JP5206507B2 - High-strength steel material for hydraulic iron pipe, manufacturing method thereof, and hydraulic iron pipe - Google Patents

Description

  The present invention relates to a high-tensile steel material for a hydraulic iron pipe, a method for producing the same, and a hydraulic iron pipe. Specifically, it has excellent weld toughness and is excellent in “weld crack resistance” without causing weld cracking even when welding without preheating (without preheating) in a high temperature and high humidity environment. The present invention relates to a tensile steel material, a manufacturing method thereof, and a hydraulic iron pipe.

  The high-tensile steel material of the present invention is mainly intended for plate-like materials having a plate thickness exceeding 10 mm, that is, “thick steel plates”. For this reason, in the following description, it may be described as “thick steel plate” as an example of “steel material”.

  Moreover, as a strength class of the high-tensile steel material of the present invention, those having a tensile strength of 610 to 750 MPa and a yield strength of 490 MPa or more are targeted.

A thick steel plate used for a large structure is manufactured by heating a slab to an austenite temperature range, that is, Ac ₃ point or higher, rolling to a predetermined thickness, and cooling.

  The properties of the thick steel plate are determined by the steel composition, heating temperature condition, rolling condition, cooling condition, etc., and by adjusting these conditions as appropriate, it is possible to produce a high-performance thick steel sheet with high added value.

  The heating temperature in the production of thick steel plates has been generally performed at a relatively high temperature of about 1150 ° C. even in the austenite temperature range. This is to reduce the rolling load in the subsequent rolling process due to the softening action of the slab by high temperature heating.

  In the production of thick steel plates, a reduction in energy intensity is always required, but due to the recent increase in the price of energy resources, a further reduction in energy intensity has come to be required. In addition, in recent years, due to environmental considerations, there is a need for a technique for producing steel sheets without producing greenhouse gases such as carbon dioxide as much as possible.

  In the manufacture of thick steel plates, it is preferable that the slab is heated and the temperature is made uniform up to the center of the slab as a heating step. For this reason, a large amount of energy is required in the heating process. Therefore, if the heating temperature is lowered, for example, the heating temperature is less than 1000 ° C. and a thick steel plate can be manufactured, the above-described requirements can be satisfied.

  The manufacturing method of a thick steel plate is disclosed by patent documents 1-3, for example.

That is, Patent Document 1 discloses an invention that includes an example in which the heating temperature is defined as Ac ₃ point or higher and heated at a temperature of less than 1000 ° C.

Further, Patent Document 2 discloses an invention including an example in which the heating temperature is defined as Ac ₃ transformation point or higher and 1200 ° C. or lower and heated at 950 ° C.

  Further, Patent Document 3 discloses an invention including an example in which the heating temperature is defined as 950 ° C. or higher and the heating is performed at 975 ° C.

  However, the techniques disclosed in these Patent Documents 1 to 3 are positively applied at a low heating temperature of less than 1000 ° C., as is clear from the fact that there are many descriptions of heating temperatures of 1000 ° C. or more in the examples. It is not a technology for manufacturing thick steel plates.

  On the other hand, steels or steel plates that are particularly useful for hydraulic iron pipes targeted by the present invention are disclosed in, for example, Patent Documents 4 to 6.

  That is, Patent Document 4 and Patent Document 5 disclose a method for producing a 60 kg class high-strength steel in which steel having a specific chemical composition is heated to 950 to 1250 ° C., rolled and cooled under specific conditions. .

  However, the techniques disclosed in these patent documents are positive as it is clear from the fact that all of the “examples of the present invention” described in the examples are manufactured at a heating temperature of 1000 ° C. or higher. In particular, this is not a technique for producing a thick steel plate at a heating temperature of less than 1000 ° C.

In Patent Document 6, after casting the steel, without cooling below Ar ₃ transformation point, or reheated to Ar ₃ transformation point or higher, rolling under certain conditions, method for producing a high tensile steel plate for cooling It is disclosed.

  However, as is clear from the technique disclosed in Patent Document 6, all of the “examples of the present invention” described in the examples are manufactured at a heating temperature of 1000 ° C. or higher. This is not a technique for actively producing a thick steel plate at a heating temperature of less than 1000 ° C.

JP-A-6-299237 JP-A-8-60239 JP 2004-2934 A JP 2001-64725 A JP 2001-64728 A JP 2006-45672 A

  High-tensile steel plates are often used in structures that support enormous force, so once a breakdown occurs, it will develop into a major accident that affects citizens' lives. Since this fracture often occurs in the weld zone, excellent “weld zone toughness” is required for the high-tensile steel plate.

  Among the high-tensile steel plates, in particular, in the case of the high-tensile steel plates used for the hydraulic iron pipes of pumped hydropower plants, in addition to excellent weld toughness, good “weldability ( Weld weldability) ”.

