JP4878219B2 - Steel sheet with excellent HAZ toughness and small reduction in strength due to heat treatment after welding - Google Patents

Description

  The present invention relates to a steel plate used for manufacturing a storage tank, an offshore structure, and the like, and more particularly, when a weld and a post weld heat treatment (PWHT) are performed, a heat affected zone (heat affected zone). , HAZ), and a steel sheet that is less likely to decrease in strength after PWHT or that has improved strength.

  When manufacturing welded structures such as crude oil, ethylene, and LPG storage tanks and offshore structures, PWHT that is maintained at about 600 ° C. for several hours may be performed in order to reduce residual stress in the welds. is there. In PWHT, an object is held at a high temperature for a long time, so that the microstructure is destroyed, and strength may decrease after PWHT. In particular, in steel materials with a reduced carbon content, a decrease in strength after PWHT becomes a problem.

  In order to ensure the strength of the steel sheet after PWHT, for example, Patent Document 1 adjusts the chemical composition (in particular, adding Cu and Ni in addition to a small amount of Nb), and controls the rolling conditions. Is disclosed. Patent Document 2 discloses adjusting the chemical composition (in particular, adding Nb and B in combination to increase the amount of bainite) and controlling rolling conditions. However, Patent Documents 1 and 2 do not intend to extremely reduce the C amount, as the upper limit of the C amount is defined as 0.18%.

In the production of a welded structure, a steel plate that can be welded with high heat input is required from the viewpoint of efficiency. However, high heat input welding has a problem that HAZ toughness is reduced. It is known that this problem can be improved by extremely reducing the C content. For example, in Patent Document 3, by adjusting the chemical component composition, the amount of Mn, Cr, Mo, V, and Nb in particular is set to 2.4 ≦ [Mn] + 1.5 × [Cr] + 2 × [Mo] ≦ 4. .5, adjusted to satisfy [V] + [Nb] ≦ 0.040, further reduced the amount of C, and added B to improve the weld crack resistance and high heat input HAZ toughness of the steel sheet Is disclosed. However, Patent Document 3 does not consider the strength reduction after PWHT.
JP-A-62-93312 JP-A-62-240713 JP 2002-47532 A

  Accordingly, an object of the present invention is to provide a steel sheet that has excellent HAZ toughness and is less likely to decrease in strength after PWHT, or conversely, has improved strength.

With the steel sheet of the present invention that can achieve the above-mentioned object,
C: 0.01 to 0.05% (meaning mass%, the same shall apply hereinafter)
Si: 1.0% or less (excluding 0%),
Mn: 0.50 to 2.0%,
P: 0.05% or less (excluding 0%),
S: 0.01% or less (excluding 0%),
Al: 0.01 to 0.07%,
Cr: 0.5 to 2.0%,
Nb: 0.005 to 0.100%,
V: 0.005-0.10%,
Ti: 0.005 to 0.03%, and N: 0.002 to 0.008%,
The balance consists of Fe and inevitable impurities,
The post-weld heat treatment is performed, wherein the X1 value represented by the following formula (1) is in the range of 0.005 to 0.020 and the bainite fraction is 90 area% or more. Steel sheet.
X1 = (9 [Nb] +4 [V]) × [C] (1)
[Wherein [Nb], [V] and [C] represent the contents (% by mass) of Nb, V and C, respectively. ]

  In the present invention, the “steel sheet subjected to post-weld heat treatment” means that the steel sheet of the present invention is used for welding and post-weld heat treatment (for example, storage tanks for crude oil or offshore structures, etc. It means that it is limited to manufacture of a welded structure.

The steel plate of the present invention may further contain Mo. Preferred steel sheet of the present invention containing Mo is
C: 0.01-0.05%
Si: 1.0% or less (excluding 0%),
Mn: 0.50 to 2.0%,
P: 0.05% or less (excluding 0%),
S: 0.01% or less (excluding 0%),
Al: 0.01 to 0.07%,
Cr: 0.5 to 2.0%,
Nb: 0.005 to 0.100%,
V: 0.005-0.10%,
Mo: 0.5% or less (excluding 0%)
Ti: 0.005 to 0.03%, and N: 0.002 to 0.008%,
The balance consists of Fe and inevitable impurities,
The X2 value represented by the following formula (2) is in the range of 0.005 to 0.020, and a bainite fraction is a structure having a area of 90 area% or more. Steel sheet.
X2 = (9 [Nb] +4 [V] + [Mo]) × [C] (2)
[Wherein [Nb], [V], [Mo] and [C] represent the contents (mass%) of Nb, V, Mo and C, respectively. ]

