JP5213517B2 - Steel with excellent weld heat affected zone toughness - Google Patents

Description

  The present invention relates to a steel material excellent in weld heat affected zone (Heat Affected Zone: HAZ) toughness. The steel material of the present invention has good toughness particularly in a high heat input welding heat-affected zone, and is used for various welded steel structures such as buildings, bridges, shipbuilding, line pipes, construction machinery, offshore structures, tanks, etc. .

  In recent years, from the viewpoint of preventing brittle fracture of welded structures such as shipbuilding and construction, many studies have been made on the suppression of the occurrence of brittle fracture from the welded portion, that is, the improvement of the HAZ toughness of the steel sheet used.

  In general, the coarsening of crystal grains causes a decrease in brittleness in the HAZ of a steel sheet. In contrast to this coarsening of crystal grains, for example, Patent Document 1 discloses that fine TiN is finely dispersed in steel, whereby the coarsening of crystal grains is achieved by the pinning effect of prior austenite grains (hereinafter referred to as former γ grains). Preventing is disclosed. And in patent document 2, the countermeasure which prevents the coarsening of a crystal grain by the pinning effect of an old gamma grain is taken by finely dispersing fine ZrN in steel.

  However, such a nitride is dissolved in the base material in a region heated to a high temperature of 1400 ° C. or more, and the effect is reduced. For this reason, when welding with a large heat input is performed, the pinning of the old γ grains does not work sufficiently, and there is a problem that the effect of improving toughness may not be obtained.

  Considering the problem of improving the toughness of HAZ by using such nitrides, various studies have been made on stabilizing the HAZ toughness by using oxides that have higher stability at high temperatures than nitrides.

  For example, in Patent Document 3, Patent Document 4, Patent Document 5, and Patent Document 6, an intragranular transformed ferrite centered on a Ti oxide or a composite precipitate of TiN and MnS is actively incorporated in coarse γ grains. And a method for improving the HAZ toughness is disclosed.

  Further, Patent Document 7 discloses a method of achieving suppression of coarsening of γ grains by a pinning effect and refinement of a structure by intragranular transformation by using an oxide containing Mg. Patent Document 8 discloses a means for securing HAZ toughness by using oxides and nitrides of Ti and Zr. Further, Patent Document 9 discloses a method for improving HAZ toughness by coexistence of REM or Ca oxide and Zr oxide.

JP-A-55-26164 JP 52-17314 A JP 60-245768 A JP 60-152626 A JP 63-210235 A JP-A-2-250917 JP 2002-212670 A JP 2003-213366 A Japanese Patent Laid-Open No. 2007-1001000

  In any of the above techniques, the dispersion amount and form of oxides and nitrides are regarded as important, and the HAZ toughness is improved. However, when the present inventors examined these techniques, it became clear that the HAZ toughness cannot be stabilized only by optimizing dispersed particles such as oxides and nitrides.

  In view of the background as described above, the problem to be solved by the present invention is a steel material excellent in weld heat-affected zone toughness that can stably obtain good HAZ toughness even when carrying out welding with a large heat input. Is to provide.

  With the aim of developing a steel material having good HAZ toughness, the present inventors have intensively studied the amount of addition of Ti, Zr and Mg, which are oxide forming elements, and the existing state in the steel material.

  The inventors of the present invention have increased solid solution components even when γ grain coarsening suppression and intragranular transformation are promoted by pinning by dispersion control of stable crystals and precipitates such as oxides. Acquired the knowledge that it would not lead to improvement in toughness when lath elongation was promoted by this, and focused on the composition and morphology control of stable crystals and precipitates even at high temperatures such as oxides. We focused on the effect of the solid solution components of the elements used for the composition and morphology control of the materials and precipitates on the HAZ toughness. And it succeeded in obtaining the steel material which has the HAZ toughness stably stably by evaluating the influence of a solid solution component quantitatively through the analysis according to the state by an electrolytic extraction method.

  The inventors of the present invention have made various studies and experiments on HAZ toughness, focusing on the elements of Ti, Zr and Mg, which are conventionally known as oxide-forming elements and have been reported to improve HAZ toughness. . As a result, the following important findings (a) to (e) were obtained.

