Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a steel member and a method of manufacturing the steel member. Specifically, the invention relates to a steel member produced through performing welding and post-welding heat treatment (PWHT) on a thick steel plate, and particularly relates to a steel member of which the thicknesswise central portion has high strength and high toughness even after high-temperature and long PWHT, and relates to a method of manufacturing the steel member.
• PWHT welding and post-welding heat treatment
• the steel plate is subjected to normalizing and/or quenching so as to have such properties including high strength. However, if the steel plate has a large thickness, the inside of the steel plate (particularly a thicknesswise central portion) is slowly cooled in the normalization or the quenching, and the steel plate is less likely to have the properties including high strength.
• the steel member for the pressure vessel or the like is produced through welding of the steel plate followed by stress relief annealing (post-welding heat treatment, hereinafter sometimes referred to as "PWHT") for relieving stress. If the steel plate has a large thickness, long PWHT is necessary for relieving stress.
• the steel plate subjected to long PWHT is disadvantageously degraded in toughness or the like.
• quenching is performed in place of normalizing that has been performed so that the thicknesswise central portion is rapidly cooled.
• quenching is performed in place of normalizing that has been performed so that the thicknesswise central portion is rapidly cooled.
• quenching is performed in place of normalizing that has been performed so that the thicknesswise central portion is rapidly cooled.
• such an approach also cannot sufficiently increase the cooling rate, i.e., does not sufficiently meet the demand of high strength and high toughness.
• the amount of alloy elements is increased.
• Cr-Mo steel containing Cr and Mo as alloy elements is used for the steel member for the pressure vessel or the like. It is known that when 2.25Cr-1.0Mo steel is, for example, used as the Cr-Mo steel, good toughness is exhibited even in the thicknesswise central portion of a thick steel plate while such a portion is in general difficult to have good toughness. In recent years, however, there is an increased trend toward resources saving and cost reduction.
• PTL 1 and PTL 2 each disclose a technique for improving low-temperature toughness of steel having a composition of 1.25Cr-0.5Mo level that is difficult to have good toughness.
• PTL 1 discloses a technique for providing good hardenability by adding Nb and Ca, and suppressing degradation in properties during stress relief (SR) annealing.
• SR stress relief
• PTL 2 discloses a technique for decreasing austenite grain size by performing controlled rolling or controlled rolling combined with accelerated cooling before quenching in a manufacturing process, and thereby providing good low-temperature toughness. This technique however is difficult to be practically used since the controlled rolling extremely lowers productivity of a rolling line for manufacturing an extremely thick steel plate having a thickness of more than 100 mm.
• An object of the invention which has been made in light of the above-described circumstances, is to provide a steel member produced using a thick steel plate, of which the inside (thicknesswise central portion) has high strength and high toughness even after being subjected to welding followed by long (particularly high-temperature and long) PWHT in the manufacturing process of the steel member, and provide a method of manufacturing the steel member.
• a steel member of the invention which has succeeded in solving the above-described problem, consisting of
• the invention also includes a method of manufacturing the steel member.
• the method includes performing hot rolling on a slab having a chemical composition of the above-described steel member, after the hot rolling, performing quenching under a condition of heating temperature of 900 to 950°C and holding time after the quenching, performing welding and post-welding heat treatment.
• tempering may be further performed at a temperature of 620°C to A c1 point.
• a steel member produced using a thick steel plate of which the inside (thicknesswise central portion) has high strength and high toughness even after being subjected to welding followed by long (particularly high-temperature and long) PWHT in the manufacturing process of the steel member. Consequently, it is possible to provide a medium- or high- temperature pressure vessel or the like that is produced using a thick steel plate, and has high strength and high toughness even after being subjected to high-temperature and long PWHT.
• the steel member of the invention is controlled to be low in amount of alloy elements, and therefore contributes to resources saving and cost reduction.
• steel plate that is composed of Cr-Mo steel (for example, 1.25Cr-0.5Mo steel) having a lower amount of alloy elements than the 2.25Cr-1.0Mo steel and has a thickness of 90 mm or more, the thicknesswise central portion of the steel plate having high toughness (low-temperature toughness) and high strength even if the thick steel plate is subjected to long PWHT.
• microstructure of the thicknesswise central portion is simply referred to as “microstructure”.
• the following properties, i.e., strength and toughness (low-temperature toughness) mean properties of at least the thicknesswise central portion of the steel member (i.e., the thick steel plate subjected to welding and PWHT).
