KR101164729B1 - Steel material excellent in toughness of weld heat affected zone - Google Patents

Abstract

The steel material of this invention satisfy | fills predetermined chemical composition, and when the length of the largest width | variety is perpendicular | vertical to the maximum projection length of the inclusions when it observes under a microscope, the inclusions are 2 micrometers or more in size. In addition to being distributed, the inclusion satisfies the relationship of Equations 1 and 2 below. By this structure, formation of coarse TiN is suppressed and it is excellent in HAZ toughness.
[Equation 1]
0.01≤ [Ca] / [Al] ≤0.50
[Equation 2]
0.08≤ ([CaO] + [CaS]) / ([Al ₂ O ₃ ]) ≤1.80
However, [Ca], [Al], [CaO], [CaS], and [Al ₂ O ₃ ] represent content (mass%) of Ca, Al, CaO, CaS, and Al ₂ O _{3 in} the inclusions, respectively.

Description

STEEL MATERIAL EXCELLENT IN TOUGHNESS OF WELD HEAT AFFECTED ZONE}

TECHNICAL FIELD The present invention relates to steel materials applied to welded structures, such as bridges, high-rise buildings, ships, and the like, and more particularly, to steel materials having excellent toughness of heat affected zones (hereinafter sometimes simply referred to as "HAZ") after high heat input welding. will be.

In recent years, the various types of welding structures have been enlarged and steel has been thickened. From the standpoint of reducing manufacturing costs and increasing the efficiency of work, high heat input welding of 400 kJ / cm is applied. It is becoming.

However, when the high heat input welding is performed, HAZ is sustained in a high temperature austenite region for a long time and then cooled slowly. Thus, it is represented by γ grain growth during heating and coarse ferrite (α) grain formation in the cooling process. As mentioned above, there is a problem that tissue coarsening is likely to be caused, and the toughness of the portion is likely to deteriorate. In terms of safety, there is a need for a technique for stably maintaining the toughness (hereinafter sometimes referred to as "HAZ toughness") in HAZ at a high level even in high heat input welding.

As a main means for securing the HAZ toughness, γ grain growth pin fixation by inclusion particles such as oxides, nitrides, and sulfides (hereinafter abbreviated as γ pin fixation) and intragranular α based on inclusion particles Tissue miniaturization by production | generation is proposed. As such a technique, for example, as disclosed in Japanese Patent Publication No. 1980-26164, by dispersing fine TiN in steel as γ pin-fixed particles, coarsening of austenite grains generated in HAZ when high heat input welding is performed is suppressed. A technique for suppressing deterioration of HAZ toughness has been proposed. However, in this technique, in addition to the need for reheating at an appropriate temperature after casting, it does not consider the form of inclusions in the steel material, so that coarse TiN is produced using the inclusions as a nucleus, so that the HAZ toughness is not necessarily good.

On the other hand, Japanese Patent Application Laid-Open No. 2001-98340 also proposes a technique for suppressing deterioration of HAZ toughness by optimizing the amount of Ti and N added, optimizing the dispersion state of TiN, and suppressing coarsening of austenite grains. . However, only by controlling the addition amount of Ti and N, there is a problem that the concentration of Ti and N becomes nonuniform during the solidification process of molten steel, so that a sufficient dispersion effect is difficult to be obtained. In particular, in the final solidification part, Ti and N are concentrated and coarse TiN precipitates in molten steel, so that the HAZ toughness may deteriorate.

This invention is made | formed in view of such a situation, and the objective is to provide the steel material excellent in HAZ toughness by suppressing generation | generation of coarse TiN.

The steel material which concerns on this invention which could solve the said subject is C: 0.02-0.15% (The meaning of "mass%", as for chemical components are the same below), Si: 0.03-1.0%, Mn: 1.0-2.0%, P: 0.02% or less, S: 0.0002 to 0.01%, Al: 0.005 to 0.08%, Ti: 0.003 to 0.03%, Ca: 0.0003 to 0.005%, N: 0.001 to 0.01% and O: 0.004% or less, respectively. , The remainder is composed of iron and unavoidable impurities, and the inclusions having a size of 2 μm or more are dispersed when the length of the largest one in the direction perpendicular to the maximum projection length of the inclusions under a microscope is taken as the size of the inclusions. In addition, the inclusion satisfies the relationship of Equations 1 and 2 below.

However, [Ca], [Al], [CaO], [CaS], and [Al ₂ O ₃ ] represent content (mass%) of Ca, Al, CaO, CaS, and Al ₂ O _{3 in} the inclusions, respectively.