  (a) HAZ toughness is improved when an oxide containing Ti and Zr is formed in the steel material by adding Ti and Zr in combination and reacting with O (oxygen) in the steel.

  (b) The content of O (oxygen) constituting the oxide in the steel is considered to be that the oxide composition control and the dispersion amount control contribute to the HAZ toughness. It is necessary to make it 0.01%.

  (c) Regarding Ti, the abundance of solute Ti and insoluble solute Ti is important. Ti reacts with O (oxygen) in the steel to form an oxide. Ti in which such an oxide is formed exists as insoluble Ti, and contributes to HAZ toughness as shown in (a) above. On the other hand, solid solution Ti existing in a solid solution state in the steel material without contributing to the formation of oxides contributes to the steel material as a deoxidation material. When the solid solution Ti amount is less than 0.004%, deoxidation is insufficient. , HAZ toughness decreases. Further, even if the amount of solid solution Ti is large, the elongation of the lath structure is promoted, and if the amount of solid solution Ti exceeds 0.020%, the HAZ toughness decreases. That is, material design considering the balance between solute Ti and insoluble Ti is important.

  (d) Regarding Zr, control of its existence form is particularly important. Zr also reacts with O (oxygen) in the steel to form an oxide. Similarly to Ti, Zr, which forms an oxide, contributes to HAZ toughness as non-solid Zr. On the other hand, solid solution Zr existing in a solid solution state in a steel material without contributing to the formation of oxides, unlike solid solution Ti, promotes elongation of the lath structure even if it is a small amount, and has a great influence on toughness. It became clear to give. Therefore, the solid solution Zr should be eliminated as much as possible, but the solid solution Zr amount is acceptable up to 0.005%.

  (e) Contrary to expectations, Mg has a very serious adverse effect on the reduction of HAZ toughness. Therefore, Mg should be excluded from steel materials as much as possible. However, the amount of Mg is acceptable up to 0.001%.

  The present invention has been completed on the basis of the above findings, and the gist thereof lies in the steel materials excellent in weld heat affected zone toughness of the following (1) to (5).

(1) By mass%, C: 0.03 to 0.20%, Si: 0.01 to 0.3%, Mn: 0.5 to 2.5%, P: 0.02% or less, S: 0.01% or less, Ti: 0.005-0.025%, Zr: 0.001-0.020%, N: 0.01% or less, O (oxygen): 0.001-0.010%, Mg: 0.001% or less, the balance being Fe and impurities, 0.0036 to 0.0201% of Ti as solid solution Ti , and 0.0002 to 0.005% of non-solid solution Ti exist, 0.005% or less as a solid solution Zr, and excellent steel in the weld heat-affected zone toughness, characterized in that more than 0.001% is present as non-soluble Zr of Zr as.

  (2) by mass%, further containing Al: 0.040% or less, wherein the mass ratio Al / O of insoluble Al to O (oxygen) is 1.0 or less (1) ) Steel material with excellent weld heat affected zone toughness.

  (3) By mass%, Cu: 1.0% or less, Ni: 1.5% or less, Cr: 1.0% or less, and Mo: 1.0% or less (1) or (2), a steel material excellent in weld heat affected zone toughness.

  (4) Any one of the above (1) to (3), further comprising one or two of Nb: 0.1% or less and V: 0.1% or less in mass% Steel material with excellent weld heat affected zone toughness.

  (5) A steel material excellent in weld heat-affected zone toughness according to any one of (1) to (4) above, characterized by containing, in mass%, B: 0.0020% or less.

  The steel material of the present invention is excellent in HAZ toughness, and good HAZ toughness can be stably obtained even when welding with a large heat input is performed.

  Next, the chemical composition of the steel of the present invention will be described. The reasons for limiting the components of the steel material excellent in HAZ toughness according to the present invention are as follows. Here, “%” representing the component content means “% by mass”.

C: 0.03-0.20%
C is an element necessary for securing the strength of the steel material. However, if the C content is less than 0.03%, sufficient strength cannot be secured, and if it exceeds 0.20%, an excessive increase in hardness occurs in HAZ and the toughness is impaired. For this reason, C content is made into 0.03 to 0.20%. Preferably, it is 0.03 to 0.16%.

