JP5842574B2 - Manufacturing method of steel for large heat input welding - Google Patents

Description

  The present invention is suitable for steel materials with a tensile strength (TS) of 510 MPa class and a plate thickness of up to 70 mm used in various structures such as shipbuilding, construction and civil engineering, especially for high heat input welding where the heat input of welding exceeds 300 kJ / cm. The present invention relates to a method for manufacturing a steel material for large heat input welding.

  Steel materials used in the fields of shipbuilding, construction, civil engineering, etc. are generally finished into a desired shape structure by welding. From the viewpoint of safety, these structures are required to have excellent welded toughness as well as the performance of the base material of the steel used.

  In recent years, these structures and ships have become larger and the thickness of the steel materials used has increased, and high efficiency heat input welding methods such as submerged arc welding, electrogas welding and electroslag welding have been used for welding. Has been applied. For this reason, even when welding is performed by these large heat input welding methods, a steel material having excellent characteristics that does not lower the toughness of the welded portion is required.

  However, it is generally known that as the welding heat input increases, the structure of the weld heat affected zone becomes coarser, so that the toughness of the weld heat affected zone decreases. Many measures have been proposed so far in order to prevent a decrease in the toughness of the weld heat affected zone due to the adoption of such a high heat input welding method.

  For example, a technique utilizing the effect of suppressing the coarsening of austenite grains by fine dispersion of TiN and acting as a ferrite transformation nucleus has been put into practical use. In addition, Patent Document 1 discloses a technique for dispersing an oxide of Ti. A technique for combining the ferrite nucleation ability of BN is disclosed.

  Further, technologies for controlling the form of sulfides in steel are disclosed in Patent Document 3 and Patent Document 4, and steel materials for high heat input welding that achieve high toughness of the weld heat affected zone by adding Ca and REM, respectively. It is disclosed.

  On the other hand, there is also a technical request for increasing the strength of these high heat input welding steel materials, and in response to this, it is necessary to stably maintain the base material performance, particularly the base material toughness, while maintaining the high strength. . In general, it is known that when the strength and thickness of a steel material are increased, the toughness of the base material decreases with it. In addition, in steel materials for high heat input welding, trace elements (microalloys) such as Ti, Nb, B, and N are utilized to ensure the toughness of the weld heat affected zone (sometimes simply referred to as “HAZ”). In many cases, the effect of application of these trace elements to the steel sheet has a problem that the strength and toughness of the steel sheet base material (also simply referred to as “base material”) varies and decreases.

  Several countermeasures have been proposed to date for such variations and reductions in base material toughness. For example, Patent Document 5 and Patent Document 6 disclose a technique for controlling the rolling condition precisely so that the grain size of the base material structure is not more than a certain size, thereby stabilizing the toughness.

  Patent Document 7 also discloses a technique of making Ti-based carbonitrides and Nb-based carbonitrides finer and improving the base metal low-temperature toughness by controlling the width of the δ temperature region of steel narrowly. Yes.

  On the other hand, in Patent Document 8, a DQT method (direct quenching-tempering) is selected as a steel plate manufacturing method instead of the TMCP method, thereby forming a bainite structure in which C (carbon) distribution in the transformation process is uniform, A technique for stably securing the toughness of a steel sheet is disclosed.

JP 57-51243 A JP-A-62-170459 JP 60-204863 A Japanese Patent Publication No. 4-14180 JP 2009-68050 A JP 2009-74111 A JP 2007-239090 A Japanese Patent No. 3599556 Japanese Patent No. 3546308

  It is known that trace elements such as Ti and B which are important in the components of high heat input welding steel materials are greatly affected by the amount of N. The variation in the amount of N in the steel affects the amount of TiN formation and the amount of solute B, which in turn is a major factor in variation in the strength and toughness characteristics of the base metal. In the steel manufacturing process, the amount of N can be accurately controlled within a narrow range by adopting on-site control in the steel making process, but still varies within a range of ± 10 ppm.

  Patent Document 5 and Patent Document 6 disclose precise rolling condition control, and Patent Document 7 discloses temperature management based on δ temperature range control. If there is a variation in the amount of N between the steel charges, it is difficult to reduce the variation in the base metal toughness due to this variation.