In addition, although the magnitude | size of the said interference | inclusion means the length of the location largest width | variety in the direction perpendicular | vertical to the maximum projection length of an inclusion, this size can be easily measured by image processing, such as SEM and EPMA. [Ca], [Al], [CaO], [CaS], and [Al ₂ O ₃ ] in the above formulas (1) and (2) indicate the composition of all inclusions having a size of 2 µm or more in the steel. It means content when it measures and converts as a single inclusion. Further, being dispersed means a state in which inclusions having a size of 2 µm or more are scattered almost uniformly independently in the base material.

In the steel of the present invention, in the form of inclusions, the Ti-containing inclusions containing 10% or more of Ti, wherein the number of the particles having a size of 2 µm or more is preferably 7% or less of the number of all inclusions having the size of 2 µm or more. Do.

In the steel materials of the present invention, if necessary, (a) B: 0.005% or less, Nb: 0.06% or less, V: 0.1% or less, Cu: 1.5% or less, Ni: 3.5% or less, Cr: 1.5% or less At least one element selected from the group consisting of Mo: 1.5% or less, (b) Zr: 0.05% or less, REM: 0.005% or less, and Mg: at least one element selected from the group consisting of 0.005% or less; It is also useful to contain N, and by including such an element, the properties of the steel are further improved depending on the type thereof.

According to the present invention, while limiting the chemical composition of the steel material within an appropriate range, by appropriately dispersing the inclusion having a predetermined chemical composition of 2㎛ or more size, the welding heat affected zone ( HAZ) steels aimed at improving the toughness are realized, and such steels are very useful as applied to welded structures such as bridges, high-rise buildings, ships, and the like.

1 is a graph showing the relationship between ([CaO] + [CaS]) / ([Al ₂ O ₃ ]) and the wavefront transition temperature vTrs.
2 is a graph showing the relationship between the Ti-containing inclusion ratio and the wavefront transition temperature vTrs.
3 is No. It is explanatory drawing which showed the form of the inclusion in 25 steel plates.
4 is No. It is explanatory drawing which showed the form of the interference | inclusion in the steel plate of 27.
5 is No. It is explanatory drawing which showed the form of the inclusion in 35 steel plates.
6 is No. It is explanatory drawing which showed the form of the inclusion in 37 steel plate.

The present inventors examined the factors which affect HAZ toughness at the time of high heat input welding from various angles. As a result, the main cause of the deterioration of the HAZ toughness is the presence of coarse Ti-containing inclusions, and it has been found that such Ti-containing inclusions are TiN or TiO ₂ which crystallizes out mainly at the stage where the molten steel solidifies.

As for the production process of the coarse Ti-containing inclusions as described above, and the factors causing such inclusions to deteriorate the HAZ toughness, it was considered as follows. First, as a result of deoxidation by adding Al in molten steel, solids of Al ₂ O ₃ having a distorted (ellipse) shape are dispersed in molten steel, so that the solid phase or The γ phase is formed, and Ti, N, and S are concentrated in the remaining liquid phase. When the solubility product of Ti and N is exceeded, solid Al ₂ O ₃ is used as the nucleus to precipitate TiN, and S is concentrated in the unsolidified portion, thereby lowering the solidus temperature. Finally, it is thought that TiN grows in the temperature range (for example, about 1535-1490 degreeC) between a liquidus line and a solidus line.

In view of such a production mechanism, the present inventors further examined the requirements for obtaining good HAZ toughness while suppressing the production of Ti-containing inclusions. As a result, when the chemical composition of the steel material (base material) is appropriately adjusted, the relationship of the following formulas (1) and (2) is satisfied, and the inclusions having a size of 2 µm or more defined as described above are dispersed in Ti content. While suppressing the formation of inclusions, it was found that good HAZ toughness was obtained and completed the present invention.

[Equation 1]

0.01≤ [Ca] / [Al] ≤0.50

[Equation 2]

0.08≤ ([CaO] + [CaS] / [Al ₂ O ₃ ]) ≤1.80

However, [Ca], [Al], [CaO], [CaS], and [Al ₂ O ₃ ] represent content (mass%) of Ca, Al, CaO, CaS, and Al ₂ O _{3 in} the inclusions, respectively.

In the steel of the present invention, the inclusions to be dispersed should satisfy the above formulas (1) and (2), but the reason for defining these requirements is as follows.