• Each of the tempered bainite and the tempered martensite is a fine microstructure, and is particularly effective for providing high strength and high toughness of the thicknesswise central portion of an extremely thick steel plate.
• the microstructure of the steel member of the invention is at least one of tempered bainite and tempered martensite, and does not substantially include other phases such as polygonal ferrite, retained austenite, and perlite.
• the microstructure mainly includes an upper bainite structure having a large grain size, so that good toughness cannot be provided.
• the microstructure of the thicknesswise central portion is controlled to be at least one of tempered bainite and tempered martensite, thereby the microstructure can be refined.
• a large-angle grain boundary size of the microstructure (i.e., at least one of tempered bainite and tempered martensite) of the thicknesswise central portion is controlled to be 20 ⁇ m or less to achieve high toughness through steady refinement of the microstructure.
• a so-called large-angle grain boundary which has a crystal misorientation (crystal misorientation) of 15° or more between two adjacent grains in most cases, has a large crystal misorientation between two adjacent grains.
• brittle fracture is curvedly propagated, and a surface unit of the brittle fracture is reduced, contributing to improvement in toughness in the microstructure including tempered bainite and tempered martensite.
• the large-angle grain boundary size (the mean equivalent circle diameter of grains each being surrounded by the large-angle grain boundary) is controlled to be 20 ⁇ m or less as described above to increase the large-angle grain boundaries in a certain region in order to sufficiently improve toughness.
• the large-angle grain boundary size can be determined by an electron back scattering pattern (EBSP) method as described later in an embodiment.
• EBSP electron back scattering pattern
• the large-angle grain boundary size is preferably 15 ⁇ m or less, and more preferably 13 ⁇ m or less.
• the lower limit of the large-angle grain boundary size is roughly 10 ⁇ m due to manufacturing reasons.
• the steel member of the invention is subjected to PWHT (particularly long PWHT, and further particularly high-temperature and long PWHT).
• PWHT high-temperature and long PWHT
• grain boundary carbide of M 23 C 6 is typically formed.
• the PWHT is performed under a severe condition such as high temperature and long time, the grain boundary carbide is coarsened and thus tends to be a fracture origin, causing degradation in toughness.
• the maximum size of the grain boundary carbide is controlled to be 0.8 ⁇ m or less in the thicknesswise central portion of the steel member, thereby the steel member has good toughness.
• the maximum size of the grain boundary carbide is preferably 0.6 ⁇ m or less, and more preferably 0.5 ⁇ m or less.
• the lower limit of the maximum size of the grain boundary carbide is roughly 0.2 ⁇ m within a range of each of the composition and the manufacturing condition defined in the invention.
• the fraction of the grain boundary carbide (the proportion of the grain boundary carbide in the entire microstructure of the thicknesswise central portion as described later in the embodiment) is controlled to be 1.0 area% or more.
• the fraction of the grain boundary carbide is preferably 2.0 area% or more.
• the fraction of the grain boundary carbide increases with an increase in the content of C, the increased C content coarsens the carbide, and tends to degrade toughness. Consequently, from the viewpoint of providing good toughness, the upper limit of the C content is defined as described later, and the upper limit of the fraction of the grain boundary carbide is about 5.0 area% within the range of the C content.
• the microstructure of the thicknesswise central portion must be controlled as described above, the microstructure of any other region (for example, a thicknesswise surface portion) is not limited.
• a portion closer to the surface than the thicknesswise central portion is in general rapidly cooled in quenching compared with the thicknesswise central portion; hence, such a portion tends to have a finer microstructure than the thicknesswise central portion, and tends to be better in both strength and toughness than the thicknesswise central portion.
• B is contained as a chemical component in the amount as described later so as to exist in a form of free B (dissolved B) to improve hardenability.
• Al is added in the amount as described later so that N, which is easily bonded to B and form BN, is fixed in a form of AlN (that is useful for suppressing coarsening of prior austenite (y) grains during quenching to form a fine microstructure) in order to provide a sufficient amount of free B.
• it is important to appropriately control the manufacturing condition such as heating temperature and heating retention time in quenching.
• C is an element necessary for forming at least one of tempered bainite and tempered martensite during quenching of a thick steel plate even in the thicknesswise central portion of the steel plate while such a portion is slowly cooled in the quenching.
• C is an element necessary for forming the grain boundary carbide to provide sufficient strength of a base metal.
• the C content is 0.12% or more.
• the C content is preferably 0.13% or more, and more preferably 0.15% or more.
