JP2005213534A - Method for producing steel material excellent in toughness at welding heat affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a steel material excellent in the toughness at a welding heat-affected zone. <P>SOLUTION: To a steel blank having composition composed by mass% of 0.01-0.15% C, 0.05-0.70% Si, 0.6-2.5% Mn, ≤0.030% P, ≤0.0060% S, 0.005-0.10% Al and 0.005-0.040% Ti, 0.0015-0.0100% N under condition of satisfying Ti/N: 0.8-4.0, and the balance Fe with inevitable impurities, after heating at a temperature in the range of 1000-1300°C for ≥5 hr, the steel blank treatment for cooling to ≤700°C is applied and thereafter, the hot-working is applied. Thereby, in the whole portion in the plate thickness direction, the toughness as the welding heat affected zone is improved over the wide range of welding heat input. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing steel materials suitable for use in shipbuilding, bridges, architecture, civil engineering, construction machinery, pressure vessels, and the like, and more particularly to a method for producing steel materials suitable for use in which welding is performed. In addition, although the steel materials as used in the field of this invention mainly point to a thick steel plate, they shall also include the shape steel, steel pipe, etc. which are welded.

  In the fields of shipbuilding, offshore structures, bridges, architecture, civil engineering, construction machinery, pressure vessels, etc., steel materials are generally welded to finish a desired shape of the structure. In such a structure, from the viewpoint of safety, it is important to secure not only the strength and toughness of the base material of the steel material used, but also the characteristics of the welded joint part to a predetermined level or more. Steel sheets manufactured using TMCP (Thermo-Mechanical Control Process) technology developed in recent years have a matrix structure with excellent strength and toughness. However, in the welded joint portion, the base material structure tends to be reheated and transformed by the welding heat cycle, resulting in a structure with deteriorated characteristics. The strength can be an appropriate joint strength by a combination of a weld metal and a base material. However, in the weld heat affected zone (hereinafter also referred to as HAZ), it is exposed to a high temperature just below the melting point during welding, the austenite crystal grains tend to coarsen, and the subsequent cooling tends to transform into a fragile upper bainite structure. There is concern about deterioration of HAZ toughness. In multi-layer welding, HAZ is reheated to a two-phase region by subsequent reheating, and the HAZ toughness is significantly deteriorated due to the formation of island martensite and precipitation embrittlement. For this reason, it has been left as a difficult problem to ensure HAZ toughness stably above a predetermined level.

  In response to such problems, a method for effectively improving the toughness of HAZ has been put into practical use by utilizing the austenite grain coarsening suppression technology and the ferrite formation promotion technology, which have been studied for a long time. In particular, the austenite grain coarsening suppression technology by the dispersion of TiN has been used as a basic technology for HAZ structure control, and various proposals have been made to further improve this function.

For example, Patent Document 1 contains one or both of Ti and Zr 0.002 to 0.10%, one or both of Ce and La 0.001 to 0.1%, and one or both of Ca and Mg 0.004% or less, and Ti and Ca Steels for high heat input welded structures that improve the toughness in the heat affected zone of high heat input welding, particularly in the vicinity of the bond, have been proposed by utilizing the combined effect of C and Ce. In Patent Document 2, N: 0.006% or less, B: 0.0003 to 0.0020%, Ti: 0.02% or less, (N-0.3Ti): 5 ppm or more, (N-0.5Ti): 25 ppm or more satisfied Thus, a steel for high heat input welded structure containing B and considering the balance between Ti content and N content has been proposed. Patent Document 3 proposes a welding steel containing 0.004 to 0.06% of TiO _x (x: 0.65 to 1.3) having a particle size of 5 μm or less. In the technique described in Patent Document 3, among Ti oxides that are stable at a higher temperature than TiN, TiO _x (x: 0.65 to 1.3) is effective for nucleation of fine acicular ferrite. It is said that the HAZ of heat welds can be made tough. Patent Document 4 includes welding that includes REM: 0.005 to 0.015%, N: 0.0008 to 0.0030%, and Ti: 0.001 to 0.004% so that Ti / N satisfies the relationship of 1.0 to 3.4. Steels for high heat input welding with excellent bond part toughness have been proposed. In the technique described in Patent Document 4, REM and Ti are added together, and in low N steel of 0.0030% or less, it is effective to improve the toughness of high heat input welds by making Ti 0.004% or less. Yes.