  That is, since the on-site welding of a hydraulic iron pipe is performed in an environment where high temperature and humidity are likely to cause weld cracking, it is necessary to perform welding at a preheating temperature higher than usual. However, preheating at a high temperature not only increases the welding cost but also increases the work period, and increases the burden on workers on site.

  For this reason, it has excellent weld toughness for hydraulic iron pipes, and it can be welded on-site without causing cracks even in high-temperature and high-humidity environments without preheating. There is a growing demand for technology that can supply steel sheets.

  On the other hand, as described above, a technique for manufacturing a thick steel plate by heating a slab, which is a rolling material, in a low temperature range is required from the viewpoint of rising energy resource prices and prevention of greenhouse gas emissions.

  Therefore, the present invention is not only capable of manufacturing at a low cost while suppressing soaring energy costs, but also has high weldability in a high-temperature and high-humidity environment and also has excellent weld toughness, a method for manufacturing the same, and a hydraulic iron pipe The purpose is to provide.

In addition, the specific target of the high-tensile steel material for hydraulic iron pipes of the present invention is
<1> Tensile properties of the base material:
Tensile strength: 610 to 750 MPa (about 62 to about 76 kgf / mm ² or more),
Yield strength: 490 MPa or more (about 50 kgf / mm ² or more),
<2> Weld toughness:
-Absorbed energy at -20 ° C (vE-20): 47J or more,
<3> Weld crack resistance:
・ Weld cracks will not occur even if welding is done without preheating.
It is.

  However, the toughness described above is a V-notch specimen (hereinafter referred to as “2 mm V notch Charpy impact test”) having a width of 10 mm, a notch angle of 45 °, a notch depth of 2 mm, and a notch bottom radius of 0.25 mm as defined in JIS Z 2242 (2005). It refers to a value obtained by a Charpy impact test using a piece.

  In order to solve the above-described problems, the present inventors first considered lowering the slab heating temperature from the viewpoint of reducing the energy intensity and suppressing the generation of greenhouse gases.

  That is, as already described, the slab is usually heated at about 1150 ° C. If this temperature is lowered, for example, the amount of heating gas introduced into the heating furnace such as coke oven gas (C gas) can be reduced. Naturally it can be reduced.

  In particular, if the slab heating temperature is suppressed to a low temperature of less than 1000 ° C., the amount of gas used can be reduced, and the generation of greenhouse gases can be suppressed at the same time.

  However, when a thick steel plate manufactured by heating to less than 1000 ° C. was welded, the toughness of the weld heat affected zone (hereinafter referred to as “HAZ”) decreased. Therefore, various investigations were made on this cause, and the following findings (a) and (b) were obtained.

  (A) When a thick steel plate is welded, the “melting line part” which is a part of the HAZ is heated to 1350 ° C. or higher and temporarily becomes a molten state. And when this solidifies, Nb carbide precipitates. Since this Nb carbide is a very hard precipitate, the toughness is reduced by the notch effect starting from the Nb carbide. Therefore, if the Nb content is reduced, Nb carbide is not formed, and toughness reduction can be prevented.

  (B) On the other hand, Nb contributes to an increase in the strength of the thick steel plate, so reducing the Nb content decreases the strength of the thick steel plate, but other elements such as Cu, Ni, Cr, Mo, V, B, etc. By containing a specific amount, the strength of the thick steel plate can be supplemented.

  The present invention has been completed based on the above findings, and the gist thereof is as follows. (1) to (3) High tensile steel materials for hydraulic iron pipes, (4) and for hydraulic iron pipes shown in (5) It exists in the manufacturing method of high-tensile steel materials, and the hydraulic iron pipe shown in (6).

  (1) By mass%, C: 0.04 to 0.09%, Si: 0.05 to 0.6%, Mn: 1.0 to 1.8%, P: 0.02% or less, S: 0.01% or less, Nb: less than 0.005%, Al: 0.002 to 0.07%, N: 0.001 to 0.005%, with the balance being Fe and impurities. A high-tensile steel material for a hydraulic iron pipe manufactured by hot working after heating to a temperature of 850 ° C. or higher and lower than 1000 ° C.

  (2) The chemical composition is mass%, and Cu: 2.0% or less, Ni: 4.0% or less, Cr: 2.0% or less, Mo: 1.0% or less, V: 0.5 % Or less, Ti: 0.05% or less, and B: One or more elements selected from 0.003% or less, and for a hydraulic iron pipe according to (1) above High tensile steel.

  (3) The chemical composition is mass% and further contains one or more elements selected from Ca: 0.02% or less, Mg: 0.02% or less, and REM: 0.02% or less. The high-tensile steel material for hydraulic iron pipes according to (1) or (2) above,

(4) A method for producing a high-tensile steel material for hydraulic iron pipes according to any one of (1) to (3) above, wherein hot working after heating to a temperature of 850 ° C. or higher and lower than 1000 ° C. is performed (Ar ₃ After ending at a temperature of point -30 ° C or higher, cooling is started at an average cooling rate of 5 to 80 ° C / s from a temperature of 700 ° C or higher and stopped at a temperature of 500 ° C or lower. The manufacturing method of the high strength steel materials for the hydraulic iron pipe.

