JP2013053368A - Steel product for high heat input welding - Google Patents

Abstract

The present invention provides a steel material used in various structures such as shipbuilding, construction, and civil engineering, particularly a steel material suitable for high heat input welding with a heat input of welding exceeding 300 kJ / cm.
SOLUTION: Steel composition is mass%, C: 0.03-0.10%, Si: 0.01-0.15%, Mn: 0.2-2.5%, P: 0.008% Hereinafter, S: 0.0005 to 0.0040%, Al: 0.003% or less, Nb: 0.003 to 0.03%, Ti: 0.010 to 0.080%, Cr: 2.5 to 6 0.0%, N: 0.0020 to 0.0100%, B: 0.0003 to 0.0025%, O: 0.0025 to 0.0120%, Cu, Ni, Mo, V, Ca as necessary Number of oxide-containing inclusions containing Ti oxide and / or Ti having a particle size of 1 μm or less in steel, containing one or more of Mg, Zr, REM, balance Fe and inevitable impurities a density of 300 pieces / mm ² or more, the thermal influence of the bond near beyond the welding heat input 300 kJ / cm Steel prior austenite grain size in a tissue is 150μm or less.
[Selection figure] None

Description

  The present invention relates to a steel material used in various structures such as shipbuilding, construction, and civil engineering, and more particularly to a steel material suitable for high heat input welding with a welding heat input exceeding 300 kJ / cm.

  Steel materials used in the fields of shipbuilding, construction, civil engineering and the like are generally finished into a structure having a desired shape by welding. In these structures, from the viewpoint of safety, not only the base material toughness of the steel material used but also the toughness of the welded portion is required to be excellent.

On the other hand, these structures and ships are becoming larger and more efficient, such as submerged arc welding, electrogas welding, and electroslag welding, as the steel materials used become stronger and thicker. Heat input welding is applied. For this reason, when welding is performed by high heat input welding, a steel material excellent in toughness of the welded portion is required.
However, generally, when the heat input of welding becomes large, the structure of the weld heat affected zone (heat affected zone, sometimes HAZ) becomes coarse, so the toughness of the weld heat affected zone (sometimes called HAZ toughness) It is known to decline. Many countermeasures have been proposed for the reduction of toughness due to such high heat input welding. For example, a technology that uses austenite grain coarsening suppression and action as a ferrite transformation nucleus by fine dispersion of TiN has already been put into practical use.

  However, in the technique mainly using TiN, the above-described austenite grain coarsening suppressing effect of Ti is lost in the weld heat affected zone heated to a temperature range where TiN dissolves, and the ground structure is dissolved in Ti. Further, there is a problem that the toughness is remarkably lowered due to embrittlement by the solute N.

  Further, as a technique using Ti oxide, it is possible to disperse Ti oxide and improve heat-affected zone toughness by adding no trace of Al and adding a small amount of Ti. Although described in Patent Document 3, it is difficult to uniformly and finely disperse the Ti oxide, and it has not been solved until now.

JP 57-51243 A Japanese Patent Laid-Open No. 7-278738 JP-A-6-293935

  The present invention has been made in view of the above situation, and Ti oxides and many oxide-containing inclusions containing Ti are finely dispersed in steel, and austenite grains are coarsened by the pinning effect of the fine oxides. An object of the present invention is to provide a steel material for high heat input welding that has excellent HAZ toughness in high heat input welding in which the amount of welding heat input exceeds 300 kJ / cm and suppresses and improves the HAZ toughness of the weld heat affected zone.

