JP2004360074A - Steel having superhigh heat input welding characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel which has little deterioration in toughness even in high heat input welding, and can contribute to reduction in the operation cost of welded structures. <P>SOLUTION: The steel has a composition consisting of, by mass, 0.03 to 0.1% C, 0.01 to 0.5% Si, 0.5 to 2% Mn, >0.8 to 2% Cu, 0.005 to 0.025% Ti, 0.002 to 0.008% N, 0.002 to 0.05% sol.Al and ≤0.0035% O (oxygen), and the balance Fe with impurities, wherein the occupation ratio of a ferritic structure in the metallic structure is ≤50%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a steel material for ultra-high heat input welding used as a welding structure such as a building, civil engineering, a construction machine, a shipbuilding, a pipe, a tank, and an offshore structure.

  BACKGROUND ART Steel materials for welded structures typified by thick steel plates have been used in the various fields described above and have been improved in properties such as higher strength and higher toughness.

  Patent Literature 1 discloses an ultra-low CB addition steel in which C is reduced to 0.03% or less and a small amount of B is added as a steel mainly used for line pipes. It is shown that the martensite is reduced, the toughness of the weld is improved, and the addition of a small amount of B makes the base material structure bainite, thereby obtaining a high-strength, high-toughness base material.

  Patent Document 2 discloses an ultra-low CB-added bainite steel in which the amounts of Mn and Nb with small material variations are adjusted, and a method for producing the same.

  Patent Literature 3 discloses an ultra-low CB-added high toughness steel material utilizing precipitation strengthening by adding Cu, which has less variation in material and excellent fatigue resistance, and a method for producing the same.

  When manufacturing a welded structure, it is desirable to apply large heat input welding from the viewpoint of reducing welding construction costs. Each of the above publications shows that reduction of island martensite due to low C in ultra-low CB added steel is effective in improving weld toughness. It is limited to a region having a relatively small heat input of less than 100 kJ / cm, and there is a problem that the contribution to the reduction of the construction cost is small.

JP-A-54-132421 JP-A-8-144019 Japanese Patent Application Laid-Open No. 9-249915

An object of the present invention is to provide a steel material that has less toughness deterioration even in large heat input welding and can contribute to reduction in the construction cost of a welded structure. Specifically, as a characteristic of a base material, a yield stress is 450 MPa or more or tensile strength of more than 570 MPa, JIS Z 2202 with the provisions of width 10mm of V-notch Charpy impact test piece fracture appearance transition temperature in impact testing using _{(V T S)} is -40 ℃ or less, the absorbed energy of the weld bond It is to provide a steel material of 100 J or more at -10 ° C.

  The present inventors have conducted various experiments and studies to solve the above problems, and as a result, have obtained the following findings.

  In order to increase the tensile strength of the base material to 570 MPa or more and to secure the toughness of the weld bond when large heat input welding is performed, it is effective to contain Cu in excess of 0.8% to enhance precipitation strengthening. The ferrite structure fraction is preferably set to 50% or less.

  The present invention has been made based on such knowledge, and the gist of the present invention is a steel material having the following excellent super heat input weldability.

  (1) In mass%, C: 0.03 to 0.1%, Si: 0.01 to 0.5% or less, Mn: 0.5 to 2%, Cu: more than 0.8 to 2%, Ti : 0.005 to 0.025%, N: 0.002 to 0.008%, sol. Al: 0.002 to 0.05%, O (oxygen): 0.0035% or less, with the balance being A steel material which is composed of Fe and impurities and has excellent ultra-high heat input welding characteristics in which the ratio of the metal structure to the ferrite structure is 50% or less.

  (2) Ni: 0.2-2%, Cr: 0.05-1%, Mo: 0.05-1%, V: 0.01-0. The steel material having excellent ultra-high heat input welding characteristics according to the above (1), containing 1% or more of Nb: 0.005 to 0.07%.

  According to the present invention, TS is 570 MPa or more, YS is 450 MPa or more, 10 mm width specified in JIS Z 2202, vTs in an impact test using a V notch Charpy impact test specimen is -40 ° C. or less, and 300 kJ / mm. A steel having an absorption energy at −10 ° C. of 100 J or more at the time of welding can be obtained, and exhibits excellent effects in various welded structures.

  Hereinafter, embodiments of the present invention will be described in detail. In addition, "%" of the content of each element is all expressed as "% by mass".

C: 0.03 to 0.1%
C is contained for the purpose of ensuring the strength of the base material. If it is less than 0.03%, a strength of 570 MPa or more cannot be secured. On the other hand, if it exceeds 0.1%, the toughness of the weld heat affected zone near the melting line deteriorates. Therefore, the content of C is set to 0.03 to 0.1%. Note that the content is preferably 0.08% or less, and more preferably 0.06% or less. A desirable lower limit is 0.04%.

