KR20110117669A - Steel material excellent in fatigue crack progression resistance and corrosion resistance, and method for manufacturing the same - Google Patents

Abstract

By mass%, C: 0.01 ~ 0.14%, Si: 0.04 ~ 0.6%, Mn: 0.5 ~ 2.0%, P: 0.01% or less, S: 0.003% or less, Cu: less than 0.2%, B: 0.0007% Less than 0.005%, less than 0.05% of Al, less than 0.007% of N, less than 0.003% of O and less than 0.03% to 0.50% of Sn; the remainder consists of Fe and impurities, and has a Cu / Sn ratio of 1 or less It has a composition, and the Bq value is 0.003 or less, Ceq value is 0.15 to 0.35, and the number of oxide crack propagation characteristics, characterized in that the number of oxides in the region within 2mm from the surface layer of 5 × 10 ⁴ or less per square mm and It is a steel with excellent corrosion resistance. Moreover, you may contain 1 type, or 2 or more types of Mo, V, Nb, Ni, Cr, Ti, Ca, and Mg.

Description

STEEL MATERIAL EXCELLENT IN FATIGUE CRACK PROGRESSION RESISTANCE AND CORROSION RESISTANCE, AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to steel materials suitable for use in hulls, civil constructions, construction machinery, hydraulic steel pipes, offshore structures, line pipes and other welded structures requiring crack propagation characteristics and corrosion resistance.

In recent years, the tendency to enlarge a welded structure has become remarkable, and high strength and weight reduction are desired. However, when using high-strength steel, the design stress rises, so that fatigue breakage easily occurs from the welded portion, and the improvement is an important problem. In thick steel sheets, such as structural steel, welding is generally performed, and there is a possibility that fatigue cracking occurs from the welded portion. Therefore, it is effective for extending the fatigue life of the structure as long as the fatigue crack generated and developed from the welded portion can be retained in the steel. For this reason, various steel materials which have a fatigue crack growth suppression effect are proposed.

For example, Patent Document 1 proposes a technique of reducing the high temperature strength and relieving the welding residual stress in the weld joint by utilizing a ferrite phase having a lower strength than the austenite phase at the same temperature. That is, in the weld part, since it is quenched after welding, the temperature range of the austenite single phase is wide, and a high level of residual stress is generated by the heat shrink of the weld metal. Therefore, in the invention described in Patent Document 1, 0.5-2.0% of Al is contained in steel as a ferrite generating element, ferrite is produced in a temperature range of 800 to 600 ° C, and residual stress is reduced by plastic deformation of ferrite of low strength. It is easing.

In addition, Patent Document 2 discloses a technique for improving fatigue strength by suppressing grain boundary ferrite generated from austenite grain boundaries using the structure of the weld heat affected zone (HAZ) of high tensile strength steel sheets having a tensile strength of 490 to 780 MPa as the bainite. Proposed. In this technique, in order to suppress the formation of grain boundary ferrite, 0.0005 to 0.01% of B is added, and in order to strengthen the entire structure including bainite and martensite, a carbon equivalent (Ceq) is formed.

On the other hand, welded steel structures are often used in environments with high salt content, such as beach areas and areas where snow melt is sprayed, and in sea water splash environments in shipbuilding.

Generally, when weathering steel is exposed in an atmospheric corrosion environment, a protective rust layer is formed on the surface, and subsequent steel corrosion is suppressed. Therefore, weather resistant steel is used for structures, such as a bridge, as a minimum maintenance steel material which can be used as it is, without painting.

However, not only the beach area but also the inland area where the salt content is high, such as the area where snow salts and cryoprotectants are sprayed, protective rust layer is hard to form on the surface of weathering steel, so the effect of suppressing corrosion is exhibited. It's hard to be. Therefore, it is not possible to use the weather resistant steel as it is in these areas, and the use of the coating of ordinary steel which coats and uses ordinary steel is common. However, in the case of the use of the coating of ordinary steel, it is necessary to repaint every ten years because of the deterioration of the coating film due to corrosion, and the cost for maintenance is enormous.

In recent years, weather resistant steel (JIS G 3114: weather resistant hot rolled steel for welded structures), which has been standardized to Japanese Industrial Standards (JIS), has an aliphatic salt content of NaCl as 0.05 mg / dm ² / day (0.05mdd) or more, for example, beaches. In the area, corrosion loss is large due to scale rust and layer rust, so it cannot be used in the unpainted ground (Construction Civil Engineering Research Institute, Steel Club, Japan Bridge Construction Association: Weatherproof Steel Joint Research Report on Application to Bridges (XX)-See Design and Construction of Unpainted Weathering Bridges (Rev. 1993.3).

As described above, in a salty environment such as a beach area, the steel is usually coated and coped. However, it is presently present that bridges built on roads such as beach areas near estuaries and mountain roads spraying snow melt are markedly corroded and must be repainted. Since these repaints require enormous process water, there is a strong demand for steel materials that can be used in unpainted coatings.

Recently, Ni-based highly weather resistant steels with Ni added in amounts of about 1 to 3% have been developed. However, in areas where the salt content exceeds 0.3 to 0.4mdd, it has proved difficult to apply to steel materials which can be used for unpainting only with such Ni addition.

Corrosion of steel materials is intensified as the amount of fly salt increases, and therefore, from the viewpoint of corrosion resistance and economical efficiency, weather resistant steels according to the amount of fly salt are required. Moreover, even if it is a bridge, the corrosion environment of steel materials is not the same according to the place and site | part used. For example, outside the trees, they are exposed to rainfall, condensation and sunlight. On the other hand, inside the tree tree, it is exposed to condensation water but does not encounter rain. In general, it is said that in the environment where the amount of salt in the air is high, the inner side of the tree is more corrosive than the outside of the tree.

In addition, in an environment in which a snow melting salt or a cryoprotectant is sprayed on a road, the salt is wound up on a running car and adhered to a bridge supporting the road, resulting in a severe corrosion environment. In addition, even under the eaves slightly away from the coast, it is exposed to a severe salty environment, and in such a region, it becomes a severe corrosive environment with a salt content of 1mdd or more.

