JP2001026839A - Steel product excellent in toughness in weld-heat affected zone, and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel product having superior HAZ toughness even at large heat input welding where weld heat input exceeds 20 kJ/mm. SOLUTION: The steel product has a chemical composition consisting of, by mass, 0.03 to 0.2% C, <=0.4% Si, 0.5 to 2% Mn, <=0.015% P, <=0.006% S, >0.01 to 0.03% Al, 0.007 to 0.02% Ti, >0.001 to 0.006% Mg, 0.001 to 0.004% O, 0.0025 to 0.006% N, and the balance Fe with inevitable impurities. In this steel product, the number of TiN of 0.01 to <0.5 μm, containing an Mg- and Al-bearing oxide, is regulated to >=10000 pieces/mm2, and further, the mean value of the sum of Mg content and Al content in the oxide of 0.5 to 5 μm is regulated to >=30%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a heat affected zone (Heat).
(Affected Zone: HAZ) It relates to a steel material having excellent toughness. Since the steel material of the present invention has good HAZ toughness under a wide range of welding conditions from small heat input welding to very large heat input welding, it can be used for various types of construction, bridges, shipbuilding, line pipes, construction machinery, marine structures, tanks, etc. Used for welded steel structures.

[0002]

2. Description of the Related Art In a HAZ, the heating temperature at the time of welding becomes higher as it approaches the melting line.
In a region heated to 0 ° C. or higher, the heated austenite (γ) is significantly coarsened, and the HAZ structure after cooling is coarsened to deteriorate toughness. This tendency is more remarkable as the welding heat input increases.

[0003] As means for solving such problems,
JP-A-60-245768, JP-A-60-152
626, JP-A-63-210235, JP-A-63-210235, JP-A-2-250917
Japanese Patent Application Laid-Open No. H07-73320 discloses a method of positively generating intragranular transformation ferrite having nuclei of Ti oxide or a composite precipitate of TiN and MnS inside coarse γ grains,
The toughness has been improved. However, steels manufactured by these techniques also have a welding heat input of 20 kJ / mm.
It has been difficult to obtain sufficient toughness in a large heat input welding HAZ that exceeds.

[0004]

An object of the present invention is to provide a steel material having good HAZ toughness even in a large heat input welding where the welding heat input exceeds 20 kJ / mm, and a method for producing the same. It is to be.

[0005]

Means for Solving the Problems The inventors of the present invention aimed at improving the HAZ toughness of a large heat input weld having a heat input of more than 20 kJ / mm to suppress the growth of the heated γ grains and the proper form of Ti and N. The present inventors have made intensive studies and found a new metallurgical effect, leading to the present invention.

The gist of the present invention is as follows.

(1) C: 0.03% to 0.
2%, Si: 0.4% or less, Mn: 0.5 to 2%, P:
0.015% or less, S: 0.006% or less, Al: 0.
More than 01% to 0.03% or less, Ti: 0.007% to 0.1%.
02%, Mg: more than 0.001% to 0.006% or less,
O: 0.001 to 0.004%, N: 0.0025 to
0.006%, with the balance having a chemical composition of Fe and unavoidable impurities, and containing 0.01 to less than 0.5 μm of TiN containing an oxide of Mg and Al.
000 / mm ² or more, and 0.5 to 5 μm
The average value of the sum of the Mg content and the Al content in the oxide having a size of 30% by mass or more is 30% or more.

(2) In mass%, Cu: 1.5
%, Ni: 1.5% or less, Mo: 1% or less, Cr:
The welding according to the above (1), wherein one or two or more of 1% or less, Nb: 0.05% or less, V: 0.05% or less, and B: 0.002% or less are contained. Steel material with excellent heat-affected zone toughness.

(3) mass%, and Ca: 0.0
The steel material having excellent toughness of the weld heat-affected zone according to the above item (1) or (2), which contains one or both of not more than 04% and not more than REM: 0.003%.

