JP2008274367A - Bolt steel, and bridge using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bolt steel which has excellent weatherability and coating peeling resistance and can be used as a material requiring minimum maintenance even under an environment containing large amounts of flying salt, such as a seaside area and an area where snow-melting salt is sprayed, and also to provide a bridge using a bolt made of the bolt steel. <P>SOLUTION: The bolt steel, having a composition which consists of, by mass, 0.15 to 0.6% C, 0.05 to 0.5% Si, 0.5 to 3.5%, in total, of Mn and Cr, ≤0.05% P, ≤0.03% S, <0.3% Cu, <1% Ni, ≤0.01% O, 0.05 to 0.50% Sn and the balance Fe with impurities and in which a ratio between Cu and Sn satisfies Cu/Sn≤1, is provided. The bridge using a bolt made of the bolt steel is also provided. Besides the above alloying elements, one or more elements among Al, Mo, W, V, Nb, Ti, Zr, B, N, Ca, Mg and REM can be further contained. Moreover, the surface can be coated with a corrosion protective film. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is a bolt steel excellent in weather resistance and paint peeling resistance, which can be used as a minimum maintenance material even in an environment where the amount of incoming salt is large, such as in a beach area or an area where snow melting salt is sprayed. The present invention relates to a bridge using bolts made from steel for bolts.

  In general, when weathering steel is exposed to an atmospheric corrosive environment, a protective rust layer is formed on the surface, and the subsequent corrosion of steel is suppressed, thereby exhibiting weather resistance. Therefore, the weather-resistant steel is used for structures such as bridges as a minimum maintenance steel that can be used as it is without being painted.

  However, not only in the beach area, but also in inland areas where snow melting salt and anti-freezing agents are sprayed, in environments where the amount of incoming salt is high, it is difficult to form a protective rust layer on the surface of weathering steel, It is difficult to exert the effect of suppressing corrosion. Therefore, in these areas, it is not possible to use bare weatherproof steel, and it is necessary to paint it.

Even in weathering steel standardized by Japanese Industrial Standards (JIS) (JIS G3114: weathering hot rolled steel for welded structures), the amount of incoming salt as NaCl is 0.05 mg / dm ² / day as in the beach area. (0.05mdd) and higher areas have a large weight loss due to the occurrence of scale-like rust, layered rust, etc., so they cannot be used without painting (Ministry of Construction, Civil Engineering Research Institute, Steel Club, ( Japan Association for Bridge Construction: Joint Research Report on the Application of Weatherproof Steel to Bridges (XX)-Design and Construction Guidelines for Uncoated Weatherproof Bridges (Ref. Rev. 1993.3)).

  For this reason, in a salty environment such as a beach area, it is common to use ordinary steel that is coated with ordinary steel. However, bridges constructed in the coastal area near the estuary, and bridges constructed in roads such as mountainous areas where snowmelt salt and anti-freezing agents are sprayed are extremely corroded, and the coating film deteriorates due to corrosion. It is necessary to repaint. Since these repaints require a great amount of man-hours and enormous costs for maintenance, there is a strong demand for steel materials with excellent beach weather resistance that can be used without painting even in beach areas.

  Recently, Ni-based high weathering steel with about 1 to 3% Ni added has been developed and put to practical use. However, the amount of incoming salt exceeds 0.3 to 0.4 mdd only with such addition of Ni. It has proven difficult to apply locally.

  Since corrosion of steel materials becomes more severe as the amount of flying salt increases, a weather-resistant steel material corresponding to the amount of flying salt is required from the viewpoint of corrosion resistance and economy. Moreover, even if it is called a bridge, the corrosive environment of steel materials changes with places and parts used. For example, outside the girder, it is exposed to rainfall, condensed water and sunlight. On the other hand, inside the girders, they are exposed to condensed water, but there is no rain. In general, in an environment with a large amount of incoming salt, it is said that the inside of a girder without rain is more corrosive than the outside of the girder washed with rain.

  In addition, in an environment where snow melting salt or an antifreezing agent is sprayed on the road, the salt is wound up on a running car and adheres to a bridge that supports the road, resulting in a severe corrosive environment. Furthermore, the eaves under the eaves a little away from the coast are also exposed to severe salt damage environments, and in such areas, the amount of incoming salt becomes a severe corrosive environment with 1 mdd or more.