  (5) The method for producing a high-tensile steel material for a hydraulic iron pipe according to (4), wherein the cooling is stopped at a temperature of 500 ° C. or lower, and then tempering is further performed at a temperature of 300 to 700 ° C.

  (6) A hydraulic iron pipe manufactured using the high-tensile steel material for a hydraulic iron pipe according to any one of (1) to (3) above.

  In the present invention, “REM” is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of one or more elements of REM.

  The “impurities” in the remaining “Fe and impurities” refers to those that are mixed due to various factors in the manufacturing process, including raw materials such as ore or scrap, when industrially producing steel materials. .

  Moreover, heating temperature points out the temperature in the plate | board thickness center part ((1/2) part of plate | board thickness t) of a hot work material. Since there is no means for directly measuring the temperature at the center of the plate thickness, the calculated temperature management based on the heat transfer calculation may be performed and the calculated temperature may be adopted as the temperature at the center of the plate thickness, that is, the heating temperature.

  The temperature at which hot working is terminated refers to the temperature at the surface of the hot work material. The temperature at which cooling starts and stops also refers to the temperature at the surface of the steel material that has been hot-worked. And the average cooling rate prescribed | regulated by this invention points out the value calculated | required from the surface temperature of the steel materials which completed hot working.

Further, the Ar ₃ point is the content of C, Mn, Cu, Cr, Ni, and Mo in mass% of each element, and t is the thickness after the end of hot working in mm units of steel ( Hereinafter referred to as “finish thickness”)
[910-310 × C-80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo + 0.35 × (t−8)]
The value obtained by the formula of

  The high-strength steel material for hydraulic iron pipes of the present invention is easy to manufacture on an industrial scale at a low cost while suppressing soaring energy costs, and has excellent weld toughness and weldability in high-temperature and high-humidity environments. Since welding cracks do not occur even when welding without preheating, it is possible to reduce welding construction costs, shorten the construction period, and further reduce the burden on workers on site. For this reason, the high-tensile steel material for hydraulic iron pipes of the present invention can be used very suitably as a raw material for hydraulic iron pipes of a pumped-up hydroelectric power plant. Furthermore, since the energy consumption at the time of manufacture of this high tensile steel material for hydraulic iron pipes may be small, the effect that emission of greenhouse gases, such as a carbon dioxide, can be suppressed is also acquired. This high-tensile steel material for a hydraulic iron pipe can be manufactured by the method of the present invention.

  Hereinafter, each requirement of the present invention will be described in detail. In the following description, “%” of the content of each element means “mass%”.

(A) About chemical composition:
C: 0.04 to 0.09%
C is added to ensure the strength of the steel sheet. If the C content is less than 0.04%, the hardenability is insufficient, it is difficult to ensure a tensile strength of 610 MPa or more, and the toughness is not sufficient. On the other hand, if it exceeds 0.09%, not only the toughness of the base material is lowered, but also the hardness of the HAZ is increased and the weld cracking sensitivity is increased. Therefore, the content of C is set to 0.04 to 0.09%. The C content is preferably 0.05% or more and less than 0.07%.

Si: 0.05-0.6%
Si has a deoxidizing action and a strength improving action. However, if the Si content is less than 0.05%, it is difficult to ensure such effects. On the other hand, if it exceeds 0.6%, the toughness of the base material and HAZ is reduced. Therefore, the Si content is set to 0.05 to 0.6%. The desirable lower limit of the Si content is 0.1%, and the desirable upper limit is 0.45%.

Mn: 1.0 to 1.8%
Mn is added to improve the hardenability of the steel sheet and increase the strength. However, if the content is less than 1.0%, it is difficult to ensure the strength. On the other hand, if it exceeds 1.8%, the toughness of both the base material and the HAZ is lowered. Therefore, the Mn content is set to 1.0 to 1.8%. The desirable lower limit of the Mn content is 1.2%, and the desirable upper limit is 1.7%.

P: 0.02% or less P is unavoidably present in steel as an impurity. If the P content exceeds 0.02%, not only does it segregate at the grain boundaries to lower the toughness, but it also causes cracking during welding. Therefore, the content of P is set to 0.02% or less. A desirable upper limit of the P content is 0.015%.

S: 0.01% or less If the content of S is too large, center segregation is promoted or a large amount of stretched MnS is generated, so that the mechanical properties of the base material and the HAZ deteriorate. Therefore, the S content is set to 0.01% or less. A desirable upper limit of the S content is 0.005%. The lower the content of S, the better. Therefore, the lower limit is not particularly specified.

