JP5172391B2 - Steel sheet with excellent toughness and uniform elongation of weld heat affected zone - Google Patents

Description

  The present invention relates to a steel plate applied to a welded structure such as a building, a ship, and an offshore structure, and is particularly referred to as a portion (hereinafter referred to as “HAZ”) that is affected by heat when performing high heat input welding. A) steel sheet having excellent toughness and uniform elongation.

  Structures in various fields such as ships, buildings, and offshore structures are generally constructed by joining steel materials by welding, but steel materials used in such structures have a viewpoint of ensuring safety. Therefore, it is required that not only the steel material strength but also the toughness of the welded portion is good.

  In recent years, with the increase in size of welded structures, from the viewpoint of improving the construction efficiency of the structure and reducing the construction cost, improvement in welding construction efficiency is required, and an increase in welding heat input is directed. In particular, there is a tendency that high heat input welding is performed such that the welding heat input is 20 kJ / mm or more.

  In carrying out the high heat input welding as described above, the HAZ [base metal side number of the weld metal-base metal interface (bond part) is affected by the heat of the weld base metal (steel plate as the welded material). The toughness at the position of mm] becomes a problem. In this HAZ, the base material is exposed to a high temperature just below the melting point during welding, the austenite grains in the metal structure tend to be coarse, and the cooling rate is also slowed due to the increase in welding heat input, so a coarse structure is likely to be formed. For these reasons, there has been a problem that the HAZ toughness tends to decrease.

  Various steel plates have been proposed so far to suppress HAZ toughness deterioration when the high heat input welding method is adopted. For example, Patent Documents 1 and 2 finely disperse TiN in the steel plate. In addition, a technique for improving HAZ toughness by complex precipitation of MnS and suppressing austenite grain coarsening has been proposed. Patent Documents 3 and 4 propose a technique for improving the toughness in the vicinity of the weld bond portion by finely depositing Ti oxide and using it as a nucleation site for ferrite transformation.

  Patent Document 5 proposes a technique for improving HAZ toughness by using BN precipitated on TiN or the like in the cooling process during welding as a nucleation site for ferrite transformation.

  By the way, it is also known that the HAZ toughness deteriorates when the amount of the solid solution N is excessive, and it is common to reduce the N to improve the HAZ toughness (for example, Non-Patent Document 1). Moreover, in patent document 6, from the viewpoint of reducing solid solution N thoroughly, by containing Ti and sufficient quantity of Al, and also using Ca oxide as a fine oxide, HAZ in super-high heat input welding is used. Techniques for improving toughness have also been proposed.

On the other hand, Patent Document 7 also proposes a technique for improving HAZ toughness in high heat input welding by utilizing CaS.
JP-A-2-250917 JP-A-2-254118 JP 60-245768 A JP 61-79745 A JP-A-61-253344 Japanese Patent Laid-Open No. 2001-107177 JP 2002-256379 A Journal of the Japan Welding Society, vol. 13, no. 4, P758-766 (issued in November 1985)

  However, none of the techniques that have been proposed so far has fundamentally improved the HAZ toughness, and each has the following problems.

  In the technology of finely dispersing TiN in steel (Patent Documents 1, 2, and 5), when large heat input welding is performed, the vicinity of the weld bond portion is heated to a high temperature for a long time. The actual situation is that it is dissolved and the coarsening of crystal grains cannot be suppressed, and good HAZ toughness cannot be obtained.

  Further, the technique of finely depositing Ti oxide (Patent Documents 3 and 4) has a problem that the HAZ toughness cannot be improved because it is difficult to uniformly disperse the oxide in the steel. In the technique for reducing the solid solution N (Patent Document 6 and Non-Patent Document 1), if excessive Ti is contained, the amount of solid solution Ti increases, and conversely, an embrittled structure is generated. is there.

  Moreover, in the technique using CaS (the said patent document 7), CaS becomes comparatively coarse and has not led to improving HAZ toughness. In this technology, the use of TiN is also considered, but the synergistic effect with the ferrite forming ability of TiN is not fully utilized, and the effect of improving the HAZ toughness in high heat input welding is not sufficient. There is also a problem of not.

  By the way, in the above steel plates, especially when used for building structures and steel structures, high uniform elongation is also required from the viewpoint of improving earthquake resistance. In other words, this uniform elongation means the elongation until the local contraction starts in the middle of the steel sheet breaking, and is an indicator of stability when the steel sheet is deformed. It is said that better earthquake resistance is obtained at higher values.

