JP2010047815A - Steel sheet less in welding deformation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet capable of inexpensvely and securely suppressing welding deformation in a fillet welding. <P>SOLUTION: The steel sheet having reduced welding deformation has a chemical composition containing, by mass, 0.02 to 0.25% C, 0.01 to 0.7% Si, 0.3 to 2% Mn, ≤0.05% P, ≤0.008% S, 1 to 2.5% Cr, ≤0.05% Mo, 0.005 to 0.1% Nb, 0.003 to 0.1% Al and ≤0.01% N, and the balance Fe with impurities, and has a metallic structure composed of a ferritic structure of 10 to 60% and a bainitic structure and/or a martensitic structure of 40 to 90%, and in which the average grain size of the ferritic structure is ≤30 μm and the ratio between the hardness of the bainitic structure and/or martensitic structure and the hardness of the ferritic structure is ≥1.5. The steel sheet may further comprise one or more kinds selected from Ti, Cu, Ni, V, B, Zr, Ca, Mg and rare earth metals. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steel plate having a small welding deformation used in the fields of shipbuilding, offshore structures, building structures, bridges, civil engineering, and the like. In particular, the present invention relates to a thick steel plate with small welding deformation that occurs during fillet welding.

  In general, during the production of various welded steel structures, deformation occurs due to solidification shrinkage of the weld metal and subsequent shrinkage and expansion due to cooling and phase transformation. As a typical welding deformation, there is an angular deformation of a T-type fillet weld. If a structure is manufactured with the angular deformation left, the buckling strength is greatly reduced due to the deformation of the member, or the fracture characteristics are deteriorated. Therefore, the structure is not aimed by the designer. In order to prevent such a situation, various measures have been taken to prevent it.

  The welding deformation prevention measures currently applied are roughly divided into the following three (i) to (iii).

(i) Design ingenuity (method to increase the rigidity of the deformed member)
The reason why the welding deformation remains is that the vicinity of the weld toe of the weld metal or the base metal is subjected to plastic deformation. The part that has undergone plastic deformation tends to elastically deform its outer part, but if the rigidity is high, that is, the cross-sectional area is large, the amount of deformation is small. Therefore, changing the design to increase the cross-sectional area can be one preventive measure. However, the design change to increase the cross-sectional area has a lot of loss in terms of cost increase, weight increase and construction period extension of the steel material used.

(ii) Device for welding Welding deformation can be prevented by making some device for welding. There are several methods, but the first is to bend in the opposite direction before welding. Although angular deformation occurs after welding, there is a possibility that it will be finished in a desired shape by bending it in the opposite direction in advance. There is also a method in which the end is constrained during welding and deformation is not allowed. Further, there is a case where a backward torch is installed and bent back by reheating an appropriate position after welding. However, all of them are accompanied by a significant increase in man-hours, which causes a cost increase.

(iii) Straightening after welding There are mechanical straightening and linear heating straightening as methods for straightening after welding. However, these methods also require a significant increase in man-hours and require highly skilled skills.

  All of the measures (i) to (iii) above are contrivances in production. For example, Patent Document 1 proposes reducing welding deformation by contriving with welding materials. However, there are many problems that the cost increase of the welding material hinders the economic efficiency and the effect is insufficient, and the application is not practically progressing.

  On the other hand, there is an example of trying to suppress welding deformation by devising a steel material as a base material, and some have been proposed as follows.

  Patent Document 2 discloses a method of increasing yield stress by promoting precipitation in welding heat history by adding Nb and Mo in combination. However, since addition of Mo brings about a significant cost increase, it is poor in versatility.

  In Patent Documents 3 and 4, by controlling the fraction of bainite and / or martensite of the steel material as the base material to 20% or more and further defining the dispersion state of carbonitride, the yield stress is increased, and There is a description of suppressing welding deformation. However, it has not yet reached a practically sufficient weld deformation reduction effect.