  Patent Document 8 attempts to reduce such a variation in base material toughness by selecting the DQT method (direct quenching-tempering). However, from the viewpoint of manufacturing efficiency, cost, and short delivery time, it is manufactured by the TMCP method. Is desired.

  Accordingly, the present invention is based on the premise of manufacturing by the TMCP method, and sets an appropriate rolling finish temperature (FT) according to the N amount variation between the steel output charges, and performs welding with a welding heat input of 300 kJ / cm or more. The present invention also provides a method for producing a steel material for high heat input welding in which the toughness of the heat affected zone is not lowered.

  The present inventors have solved these problems, and produced a steel material for high heat input welding having excellent HAZ toughness even if high heat input welding having excellent base metal toughness and exceeding 300 kJ / cm is performed. The method was intensively studied.

  First, in the present invention, for securing the HAZ toughness of the welded part in the high heat input welding, Ti, B, N, and Ca are appropriately contained in consideration of the knowledge disclosed in Patent Document 9, and further improvement studies are made. To complete the present invention. According to the present invention, it is possible to suppress the coarsening of γ grains during high heat input welding and to finely disperse ferrite transformation nuclei that do not dissolve even at high temperatures. High toughness can be achieved as a pearlite structure.

  First, the present inventors diligently studied a TMCP manufacturing method for ensuring stable base metal toughness of high heat input welding steel under the restriction of alloy elements of Ceq ≦ 0.36. Here, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and each element symbol represents the content (mass%) of each element, and is 0 when not contained. In the following description, when it is simply described as Ceq, it means that obtained by the above formula.

As a result of the study, the inventors have obtained the knowledge that an appropriate rolling finish temperature (FT) may be set according to the variation in the N amount between the steel charges, and have reached the present invention. Specifically, the finishing temperature is FT ≧ 790 ° C. when Ti / N ≦ 2.2, FT ≧ (1065−125 × Ti / N) when Ti / N> 2.2 (however, the Ar ₃ transformation point or higher) ) Is important. Here, Ti and N are amounts (mass%) contained in the steel sheet, respectively, but may be obtained from a ladle analysis value (mass%).

This invention is completed based on said knowledge, The summary is as follows. (1) The component composition of steel is the mass%, C: 0.03-0.1%, Si: 0.01 to 0.3%, Mn: 1.2 to 2.0%, P: 0.015% or less, S: 0.0005 to 0.0040%, Al: 0.005 to 0.1%, Ni: 0.1-0.6%, Nb: 0.005-0.02%, Ti: 0.005-0.02%, N: 0.0035-0.0070%, Ca: 0.0005 0.0030%, B: 0.0003-0.0025%, Ceq ≦ 0.36 is satisfied, and the balance is Fe and unavoidable impurities. After heating the steel material, the steel sheet surface temperature is 850 ° C. or less. Rolling at a cumulative reduction rate of 40% or more in the temperature range of
Finishing temperature of the rolling: When FT (° C.) is Ti / N ≦ 2.2, FT (° C.) ≧ 790 ° C.
When Ti / N> 2.2, FT (° C.) ≧ (1065−125 × Ti / N) (° C.) and FT (° C.) ≧ Ar ₃ transformation point (° C.)
Thereafter, the cooling start temperature is set to (Ar ₃ -30) ° C. or higher, the cooling stop temperature is set to a temperature within the range of 300 to 500 ° C., and accelerated cooling is performed. Method.

Here, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and each element symbol represents the content (mass%) of each element, and is 0 when not contained.
(2) The steel material is further mass%, Cr: 0.1 to 0.5%, Mo: 0.01 to 0.3%, V: 0.01 to 0.2%, Cu: 0 The method for producing a steel material for high heat input welding according to (1), comprising at least one selected from 1 to 0.5%.
(3) The steel material is further selected by mass% from Mg: 0.005% or less, Zr: 0.02% or less, Ta: 0.02% or less, REM: 0.01% or less. One or more types are contained, The manufacturing method of the steel material for large heat input welding as described in (1) or (2) characterized by the above-mentioned.