Nb: less than 0.005% Nb contributes to the strength increase of the base metal and the fine grain of the structure, but during welding, it precipitates as Nb carbide in the melted line part, and due to the notch effect The toughness of the weld is reduced. Therefore, the Nb content is set to less than 0.005% in order to prevent precipitation of Nb carbide during welding and to suppress a decrease in toughness of the welded portion. The preferred Nb content is less than 0.003%.

Al: 0.002 to 0.07%
Al has a deoxidizing action. Al combines with N to form AlN, and has the effect of making the austenite grains fine. In order to obtain these effects, the Al content needs to be 0.002% or more. On the other hand, if the Al content exceeds 0.07%, coarse cluster-like alumina inclusion particles are likely to be formed, so that the toughness tends to deteriorate particularly in HAZ. Therefore, the content of Al is set to 0.002 to 0.07%. A desirable lower limit of the Al content is 0.010%, and a desirable upper limit is 0.05%. In the present invention, Al means so-called “sol.Al (acid-soluble Al)”.

N: 0.001 to 0.005%
N has a function of forming nitrides in combination with Al and Ti to refine austenite grains, so it is necessary to contain N 0.001% or more. However, when the N content increases and exceeds 0.005%, the toughness of the base material and the HAZ is significantly reduced. Therefore, the N content is set to 0.001 to 0.005%.

  One of the high-tensile steel materials for a hydraulic iron pipe of the present invention has a chemical composition in which the balance is composed of Fe and impurities in addition to the above elements. In addition, as already stated, the “impurities” in the “Fe and impurities” as the balance means that when manufacturing steel materials industrially, starting from raw materials such as ore or scrap, This refers to those that are mixed in due to the above factors.

  Another one of the high-tensile steel materials for hydraulic iron pipes according to the present invention is one selected from Cu, Ni, Cr, Mo, V, Ti, B, Ca, Mg, and REM in addition to the above elements. It has a chemical composition containing the above elements. Hereinafter, the effect of these arbitrary elements and the reason for limiting the content will be described.

  Cu, Ni, Cr, Mo, V, Ti, and B have an effect of improving hardenability. For this reason, when it is desired to ensure greater hardenability, these elements may be contained. Hereinafter, the above Cu, Ni, Cr, Mo, V, Ti, and B will be described in detail.

Cu: 2.0% or less Since Cu has an effect of improving hardenability, Cu may be contained for improving hardenability. However, when the Cu content exceeds 2.0%, not only the toughness of the base material and the HAZ is impaired, but also the hot ductility is greatly reduced. Therefore, the content when Cu is contained is set to 2.0% or less. Note that the Cu content is preferably less than 0.7%, and more preferably less than 0.5%.

  On the other hand, in order to surely obtain the effect of improving the hardenability of Cu described above, the lower limit of the Cu content is preferably 0.02%, and more preferably 0.05%.

Ni: 4.0% or less Ni has an effect of improving hardenability. Ni also has the effect of increasing the toughness and weldability of the high-tensile steel material for hydraulic iron pipes. Therefore, Ni may be contained to obtain the above effect. However, if the Ni content exceeds 4.0%, the cost for improving the effect is small for the cost increase, which is uneconomical. Therefore, the content when Ni is contained is set to 4.0% or less. Note that the Ni content is preferably less than 3.2%, and more preferably less than 2.5%.

  On the other hand, in order to surely obtain the effect of Ni described above, the lower limit of the Ni content is preferably 0.03%, and more preferably 0.05%.

Cr: 2.0% or less Cr increases the hardenability and precipitates during tempering to improve the strength and toughness. Therefore, in order to acquire said effect, you may contain Cr. However, if the Cr content exceeds 2.0%, the strength is excessively increased and the toughness of the base material and the HAZ is impaired. Therefore, the content when Cr is contained is set to 2.0% or less. Note that the Cr content is preferably 1.0% or less, and more preferably 0.5% or less.

  On the other hand, in order to reliably obtain the effect of Cr, the lower limit of the Cr content is preferably 0.05%, and more preferably 0.10%.

Mo: 1.0% or less Mo increases the hardenability, and precipitates during tempering to improve the strength and toughness. Therefore, you may contain Mo in order to acquire said effect. Note that Mo has a larger hardenability improving effect and precipitation hardening effect than Cr compared with the same amount, and particularly when coexisting with B, the hardenability improving effect appears remarkably. However, if the Mo content exceeds 1.0%, “baking” is excessively entered in the surface layer portion and is hardened greatly, and the toughness of the surface layer portion is deteriorated. Therefore, the content when Mo is contained is set to 1.0% or less. Note that the Mo content is preferably 0.7% or less, and more preferably 0.5% or less.