[[Ca] / [Al]: 0.01 to 0.50; Relationship of Equation 1]

The addition ratio of Ca and Al is an optimal ratio of (CaO) and (Al ₂ O ₃ ) in order to promote the low melting point of inclusions, suppress the low melting point of molten steel, and to suppress the crystallization of TiN by CaS. You have to control to get it. In order to obtain these effects, [Ca] / [Al] must be controlled within the range of 0.01 to 0.50. If the value of [Ca] / [Al] is less than 0.01, the inclusions do not sufficiently lower the melting point, and coarse TiN is crystallized to deteriorate the HAZ toughness. On the other hand, when the value of [Ca] / [Al] exceeds 0.50, the cleanliness of the steel is lowered, and since TiO ₂ precipitates as the amount of Al added decreases, HAZ toughness deteriorates. On the other hand, the upper limit with a preferable value of [Ca] / [Al] is 0.40.

[([CaO] + [CaS]) / ([Al ₂ O ₃ ]): 0.08 to 1.80; Relationship of Equation 2]

Since the inclusions (basically oxide-based inclusions) contained in the steel are made into composite inclusions containing CaO, CaS, and Al ₂ O ₃ , the inclusions are lowered in melting, and the decrease in melting point of molten steel is suppressed. Crystallization of coarse TiN can also be suppressed. In addition, since precipitation of CaS can reduce the ability to generate TiN from the composite inclusions containing Ca and Al, coarse crystallization of TiN can be suppressed. In order to exert these effects, the value of ([CaO] + [CaS]) / ([Al ₂ O ₃ ]) of inclusions having a size of 2 μm or more in steel materials should be controlled within a range of 0.08 to 1.80. If the value of ([CaO] + [CaS]) / ([Al ₂ O ₃ ]) is less than 0.08, the inclusions do not sufficiently lower the melting point, and the amount of precipitation of CaS is also suppressed. As a result, the HAZ toughness is degraded. On the other hand, ([CaO] + [CaS ]) / If the value of ([Al ₂ O _3]) exceeds 1.80, with Sikkim lower the cleanness of the steel, afterwards unless a low Al addition amount, the amount of precipitation of TiO ₂ As it increases, the HAZ toughness deteriorates. On the other hand, the lower limit of the value of ([CaO] + [CaS]) / ([Al ₂ O ₃ ]) is preferably 0.10, and the upper limit is preferably 1.1.

In the steel of the present invention, the inclusions (composite inclusions) satisfying the above requirements are suitably dispersed, so that the HAZ toughness is improved, but as is apparent from the above-mentioned purpose, the Ti-containing inclusions represented by TiN (for example, TiN) In addition, TiO ₂ ), as small as possible, can be coarse in size. From this point of view, in the steel material of the present invention, it is preferable that the number of Ti-containing inclusions having a size of 10 μm or more and containing Ti of 2 μm or more is 7% or less of the number of all inclusions having a size of 2 μm or more. Do. On the other hand, in fine Ti-containing inclusions whose size does not occupy 2 micrometers (especially 0.5 micrometer or less), it does not affect HAZ toughness very much regardless of the number.

Not all of the mechanisms for improving the HAZ toughness of steel materials by dispersing inclusions satisfying the above requirements can be elucidated. However, considering the mechanism for generating coarse TiN described above, You can think together.

If Al is added to the molten steel, alumina (Al ₂ O ₃ ) is formed, but if Ca is contained at the same time, it becomes a composite inclusion, the melting point is lowered, and the liquid state. This state continues even at the stage where iron solidification has started. In such a state, there is no nuclei that can be crystallized even when TiN is precipitated, and precipitation of TiN is suppressed. After that, even if TiN precipitates, since the iron is solidified, TiN no longer coarsens.

Further, Al ₂ O ₃ and CaO become solids (precipitate out of the liquid phase), so that S is concentrated in the liquid phase of iron. At this time, when S concentrates in the liquid phase of iron, iron becomes difficult to solidify. In this state, TiN promotes coarsening, but it is thought that TiN becomes difficult to coarsen without S being concentrated due to the presence of CaS (S becomes CaS).

In addition, CaS is also known to have poor lattice matching with TiN (for example, "a study on the phenomenon and control mechanism of structure and material control by inclusions in steel", published by the Japan Iron and Steel Institute Basic Research Group, September 1995), CaS Is difficult to become a crystallization site of TiN, it is considered that the presence of CaS causes the coarsening of TiN to be suppressed.