  In the steel plate of the present invention, in addition to the above components, if necessary, (a) B: 0.0040% or less (not including 0%), (A) Cu: 3.0% or less (0% And / or Ni: 3.0% or less (not including 0%), (c) W: 0.5% or less (not including 0%), (d) Ca: 0.005% or less ( 0% not included) and / or rare earth elements: 0.003% or less (not including 0%), (f) Zr: 0.005% or less (not including 0%), and / or (g) Mg : It is also effective to contain 0.005% or less (not including 0%), etc., and the characteristics of the steel sheet are further improved depending on the type of the component to be contained.

  By making the amount of C extremely low and adding an appropriate amount of Nb and V, and Mo as required, it is excellent in high heat input HAZ toughness, and the strength is less likely to decrease after PWHT, or conversely the strength An improved steel sheet can be produced.

  Conventionally, it has been known that a steel sheet containing a relatively large amount of C contains a precipitation strengthening element in order to prevent a decrease in strength after PWHT. However, it is considered that precipitates (particularly carbides) are unlikely to form in steel sheets with extremely low C content in order to improve high heat input HAZ toughness, and so far the strength reduction after PWHT and precipitation strengthening elements The relationship was not clear. For this reason, an extremely low C steel sheet having excellent HAZ toughness and reduced strength reduction after PWHT has not been obtained.

  In order to produce a steel sheet that combines these two characteristics, the influence of various precipitation strengthening elements was examined. In an extremely low C steel sheet, Nb, V, and Mo are particularly preferable in order to suppress a decrease in strength after PWHT. I found it effective.

Nb, V and Mo form carbides with C in particular, and are considered to contribute to the suppression of strength reduction after PWHT. Therefore, a parameter X3 represented by the following formula (3) in which the product of the amount of C and Nb, V, or Mo is summed has been devised:
X3 = (α [Nb] + β [V] + γ [Mo]) × [C] (3)
[Wherein [Nb], [V], [Mo] and [C] represent the contents (% by mass) of Nb, V, Mo and C, respectively, and α, β and γ represent constants. ]

And in order to determine the coefficients (α, β and γ) for the respective contents of Nb, V and Mo in the above formula (3), only one of these contents is changed, and the other elements are the same. The change margin of tensile strength (TS) before and after PWHT (ΔTS = TS before TS-PWHT before PWHT) was measured using the steel plate (TS is the same as that described in the following examples) It was measured). A straight line graph is created with the content of each element (Nb, V or Mo) on the horizontal axis and ΔTS on the vertical axis (FIG. 1). From the ratio of the slopes of these lines, α = 9, β = 4 And γ = 1, and a parameter X represented by the following formula (4) was determined:
X = (9 [Nb] +4 [V] + [Mo]) × [C] (4)
[Wherein [Nb], [V], [Mo] and [C] represent the contents (mass%) of Nb, V, Mo and C, respectively. ]

  Next, in order to identify the steel sheet that is suppressed in strength reduction after PWHT and excellent in both high heat input HAZ toughness by the parameter X (the above formula (4)), the component has a strength of 60 kg or more. The steel pieces with the adjusted composition (Table 1) were heated to 1100 ° C., finished hot rolling at a rolling finish temperature of 800 ° C., and then air-cooled to produce a steel plate having a thickness of 20 mm.

Using these steel sheets, ΔTS before and after PWHT, and absorption energy (vE _-50 ) in a Charpy impact test at -50 ° C. as an index of HAZ toughness were measured (TS and vE _-50 are the following examples) In the same manner as described in 1). The results are shown in Table 2. The X value and ΔTS, and the X value and vE _-50 are in the relationship shown in FIGS. 2 and 3 below.