• the C content is if the C content is excessive, the grain boundary carbide is coarsened after long PWHT, and toughness is degraded. In addition, weld cracking easily occurs during welding of the steel plate. Consequently, the C content is 0.18% or less.
• the C content is preferably 0.17% or less, and more preferably 0.16% or less.
• Si is an element effective for increasing strength of a base metal (i.e., strength of the thicknesswise central portion) of the steel member. Si is also an element used as a deoxidizer. To allow such effects to be exhibited, the Si content is 0.50% or more. The Si content is preferably 0.55% or more, and more preferably 0.60% or more. However, if the Si content is excessive, temper embrittlement sensitivity increases, and toughness is degraded. Hence, the Si content is 0.80% or less. The Si content is preferably 0.75% or less, and more preferably 0.70% or less.
• Mn is an element effective for stabilizing austenite and lowering transformation temperature, and thus improving hardenability and forming a fine microstructure, and consequently providing high strength and high toughness.
• 0.40% or more of Mn is contained.
• the Mn content is preferably 0.45% or more, and more preferably 0.48% or more.
• the upper limit of the Mn content is 0.70%.
• the Mn content is preferably 0.65% or less, and more preferably 0.60% or less.
• the P content is controlled to be 0.015% or less to prevent such disadvantages.
• the P content is preferably 0.010% or less.
• the S content is preferably small as much as possible, and is controlled to be 0.005% or less, preferably 0.003% or less.
• Al is an important element in the invention, and is necessary for fixing N in a form of AlN during quenching to provide good hardenability by free B. AlN is also useful for suppressing coarsening of prior y grains during quenching and forming a fine microstructure. Furthermore, Al is an element necessary for deoxidation. To allow such effects to be exhibited, the Al content is 0.040% or more. The Al content is preferably 0.045% or more, and more preferably 0.050% or more. If the Al content is excessive, coarse alumina-based inclusions are formed, and toughness is degraded. Consequently, the Al content is 0.080% or less. The Al content is preferably 0.075% or less, and more preferably 0.071% or less.
• Cu and Ni are each an element effective for increasing strength without significantly degrading toughness.
• Cu is contained in the amount of 0.05% or more (preferably 0.10% or more, more preferably 0.11% or more, and further preferably 0.20% or more).
• Ni is contained in the amount of 0.05% or more (preferably 0.10% or more, more preferably 0.15% or more, and further preferably 0.16% or more).
• the Cu content is more preferably 0.37% or less, and further preferably 0.30% or less.
• the Ni content is more preferably 0.38% or less, and further preferably 0.30% or less.
• Cr is an element effective for suppressing coarsening of carbide due to PWHT, and providing good toughness of the steel member. Cr is also an element effective for providing high strength in a medium- or high- temperature region, and further effective for improving corrosion resistance. To allow such effects to be exhibited, Cr is contained in the amount of 1.25% or more. The Cr content is preferably 1.35% or more, and more preferably 1.39% or more. If Cr is excessively contained, temper embrittlement sensitivity increases, and grain boundary fracture easily occurs after PWHT, leading to an adverse effect on toughness. In addition, excessive Cr degrades workability and weldability, and increases manufacturing cost. Consequently, the Cr content is 1.50% or less. The Cr content is preferably 1.45% or less, and more preferably 1.40% or less.
• Mo is an element effective for improving hardenability and reducing temper embrittlement. To allow such effects to be exhibited, Mo is necessary to be contained in the amount of 0.45% or more.
• the Mo content is preferably 0.50% or more, and more preferably 0.55% or more. When the Mo content exceeds 0.65%, the effects are not so enhanced, and manufacturing cost is increased; hence, the upper limit of the Mo content is 0.65%.
• the Mo content is preferably 0.62% or less, and more preferably 0.60% or less.
• N is an important element in addition to Al in the invention. N is fixed during quenching through formation of AlN, thereby the effect of improving hardenability by free B can be maximally exhibited. AlN is also useful for suppressing coarsening of prior Y grains during quenching and forming a fine microstructure. If the N content is less than 0.0030%, AlN becomes insufficient, and the prior Y grains are coarsened. As a result, the fine microstructure is not formed, and toughness is degraded. Consequently, the N content is 0.0030% or more. The N content is preferably 0.0035% or more, and more preferably 0.0040% or more.
• the N content exceeds 0.0060%, the effect of fixing N by Al is substantially not exhibited, and BN is formed, thereby the effect of improving hardenability by free B is prevented. As a result, the microstructure is coarsened, and toughness is degraded. Consequently, the N content is 0.0060% or less.
• the N content is preferably 0.0055% or less, and more preferably 0.0050% or less.