(4) Further, the effective Ti amount calculated by the following equation (1) or (2) using the mass% is-
The steel material excellent in toughness of the weld heat-affected zone according to any one of the above items (1) to (3), which is in the range of 0.01% to + 0.005%. O-0.17 x REM-0.4 x Ca-0.66 x Mg
When −0.89 × Al ≧ 0, effective Ti amount = Ti−2 × (O−0.17 × REM−0.4 × Ca−0.66 × Mg−0.89 × Al) −3.4 × N · · · (1) O-0.17 x REM-0.4 x Ca-0.66 x Mg
When −0.89 × Al <0, effective Ti amount = Ti−3.4 × N (2)

(5) T. in slag before addition of Mg and Ca The method for producing a steel material having excellent toughness in a weld heat-affected zone according to any one of the above items (1) to (4), wherein Fe + MnO is 10% by mass.

(6) The above (1), wherein Mg and Ca are added after adding elements other than Mg and Ca.
Item 4. The method for producing a steel material having excellent toughness of a weld heat-affected zone according to any one of Items 4 to 4.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The new metallurgical effects discovered by the present invention will be described below.

First, suppression of heating γ grain growth will be described. Since the heating temperature of the HAZ near the welding line reaches 1400 ° C., the force for pinning the movement of the γ grain boundary is significantly reduced due to dissolution and coarsening of carbides and nitrides, and the growth of γ grains can be avoided. Did not. Therefore, suppression of γ grain growth by pinning with an oxide that is thermally stable even at a high temperature of 1400 ° C. or higher was studied. as a result,
By adding trace amounts of Mg and Al to steel, 0.01%
Extremely fine (M
(g, Al) oxide was found to be produced in large quantities.
Further, fine TiN having a size of 0.01 or more and less than 0.5 μm is compositely deposited on the (Mg, Al) oxide, and
It has been clarified that at a high temperature of 400 ° C. or more, a very strong pinning force, which has never been seen before, is exhibited. The TiN composite particles preferably have a thickness of more than 0.01 and 0.2 μm or less.

Since this (Mg, Al) oxide has good lattice matching with TiN, it effectively acts as a precipitation nucleus of TiN. Then, by combining TiN with (Mg, Al) oxide of 0.01 to 0.1 μm, its surface area increases, and a stronger pinning force is developed. FIG. 1 shows the influence of the γ grain size on the HAZ toughness when the cooling time from 800 ° C. to 500 ° C. during welding cooling is 330 s. This cooling time corresponds to the case of electroslag welding of a steel material having a thickness of 80 mm with a welding heat input of about 70 kJ / mm. From FIG. 1, the HAZ toughness is improved as the γ grains are refined. This is because the grain boundary ferrite or ferrite side plate that transforms from the γ grain boundary as the γ grains are refined becomes smaller, and HA
This is because the Z structure is refined. Such an effect is γ
This is remarkable when the particle size is 150 μm or less. FIG. 2 shows 140
0.01 μm effect on γ grains when held at 0 ° C for 30 seconds
The effect of the number of composite precipitated TiN having a length of m to less than 0.5 μm is shown. This heating condition is as follows.
It corresponds to the HAZ near the melting line when electroslag welding was performed with a welding heat input of J / mm. From FIG. 2, the composite deposited TiN
If the number is less than 10,000 / mm ² , the γ particle size will be 150 μm or more, and the HAZ structure will not be sufficiently refined, so that good toughness cannot be obtained. The dispersed state of such composite precipitated TiN effective for suppressing γ grain growth is Mg, A
This is achieved by controlling the amounts of l, Ti, O, and N within the range of the present invention.

Next, an oxide composition of 0.5 to 5 μm,
The relationship with the number of composite precipitated TiN of 0.01 μm or more and less than 0.5 μm will be described. FIG. 3 shows 0.5 to 5 μm
Sum of Mg content and Al content expressed in mass% of the oxide composition of Mg + Al, and 0.01 μm or more and 0.5 μm or more
2 shows the relationship with the number of composite precipitated TiN less than. Mg + A
When 1 is 30% or more, the number of composite precipitated TiN is 100
It becomes 00 pieces / mm ² or more. When Al and Mg in the molten steel fall within the range of the present invention, Mg in the oxide of 0.5 to 5 μm is used.
By increasing + Al, the M in oxides of this size
g increases, but at the same time, 0.01-0.5 μm
This is because the number of (Mg, Al) oxides increases.

Next, the reasons for limiting each chemical component will be described.

