JP5223706B2 - Steel material excellent in toughness of heat-affected zone with high heat input and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel material excellent in weld heat-affected zone (hereinafter referred to as HAZ) toughness with a high heat input exceeding 200 kJ / cm and a method for producing the same. In particular, the present invention relates to a steel material that is used for large structures such as buildings and bridges, and to which the toughness of a high heat input welding heat-affected zone is required to stably exhibit a high value, and a method for manufacturing the same.

  The demands for material properties of welded structural steel materials used for large structures such as buildings and bridges are becoming stricter year by year. As with the base material toughness, the HAZ toughness has become more severe. Yes. In particular, if the HAZ toughness is significantly inferior to the base metal toughness, there is a risk that a crack will occur from the HAZ due to a large earthquake and the building will collapse.

  Regarding steel materials for high heat input welding having excellent HAZ toughness, for example, Patent Document 1 discloses welding heat when performing high heat input welding by adjusting the contents of Ca, O, and S in the steel material. It is described that the affected part has a fine structure.

  Here, CaS is crystallized in the solidification stage when the steel sheet is melted, MnS is further precipitated on the surface of CaS, and further, ferrite-forming nuclei such as TiN, BN, AlN, and VN are formed on MnS. By precipitating, the ferrite transformation nuclei that do not dissolve even at high temperatures during high heat input welding are finely dispersed, and the HAZ structure is made fine ferrite-pearlite structure to achieve high toughness.

  Patent Document 2 describes a method for producing a steel material having excellent weld joint toughness by adding a deoxidizing material in the order of Ti, Al, and Ca in the steelmaking stage and then further adding Al. Yes.

  Here, steel materials with excellent HAZ toughness are manufactured by finely dispersing and increasing the number of complex oxides containing any two of Ca, Al, and Ti, and by making austenite grains fine and producing fine ferrites. I can do it.

JP 2002-256379 A JP 2001-288509 A

  Conventionally, if the specified value is cleared by the average value of absorbed energy in the Charpy test, the requirement can be satisfied. However, there is an ever-increasing demand for ensuring the safety of steel structures such as buildings and bridges. Recently, stable and high toughness is required, and there is a problem with variations in individual values of Charpy absorbed energy in large heat input welding HAZ. Has come to be.

  However, the inventions described in Patent Documents 1 and 2 cannot satisfy such a requirement.

  Specifically, there are the following two points.

  (i) Even if the average value of the absorbed energy in the Charpy test is high, the fact that the individual values are uneven means that the toughness is essentially not stable, and when the test is repeated, the average value shows a low value. Being considered to have a high probability.

  (ii) For important steel structures, it may be required to pass a Charpy test once per 100m of weld length, for example, to confirm safety. In such a case, it is difficult to pass all the repeated tests unless stable high toughness is obtained at the individual Charpy absorbed energy level.

  Furthermore, in order to be used as a steel material for steel structures such as buildings and bridges, it is also required to have high safety against large-scale earthquakes. For this reason, it is necessary that the yield ratio, which is the ratio between the yield strength and the tensile strength of the steel material, is low.

  In view of such circumstances, the object of the present invention is to secure high absorbed energy stably in the Charpy test in HAZ, and to be used for steel structures such as buildings and bridges. The object is to provide a steel material excellent in toughness of a heat-affected zone.

  The present inventors conducted various examinations and experiments in order to provide a steel material having stable high toughness even in a heat-affected zone with high heat input welding. As a result, as shown in the following (a) to (e), new metallurgical effects were obtained by combining appropriate inclusion control and hard second phase structure control techniques.

  (a) In general, Al killed steel obtained by removing oxygen in steel with Al is used for important structures that require high toughness in the heat affected zone. However, as described above, Al killed steel has a large variation in individual values in the Charpy test, etc. in the weld heat affected zone. Is required.