The present inventors conducted intensive studies to improve the toughness of the high heat input heat affected zone where the welding heat input exceeds 300 kJ / cm, targeting high strength steel with a yield strength of 460 N / mm ^2. 1. Pinning of austenite grains by Ti oxide and / or oxide-containing inclusions containing Ti (also referred to as Ti-containing oxides) is effective in suppressing the coarsening of austenite grains to improve HAZ toughness. In order to exhibit the effect sufficiently, it is necessary to finely disperse Ti oxide and Ti-containing oxide to 300 pieces / mm ² or more. For this purpose, it was found that increasing the Cr concentration in the steel is effective. The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. Steel composition is C: 0.03-0.10 mass%, Si: 0.01-0.15 mass%, Mn: 0.2-2.5 mass%, P: 0.008 mass% or less, S: 0.00. 0005 to 0.0040 mass%, Al: 0.003 mass% or less, Nb: 0.003 to 0.03 mass%, Ti: 0.010 to 0.080 mass%, Cr: 2.5 to 6.0 mass%, N: 0.0020 to 0.0100 mass%, B: 0.0003 to 0.0025 mass%, O: 0.0025 to 0.0120 mass%, consisting of the remainder Fe and unavoidable impurities, particle size in steel of 1 μm the following Ti oxide and / or the number density of the oxide-containing inclusions containing Ti is 300 / mm ² or more, the weld heat input 300 kJ / cm beyond the vicinity of the bond of the heat-affected zone structure Definitive large heat input welding steel material, wherein the prior austenite grain size is 150μm or less.
2. In addition to the above component composition, it further comprises Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Mo: 0.4 mass% or less, which is selected from one or more. The steel material for large heat input welding according to 1.
3. The steel material for high heat input welding according to 1 or 2, further containing V: 0.03 mass% or less in addition to the above component composition.
4). In addition to the above component composition, Ca: 0.0003 to 0.0030 mass%, Mg: 0.0002 to 0.0020 mass%, Zr: 0.001 to 0.02 mass%, REM: 0.001 to 0.02 mass% The steel material for high heat input welding according to any one of 1 to 3, wherein the steel material contains one or more selected from among the above.

  According to the present invention, a steel material having excellent weld heat-affected zone toughness can be obtained even when high heat input welding exceeding 300 kJ / cm is performed. Therefore, high heat input such as submerged arc welding, electrogas welding, electroslag welding, etc. It greatly contributes to improving the quality of large structures constructed by welding.

In the present invention, the component composition and the microstructure are defined.
[Ingredient composition]
C: 0.03-0.10 mass%
The amount of C needs to be 0.03 mass% or more in order to obtain the strength required for structural steel. On the other hand, if it exceeds 0.10 mass%, the amount of island martensite generated in the HAZ part during welding increases. Since the toughness deteriorates, the content is set to 0.03 to 0.10 mass%.

Si: 0.01-0.15 mass%
Si needs to be 0.01 mass% or more on steel making. On the other hand, if it exceeds 0.15 mass%, the toughness of the base metal and the heat-affected zone of the high heat input welding are deteriorated. .15 mass%.

Mn: 0.2 to 2.5 mass%
Mn is an element necessary for securing the strength of the base material, and is contained in an amount of 0.2 mass% or more in order to exert its effect. However, if it exceeds 2.5 mass%, the toughness of the welded portion is deteriorated conversely, so 0.2 to 2.5 mass%. Preferably, it is 0.5 to 2.5 mass%.

P: 0.008 mass% or less, S: 0.0005 to 0.0040 mass%
P is an unavoidable impurity in the present invention, and if P is contained in excess of 0.008 mass%, the toughness of the steel material deteriorates, so the content is made 0.008 mass% or less. Although S is an unavoidable impurity in the present invention, S is contained in an amount of 0.0005 mass% or more in order to produce the required MnS or CaS, but if it exceeds 0.0040 mass%, the toughness of the base material is deteriorated. 0005 to 0.0040 mass%.

Al: 0.003 mass% or less Al is an unavoidable impurity in the present invention, and Ti oxide and Ti-containing oxide are generated and the oxide is finely dispersed during solidification. It may be contained up to 003 mass%.

Cr: 2.5-6.0 mass%
Cr is important for increasing the number of oxides having an average particle size of 1 μm or less and finely dispersing inclusions, and it is important to contain 2.5 mass% or more, preferably 3.0 mass% or more. On the other hand, if added to 6.0 mass% or more, the toughness is adversely affected, so the upper limit is made 6.0 mass%.