In order to realize the dispersion state of the inclusions described above, steel materials may be manufactured in the following order. First, Al is added and deoxidized so that the Al concentration in molten steel is 0.005 to 0.08%, and then degassing is performed for 10 minutes or more with a degassing apparatus (for example, an RH apparatus). Thereafter, Ti is added 5 minutes before degassing is completed, and then refluxed for 1 minute or more. And after completion of the degassing treatment the addition of Ca, with respect to each of the inclusions is less than the size _{2㎛, Al 2 O 3, MnS} , MnO, CaO, total occupied by a mass (100%) Al ₂ O ₃ of CaS, CaO And the standard deviation sigma of the mass% of CaS becomes sigma (Al ₂ O ₃ ) ≤ 30 mass%, sigma (CaO) ≤ 15 mass% and sigma (CaS) ≤ 20 mass%, respectively. Here, sigma (Al ₂ O ₃ ) is the ratio of Al ₂ O ₃ content in each inclusion (Al ₂ O ₃ , MnS, MnO, CaO, CaS of inclusions) when irradiating a large number of inclusions in one steel material. Standard Deviation for the distribution of mass% to total mass). σ (CaO) and σ (CaS) have the same meaning. The composition of each inclusion can be investigated by the method described in [Measurement of composition of inclusions] in description of a later Example. Such measurements may be made on a large number of inclusions, and standard deviation may be obtained from the obtained data.

Al initially added has a strong deoxidation force and combines with oxygen in molten steel to form Al ₂ O ₃ . Next, impurities such as coarse Al ₂ O ₃ are separated by a degassing apparatus. The yield of Ti is improved 5 minutes before the degassing is completed, and then refluxed for 1 minute or more, thereby dispersing Ti uniformly in the molten steel.

After the degassing was completed, Ca was added to form CaO and CaS in the molten steel, and then the standard deviation σ of the mass% of Al ₂ O ₃ , CaO and CaS was σ (Al ₂ O ₃ ) ≦ 30 mass, respectively. By setting%, sigma (CaO) ≤ 15 mass% and sigma (CaS) ≤ 20 mass%, CaO and CaS are uniformly bonded with Al ₂ O _3, and (CaO + CaS) and Al ₂ O as described above. A composite inclusion containing Al ₂ O ₃ , CaO, and Ca having a good balance of _trivalent [Formula 2 above] is formed.

On the other hand, it is useful to manufacture the steel materials of the present invention under the above conditions at the stage of molten steel, but it is preferable to appropriately control the conditions even in the hot rolling step. That is, in hot rolling a steel raw material (slab), it is preferable to make the heating temperature about 1000-1250 degreeC. If this temperature is less than 1000 degreeC, it will cause temperature fall during hot rolling, and it will become difficult to complete rolling at an appropriate temperature, and it will become difficult to crimp the casting defect in a slab. On the other hand, when this heating temperature exceeds 1250 degreeC, austenite will coarsen and TiN will also coarsen, and HAZ toughness will deteriorate.

The slab heated in the above temperature range may then be hot rolled at a temperature range of Ar ₃ transformation point (Ar ₃ transformation point 1100 ° C.) so that the cumulative reduction ratio is 30% or more. By carrying out hot rolling on the conditions mentioned above, it is useful in making crystal grain size of steel materials refine | miniaturize and ensuring favorable base material toughness. After completion of hot rolling under the above conditions, cooling is carried out to 500 ° C or lower at a cooling rate of 3 ° C / sec or more at the surface temperature of the steel (steel sheet). The upper limit of the rolling finish temperature falls within the range of the heating temperature. In addition, the said Ar ₃ transformation point is a value calculated | required by following formula (3).

However, [C], [Si], [Mn], [Ni], [Cu], and [Cr] represent the contents (mass%) of C, Si, Mn, Ni, Cu, and Cr, respectively. When not added, it is calculated that there is no term.

Next, the chemical component composition in the thick steel plate (base material) of this invention is demonstrated. Even if the dispersion state of an interference | inclusion is appropriate, the steel material of this invention cannot improve the characteristic of a base material and HAZ, unless the chemical composition of a steel material exists in an appropriate range. Therefore, in the steel of this invention, it is also necessary that the quantity of each chemical component exists in the appropriate range as described below. In addition, content of the element (for example, Al, Ca, Ti, etc.) which forms an interference | inclusion among these components includes the quantity which comprises an interference | inclusion as evident from the effect.

[C: 0.02 to 0.15%]

C is an element which cannot be removed in order to secure the strength of the steel plate (base metal) and the welded part (welded metal). In order to exhibit such an effect, it is necessary to make C content into 0.02% or more. Preferably it is 0.03% or more. However, when the C content is excessive, the HAZ toughness and weldability deteriorate, so it is necessary to suppress the content to 0.15% or less (preferably 0.13% or less).

[Si: 0.03 to 1.0%]

Si is an element effective in improving the strength of the base metal and the welded part while having a deoxidizing action. In order to exert such an effect, it is necessary to contain Si 0.03% or more. Preferably it is 0.05% or more. However, when Si contains too much, since weldability and the toughness of a base material deteriorate, it is necessary to be 1.0% or less. Preferably it is 0.7% or less.