In order to cope with such a problem, the development of the steel which prevents corrosion in the environment where abundant salt content is high has conventionally progressed.

For example, Patent Document 3 proposes a weather resistant steel having increased chromium (Cr) content, and Patent Document 4 proposes a weather resistant steel having increased nickel (Ni) content.

Japanese Patent Publication No. 2004-211150 Japanese Patent Publication No. 2003-171731 Japanese Patent Application Laid-open No. Hei 9-176790 Japanese Patent Laid-Open No. 5-118011

However, the steel materials having the proposed fatigue crack growth inhibiting effect and the steel materials having corrosion resistance have problems as follows.

The technique proposed in Patent Literature 1 makes a high concentration of Al addition essential for the ferrite phase to exist in a wider temperature range. However, Al contributes to the formation of the ferrite phase, but is an element that significantly lowers toughness, which is one of the basic characteristics required for structural steel materials. For this reason, according to this technique, although the residual stress of a weld part can be suppressed and a fatigue strength improvement can be expected, the toughness itself with respect to a static load falls short. The design of the shape and dimensions of the structural material needs to be carried out not only from the viewpoint of fatigue strength, but also from the standpoint of preventing brittle fracture against static load. In the technique proposed in Patent Literature 1, the strength and health are balanced well. Can't improve

In the technique proposed in Patent Literature 2, the hardenability at the grain boundary is increased and the production of grain boundary ferrite is suppressed by adding B which can effectively suppress grain boundary ferrite. However, since B is an element which lowers the toughness of the weld heat affected zone, its use requires attention. In the weld joint part, not only the fatigue characteristics with respect to a cyclic load but also the toughness is important to prevent brittle fracture by a static load. In particular, the majority of the member dimensions are determined by the latter toughness, and toughness is also important in the fatigue design system of the present situation in which fatigue investigation for confirming the fatigue fracture prevention is performed on necessary parts. . In this sense, considering the case where the welding conditions, for example, the weld heat input is varied, it can be said that it is extremely difficult to achieve both the generation of grain boundary ferrite and the toughness of the weld heat affected portion only by the same method as the addition of B.

In addition, weather resistant steels having an increased content of chromium (Cr) proposed in Patent Document 3 can improve weather resistance in a region of a specific amount of fly salt, but deteriorate weather resistance in a severe salt environment exceeding that. .

In the case of weather resistant steels in which the nickel (Ni) content proposed in the patent document 4 is increased, the weather resistance is improved to some extent, but the cost of the steel itself becomes high, and it is expensive as a material used for applications such as bridges. In order to avoid this, when the Ni content is small, the weather resistance is not improved so much, and when the salt content is large, layered peeling rust is formed on the surface of the steel, corrosion is remarkable, and there is a problem that it cannot endure long-term use. Occurs.

In addition, in the welded steel structure that is used in an environment where the salt content is high, coating coating peeling resistance is a big problem. That is, as shown above, in a coastal environment in which a large amount of chloride flows, or in an environment in which a snow melting agent or an antifreezing agent is sprayed, there is a problem that the coating is peeled off early and corrosion progresses. It is necessary to repaint every decade. Moreover, when repainting, it is necessary to provide a scaffolding to the bridge which once corroded, and to carry out the reblasting process, and it costs enormous cost. And since it is difficult to remove rust completely even if it carries out reblasting process, even if it coats again on the steel material from which rust was not removed completely, paint life becomes remarkably short. Coating resistance peelability is also large by the characteristic which included the corrosion resistance of the steel material which is a base.

Therefore, it is strongly desired to extend the service life of the coating and to greatly extend the maintenance coating interval. In other words, even in the field of ships and bridges where painting is required, the demand for minimizing the life cycle cost is high, and extending the painting life becomes very important in considering the life cycle management of bridges.

The present invention has been made to solve such a problem, and an object of the present invention is to provide a steel material having excellent fatigue crack propagation characteristics and corrosion resistance and a manufacturing method thereof without containing a large amount of elements that inhibit toughness such as Al and B. Doing.

Here, the corrosion resistance includes corrosion resistance in a high chloride environment (the coating is not peeled off and the corrosion in the coating defect is suppressed to maintain the corrosion resistance (paint resistance) and weather resistance at the time of unpainting. Means).

In order to achieve the above object, the present inventors first conducted a study paying attention to the correlation between the fatigue properties of weld joints and the cleanliness of inclusions present in steel materials. It did not matter.

Therefore, paying attention to the steel surface with a large amount of displacement, and examining it in more detail, the joint fatigue characteristic largely depends on the characteristic of the steel surface, and it turned out that the joint fatigue characteristic improves by raising the cleanness. More specifically, when the inclusion analysis was limited to an area from the surface of the steel sheet to the depth of 2 mm in the plate thickness direction, the cleanliness was determined for each steel sheet, and the correlation with the joint fatigue characteristics was examined. As a reason for such a correlation being recognized, it can be considered that the steel surface has a large displacement amount and tends to cause fatigue cracking.

However, since the inclusions have high hardness, they do not deform even under high stress. On the other hand, since the displacement amount of the steel surface is large, it can be considered that cracking occurs at the interface between the inclusions and the substrate and the fatigue characteristics deteriorate. Therefore, although the cleanliness of an interference | inclusion generally becomes a sheet thickness center part of steel materials, the cleanliness of steel surface becomes a problem regarding fatigue characteristics.

On the other hand, the present inventors examined corrosion in an environment having a large amount of salt, and as a result, in such an environment, repeated wet and dry of the FeCl ₃ solution became an essential condition of corrosion, and the pH was lowered by the hydrolysis of Fe ³⁺ . In the state, it was also found that the corrosion is accelerated by Fe ³⁺ acting as an oxidant.

The corrosion reaction at this time is as showing below.

As a cathode reaction, the following reaction mainly occurs.

^{Fe 3+ + e - → Fe 2+} ( reduction of the Fe ³⁺⁾

In addition to this reaction, the following cathode reaction is also combined.

_{2H 2 O + O 2 + 2e} - → 4OH -,

2H ^{+ +} 2e ^- → H ₂

On the other hand, the following anode reaction occurs with respect to the reduction reaction of Fe ³⁺ .