  And patent document 5 has description of suppressing welding deformation by ensuring the bainite rate of the steel materials used as a base material to 70% or more, and also ensuring 0.0040% of solid solution Nb amount. However, when the bainite ratio is 70% or more, not only does the strength of the base material deviate from the general-purpose range, but there is a concern that inhibition of weld cracking by Nb may become a problem.

Japanese Unexamined Patent Publication No. 7-9191 Japanese Laid-Open Patent Publication No.7-138715 JP 2003-268484 A JP 2006-2211 A Japanese Unexamined Patent Publication No. 2006-2198

  Thus, the conventional methods have difficulty from the viewpoints of economy and practical reproducibility, and there is much room for improvement in practical use.

  In particular, in a welded structure manufactured using a thick steel plate having a thickness of 15 mm or more, even if the amount of deformation at each weld location is small, large deformation can occur as a whole welded structure. It is necessary to do. In addition, although the upper limit of thickness is not specifically limited, It is preferable to handle a thing to 50 mm.

  In view of the above circumstances, an object of the present invention is to establish a technique for reliably suppressing welding deformation in fillet welding at low cost, and to provide a steel plate having small welding deformation. In addition, the target value of the amount of welding deformation was set to 1/2 that of the conventional steel.

  As a result of various studies, the present inventors have specified the chemical composition of the steel sheet and the metal structure thereof in order to solve such problems. FIG. 1 shows the independent influence of the physical property values of each material obtained by the thermal coupled FEM analysis conducted in conjunction with the experiment. Moreover, the calculation conditions of FEM analysis are shown in FIG.

  In FIG. 1, the horizontal axis represents thermal conductivity (white circle plot), transformation point Ac1 (black circle plot), and strength TS (square plot), and the vertical axis represents the amount of angular deformation. From FIG. 1, it can be seen that the amount of angular deformation does not change even when the thermal conductivity of the steel sheet is increased, the amount of angular deformation increases as the transformation point increases, and the amount of angular deformation decreases as the strength increases. Therefore, the welding deformation particularly depends largely on the strength and transformation point, and the target value of the welding deformation amount (angular deformation amount) is ½ that of conventional steel (angular deformation amount is about 0.8 mm), that is, 0.4 mm. The strength becomes extremely high and deviates from the general-purpose strength class. Deviations from the general strength class are not desirable because they are not only subject to general commercial transactions, but may also cause structural design problems and weldability problems.

  Therefore, the present inventors aimed to develop a steel type that increases the high-temperature strength while maintaining the normal-temperature strength suitable for the general-purpose strength class.

  Note that Mo, which has been conventionally said to be effective in high-temperature strength, is not realistic because it causes a cost increase due to a rise in alloy costs. Accordingly, the inventors focused on Cr, which is relatively inexpensive and has a small adverse effect on weldability, and conducted various tests. As a result, the following findings (a) to (d) were obtained.

  (a) Cr can increase the high-temperature strength. When Cr is contained in an amount of 1.0% or more, high-temperature strength can be secured without coexisting Mo, and welding deformation can be sufficiently suppressed. However, if the Cr content is less than 1.0%, the high temperature strength cannot be ensured sufficiently if Mo is not allowed to coexist.

  (b) It is essential to contain Nb. If Nb is not contained, securing of high temperature strength is insufficient. However, the amount of Nb added may be small and may be 0.005% or more.

  (c) In order to conform to the general-purpose strength level, it is essential to include a ferrite structure. From the viewpoint of toughness, the crystal grain size of the ferrite structure needs to be 30 μm or less. In addition, in order to minimize welding deformation, the hardness of the hard phase composed of bainite or martensite is better, and the hardness ratio of the hard phase to the soft phase needs to be 1.5 or more.

  (d) The manufacturing method of the steel sheet may be under general conditions, but since it tends to be harder than normal steel, it is preferable to devise in order to adapt it to the general-purpose strength level.

  The present invention has been completed on the basis of the above knowledge, and the gist thereof is a steel sheet with small welding deformation shown in the following (1) to (4).