  As a means for improving uniform elongation, it is known to increase the amount of residual γ (residual austenite) (for example, TRIP steel plate using martensite transformation-induced plasticity), but when the residual γ is increased, island-like There has been a problem that martensite (MA) also increases and the base material toughness decreases. For these reasons, it is actually desired to establish a technique for improving uniform elongation while ensuring good base material toughness.

  The present invention has been made to solve such problems in the prior art, and its purpose is, of course, when large heat input welding is performed in which the welding heat input is 20 kJ / mm or more, For example, an object of the present invention is to provide a steel sheet that exhibits excellent HAZ toughness and excellent uniform elongation even when welding is performed such that the heat input is relatively small such that the heat input is 5 kJ / mm or more.

  The steel sheet of the present invention that can achieve the above object is C: 0.03 to 0.150% (meaning mass%; the same applies to chemical composition), Si: 0.50% or less (0% Mn: 1.0 to 2.0%, P: 0.015% or less (not including 0%), S: 0.005% or less (not including 0%), Al: 0.005 -0.06%, Ti: 0.008-0.030%, N: 0.0050-0.010%, Ca: 0.0010-0.0035% and O: 0.003% or less (0% Not included), the volume fraction of residual γ is 2 to 10%, and the average equivalent circle diameter of the island-shaped martensite (MA) is 3.0 μm or less, and the following (1), ( 2) It has a gist in that it satisfies the relationship defined by the equation.

1.0 ≦ [Ti] / [N] ≦ 2.5 (1)
However, [Ti] and [N] indicate the contents (mass%) of Ti and N, respectively.
2.0 ≦ 1000 × ([Ca] + 2 × [S] + 3 × [O]) ≦ 13.0 (2)
However, [Ca], [S] and [O] indicate the contents (mass%) of Ca, S and O, respectively.

  In the steel sheet of the present invention, if necessary, (a) B: 0.0035% or less (not including 0%), (b) Cu: 2.0% or less (not including 0%), Ni: 2. One or more selected from the group consisting of 0% or less (not including 0%) and Cr: 1.50% or less (not including 0%), (c) Mo: 0.5% or less (including 0%) (D) Nb: 0.035% or less (not including 0%) and / or V: 0.10% or less (not including 0%), (e) Mg: 0.005% or less (0 %), (F) Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less (not including 0%), (g) Co: 2.5% Or less (not including 0%) and / or W: 2.5% or less (not including 0%), (h) REM: 0.010% or less (not including 0%), etc. Is effective also contain, it is possible to further improve the properties of the steel sheet in accordance with the components to which they are contained.

  In the steel sheet of the present invention, for elements that affect the HAZ toughness, the chemical component composition is strictly defined and satisfied while satisfying a predetermined relational expression, thereby achieving good HAZ toughness and residual γ By controlling the amount and the particle size of island martensite (MA), uniform elongation can be improved while ensuring good base material toughness, which is extremely useful as a material for various building structures and the like.

  First, in order to achieve the above-mentioned problem of “exhibiting excellent HAZ toughness”, the inventors of the present invention repeatedly studied the factors affecting the HAZ toughness when performing high heat input welding from various angles. As a result, the HAZ toughness of the steel sheet is greatly influenced by the presence or absence of the formation of an embrittlement structure, and the formation of this embrittlement structure suppresses the austenite coarsening in a region heated to a high temperature, and ferrite transformation during cooling. It was found that it can be prevented by fine dispersion of transformation nuclei that promotes nucleation. Conventionally, it was considered that the toughness of HAZ could not be stably improved because these were insufficient.

  Therefore, the present inventors have further studied under the idea of effectively using CaS, TiN, and MnS generated using them as nuclei in the solidification stage during casting in order to finely disperse ferrite-forming nuclei. . CaS and TiN exist alone or in a composite precipitate with MnS, but in order to finely disperse them and disperse many ferrite-forming nuclei, the chemical composition of the steel sheet is adjusted appropriately. Thus, it has been clarified that it is effective to satisfy the relationship of the following expressions (1) and (2).

  Conventionally, it has been generally attempted to reduce N due to a decrease in toughness due to solute N (Non-Patent Document 1). However, in the present invention, [Ti] / [N An important point is that the effect of solid solution N, which increases when the ratio is made low (actively high N), can be reduced, and TiN itself is finely dispersed to improve the HAZ toughness. From these viewpoints, the following formulas (1) and (2) are defined. The reasons for defining these formulas are as follows.