  On the other hand, in order to reliably obtain the effect of Mo described above, the lower limit of the Mo content is preferably 0.05%, and more preferably 0.07%.

V: 0.5% or less V has an effect of improving hardenability. V also precipitates during tempering, increases the temper softening resistance by precipitation hardening, enables tempering at high temperatures, and has the effect of improving the balance between strength and toughness. Therefore, you may contain V in order to acquire said effect. However, when the V content exceeds 0.5%, the base material and the HAZ toughness are significantly deteriorated. Therefore, the content when V is contained is set to 0.5% or less. Note that the V content is preferably 0.4% or less, and more preferably 0.3% or less.

  On the other hand, in order to surely obtain the effect of V described above, the lower limit of the V content is preferably 0.005%, and more preferably 0.01%.

Ti: 0.05% or less Ti has an effect of improving hardenability. Further, Ti has a deoxidizing action, and the structure can be refined by forming an oxide phase composed of Al, Ti, and Mn. Therefore, Ti may be contained to obtain the above effect. However, if the Ti content exceeds 0.05%, the oxide formed will be Ti oxide or Ti-Al oxide, and the dispersion density will decrease, especially in the HAZ when small heat input welding is performed. The effect of refining the tissue is lost. Therefore, the content when Ti is contained is set to 0.05% or less. The Ti content is preferably 0.04% or less, and more preferably 0.03% or less.

  On the other hand, in order to reliably obtain the effect of Ti, the lower limit of the Ti content is preferably 0.005%, and more preferably 0.010%.

B: 0.003% or less B has an effect of improving hardenability and increasing strength. Therefore, you may contain B in order to acquire said effect. However, if the B content exceeds 0.003%, the effect of increasing the strength based on the improvement in hardenability is saturated, and the tendency of deterioration of toughness becomes remarkable in both the base material and HAZ. Therefore, the content when B is contained is set to 0.003% or less. The B content is preferably 0.0025% or less, and more preferably 0.0020% or less.

  On the other hand, in order to surely obtain the effect of B described above, the lower limit of the B content is preferably 0.0005%, and more preferably 0.0007%.

  In addition, said Cu, Ni, Cr, Mo, V, Ti, and B can be contained only in any 1 type in them, or 2 or more types of composites. The total content of these elements is preferably 5.0% or less, and more preferably 3.0% or less.

  Ca, Mg, and REM have an effect of improving hot workability. For this reason, when it is desired to ensure greater hot workability, these elements may be contained. Hereinafter, the above Ca, Mg, and REM will be described in detail.

Ca: 0.02% or less Ca has an effect of improving hot workability. In addition, the acid and sulfide (oxysulfide) which Ca reacts with S in steel and forms in molten steel, unlike MnS etc., may extend in the rolling direction by rolling which is one form of hot working. In addition, since it is spherical after rolling, it has the effect of suppressing welding cracks and hydrogen-induced cracks starting from the ends of the stretched inclusions. Therefore, Ca may be contained to obtain the above effect. However, if the Ca content exceeds 0.02%, the toughness may be deteriorated. Therefore, the content when Ca is contained is set to 0.02% or less. The Ca content is preferably 0.01% or less, and more preferably 0.008% or less.

  On the other hand, in order to reliably obtain the effect of Ca described above, the lower limit of the Ca content is preferably 0.0010%, and more preferably 0.0020%.

Mg: 0.02% or less Mg has an effect of improving hot workability. Mg also has the effect of finely dispersing TiN by generating Mg-containing oxides and forming TiN generation nuclei. Therefore, Mg may be contained to obtain the above effect. However, if the Mg content exceeds 0.02%, the amount of oxide becomes excessive, resulting in a decrease in ductility. Therefore, the content when Mg is contained is set to 0.02% or less. Note that the Mg content is preferably 0.015% or less, and more preferably 0.01% or less.

  On the other hand, in order to reliably obtain the above-described effect of Mg, the lower limit of the Mg content is preferably 0.002%, and more preferably 0.003%.

REM: 0.02% or less REM has an effect of improving hot workability. REM also has the effect of refining the HAZ structure. Therefore, you may contain REM in order to acquire said effect. However, if the content of REM increases, it becomes inclusions and reduces cleanliness. However, inclusions formed by the addition of REM have a relatively small influence on deterioration of toughness. If it exists, even if it contains REM, the fall of the toughness of a base material is accept | permitted. Therefore, the content when REM is contained is set to 0.02% or less. Note that the REM content is preferably 0.01% or less, and more preferably 0.009% or less.

  On the other hand, in order to reliably obtain the effect of the REM described above, the lower limit of the REM content is preferably 0.002%, and more preferably 0.003%.

  As described above, “REM” in the present invention is a generic name for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of one or more elements of REM. .

  In addition, said Ca, Mg, and REM can be contained only in any 1 type in them, or 2 or more types of composites. The total content of these elements is preferably 0.05% or less, and more preferably 0.03% or less.