  Steel materials that prevent corrosion in environments with a high amount of incoming salt have also been developed.

  For example, Patent Document 1 proposes a weather-resistant steel material having an increased chromium (Cr) content, and Patent Document 2 proposes a weather-resistant steel material having an increased nickel (Ni) content.

  In addition, the present inventors have previously proposed a steel material having beach weather resistance in which Cu, Ni, and Cr are combined and contained in Patent Document 3.

  However, steel structures such as bridges are constructed as bolts (general-purpose bolts and torcia-type bolts, etc.), and the head of the bolts are particularly poorly covered or edged when painted. It is difficult to attach a film thickness to the portion, and it has been found that it is the bolt portion that first has a problem of corrosion resistance. Therefore, in the recent steel road bridge painting and anticorrosion manual [edited by the Japan Road Association Bridge Committee (2005)], the coating specifications are also described in the bolt, and the anticorrosion of the bolt has been regarded as important. However, in reality, there is a problem of kelen and shape peculiar to the bolt as described above, and long-term durability is insufficient.

  Further, in the case of a high-strength bolt that can withstand a high tightening force, for example, as shown in Patent Document 4, it is known that so-called delayed fracture is likely to occur. In this patent document 4, it is said that Sn which raises a hydrogen overvoltage has a bad influence on delayed fracture property. Delayed fracture is a phenomenon in which steel placed under a static load breaks brittlely after a certain period of time.It is considered as a type of hydrogen embrittlement due to hydrogen entering the steel due to corrosion. It is pointed out that deterioration of corrosion resistance may lead to the above-mentioned delayed fracture in a severely corrosive environment.

  As a countermeasure for these, as shown in Patent Document 5, for example, steel having an alloy component to which Cu, Cr, and Mo are added at a specific ratio has been proposed.

JP-A-9-176790 JP-A-5-51668 JP 2000-297343 A JP 2004-231992 A JP-A-9-78182

  Considering the use of the weathering steel material proposed in each of the above patent documents for bolts, first, the weathering steel material with an increased content of chromium (Cr) proposed in the above patent document 1 has a certain degree of Although the weather resistance can be improved in the range of the amount of incoming salt, the weather resistance is deteriorated conversely in a severe salinity environment exceeding that.

  Next, in the case of the weathering steel material with the increased nickel (Ni) content proposed in Patent Document 2, the weathering resistance is improved to some extent, but the cost of the steel material itself is increased and used for applications such as bridges. The material used is expensive. In order to avoid this, if the Ni content is reduced, the weather resistance will not be improved so much, and if the amount of incoming salt is high, layered peeling rust will form on the surface of the steel material, corrosion will be remarkable, and it will be used for a long time. The problem of being unbearable arises.

  And in the steel material which has the beach weather resistance which compounded Cu, Ni, and Cr proposed by the said patent document 3, and contains these elements about several percent, compared with JIS weather-resistant steel, Although the weather resistance can be improved, sufficient weather resistance cannot be exhibited in a very severe environment where the amount of incoming salt exceeds 1 mdd, and further improvement is necessary.

  Furthermore, the above-mentioned Patent Document 4 merely proposes improvement of delayed fracture resistance of steel sheets for automobiles, and does not touch on weather resistance. And in the steel which has the alloy component which added Cu, Cr, and Mo proposed by the said patent document 5 in the specific ratio, sufficient weather resistance is exhibited in the very severe environment where the amount of incoming salt exceeds 1 mdd. And the paint life is short.

  Furthermore, paint peeling resistance in the bridge field is a major problem. That is, as described above, in a coastal environment where a large amount of chlorides fly or an environment where a snow melting agent or an antifreezing agent is sprayed, there is a problem that the coating peels off early and corrosion progresses. Yes, it is necessary to repaint every few to a dozen years. In particular, the number of repaints increases in the shape of a bolt. However, in order to repaint the paint, it is necessary to assemble a scaffold on the once corroded bridge and perform a reblast treatment, which is very expensive. And even if it re-blasts, a rust cannot be removed completely, and even if it repaints on the steel material which has not removed rust completely, a coating life will become remarkably short.