  (1) By mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.7%, Mn: 0.3 to 2%, P: 0.05% or less, S: 0.00. 008% or less, Cr: 1 to 2.5%, Mo: 0.05% or less, Nb: 0.005 to 0.1%, Al: 0.003 to 0.1%, and N: 0.01% or less The metal structure is composed of 10-60% ferrite structure and 40-90% bainite structure and / or martensite structure, and the average grain size of the ferrite structure is A steel plate having a small welding deformation, wherein the ratio of the hardness of a bainite structure and / or martensite structure to the hardness of a ferrite structure is 1.5 or more, and is 30 μm or less.

  (2) The steel plate having a small weld deformation according to the above (1), further comprising Ti: 0.1% or less by mass%.

  (3) By mass%, Cu: 2% or less, Ni: 3.5% or less, V: 0.1% or less, B: 0.004% or less, and Zr: 0.02% or less The steel plate having a small weld deformation according to the above (1) or (2), comprising a seed or two or more kinds.

  (4) The above-mentioned, characterized by further containing one or more of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less in mass%. A steel plate with small welding deformation according to any one of (1) to (3).

  Considering the present invention from the viewpoint of a welding method with small weld deformation in a steel plate, the weld deformation in the steel plate is substantially a weld deformation in the weld heat affected zone, and therefore satisfies a predetermined requirement in the weld heat affected zone. In addition, if welding is performed, the ability to suppress welding deformation is considered to be improved.

Therefore, from the viewpoint of the welding method, the present invention
“In mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.7%, Mn: 0.3 to 2%, P: 0.05% or less, S: 0.008% Hereinafter, including Cr: 1 to 2.5%, Mo: 0.05% or less, Nb: 0.005 to 0.1%, Al: 0.003 to 0.1%, and N: 0.01% or less , A method of welding a steel sheet having a chemical composition comprising the balance Fe and impurities, wherein the metal structure of the part that becomes the weld heat affected zone in the steel sheet before welding is a ferrite structure of 10 to 60% and a bainite structure and / or a martensite structure The average grain size of the ferrite structure is 30 μm or less, and the ratio of the hardness of the bainite structure and / or martensite structure to the hardness of the ferrite structure is 1.5 or more. Features welding method. "
It can also be grasped.

Of course, the steel sheet is in% by mass, Ti: 0.1% or less, Cu: 2% or less, Ni: 3.5% or less, V: 0.1% or less, B: 0.004% or less, Zr : 0.02% or less, Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less may be contained.
Here, as will be described later, the steel plate as a base material may be welded after being manufactured so as to satisfy the above requirements, or the part to be welded (welding heat effect in the steel plate as a base material). It is also possible to weld only after satisfying the above requirements for the part by processing only the part to be a part.

  And this welding method is applicable also in the case of fillet welding with a large welding deformation. Fillet welding is performed on lap joints, T joints, cruciform joints, etc., but this welding method involves fillet welding on T joints and cruciform joints, which cause particularly large welding deformations due to the relative positional relationship of the joint base material. Is particularly effective.

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate which can suppress a welding deformation in fillet welding reliably at low cost can be provided.

  In the present invention, the reason for limiting the chemical composition and metal structure of the steel sheet with small welding deformation is as follows.

(A) Chemical composition of steel sheet The effects of each component of the steel sheet and the preferred contents of each component are as follows. In addition, "%" regarding content means "mass%".

C: 0.02-0.25%
C is the most effective element for improving the strength and is an inexpensive element. However, if it is less than 0.02%, it is necessary to guarantee strength by using other elements in combination, resulting in an increase in cost. Moreover, when it contains exceeding 0.25%, weldability will be inhibited remarkably. Therefore, the C content is 0.02 to 0.25%.

Si: 0.01 to 0.7%
Si is an element contributing to strength improvement. However, if it is less than 0.01%, the required strength cannot be ensured. Moreover, if added over 0.7%, the base metal toughness and weldability toughness will be remarkably deteriorated. Therefore, the Si content is set to 0.01 to 0.7%.