  According to the present invention, a steel plate having a tensile strength (TS) of 510 MPa class having excellent weld heat-affected zone toughness even in large heat input welding of 300 kJ / cm or more is industrially stable. Can be manufactured.

  Therefore, the present invention greatly contributes to improving the quality of large structures constructed by high heat input welding such as submerged arc welding, electrogas welding, and electroslag welding.

  A mode for carrying out the present invention will be described. In the following description, the notation of% of the element unit means mass%.

First, regarding the component composition of steel, in the present invention, the carbon equivalent Ceq is defined as follows.
Ceq: 0.36% or less The higher Ceq, the higher the strength and the lower the elongation. Ceq also serves as an index of the toughness of the weld heat affected zone. When it exceeds 0.36%, the toughness of the weld heat affected zone at the time of high heat input welding deteriorates. For this reason, Ceq is made 0.36% or less.
Here, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, and each element symbol represents the content (% by mass) of each element, and is 0 when not contained.

  Hereinafter, the reason for limitation of each component is demonstrated.

C: 0.03-0.1%
C is necessary to obtain the strength required for structural steel, and the C content is required to be 0.03% or more, but if it is excessively contained, island martensite is generated in the weld heat affected zone. Therefore, in order to suppress the production amount, the upper limit of the C content is set to 0.1%.

Si: 0.01 to 0.3%
Si is added as a deoxidizing element during steelmaking, but the Si content should be 0.01% or more. Island-like martensite is generated in the heat-welded heat-affected zone to deteriorate toughness. Therefore, the Si content is set to 0.01 to 0.3%.

Mn: 1.2 to 2.0%
In order to ensure the strength of the base material, the Mn content needs to be 1.2% or more. However, if it exceeds 2.0%, the toughness of the weld heat-affected zone deteriorates conversely. Therefore, the Mn content is 1.2 to 2.0%.

P: 0.015% or less When P exceeds 0.015%, the formation of island martensite is promoted in the structure of the weld heat affected zone to deteriorate the toughness of the weld heat affected zone. Is 0.015% or less.

S: 0.0005 to 0.0040%
S needs to be 0.0005% or more in order to contribute to the generation of CaS and MnS necessary for improving the structure of the heat affected zone, and if it exceeds 0.0040%, the toughness of the base material is deteriorated. , S content is 0.0005 to 0.0040%.

Al: 0.005 to 0.1%
Al is a deoxidizing element, and the Al content needs to be 0.005% or more. However, if it exceeds 0.1%, the toughness of the base metal is lowered and the toughness of the weld metal is also deteriorated. Therefore, the Al content is 0.005 to 0.1%. Preferably, the Al content is 0.01% or more.

Ni: 0.1 to 0.6%
Ni is an element effective for increasing the strength and toughness of the base material, but in order to obtain the effect, 0.1% or more is necessary, and if it exceeds 0.6%, the alloy cost increases. Therefore, the Ni content is 0.1 to 0.6%.

Nb: 0.005 to 0.02%
Nb is an element effective for ensuring the strength and toughness of the base steel sheet and the strength of the joint, but particularly important in the present invention as an element that can improve the balance between the strength and elongation of the base material. is there. In order to improve the elongation of the base steel sheet, it is necessary to increase the cooling stop temperature. On the premise of this, it is necessary to contain Nb in order to ensure the strength of the base material. However, if the content is less than 0.005%, the effect is small. Further, when Nb is contained in an amount exceeding 0.02%, the toughness of the weld heat affected zone deteriorates. Therefore, the Nb content is 0.005 to 0.02%.

Ti: 0.005-0.02%
Ti becomes TiN during the solidification of the molten steel and precipitates in the steel sheet, thereby suppressing the austenite coarsening in the weld heat-affected zone and increasing the toughness as a ferrite transformation nucleus. However, if the Ti content is less than 0.005%, the effect is small, and if it exceeds 0.02%, the precipitate of TiN becomes coarse and the above expected effect cannot be obtained. Is 0.005 to 0.02%.

N: 0.0035 to 0.0070%
N is an important element for securing the necessary amount of TiN precipitates. If the N content is less than 0.0035%, a sufficient amount of TiN precipitates cannot be obtained. If the N content exceeds 0.0070%, TiN present in the region subjected to the thermal cycle during welding is dissolved, and the solid solution N amount in that region increases, so that the toughness is significantly reduced. Therefore, the N content is in the range of 0.0035 to 0.0070%.