Si: 0.01-0.5%
Si is an element added as a deoxidizing agent. In order to obtain the effect, it is necessary to contain 0.01% or more. On the other hand, when the content increases, the residual γ suppresses the reaction of decomposing into cementite in the welding cooling process and increases the amount of island martensite. 0.5%. A preferred upper limit is 0.3%, more preferably 0.15% or less.

Mn: 0.5-2%
Mn is effective in securing the strength of the base material and in the hardenability of the heat affected zone by welding, and is also effective as a deoxidizing element. If it is less than 0.5%, ferrite is formed in the heat affected zone and the toughness deteriorates, so it is necessary to contain 0.5% or more. On the other hand, if it exceeds 2%, the base material characteristics become non-uniform and the toughness deteriorates in the thickness direction due to the center segregation. Therefore, the upper limit of Mn is set to 2%.

Cu: more than 0.8 to 2%
Cu has the effect of precipitating alone as an ε precipitate and improving the strength of the base material. The precipitate can be precipitated by subjecting the steel material to an aging treatment or a treatment such as slow cooling during cooling. The precipitated Cu has the effect of improving the strength in the base metal and reducing the hardness of the welded portion, which tends to become excessively hard because Cu is redissolved in the heat affected zone, and improving the toughness of the welded portion. In order to effectively increase the strength by precipitating the ε precipitate, the amount of Cu exceeding 0.8% is necessary. However, if the content exceeds 2%, undissolved precipitates increase in the heat affected zone and the toughness decreases, so that the content needs to be 2% or less.

Ti: 0.005 to 0.025%
Ti is effective in improving the strength of the base material and at the same time preventing lateral cracking of the continuously cast slab. Further, TiN formed by combining with solid solution N has the effect of suppressing the coarsening of γ crystal grains during heating and improving the toughness of the base metal and the welded portion. In order to exhibit these effects, it is necessary to contain 0.005% or more. However, if the content exceeds 0.025%, the toughness of the base material deteriorates, so the upper limit was made 0.025%.

N: 0.002 to 0.008%
N combines with Ti to form TiN, suppresses coarsening of γ grains during heating, and improves the toughness of the base metal and the weld heat affected zone. For this purpose, it is necessary to contain 0.002% or more. However, when N does not contain B and N exceeds 0.008%, excess N is present in the structure and deteriorates toughness. From the viewpoint of grain refinement of γ grains, the N content is desirably contained slightly more than one-third of the Ti content.

sol.Al: 0.002-0.05%
Al is added for the purpose of deoxidation. In order to obtain the effect, it is necessary to contain 0.002% or more. On the other hand, when Al content increases, as in the case of Si, the reaction to decompose residual austenite into cementite in the welding cooling process is suppressed, and island martensite increases. Therefore, from the viewpoint of securing the toughness of the weld heat affected zone, it is desirable that the content is small, but there is no problem if the content is up to 0.05%. A desirable upper limit is 0.03%, more preferably 0.01%.

O: 0.0035% or less Oxygen is an element that may be contained as necessary, and if contained, has an effect of forming an oxide to refine the structure of the weld heat affected zone. Although this effect is effective even when the amount exceeds the impurity amount, it is preferable that the amount exceeds 0.001% when it is contained. On the other hand, an excessive addition adversely affects the toughness due to the formation of a coarse oxide, so that the upper limit in the case where it is contained is set to 0.0035%.

  When it is desired to further increase the strength of the base material in the Cu-containing steel material having the above chemical composition, it is effective to include at least one of Ni, Cr, Mo, V, and Nb in each steel material. Hereinafter, each element will be described.

Ni: 0.2 to 2%
Ni is effective for minimizing degradation of toughness and increasing the strength of the base material, and is preferably added at 0.2% or more. On the other hand, even if the content exceeds 2%, the effect of improving the strength and toughness corresponding to the cost increase is not seen, so the upper limit is set to 2%.

Cr: 0.05-1%
Cr is contained for the purpose of ensuring the strength of the base material. For that purpose, it is necessary to contain at least 0.05%. On the other hand, if the content is 1% or more, the toughness is deteriorated, so the upper limit is set to 1%. A preferred upper limit is 0.5% or less.

Mo: 0.05-1%
Mo is contained for the purpose of ensuring the strength of the base material. For that purpose, it is necessary to contain at least 0.05%. On the other hand, if the content exceeds 1%, the toughness deteriorates, so the upper limit was made 1%. A preferred upper limit is 0.5%.

V: 0.01-0.1%
V is contained for the purpose of ensuring the strength of the base material. For that purpose, it is necessary to contain at least 0.01%. On the other hand, when the content exceeds 0.1%, the toughness is deteriorated, so the upper limit is set to 0.1%. A preferred upper limit is 0.05% or less, more preferably 0.03% or less.