Mn: 0.3-2%
Mn is an element necessary for ensuring strength. However, if it is less than 0.3%, the required strength cannot be ensured. On the other hand, if it exceeds 2%, weldability deteriorates. Therefore, the Mn content is 0.3-2%.

P: 0.05% or less P is an element present in steel as an impurity. If the P content exceeds 0.05%, it not only segregates at the grain boundaries and lowers the toughness, but also causes hot cracking during welding, so the P content is 0.05% or less.

S: 0.008% or less S is an element present in steel as an impurity. If the S content exceeds 0.008%, center segregation is promoted or a large amount of stretched MnS is generated, so that the mechanical properties of the base material and the HAZ deteriorate. Therefore, the upper limit of the S content is 0.008%.

Cr: 1 to 2.5%
Cr is an effective element for increasing the strength by improving the hardenability. In order to obtain this effect, addition of 1% or more is necessary. However, if it exceeds 2.5%, the toughness deteriorates. Therefore, the Cr content is 1 to 2.5%. In addition, preferable content is 1-1.8%.

Mo: 0.05% or less Mo is not added because it causes a significant increase in cost. Although impurities may be mixed in, the Mo content should be 0.05% or less even in that case.

Nb: 0.005 to 0.1%
Nb has the effect of delaying recrystallization of the metal structure of the steel sheet. However, if the content is less than 0.005%, the effect cannot be obtained. On the other hand, if it exceeds 0.1%, the above effect is saturated while the toughness of the HAZ is significantly impaired. Therefore, the Nb content is 0.005 to 0.1%. In addition, preferable content is 0.008 to 0.020%.

Al: 0.003-0.1%
Al is an essential element for deoxidation. In order to reliably perform deoxidation, a content of 0.003% or more is necessary. However, if it exceeds 0.1%, the toughness tends to deteriorate particularly in HAZ. This is presumably because coarse cluster-like alumina inclusion particles are easily formed. Therefore, the Al content is 0.003 to 0.1%.

N: 0.01% or less N is an element present in steel as an impurity. If the N content exceeds 0.01%, the base material toughness and the HAZ toughness are deteriorated. Therefore, the upper limit of the N content is 0.01%.

  The steel plate with small welding deformation according to the present invention can contain one or more components selected from at least one group from the following first group to the third group, if necessary, in addition to the above components. . Hereinafter, components belonging to these groups will be described.

Group 1 ingredients: Ti
Ti: 0.1% or less Since Ti mainly acts as a deoxidizing element, it can be contained if necessary. However, since deoxidation can be performed with Al, it is not always necessary to contain it. However, since Ti oxide or Ti—Al oxide is formed when the Ti content is large, the ability to refine the structure particularly in the heat-affected zone of the small heat input weld zone is lost. For this reason, Ti content in the case of making it contain as needed is 0.1% or less. In addition, in order to acquire the deoxidation effect by containing Ti reliably, it is preferable to make the content into 0.01% or more.

Second group of components: Cu, Ni, V, B, Zr
Cu: 2% or less Cu can improve the strength without significantly degrading toughness, and can be contained as required. However, if the Cu content exceeds 2%, turtle shell cracks are generated during hot rolling, so the Cu content in the case of inclusion if necessary is 2% or less. In addition, in order to acquire the strength improvement effect by containing Cu reliably, it is preferable that the content shall be 0.05% or more. A more preferable content is 0.2% or more.

Ni: 3.5% or less Ni is an element that improves the toughness of the base material and contributes to the improvement of the strength by improving the hardenability, and can be contained as necessary. However, since Ni is an expensive element, if Ni is excessively contained, it causes a large cost increase. For this reason, the upper limit of the content of Ni when it is contained as necessary is set to 3.5% or less. In addition, in order to acquire the said effect by containing Ni reliably, it is preferable to make the content into 0.05% or more.