  The relationship between the Cr concentration in the molten steel and the number of oxides was investigated using a test steel in which the Ti deoxidation component composition was the basic component system and the Cr addition amount was changed. As a result, when the Cr concentration in the molten steel is increased, the dissolved oxygen concentration before solidification is increased, the number of oxides generated during solidification is remarkably increased, and the effect of pinning by Ti oxide works during high heat input welding. As a result, the growth of austenite grains in the vicinity of the weld bond was suppressed.

  When the Cr concentration increases, the activity coefficient of oxygen in the molten steel increases, and the dissolved oxygen concentration with respect to the oxygen activity in equilibrium with the Ti concentration in the molten steel increases. As a result, the amount of deoxidation products produced during solidification increases, the number density of Ti-containing oxides and oxide-containing inclusions containing Ti increases, and the oxide-containing inclusions are finely dispersed. Guessed.

Nb: 0.003 to 0.03 mass%
Nb is an element effective for ensuring the strength and toughness of the base material and the strength of the joint, but the effect is small at less than 0.003 mass%. On the other hand, when it contains exceeding 0.03 mass%, the toughness of a heat affected zone will deteriorate. Desirably, it is 0.02 mass% or less.

Ti: 0.010-0.080 mass%
In the present invention, Ti is added in an amount of 0.010 mass% or more for deoxidation, and the secondary deoxidation product during solidification is dispersed in Ti oxide and Ti-containing composite oxide. Suppresses the coarsening of austenite grains in the heat affected zone. TiN also becomes a ferrite transformation nucleus and improves toughness. If the amount is less than 0.010 mass%, the effect is small. On the other hand, if it exceeds 0.080 mass%, the expected effect is obtained by reducing the amount of Ti oxide due to the decrease in dissolved oxygen concentration in the steel before solidification, and coarsening of TiN particles. Therefore, it is set to 0.010 to 0.080 mass%.

B: 0.0003 to 0.0025 mass%
B generates BN in the weld heat affected zone, reduces solid solution N, and improves the weld heat affected zone toughness with an element that acts as a ferrite transformation nucleus. In order to acquire such an effect, 0.0003 mass% or more is contained. On the other hand, if it exceeds 0.0025 mass%, the hardenability is increased and the toughness is deteriorated, so that the content is set to 0.0003 to 0.0025 mass%.

O: 0.0025 to 0.0120 mass%
O is necessary for securing a fine oxide, and is 0.0025 mass% or more, preferably 0.0035 mass% or more in order to obtain a number density of oxide-containing inclusions of 300 / mm ² or more. Although it is necessary, if it is contained in excess of 0.0120 mass%, coarse inclusions are generated and the toughness is reduced, so 0.0025 to 0.0120 mass%.

N: 0.0020 to 0.0100 mass%
N is an element necessary for securing the necessary amount of TiN, but is contained in the steel, but if the content is less than 0.0020 mass%, a sufficient amount of TiN cannot be obtained. On the other hand, if it exceeds 0.0100 mass%, Since the toughness is remarkably lowered by increasing the amount of solid solution N in the region where TiN is dissolved, the content is set to 0.0020 to 0.0100 mass%, preferably 0.0020 to 0.0080 mass%.

  The above is the basic component composition, but in the present invention, at least one or more selected from Cu, Ni, and Mo having functions such as strength improvement, and / or V can be contained.

Cu: 1.0 mass% or less Cu is an element effective for increasing the strength of the base material. However, when added in a large amount, it has an adverse effect on toughness, so the upper limit is set to 1.0 mass%.

Ni: 1.0 mass% or less Ni is an element effective for increasing the strength of the base material. However, if added in a large amount, the upper limit is set to 1.0 mass% when added in order to adversely affect toughness.

Mo: 0.4 mass% or less Mo is an element effective for increasing the strength of the base material. However, when added in a large amount, it has an adverse effect on toughness, so the upper limit is made 0.4 mass%. Desirably, it is 0.1 mass% or less.

V: 0.03 mass% or less V increases the strength and toughness of the base metal and acts as a ferrite nucleation by forming VN. However, if it exceeds 0.03 mass%, V is added because it causes a decrease in toughness. In this case, it is 0.03 mass% or less, preferably 0.02 mass% or less.