  Therefore, it has been strongly desired to extend the service life of the coating and greatly extend the interval between repair coatings. In other words, even for bridges that require painting, there is a high demand for minimizing the life cycle cost, and extending the life of painting is very important in considering bridge life cycle management. In particular, as described above, it is known that the life of the coating of the bolt portion is remarkably shortened due to its kelen and shape. Furthermore, there is a concern about delayed fracture due to corrosion in a high-flying salinity environment.

  In order to solve the above-mentioned problems included in conventional steel materials, the present invention provides a bolt steel excellent in weather resistance and paint peeling resistance in a high chloride environment, and a bridge made of the bolt steel. The purpose is to provide.

As already reported by one of the present inventors (see “Materials and Environment”, Vol. 43 (1994), No. 1, p. 26), the rust layer has a protective property in the weathering steel. This is because a rust layer made of fine α- (Fe _1-x Cr _x ) OOH partially substituted with Cr is formed.

  However, as described above, the addition of Cr is effective in improving the weather resistance in an environment where the amount of incoming salt is relatively small, but in the environment where the amount of incoming salt is large, the weather resistance is deteriorated conversely. On the other hand, it has been considered that the addition of Ni is effective for improving the weather resistance in an area where the amount of incoming salt is large.

Based on these findings, the inventors of the present invention have studied corrosion in an environment with a large amount of incoming salt. As a result, in such an environment, repeated drying and wetting of the FeCl ₃ solution becomes an essential condition for corrosion. It has been found that corrosion is accelerated when pH decreases due to hydrolysis of ³⁺ and Fe ³⁺ acts as an oxidizing agent.

The corrosion reaction at this time is as follows.
As the cathode reaction, the following reaction mainly occurs.
Fe ³⁺ + e ⁻ → Fe ²⁺ (reduction reaction of Fe ³⁺ )
In addition to this reaction, the following cathode reaction also occurs.
2H ₂ O + O ₂ + 2e ⁻ → 4OH ⁻ ,
2H ⁺ + 2e ⁻ → H ₂
On the other hand, the following anodic reaction occurs with respect to the above Fe ³⁺ reduction reaction.
Anode reaction: Fe → Fe ²⁺ + 2e ⁻ (Fe dissolution reaction)
Therefore, the overall reaction of corrosion is as shown in the following equation (1).
2Fe ³⁺ + Fe → 3Fe ²⁺ (1)

Fe ²⁺ generated by the reaction of the above formula (1) is oxidized to Fe ³⁺ by air oxidation, and the generated Fe ³⁺ acts again as an oxidant 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 concentrated chloride solution, and Fe ³⁺ is easily generated. It has been found that due to such a cyclic reaction, in an environment where the amount of incoming salt is very large, Fe ³⁺ is always supplied, corrosion of steel is accelerated, and corrosion resistance is significantly deteriorated.

  Thus, in an environment where the amount of incoming salt is very high, protection by the rust layer cannot be expected. Therefore, it is effective to delay the corrosion of the steel by slowing the anodic dissolution reaction of the steel itself. That is, in an environment where the amount of flying salt is very large, it is presumed that the steel containing Cr accelerates the anodic dissolution reaction, so that the weather resistance deteriorates, whereas the steel containing Ni does not perform the anodic dissolution reaction. Since it is delayed, it can be expected that the weather resistance is improved.

  As a result of examining the influence of various alloy elements on the weather resistance on the basis of the mechanism of corrosion in the above-described salinity environment, the following findings (a) to (i) were obtained.

(A) Sn dissolves as ^{Sn 2+,} by lowering the concentration of ^{Fe 3+} by ^{2Fe 3+ + Sn 2+ → 2Fe 2+} + Sn 4+ becomes reactions, (1) inhibit the reaction of formula. Sn also has an effect of suppressing anodic dissolution.

  (B) Cu is an element that has conventionally been regarded as the basis for the effect of improving the corrosion resistance in an environment where the amount of flying salt is high, and the effect of improving the corrosion resistance is seen in an environment having a relatively long wet time. However, an environment where the chloride concentration is further increased and the pH is locally lowered, for example, a relatively dry environment in which salt is attached and the humidity is changed, resulting in repeated drying and wetting to produce β-FeOOH. Then, it was found that Cu rather promotes corrosion.