V: 0.1% or less V is an element effective for improving the strength, and can be contained as necessary. However, if the V content exceeds 0.1%, the toughness is greatly deteriorated. Therefore, the V content in the case where V content is included is 0.1% or less. In addition, in order to acquire the strength improvement effect by containing V reliably, it is preferable that the content shall be 0.005% or more.

B: 0.004% or less B has an effect of improving hardenability and increasing strength, and can be contained as required. However, when the content of B exceeds 0.004%, the effect of increasing the strength is saturated, and the tendency of toughness deterioration becomes remarkable in both the base material and HAZ. Therefore, the content of B in the case where it is contained as necessary is 0.004% or less. In addition, in order to acquire the effect which improves hardenability and intensity | strength by containing B reliably, it is preferable that content of B shall be 0.0003% or more.

Zr: 0.02% or less Zr has the effect of finely dispersing and precipitating nitrides in steel and improving the strength, and can be contained as required. However, if added over 0.02%, coarse precipitates are formed and the toughness is deteriorated, so the Zr content in the case of inclusion if necessary is 0.02% or less. In addition, in order to acquire the strength improvement effect by containing Zr reliably, it is preferable that content of Zr shall be 0.0003% or more.

Group 3 components: Ca, Mg, REM
Ca: 0.004% or less Ca reacts with S in steel to form oxysulfide (oxysulfide) in molten steel. Unlike the stretched inclusions such as MnS, this oxysulfide does not extend in the rolling direction during rolling and is spherical after rolling. Therefore, welding with the tip of the stretched inclusions as the starting point of cracking Since there exists an effect | action which suppresses a crack and a hydrogen induction crack, it can be made to contain as needed. However, if its content exceeds 0.004%, toughness may be deteriorated. Therefore, the Ca content in the case of inclusion as necessary is 0.004% or less. In addition, in order to acquire the effect which suppresses a weld crack or a hydrogen induction crack reliably, it is preferable that content of Ca shall be 0.0003% or more.

Mg: 0.002% or less Mg forms an Mg-containing oxide, serves as a generation nucleus of TiN, and has an effect of finely dispersing TiN. Therefore, Mg can be contained as necessary. However, when the content exceeds 0.002%, the amount of oxide becomes excessive and ductility is reduced. Therefore, the upper limit of the content of Mg in the case where it is contained as necessary is set to 0.002%. In order to surely obtain the effect of finely dispersing TiN, the Mg content is preferably 0.0003% or more.

REM: 0.002% or less REM contributes to the refinement of the structure of the weld heat affected zone and the fixation of S, and can be contained as necessary. However, if the content exceeds 0.002%, REM becomes an inclusion that adversely affects the toughness of the base material, so the content of REM in the case of inclusion is set to 0.002% or less as required. In addition, in order to acquire the refinement | miniaturization of a structure | tissue and the fixing effect of S reliably, it is preferable that content of REM shall be 0.0003% or more. Note that REM is a generic name for 17 elements in which Y and Sc are combined with 15 elements of lanthanide, and one or more of these elements can be contained. Further, the content of REM means the total content of these elements.

(B) Metal structure The ferrite fraction of a metal structure shall be 10 to 60%. From the viewpoint of preventing welding deformation, it is better that the amount of ferrite, which is a structure that tends to yield, is better. The average grain size of the ferrite structure is preferably small from the viewpoint of fracture toughness. And since the sufficient fracture toughness cannot be obtained when the average particle diameter of a ferrite structure exceeds 30 micrometers, the upper limit was made into 30 micrometers.

  The structure other than the ferrite structure is a bainite structure and / or a martensite structure. That is, a bainite structure, a martensite structure, or a (bainite + martensite) structure. Here, the ferrite structure is called a soft phase, and the bainite structure and / or the martensite structure is called a hard phase. Since it is necessary to prevent the material from yielding at various temperatures as much as possible, it is desirable that the hardness of the hard phase is high. On the other hand, the presence of the soft phase makes it possible to adjust the yield strength and tensile strength of structural steel to a range that conforms to standards and the like. However, as described above, since the welding deformation is suppressed by keeping the hard phase hard, here, the index of the hardness ratio between the hard phase and the soft phase is used to define the ability to suppress welding deformation. . According to the study by the inventors, when the hardness of the hard phase is 1.5 times or more of the hardness of the soft phase, the improvement in the ability to suppress welding deformation becomes remarkable, so the hardness ratio is 1.5 times or more.