The anode ^{reaction: Fe → Fe 2+ + 2e -} (Fe dissolution of the reaction)

Therefore, the overall reaction of corrosion is as follows (1) Formula.

2Fe ³⁺ + Fe → 3Fe ²⁺ ... … (1) expression

Fe ²⁺ produced by the reaction of the above formula (1) is oxidized to Fe ³⁺ by air oxidation, and the generated Fe ³⁺ acts as an oxidant again to accelerate corrosion. At this time, the reaction rate of air oxidation of Fe ²⁺ is generally slow in a low pH environment, but is accelerated in a rich chloride solution, and Fe ³⁺ is easily generated. Because of this cyclic reaction, it has been found that in an environment where the amount of salt in the air is very high, Fe ³⁺ is continuously supplied all the time, and the corrosion of the steel is accelerated to significantly deteriorate the corrosion resistance.

MEANS TO SOLVE THE PROBLEM The present inventors acquired the knowledge shown to the following (a)-(c) as a result of examining the influence on the weather resistance of various alloy elements based on the mechanism of corrosion in such a salt environment.

(a) Sn is dissolved as Sn ²⁺ , and the reaction of formula (1) is suppressed by decreasing the concentration of Fe ³⁺ by the reaction of 2Fe ³⁺ + Sn ²⁺ → 2Fe ²⁺ + Sn ⁴⁺ . . Sn also has the effect of suppressing anode dissolution.

(b) Cu is an element which has conventionally been the basis of the corrosion resistance improvement effect in an environment with a large amount of salt, and shows an improvement in corrosion resistance in an environment with a relatively long wet time. However, in environments with higher chloride concentrations, locally lowering pH, for example, salt deposits, humidity changes as humidity changes, and in relatively dry environments where β-FeOOH is produced, Cu rather promotes corrosion. It turned out to be.

(c) Thus, the steel material which contains Sn actively and suppressed Cu content can expect high corrosion resistance. Moreover, since corrosion resistance is high, even if it coats steel materials, since the peeling of the coating resulting from corrosion of steel materials is few, the corrosion of a coating defect part can be suppressed, and the anticorrosive effect by a coating film can also be expected. The effect of corrosion resistance can be expected further. Therefore, in addition to corrosion resistance, the service life of the coating can be extended, and the maintenance coating interval can be extended significantly. In particular, it is effective in the improvement of the coating peelability in the ship field or the bridge field.

This invention is completed based on this knowledge, The summary is the steel material excellent in the fatigue fatigue crack growth characteristics and corrosion resistance shown to following (1)-(6), and following (7) and (8) The fatigue resistance crack growth characteristics and corrosion resistance shown in the manufacturing method of steel materials are shown.

(1) In mass%, C: 0.01 to 0.14%, Si: 0.04 to 0.6%, Mn: 0.5 to 2.0%, P: 0.01% or less, S: 0.003% or less, Cu: less than 0.2%, B: 0.0007% More than 0.005%, Al: less than 0.05%, N: 0.007% or less, O: 0.003% and Sn: 0.03 to 0.50% or less, and the balance is made of Fe and impurities, and the Cu / Sn ratio It has a chemical composition of 1 or less, and the Bq value obtained from the following formula (1) is 0.003 or less, the Ceq value obtained from the following formula (2) is 0.15 to 0.35, and the number of oxides in the region within 2 mm from the surface layer is 1 Steel having excellent fatigue crack propagation characteristics and corrosion resistance, characterized by less than 5 × 10 ⁴ pieces per square mm.

However, each element symbol in the said formula means content (mass%) of each element. In addition, when content of each element is an impurity level, 0 (zero) shall substitute.

(2) In addition, the fatigue fatigue crack growth characteristics according to the above (1), wherein the mass% contains at least one element selected from Mo: 1.0% or less, V: 0.1% or less and Nb: 0.1% or less. And steel having excellent corrosion resistance.

(3) Furthermore, the steel material which is excellent in the crack propagation characteristics and corrosion resistance of said (1) or (2) characterized by containing 1.5% or less of Ni by mass%.

(4) Further, the steel is excellent in crack propagation characteristics and corrosion resistance in any of the above (1) to (3), wherein the mass% contains Cr: 1.2% or less.

(5) In addition, the steel material excellent in crack propagation characteristics and corrosion resistance in any of the above (1) to (4), wherein the mass% contains 0.05% of Ti or less.

(6) Further, the fatigue propagation cracking resistance and corrosion resistance of any one of (1) to (5) above, wherein the mass% contains one or both of Ca: 0.003% or less and Mg: 0.003% or less. This excellent steel.

(7) Manufacture of steel excellent in fatigue fatigue crack propagation characteristics and corrosion resistance, comprising the following steps A to D and further comprising a recuperation temperature width after completion of cooling in step D of 70 ° C or less. Way.

Process A: injecting inert gas into molten steel under conditions satisfying the following formula (3),

Process B: The process of continuously casting the obtained molten steel and obtaining the steel piece which has a chemical composition in any one of said (1)-(6),

Step C: after heating the obtained steel strip to 900 to 1180 ° C, performing hot rolling under the condition that the finishing temperature is 650 to 1000 ° C to obtain a hot rolled material, and

Process D: The obtained hot rolled material is accelerated-cooled from the temperature range of 620-950 degreeC on condition that the average cooling rate in the temperature range of 620-500 degreeC will be 5-50 degreeC / sec, and the temperature range of 500 degrees C or less. Terminating cooling in the process.

However, the definition of the symbol in said (3) formula is as follows.

G ₁ : flow rate of inert gas injected into molten steel (NL / min)

H ₁ : Distance from the tip of the inert gas injection nozzle to the molten steel bath surface (m)

t ₁ : inert gas injection time (min)

S ₁ : Ladle molten steel (ton)

D ₁ : ladle inner diameter (m)

(8) A method for producing a steel having excellent fatigue crack propagation characteristics and corrosion resistance, comprising the following steps A1 to D, and a recuperation temperature width after completion of cooling in step D is 70 ° C. or less.