Si: 0.01 to 0.3%
Si is an element effective as a deoxidizing material. In order to obtain the effect as a deoxidizer, it is necessary to contain 0.01% or more. However, if it exceeds 0.3%, the HAZ toughness is lowered due to an increase in island martensite. Therefore, the Si content is set to 0.01 to 0.3%. Preferably, it is 0.05 to 0.2%.

Mn: 0.5 to 2.5%
Mn is an element effective as a deoxidizing material and an element necessary for ensuring strength. Moreover, it has the effect of stabilizing the HAZ toughness. In order to exhibit these effects, it is necessary to contain 0.5% or more. However, if the content exceeds 2.5%, center segregation is promoted, and the toughness at the center of the thickness of the base material is reduced. Therefore, the Mn content is 0.5 to 2.5%. Preferably, it is 0.5 to 2.0%.

P: 0.02% or less P is contained as an impurity in steel. The smaller the fracture toughness, the better. However, considering the cost of reducing the content in steel, the allowable upper limit is set to 0.02%.

S: 0.01% or less S, like P, is contained as an impurity in the steel. Since S has a high segregation rate and causes a low melting point material to cause solidification cracking, it is better to reduce the amount as much as possible. However, considering the cost of reducing the content in steel, the allowable upper limit is 0.01%. To do.

Ti: 0.005-0.025%
Solid solution Ti: 0.0036 to 0.0201%
Ti is an element that greatly contributes to the improvement of HAZ toughness. In particular, Ti contributes to the improvement of HAZ toughness in both solid and non-solid forms. Ti reacts with O (oxygen) in the steel to form an oxide in the steel material and exists as insoluble Ti. This oxide contributes to the improvement of HAZ toughness, and in order to obtain this effect, it is necessary that non-solid Ti is present in the steel in an amount of 0.0002% or more. Further, when the amount of non-solid Ti increases, the formation of non-solid Zr is inhibited through the consumption of oxygen and the increase of solid Zr is promoted, so the non-solid Ti is made 0.005% or less. On the other hand, solute Ti contributes as a deoxidizer, and when the amount of solute Ti is less than 0.0036% , deoxidation becomes insufficient and HAZ toughness is reduced. Therefore, the solid solution Ti amount is set to 0.0036% or more. More preferably, the amount of solid solution Ti is 0.005% or more. Further, since the toughness is lowered by the promotion of the elongation of the lath structure when the solid solution Ti increases, the solid solution Ti amount is set to 0.0201% or less. More preferably, the amount of solid solution Ti is 0.015% or less.

  From the above, the total Ti content of solid solution / non-solid solution is set to 0.005 to 0.025%.

Zr: 0.001 to 0.020%
Solid solution Zr: 0.005% or less Zr is also an element that greatly contributes to the improvement of HAZ toughness, but control of its presence is important for the improvement. Zr also reacts with O (oxygen) in the steel to form an oxide like Ti, and is present as non-solid solution Zr, contributing to the improvement of HAZ toughness. Therefore, the amount of non-solid solution Zr needs to be 0.001% or more. Preferably, the amount of insoluble Zr is 0.002% or more. Although the reason is not clear, in the experimental examples described later, the non-solid solution Zr was set to 0.015% or less from the evaluation of absorbed energy at −50 ° C. performed under the most severe conditions.

  On the other hand, solid solution Zr existing in a solid solution state in a steel material without contributing to the formation of oxides, unlike solid solution Ti, promotes elongation of the lath structure and has a great influence on toughness even if it is a small amount. Therefore, the solid solution Zr should be eliminated as much as possible, and the presence of the solid solution Zr is preferably as small as possible. Therefore, the solid solution Zr amount is set to 0.005% or less. Preferably, the amount of solid solution Zr is 0.002% or less.

  From the above, the total Zr amount of solid solution / non-solid solution is set to 0.001 to 0.020%.

N: 0.01% or less N, like P and S, is an impurity. Although it is better to make it as small as possible, the allowable upper limit is made 0.01% in consideration of the cost of reducing the content in steel. A preferable upper limit is 0.006%.