  Next, conditions for rolling and heat treatment for obtaining a steel sheet according to the present invention will be described.

Prior to hot rolling, the steel ingot is first heated, but if the heating temperature at this time is set to Ac ₃ or higher, it can be completely austenitic phase and homogenized without any untransformed part. It is preferable that the temperature be Ac ₃ point or higher. Specifically, it is preferable to heat to 900 to 1200 ° C. When the rolling finish temperature at the thin-walled end is set to 900 ° C. or lower during hot rolling, the crystal grains become an appropriate size and the fracture toughness of the material becomes sufficient. The lower limit of the rolling finishing temperature is not particularly defined, and any conditions may be used as long as the strength can be adapted to the general-purpose strength range. However, when the rolling finishing temperature is set to 700 ° C. or higher, anisotropy due to two-phase region processing is not noticeable, which is desirable. Subsequent to rolling, accelerated cooling may be performed. In the case of performing accelerated cooling, it is preferable to control the cooling rate of the central part to 0.5 to 20 ° C./s immediately after rolling or after some standing time. The cooling stop temperature is preferably controlled using 150 to 500 ° C. as a guide. Moreover, you may implement heat processing suitably after rolling. When the heat treatment is performed, it is preferable to perform a normalizing process or a tempering process, and it is preferable to select temperature ranges of 800 to 1100 ° C. and 300 to 700 ° C., respectively.

  An example of the steel plate concerning this invention is shown. Steel ingots having the composition components shown in Table 1 were produced under the heating temperature, finishing temperature, accelerated cooling, and heat treatment conditions shown in Table 2. The plate thickness of the steel plate was 16 mm.

  Table 3 shows the yield point YP, tensile strength TS, transition temperature vTrs, ferrite fraction, ferrite average grain size, hardness ratio of hard phase to soft phase, and welding angle deformation of the steel sheet thus obtained. Each is shown.

In addition, in order to measure the tensile property of the obtained steel plate, the test piece was extract | collected according to the test method as described in JIS-Z-2201. The sampling position was set to around ¼ of the plate thickness direction and the L direction (parallel to the rolling direction). The yield point was determined as a test speed of 10 N / mm · s, and the yield point was 0.2% proof stress when no clear yield point appeared. The target value of the tensile properties, the yield point YP is 350 N / mm ² or more, and the tensile strength TS has a 490~720N / mm ^2.

  Moreover, in order to measure the impact characteristic of the obtained steel plate, the test piece was extract | collected according to the test method as described in JIS-Z-2202. The sampling position was around 1/4 in the plate thickness direction and the L direction (parallel to the rolling direction), and a 2 mmV notch Charpy test piece was measured. The brittle fracture surface ratio at various temperatures was measured to determine the transition temperature. The target value of the Charpy characteristic is that the transition temperature is 0 ° C. or lower. Tissue observation was performed with an optical microscope. The image obtained by observation was subjected to image analysis. For example, when calculating the particle diameter, the short diameter and the long diameter were measured, and the particle diameter was obtained from 1/2 of the sum. The arithmetic average of the particle diameters of the individual particles obtained by observing 100 visual fields in this manner was defined as “average particle diameter”. In addition, the ferrite fraction of the metal structure was obtained by calculating the area ratio of ferrite with respect to the area for 100 field observations obtained by the same observation method as described above. The same applies to the bainite fraction and martensite fraction, but Table 3 shows only the ferrite fraction.

  And the welding angle deformation amount by fillet welding was evaluated in the following manner.