1.0 ≦ [Ti] / [N] ≦ 2.5 (1)
However, [Ti] and [N] indicate the contents (mass%) of Ti and N, respectively.

  In order to finely disperse TiN and generate many ferrite nuclei, it is necessary to keep the balance of addition of Ti and N within this range. By adjusting to this balance, ferrite formation nuclei with CaS, MnS, etc. can be increased, and good HAZ toughness at high heat input can be ensured. If the value of [Ti] / [N] (hereinafter referred to as “P value”) exceeds 2.5, TiN becomes coarse, and if it is less than 1.0, the TiN generation amount itself decreases. From such a viewpoint, the above equation (1) is defined. In addition, the preferable minimum of P value is 1.5, and a preferable upper limit is 2.3.

2.0 ≦ 1000 × ([Ca] + 2 × [S] + 3 × [O]) ≦ 13.0 (2)
However, [Ca], [S] and [O] indicate the contents (mass%) of Ca, S and O, respectively.

  Formula (2) shows that there is a strong tendency to finely and uniformly disperse ferrite-forming nuclei in the order of Ca, S and O under the range of chemical components defined in the present invention. By setting the value of [1000 × ([Ca] + 2 × [S] + 3 × [O])] (hereinafter referred to as “Q value”) in the range of 2.0 to 13.0, large heat input Thus, a large number of ferrite-forming nuclei effective for securing the HAZ toughness can be introduced, and good HAZ toughness can be obtained. If the Q value exceeds 13.0, the ferrite nuclei become coarse, and if it is less than 2.0, the amount of ferrite nuclei itself decreases. A preferable upper limit of the Q value is 10.0, and a preferable lower limit is 3.0.

  In order to achieve another object of the present invention, which is to improve uniform elongation while ensuring good base material toughness, the present inventors control the amount of residual γ (residual austenite). It was clarified that it is effective to set the average particle size of the island-like martensite (MA) to a certain value or less.

The residual γ volume fraction is 2 to 10% and the MA average circle equivalent diameter is 3.0 μm or less. As described above, particularly when used for a building structure or a steel structure, from the viewpoint of improving the earthquake resistance. The uniform elongation is required to be large. As a means for improving the uniform elongation, it is conceivable to increase the amount of residual γ in the steel structure. Generally, however, when the amount of residual γ is increased, the island-like martensite (MA) is also coarsened. The toughness of the material decreases. In the present invention, the chemical component composition is strictly controlled, and a specific manufacturing method as shown in the examples described later is adopted, so that the martensite (MA) is prevented from becoming coarse while remaining. Succeeded in increasing the amount of γ, it is possible to achieve both base material toughness and uniform elongation.

  The volume fraction of residual γ with respect to the entire tissue is 2% or more, preferably 2.5% or more, more preferably 3.5% or more. The uniform elongation can be improved as the volume fraction of the residual γ is increased. However, if the volume fraction of residual γ becomes too large, the toughness and elongation decrease. Accordingly, the volume fraction of residual γ is 10% or less, preferably 7% or less, and more preferably 5% or less.

  The residual γ volume fraction was measured as follows. The peak intensity of the α-Fe (200) plane and the γ-Fe (200) plane by X-ray diffraction and the Liberty method of a test piece mirror-polished at a depth t / 4 position (t: plate thickness) of each steel plate. The theoretical strength ratio was determined from the ratio by calculation, and the residual γ fraction was determined. As the X-ray diffractometer, “RAD-RU300” manufactured by Rigaku Denki was used, the target was Co, and the target output was 40 kV, 200 mA.

  The average equivalent circle diameter of the island martensite (MA) is 3 μm or less, preferably 2.8 μm or less, and more preferably 2.3 μm or less. As the average equivalent circle diameter of the island martensite (MA) decreases, the base material toughness can be improved, and it is not necessary to set a lower limit, but a range that can be easily achieved is desirable, for example, 0.5 μm or more, preferably May be 1.0 μm or more, more preferably 1.5 μm or more.

  The average equivalent circle diameter of island martensite (MA) was measured as follows. Specular corrosion of the specimens mirror-polished at the depth t / 4 position (t: plate thickness) of each steel sheet, observing the structure with an optical microscope, photographing 10 areas of 1000 × magnification and 50 μm square, image analysis By processing with an apparatus (Media Cybernetics: Image-Pro Plus), the equivalent circle diameter of each island martensite (MA) is calculated, and the arithmetic average (arithmetic mean) is obtained.