(b) As a result of intensive research on the dispersion of individual values of absorbed energy in the Charpy test, the inventors have found that aggregates of coarse inclusions in steel or inclusions formed in the form of dotted lines by rolling are welded heat affected zone. It has been found that, during the Charpy test, crack initiation and propagation are promoted, causing variations in absorbed energy, and these variations cannot be suppressed only by reducing the HAZ structure. Note that coarse inclusions in steel are mainly Al ₂ O ₃ inclusions. In addition, Al ₂ O ₃ inclusions and sulfide inclusions such as MnS elongated by rolling are crushed by rolling, forming a series of inclusions in a dotted line shape.

  (c) If such a coarse inclusion or a group of inclusions in the form of a line is present in the vicinity of the notch of the Charpy test piece, the occurrence of cleavage fracture and crack propagation will occur due to the stress concentration effect around it. It becomes easier and the absorbed energy of the Charpy test is significantly reduced. Therefore, controlling the form of inclusions in the steel material to prevent the formation of these group of inclusions in the form of dots is an important factor for preventing variations in individual values of absorbed energy.

  (d) On the other hand, however, in the high heat input welding HAZ with a heat input exceeding 200 kJ / cm, not only the formation of these inclusion groups but also the formation of a hard second phase structure also causes the occurrence of cleavage fracture and propagation of cracks. It was found to encourage. In other words, it was newly found that when a hard second phase structure is formed, the effect of improving Charpy absorbed energy by inclusion shape control is lost.

  (e) The reason is presumed to be that an island-like martensite structure in which a martensite structure and an austenite phase are combined is formed based on the result of observing the hard second phase structure generated in these HAZs. . This is because such complex island martensite causes a significant decrease in Charpy absorbed energy. Therefore, in order to improve the toughness of the high heat input welding HAZ exceeding 200 kJ / cm, it is only necessary to prevent the formation of island martensite in addition to the inclusion form control in the steel.

  The present invention has been made on the basis of the above knowledge, the gist of which is the following (1) to (3) steel material excellent in toughness of the heat input heat affected zone of (1) to (3) and (4) production of the steel material Is in the way.

(1) By mass%, C: 0.01 to 0.2%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: Less than 0.01%, Al: more than 0.005 to 0.08%, Ti: 0.0005 to 0.02%, Ca: 0.0003 to 0.02%, N: 0.002 to 0.009% And O (oxygen): 0.001 to 0.0035%, with the balance being Fe and impurities, satisfying the following formulas (1), (2) and (3), and in the thickness direction The ferrite area ratio at the 1/4 position is 15% or more, CaO.Al ₂ O ₃ inclusions having a particle size of 0.5 to 5 μm are present in the steel, and the aspect ratio of the inclusions is 1.9. A steel material excellent in toughness of a heat-affected zone with high heat input welding, characterized by the following:

0.50 ≦ Ca / O ≦ 1.30 Equation (1) Ti / N <3.4 Equation (2)
Pcm ^* ≦ 0.23 (3) where
Pcm ^* = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 3 + Nb / 2 + 23 {B- (10.8 / 14.1) (N-Ti /3.4)}
However, when B- (10.8 / 14.1) (N-Ti / 3.4) ≦ 0, it is handled as B- (10.8 / 14.1) (N-Ti / 3.4) = 0.

  In addition, the element symbols in the expressions (1), (2) and (3) indicate the content (% by mass) of the element, and the aspect ratio is an observation observed in a cross section parallel to the rolling direction of the steel material. It means the value obtained by dividing the major axis of the product by the minor axis.

  (2) Instead of a part of Fe, in mass%, B: 0.005% or less, Nb: 0.05% or less, V: 0.1% or less, Cu: 1.5% or less, Ni: 6 0.0% or less, Cr: 1.0% or less, and Mo: 0.8% or less, one or more selected from the steel materials according to (1) above.

  (3) The steel material as described in (1) or (2) above, which is used for construction steel pipes.

  (4) Al is added and deoxidized so that the Al content in the molten steel is in the range of 0.005 to 0.08% by mass, and after further treatment with a degassing apparatus for 15 minutes or more, the molten steel temperature is 1600. Any one of the above (1) to (3), wherein Ca is added in a state maintained at ± 70 ° C., the cast slab is hot-rolled, and then water cooling is started from a temperature of 750 ° C. or lower. Steel manufacturing method.