  (C) Furthermore, when Ni is added in combination with Sn, it has been found that there is no effect of improving the corrosion resistance in an environment with a large amount of incoming salt, and if it is added in a large amount, the weather resistance is deteriorated. This behavior of Ni is contrary to the conventional knowledge that weather resistance improves as the amount of Ni added increases.

  (D) When Cr is added alone, it degrades the weather resistance in an environment with a large amount of incoming salt, but when combined with Sn, Cr improves the weather resistance in an environment with a large amount of incoming salt. Demonstrate.

(E) N dissolves as ammonia and has the effect of increasing the pH of the corrosion interface. In an environment with a large amount of incoming salt, the pH decreases due to the hydrolysis of the Fe ³⁺ , but the inclusion of N suppresses the pH decrease at the corrosion interface and improves the weather resistance and the coating film peelability.

  (F) The material containing the alloy elements described in (a) to (e) above further contains one or more selected from Ti, Nb, Mo, W, V, Ca and Mg. However, it is effective in improving beach weather resistance.

  (G) Furthermore, when REM is contained, the weldability of the steel for bolts is improved.

  (H) These steels for bolts excellent in beach weather resistance may be coated on the surface with an anticorrosive film, and even if kelen is insufficient, the effect of extending the coating life is great.

  (I) Further, Sn that increases hydrogen overvoltage has been conventionally considered to have an adverse effect on delayed fracture properties (see Patent Document 4 above), but unexpectedly hydrogen permeation in a dry and wet environment in the presence of chloride. It has been found that the coefficient is significantly reduced.

  The present invention has been made on the basis of the above findings, and the gist of the present invention is the following bolt steels (1) to (7) and bridges (8). Hereinafter, the present invention may be collectively referred to as the present invention.

  (1) By mass%, C: 0.15 to 0.6%, Si: 0.05 to 0.5%, Mn and Cr: 0.5 to 3.5% in total, P: 0.05% Hereinafter, S: not more than 0.03%, Cu: less than 0.3%, Ni: less than 1%, O: not more than 0.01% and Sn: 0.05 to 0.50%, with the balance being Fe and A bolt steel comprising an impurity and having a composition with a Cu / Sn ratio of 1 or less.

  (2) The steel for bolts according to the above (1), further comprising Al: 0.3% or less by mass%.

  (3) Furthermore, it contains one or more selected from the group consisting of Mo: 3.0% or less, W: 3.0% or less, and V: 0.3% or less in terms of mass%. The bolt steel according to (1) or (2) above.

  (4) Furthermore, it contains one or more selected from the group consisting of Nb: 0.1% or less, Ti: 0.1% or less, and Zr: 0.1% or less in mass%. The steel for bolts according to any one of (1) to (3) above.

  (5) Further, by mass%, one selected from the group consisting of B: 0.0030% or less, N: 0.03% or less, Ca: 0.01% or less, and Mg: 0.01% or less The steel for bolts according to any one of (1) to (4) above, which contains two or more kinds.

  (6) The steel for bolts according to any one of (1) to (5) above, further containing 0.02% or less of REM in mass%.

  (7) The steel for bolts according to any one of (1) to (6) above, wherein the surface is coated with an anticorrosive film.

  (8) A bridge using a bolt manufactured from the bolt steel according to any one of (1) to (7) above.

  The bolt steel of the present invention has sufficient weather resistance even in an environment where the amount of incoming salt is large. As a minimum maintenance material used for structures such as bridges in beach areas and areas where snow melting salt and antifreezing agents are sprayed, it can be widely applied in the civil engineering and construction fields. Furthermore, when coated and used for steel structures such as ships and bridges, it can be widely applied as a material that contributes to maintenance minimization because it significantly suppresses corrosion from defects and the like.

  Below, the reason for prescription | regulation of the chemical composition of the steel for bolts of this invention is demonstrated. In addition, the following "%" displays all mean "mass%".