The lower limit of C is 0.03%, which is the minimum amount for securing the strength and toughness of the base metal and the welded portion. But,
If the amount of C is too large, the toughness of the base material and the HAZ is reduced and the weldability is deteriorated. Therefore, the upper limit is set to 0.2%.

Although Si is contained in steel for deoxidation, if it is too much, weldability and HAZ toughness deteriorate, so the upper limit is made 0.4%. The deoxidation of the present invention can be sufficiently performed only with Ti, and in order to obtain good HAZ toughness, Si is added to 0.1%.
It is desirable to make it 3% or less.

Mn is indispensable for securing the strength and toughness of the base material and the welded portion, and the lower limit is made 0.5%. But M
If n is too large, HAZ toughness is degraded, center segregation of the slab is promoted, and weldability is degraded, so the upper limit is made 2%.

P is an impurity element in the steel of the present invention,
0.015% or less. The reduction of P improves the mechanical properties of the base metal and HAZ through the reduction of slab center segregation,
Furthermore, the grain boundary destruction of HAZ is suppressed.

If the content of S is too large, the center segregation is promoted and a large amount of stretched MnS is generated, so that the mechanical properties of the base material and the HAZ are deteriorated. Therefore, the upper limit is made 0.006%.

Al is important in controlling the number of (Mg, Al) oxides of 0.01 to 0.1 μm, which are precipitation nuclei of composite precipitation TiN, which is pinning particles for γ grain growth. When Al is less than 0.01%, Mg + Al in the oxide of 0.5 to 5 μm becomes less than 30%, and
Number of 0.1 μm (Mg, Al) oxides is 10,000
Particles / mm ² or less, and the number of composite precipitated TiN is insufficient, so that the γ grains are not sufficiently refined and good HAZ toughness cannot be obtained. On the other hand, even if Al is added in excess of 0.03%, the effect is saturated. Therefore, Al is 0.0
More than 1% and 0.03% or less.

Ti is important in controlling the dispersion state of the composite precipitated TiN as pinning particles, and must be limited to a narrow range in conjunction with an appropriate range of the effective Ti concentration described later. When Ti is less than 0.007%,
The number of TiN that is compositely deposited on the (Mg, Al) oxide is less than 10,000 / mm ^2, and the effect of suppressing γ grain growth required for improving the HAZ toughness cannot be obtained. On the other hand, when Ti is 0.
If it exceeds 02%, even if the effective Ti is within an appropriate range, TiC is substantially excessively generated, and the HAZ toughness is reduced. TiN also contributes to refinement of the base metal structure by suppressing the growth of γ grains during slab heating in thick plate rolling, and to improve the strength and toughness of the steel material.

Here, the proper form of Ti and N will be described. Ti in the steel combines with O to form an oxide, the remaining Ti combines with N to form TiN, and if there is any remaining Ti, combines with C to form TiC, TiC causes precipitation embrittlement. On the other hand, Ti in steel
Is consumed as oxides and TiN, Ti
Excess N, which could not be bonded to the iron, forms a solid solution in the base iron, but the solute N also causes embrittlement. As described above, the existence form of Ti and N differs depending on whether or not the remaining Ti consumed as oxides and nitrides is present, and this greatly affects toughness. In the present invention, the remaining Ti amount consumed as oxides and nitrides is defined by the formulas (1) and (2) using mass% as the “effective Ti amount”.

O-0.17 × REM-0.4 × Ca-
0.66 × Mg−0.89 × Al ≧ 0, effective Ti amount = Ti−2 × (O−0.17 × REM−0.4 × Ca−0.66 × Mg−0.89 × Al ) −3.4 × N (1) O−0.17 × REM−0.4 × Ca−0.66 × Mg
When −0.89 × Al <0, effective Ti amount = Ti−3.4 × N (2)