Step A1: a step of performing vacuum refining treatment on molten steel under conditions satisfying the following formula (4),

Process B: The process of continuously casting the obtained molten steel and obtaining the steel piece which has a chemical composition in any one of said (1)-(6),

Step C: after heating the obtained steel strip to 900 to 1180 ° C., performing hot rolling under the condition that the finishing temperature is 650 to 1000 ° C. to obtain a hot rolled material, and

Process D: The obtained hot rolled material is accelerated-cooled from the temperature range of 620-950 degreeC on condition that the average cooling rate in the temperature range of 620-500 degreeC will be 5-50 degreeC / sec, and the temperature range of 500 degrees C or less. Terminating cooling in the process.

However, the definition of the symbol in said (4) formula is as follows.

G ₂ : Inert gas flow rate (NL / min) used for reflux of molten steel

D ₂ : Internal diameter of immersion pipe (m)

t ₂ : vacuum treatment time (min)

S ₂ : Ladle molten steel (ton)

The steel of the present invention is excellent in fatigue fatigue crack propagation characteristics and corrosion resistance, and thus is suitable for use in hulls, civil constructions, construction machinery, hydraulic steel pipes, offshore structures, line pipes and other welded structures requiring fatigue fatigue crack growth characteristics. Do.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the shape and dimension of a joint test body.

A. About the chemical composition of the steel of the present invention

First, the chemical composition and others of the steel of the present invention will be described. In the following description, "%" with respect to content means "mass%."

C: 0.01 to 0.14%

C is an element necessary for securing strength. If the content is less than 0.01%, the required strength cannot be secured. However, when the content exceeds 0.14%, it becomes difficult to secure the toughness of both the weld heat affected zone (HAZ) and the base material when welding. Therefore, content of C is made into 0.01 to 0.14%. The minimum with preferable C content is 0.03%, and a preferable upper limit is 0.10%.

Si: 0.04 ~ 0.6%

Si not only has a deoxidation effect but also contributes to an increase in strength of the steel. In order to acquire these effects, it is necessary to contain Si 0.04% or more. However, when the content exceeds 0.6%, the toughness is reduced. Therefore, content of Si is made into 0.04 to 0.6%.

Mn: 0.5 ~ 2.0%

Mn has the effect of raising the hardenability of steel and is an effective component for securing strength. If the content is less than 0.5%, hardenability is insufficient and desired strength and toughness are not obtained. However, when Mn is contained in excess of 2.0%, segregation increases and hardenability becomes too high, and the toughness of both the weld heat affected part and the base material decreases during welding. Therefore, content of Mn is made into 0.5 to 2.0%.

P: 0.01% or less

P inevitably exists in steel as an impurity. When the content exceeds 0.01%, the segregation at the grain boundary not only lowers the toughness but also causes high temperature cracking during welding. Therefore, the content of P needs to be limited to 0.01% or less. The smaller P is, the more preferable.

S: 0.003% or less

S inevitably exists in steel as an impurity. If the content is too large, the center segregation is encouraged, or a large amount of elongated MnS is generated, thereby deteriorating the mechanical properties of the base metal and the weld heat affected zone. Therefore, the content of S needs to be limited to 0.003% or less. The smaller S is, the more preferable.

Cu: less than 0.2%

Cu generally becomes a basic element which improves weather resistance and is added to all seaside weather resistant steels and corrosion resistant steels, but in a relatively dry environment under high boiling salt, corrosion resistance is rather deteriorated. If it coexists with Sn, cracking occurs during rolling. Therefore, it is necessary to reduce the content of Cu. Even if it contains as an impurity, Cu content needs to be less than 0.2%. Preferably less than 0.1%.

B: more than 0.0007% and less than 0.005%

B is an element which is effective in improving hardenability and raising strength. In order to acquire this effect, it is necessary to contain exceeding 0.0007%. However, when the content exceeds 0.005%, fatigue characteristics deteriorate. Therefore, content of B exceeds 0.0007% and may be 0.005% or less.

Al: less than 0.05%

Al is an element having a deoxidation action. However, when the content is 0.05% or more, the toughness tends to deteriorate mainly in the weld heat affected zone. This is considered to be because coarse cluster-like alumina inclusion particles are easily formed. Therefore, Al content is made into less than 0.05%. However, when deoxidation is performed by Si which has a deoxidation effect, it does not need to contain especially. Moreover, in order to exhibit the deoxidation effect by Al stably, it is preferable to contain 0.001% or more.

N: 0.007% or less

N is an element which inevitably exists in steel as an impurity. When present in a large quantity, it becomes a cause of deterioration of the toughness of a base material and a weld heat influence part. Therefore, N content is made into 0.007% or less. The smaller N is, the more preferable.

O: 0.003% or less

O (oxygen) is an element which inevitably exists in steel as an impurity. If the content exceeds 0.003%, it adversely affects the ductility of the base metal toughness and elongation shrinkage. Therefore, O content is limited to 0.003% or less.

Sn: 0.03 to 0.50%

Sn dissolves and becomes Sn ²⁺ , and has a function of inhibiting corrosion by an inhibitor action in an acid chloride solution. In addition, Fe ³⁺ is rapidly reduced to reduce the Fe ³⁺ concentration as an oxidizing agent, thereby suppressing the corrosion promoting action of Fe ³⁺ , thereby improving weather resistance in a high boiling salt environment. In addition, Sn has an effect of suppressing the anode dissolution reaction of steel to improve corrosion resistance. Moreover, by containing Sn, the effect of improving the weather resistance of Cr is exhibited also in the environment where abundant salts exist. These effects are obtained by containing 0.03% or more of Sn. If it exceeds 0.50%, it is saturated. Therefore, content of Sn is made into 0.03 to 0.50%. The range with preferable content of Sn is 0.03 to 0.20%.

Cu / Sn ratio: 1 or less

In the case of steel containing Sn, the fall of corrosion resistance by the containing of Cu is remarkable. Moreover, when manufacturing steel materials, it may be a cause of the rolling crack by containing Cu. For this reason, it is necessary to make Cu / Sn ratio, ie, ratio of Cu content with respect to Sn content, 1 or less.