  According to the present invention, stable high absorption energy can be secured in the Charpy test in the high heat input welding heat-affected zone, so that it is possible to provide a steel material having excellent toughness suitable for steel structures such as buildings and bridges. .

  Hereinafter, regarding the steel material according to the present invention, its chemical composition, ferrite area ratio, and inclusions will be described, and the manufacturing method according to the present invention will also be described. In addition, "%" regarding content means "mass%".

(A) Chemical composition C: 0.01 to 0.2%
C needs to be contained in an amount of 0.01% or more in order to ensure the strength and toughness of the base material and the weld. However, if the C content is too large, the formation of island martensite is promoted, the HAZ toughness is lowered and the weldability is deteriorated, so the upper limit is made 0.2%.

Si: 0.03-0.5%
Si needs to be contained in an amount of 0.03% or more because Si is effective for preliminary deoxidation of steel and is effective in securing the strength of the base material. However, if contained excessively, the formation of island martensite is promoted and the HAZ toughness is deteriorated, so the upper limit is made 0.5%. A preferable content of Si is 0.4% or less.

Mn: 0.5 to 2.0%
In order to ensure the strength and toughness of the base material and the HAZ part, Mn needs to be contained by 0.5% or more. However, if the Mn content is too large, the HAZ toughness is deteriorated and weldability is deteriorated due to the promotion of center segregation of the slab, so the upper limit is made 2.0%.

P: 0.02% or less P is an impurity element in the present invention. If it is too much, slab center segregation is promoted, and HAZ grain boundary fracture is promoted, so that the mechanical properties of the base material and HAZ are increased. Reduce. Therefore, it is preferable that the amount is as small as possible. However, if the P content is 0.02% or less, the toughness of the base material and the HAZ can be secured, so the content is specified to 0.02% or less.

S: Less than 0.01% S is an impurity element in the present invention. If it is too much, a large amount of MnS stretched at the center of the plate thickness is generated, so that the toughness of the base material and the HAZ is lowered. Moreover, since the affinity with Ca is large and CaS is produced, production of an appropriate complex oxide is inhibited. Therefore, it is preferable that the amount is as small as possible. However, if the S content is less than 0.01%, the toughness of the base material and the HAZ can be secured, and the formation of an appropriate composite oxide may be inhibited. Therefore, it is specified to be less than 0.01%. In addition, Preferably it is less than 0.001%, More preferably, it is less than 0.0004%.

Al: more than 0.005% and 0.08% or less Al is one of the important elements in the present invention. When Al is added to molten steel, it acts as a deoxidizer and produces Al ₂ O ₃ . Al ₂ O ₃ forms clusters in the molten steel, and when rolled, these clusters are separated and dispersed in the steel material in the form of dotted lines. In this case, the Al ₂ O ₃ gathered in the form of dots serves as a starting point of crack generation during the Charpy test, and deteriorates the toughness of the base material. Moreover, since Al ₂ O ₃ is a stable oxide, it does not change even by welding, and finally remains in the HAZ, so that the HAZ toughness is also deteriorated.

However, when CaO · Al ₂ O ₃ inclusions are generated in steel by adding Ca together with Al, it is prevented from acting as a fracture starting point. Therefore, Al needs to be contained exceeding 0.005%. On the other hand, when a large amount of Al is contained, the amount of Al dissolved in the steel increases, which suppresses the decomposition reaction of retained austenite to cementite in the welding cooling process, increases island martensite, and improves the toughness of the weld. Reduce. Therefore, the Al content is 0.08% or less.

Ti: 0.0005 to 0.02%
Ti reacts with N in the steel to precipitate as TiN, suppresses austenite coarsening in HAZ, and acts as a core of ferrite transformation to refine the intragranular structure, so HAZ toughness To improve. In order to obtain this effect, it is necessary to contain 0.0005% or more of Ti. On the other hand, when the Ti content is increased, the solid solution Ti is increased and the HAZ toughness is decreased.