• B is contained in a form of free B (dissolved B) and thus improves hardenability, which in particular makes it possible to form a fine microstructure even in the thicknesswise central portion of the thick steel plate while such a portion is slowly cooled in quenching. As a result, the thicknesswise central portion is allowed to have good toughness.
• 0.0003% or more of B is necessary though it is premised that the Al content and the N content are controlled as described above, and a quenching condition is controlled as described later.
• the B content is preferably 0.0005% or more, and more preferably 0.0007% or more.
• the B content is preferably 0.0009% or less, and more preferably 0.0008% or less.
• the steel member of the invention contains the above-described components with the remainder consisting of iron and inevitable impurities.
• the steel member may further contain V in an appropriate amount as described below in addition to such elements.
• V is an element that contributes to increasing strength through formation of carbide and nitride, and is effective for improving hardenability and forming a fine microstructure. To allow such effects to be exhibited, V is preferably contained in the amount of 0.005% or more.
• the V content is more preferably 0.010% or more. Excessive addition of V causes an increase in cost; hence, the upper limit of the V content is preferably 0.030%.
• the V content is more preferably 0.028% or less, and further preferably 0.020% or less.
• a slab having the above-described chemical composition of the steel member is hot-rolled in a usual manner to produce a thick steel plate. Subsequently, the thick steel plate is subjected to hardening (and tempering as necessary).
• the thick steel plate has a thickness of 90 mm or more (particularly 100 mm or more, and further particularly 120 mm or more).
• the thick steel plate to be used for the steel member must be subjected to hardening under the following condition.
• Heating temperature in hardening is controlled to be 900 to 950°C (in particular, controlled to be 900°C or more), and heating retention time therein is controlled to be 60 min or more, thereby the prior Y grains can be somewhat grown. As a result, hardenability is improved, and a fine microstructure can be formed.
• the heating temperature in hardening is below 900°C, the prior Y grains are still fine during the hardening; hence, the fine microstructure is not formed in a slowly cooled portion such as the thicknesswise central portion of the thick steel plate, and good toughness cannot be provided. Consequently, the heating temperature in hardening is 900°C or more.
• the heating temperature is preferably 910°C or more. If the heating temperature exceeds 950°C, some of N that has been fixed in a form of AlN is dissolved, and is bonded to B and formed into BN, so that the effect of improving hardenability by free B is not exhibited. As a result, the fine microstructure is not formed, and toughness is degraded. Consequently, the heating temperature in hardening is 950°C or less.
• the heating temperature is preferably 940°C or less.
• the heating retention time at the heating temperature (heating retention time) of shorter than 60 min allows the prior Y grains to be still fine. Hence, sufficient hardenability is not provided even if the predetermined amount of B is contained. As a result, the microstructure is coarsened and toughness is degraded. Consequently, the heating retention time is 60 min or more.
• the heating retention time is preferably 80 min or more.
• the upper limit of the heating retention time is about 150 min from the viewpoint of productivity or the like.
• the fine microstructure is preferably easily formed.
• the tempering is recommended to be performed under the following condition.
• the tempering temperature is preferably 620°C to A c1 point.
• the tempering temperature of 620°C or more allows the hardness of the surface to be sufficiently lowered, and allows good workability to be maintained.
• the tempering temperature is more preferably 700°C or more.
• the tempering temperature exceeds the A c1 point, some of the microstructure is reversely transformed and then air-cooled; hence, polygonal ferrite is mixedly formed in the microstructure. As a result, strength is lowered, and toughness is also degraded due to the coarse microstructure of the reversely transformed region. Consequently, the upper limit of the tempering temperature is preferably equal to the A c1 point.
• the tempering temperature is more preferably 750°C or less.
• the steel member of the invention is produced as follows: the thick steel plate produced through the hardening (and tempering as necessary) is subjected to welding in a usual manner, and further subjected to post-welding heat treatment (PWHT) for removing strain as described above to produce the steel member.
• PWHT post-welding heat treatment
• heating temperature is 600 to 690°C
• heating time is 5 to 22 hours.
• the invention covers a thick steel plate of which the thicknesswise central portion is in general difficult to have high strength and high toughness after PWHT (particularly high-temperature and long PWHT).
• the invention therefore also covers a steel member, which is produced by such a thick steel plate, having a thickness of 90 mm or more (particularly 100 mm or more, and further particularly 120 mm or more).
• the steel member of the invention can be used for a medium- or high- temperature pressure vessel and the like for use in chemical industry including oil refining.