The coefficients of the respective elements in the equations (1) and (2) are values stoichiometrically determined from assumed oxides and nitrides. H near the melting line exceeding 1400 ° C
In AZ, the form of existence of Ti and N is more complicated. The reason is that most of TiC and TiN are once dissolved in the base iron at the time of welding heating, and the dissolved Ti, N and C become T
This is because they are re-precipitated as iN or TiC, and partly exists as a solid solution. In order to improve the HAZ toughness by controlling the existing form of Ti and N, it is necessary to define the respective amounts of Ti and N and to increase the effective Ti.
It is important to balance with other components using the concept of. FIG. 4 shows the effect of the effective Ti amount on the 1400 ° C. heating reproduction HAZ toughness simulating the case where the welding heat input is 50 kJ / mm. Effective Ti concentration is -0.01%
Good toughness is shown in the range of + 0.005%. That is, this range indicates that the range is an appropriate component range in which both the precipitation embrittlement of TiC and the solid solution embrittlement of N can be avoided.
If the effective Ti amount is less than g-0.01%, the amount of dissolved N becomes excessive. If the effective Ti amount exceeds + 0.005%, T
The iC precipitation amount becomes excessive, and the HAZ toughness deteriorates.

By considering the effective Ti in this way, a better HAZ toughness can be obtained.

Mg is a characteristic element of the present invention and has the most important role. By containing Mg in an appropriate amount, the dispersed state of the oxide in the present invention can be achieved. M
When g is 0.001% or less, Mg is consumed by the oxide of 0.5 to 5 μm and is a precipitation nucleus of TiN (Mg, A
l) Insufficient number of oxides. On the other hand, 0.006% of Mg consumed as an oxide is sufficient, and Mg exceeding this has no metallic effect. Since Mg is a very active element having a high vapor pressure and a strong oxidizing power, it is not preferable to include Mg more than necessary in the steel because it increases the production cost.

O is a precipitation nucleus of TiN (Mg, A
l) Necessary for securing the number of oxides. O is 0.
If it is less than 001%, the number of oxides will be insufficient, and the HAZ toughness will deteriorate. On the other hand, when O exceeds 0.004%,
The cleanliness of the steel decreases and the mechanical properties deteriorate.

N is a compound precipitation Ti which is a pinning particle.
It is important in securing the number of N, and must be limited to a narrow range in conjunction with an appropriate range of the effective Ti amount.
When N is less than 0.0025%, the number of TiN cannot be secured. On the other hand, when N exceeds 0.006%, the effective T
Even if the i amount is within the proper range, the amount of dissolved N becomes substantially excessive, and the HAZ toughness is reduced.

Subsequently, Cu, Ni, Mo, Cr, Nb,
The reason for adding V, B, Ca, and REM will be described.

Cu and Ni improve the strength and toughness of the base material without adversely affecting weldability and HAZ toughness.
However, if it exceeds 1.5%, the weldability and the HAZ toughness deteriorate.

Mo and Cr improve the strength and toughness of the base material. However, if it exceeds 1%, the toughness, weldability and HAZ toughness of the base material deteriorate.

Nb is an element effective for refining the structure of the base material, and deducts the mechanical properties of the base material. However, 0.05
%, HAZ toughness deteriorates.

V improves the toughness of the base material. But 0.
If it exceeds 05%, the weldability and the HAZ toughness deteriorate.

B enhances the quenching strength and improves the mechanical properties of the base material and HAZ. However, if it exceeds 0.002%, Z toughness and weldability deteriorate.

Ca and REM form oxides and sulfides to improve the material. Here, REM indicates a rare earth metal element such as La or Ce. Even if Ca is added in excess of 0.004%, the material improvement effect is saturated. REM 0.00
Even if added in excess of 3%, the effect of improving the material is similarly saturated. If it is added more than necessary, the production cost increases, which is not preferable. The effect is the same even if both Ca and REM are added.

The steel of the present invention is manufactured by adjusting a predetermined chemical composition in a steelmaking process of the steel industry, performing continuous casting, reheating a slab, and then giving a shape and a base material by rolling. If necessary, the steel material may be subjected to various heat treatments to control the material of the base material. Hot charge rolling can be performed without reheating the slab.