Ca: 0.0003 to 0.02%
Ca is one of the important elements in the present invention, and in order to achieve spheroidization of inclusions, the content of Al and O must be strictly controlled. When Ca is added to the molten steel, it acts as a deoxidizer and forms CaO.Al ₂ O ₃ inclusions in the steel together with Al. Therefore, it is an element necessary for controlling the inclusion form. . Therefore, it is necessary to contain 0.0003% or more of Ca. However, if added in a large amount, the cleanliness of the steel is lowered, and the toughness of the base material and the HAZ is deteriorated. For this reason, content of Ca shall be 0.02% or less.

N: 0.002 to 0.009%
N is an extremely important element for the precipitation of TiN, reacts with Ti in the steel and precipitates as TiN, suppresses the coarsening of austenite in HAZ, and acts as a nucleus of ferrite transformation to form an intragranular structure. HAZ toughness is improved. In order to acquire this effect, it is necessary to contain N 0.002% or more. On the other hand, if the N content is less than 0.002%, the amount of TiN deposited is insufficient, and harmful Ti carbides are produced during cooling. Therefore, it is necessary to contain N in an amount of 0.002% or more. On the other hand, when the N content increases, the solid solution N increases and, as a result, the HAZ toughness deteriorates, so the upper limit of the N content is 0.009%.

O (oxygen): 0.001 to 0.0035%
O (oxygen) is one of the important elements in the present invention along with Al and Ca. O, together with Al and Ca, generates CaO.Al ₂ O ₃ inclusions in the steel, thereby preventing it from acting as a fracture starting point, and is directly related to the number of dispersed inclusions and the particle size. . Therefore, O (oxygen) needs to be contained by 0.001% or more. On the other hand, if O (oxygen) is contained excessively, a coarse oxide is formed, the number of inclusions is increased more than necessary, and the cleanliness of the base material is lowered, which adversely affects the toughness of the base material and the HAZ. Effect. Therefore, the upper limit is made 0.0035%.

  The steel material according to the present invention may further contain one or more selected from B, Nb, V, Cu, Ni, Cr and Mo in addition to the above chemical components. The reason why these elements may be contained and the contents at that time are as follows.

B: 0.005% or less B can be contained as necessary. If contained, it has the effect of enhancing the hardenability and improving the mechanical properties of the base material and HAZ. However, if the content exceeds 0.005%, the HAZ toughness and weldability are deteriorated, so the upper limit of the B content is set to 0.005%. A preferable upper limit is 0.002%. In order to enhance the hardenability and to ensure the effect of improving the mechanical properties of the base material and HAZ, it is preferable to contain 0.0003% or more.

Nb: 0.05% or less Nb can be contained as necessary. If contained, it has the effect of promoting the refinement of the matrix structure and improving the mechanical properties of the matrix. However, if the content exceeds 0.05%, the toughness of the base material and the HAZ is reduced, so the upper limit of the Nb content is set to 0.05%. In order to promote the refinement of the base material structure and to ensure the effect of improving the mechanical properties of the base material, it is preferable to contain 0.004% or more.

V: 0.1% or less V can be contained as necessary. If contained, it has the effect of improving the strength of the base material by carbonitride precipitation during tempering. However, if the content exceeds 0.1%, the toughness of the base material is lowered, so the upper limit of the V content is 0.1%. In order to ensure the effect of improving the strength of the base material by carbonitride precipitation during tempering, it is preferable to contain 0.005% or more.

Cu: 1.5% or less Cu can be contained as necessary. If contained, it has the effect of increasing strength without degrading toughness. However, if the content exceeds 1.5%, the hardenability of the steel is excessively increased and the tendency to impair the HAZ toughness becomes strong, so the upper limit of the Cu content is set to 1.5%. In order to ensure the effect of increasing the strength without deteriorating the toughness, it is preferable to contain 0.1% or more.