That is, as the X value represented by the above formula (4) increases, the strength decreases after PWHT, or the strength increases. However, the higher the X value, the lower the HAZ toughness (vE _-50 ). Therefore, in order to obtain an ultra-low C steel sheet excellent in both suppression of strength reduction after PWHT and HAZ toughness, the X value in the present invention is 0.005 or more, preferably 0.007 or more, more preferably 0.010. The above is defined as 0.020 or less, preferably 0.018 or less, and more preferably 0.015 or less. In addition, when the steel plate of this invention does not contain Mo, the X value of said Formula (4) corresponds with the X1 value of said Formula (1), and when it contains Mo, X value is X2 of said Formula (2). Matches the value. Hereinafter, the X1 value and the X2 value will be collectively described as “X value”.

  The steel sheet of the present invention is also characterized by being based on a bainite structure. Such a bainite structure is useful for securing a strength of 570 MPa or more despite the extremely low C. Generally, in line pipes and the like, high strength is realized by mainly using a ferrite structure, but in a ferrite structure, it is necessary to realize high strength as fine ferrite by performing low temperature rolling. . On the other hand, the bainite structure can achieve high strength even at high temperature rolling, and is useful for improving productivity. However, in order to exhibit these effects, 100 area% does not necessarily need to be a bainite structure, and what is necessary is just 90 area% or more by a bainite fraction. Examples of structures other than bainite include martensite and ferrite.

In addition to the upper or lower bainite, the bainite structure in the present invention is “steel bainite photo collection-1” [Japan Iron and Steel Institute, Bainite Research Group: (1992). 4] and bainitic ferrite or granular bainitic ferrite introduced in 4]. These bainite structures with extremely low amounts of C (very low C bainite structures) are excellent in strength and toughness, and can be obtained by making them within the chemical composition range defined in the present invention and by producing them under appropriate conditions. it can.

  The present invention intends to provide a steel sheet in which the X value is appropriately defined as described above and the HAZ toughness and strength reduction after PWHT are suppressed by making the structure mainly composed of bainite. is there. However, in order to achieve these properties, as well as excellent strength and base material toughness, it is important to appropriately adjust not only the X value and the bainite fraction, but also the chemical composition of the steel sheet. Therefore, below, the chemical component composition of the steel plate of this invention is demonstrated.

<C: 0.01 to 0.05%>
C is an element effective for increasing the strength of steel. Further, it is an element necessary for precipitating carbides and suppressing a decrease in strength after PWHT. In order to secure a desired strength and to sufficiently suppress a decrease in strength after PWHT, the amount of C is 0.01% or more, preferably 0.02% or more. However, if C is excessively contained, the carbides become coarse, and there is a possibility that a large amount of island-like martensite phase (MA phase) and cementite are formed and the toughness is lowered. Therefore, the C content is 0.05% or less, preferably 0.04% or less.

<Si: 1.0% or less (excluding 0%)>
Si is an effective element for increasing the strength of steel by solid solution strengthening regardless of cooling conditions. In order to fully exhibit this effect, it is recommended to contain Si in an amount of 0.1% or more, preferably 0.2% or more. However, if it is contained excessively, a large amount of MA phase is precipitated in the steel material (base material) and the toughness is deteriorated. Therefore, the upper limit of the Si amount is set to 1.0%. A preferable upper limit is 0.5%.

<Mn: 0.50 to 2.0%>
Mn is an element effective for strengthening steel by generating an extremely low C bainite structure. In order to sufficiently exhibit these effects, the amount of Mn is 0.50% or more, preferably 0.7% or more. However, when Mn is contained excessively, the toughness of the base material is deteriorated. Therefore, the amount of Mn is 2.0% or less, preferably 1.8% or less.

<P: 0.05% or less (excluding 0%)>
P is an unavoidable impurity that segregates in crystal grains and adversely affects ductility and toughness. Therefore, P is preferably as small as possible, but is inevitably mixed into the steel material. Therefore, it is recommended that the P content be 0.05% or less, preferably 0.01% or less.

<S: 0.01% or less (excluding 0%)>
Since S is an impurity that reacts with alloy elements in the steel material to form various inclusions and adversely affects the ductility and toughness of the steel material, it is preferably as small as possible. However, S is inevitably mixed in as with P. Therefore, it is recommended that the S content be 0.02% or less, preferably 0.005% or less.