The steel material which concerns on this invention has said chemical composition, and remainder consists of Fe and an impurity. Here, an impurity is a component mixed by various factors of a manufacturing process, such as raw materials, such as an ore and scrap, when manufacturing steel materials industrially, and means that it is acceptable in the range which does not adversely affect this invention.

The steel material of this invention can be made to contain 1 or more types of components selected from the at least 1 group from the following 1st group to 5th group as needed. Hereinafter, the component which belongs to these groups is described.

Ingredients of Group 1: Mo, V, Nb

Mo: 1.0% or less

Since Mo has the effect of improving the strength and toughness of the base metal, you may contain Mo as needed. However, when it contains exceeding 1.0%, the hardness of a weld heat influence part mainly becomes high, and damages toughness and SSC resistance. Therefore, when it contains Mo, it is preferable to make the content into 1.0% or less. In addition, in order to acquire this effect stably, it is preferable to contain 0.05% or more.

V: 0.1% or less

Since V has the effect of improving the strength of a base material mainly by carbonitride precipitation at the time of tempering, you may contain V as needed. However, when it contains exceeding 0.1%, the performance improvement effect of a base material will be saturated and a toughness will be deteriorated. Therefore, when it contains V, it is preferable to make the content into 0.1% or less. In addition, in order to acquire this effect stably, it is preferable to contain 0.005% or more.

Nb: 0.1% or less

Since Nb has the effect of improving the strength and toughness of the base material by refining and carbide precipitation, it may be included as necessary. However, when the content exceeds 0.1%, the above effects are saturated and the toughness of the weld heat affected zone is remarkably impaired. Therefore, when it contains Nb, it is preferable to make the content into 0.1% or less. In addition, in order to acquire this effect stably, it is preferable to contain 0.005% or more.

Group 2: Ni

Ni: 1.5% or less

Since Ni has the effect of improving the toughness of the steel matrix (dough) in the solid solution state, it may be contained as necessary. However, even if it contains more than 1.5%, the improvement of the characteristic suitable for raise of an alloy cost cannot be acquired. Moreover, corrosion resistance may deteriorate by coexistence of Sn and Ni. Therefore, when it contains Ni, it is preferable to make the content into 1.5% or less. In addition, in order to acquire this effect stably, it is preferable to contain 0.05% or more.

Group 3: Cr

Cr: 1.2% or less

Cr has the effect of improving carbonic acid gas corrosion resistance and hardenability, and may be contained as needed. However, when it exceeds 1.2%, even if it satisfy | fills another component condition, not only hardening suppression of a weld heat influence part becomes difficult, but the carbonic acid gas corrosion resistance improvement effect is also saturated. Therefore, when it contains Cr, it is preferable to make the content into 1.2% or less. In addition, in order to acquire this effect stably, it is preferable to contain 0.05% or more.

Group 4: Ti

Ti: 0.05% or less

Ti acts as a deoxidation element, forms an oxide phase composed of Ti and Mn, and in particular, makes the structure in the heat affected zone of the high heat input welding fine, and thus the effect of improving fatigue characteristics can be obtained. You may contain accordingly. However, if it contains more than 0.05%, the oxide formed will become a Ti oxide or a Ti-Al oxide, and a dispersion density will fall, and the ability to refine | miniaturize the structure in the heat affected zone of a high heat input welding part is lost. For this reason, when Ti is contained, it is preferable to make the content into 0.05% or less. More preferred is less than 0.02%. More preferably, it is 0.018% or less. In addition, in order to form this oxide phase stably in steel, it is preferable to make the total amount of Ti in steel into 0.003% or more.

Fifth Group: Ca, Mg

Ca: 0.003% or less

Ca reacts with S in the steel to form an oxysulfide (an oxysulfide) in molten steel. Unlike the MnS and the like, this oxysulfide is not elongated in the rolling direction by rolling but is spherical even after rolling, and thus the effect of suppressing welding cracking or hydrogen organic cracking, which is the starting point of the crack of the elongated inclusion, etc. There is this. Therefore, you may contain as needed. However, when the content exceeds 0.003%, deterioration of toughness may be caused. Therefore, when it contains Ca, it is preferable to make the content into 0.003% or less. In addition, in order to acquire this effect stably, it is preferable to contain 0.0005% or more.

Mg: 0.003% or less

Mg generates an Mg-containing oxide, becomes a nucleus of TiN, and has an effect of finely dispersing TiN. Therefore, Mg may be contained as necessary. However, when the content is more than 0.003%, the amount of oxide is too large to cause ductility decrease. Therefore, when it contains Mg, it is preferable to make the content into 0.003% or less. In addition, in order to acquire this effect stably, it is preferable to contain 0.0005% or more.

The steel material of the present invention needs to have a Bq value of 0.003 or less obtained from the following formula (1), and a Ceq value of 0.15 to 0.35 obtained from the following formula (2).

However, each element symbol in the said formula means content (mass%) of each element. In addition, when content of each element is an impurity level, 0 (zero) shall substitute.

Bq: 0.003 or less

In order to exhibit the hardenability improvement effect by B, it is necessary to eliminate the influence of N in steel. It is because B is easy to combine with N, and when there is free N in steel, BN is easy to produce | generate by combining with N. For this reason, B is made to exist in steel by adding Ti according to N content and fixing it as TiN. As the B content increases, the hardenability by B improves. However, when Bq value calculated | required from Formula (1) exceeds 0.003, coarse iron-carbon boride will form and lead to deterioration of a fatigue characteristic. Therefore, Bq value needs to be 0.003 or less.

In addition, in order to acquire the hardenability improvement effect by B stably, it is preferable to make Bq value prescribed | regulated by said Formula (1) more than 0.0001. It is more preferable to set it as 0.0005 or more, and it is further more preferable to set it to 0.001 or more.

Ceq: 0.15 ~ 0.35

Ceq calculated | required from said Formula (2) is what is called carbon equivalent, and it is an index which evaluates the hardenability and weldability of steel materials, and is generally used widely.