Ni: 6.0% or less Ni can be contained as necessary. If contained, the weldability and HAZ toughness are not adversely affected, and the hardenability is improved and the strength and toughness of the base material are improved. However, if the content exceeds 6.0%, the cost of the structural steel material is extremely high, so the upper limit of the Ni content is 6.0%. In addition, in order to improve hardenability improvement and to express the effect which improves the intensity | strength of a base material and toughness reliably, it is preferable to make it contain 0.1% or more. In particular, when Cu is contained, it is preferable to contain 0.1% or more of Ni in order to prevent cracking (Cu checking) during rolling.

Cr: 1.0% or less Cr can be contained as necessary. If contained, it has the effect of enhancing the hardenability and improving the mechanical properties of the base material and HAZ. However, if the content exceeds 1.0%, the HAZ toughness decreases even if other component conditions are satisfied, so the upper limit of the Cr content is 1.0%. In order to enhance the hardenability and to ensure the effect of improving the mechanical properties of the base material and HAZ, it is preferable to contain 0.05% or more.

Mo: 0.8% or less Mo can be contained as necessary. If contained, it has the effect of improving the strength and toughness of the base material. However, if the content exceeds 0.8%, the hardness of the HAZ increases and the toughness is impaired, so the upper limit of Mo is set to 0.8%. In order to ensure the effect of improving the strength and toughness of the base material, it is preferable to contain 0.05% or more.

  The steel material according to the present invention contains the above-mentioned essential elements or the above-mentioned optional elements, with the balance being Fe and impurities. Here, the impurity means a component mixed from raw material ore, scrap or the like, and is allowed within a range that does not adversely affect the present invention.

  The steel material according to the present invention must further satisfy the following formulas (1) to (3).

0.50 ≦ Ca / O ≦ 1.30 (1) In the CaO · Al ₂ O ₃ inclusions produced in the molten steel, CaO and Al ₂ O _{3 are} approximately 1: 1. When coexisting, the melting point of the CaO · Al ₂ O ₃ inclusions decreases below the molten steel temperature and liquefies. At this time, surface tension acts on the CaO.Al ₂ O ₃ inclusions, and the inclusions become spherical. However, in the formula (1) (where the element symbol indicates the content (mass%) of the element), when Ca / O exceeds 1.30, CaO is more than Al ₂ O ₃ In addition, when Ca / O is less than 0.50, Al ₂ O ₃ is more than CaO, and in any case, the melting point of the CaO · Al ₂ O ₃ inclusion is the molten steel temperature. Therefore, it is difficult to spheroidize CaO · Al ₂ O ₃ inclusions. Therefore, in order to control the shape so that the CaO.Al ₂ O ₃ -based inclusions are spheroidized, it is necessary to set Ca / O to 0.50 to 1.30. In order to further promote spheroidization, Ca / O is preferably 0.63 to 1.13.

Ti / N <3.4 (2) As described above, Ti and N form TiN in the steel material, so the effect of suppressing the coarsening of austenite heated during welding and the promotion of intragranular ferrite transformation HAZ toughness is improved by refinement due to the effect. However, if Ti becomes excessive and solute Ti increases, the HAZ toughness significantly decreases, so it is necessary to lower the atomic stoichiometric ratio of 3.4 for TiN formation. Therefore, in the formula (2) (where the element symbol in the formula indicates the content (mass%) of the element), it is defined as Ti / N <3.4. In addition, when Ti / N is lowered, TiN is brought to a higher temperature, so that the austenite coarsening suppressing effect is improved. Ti / N is preferably less than 2.5.

Pcm ^* ≦ 0.23 (3) The formation of island martensite in high heat input welding HAZ is facilitated not only by the increase of C but also other alloy elements. When the hardness of the HAZ increases, the structure changes from a ferrite-based structure to a bainite-based structure, and island martensite also increases.