<Al: 0.01 to 0.07%>
Al is an element effective as a deoxidizer. Al is also an element that helps improve the hardenability of B by increasing the solid solution amount of B by fixing N in the steel. In order to exhibit such an effect, the amount of Al is 0.01% or more, preferably 0.02% or more. However, if it is contained excessively, as in the case of Si, a large amount of island-like martensite phase (MA phase) is precipitated in the matrix structure, and toughness is deteriorated. Therefore, the Al content is 0.07% or less, preferably 0.05% or less.

<Cr: 0.5 to 2.0%>
Cr is important for obtaining an extremely low C bainite structure, and is an effective element for reducing the bainite block size in the HAZ structure. Further, it is an element effective in improving the hardenability and ensuring the strength of the steel material. In order to sufficiently exhibit such effects, the Cr content is 0.5% or more, preferably 0.7% or more. However, when the amount of Cr becomes excessive, the base material toughness deteriorates. Therefore, the Cr content is 2.0% or less, preferably 1.8% or less.

<Nb: 0.005 to 0.100%>
Nb is effective for obtaining an extremely low C bainite structure, and is an important element for suppressing a decrease in strength after PWHT. In order to sufficiently exhibit such effects, the Nb content is 0.005% or more, preferably 0.008% or more. However, when the amount of Nb is excessive, the precipitate becomes coarse and the toughness is deteriorated. Therefore, the Nb content is 0.100% or less, more preferably 0.07% or less.

<V: 0.005-0.10%>
V is an effective element for securing the strength and is an important element for suppressing the strength reduction after PWHT. In order to sufficiently exhibit these effects, the V amount is 0.005% or more, preferably 0.008% or more. However, when the amount of V becomes excessive, precipitates are formed in the HAZ and the HAZ toughness deteriorates. Therefore, the V amount is 0.10% or less, more preferably 0.08% or less.

<Mo: 0.5% or less>
Mo is an element effective for improving the strength and effective for suppressing a decrease in strength after PWHT, but is not an essential element in the steel sheet of the present invention, and can be contained as necessary. In order to sufficiently exhibit the effect, it is recommended that Mo is contained in an amount of preferably 0.03% or more, more preferably 0.06% or more. However, when the amount of Mo becomes excessive, toughness deteriorates. Therefore, when Mo is contained, the amount is 0.5% or less, preferably 0.2% or less.

<Ti: 0.005-0.03%>
Ti is an element effective for suppressing the coarsening of the prior austenite grains during high heat input welding and improving the HAZ toughness by forming a nitride together with N. In order to exert such an effect, the Ti amount is 0.005% or more, preferably 0.010% or more. However, when Ti is excessively contained, coarse inclusions are precipitated, and the HAZ toughness is deteriorated. Therefore, the Ti content is 0.03% or less, preferably 0.025% or less.

<N: 0.002 to 0.008%>
N is an element that forms fine TiN together with Ti and is effective in improving the HAZ toughness by preventing coarsening of prior austenite grains in high heat input welding. In order to exert such an effect, the N amount is 0.002% or more, preferably 0.003% or more. However, when the amount of N becomes excessive, coarse TiN precipitates and the toughness deteriorates. Therefore, the N content is 0.008% or less, preferably 0.006% or less.

  The basic component composition of the steel sheet of the present invention is as described above, and the balance is substantially Fe. However, as a matter of course, it is permissible for the steel sheet to contain inevitable impurities brought in depending on the situation of raw materials, materials, manufacturing equipment, and the like. Furthermore, the steel plate of this invention may contain the following arbitrary elements as needed.

<B: 0.0040% or less>
B is an element effective for obtaining an extremely low C bainite structure and for improving the strength, and can be contained as required. In order to exert such an effect, it is recommended that B is contained in an amount of preferably 0.0005% or more, more preferably 0.0010% or more. However, when B is contained excessively, not only the effect is saturated, but also HAZ toughness is lowered. Therefore, when B is contained, the amount is 0.0040% or less, preferably 0.0025% or less.