  The structure of the steel sheet of the present invention is a structure mainly composed of bainite or a structure mainly composed of ferrite and bainite. The main body means an area ratio of 70% or more, and the remaining structure includes pearlite, martensite, cementite, etc. in addition to the above-mentioned residual γ (residual austenite) and island martensite (MA). Sometimes.

  In the steel plate of the present invention, it is also an important requirement to control its chemical composition to an appropriate range in order to exhibit its characteristics. The reasons for limiting the range including the elements (Ti, N, Ca, S and O) involved in the above formulas (1) to (2) are as follows.

[C: 0.03-0.150%]
C is an element necessary for ensuring the strength of the steel sheet (welding base metal), and it is necessary to contain 0.03% or more in order to ensure the desired strength. However, if C is contained excessively, the HAZ toughness is reduced instead. For these reasons, the upper limit needs to be 0.150%. In addition, the minimum with preferable C content is 0.05%, and a preferable upper limit is 0.08%.

[Si: 0.50% or less (including 0%)]
Si is an effective element for securing the strength of the steel sheet, and is contained if necessary. However, if it is contained excessively, a large amount of island martensite phase (MA phase) is precipitated in the steel material (base material) to deteriorate the HAZ toughness. For these reasons, the upper limit was made 0.50%. In addition, the minimum with preferable Si content is 0.1%, and a preferable upper limit is 0.4%.

[Mn: 1.0 to 2.0%]
Mn is an element effective in improving the hardenability and ensuring the strength of the steel sheet. In order to exert such effects, it is necessary to contain Mn in an amount of 1.0% or more. However, if Mn is contained excessively, the HAZ toughness of the steel sheet deteriorates, so the upper limit is made 2.0%. The minimum with preferable Mn content is 1.3%, and a preferable upper limit is 1.8%.

[P: 0.015% or less (excluding 0%)]
P is an impurity that is inevitably mixed in, and adversely affects the toughness of the steel sheet and HAZ. From such a viewpoint, P is preferably suppressed to 0.015% or less. The upper limit with preferable P content is 0.01%.

[S: 0.005% or less (excluding 0%)]
S is an element that works effectively for ferrite formation in the HAZ part by forming MnS on the CaS after welding by forming CaS in the steel plate during solidification of the steel plate during casting. Such an effect increases as its content increases, but if it is contained in excess of 0.005%, the toughness of the base material and HAZ deteriorates. In addition, in order to exhibit the said effect by S, it is preferable to make it contain 0.0003% or more, and a preferable upper limit is 0.0020%, More preferably, it is 0.0010%. In order to reduce this S to a predetermined range, the desulfurization time may be made relatively long (for example, 25 minutes or more).

[Al: 0.005 to 0.06%]
Al is an element effective as a deoxidizing agent, and also exhibits the effect of improving the toughness of the base material by refining the microstructure of the steel sheet. In order to exert such effects, the Al content needs to be 0.005% or more. However, if it is contained excessively, a large amount of island martensite phase (MA phase) is precipitated on the steel plate (base material) to deteriorate the HAZ toughness. For these reasons, the upper limit was made 0.06%. In addition, the minimum with preferable Al content is 0.01% (more preferably 0.02% or more), and a preferable upper limit is 0.04%.

[Ti: 0.008 to 0.030%]
Ti is an element that forms nitrides, suppresses coarsening of prior austenite grains during high heat input welding, and improves HAZ toughness. In order to exert such effects, the Ti content needs to be 0.008% or more. However, if Ti is excessively contained, coarse inclusions are precipitated, and the HAZ toughness is deteriorated. Therefore, the upper limit is made 0.030%. In addition, the minimum with preferable Ti content is 0.01%, and a preferable upper limit is 0.025%.

[N: 0.0050 to 0.010%]
In order to ensure high toughness in the HAZ during large heat input welding, it is effective to prevent coarsening of the prior austenite grains by finely depositing TiN in the prior austenite grains. In order to exert such effects, the N content needs to be 0.0050% or more. However, if the N content is excessive and exceeds 0.010%, coarse TiN precipitates and the HAZ toughness decreases. In addition, the minimum with preferable N content is 0.006%, and a preferable upper limit is 0.009% (more preferably 0.008%).