C: 0.15 to 0.6%
C enhances the hardenability of the steel and has the effect of improving the strength after quenching. In order to obtain sufficient hardenability, it is necessary to contain 0.15% or more. However, when the content exceeds 0.6%, excessive fine carbides are formed, and not only the moldability (cold forgeability) of the bolt is greatly lowered, but also the corrosion resistance tends to be lowered. Therefore, the C content is 0.15 to 0.6%. A preferable range is 0.15 to 0.4%.

Si: 0.05-0.5%
Si is an alloy element necessary for deoxidation during steel making, and is also an alloy element that improves weather resistance. From these viewpoints, it is necessary to contain Si by 0.05% or more. On the other hand, even if added excessively, the effect is saturated, so 0.5% is made the upper limit.

Mn and Cr: 0.5 to 3.5% in total,
Mn and Cr are elements necessary for ensuring the hardenability of the steel and ensuring the strength as a bolt. For this purpose, it is necessary to contain 0.5 to 3.5% of these elements in total.

  When the S content in the steel is low, Mn generally has the effect of improving the weather resistance in a high flying salinity environment. In order to obtain this effect, it is preferable to contain 0.1% or more of Mn. More preferably, it is 0.5%. However, it combines with S in the steel to form MnS, and this MnS becomes a starting point of corrosion, so that the corrosion resistance and consequently the weather resistance are deteriorated. Further, the mechanism is unknown, but when it coexists with Ni, the weather resistance deteriorates when the Mn content exceeds 2.0%. Therefore, the Mn content is preferably 2.0% or less.

  Cr can be expected to improve the corrosion resistance due to the formation of protective rust in an environment where the amount of flying salt is not so large, but it promotes the anodic dissolution reaction of steel and degrades the weather resistance in an environment where the amount of flying salt is large. However, when Sn is contained, the effect of improving weather resistance due to the Cr content is exhibited even in an environment with a large amount of incoming salt. This effect is exhibited when the Cr content is 0.01% or more, but when it exceeds 3.0%, the local corrosion sensitivity increases. Therefore, the Cr content is preferably 0.01 to 3.0%.

P: 0.05% or less P is contained as an impurity, but excessive P content in a concentrated chloride environment deteriorates the weather resistance, so it is necessary to reduce it as much as possible. Further, P segregates at the grain boundaries and reduces delayed fracture resistance. If the content exceeds 0.05%, the effect becomes significant, so the upper limit is made 0.05%.

S: 0.03% or less S is contained as an impurity, but when combined with Mn, it forms MnS as a non-metallic inclusion, which tends to be a starting point of corrosion, and deteriorates weather resistance. S, like P, segregates at the grain boundaries, reducing delayed fracture resistance. If the content exceeds 0.03%, the effect becomes significant, so the upper limit is made 0.03%.

Cu: Less than 0.3% Cu is generally regarded as a basic element for improving weather resistance, and is added to all beach weather resistant steels and corrosion resistant steels, but in a relatively dry environment under high flying salt content. Rather, it reduces the corrosion resistance. Moreover, when it coexists with Sn, cold forgeability falls. Therefore, it is necessary to reduce the Cu content as much as possible, and even if it is contained as an impurity, the Cu content needs to be less than 0.3%.

Ni: Less than 1% Ni has been conventionally added to steel as an element that remarkably improves beach weather resistance in environments with a large amount of incoming salt, and has been developed and put into practical use as Ni-based weather-resistant steel. ing. However, although the reason is not clear, when it is added in combination with Sn, there is not only an effect of improving the corrosion resistance but also an adverse effect of reducing the effect of improving the weather resistance by Sn. Therefore, it is necessary to reduce the Ni content as much as possible, and even if it is contained as an impurity, the Ni content needs to be less than 1%.

O (oxygen): 0.01% or less O (oxygen) is present in the steel as an impurity, and when its content exceeds 0.01%, a coarse oxide is formed to reduce fatigue characteristics and the like. Therefore, the upper limit is made 0.01%.

Sn: 0.05-0.50%
Sn dissolves as Sn ²⁺ and has an action of inhibiting corrosion by an inhibitor action in an acidic chloride solution. Further, rapidly to reduce the Fe ^3+, by have the effect of reducing the Fe ³⁺ concentration of the oxidizing agent, since inhibit corrosion promoting effect of Fe ^3+, thereby improving the weather resistance in high airborne salt environments.
Moreover, Sn has the effect | action which suppresses the anodic dissolution reaction of steel and improves corrosion resistance. Furthermore, by containing Sn, the effect of improving the weather resistance of Cr is exhibited even in an environment with a large amount of incoming salt.