Nb: 0.005 to 0.07%
Nb has the effect of increasing the strength of the base material and at the same time improving the low-temperature toughness of the base material through refinement of the structure. In order to obtain these effects, the content needs to be 0.005% or more. On the other hand, if it is contained in excess of 0.07%, coarse carbides and nitrides are formed and the toughness is reduced. Therefore, the upper limit is set to 0.07%. From the viewpoint of a balance between strength and toughness, a preferred upper limit is 0.05%, and more preferably 0.03%.

Ferrite structure fraction: 50% or less As described above, in order to secure the strength of 570 MPa or more in the steel of the present invention, it is necessary to make the chemical composition as described above and further secure an appropriate ferrite structure fraction. The ferrite structure fraction may be 50% or less so that strengthening by aging precipitation can be achieved.

  The steel material of the present invention has a bainite or martensite structure except for a metal structure other than a ferrite structure. In this case, the average length of the lath of the bainite structure is preferably 50 μm or less for the reason of improvement in toughness. From the viewpoint of improving the toughness, the shorter the length of the bainite lath, the better. In order to realize this, generally, for example, high-pressure rolling may be performed in a non-recrystallization temperature region of austenite. However, in a steel having a relatively low hardenability such as the steel material of the present invention, if the average lath length of bainite is set to 15 μm or less, the frequency of formation of proeutectoid α is simultaneously increased by strong rolling, and the bainite is reduced. And the organizational fraction of martensite is insufficient. If the structure fraction of bainite and martensite is insufficient, the desired strength is not ensured. Therefore, the average lath length of bainite is preferably adjusted to 15 μm or more.

  Thirty-three steels having the chemical compositions shown in Table 1 were melted using a 180 kg vacuum melting furnace. Symbols 1 to 12 in the table are examples of the present invention, and symbols X1 to X7 are comparative examples.

  Each of these 180 kg steel ingots was forged into a steel slab having a thickness of 160 mm. Then, it was heated to each temperature shown in Table 2, hot-rolled, finished at each temperature, and cooled. Thereafter, the steel sheet was kept at a temperature in the range of 600 to 630 ° C. shown in these tables for 1 hour and subjected to tempering heat treatment to obtain a steel sheet having a thickness of 40 mm.

  From the center of the thickness of each steel sheet obtained in this way, a JIS No. 4 tensile test piece and a V-notch Charpy impact test piece having a width of 10 mm specified in JIS Z 2202 were sampled in a direction parallel to the rolling direction. Was investigated for its mechanical properties. In addition, each steel sheet was corroded with nital to reveal a structure, and then the area ratio was determined by observing 20 visual fields with an optical microscope, and the ferrite structure fraction was examined.

  In addition, in order to examine the toughness of each steel plate when large heat input welding was performed, the same Charpy impact test specimen was collected by performing electrogas arc welding at a heat input of 300 kJ / mm so that a notch could be formed in the weld bond. Then, the absorbed energy of the bond at the weld was measured.

  Table 2 summarizes the test results.

  The targets of strength and toughness of the base material were 450 MPa or more for YS, 570 MPa or more for TS, and -40 ° C. or less for vTs. Further, in the Charpy impact test, the target value of the absorbed energy of the weld bond portion was 100 J or more at -10 ° C.

As is evident from Table 2, all of Examples 1 to 12 of the present invention have a strength of 570 MPa or more in TS, a strength of 450 MPa or more in YS, and a vTs of -40 ° C or less. Further, the energy at −10 ° C. by welding at 300 kJ / mm became 100 J or more.
Symbols X1 to X7 of Comparative Examples in which any of the components deviated from the range specified in the present invention did not attain at least one of strength, toughness, and weldability.

TS is 570 MPa or more, YS is 450 MPa or more, 10 mm width specified in JIS Z 2202, vTs is −40 ° C. or less in an impact test using a V notch Charpy impact test piece, and −10 at the time of welding at 300 kJ / mm. A steel having an absorption energy at 100C of 100 J or more is obtained, and can be used for various welded structures.

Claims (2)

  In mass%, C: 0.03 to 0.1%, Si: 0.01 to 0.5%, Mn: 0.5 to 2%, Cu: more than 0.8 to 2%, Ti: 0.005 0.025%, N: 0.002 to 0.008%, sol. Al: 0.002 to 0.05%, O (oxygen): 0.0035% or less, with the balance being Fe and impurities A steel material having excellent ultra-high heat input welding characteristics, wherein the ratio of the metal structure to the ferrite structure is 50% or less. Ni: 0.2 to 2%, Cr: 0.05 to 1%, Mo: 0.05 to 1%, V: 0.01 to 0.1% by mass% instead of part of Fe The steel material according to claim 1, wherein the steel material contains one or more of Nb: 0.005 to 0.07%.