Ca: 0.0005 to 0.0030%
Ca is an element having an effect of improving toughness by fixing S. In order to exhibit such an effect, it is preferable to contain at least 0.0005% or more, but even if it exceeds 0.0030%, the effect is saturated. For this reason, in this invention, content of Ca is limited to the range of 0.0005%-0.0030%.

B: 0.0003 to 0.0025%
B is an element that reacts with N in the weld heat affected zone to generate BN, reduces the solid solution N present in the weld heat affected zone, and acts as a ferrite transformation nucleus. In order to obtain such an effect, the B content needs to be 0.0003% or more. However, if added over 0.0025%, the solid solution B tends to increase, the hardenability increases, and the weld heat affected zone increases. The toughness of the steel deteriorates. Therefore, the B content is in the range of 0.0003 to 0.0025%.

  In the present invention, at least one selected from the group consisting of Cr, Mo, V, and Cu, which is a first element group having a function of improving the strength, can be optionally and selectively contained within the following range. it can. Their contents are as follows.

Cr: 0.1 to 0.5%
Cr is an element effective for increasing the strength of the base material, but 0.1% or more is necessary to obtain the effect, and if it exceeds 0.5%, the toughness may be adversely affected. When added, the upper limit is preferably 0.5%.

Mo: 0.01 to 0.3%
Mo is an element effective for increasing the strength of the base material, but in order to obtain its effect, it is necessary to be 0.01% or more, and if added in a large amount, it may adversely affect toughness. In this case, the upper limit is preferably set to 0.3%.

V: 0.01 to 0.2%
V is an element effective for increasing the strength of the base material and works as a ferrite formation nucleus as VN. To obtain the effect, V content of 0.01% or more is required. If it exceeds 2%, the toughness may be adversely affected. Therefore, when added, the upper limit is preferably made 0.2%.

Cu: 0.1 to 0.5%
Cu is an element effective for increasing the strength of the base material. In order to obtain the effect, the Cu content needs to be 0.1% or more, and if it exceeds 0.5%, the toughness of the base material is increased. Since an adverse effect may be caused, when it is added, the upper limit is preferably set to 0.5%.

  In the present invention, if necessary, at least one selected from Mg, Zr, Ta, and REM, which is a second element group that improves toughness, is optionally and selectively contained within the following range. Can do. Their contents are as follows.

Mg: 0.005% or less Mg contributes to the formation of MgO, and this MgO has an effect of suppressing the coarsening of HAZ austenite grains, thereby improving the toughness of HAZ. In order to obtain such an effect, the Mg content is preferably 0.0001% or more. However, if the Mg content exceeds 0.005%, the base metal toughness and the HAZ toughness may be deteriorated. Therefore, when added, the upper limit is preferably made 0.005%.

Zr: 0.02% or less Zr is an element effective for improving HAZ toughness because it forms a nitride like Ti and suppresses the coarsening of austenite grains of HAZ during welding. In order to sufficiently exhibit such an effect, the Zr content is preferably 0.001% or more. However, if this amount exceeds 0.02%, the base metal toughness and the HAZ toughness may be lowered. Therefore, when added, the upper limit is preferably made 0.02%.

Ta: 0.02% or less Ta, like Nb, forms carbonitrides in steel and suppresses the coarsening of HAZ austenite grains during welding, and is therefore an effective element for improving HAZ toughness. In order to sufficiently exhibit such an effect, the Ta content is preferably 0.001% or more. However, if the content of Ta exceeds 0.02%, the base metal toughness and the HAZ toughness may be lowered. Therefore, when added, the upper limit is preferably made 0.02%.

REM: 0.01% or less REM is an element having an effect of improving HAZ toughness like Ca. REM has a spheroidizing effect of MnS, in other words, has an effect of reducing anisotropy by controlling the form of inclusions, and improves HAZ toughness. In order to sufficiently exhibit such an effect, it is preferable to contain 0.0005% or more of REM in the steel sheet. However, if the content of REM exceeds 0.01%, the base material toughness and the HAZ toughness may be deteriorated. Therefore, when added, the upper limit is preferably made 0.01%.