The dispersion state of the oxide specified in the present invention is quantitatively measured, for example, by the following method. 0.0
(Mg, Al) oxide and TiN of 1 to less than 0.5 μm
In the dispersion state of the composite precipitate of the above, an extracted replica sample was prepared from an arbitrary position of the base steel material, and this was sampled by a transmission electron microscope (T
(EM) at a magnification of 10,000 to 50,000 times over an area of at least 1000 μm ² , the number of composite precipitates of a target size is measured, and the number is converted into the number per unit area. At this time, identification of the (Mg, Al) oxide and TiN is performed by a composition analysis by energy dispersive X-ray spectroscopy (EDS) attached to the TEM and a crystal structure analysis of an electron diffraction image by the TEM. When it is complicated to perform such identification for all the composite precipitates to be measured, the following procedure is simply used. First, a square-shaped precipitate is regarded as TiN, and the number of oxides compounded in TiN of a target size is measured as described above. Next, at least 10 or more of the composite precipitates whose number has been measured by such a method are identified in the above-described manner, and the ratio of (Mg, Al) oxide and TiN present as a composite is calculated. . Then, this ratio is multiplied by the number of composite precipitates measured first. If carbide in steel hinders the above TEM observation, 500
Carbide can be aggregated and coarsened by heat treatment at a temperature of not more than ° C, and observation of the target composite precipitate can be facilitated.

The following is an example of measuring the composition of the oxide of 0.5 to 5 μm. A small piece sample is cut out from an arbitrary location of the base steel material, and is kept at 1400 to 1450 ° C. for 10 minutes or more to form a solution of 0.5 to 5 μm inclusions other than oxides, and then cooled with water. This is mirror-polished and observed with an optical microscope at a magnification of 1000 times over an area of at least 1 mm ² or more. For at least 10 or more of the target oxides, an X-ray microanalyzer (E
The composition is analyzed using wavelength dispersion spectroscopy (WDS) attached to PMA), and the contents of Mg and Al in the average composition of the oxides are determined by mass%. In the case of inclusions, when Fe of ground iron is detected in the analysis value of the oxide, the average composition of the oxide is obtained by excluding Fe from the analysis value.

Since Mg and Ca have a strong affinity for oxygen and a high vapor pressure, they are oxidized, removed from the molten steel as oxides, or evaporated and lost. Therefore, the addition yield is low. In order to improve the yield, it is important to minimize oxidation loss and evaporation loss.

In order to reduce the oxidation loss, Mg or C
a) O2 in molten steel before Fe addition and FeO concentration in slag and Mn
It is important to reduce the O concentration. The steel material of the present invention contains deoxidizing elements such as Si, Mn, Al, and Ti. By adding Mg or Ca after adding these elements, the oxidation loss can be reduced.
That is, since elements other than Mg and Ca are added to lower the oxygen concentration in the molten steel, the oxidation loss of Mg and Ca is reduced.

By supplying oxygen from the slag, Mg or Ca
It is effective to reduce the FeO concentration and the MnO concentration in the slag in order to suppress oxidation loss of the slag. Mg and C
a in the slag before the addition of Fe + MnO is 1% by mass.
If it exceeds 0%, the yield of Mg or Ca is significantly reduced. Therefore, T. Fe + MnO is set to 10% or less. The smaller this value is, the more effective it is in preventing the oxidation loss of Mg, and desirably 5% or less.

In order to suppress the evaporation loss of Mg and Ca, it is advantageous to add as much as possible at the end of the refining process.
Therefore, it is preferable to add the other elements after adding them in the refining process. This is advantageous from the viewpoint of suppressing the oxidation loss as described above. However, in order to finely adjust the components, a small amount of an element other than Mg and Ca may be added after Mg and Ca are added.

To add Mg to molten steel, one or more of Mg-containing alloys and MgO-containing oxides are used.

The method of adding the Mg-containing alloy and the MgO-containing oxide to the molten steel includes a method of blowing the powdered Mg alloy and the MgO-containing oxide into the molten steel in the ladle using an inert gas as a carrier gas, A method in which the molten steel in the ladle, RH, DH, etc. is added upward to the molten steel in the vacuum chamber, and the powdery steel is coated with iron, for example, and is made into a wire, and the molten steel in the ladle or / and the molten steel in the tundish or And / or a method of adding to the molten steel in the mold. Any of these methods may be used, and the effects are equivalent. Further, these methods may be combined.

As Ca, any alloy containing Ca may be used. Generally, a Ca-Si alloy is used.

The Mg may be added either before Ca addition, simultaneously with Ca addition, after Ca addition, or any combination thereof.

