it exceeds 0.015%, the toughness deteriorates, so the REM content is 0.015%. It is defined below. Preferably it is 0.001 to 0.007%.

N: 0.006% or less 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 prior austenite grains in the weld heat affected zone and affecting the heat of welding. This greatly contributes to improving the toughness of the part. Addition exceeding 0.006% leads to deterioration of base metal toughness due to coarsening of precipitates and increase in solute N, and toughness deterioration of weld metal in steel pipes, so the N content is 0.006% or less. It was.

Nb _eff. : 0.025 or more Nb _eff. = 0.002 × (1-25Ti) / (C + 0.86N) (1)
However, Ti, C, and N are made into content (mass%) in steel.

This parameter equation is an important factor for making the steel having the above-mentioned composition range into a steel having excellent strength in the middle temperature range, and Nb _eff. When (%) is less than 0.025%, the amount of finely dispersed carbide that precipitates during reheating after cooling is small, and it is difficult to ensure strength, particularly strength in the middle temperature range, so Nb _eff. (%) Was 0.025% or more.

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

Cu: 0.5% or less Cu is one of the elements effective for improving toughness and increasing strength. However, when Cu is added in excess of 0.5%, weldability is impaired. 0.5% or less.

Ni: 0.5% or less Ni is one of elements effective in improving toughness and increasing strength. However, if it exceeds 0.5%, the effect is saturated and the manufacturing cost is increased, so Ni is added. In the case, it was 0.5% or less.

Cr: 0.5% or less Cr is one of the elements effective in increasing the strength, but if added over 0.5%, the weldability is adversely affected. Therefore, when Cr is added, 0.5% 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 when V is added, the content was made 0.08% or less.

Ca: 0.0005 to 0.004%
Ca improves the toughness by controlling the form of sulfide inclusions, but its effect appears at 0.0005% or more, and when it exceeds 0.004%, 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 0.004%.

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

Heating temperature: 1100-1200 ° C
At the time of hot rolling, the heating temperature of the steel slab is set to 1100 ° C. or more in order to sufficiently advance austenitization and solid solution of carbides and obtain sufficient strength at room temperature and in the middle temperature range.

  On the other hand, when the heating temperature exceeds 1200 ° C., the austenite grains grow remarkably and the base material toughness deteriorates, so the heating temperature was set to 1100 to 1200 ° C.

Cumulative rolling reduction at 900 ° C. or less ≧ 50% and rolling end temperature: 850 ° C. or less This process is an important production condition of the present invention. Rolling is performed cumulatively in a temperature range of 900 ° C. or less, and by setting the finishing temperature to 850 ° C. or less, austenite grains are expanded and become fine grains in the plate thickness and width directions, and grains introduced by rolling The dislocation density inside 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 end temperature exceeds 850 ° C., the austenite grains are not sufficiently refined, and the amount of increase in dislocations in the grains is small. Since strength and toughness deteriorate, the cumulative rolling reduction at 900 ° C. or lower is set to 50% or more and the rolling end temperature is set to 850 ° C. or lower.

Cooling rate of accelerated cooling: 5 ° C./second 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./second, pearlite is generated during cooling and MA is not generated, the uniform elongation is small at room temperature and medium temperature range, and the recovery of dislocation proceeds during cooling. Sufficient strength cannot be obtained at room temperature and medium temperature. Therefore, the cooling rate of accelerated cooling is set to 5 ° C./second or more.

Cooling stop temperature for accelerated cooling: 550 ° C. and below 650 ° C. As the cooling stop temperature for accelerated cooling decreases, the dislocation density increases and the steel sheet strength tends to increase, but the cooling stop temperature for accelerated cooling exceeds 650 ° C. In this case, a sufficient dislocation density cannot be obtained, and further, the growth of carbides is promoted, so that a sufficient strength, particularly in the middle temperature range, cannot be obtained.

  On the other hand, when the cooling stop temperature is 550 ° C. or lower, the bainite transformation proceeds and the amount of untransformed austenite decreases, so that the amount of carbon concentrated in untransformed austenite during reheating in the subsequent process decreases, and the subsequent air cooling process Since the amount of MA obtained from untransformed austenite enriched with carbon decreases, the uniform elongation decreases at room temperature and in the middle temperature range. For this reason, the cooling stop temperature of accelerated cooling is set to 550 ° C. and 650 ° C. or less.

Temperature increase rate after accelerated cooling: speed 0.5 ° C / second or more, reheating temperature: 610-720 ° C
This process is important in the present invention. Fine precipitates that contribute to strengthening at room temperature and medium temperature range are precipitated during reheating, and further, from untransformed austenite during reheating to ferrite transformation and accompanying untransformed austenite. By discharging carbon, untransformed austenite in which carbon is concentrated during air cooling after reheating is transformed into MA.

  When the reheating temperature is lower than 610 ° C., sufficient precipitation driving force cannot be obtained, and pearlite is generated, so that fine precipitates cannot be dispersed and MA cannot be obtained. I can't get like growth.

  On the other hand, if the temperature exceeds 720 ° C., the precipitates become coarse and sufficient strength cannot be obtained at room temperature and in the middle temperature range.