[Mn: 1.0 to 2.0%]

Mn is an element effective for improving the strength of the base metal and the welded portion, and in order to effectively exhibit such an effect, it is required to contain 1.0% or more. Preferably it is 1.2% or more. However, excessive Mn content deteriorates the HAZ toughness and weldability, and therefore it is necessary to make it 2.0% or less. Preferably it is 1.8% or less.

[P: 0.02% or less]

P is an impurity element inevitably contained in steel, and since its deterioration of HAZ toughness becomes remarkable when its content exceeds 0.02%, it should be suppressed to 0.02% or less, Preferably it is 0.015% or less. However, industrially, it is difficult to make P in steel 0%.

[S: 0.0002 to 0.01%]

S has an effect of forming CaS and suppressing generation of coarse TiN. In order to exhibit such an effect, it is necessary to contain S content 0.0002% or more. However, when the S content is too large, stretched MnS is generated in a large amount, and the deterioration of the HAZ toughness becomes remarkable. Therefore, the S content should be suppressed to 0.01% or less, preferably 0.008% or less.

[Al: 0.005 to 0.08%]

Al is a strong deoxygenation element, and since a small amount of Al generates TiO ₂ in a large amount, the content needs to be 0.005% or more. However, when the Al content is too high, an appropriate composition with the formula (CaO + CaS) [Equation 2] cannot be ensured. Therefore, it is necessary to suppress the content to 0.08% or less. The upper limit with preferable Al content is 0.06%.

[Ti: 0.003 to 0.03%]

Ti is an element necessary for improving HAZ toughness because precipitation of TiN in steel prevents coarsening of austenite grains in HAZ during welding and promotes ferrite transformation. In order to exhibit such an effect effectively, Ti needs to be contained 0.003% or more, and preferably 0.005% or more. However, when Ti is excessively contained, the amount of solid solution Ti increases and TiC precipitates, which deteriorates the toughness of the base metal and HAZ. Therefore, the content should be suppressed to 0.03% or less. Preferably, you may be 0.02% or less.

[Ca: 0.0003 to 0.005%]

Ca is an element which occupies the most important position in the steel of the present invention, and a low melting point of steel by reducing the solid solution S in addition to lowering the inclusions by forming a composite inclusion together with Al ₂ O ₃ as CaO · CaS. Since it suppresses ignition, there exists an effect of approaching melting | fusing point of an interference | inclusion and steel, and coarse crystallization of TiN can be suppressed. In addition, the precipitation of CaS reduces the ability to form TiN from the composite inclusions containing Ca and Al, and therefore coarse crystallization of TiN can be suppressed. In order to exhibit these effects, it is necessary to contain Ca 0.0003% or more. Preferably it is 0.0005% or more. However, when Ca content becomes too much, since the cleanliness of steel materials will fall and melt loss of a nozzle will arise at the time of casting, it is necessary to be 0.005% or less. Preferably it is 0.004% or less.

[N: 0.001-0.01%]

N is an element which precipitates as TiN in steel structure, suppresses coarsening of austenite grains of HAZ, and promotes ferrite transformation, thereby improving HAZ toughness. In order to exhibit such an effect, it is necessary to contain N 0.001% or more. Preferably it is 0.003% or more. However, when N content becomes too much, the solid solution N amount will increase and the toughness of HAZ will deteriorate. For this reason, N content needs to be suppressed to 0.01% or less, Preferably you may be 0.008% or less.

[O: 0.004% or less]

O is an element constituting the inclusions in the present invention and must be managed. Excess O exceeding 0.004% lowers the cleanliness of the steel, generates a large amount of coarse inclusions that are the starting point of fracture, and lowers the base metal toughness and the HAZ toughness. Therefore, O is made into 0.004% or less. Preferably it is 0.003% or less. On the other hand, since the lower the O content, the more preferable. Therefore, there is no need to set the lower limit, but industrially, there is a limit to the reduction of O, and usually 0.0005% or more is included.

The containing elements defined in the present invention are as described above, and the balance is iron and unavoidable impurities, and as the unavoidable impurities, elements carried in accordance with the situation of raw materials, materials, manufacturing facilities, etc. (for example, Co, Zn, Pb, etc.). ) May be allowed. Moreover, it is also effective to actively contain the following element further, and the characteristic of a steel plate further improves according to the kind of component contained.