  As shown in FIG. 3, a T-shaped welding test piece was prepared for the steel plate, one side was constrained with a triangular steel plate having high rigidity, and the other side was subjected to 1-pass fillet welding. The welding material used was a general 50-kilo steel flux cored wire, and the welding conditions were 10.4 kJ / cm (200 A-26 V-30 cm / min). When a sufficient time has elapsed after welding, place the test piece on the surface plate, and measure the angular deformation θ defined in Fig. 4 with three clearance gauges at the welding start position, center position and end position. The average value thereof was taken as the welding angle deformation. In addition, the welding angle deformation amount of ordinary general-purpose 50 kg steel measured by this method is about 1 °, and the target welding angle deformation level of the present invention is 0.5 °.

  As a result, in the Mark 1-e steel plate (comparative example), the finishing temperature is as high as 910 ° C. and the cooling condition is air cooling, so the amount of ferrite produced is large, and the produced ferrite grows and the average grain size grows. Became larger. For this reason, the tensile strength was reduced. Moreover, although the hardness ratio between the hard phase and the soft phase is within the range of the present invention, the welding angle deformation amount is also increased. This is thought to be because the balance between the amount of ferrite as the soft phase and the amount of the hard phase is lost. As described above, the steel sheet of Mark 1-e has a low tensile strength and a large amount of welding angle deformation, and is therefore an inappropriate steel material as a structural steel sheet.

  Next, in the steel plate of Mark 1-f (comparative example), the cooling rate after heating was set to 25 ° C./sec. The toughness was also greatly reduced. Although the welding angle deformation is small, it is an inappropriate steel material for structural steel plates.

  Moreover, in the Mark12-b steel sheet (comparative example), the water cooling stop temperature was set to 120 ° C., and the steel was quenched to a relatively low temperature, so the hardness ratio between the hard phase and the soft phase was reduced and the welding angle deformation was increased. .

  Furthermore, in the steel plates of Mark 40 to 46 (comparative examples), the steel composition defined in the present invention was not satisfied, and the toughness of the steel plate itself was lowered. It is an inappropriate steel material as a structural steel plate.

On the other hand, in the steel sheets according to the examples of the present invention indicated by other marks, the tensile properties are all such that the yield point YP is 350 N / mm ² or more and the tensile strength TS is 490 to 720 N / mm ² class. General purpose steel with transition temperature vTrs, ferrite fraction, ferrite average particle size, hardness ratio of hard phase to soft phase, and proper welding angle deformation within 0.5 ° of target. Therefore, it turns out that it is suitable as a structural steel plate.

  As described above, according to the present invention, it is possible to provide a steel plate that can reliably suppress welding deformation in fillet welding at low cost.

It is a FEM calculation result which shows the influence of each material physical property value on the amount of welding angle deformation. It is a schematic diagram which shows the calculation conditions of FEM analysis. It is a figure which shows the test piece used in evaluating the welding angle deformation. It is a figure which shows the definition of the welding angle deformation amount.

Claims (4)

  In mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.7%, Mn: 0.3 to 2%, P: 0.05% or less, S: 0.008% or less Cr: 1 to 2.5%, Mo: 0.05% or less, Nb: 0.005 to 0.1%, Al: 0.003 to 0.1% and N: 0.01% or less, It has a chemical composition composed of the remaining Fe and impurities, the metal structure is composed of a ferrite structure of 10 to 60% and a bainite structure and / or a martensite structure of 40 to 90%, and the average grain size of the ferrite structure is 30 μm or less. A steel sheet with small welding deformation, wherein the ratio of the hardness of the bainite structure and / or martensite structure to the hardness of the ferrite structure is 1.5 or more.   The steel plate with small welding deformation according to claim 1, further comprising Ti: 0.1% or less by mass%.   In addition, one or two of Cu: 2% or less, Ni: 3.5% or less, V: 0.1% or less, B: 0.004% or less, and Zr: 0.02% or less. The steel plate with small weld deformation according to claim 1 or 2, wherein the steel plate contains seeds or more.   2. From mass%, further comprising one or more of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less. A steel plate having a small weld deformation according to any one of 3 to 3.

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