[Ca: 0.0010 to 0.0035%]
Ca is an element that contributes to the improvement of HAZ toughness by controlling the form of sulfide. In order to exhibit such an effect, it is necessary to contain 0.0010% or more, but even if it contains over 0.0035%, HAZ toughness will deteriorate on the contrary. In addition, the minimum with preferable Ca content is 0.0015% or more (more preferably 0.0020% or more), and a preferable upper limit is 0.0030%.

[O: 0.003% or less (not including 0%)]
O is contained as an unavoidable impurity, but exists as an oxide in steel. However, if the content exceeds 0.003%, coarse CaO is generated and the HAZ toughness deteriorates. For these reasons, the upper limit of the O content is set to 0.0030%. A preferable upper limit of the O content is 0.0020% (more preferably 0.0015%).

  In the steel sheet of the present invention, the other balance of the above components is composed of Fe and unavoidable impurities (for example, Sb, Se, Te, etc.), but there are also trace components (allowable components) that do not hinder the properties thereof. Such a steel sheet is also included in the scope of the present invention. If necessary, (a) B: 0.0035% or less (excluding 0%) (b) Cu: 2.0% or less (excluding 0%), Ni: 2.0% or less (0% 1) or more selected from the group consisting of Cr: 1.50% or less (not including 0%), (c) Mo: 0.5% or less (not including 0%), (d) Nb : 0.035% or less (not including 0%) and / or V: 0.10% or less (not including 0%), (e) Mg: 0.005% or less (not including 0%), ( f) Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less (not including 0%), (g) Co: 2.5% or less (not including 0%) ) And / or W: 2.5% or less (not including 0%), (h) REM: 0.010% or less (not including 0%), etc. To be the case. The reasons for limiting the range when these components are contained are as follows.

[B: 0.0035% or less (excluding 0%)]
B is an element effective for improving the HAZ toughness, in addition to generating intragranular ferrite with BN as a nucleus in the vicinity of the bond portion of high heat input welding and also having a fixing action of solute N. However, if the content of B is excessive, the bond portion has a coarse bainite structure, which adversely degrades the HAZ toughness. For these reasons, when B is contained, the upper limit is preferably made 0.0035%. A preferred range is 0.0010 to 0.0025%.

[Selected from the group consisting of Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%) and Cr: 1.50% or less (not including 0%) One or more
Cu, Ni and Cr are all effective elements for improving the hardenability and improving the strength, and are contained as necessary. However, if the content of these elements is excessive, the HAZ toughness is decreased, so that Cu and Ni are 2.0% or less (more preferably 1% or less), and Cr is 1.50% or less (more preferably). 1% or less). A preferable lower limit for exhibiting the above effect is 0.20% (more preferably 0.40%).

[Mo: 0.5% or less (excluding 0%)]
Mo is effective for securing the strength by improving the hardenability, and is appropriately used for preventing temper brittleness. Such an effect increases as the content thereof increases. However, if the Mo content becomes excessive, the HAZ toughness deteriorates, so that the content is preferably 0.5% or less. More preferably, it is good to set it as 0.3% or less.

[Nb: 0.035% or less (not including 0%) and / or V: 0.10% or less (not including 0%)]
Nb and V exhibit the effect of improving the hardenability and improving the base metal strength. V also has the effect of increasing the temper softening resistance. However, since HAZ toughness deteriorates when contained in a large amount, Nb is 0.035% or less (more preferably 0.030% or less), and V is 0.10% or less (more preferably 0.05% or less). It is good to do. The contents for effectively exhibiting these effects are 0.005% or more for Nb and 0.01% or more for V.

[Mg: 0.005% or less (excluding 0%)]
Since Mg has the effect of improving HAZ toughness by forming MgO and suppressing the coarsening of austenite grains in the HAZ, it is contained as necessary. However, if the Mg content is excessive, the inclusions become coarse and the HAZ toughness deteriorates. Therefore, the content is preferably 0.005% or less (more preferably 0.0035% or less).

[Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less (not including 0%)]
Zr and Hf are elements effective for improving the HAZ toughness by forming a nitride with N as in the case of Ti, refining the austenite grains of the HAZ during welding. However, if excessively contained, the HAZ toughness is reduced. Therefore, when these elements are contained, Zr is set to 0.1% or less and Hf is set to 0.05% or less.