At least 1 selected from the group consisting of: B: 0.005% or less, Nb: 0.06% or less, V: 0.1% or less, Cu: 1.5% or less, Ni: 3.5% or less, Cr: 1.5% or less and Mo: 1.5% or less More than one element]

B, Nb, V, Cu, Ni, Cr, and Mo are all useful elements in improving the strength of steel materials, and may contain 1 type, or 2 or more types as needed. Among them, B increases the hardenability of the steel material, increases the strength of the base metal and the welded portion, and in the process of cooling the HAZ heated during welding, combines with N to precipitate BN to promote ferrite transformation in the austenite grain. Therefore, HAZ toughness is improved. Although such an effect increases as its content increases, in order to exhibit such an effect effectively, it is preferable to contain 0.0002% or more (more preferably 0.0005% or more). However, when B content becomes excessive, since the toughness and weldability of a base material and HAZ deteriorate, it is preferable to set it as 0.005% or less. More preferably, it is good to set it as 0.004% or less.

Nb is an element useful for increasing strength and base metal toughness, and the effect thereof increases as the content thereof increases. However, in order to effectively exhibit such an effect, it is preferable to contain Nb by 0.005% or more. However, when the Nb content is excessive, the toughness of the base material and the HAZ is deteriorated, so it is preferable to suppress the content to 0.06% or less.

V is an element useful for increasing the strength, and its effect increases as its content increases, but in order to effectively exhibit such an effect, it is preferable to contain 0.005% or more. However, when the V content is too high, the HAZ toughness and weldability deteriorate, so it is preferable to suppress the content to 0.1% or less.

Cu is an effective element for increasing strength by increasing the hardenability of steel materials. Although such an effect increases as its content increases, in order to exhibit such an effect effectively, it is preferable to contain 0.05% or more. However, when the Cu content is too high, the toughness of the base material and the HAZ deteriorates, so it is preferable to be 1.5% or less.

Ni is an element useful for increasing the strength and toughness of steel materials and welded portions, and the effect thereof increases as the content thereof increases. However, in order to effectively exhibit such an effect, it is preferably contained at 0.05% or more. However, when the Ni content is excessively high, it becomes very expensive as a structural steel material, and therefore it is preferable to suppress it to 3.5% or less from the viewpoint of economy.

Cr is an effective element for increasing the strength of steel. Although such an effect increases as its content increases, in order to exhibit such an effect effectively, it is preferable to contain 0.01% or more. However, when Cr content becomes too much, since HAZ toughness deteriorates, it is preferable to set it as 1.5% or less.

Mo is an effective element for increasing the strength and toughness of steel materials. Although such an effect increases as its content increases, in order to exhibit such an effect effectively, it is preferable to contain 0.01% or more. However, when the Mo content is excessively high, the HAZ toughness and weldability deteriorate, so it is preferable to be 1.5% or less.

[At least one element selected from the group consisting of Zr: 0.05% or less, REM: 0.005% or less and Mg: 0.005% or less]

Zr, REM (rare earth element) and Mg are effective elements for improving the HAZ toughness, and may contain one or two or more kinds as necessary. Among them, Zr and REM form oxides and finely disperse to improve HAZ toughness. Although such an effect increases as its content increases, in order to exhibit such an effect effectively, it is preferable to contain all 0.001% or more. However, when these contents become excessive, the optimum balance of (CaO + CaS) / (Al ₂ O ₃ ) cannot be maintained and HAZ toughness deteriorates. For this reason, it is preferable to set it as 0.05% or less in Zr and 0.005% or less in REM. In the present invention, REM (rare earth element) means a lanthanoid element (15 elements from La to Ln), and Sc (scandium) and Y (yttrium).

Mg improves HAZ toughness through refinement of crystal grains. Although such an effect increases as Mg content increases, in order to exhibit such an effect effectively, it is preferable to contain 0.001% or more. However, even if it contains Mg too much, since the effect is saturated, it is preferable to set it as 0.005% or less.

Although the steel material of this invention basically assumed the raw material of a thick steel plate, a thick steel plate generally means that plate | board thickness is 3.0 mm or more as defined by JIS. The steel material of the present invention exhibits good HAZ toughness even when a large heat input weld of 400 kJ / cm is applied to a thick steel plate having a plate thickness of 30 mm or more, and therefore, it is preferable to apply it to a steel sheet having such a thickness. Although it is an aspect, the thickness of a steel plate is not limited to 30 mm or more, and does not exclude the application to the steel plate used below.

The thick steel sheet thus obtained can be used as a material for structures such as bridges, high-rise buildings, ships, and the like, and can prevent the deterioration in the toughness of the weld heat-affected zone even in small to medium heat welding.

Example

Hereinafter, although an Example demonstrates this invention further in detail, the following example is not a property which limits this invention, It is also possible to change suitably and to implement in the range which may be suitable for the purpose of the previous and the later, Any of them is included in the technical scope of the present invention.