O (oxygen): 0.001 to 0.010%
When O (oxygen) reacts with Ti and Zr in steel to form an oxide containing Ti and Zr in the steel material, the HAZ toughness is improved. O (oxygen) is considered to contribute to the HAZ toughness by controlling the composition and amount of dispersion of this oxide, so the content of O (oxygen) is set to 0.001 to 0.010%.

Mg: 0.001% or less Mg is an element having extremely strong reactivity with O (oxygen). Contrary to expectation, it has been found that the inclusion of Mg has a very large adverse effect on the HAZ toughness. Therefore, addition or mixing of Mg should be eliminated as much as possible, and it is preferable that the presence of Mg is as small as possible. On the other hand, since Mg oxide is a general-purpose material used as a refractory for a furnace body and a furnace wall, there is a concern about contamination due to reaction with molten steel. Therefore, Mg needs to be positively excluded. Even if it exists as an impurity, Mg in the steel material is made 0.001% or less. Preferably it is 0.0005% or less, More preferably, it is 0.0003% or less.

Al: 0.040% or less Al is an element effective for deoxidation of steel. In the present invention, since Si and Mn are contained as deoxidizing materials, it is not always necessary to perform deoxidation treatment by adding Al. However, in addition to Si and Mn, Al can further be contained to perform composite deoxidation. Since Al is an element highly reactive with oxygen, care must be taken when adding Al. When Al forms an oxide, oxygen is consumed and the amount of oxygen in the steel that can be combined with Zr is reduced. The stable Al oxide formed in steel is Al ₂ O ₃ and the stoichiometric ratio of oxygen to Al is 1.12. Therefore, when Al is included, it does not interfere with the formation of Zr oxide. In addition, it is necessary that the non-solid Al is 1.0 or less in mass ratio Al / O with respect to the oxygen content in the steel. Preferably it is 0.7 or less. On the other hand, if the Al content exceeds 0.040%, the amount of solid solution Al increases, which promotes the formation of a hardened phase called island martensite and adversely affects the HAZ toughness. Therefore, the Al content is preferably 0.040% or less. More preferably, it is 0.020% or less, More preferably, it is 0.010% or less. In order to surely obtain the effect of Al as a deoxidizer, the content is preferably 0.0004% or more.

  The steel material excellent in HAZ toughness according to the present invention further includes Cu: 1.0% or less, Ni: 1.5% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.00%. One or more of 1% or less, V: 0.1% or less, and B: 0.0020% or less can be contained. Hereinafter, these components will be described.

Cu: 1.0% or less Cu has an effect of improving the strength of the base material. Therefore, when this effect is desired to be expressed, Cu can be contained. However, if the content exceeds 1.0%, the slab quality is deteriorated, so the content is preferably 1.0% or less. More preferably, it is 0.6% or less. In addition, in order to acquire this effect reliably, it is preferable that the content shall be 0.1% or more.

Ni: 1.5% or less Ni has an effect of improving the strength of the base material. Therefore, when this effect is desired to be expressed, Ni can be contained. However, if the content exceeds 1.5%, the slab quality is deteriorated, so the content is preferably 1.5% or less. In addition, in order to acquire this effect reliably, it is preferable that the content shall be 0.1% or more.

Cr: 1.0% or less Cr has an effect of improving the strength of the base material. Therefore, when it is desired to express this effect, Cr can be contained. However, if the content exceeds 1.0%, the slab quality is deteriorated, so the content is preferably 1.0% or less. In addition, in order to acquire this effect reliably, it is preferable that the content shall be 0.1% or more.

Mo: 1.0% or less Mo has an effect of improving the strength of the base material. Therefore, when this effect is desired to be expressed, Mo can be contained. However, if the content exceeds 1.0%, the slab quality is deteriorated, so the content is preferably 1.0% or less. In addition, in order to acquire this effect reliably, it is preferable that the content shall be 0.1% or more.

Nb: 0.1% or less Nb has the effect of improving the strength and toughness of the base material. Therefore, Nb can be contained when it is desired to express this effect. However, if the content exceeds 0.1%, the HAZ toughness is adversely affected, so the content is preferably 0.1% or less. More preferably, it is 0.05% or less. In addition, in order to acquire this effect reliably, it is preferable that the content shall be 0.005% or more.