(B) Microstructure:
The high-strength steel material for hydraulic iron pipes of the present invention does not define a microstructure, but is preferably a structure mainly composed of bainite. The structure mainly composed of bainite means that a part of the other phase such as ferrite and pearlite may be used as long as the target characteristics of the high-tensile steel material of the present invention are satisfied.

  As shown in the next item (C), the heating temperature at the time of hot working is a low temperature of 850 ° C. or more and less than 1000 ° C. Therefore, in the case where the above-described bainite is a main structure, the bainite lath is used. The width does not grow and becomes 1.5 μm or less.

(C) About manufacturing conditions:
(C-1) Heating process:
The high-tensile steel material for a hydraulic iron pipe according to the present invention is a steel having the chemical composition described in the item (A), as a starting material, that is, a heated material to be heated to a temperature of 850 ° C. or more and less than 1000 ° C. It is manufactured by hot working.

  The reason why the hot work material is heated to 850 ° C. or more is to transform it into austenite to form a uniform structure. On the other hand, the heating temperature is less than 1000 ° C. for the purpose of reducing energy consumption and considering the natural environment. In addition, since a coarsening of austenite can be prevented by making heating temperature less than 1000 degreeC, toughness improves. The heating temperature is preferably less than 975 ° C, more preferably less than 950 ° C. The lower limit of the heating temperature is preferably 875 ° C, more preferably 900 ° C.

  In addition, in order to make temperature uniform to the center part of a hot work material, it is preferable that the heating time in the said temperature range shall be 2 hours or more. However, for the purpose of the present invention, the upper limit of the heating time is preferably about 9 hours.

  In addition, it is not necessary to prescribe | regulate conditions in particular about manufacture of the hot work material as a starting material of the high strength steel materials for hydraulic iron pipes concerning this invention.

  For example, in the case of “thick steel plate” as an example of “steel material”, a slab may be produced by a usual continuous casting method and used as a starting material. However, it is preferable that the slab has a certain size in advance.

  That is, when the amount of processing is large in the hot processing step, the processing end temperature for finishing into a desired shape at the time of hot processing is lowered, so that the slab is preferably thin. Specifically, it is preferable to use a slab having a slab thickness of 300 mm or less as a starting material. In addition, in order to manufacture such a thin slab, the thick slab may be thinly processed in advance to form a slab as a starting material of the present invention.

(C-2) Hot working process:
The hot work material as a starting material is heated under the conditions described in the above section (C-1) and then hot worked to finish it into a desired shape. In order to perform the reduction in the recrystallization region and make the structure finer, it is preferable to end at a temperature of (Ar ₃ point −30 ° C.) or higher. More preferably, the process is terminated at a temperature not lower than the Ar ₃ point. The hot working end temperature may be controlled by a normal method. However, as described in the section (C-1), since the heating temperature is relatively low, the hot work material as the starting material is heated. It is preferable to perform hot working as soon as possible after removal from the furnace.

(C-3) Cooling step:
After finishing the hot working step (C-2) above, cooling is started at an average cooling rate of 5 to 80 ° C./s from a temperature of 700 ° C. or higher, and is stopped at a temperature of 500 ° C. or lower. It is preferable.

  When the temperature at which cooling starts is below 700 ° C., the target value of tensile properties can be obtained, but the value will be low.

  When it is cooled at an average cooling rate of less than 5 ° C./s, it is not preferable because growth of crystal grains after completion of hot working cannot be suppressed, and the grain size of the steel material becomes large and causes a decrease in strength. . On the other hand, when the cooling is performed at an average cooling rate exceeding 80 ° C./s, “baking” is excessively performed, and the amount of the hard phase in the steel material is increased, resulting in deterioration of toughness.

  The cooling at an average cooling rate of 5 to 80 ° C./s or higher from the temperature of 700 ° C. or higher is preferably stopped at a temperature of 500 ° C. or lower. When cooling is stopped, reheating is observed in the steel material. However, if such cooling is performed, there will be no rise to a temperature that promotes the growth of crystal grains even if reheating occurs. Bond portion toughness is ensured, and therefore there is no problem as a steel material for a hydraulic iron pipe.

  Although the cooling stop temperature described above may exceed 500 ° C., in that case, although the target value of the weld zone toughness of the bond portion can be obtained, the value becomes low.

  After the cooling is stopped, an appropriate measure such as cooling to the room temperature in the atmosphere may be taken. Of course, you may cool to room temperature with said average cooling rate. Tempering may be performed by raising the temperature directly from the cooling stop temperature to the temperature described in the next item (C-4).

  Examples of the method for obtaining an average cooling rate of 5 to 80 ° C./s by cooling from a temperature of 700 ° C. or higher after finishing the hot working step include water cooling and mist cooling.