• a slab satisfying the (chemical) composition shown in Table 1 (the remainder consisting of iron and inevitable impurities, and each blank in Table 1 indicating no element being added) was hot-rolled in a usual manner, and then subjected to hardening under the condition shown in Table 2 to produce steel plates each having a thickness (also being a thickness of a test specimen simulating the steel member) shown in Table 2.
• each steel plate was further subjected to tempering under the condition shown in Table 2 or 3.
• the heating temperature at each of hardening and tempering refers to the temperature of the thicknesswise central portion of the steel plate, which was calculated by calculus of finite differences based on the furnace atmosphere temperature of the heat treatment furnace and in-furnace time, or was measured using a thermocouple inserted into a dummy steel plate having the same thickness in an experimental furnace.
• the steel plate was heat-treated under a condition of heating temperature of 690°C and heating retention time of 22 hours (an extremely severe condition among currently-practiced conditions, the value P is 20.6 at the condition) in a truck-type electric furnace (air atmosphere) as a simulation of the PWHT after welding, so that a test specimen simulating the steel member was produced.
• the heating rate from room temperature to the heating temperature and the cooling rate from the heating temperature to room temperature were each 55 °C/hr or less.
• the steel plate is subjected to welding followed by PWHT.
• the welding is less likely to adversely affect the properties (particularly toughness) of the steel member (including a weld heat-affected zone).
• each test specimen was produced without being subjected to heat treatment following welding.
• test specimen produced in the above manner was used to evaluate a microstructure, and perform a tensile test and a Charpy impact test according to the following procedure.
• surface hardness was measured using the steel plate before being subjected to PWHT in order to evaluate workability (properties to be required in the manufacturing process of the steel member) of the steel plate.
• the microstructure was observed as follows.
• the tempered bainite described herein refers to a microstructure formed through tempering of upper bainite, lower bainite, or bainitic ferrite. Such phases, including tempered martensite, are typically difficult to be sorted out, and the microstructure is sufficiently tempered after PWHT. Hence, any of the phases other than polygonal ferrite was defined as at least one of tempered bainite and tempered martensite (B + M). It was also found that no perlite phase was contained in any of the test specimens used in this embodiment.
• the measurement procedure was as follows.
• the size and the fraction of the grain boundary carbide were measured as follows.
• a round-bar tensile test piece was sampled from the portion of t/2 (half the thickness) in a direction perpendicular to the rolling direction, and was subjected to a tensile test according to the procedure of ASTM A370 so that yield strength and tensile strength were measured.
• a sample having a yield strength of 310 MPa or more and a tensile strength of 515 MPa or more was evaluated to have high strength (good tensile characteristics).
• a full-size V-notch test piece was sampled from the portion of t/2 (half the thickness) in a direction perpendicular to the rolling direction, and was subjected to a Charpy impact test at a test temperature of -10°C according to the procedure of ASTM A370 so that absorbed energy was measured. The average of absorbed energy values of three test pieces was determined as that absorbed energy. A sample having an absorbed energy of 100 J or more was evaluated to have good toughness (good impact characteristics).
• a steel plate before being subjected to PWHT was subjected to a Brinell hardness test at a depth position of 1 mm from the surface of the steel plate according to the procedure of ASTM 370.
• a sample showing up to 250 HB was evaluated to be excellent ( ⁇ ) in workability, and a sample showing higher than 250 HB was evaluated to be normal ( ⁇ ) in workability.
• Tables 1 to 3 reveal the following. Specifically, in each of examples of the invention of A1-1, A1-2, A1-4, A1-5, A1-8, A1-9, A1-11 to A1-13, and A2 to A14, the steel member was composed of steel satisfying the defined composition, and was produced under the defined condition. Hence, the resultant steel member satisfied the specification of the microstructure, and exhibited high strength and high toughness at the thicknesswise central portion despite a large thickness of the steel member.
• Comparison of A1-13 with any of other examples of the invention shows that the steel member is preferably subjected to tempering under the defined condition in order to provide good workability.
• each of example Nos. other than the above-described examples did not satisfy one of the composition and the manufacturing condition, and was therefore inferior in at least one of the tensile characteristics and the impact characteristics of the thicknesswise central portion.
• B1 to B15 are examples that each do not satisfy the defined composition as described in detail below.
• B12 was excessive in Si content.
• B13 was excessive in Mn content.
• B14 was insufficient in Mo content.
• B15 was excessive in B content. Hence, any of them was increased in temper embrittlement sensitivity and degraded in toughness.

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