In addition to the above, many techniques for combining TiN with Ti oxide, REM acid / sulfide, and techniques for simultaneously adding B have been proposed for increasing the toughness of the high heat input welding heat-affected zone.
Japanese Patent Publication No.54-43970 Japanese Unexamined Patent Publication No. 60-204863 JP-A-57-51243 Japanese Examined Patent Publication No. 4-14180

However, according to the above-described prior art, it is possible to refine the HAZ structure, but there is a problem that the toughness of the HAZ may not be sufficiently improved. For example, when trying to improve HAZ toughness by dispersing TiN, in addition to strict adjustment of Ti amount and N amount, even if strict control of Ti / N ratio is performed, HAZ toughness can be improved. In some cases, it could not be obtained sufficiently. Further, even when B is contained in addition to the contents of Ti and N, there is a problem that the HAZ toughness may be deteriorated by small heat input welding.

  In addition, when TiN and oxides are combined, the problem that HAZ toughness cannot be sufficiently improved cannot be solved, and in actual operation, it is difficult to sufficiently disperse oxides. There was a problem that the expected effect could not be obtained sufficiently from the viewpoint of refining the structure.

  Furthermore, in the case of a thick steel plate using a continuously cast slab, a central segregation portion generally exists in the central portion of the plate thickness, and elements such as Mn are concentrated in this central segregation portion to provide hardenability. In order to improve, even when the above-described dispersion control of TiN or the like is performed, there is a problem that the HAZ toughness is remarkably lowered in such a central segregation portion, and the desired HAZ toughness cannot be obtained.

  As described above, in the above-described prior art, it has been left as a difficult problem to stably refine the structure and obtain high HAZ toughness at all positions in the thickness direction over a wide range of welding heat input. .

 The present invention solves the above-mentioned problems of the prior art, contains appropriate amounts of Ti and N, and exhibits high HAZ toughness at all positions in the thickness direction over a wide range of welding heat input. It aims at proposing the manufacturing method of the steel material excellent in.

  In order to achieve the above-mentioned problems, the present inventors have made a wide range of welding heat input from small heat input welding to large heat input welding for steel materials containing Ti and N, at all positions in the plate thickness direction. Research and examination of various factors affecting weld heat-affected zone toughness were conducted. As a result, when TiN is dispersed to improve HAZ toughness, precipitation is dispersed even if the Ti / N ratio is further controlled in addition to the strict adjustment of the Ti amount and N amount. It came to mind that TiN grains have a certain size distribution. The average value of the TiN grain size is almost determined by the Ti content, the N content, and the Ti / N ratio. I found that there is a big range.

  And the present inventors guessed that very fine TiN grain melt | dissolves with welding heat at the time of welding, increases the amount of solid solution N, and reduces the toughness of a welding heat affected zone remarkably. In particular, it has been found that fine TiN particles of about 50 nm or less dissolve even in a welding heat cycle such as the vicinity of a small heat input weld bond, increase the amount of solute N, and deteriorate the HAZ toughness.

  For such an increase in the amount of solute N in the weld heat affected zone, it is conceivable to add B and fix the solute N in the form of BN. However, the present inventors have no time margin for precipitation of BN when the welding heat input is small, and cannot fix solute N, and the added B acts as a hardenable element, and the weld heat affected zone. Since the structure was a hardened structure, it was thought that the weld heat-affected zone toughness deteriorated.

  And in order to maintain the welding heat-affected zone toughness of a steel material containing Ti and N stably and high over a wide range of welding heat input, the present inventors have a melting point of 50 nm or less. I came up with the idea that the fine TiN particles had to be removed beforehand at the stage of the steel material before hot rolling.