[Co: 2.5% or less (not including 0%) and / or W: 2.5% or less (not including 0%)]
Co and W are contained as necessary because they have the effect of improving the hardenability and increasing the strength of the base material. However, if the content is excessive, the HAZ toughness deteriorates, so the upper limit is made 2.5% or less.

[REM: 0.010% or less (excluding 0%)]
REM (rare earth element) is an element that contributes to improving the toughness of HAZ by refining and spheroidizing the shape of inclusions (oxides, sulfides, etc.) inevitably mixed in the steel material, Contains if necessary. Such an effect increases as the content thereof increases. However, if the content of REM becomes excessive, inclusions become coarse and the HAZ toughness deteriorates. Therefore, the content is preferably suppressed to 0.010% or less. In the present invention, REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium).

  In order to manufacture the steel sheet of the present invention, the chemical composition of the steel sheet is appropriately adjusted, and steel that satisfies the requirements of the P value and the Q value is melted by a normal melting method, and the molten steel is cooled. And slab. Then, after heating and hot rolling, it is necessary to quench by a predetermined method. In addition, you may temper the hardened steel plate as needed.

  First, about cooling of molten steel, it cools from 1500 degreeC to 1100 degreeC with the cooling rate of 0.1-2.0 degree-C / sec, and forms a slab. TiN can be made sufficiently small even when cooled under such normal conditions, but in order to form finer TiN, the amount of cooling water in the casting machine and the cooling method are changed to improve the cooling rate during solidification. It is preferable.

  Next, the heating and finishing temperature of the hot rolling can be selected from a normal range. The heating temperature can be set from a range of about 950 to 1250 ° C., for example, and the finishing temperature can be set from a range of about 750 to 950 ° C., for example.

  And the most important thing in the manufacturing process of this invention is the hardening method after hot rolling. This quenching is performed in order to increase retained austenite while preventing coarsening of island martensite. There are two types of quenching methods (quenching method A and quenching method B). The quenching method A is a method in which a hot-rolled steel sheet is reheated directly or offline, and then subjected to a first quenching, heating again to perform a second quenching, and tempering. In the quenching method B, the hot-rolled steel sheet is reheated directly or offline, and then accelerated cooling (referred to as first accelerated cooling) halfway. This is called accelerated cooling. Detailed conditions of quenching method A and quenching method B are as follows.

[Quenching method A]
In the first quenching in the quenching method A, the cooling start temperature is 750 ° C. or higher, preferably 800 ° C. or higher, more preferably 850 ° C. or higher. If the cooling start temperature is too low, baking will not be sufficient. The cooling stop temperature of the first quenching is the same as that of normal quenching, and is, for example, 200 ° C. or less.

  The cooling start temperature in the second quenching is 850 ° C. or lower, preferably 800 ° C. or lower, more preferably 770 ° C. or lower, and is equal to or higher than the temperature at which ferrite-austenite becomes a two-phase (for example, 700 ° C. or higher). If the cooling start temperature is too high, the retained austenite becomes coarse. On the other hand, if the cooling start temperature is too low, quenching from the two-phase region does not occur, residual austenite becomes insufficient, and uniform elongation decreases. The cooling stop temperature of the second quenching is, for example, 200 ° C. or less.

  In both the first quenching and the second quenching, the cooling rate is the same as in the normal quenching, for example, 1 ° C./second or more, preferably 3 ° C./second or more, more preferably 5 ° C./second or more. is there.

  In this quenching method A, the reheating temperature of the second quenching is also important. The reheating temperature is, for example, about 700 to 900 ° C. If the reheating temperature is too low, the cooling start temperature becomes too low. On the other hand, if the reheating temperature is too high, it takes too much time to start cooling, resulting in insufficient residual austenite. The holding time is 15 minutes or longer.

  Tempering conditions can be set in a normal range, for example, hold at 400 to 600 ° C. for 10 to 30 minutes, and then cool.

[Quenching method B]
In the first accelerated cooling in the quenching method B, the cooling start temperature is 900 ° C. or lower (preferably 850 ° C. or lower, more preferably 800 ° C. or lower), 700 ° C. or higher (preferably 750 ° C. or higher, more preferably 800 ° C. or higher). ). The cooling stop temperature in the first accelerated cooling is 750 ° C. or lower (preferably 700 ° C. or lower, more preferably 650 ° C. or lower), or 550 ° C. or higher (preferably 600 ° C. or higher). If the cooling stop temperature is too high, residual austenite is insufficient. On the other hand, when the cooling stop temperature is too low, residual austenite is insufficient.