  In addition, after the hot working process of the above (C-2) section, before performing the cooling process of (C-3), a process such as descaling, distortion correction, and temperature equalization heating may be performed.

(C-4) Tempering step:
After finishing the cooling step of the above (C-3), in order to improve the balance between strength and toughness by removing strain caused by cooling and precipitating fine carbides, at a temperature of 300 to 700 ° C. Tempering may be performed.

  When the tempering temperature is less than 300 ° C., a sufficient tempering effect may not be obtained. On the other hand, when it exceeds 700 ° C., strength may be lowered. The holding time in the tempering may be appropriately determined in consideration of the balance between strength and toughness. As a standard of the tempering time, for example, a condition satisfying holding time (minutes) ≧ plate thickness (mm) / 2 can be cited.

  In addition, when the cooling stop temperature in the said (c-3) term is less than 300 degreeC, you may reheat to the temperature of 300-700 degreeC directly from the temperature, and may perform tempering, and after cooling stop, Tempering may be performed by reheating to a temperature of 300 to 700 ° C. after cooling to room temperature by taking the above means.

  On the other hand, when the cooling stop temperature in the above (c-3) is 300 ° C. or higher, after cooling is stopped, the cooling is performed from 300 ° C. to room temperature by taking appropriate measures, and then 300 to 700 ° C. It may be tempered by reheating to the temperature, or after cooling is stopped, for example, by taking measures such as “pyling”, “covering”, “charging in the heat-retaining furnace”, “500-300 ° C.” So-called “self-tempering” may be performed in the temperature range.

  The above manufacturing condition is one of methods for economically realizing the high-strength steel material for hydraulic iron pipes of the present invention on an industrial scale, and the technical scope of the high-tensile steel material for hydraulic iron pipes itself. Is not defined by this manufacturing condition.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Slabs were produced by melting and continuously casting steels 1 to 23 and x1 to x11 having chemical compositions shown in Tables 1 and 2 by an ordinary method.

Steels 1 to 33 in Tables 1 and 2 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels x1 to x11 in Table 2 are steels of comparative examples whose chemical compositions deviate from the conditions defined in the present invention. In Tables 1 and 2, the value of Ar ₃ point and the steel finish thickness in Table 3 described later are shown.

Using these various steel slabs, various steel materials having a finishing thickness of 30 to 100 mm were manufactured by hot rolling based on the manufacturing conditions shown in Table 3. Cooling after the completion of rolling was performed by water cooling, water cooling was stopped at the “cooling stop temperature” shown in Table 3, and then the mixture was allowed to cool to room temperature in the air. Tempering was performed by reheating from room temperature. In Table 3, the value of [Ar ₃ point-30 ° C.] is also shown.

  First, the microstructure of each steel material obtained as described above was examined.

  That is, the (1/4) part (hereinafter referred to as “(1/4) t part”) of the thickness t of each steel material so that a so-called “L cross section” that is a plane parallel to the rolling surface becomes the test surface. )), And then the specimen was embedded in resin and mirror-polished, then corroded by nital, and observed with an optical microscope at a magnification of 200 times. The microstructure was examined by image analysis. As a result, in any of the test numbers, bainite was a main structure, that is, 70% or more of the structure was bainite, and the balance was a structure such as ferrite. The lath width of bainite was 1.5 μm or less in all cases.

  Next, in order to investigate the properties of the obtained steel materials as the base material, the tensile properties as mechanical properties in the L direction were investigated.

  For tensile properties, tensile test specimens according to JIS Z 2201 (1998) were collected around (1/4) t part and subjected to tensile tests at room temperature according to the method described in JIS Z 2241 (1998). The yield strength (hereinafter referred to as “YS”) and tensile strength (hereinafter referred to as “TS”) were measured. YS was determined from the lower yield point when the tensile test was performed at a tensile test speed of 10 N / (mm · s), and when a clear yield point did not appear, YS was defined as 0.2% proof stress.

  In addition, the target value of the tensile property was TS: 610-750 MPa, YS: 490 MPa or more.

  For evaluation of weldability (weld cracking property) and weld zone toughness, a steel material centered on (1/4) t portion is cut out, and 3 to 8 layers on one side at a heat input of 45 kJ / cm in a lathe groove. A test piece subjected to submerged arc welding was prepared, and a 2 mmV notch Charpy impact test piece was collected from the test piece and subjected to a Charpy impact test at −20 ° C. to measure the absorbed energy.

  The 2 mm V notch Charpy impact test piece described above is obtained when the notch bottom is put in a so-called “bond part” that is an interface between the weld metal and the HAZ, and when it is put in a HAZ containing 3 mm from the bond part to the base metal side. Both were collected.

  The target of the Charpy impact property was that the toughness of the welded portion was excellent when “vE-20”, which is the absorbed energy at −20 ° C., was 47 J or more for any of the above test pieces, and this was satisfied.