<Cu: 3.0% or less and / or Ni: 3.0% or less>
Cu and Ni are elements that improve the strength of the base material without impairing the HAZ toughness, and can contain one or both of them as necessary. In order to sufficiently exert such an effect, Cu is preferably 0.05% or more, more preferably 0.10% or more, further preferably 0.5% or more, and Ni is preferably 0.05% or more. %, More preferably 0.10% or more, and still more preferably 0.5% or more. However, if these elements are contained excessively, the formation of island martensite phase (MA phase) is promoted during welding, and the HAZ toughness deteriorates. Therefore, when Cu and / or Ni is contained, the Cu content is 3.0% or less, preferably 1% or less, and the Ni content is 3.0% or less, preferably 2% or less.

<W: 0.5% or less>
W is an element effective for improving the corrosion resistance, and can be contained as necessary. In particular, W preferably coexists with Ti and Ni. In order to sufficiently exhibit such an effect, it is recommended that W is contained in an amount of preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more. However, even if the amount of W becomes excessive, the effect is saturated. Therefore, when W is contained, the amount is 0.5% or less, preferably 0.3% or less.

<Ca: 0.005% or less and / or rare earth element: 0.003% or less>
Ca and rare earth elements (hereinafter abbreviated as “REM”) are effective elements for reducing the anisotropy of inclusion shapes and improving the HAZ toughness, and may be contained as necessary. it can. In order to sufficiently exhibit such an effect, Ca is preferably 0.0005% or more, more preferably 0.0010% or more, and REM is preferably 0.0003% or more, more preferably 0.0006. It is recommended that the content be at least%. However, when these elements are contained excessively, inclusions become coarse and the HAZ toughness is deteriorated. Therefore, when Ca and / or REM is contained, the Ca content is 0.005% or less, preferably 0.004% or less, and the REM content is 0.003% or less, preferably 0.002% or less. is there.

<Zr: 0.005% or less>
Zr is an element effective for improving the HAZ toughness by forming nitrides and oxides and suppressing coarsening of the prior austenite grains of the HAZ, and can be contained as necessary. In order to sufficiently exhibit such effects, it is recommended that Zr is contained in an amount of preferably 0.001% or more, more preferably 0.002% or more. However, if the amount of Zr is excessive, the HAZ toughness is deteriorated. Therefore, when Zr is contained, the amount is 0.005% or less, preferably 0.004% or less.

<Mg: 0.005% or less>
Mg is an element effective for finely dispersing an oxide serving as a nucleus of TiN precipitation and improving the HAZ toughness, and can be contained as necessary. In order to sufficiently exhibit such effects, it is recommended that Mg is contained in an amount of preferably 0.001% or more, more preferably 0.002% or more. However, if the amount of Mg is excessive, coarse inclusions are formed and the toughness deteriorates. Therefore, when Mg is contained, the amount is 0.005% or less, preferably 0.004% or less.

In order to manufacture the steel sheet of the present invention, basically, a slab or steel slab that satisfies the above chemical composition is produced by continuous casting or ingot casting, and this is subjected to hot rolling, cooling, and heat treatment. Although it can manufacture by a normal method, in order to obtain a very low C bainite structure especially, manufacturing by the method including the process of the following (A) or (B) is preferable.
(A) The slab or steel slab is heated to 950 to 1300 ° C., and after hot rolling is completed at a rolling finishing temperature of 700 ° C. or higher, air cooling is performed.
(B) The slab or steel slab is heated to 950 to 1300 ° C., and after hot rolling is finished at a rolling finish temperature of 700 ° C. or higher, the steel is cooled to 500 ° C. or lower at a cooling rate of 1 to 50 ° C./second.

  In the above steps (A) and (B), if the heating temperature is too low, the alloy element is not sufficiently dissolved, and the desired effect of the alloy element may not be obtained. Is preferred. Moreover, when heating temperature is too high, an initial austenite grain will coarsen and the toughness of a steel plate will fall as a result. Therefore, the temperature is preferably set to 1300 ° C. or lower. The rolling finishing temperature is preferably 700 ° C. or higher from the viewpoint of productivity.

  After hot rolling is finished, a bainite structure is obtained because it is a component design that suppresses ferrite transformation by air cooling, but in some cases, it is accelerated to 500 ° C. or less at a cooling rate of 1 to 50 ° C./second. It may be cooled. By doing so, the structure becomes supercooled and a good ultra-low C bainite structure is obtained. In addition, when implementing accelerated cooling, since it is necessary to cool until the production | generation of a bainite structure is completed, cooling to 500 degrees C or less is recommended.