  These effects are obtained by adding 0.05% or more of Sn, and when it exceeds 0.50%, not only is saturation saturated, but also the upsetting property is lowered as described later. Therefore, the Sn content is set to 0.05 to 0.50%. A desirable range of the Sn content is 0.05 to 0.40%. Conventionally, it has been said that the addition of Sn becomes a catalyst poison like S and Pb and promotes hydrogen intrusion into steel. However, in high air salinity air environment, it can remarkably suppress the hydrogen permeability coefficient. This has been clarified by the present invention.

Cu / Sn ratio: 1 or less In the case of steel containing Sn as in the present invention, the deterioration of corrosion resistance due to the inclusion of Cu is remarkable. Moreover, when manufacturing a volt | bolt, it becomes a cause of the 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.

  In addition to the above alloy elements, the bolt steel of the present invention further includes one or more of Al, Mo, W, V, Nb, Ti, Zr, B, N, Ca, Mg, and REM. You may contain. The reason why these elements may be contained and the contents at that time are as follows.

Al: 0.3% or less Al is an element effective for deoxidation of steel, and can be contained as necessary. However, when Si is added as a deoxidizer, it does not need to be contained. However, the effect is saturated even if added excessively, so the upper limit of Al is 0.3%. In addition, in order to express the deoxidation effect by Al reliably, it is preferable to contain 0.005% or more of Al. Here, the Al content in the present invention refers to acid-soluble Al (so-called sol. Al).

Mo: 3.0% or less Mo can be contained as necessary. When Mo is contained, it has the effect of increasing the tempering temperature and improving delayed fracture resistance by forming fine carbides. Mo also has the effect of dissolving and adsorbing to rust in the form of oxyacid ions MoO ₄ ²⁻ , suppressing the permeation of chloride ions in the rust layer, and improving corrosion resistance.

  However, if the Mo content exceeds 3.0%, these effects are not only saturated, but the cost of the steel material increases, so the upper limit of the Mo content is set to 3.0%. In addition, in order to express these effects reliably, it is preferable to contain 0.1% or more of Mo.

W: 3.0% or less W can be contained as required. When W is contained, similarly to Mo, it dissolves and is adsorbed on rust in the form of oxyacid ion MoO ₄ ²⁻ , thereby suppressing the permeation of chloride ions in the rust layer and improving the corrosion resistance.

  However, if the W content exceeds 3.0%, this effect is not only saturated, but the cost of the steel material increases, so the upper limit of the content is set to 3.0%. In addition, in order to express this effect reliably, it is preferable to contain 0.1% or more of W.

V: 0.3% or less V can be contained as necessary. When V is contained, it has the effect of increasing the tempering temperature and improving delayed fracture resistance by forming fine carbides. However, since these effects are saturated when the V content exceeds 0.3%, the upper limit of the V content is set to 0.3%. In addition, in order to express this effect reliably, it is preferable to contain V 0.05% or more.

Nb: 0.1% or less Nb can be contained as necessary. When Nb is contained, it combines with C and N to form a carbonitride, which effectively works for fine graining by the pinning effect and improves delayed fracture resistance. However, if the Nb content exceeds 0.1%, these effects are saturated, so the upper limit of the Nb content is 0.1%. In addition, in order to express this effect reliably, it is preferable to contain Nb 0.002% or more.

Ti: 0.1% or less Ti can be contained as necessary. When Ti is contained, carbonitrides are formed in combination with C and N in the same manner as Nb, and it effectively works for fine graining by the pinning effect and improves delayed fracture resistance. However, since these effects are saturated when the Ti content exceeds 0.1%, the upper limit of the Ti content is set to 0.1%. In addition, in order to express this effect reliably, it is preferable to contain 0.002% or more of Ti.