  In addition, the balance is Fe and inevitable impurities, but does not prevent the inclusion of elements that do not affect the effects of the present invention.

  The present invention relates to a method for producing the above steel material, and particularly defines rolling conditions and cooling conditions. All temperatures are steel sheet surface temperatures. Hereinafter, each condition will be described.

Cumulative rolling reduction: Cumulative rolling reduction of 40% or more in the temperature range of the steel sheet surface temperature of 850 ° C. or lower In order to sufficiently refine the microstructure after transformation and improve toughness, the temperature of 850 ° C. or lower, which is the austenite non-recrystallization temperature range. It is necessary to introduce processing strain in the region. If the cumulative rolling reduction is less than 40%, the ferrite crystal grain size after transformation is not sufficiently refined, and the toughness of the steel sheet is lowered. Therefore, the cumulative rolling reduction during rolling is set to 40% or more in a temperature range of 850 ° C. or lower.

Rolling finishing temperature (abbreviated as “FT (° C.)”): when Ti / N ≦ 2.2, FT (° C.) ≧ 790 ° C., when Ti / N> 2.2, FT (° C.) ≧ (1065− 125 x Ti / N)
The rolling finish temperature can be determined by the Ti / N level found by the ladle analysis. This is the most important point in the present invention. Here, Ti and N are the contents of each element in the steel, but values based on the ladle analysis analyzed in the steel making process can be used.

  In the case of Ti / N ≦ 2.2, the amount of the solid solution B that contributes to the hardenability of the base material is reduced, so that the base material strength is likely to be lowered. On the other hand, since the base material toughness is relatively stable, the finishing temperature was set higher as FT (° C.) ≧ 790 ° C. The reason for setting the finishing temperature to 790 ° C. or higher is that it is necessary to ensure the strength of the base material.

On the other hand, in the case of Ti / N> 2.2, the amount of solute B that contributes to the hardenability of the base material increases, so that the base material strength tends to fluctuate high, and further, the toughness of the Charpy test characteristics tends to decrease. When there is a margin in the base metal strength, it is preferable to finish at as low a temperature as possible according to the Ti / N level in order to stabilize the toughness. However, if the rolling finish temperature is lowered more than necessary, the strength of the base material may be insufficient, so FT (° C.) ≧ (1065−125 × Ti / N) (° C.). However, when (1065−125 × Ti / N) (° C.) is lower than the Ar ₃ transformation point, FT (° C.) ≧ Ar ₃ transformation point (° C.). When FT (° C.) is lower than the Ar ₃ transformation point, the toughness may be lowered, so that an Ar ₃ transformation point (° C.) or higher is required.

  Note that the rolling finish temperature: FT (° C.) is as high as possible at 790 ° C. as long as Ti / N ≦ 2.2 from the viewpoint of securing the base metal toughness, and when 10/125 × 10. A temperature just above Ti / N) (° C.) is desirable. Here, the term “directly above the desired temperature” refers to a temperature of (1065−125 × Ti / N) to (1065−125 × Ti / N) + 20 ° C.

Cooling condition: Control is performed to ensure a predetermined strength by improving the strength of the accelerated cooling and hardening phase from a temperature of (Ar ₃ -30) ° C. or higher to a range of 300 ° C. to 500 ° C. Cooling start temperature is lower as the intensity is lowered, the steel material average temperature (Ar 3 _-30) less than ° C. If for a given strength can not be obtained, and (Ar 3 _-30) ℃ or higher.

  As the cooling stop temperature is lower, the strength is improved, but the toughness is decreased. Therefore, in order to ensure sufficient base material toughness, the cooling stop temperature needs to be 300 ° C. or higher. On the other hand, since predetermined intensity | strength will not be obtained when it exceeds 500 degreeC, let cooling stop temperature be the temperature within the range of 300-500 degreeC.

  The accelerated cooling rate is preferably in the range of 5 to 20 ° C./s at the position of the thickness 1/4 t (t; thickness). This is because if the cooling rate is 5 ° C./s or more, a yield strength of 390 MPa or more can be secured, and if the cooling rate exceeds 20 ° C./s, the toughness may decrease.