  The present inventors have conducted further studies, subjecting the steel material before hot rolling to a special heat treatment, once dissolved fine TiN particles, and reprecipitated in the subsequent cooling process to an appropriately sized TiN. By making it grow or Ostwald growth of undissolved TiN particles, the fine TiN disappears without solid-dissolving Ti and N in the base structure, and the optimum TiN for improving the heat affected zone toughness It has been found that a particle distribution can be obtained.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) When hot working a steel material to obtain a steel material having a predetermined size and shape, the steel material is mass%, C: 0.01 to 0.15%, Si: 0.05 to 0.70%, Mn: 0.6 to 2.5%, P: 0.030% or less, S: 0.0060% or less, Al: 0.005-0.10%, and Ti: 0.005-0.040%, N: 0.0015-0.0100%, so that Ti / N: 0.8-4.0 is satisfied And a steel material having a composition consisting of Fe and inevitable impurities in the balance, and prior to the hot working, the steel material is heated to a temperature in the range of 1000 to 1300 ° C. for 5 hours or more and then 700 ° C. or less. A method for producing a steel material having excellent weld heat-affected zone toughness, characterized by performing a steel material treatment that cools to a temperature.
(2) When the steel material is hot-rolled into a thick steel plate, the steel material is mass%, C: 0.01 to 0.15%, Si: 0.05 to 0.70%, Mn: 0.6 to 2.5%, P: 0.030. %: S: 0.0060% or less, Al: 0.005-0.10%, and Ti: 0.005-0.040%, N: 0.0015-0.0100%, so as to satisfy Ti / N: 0.8-4.0, The remainder is made of a steel material having a composition consisting of Fe and inevitable impurities. Prior to the hot rolling, the steel material is heated to a temperature in the range of 1000 to 1300 ° C. for 5 hours or more and then cooled to a temperature of 700 ° C. or less. A method for producing a thick steel plate excellent in weld heat-affected zone toughness, characterized by performing a steel material treatment.
(3) In (2), after re-heating the hot rolling to 1000 to 1200 ° C, the cumulative rolling reduction in the temperature range of 950 ° C or lower is 20% or higher, and the finish rolling finish temperature is 680 ° C or higher. A method for producing a thick steel plate, characterized by performing controlled rolling.
(4) In (2) or (3), after completion of the hot rolling, a controlled cooling is performed to cool the cooling rate to 1.0 ° C./s or higher and the cooling stop temperature to 600 ° C. or lower. Production method.
(5) The method for producing a thick steel plate according to (2) or (3), wherein a direct quenching process for rapid cooling to 250 ° C. or less and a subsequent tempering process are performed immediately after completion of the hot rolling.
(6) In any one of (2) to (5), after the hot rolling is finished, the steel plate is once cooled, then reheated and subjected to quenching and tempering treatment.
(7) In any one of (2) to (6), in addition to the above-mentioned composition, it is further mass%, Cu: 0.01 to 2.0%, Ni: 0.01 to 4.0%, Cr: 0.01 to 2.0%, Mo: 0.01 -2.0%, Nb: 0.003-0.1%, V: 0.003-0.5%, B: 0.0005-0.0040% of a thick steel plate characterized by containing one or more selected from Production method.
(8) In any one of (2) to (7), in addition to the above composition, it is further selected in mass% from Ca: 0.0001 to 0.0060%, Mg: 0.0001 to 0.0060%, and REM: 0.0001 to 0.0200% The manufacturing method of the thick steel plate characterized by setting it as the composition containing 1 type, or 2 or more types.

  According to the present invention, steel materials such as thick steel plates containing Ti and N, exhibiting high HAZ toughness at all positions in the plate thickness direction over a wide range of welding heat input, and excellent in weld heat affected zone toughness, It can be manufactured by a relatively easy method and has a remarkable industrial effect.

  First, the reasons for limiting the composition of the steel material used in the present invention will be described. Hereinafter, mass% in the composition is simply referred to as%.

C: 0.01-0.15%
C is an element that increases the strength of the steel material. In order to obtain the strength necessary for structural steel, C is required to be contained by 0.01% or more in the present invention. On the other hand, an excessive content exceeding 0.15% deteriorates the toughness of the weld. For this reason, C was limited to the range of 0.01 to 0.15%. In addition, Preferably, it is 0.05 to 0.11%.

Si: 0.05-0.70%
Si is an element that acts as a deoxidizing agent and increases the strength of the steel by solid solution, and requires 0.05% or more in steelmaking. On the other hand, the content exceeding 0.70% deteriorates the base material toughness. For this reason, Si was limited to the range of 0.05 to 0.70%. In addition, Preferably, it is 0.10 to 0.30%.

Mn: 0.6-2.5%
Mn is an element that dissolves to increase the strength of the steel material. In the present invention, the content of 0.6% or more is required to ensure the strength of the base material. On the other hand, the content exceeding 2.5% significantly deteriorates the toughness of the welded portion. For this reason, Mn is limited to a range of 0.6 to 2.5%. In addition, Preferably, it is 1.0 to 1.8%.