  Note that it is not necessary to set the temperature holding time at the reheating temperature. In addition, since precipitation proceeds with bainite transformation even in the cooling process after reheating, the cooling rate after reheating is basically air cooling.

  Heating rate after accelerated cooling specified in the present invention: A rate of 0.5 ° C./s or more is difficult to achieve in an atmospheric furnace depending on the plate thickness, and gas combustion that allows rapid heating of a steel plate as a heating device It is preferable to use a furnace or an induction heating device, and it is more preferable to install it on the transport line downstream of the cooling equipment for performing accelerated cooling.

  The induction heating device is particularly preferable because temperature control is easier than in a soaking furnace, the cost is relatively low, and the cooled steel sheet can be heated quickly.

  In addition, by arranging a plurality of induction heating devices continuously in series, even if the line speed and the type and dimensions of the steel plate are different, just set the number of induction heating devices to be energized and the supply power, The heating rate and reheating temperature can be freely controlled.

  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.

  In addition, the normal temperature strength of steel obtained by the manufacturing method according to the present invention is 550 MPa or more and 700 MPa or less.

  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 tensile test at room temperature was carried out in accordance with ISO 6892, using a full thickness API rectangular test piece. The tensile test at 350 ° C. was performed according to ISO 783, using a round bar test piece having a diameter of 8.75 mm.

  For the strength of the steel pipe, tensile test specimens were collected in the circumferential direction, and the yield strength at room temperature and 350 ° C. was obtained. The tensile test at room temperature was carried out in accordance with ISO 6892, using a full thickness API rectangular test piece.

  The tensile test at 350 ° C. was performed according to ISO 783, using a round bar test piece having a diameter of 8.75 mm.

  Regarding the deformability of the steel sheet, a tensile test piece having a diameter of 8.75 mm was taken in the rolling direction, and the test method was a tensile test in accordance with ISO 6892 at 350 ° C and ISO 783 at 350 ° C. Uniform elongation was determined. The uniform elongation was defined as the amount of elongation from the start of elastic deformation to the maximum load.

  Regarding the deformability of the steel pipe, a round bar test piece with a diameter of 6 mm collected in the direction of the pipe axis was collected, and the test method was a tensile test according to ISO 6892 at 350 ° C and ISO 783 at 350 ° C. The uniform elongation at was determined.

  The welded heat-affected zone Charpy test for steel pipes uses a steel pipe that has been seam welded at a SAW with a heat input of about 50 kJ / cm. Three full-size test pieces of 2 mmV notch were sampled in the circumferential direction from the side close to the head. The test method conformed to ISO 148, and the test temperature was -20 ° C and 350 ° C.

  The test at −20 ° C. was alcohol, and the test at 350 ° C. was maintained at a predetermined temperature using a heating furnace. The toughness value was evaluated by the average value (unit J) of the three Charpy absorbed energy. The test results of the steel plate and the steel pipe are also shown in Table 2.

  The yield strength (unit MPa) in the direction perpendicular to or circumferential to that at room temperature and 350 ° C. is 550 MPa or more, the uniform elongation in the rolling direction or the tube axis direction is 10% or more, and Charpy at −20 ° C. and 350 ° C. The case where the absorbed energy was 100 J or more was considered good.

  The present invention steels (1-12), which are within the scope of the present invention 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 steel plates and steel pipes and uniform elongation of 10% or more In addition, no influence on the dispersion precipitation and dislocation density of fine carbides in the bainite structure due to a large amount of MA precipitation is observed, and good weld heat affected zone toughness is obtained.

  On the other hand, comparative steels (13-17, 19-25) whose chemical composition or steel plate production conditions are outside the scope of the present invention have yield strength and / or uniform elongation at room temperature or 350 ° C. and / or weld heat affected zone toughness. However, it was inferior to the steel of the present invention.

  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 0.08%, Si: 0.05 to 0.2%, Mn: 1.6 to 2%, P: 0.02% or less, S: 0.002% or less , Mo: 0.05 to 0.3%, Nb: 0.03 to 0.07%, Ti: 0.02% or less, Al: 0.04% or less, REM: 0.015% or less, N: 0 0.006% or less, the balance being Fe and inevitable impurities, Nb _eff. : After heating the steel which is 0.025% or more to 1100 to 1200 ° C, the hot rolling at 900 ° C or less is 50% or more and the rolling finish temperature is 850 ° C or less, 5 ° C / second After accelerated cooling to over 550 ° C. and below 650 ° C. at the above cooling rate, immediately reheating to 610-720 ° C. at a temperature rising rate of 0.5 ° C./s or more is performed. A method for producing a high-strength, high-toughness steel sheet having excellent strength and deformability.
Nb _eff. = 0.002 × (1-25Ti) / (C + 0.86N) (1)
Ti, C, N is mass%   Furthermore, in mass%, 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% The manufacturing method of the high intensity | strength high toughness steel plate excellent in the intensity | strength in the intermediate temperature range of Claim 1, and the deformability which contains 1 type (s) or 2 or more types. A high-strength, high-toughness weld having excellent strength and deformability in a medium temperature range, characterized in that the steel plate obtained by the method according to claim 1 or 2 is cold-formed into a tubular shape and the butt portion is welded. Steel pipe manufacturing method.