MEANS TO SOLVE THE PROBLEM The present inventors need to satisfy the request | requirement that the fatigue property of a weld joint is improved, and general tensile strength (TS) as structural steel is 500 Mpa or more, and Charpy absorbed energy value vE0 in ₀ degreeC is 27 J or more. The conditions were explored. As a result, when the Ceq value was less than 0.15%, the strength was lowered. On the other hand, when the Ceq was more than 0.35%, the hardenability of the steel was increased, the hardness distribution of the joint was uneven, and it was found to adversely affect the joint fatigue strength. Moreover, when Ceq exceeds 0.35, there exists a demerit that weldability deteriorates, welding construction becomes difficult, and the use of steel materials is remarkably limited. Therefore, Ceq value is made into 0.15 to 0.35%. In addition, the minimum with preferable Ceq is 0.20%. Moreover, the upper limit with preferable Ceq is 0.30%.

In addition, the steel material of the present invention requires that the number of oxides in a region within 2 mm from the surface layer is 5 × 10 ⁴ or less per square mm. This is because, when more than 5 × 10 ⁴ oxides are present, a source of occurrence of fatigue cracks increases and the fatigue characteristics decrease.

Here, an oxide number is measured in the procedure shown to following (i)-(iii).

(i) A small piece is cut out with a cross section perpendicular to the rolling direction of the manufactured steel as an observation surface, and the observation surface is corroded with a nital solution to prepare a test piece.

(ii) The above test piece was set in a scanning electron microscope (SEM) equipped with an energy dispersive fluorescence X-ray analyzer (EDX), and a region of 0.05 mm angle was defined as 1 field of view, and the area within 2 mm from the surface layer. It observes at 2000 times the magnification with respect to 5 views, and measures the number of oxides in each visual field. At this time, the oxide and other inclusions are distinguished by composition analysis by EDX. In addition, the measurement of the number of oxides is performed by changing the depth in the area from the surface layer to a depth of 2 mm in order to avoid the deviation of the visual field.

(iii) The number of oxides in each field of view is averaged to be the number of oxides in the region within 2 mm from the surface layer.

B. About the manufacturing method of the steel material of this invention

When manufacturing the steel material which is excellent in fatigue propagation crack growth characteristics and corrosion resistance of this invention, it is preferable to make adjustment from a refining step. That is, in the refining step, the oxide of the surface layer portion can be reduced by studying the inert gas injection treatment or the vacuum refining treatment. Specifically, when performing an inert gas injection process, it is effective to inject an inert gas into molten steel on condition that (3) below is satisfied.

However, the definition of the symbol in said (3) formula is as follows.

G ₁ : flow rate of inert gas injected into molten steel (NL / min)

H ₁ : Distance from the tip of the inert gas injection nozzle to the molten steel bath surface (m)

t ₁ : inert gas injection time (min)

S ₁ : Ladle molten steel (ton)

D ₁ : ladle inner diameter (m)

If the inert gas injection treatment is performed under the condition of the above formula (3), blowing can be performed while the bath is sufficiently stirred. That is, in the beginning of the blow, silicon in the molten iron is oxidized to become silica, which reacts with slaked lime or iron oxide added in the furnace to start forming CaO-SiO ₂ -FeO slag. . At the same time, the temperature in the furnace rises, so that the solubility of the scrap begins to progress. In the initial stage of blowing, since the carbon concentration in molten iron is high, the injected pure oxygen gas reacts efficiently with carbon, becomes carbon monoxide, and decarburization advances. In this step, the supply rate of pure oxygen gas speeds up decarburization. As the decarburization proceeds, the bath temperature rises further. As decarburization proceeds and the carbon concentration decreases, the decarburization reaction causes the movement of carbon in the molten steel to decarburize. If the movement of carbon by stirring of molten steel is insufficient, the injected pure oxygen gas is used to oxidize iron rather than react with carbon, and iron oxide increases in slag, and the yield of iron falls. To prevent this, the gas injection from the bottom of the furnace is active.

On the other hand, when performing a vacuum refining process, it is preferable to inject an inert gas into molten steel on condition which satisfy | fills following (4) Formula.

However, the definition of the symbol in said (4) formula is as follows.

G ₂ : Inert gas flow rate (NL / min) used for reflux of molten steel

D ₂ : Internal diameter of immersion pipe (m)

t ₂ : vacuum treatment time (min)

S ₂ : Ladle molten steel (ton)

When performing a vacuum refining process, it is preferable to put molten steel in the pressure-reduced container on the conditions which satisfy | fill said Formula (4), and to lower the equilibrium partial pressure, and to remove the gas component in molten steel.

In order to increase the cleanliness of the steel, it is preferable to avoid most of the Al deoxidation in the refining stage. It is preferable to cast molten steel obtained by adjusting the composition other than Al with Mn, Si, etc., and further deoxidizing with Ti or the like, and then introducing Al into the trace molten steel immediately before tapping.

In the case of ingot casting, prior to hot rolling, an extra step of manufacturing a steel slab (slab) must be passed through the hot rolling, and the yield also decreases. Therefore, it is preferable to perform casting by continuous casting. In the case of continuous casting, the segregation of the steel piece also adversely affects the toughness of the weld heat-affected portion. Therefore, in the segregation portion, management is performed such that C is 0.29% or less, P is 0.30% or less, and Mn is 3.5% or less. It is good.

In addition to the above conditions, as the discharge flow rate management at the time of injection, electromagnetic brake is applied at 1000 to 5000 gauss, electromagnetic stirring treatment to the unsolidified molten steel at 250 to 1000 gauss, or the final solidification part is about 1 mm / m. The molten steel of rich segregation may be squeezed out from the final coagulation part by pressing with a gradient of. By appropriately combining the above-mentioned management items, a steel piece excellent in cleanliness and few center segregation is obtained.

Subsequently, it is good to heat the produced steel piece in the temperature range of 900-1180 degreeC, and to perform hot rolling. At this time, the steel piece once cooled to room temperature may be reheated, and may be maintained or heated at the said temperature as it is through a crack furnace as it is without cooling to room temperature after continuous casting by a so-called direct rolling process. Here, when heating temperature is less than 900 degreeC, the reverse transformation to austenite becomes inadequate at the time of slab heating, and the following characteristic will deteriorate. On the other hand, when heating temperature exceeds 1180 degreeC, austenite crystal grain will coarsen at the time of heating a steel piece, and the toughness of not only plate | board thickness center part but the whole base material falls.