  The cooling rate of the first accelerated cooling is equivalent to the cooling rate of normal quenching, and is, for example, 1 ° C./second or more, preferably 3 ° C./second or more, more preferably 5 ° C./second or more. If the cooling rate is too slow, the first accelerated cooling will not be performed substantially, the C concentration into untransformed austenite will increase too much, and it will transform into pearlite or cementite, so there will be insufficient residual austenite. To do.

  From the end of the first accelerated cooling to the start of the second accelerated cooling, it may be kept isothermal, or may be gently cooled (for example, a cooling rate of less than 1 ° C./second (such as air cooling)). The time from the end of the first accelerated cooling to the start of the second accelerated cooling (hereinafter referred to as an interval) is, for example, about 20 to 130 seconds, preferably about 30 to 100 seconds, and more preferably about 40 to 80 seconds. is there. If the interval is too short, there is too little ferrite and C concentration to untransformed austenite is not sufficient, and residual austenite is insufficient. On the other hand, if the interval is too long, the concentration of C into untransformed austenite increases too much and transforms into pearlite or cementite, resulting in insufficient residual austenite.

  The start temperature of the second accelerated cooling is 700 ° C. or lower (preferably 650 ° C. or lower, more preferably 630 ° C. or lower), 550 ° C. or higher (preferably 600 ° C. or higher, more preferably 620 ° C. or higher). The cooling stop temperature in the second accelerated cooling is 400 ° C. or lower, preferably 300 ° C. or lower, more preferably 200 ° C. or lower. If the cooling stop temperature is too high, the second accelerated cooling is not substantially performed, and the residual austenite is insufficient. The cooling rate of the second accelerated cooling is the same as that of the first accelerated cooling.

  In the quenching method B, tempering may be performed after completion of the second accelerated cooling. The tempering conditions are the same as in the quenching method A.

  In addition, although the steel plate which is the subject of the present invention is basically a thick steel plate having a thickness of 20 mm or more, it has the same characteristics even in a thickness less than that, and is the subject of the present invention. Is included. In addition, the heat input when welding the steel sheet of the present invention is assumed to be 20 kJ / mm or more. When welding is performed with such a large amount of heat, good HAZ toughness is exhibited. For example, even if the amount of heat input is 5 kJ / mm or more, good HAZ toughness is exhibited.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and any design modifications may be made in accordance with the gist of the present invention. It is included in the range.

  Steels having the compositions shown in Tables 1 to 3 below are melted by a normal melting method, and the molten steel is cooled from 1500 ° C. to 1100 ° C. at a cooling rate of 0.1 to 2.0 ° C./min. A slab was obtained (slab thickness = 270 mm). Thereafter, this slab was hot-rolled and quenched as shown in Tables 4 and 5 to obtain a steel plate having a thickness of 60 mm. Table 4 shows the conditions for the quenching method A, and Table 5 shows the conditions for the quenching method B. In Table 1, REM was added in the form of a misch metal containing about 50% La and about 25% Ce. In Tables 1 to 3, “-” indicates that no element was added. Tables 1 to 3 also show the P value ([Ti] / [N]) and the Q value [1000 × ([Ca] + 2 × [S] + 3 × [O])] defined in the present invention. It was.

  The various steel sheets thus obtained were measured for tensile properties by the following method and welded under the following conditions to create welds.

[Tensile strength and uniform elongation of steel sheet]
JIS Z 2201 No. 4 test piece was taken from the depth t / 4 position (t: thickness) of the steel sheet, and a tensile test was conducted in accordance with JIS Z 2241, and the tensile strength (TS) and total elongation (EL) were measured. It was measured. In the system of the present invention, the uniform elongation is a value of about 50% of the total elongation. Therefore, in the test example, the total elongation was evaluated. In the present invention, when the tensile strength TS was 440 MPa or more and the total elongation EL was 20% or more, it was evaluated that the tensile strength was excellent and the uniform elongation was excellent.

[Base material toughness]
V-notch standard test piece (size: 10 mm x 10 mm x stipulated in JIS Z 2242 so that the longitudinal direction of the test piece is the rolling direction of the steel sheet (L direction) at a depth t / 4 position (t: plate thickness) 55 mm) was collected, a Charpy impact test was performed at -5 ° C, and a V Charpy impact value (vE _-5 ) at -5 ° C was measured.