P: 0.030% or less P is preferably reduced as much as possible as an impurity element in the present invention. In the present invention, the content exceeding 0.030% degrades the toughness of the welded portion, so 0.030% was made the upper limit. In addition, Preferably, it is 0.020% or less.

S: 0.0060% or less S is present as an inclusion in the steel and lowers the ductility and toughness of the steel. Therefore, in the present invention, S is preferably reduced as much as possible. The content exceeding 0.0060% deteriorates the toughness of the base metal and the welded portion, so 0.0060% was made the upper limit. In addition, Preferably, it is 0.0040% or less.

Al: 0.005-0.10%
Al is an element that acts as a deoxidizer. In the present invention, it is necessary to contain 0.005% or more in terms of deoxidation of steel, but if it exceeds 0.10%, the toughness of the base metal is lowered. For this reason, Al was limited to 0.005 to 0.10% of range.

Ti: 0.005-0.040%
Ti combines with N during solidification and precipitates as TiN, suppresses the coarsening of austenite grains in the weld heat affected zone or becomes a transformation nucleus of ferrite to refine the structure and increase the toughness of the weld heat affected zone. Contribute. Such an effect is recognized when the content is 0.005% or more. On the other hand, if the content exceeds 0.040%, the TiN particles become coarse, and the desired toughness improving effect cannot be obtained. For this reason, Ti was limited to 0.005 to 0.040% of range. In addition, from a viewpoint of toughness, Preferably it is 0.008 to 0.020%, More preferably, it is 0.010 to 0.018%.

N: 0.0015-0.0100%
In order to suppress the coarsening of austenite grains in the weld heat affected zone or to secure the necessary amount of TiN as a transformation nucleus of ferrite, N is required to be contained in an amount of 0.0015% or more in the present invention. On the other hand, if the content exceeds 0.0100%, TiN particles become coarse, and the expected toughness improvement effect cannot be expected. For this reason, N was limited to the range of 0.0015 to 0.0100%. In addition, Preferably it is 0.0025 to 0.0070%, More preferably, it is 0.0040 to 0.0060%.

Ti / N: 0.8-4.0
In the present invention, Ti and N are contained so that Ti / N is 0.8 to 4.0 within the above ranges. When Ti / N is less than 0.8, the average particle diameter of TiN particles becomes too fine, and a preferable TiN particle distribution in the steel material cannot be obtained. On the other hand, when Ti / N exceeds 4.0, TiN particles become too coarse, and a preferable TiN particle distribution in the steel material cannot be obtained. For this reason, Ti / N was limited to the range of 0.8-4.0. In addition, Preferably it is 1.5-3.5, More preferably, it is 2.5-3.5.

  In addition to the basic components described above, in the present invention, Cu: 0.01 to 2.0%, Ni: 0.01 to 4.0%, Cr: 0.01 to 2.0%, Mo: 0.01 to 2.0%, Nb: 0.003 to 0.1%, V: One or more selected from 0.003 to 0.5%, B: 0.0005 to 0.0040%, and / or Ca: 0.0001 to 0.0060%, Mg: 0.0001 to 0.0060%, REM: 0.0001 to 0.0200% 1 type or 2 types or more selected from.

Cu: 0.01-2.0%, Ni: 0.01-4.0%, Cr: 0.01-2.0%, Mo: 0.01-2.0%, Nb: 0.003-0.1%, V: 0.003-0.5%, B: 0.0005-0.0040% One or more selected from
Cu, Ni, Cr, Mo, Nb, V, and B are all elements that have the effect of increasing the strength of the steel material, and can be selected from one or more as necessary.

  Cu is an element that has the effect of increasing the strength. However, such an effect is recognized at a content of 0.01% or more. However, a content exceeding 2.0% deteriorates the properties of the steel sheet surface due to hot brittleness, and is a matrix. Reduce the toughness of the material. For this reason, it is preferable to limit Cu to 0.01 to 2.0% of range. More preferably, it is 0.20 to 1.0%.