As for the conditions of hot rolling, it is good to set the finishing temperature of hot rolling to 650-1000 degreeC. When the finishing temperature is less than 650 ° C., the deformation resistance of the steel increases, so that it is difficult to finish the shape of the steel material after the hot rolling into the target shape. If the finishing temperature is high, the effect of miniaturization of the crystal grains by the controlled rolling cannot be obtained and the toughness of the base metal cannot be secured. Therefore, the upper limit of finishing temperature is limited to 1000 degreeC.

Subsequently, the obtained hot rolled material is accelerated-cooled from the temperature range of 620-950 degreeC on condition that the average cooling rate in the temperature range of 620-500 degreeC becomes 5-50 degreeC / sec, and it is the temperature range of 500 degrees C or less. It is good to end the cooling. In addition, it is preferable that the reheating temperature width | variety after completion | finish of cooling shall be 70 degrees C or less.

By cooling on such conditions, it becomes possible to improve a fatigue characteristic.

In other words, if the average cooling rate in the temperature range of 620 to 500 ° C is less than 5 ° C / sec, bainite structures and the like with coarse carbides are likely to be produced. Can't. On the other hand, when the cooling rate in the temperature range exceeds 50 ° C / sec, it is easy to be quenched in the vicinity of the surface layer portion of the steel material, so the toughness of the surface layer may decrease. Therefore, in this invention, the average cooling rate in the temperature range of 620-500 degreeC shall be 5-50 degreeC / sec.

If the cooling stop temperature in this cooling exceeds 500 degreeC, generation | occurrence | production of martensite, lower bainite, etc. will become inadequate not only in the center part of steel materials but also in the surface layer part, and strength cannot be ensured. Therefore, cooling stop temperature shall be 500 degrees C or less. By such heat treatment, martensite or bainite structure is easily obtained. In the case of the steel having the chemical composition of the present invention, it is mainly bainite structure.

[Example]

Cast steel obtained by melting steel of chemical composition shown in Table 1 in a converter, performing inert gas injection treatment or vacuum refining treatment shown in Table 2, and then performing continuous casting. ) Was hot-rolled and cooled to the appropriate plate thickness under the conditions shown in Table 3 to obtain a steel sheet for testing.

TABLE 1

TABLE 2

[Table 3]

Using the said test steel plate, the fatigue fracture life, the tensile strength and toughness of the weld heat influence part, the number of oxides, the thickness reduction amount, and the peeling area ratio were measured by the following method.

<Fatigue test>

Using the test steel plate described above, under the welding conditions shown in Table 4, a cross-welded joint of a load-non-transfer type was produced and used for a fatigue test. In addition, the shape and dimension of a joint test body are shown in FIG. Joints were made by fillet welding. 1, 1 and 2 are a base material steel plate, and 5 is a weld part. The axial force load was repeatedly applied to each joint test body, and the occurrence life of fatigue cracking in the weld female edges, that is, fatigue failure life was measured. Table 5 shows the fatigue test conditions.

[Table 4]

TABLE 5

<Tension Strength of Weld Heat Influence Part>

In the test steel sheet described above, a test piece is taken in a direction parallel to the rolling surface and perpendicular to the rolling direction, and subjected to a tensile test in accordance with the method specified in JIS Z 2241 (1998) to obtain a tensile strength (TS). Was obtained.

<Toughness of weld heat affected zone>

From the said test steel plate (plate thickness t), in a (1/4) t thickness part from a steel plate surface, a test piece is extract | collected in the direction parallel to a rolling surface, and perpendicular | vertical to a rolling direction, JIS Z 2242 (1998). According to the method prescribed | regulated by), the impact test was done and the absorption energy (vE0) in 0 degreeC was calculated | required.

<Oxide water>

Oxide number in the area | region within 2 mm was calculated | required from the surface layer according to the procedure shown to the following (i)-(iii).

(i) Small pieces were cut out by making the cross section perpendicular | vertical to the rolling direction of the produced steel as an observation surface, and the observation surface was corroded by nital solution, and the test piece was produced.

(ii) The above test piece was set in an SEM equipped with EDX and observed at a magnification of 2000 times with respect to 5 fields of view (less than 5 fields at equal intervals) within an area of 2 mm from the surface layer, with an area of 0.05 mm angle as 1 field of view. The number of oxides in each visual field was measured. At this time, the distinction between oxides and other inclusions was performed by composition analysis by EDX. In addition, the measurement of the number of oxides was performed by changing the depth in the area | region from the surface layer to a depth of 2 mm, in order to avoid the deviation of a visual field.

(iii) The number of oxides in each field of view was averaged to be the number of oxides in the region within 2 mm from the surface layer.

<Plate thickness reduction amount and peeling area ratio>

As to corrosion resistance, test pieces obtained from the obtained steel were evaluated by the Society of Automotive Engineers (SAE) J2334 test. SAE J2334 test, wet: 50 ° C., 100% RH, 6 hours, salt attachment: 0.5% NaCl, 0.1% CaCl ₂ , 0.075% NaHCO ₃ aqueous solution immersion, 0.25 hour, drying: 60 ° C., 50% RH, 17.75 hours Is an accelerated test with 1 cycle (24 hours total), and the corrosion pattern is similar to the atmospheric exposure test (Hirogan Nagano, Masato Yamashita, Hitoshi Uchida): Environmental Materials Studies, Public Publishing (2004), p. 74). In addition, this test simulates a severe corrosive environment in which the amount of fly salt exceeds 1mdd.

After completion of the 120 cycles of SAE J2334 test, the rust layer on the surface of each test piece was removed, and the plate thickness reduction amount was measured. Here, the "plate thickness reduction amount" is the plate thickness reduction amount of the test piece average, and is computed using the weight reduction before and behind a test, and the surface area of a test piece.

In addition, in order to investigate coating peeling resistance, a modified epoxy paint (Banno 200: made in China paint) was coated to a thickness of 150 μm by a dry spray on a test piece having a size of 150 × 70 mm so as to reach a steel substrate. After forming a crosscut, the same evaluation was made by the SAE J2334 test.