V: 0.1% or less V has an effect of improving the strength and toughness of the base material. Therefore, when this effect is desired to be expressed, V can be contained. However, if the content exceeds 0.1%, the HAZ toughness is adversely affected, so the content is preferably 0.1% or less. More preferably, it is 0.05% or less. In addition, in order to acquire this effect reliably, it is preferable that the content shall be 0.005% or more.

B: 0.0020% or less B has an effect of significantly improving the strength of the base material. Therefore, when this effect is to be expressed, B can be contained. However, if the content exceeds 0.0020%, an excessive increase in hardness occurs in the HAZ and the toughness is impaired. Therefore, the content is preferably 0.0020% or less. In addition, in order to acquire this effect reliably, it is preferable that the content shall be 0.0002% or more.

  Next, the manufacturing method of the steel material concerning this invention is described.

  As a steelmaking method, in a small-scale laboratory or the like, when manufacturing in a high-frequency induction furnace or the like whose atmosphere can be adjusted, the steel ingot is adjusted by adjusting the alloying amount so as to be a chemical component specified in the present invention. Can be obtained. The reason is that in the method using the induction furnace as described above, the molten steel has already been deoxidized to some extent before the component adjustment. On the other hand, the steel of the present invention can also be produced by a steelmaking process by a converter method or an electric furnace method capable of larger-scale mass production, but the following points must be taken into consideration. It is desirable that molten steel that has been decarburized and temperature-adjusted in a steelmaking furnace such as a converter or an electric furnace is accommodated in a ladle so as to suppress outflow of hot metal in the steelmaking furnace. This is because when a large amount of hot metal in the steelmaking furnace flows out into the ladle, the oxygen concentration in the molten steel in the next ladle refining will vary to a high level, and as a result, the solid solution Ti and the requirements of the present invention steel and This is because it becomes difficult to control the solid solution Zr. In ladle refining, the order of addition of Si, Mn, Ti, Zr, and Al, which are deoxidizing elements of molten steel, is adjusted after adjusting the concentration of Si, Mn, and Ti, which are weaker deoxidizing actions. It is appropriate to add and further add Al if necessary. In addition, when Al is added before Zr addition, it is desirable to temporarily reduce the Al concentration to about 0.01 mass% or less by a method in which oxygen can be added to molten steel such as oxygen gas blowing equipment. In the steel of the present invention, it is necessary to suppress the Mg concentration in the steel, and in order to expect the effect of reliably reducing the Mg concentration in the steel, a small amount of oxygen is added by the above-described oxygen gas blowing device or the like before adding Zr. The method of adding is useful. These operations are effective operations not only for controlling various deoxidation element concentrations by controlling molten steel oxygen but also for controlling solute Ti and solute Zr.

  And as a manufacturing method of steel materials, the existing rolling method in general can be applied. From the viewpoint of balance of strength and toughness of the base material, it is preferable to apply the controlled rolling / accelerated cooling method. At this time, the strength of the base material may be adjusted by adjusting the water cooling stop temperature for accelerated cooling. In addition, when it is desired to increase the strength of the base material, a lower water cooling stop temperature may be employed.

Example 1
Twenty-three kinds of steels having chemical compositions shown in Table 1 were melted using a 150 kg capacity vacuum melting furnace and cast into ingots. Here, in order to extract only the influence of the deoxidizing element, the target components of the steel sheet were constant except for Ti, Zr and Mg.

A steel plate having a thickness of 25 mm was produced from this ingot by forging and rolling. The produced steel plate was subjected to water quenching from 900 ° C. and tempering at 600 ° C., and then subjected to a welding test. The contents of solid solution / non-solid solution Ti and Zr were determined by the following method. That is, when determining the content of insoluble Ti and Zr, 10% acetylacetone-1% tetramethylammonium chloride (TMAC) -methanol electrolyte (hereinafter 10% AA) or 2% triethanolamine-1% TMAC -Extraction residue after performing constant potential electrolytic extraction separation (current density 10mA / cm ² , potential 0 to -100mV vs SEC) using methanol electrolyte (hereinafter referred to as 2% TEA) and filtering through a 0.2 micron pore filter The content of Ti and Zr contained in the composition was determined as the content of non-solid Ti and Zr. Moreover, solid solution Ti and Zr content were calculated | required by deducting the above-mentioned non-solid solution Ti and Zr content from the total Ti and Zr content in steel.