  In the present invention, at least one selected from Ca, Mg, Zr, and REM can be further contained.

Ca: 0.0003 to 0.0030 mass%
Ca is an element having an effect of improving toughness by fixing S and dispersing oxysulfides. In order to exert such an effect, it is necessary to contain at least 0.0003 mass% or more. On the other hand, even if contained over 0.0030 mass%, the effect is saturated. .0030 mass%.

Mg: 0.0002 to 0.0020 mass%
Mg is an element having an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, it is necessary to contain at least 0.0002 mass% or more. On the other hand, the effect is saturated even if contained in excess of 0.0020 mass%. 0020 mass%.

Zr: 0.001 to 0.02 mass%
Zr is an element having an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, it is necessary to contain at least 0.001 mass% or more. On the other hand, the effect is saturated even if contained in excess of 0.02 mass%. .02 mass%.

REM: 0.001-0.02 mass%
REM is an element having an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, it is necessary to contain at least 0.001 mass% or more. On the other hand, the effect is saturated even if contained in excess of 0.02 mass%. .02 mass%.
[Microstructure]
In the present invention, the number density of Ti-containing oxides having a particle diameter of 1 μm or less and / or oxide-containing inclusions containing Ti in the steel is set to 300 pieces / mm ² or more. When the oxide-containing inclusions containing Ti oxide and / or Ti in the steel have a particle size exceeding 1 μm, they do not contribute to austenite pinning, so the particle size is set to 1 μm or less. The particle size is the maximum particle size, 0.2 μm or more that can be observed with an optical microscope.

If the number density of the oxide-containing inclusions containing Ti oxide and / or Ti is less than 300 pieces / mm ² , it does not contribute to austenite pinning, so it is set to 300 pieces / mm ² or more.
In addition, the prior austenite grain size in the heat affected zone structure in the vicinity of the bond portion when welding is performed with a welding heat input exceeding 300 kJ / cm is set to 150 μm or less. If the prior austenite grain size in the heat-affected zone structure near the bond portion exceeds 150 μm, excellent toughness cannot be obtained in the heat-affected zone near the bond portion.

  The heat-affected zone in the vicinity of the bond portion refers to a heat-affected zone in the range of 500 μm or less from the bond portion. The prior austenite grain size of the heat-affected zone in the vicinity of the bond portion can be confirmed by polishing / etching the cross section of the weld and observing with an optical microscope. The structure of the heat-affected zone in the vicinity of the bond portion is a structure mainly composed of island martensite, acicular ferrite, and bainite, and includes ferrite, pearlite, and the like.

  The steel material for high heat input welding according to the present invention is manufactured as follows in order to finely disperse Ti oxide and / or oxide-containing inclusions containing Ti in the steel, and Ti deoxidize the molten steel. Is possible.

First, hot metal discharged from the blast furnace is subjected to de-P treatment and de-S treatment in a hot metal pretreatment (hot metal ladle, torpedo car, or converter), and then refined in a converter to perform decarburization and de-P.
Then, deoxidation is performed by addition of an alloy in the secondary refining and addition of a Ti alloy in the furnace, and a steel slab is obtained through a continuous casting or ingot-bundling process.

  The obtained slab is reheated and allowed to cool after hot rolling, or after the hot rolling, accelerated cooling, direct quenching-tempering, reheating quenching-tempering, reheating normalizing-tempering, etc. It is manufactured by the process. The cooling method after hot rolling is appropriately selected according to the desired characteristics.

  Hereinafter, the function and effect of the present invention will be specifically described with reference to examples.

  In a 150 kg high frequency induction melting furnace, steels of various compositions were melted to produce test steels. After adjusting C, P, and S in the molten steel, the main elements Mn, Si, Cr, Ni, Cu, and V were added, and then the molten steel was deoxidized with Ti. Then, after adding Nb, B, etc., Zr, REM, Ca, and M were added.