[HAZ toughness test]
As a HAZ toughness evaluation method simulating a thermal cycle when electroslag welding (30 kJ / mm) is performed, a heating temperature is maintained at 1400 ° C. for 30 seconds, and then a cooling time is 800 to 500 ° C. (Tc): 500 seconds. After heat-treating each test steel plate in the thermal cycle, Charpy absorbed energy (V notch) at a temperature of −15 ° C. was measured. As a test piece, a Charpy impact test piece (JIS Z 2202, rod size of size 10 mm × 10 mm × 55 mm) was sampled from a depth t / 4 position (t: plate thickness), and a depth of 2 mm at the center part was measured. What formed V notch was used. At this time, a V Charpy impact value (vE _-15 ) of 150 J or more was considered acceptable.

In addition, after heat treatment simulating welding with a heat input equivalent to 5 kJ / mm (heating temperature: held at 1400 ° C. for 5 seconds, Tc = 120 seconds), V Charpy impact value (vE _-15 ) was measured in the same manner as above. did.

  These results are shown in Tables 6 and 7.

  From these results, it can be considered as follows. First, test no. 1-43 satisfy the requirements defined in the present invention, the strength of the steel sheet (base material) satisfies the target, and the HAZ toughness sufficiently satisfies the target average of 200 J or more. It can also be seen that these exhibit sufficient HAZ toughness even under welding conditions where the heat input is 5 kJ / mm.

  In contrast, test no. Nos. 44 to 78 lack any of the requirements defined in the present invention, and any of the characteristics is deteriorated. Of these, test no. Nos. 44 to 60 are those in which the chemical composition is outside the range defined in the present invention (Test No. 57 has a large P value). 61 and 69 satisfy the chemical composition, but the P value is outside the range defined in the present invention. Test No. 62-68 satisfy the chemical composition, but the Q value is outside the range defined in the present invention. No. 70 to 78 are examples that deviate from the scope of the method of producing the steel of the present invention, and lack the requirements of the present invention in at least one of the residual γ volume fraction and the MA average equivalent circle diameter. No. Nos. 70 to 78 are inferior in uniform elongation because the amount of residual γ is small. For 76, the island-shaped martensite (MA) is coarse, so the toughness of the base metal is also inferior.

Claims (9)

C: 0.03 to 0.150% (meaning mass%. The same applies to the chemical composition).
Si: 0.50% or less (including 0%),
Mn: 1.0-2.0%,
P: 0.015% or less (excluding 0%),
S: 0.005% or less (excluding 0%),
Al: 0.005 to 0.06%,
Ti: 0.008 to 0.030%,
N: 0.0050 to 0.010%,
Ca: 0.0010 to 0.0035%,
O: 0.003% or less (excluding 0%)
Each containing
The volume fraction of residual γ is 2 to 10%, and the average equivalent circle diameter of island martensite (MA) is 3.0 μm or less,
A steel sheet excellent in the toughness and uniform elongation of the weld heat affected zone, characterized by satisfying the relationship defined by the following formulas (1) and (2).
1.0 ≦ [Ti] / [N] ≦ 2.5 (1)
However, [Ti] and [N] indicate the contents (mass%) of Ti and N, respectively.
2.0 ≦ 1000 × ([Ca] + 2 × [S] + 3 × [O]) ≦ 13.0 (2)
However, [Ca], [S] and [O] indicate the contents (mass%) of Ca, S and O, respectively.   The steel plate according to claim 1, wherein B: 0.0035% or less (not including 0%).   Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: 1.50% or less (not including 0%) The steel sheet according to claim 1 or 2, comprising one or more kinds.   The steel sheet according to any one of claims 1 to 3, wherein Mo: 0.5% or less (not including 0%) is contained.   The steel sheet according to any one of claims 1 to 4, which contains Nb: 0.035% or less (not including 0%) and / or V: 0.10% or less (not including 0%).   The steel sheet according to any one of claims 1 to 5, which contains Mg: 0.005% or less (excluding 0%).   The steel sheet according to any one of claims 1 to 6, which contains Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less (not including 0%).   The steel sheet according to any one of claims 1 to 7, which contains Co: 2.5% or less (not including 0%) and / or W: 2.5% or less (not including 0%).   The steel sheet according to any one of claims 1 to 8, which contains REM: 0.010% or less (excluding 0%).