Table 1 below, after a slab of steel of the chemical composition shown in Fig. 2 to the continuous casting machine, by heating the slab at 1000 to 1250 ℃, in the temperature range of Ar ₃ transformation point to (Ar ₃ transformation point + 100 ℃), Hot rolling was performed so that the cumulative reduction ratio was 30% or more to obtain a steel sheet having a sheet thickness of 50 mm. In Tables 1 and 2, REM was added in the form of Misch metal containing about 30% La and about 50% Ce. In addition, in Table 1, "-" has shown that the element was not added.

[Composition Measurement of Inclusion]

About each steel plate obtained as mentioned above, the cross section (cross section perpendicular | vertical to the surface in a rolling direction) was observed with "EPMA-8705" by Shimadzu Corporation, and the size defined as mentioned above is included in the inclusion of 2 micrometers or more. The component composition was quantitatively analyzed. Observation conditions at this time were made into observation magnification: 100-400 times, observation visual field: 10-70 mm <^2> , and quantitatively analyzed the component composition in an interference | inclusion center part by wavelength dispersion spectroscopy of a characteristic X-ray.

The element to be analyzed at this time may be Al, Mn, Si, Mg, Ca, Ti, Zr, Ni, Cu, V, S, Cr, or REM, and the X-ray intensity and the element of each element using known materials. The relationship of the concentration was previously determined as a calibration curve, and the element concentration of each inclusion was quantified from the X-ray intensity obtained from the inclusion and the calibration curve. According to these measurements, the content of CaS, CaO, and Al ₂ O ₃ was measured according to the following criteria, and [Ca] / [Al], ([CaO] + [CaS]) / ([Al ₂ O ₃ ]) was obtained (the measuring method is mentioned above). Incidentally, inclusions containing 10% or more of Ti were defined as Ti-containing inclusions, and the percentage (%) of Ti-containing inclusions in the total number of inclusions (measured by EPMA) was calculated for inclusions having a size of 2 μm or more. .

(1) Calculation of CaS content: Detected S binds preferentially with Mn to form MnS, and excess S forms CaS with respect to Mn.

(2) Calculation of CaO content: In the detected Ca, Ca other than CaS was assumed to exist as CaO.

(3) Calculation of Al ₂ O ₃ content: All the detected Al was supposed to exist as Al ₂ O ₃ .

[Evaluation of HAZ Toughness]

In order to evaluate the toughness of HAZ affected by heat during welding, welding input heat of 1000 kJ / cm or 600 kJ / cm was input for each steel sheet, and the welding reproducing test shown below was performed.

(1) Welding heat input amount of 1000 kJ / cm: After heating so that the whole sample cut out from the slab might be 1400 degreeC, it hold | maintained for 30 second and cooled. The cooling rate at this time was adjusted so that the cooling time in 800-500 degreeC might be 730 second.

(2) Welding heat input amount of 600 kJ / cm: After heating the whole sample cut out from the slab to 1400 degreeC, it hold | maintained for 60 second and cooled. The cooling rate at this time was adjusted so that the cooling time in 800-500 degreeC might be 500 second.

From the sample after cooling, the V notch test piece of JIS Z2202 which cut out perpendicularly to the plate surface of the position of HAZ was extract | collected, the Charpy impact test was done by the method of JIS Z2242, and the wavefront transition temperature vTrs was measured ( One measurement). And it was evaluated that HAZ toughness was favorable that wavefront transition temperature vTrs is 0 degrees C or less.

These results are collectively shown in following Tables 3 and 4. Further, according to these results, the relationship between ([CaO] + [CaS]) / ([Al ₂ O ₃ ]) and the wavefront transition temperature vTrs is shown in FIG. 1, and the relationship between the ratio of Ti-containing inclusions and the wavefront transition temperature vTrs is shown in FIG. 2 is shown.

From these results, it can consider as follows (the following No. shows the steel No. of Tables 1-4). No. 1-34 is an example which satisfy | fills the requirements prescribed | regulated by this invention, It turns out that chemical composition of a composition and dispersion | distribution of an inclusion are made suitably, and favorable HAZ toughness is obtained. In this regard, No. 35-50 is an example which deviates from the requirements of any of this invention, and it turns out that HAZ toughness deteriorates.

With respect to steel sheets (No. 2, 3, 26) from which good HAZ toughness was obtained, and steel sheets (No. 47) with deteriorated HAZ toughness, each component (Al ₂ O ₃ , MnS, MnO) in the inclusions before casting. , CaS, CaO) was investigated for the standard deviation σ. Represents the results in Table 5, as shown a good HAZ toughness, each component in the inclusions before casting it is within a predetermined range _{[σ (Al 2 O 3)} ≤30 wt%, σ (CaO) ≤15 mass %, sigma (CaS) ≤ 20% by mass], and CaO and CaS were confirmed to bond uniformly with Al ₂ O ₃ . On the other hand, when HAZ toughness deteriorates, it turns out that it is out of the said range.