Pcm ^* was originally developed as a parameter for preventing weld cold cracking of TMCP (Thermo-Mechanical Control Process) steel. However, since this parameter has a good correlation with the hardness of the HAZ, in the present invention, this parameter is used as an index for preventing the formation of island martensite. In order to prevent generation of island martensite in the high heat input welding HAZ according to the present invention, the value of Pcm ^* may be set to 0.23 or less. Furthermore, favorable weldability can also be ensured by setting the value of Pcm ^* to 0.23 or less. Pcm ^* is defined by the following formula (wherein the element symbol in the formula indicates the content (mass%) of the element).
Pcm ^* = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 3 + Nb / 2 + 23 {B- (10.8 / 14.1) (N-Ti /3.4)}
However, when B- (10.8 / 14.1) (N-Ti / 3.4) ≦ 0, it is handled as B- (10.8 / 14.1) (N-Ti / 3.4) = 0.

(B) Ferrite area ratio at 1/4 position in the plate thickness direction The steel material according to the present invention is required not only to have excellent HAZ toughness but also to have safety against large earthquakes as a steel material for buildings and bridges. For this purpose, it is desirable to lower the yield ratio, which is the ratio between the yield strength and the tensile strength of the steel material, and it is necessary to lower the yield ratio by controlling the steel material structure. As this steel material structure control, a low yield ratio can be achieved if the ferrite area ratio at the 1/4 position in the thickness direction is 15% or more.

_{(C) CaO · Al 2 O} 3 inclusions particle size and CaO · _Al 2 _{O 3} inclusions Aspect ratio is formed by a part of Al in the _Al 2 _{O 3} is replaced with Ca Refers to inclusions. In order to form CaO · Al ₂ O ₃ inclusions in steel, a steel material may be manufactured through a steel making stage as described later.

The particle size of the CaO · Al ₂ O ₃ inclusion is 0.5 to 5 μm. The reason why the particle size is 0.5 μm or more is that inclusions smaller than this have a low probability of affecting the fracture starting point and do not significantly affect the HAZ toughness. Therefore, in the present invention, regarding the inclusion having a particle size of 0.5 μm or less, the number and shape thereof are not problematic. The reason why the particle size is 5 μm or less is that when the upper limit of O is 0.0033%, many CaO · Al ₂ O ₃ inclusions having a particle size exceeding 5 μm cannot be dispersed, even if the CaO exceeds 5 μm. This is because even if Al ₂ O ₃ inclusions exist, the probability of acting as a fracture starting point during the Charpy test is extremely low.

When CaO · Al ₂ O ₃ inclusions are spheroidized and the aspect ratio (major axis / minor axis) is close to 1, stress concentration on the inclusions and surrounding tissues during the Charpy test is reduced, improving toughness. And stabilize. On the other hand, when an inclusion with a large aspect ratio and an elongated diameter is present in the vicinity of the notch of the Charpy specimen, it becomes a stress concentration source, and the toughness is remarkably reduced due to the propagation of cracks generated therefrom. In addition, an aspect ratio means the value which remove | divided the major axis of the inclusion observed by the cross section parallel to the rolling direction of steel materials by the minor axis.

CaO · Al ₂ O ₃ inclusions are spheroidized in molten steel if Ca / O is in the range of 0.50 to 1.30, and inclusions of this composition may be crushed or stretched by rolling. Therefore, the aspect ratio is close to 1. However, when Ca / O is less than 0.50 or 1.30 or more, CaO · Al ₂ O ₃ inclusions do not completely spheroidize in the molten steel, and are crushed during rolling to form a dotted line. It becomes a smooth shape and the aspect ratio is high. In this case, since it becomes a stress concentration source in the Charpy test, the upper limit of the aspect ratio is set to 1.9. At this time, the inclusions arranged in a dot array may be regarded as one extended inclusion.

Al ₂ O ₃ and CaO having a particle size of 0.5 μm or more form rolled coarse inclusions or inclusion groups connected in a dotted line by rolling, and become a stress concentration source in the Charpy test. Stability is significantly reduced. In particular, when the aspect ratio is 5 or more, it acts as the most effective stress concentration source even if other inclusions exist. At this time, similarly to the CaO.Al ₂ O ₃ -based inclusions, the inclusions arranged in a dotted line may be regarded as one extended inclusion.