  Ni is an element that can increase the strength without lowering the toughness of the base metal, and such an effect becomes remarkable when the content is 0.01% or more, but even if it exceeds 4.0%, the effect Is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, it is preferable to limit Ni to the range of 0.01 to 4.0%. More preferably, it is 0.10 to 1.50%.

  Cr and Mo are both elements that effectively increase the strength of the base material, and the effect becomes remarkable when the content is 0.01% or more. On the other hand, if the content exceeds 2.0%, the toughness is remarkably lowered. For this reason, it is preferable to limit to the range of Cr: 0.01-2.0% and Mo: 0.01-2.0%. More preferably, Cr is 0.10 to 0.50%, and Mo is 0.05 to 0.80%.

  Nb and V are both elements that improve the strength of the base material and at the same time improve the toughness, and the effect becomes remarkable when the content is 0.003% or more. On the other hand, if Nb exceeds 0.1% and V exceeds 0.5%, the toughness is lowered. For this reason, it is preferable to limit Nb to 0.003 to 0.1% and V to 0.003 to 0.5%. More preferably, Nb is 0.010 to 0.060% and V is 0.005 to 0.050%.

  B is an element that increases the strength of the steel material through the improvement of hardenability. Such an effect becomes significant when the content is 0.0005% or more, but the effect is saturated even if the content exceeds 0.0040%. An effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, it is preferable to limit B to 0.0005 to 0.0040% of range. More preferably, it is 0.0008 to 0.0020%.

Ca: 0.0001 to 0.0060%, Mg: 0.0001 to 0.0060%, REM: One or more selected from 0.0001 to 0.0200%
Ca, Mg, and REM all improve the toughness of the steel by fixing S in the steel and form fine oxides and sulfides or inclusions that combine them, acting as TiN precipitation nuclei. It is an element that effectively contributes to fine dispersion of TiN, and can be contained alone or in combination of two or more as required. Such effects become prominent with each content of 0.0001% or more, but when Ca exceeds 0.0060%, Mg exceeds 0.0060%, and REM exceeds 0.0200%, the effect is saturated and inclusions in steel Is coarsened and the amount of inclusions is increased, and on the contrary, the toughness is deteriorated. For this reason, it is preferable to limit to Ca: 0.0001-0.0060%, Mg: 0.0001-0.0060%, and REM: 0.0001-0.0200%, respectively. More preferably, they are Ca: 0.0010-0.0030%, Mg: 0.0005-0.0020%, REM: 0.0020-0.0100%.

  The balance other than the components described above consists of Fe and inevitable impurities.

  The steel material used in the present invention is a generally known casting method such as a continuous casting method, an ingot casting method, etc., after melting the molten steel having the above composition by a usual method such as a converter, electric furnace, vacuum melting furnace or the like. And made into steel material such as slab.

  The steel material thus obtained is hot-worked to obtain a steel material such as a thick steel plate. In the present invention, before hot working, these steel materials are heated to a temperature in the range of 1000 to 1300 ° C. for 5 hours or more and then subjected to a steel material treatment for cooling to a temperature of 700 ° C. or less.

  By this steel material treatment, fine, preferably 50 nm or less TiN precipitated in the steel material is dissolved and Ostwald growth is performed.

  If the heating temperature of the steel material is less than 1000 ° C., fine TiN cannot be dissolved. On the other hand, when the temperature exceeds 1300 ° C., even effective TiN is dissolved and the surface of the steel material is significantly damaged by oxidation. For this reason, the steel material processing temperature was limited to the range of 1000-1300 degreeC. In addition, Preferably, it is 1150-1250 degreeC.

  Moreover, in the steel raw material process of this invention, it hold | maintains at the temperature within an above-described range for 5 hours or more. If the holding time is less than 5 hours, the time required for the Ostwald growth of dissolved TiN is insufficient. In addition, holding for more than 40 hours for a long period of time will saturate the effect and there will be no deterioration in the material, but it will be economically disadvantageous, such as reduced productivity. Is preferred. In addition, by applying the steel material treatment in the present invention, the homogenization of the components proceeds in the central segregation portion found in the continuously cast slab and the like, and the thickness of the base material and the weld heat affected zone, which has been a problem in the past, is 1/1 Degradation of toughness at 2t part can be improved. The effect of homogenization of the center segregation portion increases if the temperature is as high as possible within the above range for a long time.