  In addition to the above manufacturing process, it is also useful to perform a tempering treatment in a temperature range of 500 to 700 ° C. as necessary, which further increases the toughness.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steel pieces satisfying the chemical composition shown in Table 3 below were prepared by melting from a vacuum melting material, and steel sheets were manufactured under hot rolling-cooling-heat treatment conditions shown in Table 4 below.

For each steel plate obtained, the bainite fraction, the tensile strength (TS) of the steel plate and the change in tensile strength due to PWHT (ΔTS), the base material toughness (fracture surface transition temperature vTrs), and the HAZ toughness (vE- ₅₀ ) Measured by the following method. These results are listed in Table 5. The X value calculated from the chemical composition is also shown in Table 5.

<Bainite fraction>
After mirror polishing from a t / 4 (t is the plate thickness) part of the steel plate, a test piece was collected, etched with a 2% nitric acid-ethanol solution (nitral solution), and then 400 times using an optical microscope in five fields of view. Observation was performed, and the bainite fraction (area%) in the steel structure was measured by image analysis. At this time, all lath structures other than ferrite were regarded as bainite.

<Tensile strength (TS) of steel sheet and change in tensile strength due to PWHT (ΔTS)>
JIS Z 2201 No. 4 test piece was taken from the t / 4 (t is the plate thickness) portion of the steel plate before PWHT, and the tensile strength (TS) was measured by conducting a tensile test according to JIS Z 2241. In this test, TS ≧ 570 MPa was regarded as acceptable.
Next, after performing PWHT twice on the steel sheet under the conditions of 600 ° C. and holding time of “plate thickness (inch) × 1 hour”, respectively, from the t / 4 (t is the plate thickness) portion of the steel plate, JIS Z 2201 No. 4 test specimens were collected, and tensile strength (TS) after PWHT was measured by conducting a tensile test according to JIS Z 2241, and the change in tensile strength due to PWHT (ΔTS = TS-PWHT after PWHT) The previous TS) was determined. In this test, ΔTS ≧ −15 MPa was regarded as acceptable.

<Base material toughness (vTrs)>
From the t / 4 (t is the plate thickness) part of the steel plate, a V-notch test piece is taken according to JIS Z 2242 and a Charpy impact test is performed. The temperature at which the brittle fracture rate of the Charpy test piece is 50% The transition temperature (vTrs) was obtained by approximation. In this test, vTrs ≦ −50 ° C. was regarded as acceptable.

<HAZ toughness (vE- ₅₀ )>
A HAZ reproduction test was conducted. Thermal cycle test in which specimens taken from steel plates (5 specimens each of 12.5 × 32 × 55 (mm) are collected) are heated to 1400 ° C. × 5 seconds and then cooled to 800-500 ° C. in 40 seconds. (Corresponding to a heat input of 5 kJ / mm). Thereafter, two Charpy test pieces were collected from each test piece, the absorbed energy at −50 ° C. was measured with 10 pieces on each steel plate, and the average value was obtained as vE- ₅₀ . In this test, vE _-50 ≧ 100 J or more was accepted.

  From the results of Tables 3 to 5, steel plate Nos. Satisfying the requirements of the present invention (chemical composition, X value and bainite fraction). In addition to exhibiting excellent tensile strength and base material toughness before welding, 1 to 11 are sufficiently suppressed in the decrease in tensile strength even after welding and PWHT, or conversely, the strength is improved, and It can be seen that the HAZ toughness is excellent.

On the other hand, a steel plate No. 1 that does not satisfy any of the requirements of the present invention. In 12-31, the favorable result is not obtained.
Specifically, steel plate No. Since No. 12 has a large amount of C, it is not an extremely low C bainitic steel, and the base metal toughness is deteriorated. Also, the HAZ toughness is poor.
Steel plate No. In No. 13, since the amount of Si is large, a large amount of MA phase is precipitated in the steel material, and the base material and the HAZ toughness are deteriorated.

Steel plate No. No. 14 is not an extremely low C bainitic steel because the amount of Mn is small, and its strength is low.
Steel plate No. Since No. 15 has a large amount of Mn, the toughness is deteriorated.
Steel plate No. Since No. 16 has a small amount of Cr, it does not become an ultra-low C bainitic steel and has low strength.
Steel plate No. Since No. 17 has a large amount of Cr, the base material toughness is deteriorated.