In addition, according to the “y groove constrained cracking test method” described in JIS Z 3158 (1993), manual welding using a welding rod of about 598 MPa class (60 kgf / mm ² class) is performed to improve weld crack resistance. evaluated. Specifically, in consideration of on-site welding, a weld bead was placed free of preheating, and a cross-section was cut out after a predetermined time to investigate the presence or absence of cracks. The above-mentioned welding bead placement was carried out in an environment where the temperature was 30 ° C. and humidity 80% after the welding rod was left to stand for 1 hour in an environment of temperature 30 ° C. and humidity 80%.

  Table 4 shows the investigation results of the tensile properties, Charpy impact properties and weld crack resistance obtained as described above.

  From Table 4, steel materials hot-rolled after heating slabs having the chemical compositions defined in the present invention with test numbers 1 to 30, 32 and 33 under the temperature conditions defined in the present invention are targeted as high-tensile steel materials for hydraulic iron pipes. It is clear that the tensile properties, weld toughness and weld crack resistance of the base metal can be secured.

  Among the above test numbers, the steel materials of Test Nos. 1 to 30 that satisfy the end temperature of hot rolling and the cooling conditions after the end of rolling satisfy the conditions specified in the present invention have the cooling start temperature after the end of rolling in the present invention. Compared to test number 32 outside the specified conditions, YS is superior, and the cooling stop temperature after the end of rolling is higher in weld joint toughness than test number 33 out of the conditions specified in the present invention. It turns out that it is excellent.

  On the other hand, in the case of the steel materials of test numbers 34 to 44 using a slab that does not satisfy the chemical composition defined in the present invention, even if the slab is hot-rolled after heating under the temperature condition defined in the present invention, It is apparent that at least one of the above target characteristics as a high strength steel material for a hydraulic iron pipe has not been obtained.

  As shown in Test No. 31, even if a slab having the chemical composition defined in the present invention is heated to 1000 ° C. or higher to produce a high strength steel material for a hydraulic iron pipe, the target tensile properties of the base metal, welding Although it is possible to ensure toughness and weld crack resistance, it is not preferable from the viewpoint of energy resource prices and environmental conservation.

  The high-strength steel material for hydraulic iron pipes of the present invention is easy to manufacture on an industrial scale at a low cost while suppressing soaring energy costs, and has excellent weld toughness and weldability in high-temperature and high-humidity environments. Since welding cracks do not occur even when welding without preheating, it is possible to reduce welding construction costs, shorten the construction period, and further reduce the burden on workers on site. For this reason, the high-tensile steel material for hydraulic iron pipes of the present invention can be used very suitably as a raw material for hydraulic iron pipes of a pumped-up hydroelectric power plant. Furthermore, since the energy consumption at the time of manufacture of this high tensile steel material for hydraulic iron pipes may be small, the effect that emission of greenhouse gases, such as a carbon dioxide, can be suppressed is also acquired. This high-tensile steel material for a hydraulic iron pipe can be manufactured by the method of the present invention.

Claims (6)

  In mass%, C: 0.04 to 0.09%, Si: 0.05 to 0.6%, Mn: 1.0 to 1.8%, P: 0.02% or less, S: 0.01 % Or less, Nb: less than 0.005%, Al: 0.002 to 0.07%, N: 0.001 to 0.005%, and the balance having a chemical composition of Fe and impurities, 850 A high-strength steel material for a hydraulic iron pipe, manufactured by hot working after heating to a temperature of not lower than 1000 ° C and lower than 1000 ° C.   The chemical composition is mass%, and Cu: 2.0% or less, Ni: 4.0% or less, Cr: 2.0% or less, Mo: 1.0% or less, V: 0.5% or less, 2. The high-tensile steel material for a hydraulic iron pipe according to claim 1, comprising at least one element selected from Ti: 0.05% or less and B: 0.003% or less.   The chemical composition contains, in mass%, one or more elements selected from Ca: 0.02% or less, Mg: 0.02% or less, and REM: 0.02% or less. The high-tensile steel material for hydraulic iron pipes according to claim 1 or 2. A method of manufacturing a high-tensile steel for penstock according to any one of claims 1 to 3, the hot working after heating to a temperature below 850 ° C. or higher 1000 ° C. (Ar ₃ point -30 ° C.) or higher After finishing at a temperature of 700 ° C. or higher, cooling is started at an average cooling rate of 5 to 80 ° C./s, and cooling is stopped at a temperature of 500 ° C. or lower. Steel manufacturing method.   The method for producing a high-tensile steel material for a hydraulic iron pipe according to claim 4, further comprising tempering at a temperature of 300 to 700 ° C after the cooling is stopped at a temperature of 500 ° C or lower.   A hydraulic iron pipe manufactured using the high-tensile steel material for a hydraulic iron pipe according to any one of claims 1 to 3.