Steel sheets (No. 2, 3, 11, 13) from which good HAZ toughness was obtained, and steel sheets (No. 35, 36) with deteriorated HAZ toughness were examined for the number density in TiN having a size of 0.5 µm or less. . The results are shown in Table 6 below, but it can be seen that the fine number density of TiN does not affect HAZ toughness very much.

In each said steel plate, the form of an interference | inclusion is shown about the typical thing. 3 is No. The form of the inclusions in the steel plate of 25 is shown (FIG. 3 (a) is a micrograph of a drawing substitute, and FIG. 3 (b) is an explanatory drawing which shows typically the form of an inclusion). The portion indicated by reference numeral 1 in FIG. 3 (a) is CaO: 36%, CaS: 29%, Al ₂ O ₃ : 63%, SiO ₂ : 1%, and the portion indicated by 2 is CaO: 24%, CaS: 6%, MgO: 2%, Al ₂ O ₃ : 68%.

4, No. The form of the inclusions in the steel plate of FIG. 27 is shown (FIG. 4 (a) is a microphotograph of a drawing substitute, and FIG. 4 (b) is an explanatory drawing which shows the form of an inclusion typically). The portion indicated by reference numeral 1 in FIG. 4 (a) is MnO: 2%, CaS: 76%, CaO: 19%, Al ₂ O ₃ : 3% portion, and the portion indicated by reference numeral 2 is CaO: 38. %, CaS: 4%, Al ₂ O ₃ : 57%, Reference 3, MnO: 1%, CaO: 23%, CaS: 18%, Al ₂ O ₃ : 20%, TiO ₂ : 38% of the time.

5 is No. The form of the inclusions in the steel plate of 35 is shown (FIG. 5 (a) is a micrograph of a drawing substitute, and FIG. 5 (b) is an explanatory drawing which shows the form of an inclusion typically). A portion indicated by reference numeral 1 in FIG. 5A is a portion having a TiN of 100%, a portion indicated by a reference numeral 2 is a portion having a MgO of 13%, Al ₂ O ₃ : 85%, and a TiN of 1%.

6 is No. The form of the inclusions in the steel plate of 37 is shown (FIG. 6 (a) is a micrograph of a drawing substitute, and FIG. 6 (b) is an explanatory drawing which shows the form of an inclusion typically). The portion indicated by reference numeral 1 in FIG. 6 (a) is MnO: 10%, CaO: 46%, CaS: 2%, MgO: 2%, Al ₂ O ₃ : 26%, SiO ₂ : 11%, TiO ₂ : Part of 2%, part indicated by 2 is MnO: 4%, CaO: 13%, MgO: 1%, Al ₂ O ₃ : 7%, SiO ₂ : 4%, TiO ₂ : 71% to be.

Claims (4)

C: 0.02 to 0.15% (the meaning of "mass%", the same as for chemical components are as follows), Si: 0.03 to 1.0%, Mn: 1.0 to 2.0%, P: 0.02% or less, S: 0.0002 to 0.01%, Al: 0.005 to 0.08%, Ti: 0.003 to 0.03%, Ca: 0.0003 to 0.005%, N: 0.001 to 0.01% and O: 0.004% or less, respectively, and the balance consists of iron and inevitable impurities,
In addition, when the length of the largest width in the direction perpendicular to the maximum projection length of the inclusions under the microscope observation was the size of the inclusions, inclusions having a size of 2 µm or more were dispersed, and the inclusions were represented by the following equations (1) and (1). Steel satisfying the relationship of 2.
[Equation 1]
0.01≤ [Ca] / [Al] ≤0.50
[Equation 2]
0.08≤ ([CaO] + [CaS]) / ([Al ₂ O ₃ ]) ≤1.80
However, [Ca], [Al], [CaO], [CaS], and [Al ₂ O ₃ ] represent content (mass%) of Ca, Al, CaO, CaS, and Al ₂ O _{3 in} the inclusions, respectively. The method of claim 1,
A Ti-containing inclusion containing 10% or more of Ti, wherein the number of the particles having a size of 2 µm or more is 7% or less of the number of all the inclusions having the size of 2 µm or more. The method according to claim 1 or 2,
Further, B: 0.005% or less, Nb: 0.06% or less, V: 0.1% or less, Cu: 1.5% or less, Ni: 3.5% or less, Cr: 1.5% or less and Mo: 1.5% or less Steel comprising at least one element. The method according to claim 1 or 2,
Further, steel materials comprising at least one element selected from the group consisting of Zr: 0.05% or less, REM: 0.005% or less and Mg: 0.005% or less.

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