When Mg and Ca are added simultaneously, Mg
Either a method of adding a mixed alloy and / or an MgO-containing oxide to a Ca-containing alloy and adding the mixed alloy or a method of adding an alloy containing both Mg and Ca may be used, and the effects are the same.

[0051]

EXAMPLES (Example 1) Table 1 shows the chemical composition of the steel material and the dispersion state of inclusions, and Table 2 shows the manufacturing conditions and mechanical properties of the steel material.

The number of pinning particles in Table 1 was measured by preparing an extracted replica sample from the center of the thickness of the steel base material and observing it with a TEM at a magnification of 30,000 times over an area of 2000 μm ² . . In addition, 0.
The measurement of the number of oxides having a size of 5 to 5 μm is performed in the same manner.
1400 ° C by cutting a small piece from the center of the thickness of the steel base material
, And water-cooled.
The observation was performed by optical microscope observation over an area of 4 mm ² at double magnification. Furthermore, by EPMA-WDS,
The composition of 20 oxides of 0.5 to 5 μm was analyzed, and the average composition was obtained by subtracting the analysis value of the base iron (Fe) to obtain the value of Mg + Al.

The steel of the present invention has a welding heat input of 20 to 100 kJ.
/ Mm of electrogas welded portion or electroslag welded portion has unprecedented good HAZ toughness. The steel of the present invention strictly controls the amounts of Al, Ti, Mg, O, and N, optimizes the form of existence of Ti and N in the HAZ using the concept of the effective Ti amount, and is effective in suppressing γ grain growth. The excellent HAZ toughness is achieved even in large heat input welding by having a suitable oxide dispersion state. On the other hand, the comparative steel is inferior in the mechanical properties of the base material and the HAZ because the dispersion state of the chemical components and oxides is not appropriate.

In Steel 12, Steel 1 was used because the amount of C was too low.
In No. 3, since the C content is too high, the toughness of the base material and HAZ is inferior. Steel 14 has inferior HAZ toughness because the amount of Si is too high. Steel 16 has Mn content too low, so steel 16
Since the amount is too high, the toughness of the base material and HAZ is inferior.
In steel 17, the toughness of the base metal and HAZ is inferior because the P content is too high. Steel 18 has inferior toughness of the base metal and HAZ because the S content is too high. In Steel 19, the Al content is too low, so that Mg + Al in the oxide of 0.5 to 5 μm is low, and the number of pinning particles is small, so that the HAZ toughness is inferior. Since the amount of Ti in the steel 20 is too low, the pinning particles T
The number of iN is small, and the HAZ
Poor AZ toughness. Since the amount of Ti in steel 21 is too high, the effective Ti amount is out of an appropriate range, and the Ti content is reduced by embrittlement of TiC.
Poor AZ toughness. Steel 22 has a too low Mg content, so Ti
The number of (Mg, Al) oxides, which are the precipitation nuclei of N, is small, and γ grains are coarsened, resulting in poor HAZ toughness. Steel 23 is O
Since the amount is too low, the number of (Mg, Al) oxides is small, the γ grains are coarsened, and the HAZ toughness is poor. Since the amount of O in steel 24 is too high, the cleanliness of the steel deteriorates, the number of fracture starting points increases, and the HAZ toughness deteriorates. In steel 25, the N content is too low, so that the number of pinning particles, TiN, is small, and the HAZ structure is remarkably coarsened to deteriorate the HAZ toughness. Since the amount of N in the steel 26 is too high, the effective Ti amount is out of the proper range, and
Is excessive and HAZ toughness is inferior. In steel 27 and steel 28, the respective elements are in an appropriate range, but the effective Ti amount is inappropriate, so that HAZ due to TiC precipitation embrittlement or solid solution N embrittlement is caused.
Poor toughness.

[0055]

[Table 1]

[0056]

[Table 2]

(Example 2) In melting a steel having the composition of the steel 1 of the present invention shown in Table 1, T.P. in ladle slag before the addition of Mg was used.
The Fe + MnO concentration was varied. FIG. 5 shows the yield of Mg in the product at that time. The yield of Mg is
T. The lower the concentration of Fe + MnO, the higher the concentration. T. Fe + Mn
By setting the O concentration to 10% or less, preferably 5% or less by mass%, the Mg yield is remarkably improved.