  In the present invention, after the treatment at the above temperature and time, the steel material is cooled to a temperature of 700 ° C. or lower. When the cooling stop temperature is higher than 700 ° C., the austenite-ferrite transformation is not completely completed, and refinement of crystal grains cannot be achieved by subsequent processing.

  After the steel material treatment described above, the steel material is then reheated and subjected to hot working such as normal thick plate rolling, shape steel rolling, piercing rolling, etc. Steel, seamless steel pipes and other steel materials. Hereinafter, preferable hot working will be described by taking the production of a thick steel plate as an example.

  In the present invention, the hot working conditions in the production of the thick steel plate, that is, the hot rolling conditions are not particularly limited as long as the desired dimensional shape can be secured, but when high strength and high toughness are required. Is preferably subjected to controlled rolling and / or controlled cooling.

  The steel material subjected to the above steel material treatment is reheated to 1000 to 1200 ° C, and the cumulative rolling reduction in the temperature range of 950 ° C or less is 20% or more, and the finish rolling finish temperature is 680 ° C or more. It is preferable to perform controlled rolling. When the heating temperature is less than 1000 ° C., the steel plate temperature becomes too low, the deformation resistance becomes high, and the rolling load becomes maximum. On the other hand, if the temperature exceeds 1200 ° C., the crystal grains become coarse and the toughness tends to deteriorate. For this reason, the heating temperature for hot rolling is preferably 1000 to 1200 ° C.

  If the cumulative rolling reduction in the temperature range of 950 ° C. or less is less than 20%, the effect of crystal grain refinement is insufficient, and it becomes difficult to ensure desired strength and toughness.

  In order to improve the strength, it is preferable to perform controlled cooling immediately after hot rolling and cooling to a cooling rate of 1.0 ° C./s or higher and a cooling stop temperature of 600 ° C. or lower. When the cooling rate after hot rolling is less than 1.0 ° C./s and the cooling stop temperature is higher than 600 ° C., the structure becomes coarse and it becomes difficult to ensure strength and toughness.

  In addition, in order to improve strength and toughness, a direct quenching process in which rapid cooling (rapid cooling) is performed immediately after hot rolling and cooling to 250 ° C. or less may be performed. It is preferable to perform tempering treatment at a temperature of 710 ° C. or less after direct quenching. Thereby, a high strength thick steel plate having both strength and toughness can be produced at low cost. Further, instead of the direct quenching and tempering treatment, a reheating quenching and tempering treatment in which the reheating and then the quenching and tempering may be performed. In addition, it is preferable to make the reheating temperature for hardening into the range of 870-1000 degreeC from a viewpoint of intensity | strength and toughness.

  Hereinafter, based on an Example, this invention is demonstrated further in detail.

  The steel material (slab; 250 mm thickness) having the composition shown in Table 1 was subjected to the steel material treatment under the conditions shown in Table 2. The obtained steel material is further subjected to hot rolling under the conditions shown in Table 3 and air-cooled, or further subjected to controlled cooling, direct quenching-tempering, reheating quenching and tempering under the conditions shown in Table 3 to obtain a plate thickness of 25 mmt. A thick steel plate was used.

  Tensile test piece (JIS No. 4 test piece) and Charpy impact test piece (V notch test piece) are collected from the thickness position of the obtained thick steel plate: 1/2 t part, in accordance with the provisions of JIS Z 2241 A tensile test and a Charpy impact test were conducted in accordance with the provisions of JIS Z 2242 to evaluate the strength and toughness of the base material. Table 4 shows the obtained results.

  Also, from the resulting thick steel plate, a pair of welded joint preparation specimens (500 l x 250 wmm) are collected, the groove is processed, and electrogas welding (heat input: 100 to 400 kJ / cm) or gas shielded arc welding is performed. (Shield gas: 20 vol.% Carbon dioxide gas-80 vol.% Ar gas) (heat input: 10 to 30 kJ / cm) was used to produce each weld joint. For electrogas welding, sliding water-cooled copper plating was used, and 1.6 mmφ corresponding to JIS Z 3319 YFEG-22C was used as the welding wire, and one-pass vertical welding was performed. In electrogas welding, the welding heat input was variously changed to 100 to 400 kJ / cm by adjusting the root gap of the groove.

  In gas shielded arc welding (heat input: 10 to 30 kJ / cm), a welding wire equivalent to JIS Z 3313 YFW-A502B was used, and a multi-layer stack of 12 layers was formed.