  In order to evaluate the toughness of the HAZ, a single-sided flux copper backing welding was performed. A commercially available low-temperature SAW wire (US-255 manufactured by Kobe Steel, AWS: A5.23 F8P5-EG-G) was used for welding, and the welding heat input amount was 150 KJ / cm with three electrodes. PFI-50 was used for the front flux, and PFI-50R (both manufactured by Kobe Steel) was used for the back flux.

  A Charpy test piece was collected from a position of 2 mm on the surface of the produced welded joint. The notch position was a portion where the weld metal and HAZ were 1: 1. The test was conducted at -30 ° C, -40 ° C, and -50 ° C, and the toughness was evaluated by the average value.

  Table 2 shows the results of the Charpy impact test.

  First, the presence or absence of the HAZ toughness improving effect by the combined addition of Ti—Mg and the combined addition of Ti—Zr was examined. In addition, the reference | standard of the improvement effect of HAZ toughness was set to 90J or more, 65J or more, and 40J or more in the Charpy impact value in -30 degreeC, -40 degreeC, and -50 degreeC, respectively.

  The steel material to which Ti-Mg is added in combination is No. 1-4 test steel in Table 1. None of them satisfy the standard value for improving the HAZ toughness. From this, it is clear that the addition of Ti-Mg composite cannot be expected to improve the HAZ toughness, and Mg should be eliminated as much as possible.

  The steel material to which Ti—Zr is added in combination is the test steel No. 5-20 in Table 1. From these test steels, it can be seen that when Ti—Zr is added in combination, an effect of improving the HAZ toughness can be expected, but some of the test steels did not improve the HAZ toughness. In the test steel No. 7-9, the total Zr in the test steel was large, and as a result, the solid solution Zr was also increased and the HAZ toughness was not improved. The test steels of Nos. 6 and 10 have a total Zr satisfying the content specified in the present invention, but the amount of solid solution Zr increases, and sufficient HAZ toughness is obtained at some or all temperatures tested. Was not obtained. Further, in the No. 17 test steel, the solute Zr satisfied the content specified in the present invention, but the total Zr was decreased, and the improvement of the HAZ toughness was not obtained.

  The steel material to which Ti—Zr—Mg was added in combination is the test steel No. 21-23 in Table 1. From these test steels, the HAZ toughness cannot be improved if the Mg content is large as in the case of No. 1-4 test steel described above. However, it can be seen that if Mg is excluded as much as possible, specifically 0.001% or less, improvement in HAZ toughness can be expected even if Mg is contained.

As described above, the HAZ toughness of each test steel was investigated. However, even if high heat input welding is possible, it cannot be used as a structural material unless it has the characteristics as a base material. Therefore, in particular, the characteristics of the base metal were investigated for No. 13-17.
Table 3 shows the characteristics of the base material for the specimen No. 13-17. As shown in Table 3, it can be seen that all the steel plates have a tensile strength of 500 PMa or more and a high elongation.

(Example 2)
Large diameter steel pipes used for natural gas pipes and the like are subjected to seam welding by submerged arc welding of one layer each from the inner and outer surfaces. For this reason, the amount of welding heat input increases with the increase in the thickness of the steel pipe base material, and a reduction in the toughness of the HAZ often becomes a problem. The strength grade of large diameter steel pipes is being standardized to X120 grade by the American Petroleum Institute (API). The X120 grade is a high-strength material having a yield strength of 830 MPa or more, and it is extremely difficult to ensure HAZ toughness due to the addition of a large amount of alloy elements.

  In Example 2, the applicability of the steel material according to the present invention to a high-strength steel pipe was confirmed.