Steel plate No. In No. 18, since the Ti amount is large, coarse inclusions are precipitated in the steel material, and the toughness is deteriorated.
Steel plate No. No. 19 has a large amount of B, so the toughness is deteriorated.
Steel plate No. Since No. 20 has a large amount of Mo, toughness is deteriorated.
Steel plate No. Since No. 21 has a large amount of V, toughness is deteriorated.

Steel plate No. Since No. 22 has a large amount of Cu, the toughness is deteriorated.
Steel plate No. Since No. 23 has a large amount of Ni, the toughness is deteriorated.
Steel plate No. Since No. 24 has a large amount of Nb, toughness is deteriorated.
Steel plate No. Since No. 25 has a large amount of Ca, toughness is deteriorated.

Steel plate No. In Nos. 26 to 28, the X value is less than the lower limit defined in the present invention, and the strength decrease due to PWHT is large (ΔTS is small).
Steel plate No. Nos. 29 to 31 have an X value exceeding the upper limit specified in the present invention, and have low HAZ toughness.

It is a graph which shows the relationship between each content (mass%) of Nb, V, or Mo, and the change margin (ΔTS = TS before PWHT-TS before PWHT) of the tensile strength (TS) before and after PWHT. It is a graph showing the relationship between X value (the said Formula (4)) and (DELTA) TS. It is a graph showing the relationship between X value (the said Formula (4)) and the absorbed energy (vE- ₅₀ ) in the Charpy impact test at _-50 degreeC.

Claims (8)

C: 0.01 to 0.05% (meaning mass%, the same shall apply hereinafter)
Si: 1.0% or less (excluding 0%),
Mn: 0.50 to 2.0%,
P: 0.05% or less (excluding 0%),
S: 0.01% or less (excluding 0%),
Al: 0.01 to 0.07%,
Cr: 0.5 to 2.0%,
Nb: 0.005 to 0.100%,
V: 0.005-0.10%,
Ti: 0.005 to 0.03%, and N: 0.002 to 0.008%,
The balance consists of Fe and inevitable impurities,
The post-weld heat treatment is performed, wherein the X1 value represented by the following formula (1) is in the range of 0.005 to 0.020 and the bainite fraction is 90 area% or more. Steel sheet.
X1 = (9 [Nb] +4 [V]) × [C] (1)
[Wherein [Nb], [V] and [C] represent the contents (% by mass) of Nb, V and C, respectively. ] C: 0.01 to 0.05% (meaning mass%, the same shall apply hereinafter)
Si: 1.0% or less (excluding 0%),
Mn: 0.50 to 2.0%,
P: 0.05% or less (excluding 0%),
S: 0.01% or less (excluding 0%),
Al: 0.01 to 0.07%,
Cr: 0.5 to 2.0%,
Nb: 0.005 to 0.100%,
V: 0.005-0.10%,
Mo: 0.5% or less (excluding 0%)
Ti: 0.005 to 0.03%, and N: 0.002 to 0.008%,
The balance consists of Fe and inevitable impurities,
The X2 value represented by the following formula (2) is in the range of 0.005 to 0.020, and a bainite fraction is a structure having a area of 90 area% or more. Steel sheet.
X2 = (9 [Nb] +4 [V] + [Mo]) × [C] (2)
[Wherein [Nb], [V], [Mo] and [C] represent the contents (mass%) of Nb, V, Mo and C, respectively. ]   Furthermore, B: The steel plate of Claim 1 or 2 containing 0.0040% or less (0% is not included).   The steel sheet according to any one of claims 1 to 3, further comprising Cu: 3.0% or less (not including 0%) and / or Ni: 3.0% or less (not including 0%).   The steel sheet according to any one of claims 1 to 4, further comprising W: 0.5% or less (not including 0%).   Furthermore, the steel plate in any one of Claims 1-5 containing Ca: 0.005% or less (excluding 0%) and / or rare earth elements: 0.003% or less (excluding 0%).   Furthermore, the steel plate in any one of Claims 1-6 containing Zr: 0.005% or less (0% is not included).   The steel sheet according to any one of claims 1 to 7, further comprising Mg: 0.005% or less (not including 0%).

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