  A Charpy impact test piece (V-notch test piece) is taken from the bond portion (1/4 t or 1/2 t) of the obtained welded joint, and subjected to a Charpy impact test in accordance with the provisions of JIS Z 3128. The surface transition temperature vTrs was obtained and the toughness of the weld heat affected zone was evaluated. Table 4 shows the obtained results.

  Each of the examples of the present invention is a thick steel plate that is excellent in the strength toughness of the base material and excellent in the weld heat affected zone toughness even when subjected to welding with a wide heat input. On the other hand, in the comparative example that is out of the scope of the present invention, the weld heat affected zone toughness is deteriorated. In the examples of the present invention, excellent weld heat affected zone toughness is exhibited even in the central portion of the plate thickness where the center segregation exists, and there is little variation in toughness at each position in the plate thickness direction.

  As described above, any steel plate manufacturing method such as controlled rolling / control cooling, direct quenching, or reheating quenching / tempering, which is usually used after the steel material treatment of the present invention is applied to the steel material. Even if is applied, the function of TiN works effectively, and the weld heat affected zone of the welded joint with wide heat input can be made much tougher than before.

Claims (8)

When the steel material is hot-worked into a steel material of a predetermined size and shape, the steel material is mass%,
C: 0.01 to 0.15%, Si: 0.05 to 0.70%,
Mn: 0.6 to 2.5%, P: 0.030% or less,
S: 0.0060% or less, Al: 0.005-0.10%,
And including
Ti: 0.005-0.040%, N: 0.0015-0.0100%
Is a steel material having a composition consisting of Fe and unavoidable impurities, with the balance being Ti / N: 0.8 to 4.0, and prior to the hot working, the steel material has a range of 1000 to 1300 ° C. A method for producing a steel material excellent in weld heat affected zone toughness, characterized by performing a steel material treatment that is heated to a temperature of 5 hours or more and then cooled to a temperature of 700 ° C. or less. When hot rolling a steel material to make a thick steel plate, the steel material is mass%,
C: 0.01 to 0.15%, Si: 0.05 to 0.70%,
Mn: 0.6 to 2.5%, P: 0.030% or less,
S: 0.0060% or less, Al: 0.005-0.10%,
And including
Ti: 0.005-0.040%, N: 0.0015-0.0100%
Is a steel material having a composition consisting of Fe and unavoidable impurities, with the balance being Ti / N: 0.8 to 4.0, and prior to the hot rolling, the steel material has a range of 1000 to 1300 ° C. A method for producing a thick steel plate having excellent weld heat affected zone toughness, characterized by performing a steel material treatment that is heated to a temperature of 5 hours or more and then cooled to a temperature of 700 ° C. or less.   The hot rolling is re-heated to 1000 to 1200 ° C, and the cumulative rolling reduction in the temperature range of 950 ° C or lower is 20% or higher, and the finish rolling finish temperature is 680 ° C or higher. The manufacturing method of the thick steel plate of Claim 2.   The method for producing a thick steel plate according to claim 2 or 3, wherein after the hot rolling is finished, controlled cooling is performed by cooling to a cooling rate of 1.0 ° C / s or higher and a cooling stop temperature of 600 ° C or lower.   4. The method for producing a thick steel plate according to claim 2, wherein a direct quenching process for rapid cooling to 250 ° C. or less and a subsequent tempering process are performed immediately after completion of the hot rolling.   The method for producing a thick steel plate according to any one of claims 2 to 5, wherein after the hot rolling is finished, the steel plate is once cooled and then re-heated and subjected to quenching and tempering treatment.   In addition to the above-mentioned composition, it is also mass%, Cu: 0.01 to 2.0%, Ni: 0.01 to 4.0%, Cr: 0.01 to 2.0%, Mo: 0.01 to 2.0%, Nb: 0.003 to 0.1%, V: 0.003 to The method for producing a thick steel plate according to any one of claims 2 to 6, wherein the composition contains one or more selected from 0.5% and B: 0.0005 to 0.0040%.   In addition to the above composition, the composition further contains at least one selected from mass: Ca: 0.0001-0.0060%, Mg: 0.0001-0.0060%, REM: 0.0001-0.0200%. A method for producing a thick steel plate according to any one of claims 2 to 7.