  In addition, when increasing the N concentration, adjustment was performed with Cr nitride and Mn nitride. The obtained molten steel was cast into a steel ingot by casting with a water-cooled mold having a thickness of 100 mm, then a slab having a thickness of 50 mm was formed by hot rolling, heated to 1150 ° C. for 2 hours, and then hot-rolled at a sheet thickness center temperature of 850. After finishing to 30 mm at 0 ° C., accelerated cooling was performed at a cooling rate of 8 ° C./sec at the center of the plate thickness.

The cooling rate of 8 ° C./sec corresponds to the cooling rate at the 1/4 position of the plate thickness of 60 mm. A sample for micro observation was taken from a part of the obtained steel sheet, and a cross section perpendicular to the rolling direction was observed with a magnification of 1000 using EPMA (the observation visual field area was equivalent to 300 mm ² ), and the maximum diameter Qualitative analysis is performed on inclusions of 0.2 μm or more and 1 μm or less, and the number of inclusions containing Ti and oxygen (including nitrides and sulfides) as Ti oxides and oxide-containing inclusions containing Ti The number density (/ mm ² ) was determined.

  In addition, after tempering a part of the steel plate at 500 ° C. for 10 minutes, a test piece having a width of 80 mm × length of 80 mm × thickness of 15 mm was taken and measured after heating to 1450 ° C. in order to measure the characteristics after the welding heat cycle. A reproducible welding heat cycle was applied to which cooling was performed at ˜500 ° C. at 270 sec (corresponding to a weld heat affected zone in the vicinity of a bond portion having an input heat amount of 400 kJ / cm in electrogas welding), and toughness was evaluated by a 2 mm V notch Charpy test.

  The prior austenite grain size in the reproducible weld heat-affected zone is obtained by tracing the old austenite grain size in photographs taken at 100 times with an optical microscope after revealing the microstructure by nital etching, and the average by image analysis The value was obtained, and the average value for five locations was defined as the prior austenite grain size.

Table 1 shows the chemical composition of the test steel, Ti oxide of 1 μm or less, oxide-containing inclusions containing Ti (in the table, the number of inclusions (pieces / mm ² )), the prior austenite grain size (in the table, the old γ grain size) and toughness of weld heat-affected zone (vTrs (° C.) in the table).

  From Table 1, in the inventive examples (Nos. 1 to 16), the prior austenite grain size was 150 μm or less, and vTrs (° C.) was −55 ° C. or less and good weld heat affected zone toughness was obtained. On the other hand, in the comparative examples (Nos. 17 to 28), the chemical components are outside the scope of the present invention, and the prior austenite grain size and / or the number of Ti-containing oxide-containing inclusions are outside the scope of the present invention. And vTrs (° C.) is −30 ° C. or higher, and the toughness of the weld heat affected zone is inferior.

Claims (4)

Steel composition is C: 0.03-0.10 mass%, Si: 0.01-0.15 mass%, Mn: 0.2-2.5 mass%, P: 0.008 mass% or less, S: 0.00. 0005 to 0.0040 mass%, Al: 0.003 mass% or less, Nb: 0.003 to 0.03 mass%, Ti: 0.010 to 0.080 mass%, Cr: 2.5 to 6.0 mass%, N: 0.0020 to 0.0100 mass%, B: 0.0003 to 0.0025 mass%, O: 0.0025 to 0.0120 mass%, consisting of the remainder Fe and unavoidable impurities, particle size in steel of 1 μm the following Ti oxide and / or the number density of the oxide-containing inclusions containing Ti is at 300 / mm ² or more, the weld heat input 300 kJ / cm beyond the bond portion near the heat affected zone assembly Steel for high heat input welding, wherein the prior austenite grain size is 150μm or less in.   In addition to the above component composition, it further comprises Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Mo: 0.4 mass% or less, which is selected from one or more. The steel for high heat input welding according to claim 1.   The steel material for high heat input welding according to claim 1 or 2, further comprising V: 0.03 mass% or less in addition to the above component composition.   In addition to the above component composition, Ca: 0.0003 to 0.0030 mass%, Mg: 0.0002 to 0.0020 mass%, Zr: 0.001 to 0.02 mass%, REM: 0.001 to 0.02 mass% The steel material for high heat input welding according to any one of claims 1 to 3, comprising one or more selected from among the above.