The aspect ratio of the CaO · Al ₂ O ₃ inclusions of the present invention may be quantitatively measured by the following method. That is, an observation sample is prepared from a cross section parallel to the rolling direction of the steel material, preferably from the center of the cross section, and at least 100 or more CaO · s at a magnification of 3000 to 10,000 times using a scanning electron microscope (SEM). observing the al ₂ O ₃ inclusions, the diameter may be calculated an average value of a value obtained by dividing the minor axis. For Al ₂ O ₃ and CaO, there is no problem in using the same measurement method.

  Moreover, there is no problem in using the average value of the major axis measured at this time as the particle size of the inclusion.

(D) Manufacturing method The manufacturing method which concerns on this invention has the characteristics in the steelmaking stage. That is, Al is added so that the Al content in the molten steel is in the range of 0.005% to 0.08% for deoxidation, and after further treatment with a degasser for 15 minutes or more, the molten steel temperature is 1600 ± 70 ° C. After adding Ca in a state kept at a high temperature and hot rolling the cast slab, water cooling is started from a temperature of 750 ° C. or lower to produce a steel sheet having excellent toughness of the heat-affected zone with high heat input welding It is.

In the production method according to the present invention, first, Al is added to the molten steel whose components have been adjusted in advance so that the Al content in the molten steel is in the range of 0.005% to 0.08% for deoxidation. Since Al added first has a strong deoxidizing power, it combines with solute oxygen in the molten steel to produce Al ₂ O ₃ . When Al in molten steel is less than 0.005%, deoxidation by Al becomes insufficient, and Al ₂ O ₃ is not generated. Moreover, when Al in molten steel exceeds 0.08%, excess Al remains as solid solution Al in steel, and the toughness of a base material and HAZ is deteriorated. In the production method according to the present invention, “adding Al” means that the introduced Al is uniformly mixed in the molten steel. The same applies to the addition of Ca.

Then, it processes for 15 minutes or more with a degassing apparatus. By degassing for a certain time or longer, coarse Al ₂ O ₃ can be levitated and separated. Thereafter, Ca is added and cast in a state where the molten steel temperature is maintained at 1600 ± 70 ° C. When Ca is added, part of Al ₂ O ₃ is reduced and Al and Ca are substituted to form CaO · Al ₂ O ₃ inclusions. At this time, by controlling the temperature of the molten steel to 1600 ± 70 ° C., the CaO · Al ₂ O ₃ inclusions liquefy in the molten steel, and the inclusions are spherical in order to minimize the surface area by the action of surface tension. Turn into. For spheroidization, it is necessary to control the Ca, Al, and O contents to be the contents according to the present invention.

  The slab thus obtained is further processed into a plate shape through a casting process and a hot rolling process. Casting and hot rolling can be performed by ordinary methods. After hot rolling, water cooling is started from a temperature of 750 ° C. or lower. By performing such water cooling, a steel material having a low yield ratio can be manufactured with a ferrite area ratio of 15% or more at a ¼ thickness position. Performing tempering after water cooling has no problem in controlling the ferrite fraction.

  Steel having the chemical composition shown in Table 1 was melted, and a steel plate having a thickness of 19 to 65 mm was produced through heating and rolling. The obtained steel sheet was subjected to a reproducible thermal cycle test simulating a temperature history of welding at a heat input of 200 to 500 kJ / cm, a Charpy test was performed at 0 ° C., and HAZ toughness was evaluated. Test numbers 1 to 23 are examples of the present invention, and a to n are comparative steels. The manufacturing conditions of the base material are as follows.