The Ar ₃ transformation point may be measured, for example, but can be obtained from the formula Ar ₃ transformation point (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo. In this case, each element symbol represents the content (% by mass) of each element, and is calculated as 0 when not contained.

  Next, this invention is demonstrated based on an Example.

  Steel having the composition shown in Table 1 was melted and formed into a slab having a thickness of 170 mm by hot rolling. This slab was heated to 1150 ° C. for 2 hours, and a TMCP steel plate having a thickness of 60 mm was obtained under the rolling and cooling conditions shown in Table 2. A round bar tensile test piece (distance between gauge points: 70 mm, parallel part diameter: 14 mm) was sampled from the 1/4 t position of the obtained steel sheet, and the strength of the base material was evaluated. The target steel sheet strengths were such that yield strength: YS was 390 MPa or more, and tensile strength: TS was 510 MPa or more. Moreover, the toughness of the steel plate was similarly evaluated by a 2 mmV notch Charpy test at a 1/4 t position. The target toughness is −60 ° C. or lower in terms of vTrs. Here, t represents the plate thickness (mm) of the steel plate.

  Further, in order to measure the characteristics after welding heat cycle from these steel plates, a test piece having a width of 80 mm, a length of 80 mm, and a thickness of 15 mm is collected, heated to 1450 ° C., and then cooled to 800 to 500 ° C. at 1 ° C./s. The welding heat cycle was applied as a reproduction heat cycle, and the toughness of the weld heat affected zone was evaluated by a 2 mmV notch Charpy test. This cooling rate corresponds to a heat cycle that is received by the weld heat affected zone having a heat input of 450 kJ / cm in electrogas welding. Further, the target toughness characteristic is −30 ° C. or less in terms of vTrs.

  Table 2 shows the mechanical properties of the base metal and the toughness of the weld heat affected zone. From Table 2, in the examples of the present invention, good base material strength and toughness were obtained, and good toughness was also obtained in the weld heat affected zone. On the other hand, in the comparative example, the base material performance and the toughness of the weld heat affected zone are inferior. In these comparative examples, the content of each component such as values of C, Si, Mn, P, Nb, Ti, N, and Ceq is out of the scope of the present invention, or the rolling / cooling conditions are out of the scope of the present invention. It was a thing.

Claims (3)

The component composition of steel is mass%, C: 0.03-0.1%, Si: 0.01-0.3%, Mn: 1.2-2.0%, P: 0.015% or less , S: 0.0005-0.0040%, Al: 0.005-0.1%, Ni: 0.1-0.6%, Nb: 0.005-0.02%, Ti: 0.005 -0.02%, N: 0.0035-0.0070%, Ca: 0.0005-0.0030%, B: 0.0003-0.0025%, and Ceq <= 0.36 And the balance is Fe and steel material, which is an unavoidable impurity, is heated, and then rolled at a cumulative rolling reduction of 40% or more in the temperature range of the steel sheet surface temperature of 850 ° C or lower,
Finishing temperature of the rolling: When FT (° C.) is Ti / N ≦ 2.2, FT (° C.) ≧ 790 ° C.
When Ti / N> 2.2, FT (° C.) ≧ (1065−125 × Ti / N) (° C.) and FT (° C.) ≧ Ar ₃ transformation point (° C.)
Thereafter, the cooling start temperature is set to (Ar ₃ -30) ° C. or higher, the cooling stop temperature is set to a temperature within the range of 300 to 500 ° C., and accelerated cooling is performed. Method.
Here, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and each element symbol represents the content (mass%) of each element, and is 0 when not contained.   The steel material is further, in mass%, Cr: 0.1-0.5%, Mo: 0.01-0.3%, V: 0.01-0.2%, Cu: 0.1 The method for producing a steel material for high heat input welding according to claim 1, comprising at least one selected from 0.5%.   The steel material is further at least one selected from the group consisting of Mg: 0.005% or less, Zr: 0.02% or less, Ta: 0.02% or less, REM: 0.01% or less. The manufacturing method of the steel material for high heat input welding of Claim 1 or 2 characterized by the above-mentioned.