Table 6 shows the chemical composition and production method of the steel, and various test results.

TABLE 6

As shown in Table 6, in Examples 1 to 10 of the present invention in which both the chemical composition and the production method satisfy the conditions of the present invention, the number of oxides in the region within 2 mm from the surface layer is 5 × 10 ⁴ / mm ² or less. In any case, the fatigue fracture life (repeated number) exceeded 5 × 10 ⁶ times, and da / dn was 5 × 10 ⁻⁵ or less, and thus had sufficient resistance to crack growth. Moreover, it also has high corrosion resistance, and although the corrosion | corrosion was seen in the crosscut part at the time of coating, since it is small in any steel plate and peeling, it turns out that the maintenance interval of coating can be extended.

On the other hand, although the chemical composition satisfies the range defined in the present invention, Comparative Examples 1 and 2 in which the production method deviates from the conditions of the present invention, and Comparative Examples 3 to 6 in which the chemical composition deviates from the range defined in the present invention, In all cases, the fatigue failure life was extremely poor at 10 ⁴ .

Particularly, in Comparative Example 3 in which the Cu content was large and the Cu / Sn exceeded 1, minute cracks occurred at the end portion during rolling. In Comparative Example 4 with less Sn, the corrosion resistance under an environment having a large amount of salt content decreased, and the peeling area ratio also became 80%.

[Industrial Availability]

The steel of the present invention is excellent in fatigue fatigue crack propagation characteristics and corrosion resistance, and thus is suitable for use in hulls, civil constructions, construction machinery, hydraulic steel pipes, offshore structures, line pipes and other welded structures requiring fatigue fatigue crack growth characteristics. Do.

1: base material steel plate
2: base material steel plate
5: welding part

Claims (8)

By mass%, C: 0.01 ~ 0.14%, Si: 0.04 ~ 0.6%, Mn: 0.5 ~ 2.0%, P: 0.01% or less, S: 0.003% or less, Cu: less than 0.2%, B: 0.0007% Less than 0.005%, less than 0.05% of Al, less than 0.007% of N, less than 0.003% of O and less than 0.03% to 0.50% of Sn; the remainder consists of Fe and impurities, and has a Cu / Sn ratio of 1 or less It has a composition, and the Bq value obtained from the following formula (1) is 0.003 or less, the Ceq value obtained from the following formula (2) is 0.15 to 0.35, and the number of oxides in an area within 2 mm from the surface layer per square mm Steel having excellent fatigue crack propagation characteristics and corrosion resistance, characterized in that 5 × 10 ⁴ or less.

However, each element symbol in the said formula means content (mass%) of each element. In addition, when content of each element is an impurity level, 0 shall be substituted. The method according to claim 1,
Further, steel having excellent fatigue crack propagation characteristics and corrosion resistance, characterized by containing at least one element selected from Mo: 1.0% or less, V: 0.1% or less, and Nb: 0.1% or less by mass%. The method according to claim 1 or 2,
Moreover, the steel material which is excellent in fatigue-fatigue crack growth characteristics and corrosion resistance containing Ni: 1.5% or less by mass%. The method according to any one of claims 1 to 3,
In addition, the steel is excellent in fatigue fatigue crack growth characteristics and corrosion resistance, characterized by containing 1.2% or less Cr in mass%. The method according to any one of claims 1 to 4,
Further, the steel is excellent in fatigue fatigue crack propagation characteristics and corrosion resistance, characterized by containing 0.05: 0.05% or less by mass%. The method according to any one of claims 1 to 5,
A steel having excellent fatigue crack propagation characteristics and corrosion resistance, characterized by containing one or both of Ca: 0.003% or less and Mg: 0.003% or less by mass%. A method for producing a steel having excellent fatigue crack propagation characteristics and corrosion resistance, comprising the following steps A to D and further comprising a recuperation temperature width after completion of cooling of the step D to 70 ° C or less.
Process A: injecting inert gas into molten steel under conditions satisfying the following formula (3),
Process B: The process of continuously casting the obtained molten steel and obtaining the steel piece which has a chemical composition of any one of Claims 1-6,
Step C: after heating the obtained steel strip to 900 to 1180 ° C, performing hot rolling under the condition that the finishing temperature is 650 to 1000 ° C to obtain a hot rolled material, and
Process D: The obtained hot rolled material is accelerated-cooled from the temperature range of 620-950 degreeC on condition that the average cooling rate in the temperature range of 620-500 degreeC becomes 5-50 degreeC / sec, and is 500 degrees C or less temperature range. Terminating cooling in the process.

However, the definition of the symbol in said (3) formula is as follows.
G ₁ : flow rate of inert gas injected into molten steel (NL / min)
H ₁ : Distance from the tip of the inert gas injection nozzle to the molten steel bath surface (m)
t ₁ : inert gas injection time (min)
S ₁ : Ladle molten steel (ton)
D ₁ : ladle inner diameter (m) A method for producing a steel having excellent fatigue crack propagation characteristics and corrosion resistance, comprising the following steps A1 to D, and a recuperation temperature width after the completion of cooling in step D is 70 ° C. or less.
Step A1: a step of performing vacuum refining treatment on molten steel under conditions satisfying the following formula (4),
Process B: The process of continuously casting the obtained molten steel and obtaining the steel piece which has a chemical composition of any one of Claims 1-6,
Step C: after heating the obtained steel strip to 900 to 1180 ° C., performing hot rolling under the condition that the finishing temperature is 650 to 1000 ° C. to obtain a hot rolled material, and
Process D: The obtained hot rolled material is accelerated-cooled from the temperature range of 620-950 degreeC on condition that the average cooling rate in the temperature range of 620-500 degreeC becomes 5-50 degreeC / sec, and is 500 degrees C or less temperature range. Terminating cooling in the process.

However, the definition of the symbol in said (4) formula is as follows.
G ₂ : Inert gas flow rate (NL / min) used for reflux of molten steel
D ₂ : Internal diameter of immersion pipe (m)
t ₂ : vacuum treatment time (min)
S ₂ : Ladle molten steel (ton)

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