  For the determination of applicability, the present invention steel aimed at X80 (yield strength 553 MPa or more), X100 (yield strength 690 MPa or more), X120 (yield strength 830 MPa or more) was produced, and its strength was measured. Four-electrode submerged arc welding simulating seam welding was performed from one side, and a notch position was determined from the surface 2 mm on the side where welding was performed so that the weld metal and HAZ were 1: 1, and a Charpy test was performed.

  As in Example 1, all steel materials were melted in a 150 kg capacity vacuum melting furnace, cast into an ingot, and then forged into steel ingots. As for the thickness of the base material, the thickness of X100 and X120 was determined so as to obtain a steel pipe having the same strength when used as a steel pipe, based on a thickness of X80 of 32 mm. The amount of welding heat input was changed according to the produced steel plate.

  Table 4 shows the chemical composition of the steel material. Table 5 shows the rolling conditions after manufacturing the slab, the thickness of the base steel sheet, the strength of the base metal, and the welding heat input during welding. Table 6 shows the results of the Charpy impact test.

  The test steels of No. 31-33 each had a base metal strength satisfying X80, X100, and X120, and at the same time sufficient HAZ toughness up to -50 ° C. By Example 2, it became clear that the steel material of the present invention can be applied as a high-strength pipe base material.

(Example 3)
Electroslag welding may be applied in the manufacture of box columns and the like. In this case, compared with the flux copper backing welding used for evaluation in Example 1, the welding heat input is larger. On the other hand, the toughness required for the box column is used for building applications, and the required toughness is carried out at a relatively high temperature of 0 ° C.

  In Example 3, confirmation was made as to whether or not the steel material according to the present invention was applicable to a box column or the like.

  As in Example 1, all steel materials were melted in a 150 kg capacity vacuum melting furnace, cast into an ingot, and then forged into steel ingots. And the steel plate of 25 mm thickness was produced by casting and rolling. The produced steel sheet was subjected to water quenching from 90 ° C. and tempering at 600 ° C., and subjected to a welding test simulating electroslag welding on a box column. The amount of heat input during welding is 468 J / cm, and other conditions such as the specimen of the Charpy test are the same as those in Example 1.

  Table 7 shows the chemical composition of the steel material. Table 8 shows the strength of the base material. Table 9 shows the results of the Charpy impact test.

  The test steel No. 41 does not contain alloy elements such as Cu and Ni, and has a carbon content increased to 0.15% to ensure strength. In addition, the test material No. 42 is a steel material whose strength is increased by increasing alloy elements such as Cu. In any case, sufficient HAZ toughness was exhibited, and it was revealed from Example 3 that the steel material of the present invention can be applied as a steel material for buildings such as box columns.

  The steel material of the present invention has excellent weld heat affected zone toughness, and can stably provide steel having good HAZ toughness even when welding with a large heat input is performed.

Claims (5)

In mass%, C: 0.03 to 0.20%, Si: 0.01 to 0.3%, Mn: 0.5 to 2.5%, P: 0.02% or less, S: 0.01 %: Ti: 0.005-0.025%, Zr: 0.001-0.020%, N: 0.01% or less, O (oxygen): 0.001-0.010%, Mg: 0 0.0001% or less, the balance being Fe and impurities, 0.0036 to 0.0201% of Ti is present as solute Ti, and 0.0002 to 0.005% is present as non-solid solute Ti A steel material excellent in weld heat-affected zone toughness, characterized in that 0.005% or less of Zr is present as solid solution Zr and 0.001% or more is present as non-solution Zr .   The mass ratio of Al / O is 0.040% or less, and the mass ratio Al / O of non-solid Al and O (oxygen) is 1.0 or less. Steel with excellent weld heat affected zone toughness.   Further, by mass%, Cu: 1.0% or less, Ni: 1.5% or less, Cr: 1.0% or less, and Mo: 1.0% or less The steel material excellent in weld heat affected zone toughness according to claim 1 or 2. In mass%, further, Nb: 0.1% or less and V: Welding of any one of which contains one or two of 0.1% or less from the claim 1, wherein up to 3 Steel material with excellent heat-affected zone toughness. The steel material excellent in welding heat-affected zone toughness according to any one of claims 1 to 4, further comprising B: 0.0020% or less in terms of mass%.