(Example 3) In melting a steel having the composition of the steel 1 of the present invention shown in Table 1, Si, Mn, Ti, Al, Mg,
The addition time of Ca was changed. M in the product at that time
Table 3 shows the results of comparing the yields of g and Ca. Mg
T. in slag before addition Fe + MnO concentration is 2%
Met.

After adding elements other than Mg and Ca, M
When g and Ca are added, the yield of both Mg and Ca is good at 10% or more, whereas Mg and Ca are good.
When other elements are added after addition of Mg or Ca,
The yield of either or both of Mg and Ca is low.

[0060]

[Table 3]

[0061]

According to the present invention, it is possible to produce a steel material having good HAZ toughness even in large heat input welding, and the safety of various welded structures is remarkably improved. In addition, the use of the steel of the present invention has broadened the application range of high-efficiency welding, and has made it possible to significantly reduce welding work costs.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of the γ grain size on HAZ toughness.

FIG. 2 is a diagram showing the effect of the number of pinning particles on the 1400 ° C. heated γ particle size.

FIG. 3 is a view showing the effect of Mg + Al in an oxide having a size of 0.5 to 5 μm on the number of pinning particles.

FIG. 4 is a graph showing the effect of the effective Ti content on the HAZ toughness heated at 1400 ° C.

FIG. 5 shows the effect of T.C. in slag on the yield of Mg addition. F
It is a figure which shows the influence of e + MnO density | concentration.

────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] May 22, 2000 (2000.5.2)
2)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claim 5

[Correction method] Change

[Correction contents]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0011

[Correction method] Change

[Correction contents]

(5) T. in slag before addition of Mg and Ca The method for producing a steel material having excellent toughness of a weld heat-affected zone according to any one of the above items (1) to (4), wherein Fe + MnO is 10% by mass or less .

Claims (6)

[Claims] 1. A mass% of C: 0.03% to 0.2%,
Si: 0.4% or less, Mn: 0.5 to 2%, P: 0.0
15% or less, S: 0.006% or less, Al: 0.01%
Ultra-less than 0.03%, Ti: 0.007% -0.02
%, Mg: more than 0.001% to 0.006% or less, O:
0.001 to 0.004%, N: 0.0025 to 0.0
1% of TiN of not less than 0.01 and less than 0.5 μm containing 0.6%, the balance having a chemical composition of Fe and unavoidable impurities, and containing an oxide of Mg and Al.
Particles / mm ² or more, and the average value of the sum of the Mg content and the Al content in the oxide having a size of 0.5 to 5 μm is 30% or more by mass%. Steel material with excellent toughness in the heat affected zone. 2. In mass%, Cu: 1.5% or less, Ni: 1.5% or less, Mo: 1% or less, Cr: 1%
Hereinafter, Nb: 0.05% or less, V: 0.05% or less,
B: The steel material having excellent toughness of the weld heat-affected zone according to claim 1, comprising one or more of 0.002% or less. 3. In% by mass, Ca: 0.004%
The steel material having excellent toughness of a weld heat-affected zone according to claim 1 or 2, further comprising one or both of REM: 0.003% or less. 4. An effective Ti content calculated by the following formula (1) or (2) using mass% is -0.01.
The steel material having excellent toughness of the weld heat-affected zone according to any one of claims 1 to 3, wherein the steel material has a range of% to + 0.005%. O-0.17 x REM-0.4 x Ca-0.66 x Mg
When −0.89 × Al ≧ 0, effective Ti amount = Ti−2 × (O−0.17 × REM−0.4 × Ca−0.66 × Mg−0.89 × Al) −3.4 × N · · · (1) O-0.17 x REM-0.4 x Ca-0.66 x Mg
When −0.89 × Al <0, effective Ti amount = Ti−3.4 × N (2) 5. The method of claim 1 wherein the slag before the addition of Mg and Ca has a T.C. F
The method for producing a steel material excellent in toughness of a weld heat-affected zone according to any one of claims 1 to 4, wherein e + MnO is 10% by mass. 6. After adding an element other than Mg and Ca, M
The method for producing a steel material excellent in toughness of a weld heat-affected zone according to any one of claims 1 to 4, wherein g and Ca are added.