  Table 2 shows the base material manufacturing method, base material characteristics, and HAZ toughness. In addition, DQ of the base material manufacturing method in Table 2 indicates that it was directly water-cooled from the water-cooling start temperature shown in the table after the end of rolling, and T indicates that it was tempered at the tempering temperature shown in the table after water-cooling. Show. Further, the degassing treatment after the addition of Al was performed for 15 minutes or more in all Examples. The Charpy test for HAZ toughness evaluation was performed with three test pieces to which a reproducible thermal cycle was applied. Table 2 shows the measured values of the three test pieces.

  As is apparent from Table 2, the steel materials of test numbers 1 to 23 according to the inventive examples have excellent HAZ toughness, and the HAZ toughness at 0 ° C. is extremely excellent at 70 J or more.

On the other hand, as for the steel materials of test numbers a to n according to the comparative example, at least one of the three HAZ toughnesses at 0 ° C. was less than 70 J. Among these, the comparative examples h and n had large variations in HAZ toughness, and the maximum difference was 200J and 180J, respectively.
Here, the steel materials of test numbers a to k are examples in which the basic components or component parameters do not satisfy the requirements of the steel of the present invention. Moreover, since the steel material of test number 1 has a high water-cooling start temperature, the particle size and aspect ratio of the CaO · Al ₂ O ₃ inclusions exceed the predetermined values of the steel of the present invention. In addition, since both of the steel materials of test numbers m and n have a Ca addition temperature deviating from the production method of the present invention, the aspect ratio of CaO · Al ₂ O ₃ inclusions is the predetermined ratio of the steel of the present invention. This is an example in which the value exceeds the value and the HAZ toughness decreases.

  From the above experimental results and the mechanism for improving the notch tensile strength, it is clear that the phenomenon that the notch tensile strength is improved by lowering S is universally realized in general carburized steel.

  According to the present invention, stable high absorption energy can be secured in the Charpy test in the high heat input welding heat-affected zone, so that it is possible to provide a steel material having excellent toughness suitable for steel structures such as buildings and bridges. .

Claims (4)

In mass%, C: 0.01 to 0.2%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.01 %: Al: more than 0.005 to 0.08%, Ti: 0.0005 to 0.02%, Ca: 0.0003 to 0.02%, N: 0.002 to 0.009%, and O ( Oxygen): 0.001 to 0.0035% contained, the balance being Fe and impurities, satisfying the following formulas (1), (2) and (3), and 1 / The ferrite area ratio at 4 positions is 15% or more, CaO · Al ₂ O ₃ inclusions having a particle size of 0.5 to 5 μm are present in the steel, and the aspect ratio of the inclusions is 1.9 or less. A steel material with excellent toughness of heat-affected zone with high heat input welding.
0.50 ≦ Ca / O ≦ 1.30 Equation (1) Ti / N <3.4 Equation (2)
Pcm ^* ≦ 0.23 (3) where
Pcm ^* = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 3 + Nb / 2 + 23 {B- (10.8 / 14.1) (N-Ti /3.4)}
However, when B- (10.8 / 14.1) (N-Ti / 3.4) ≦ 0, it is handled as B- (10.8 / 14.1) (N-Ti / 3.4) = 0.
In addition, the element symbols in the expressions (1), (2) and (3) indicate the content (% by mass) of the element, and the aspect ratio is an observation observed in a cross section parallel to the rolling direction of the steel material. It means the value obtained by dividing the major axis of the product by the minor axis. Instead of a part of Fe, by mass%, B: 0.005% or less, Nb: 0.05% or less, V: 0.1% or less, Cu: 1.5% or less, Ni: 6.0% The steel material according to claim 1, characterized in that it contains one or more selected from Cr: 1.0% or less and Mo: 0.8% or less.   The steel material according to claim 1 or 2, wherein the steel material is used for a construction steel pipe.   Al is added so that the Al content in the molten steel is in the range of 0.005 to 0.08 mass% for deoxidation, and after further treatment with a degasser for 15 minutes or more, the molten steel temperature is 1600 ± 70 ° C. The steel material according to any one of claims 1 to 3, wherein Ca is added in a state of being kept at a low temperature, and after the cast slab is hot-rolled, water cooling is started from a temperature of 750 ° C or lower. Production method.