Zr: 0.1% or less Zr can be contained if necessary. When Zr is contained, similarly to Nb, it combines with C and N to form a carbonitride, effectively works for fine graining by the pinning effect, and improves delayed fracture resistance. However, since these effects are saturated when the Zr content exceeds 0.1%, the upper limit of the Zr content is set to 0.1%. In addition, in order to express this effect reliably, it is preferable to contain 0.002% or more of Zr.

B: 0.0030% or less B can be contained if necessary. When B is contained, the hardenability of steel can be improved and the strength as a bolt can be further improved. However, since this effect is saturated when the B content exceeds 0.0030%, the upper limit of the B content is set to 0.0030%. In addition, in order to express this effect reliably, it is preferable to contain 0.0003% or more of B.

N: 0.03% or less N can be contained as necessary. When N is contained, carbonitride is formed by linking with Al, Nb, Ti, and Zr in the steel together with C, and works effectively for refining due to the pinning effect, thereby improving delayed fracture resistance. Further, N dissolves as ammonia, and has an effect of improving the weather resistance in the salt environment by suppressing the pH drop due to the hydrolysis of Fe ^{3+ in} the environment having a large amount of incoming salt.

  However, if the N content exceeds 0.03%, these effects are saturated, so the upper limit of the N content is 0.03%. In order to surely exhibit this effect, it is preferable to contain N at 0.003% or more.

Ca: 0.01% or less Ca can be contained as necessary. When Ca is contained, it combines with S in the steel to form sulfide, and has the effect of improving the SCC resistance by improving the shape of inclusions.
Moreover, there is an effect of suppressing the promotion of corrosion by suppressing the decrease in pH at the interface in the corrosion reaction part. However, since this effect is saturated when the Ca content exceeds 0.01%, the upper limit of the Ca content is set to 0.01%. In addition, in order to express these effects reliably, it is preferable to contain 0.0003% or more of Ca.

Mg: 0.01% or less Mg can be contained as required. When Mg is contained, similarly to Ca, there is an effect of suppressing the decrease in pH at the interface in the corrosion reaction part and improving the corrosion resistance. However, since this effect is saturated when the Mg content exceeds 0.01%, the upper limit of the Mg content is set to 0.01%. In addition, in order to express these effects reliably, it is preferable to contain 0.0003% or more of Mg.

REM is 0.02% or less REM can be contained as required. When REM is contained, there is an effect of improving the weldability of steel. However, since these effects are saturated when the REM content exceeds 0.02%, the upper limit of the REM content is set to 0.02%. In addition, in order to express this effect reliably, it is preferable to contain 0.0001% or more of REM. Note that REM means 17 elements in which Y and Sc are combined with 15 elements of lanthanide.

  The bolt steel of the present invention is a steel material containing the above essential elements or the above optional elements, with the balance being Fe and impurities.

  The steel for bolts of the present invention has excellent corrosion resistance under high flying salinity environment and excellent coating film peeling resistance, so that it is not only used for bridge bolts, but also for automobiles, industrial machines, and building structures. It can also be used as a bolt.

  When further improving the weather resistance, the bolt steel of the present invention preferably has its surface covered with an anticorrosive film. The anticorrosion film used in the present invention means a film applied for the purpose of anticorrosion of steel materials. Specifically, various types of rust stabilization coatings (including a chemical conversion treatment system and a coating system) well-known in weathering steel materials; anticorrosion plating coatings such as Zn plating, Al plating, Zn-Al plating; Zn spraying, Al Metal spray coatings such as thermal spraying; general anticorrosion coatings such as vinyl butyral, epoxy, urethane, and phthalic acid, and so-called C coating and I coating systems. Even when any anticorrosion film is applied, excellent weather resistance and high anticorrosion performance can be exhibited. The film thickness or adhesion amount of these anticorrosion films is not particularly limited, and may be within a normal range. Furthermore, the steel for bolts of the present invention is a rusted steel material, that is, in a situation where the surface rust cannot be completely removed during repair, and particularly exhibits the corrosion resistance of the painted part. Even if the above coating is applied with a tool or wire brush, the life can be remarkably extended. This is because when scratches are applied, the scratched part tends to become an anode, and particularly when the paint is applied in a rusted state, the phenomenon of local drop in pH becomes prominent. It is thought that it is demonstrated.

  As described above, the steel for bolts of the present invention exhibits excellent beach weather resistance in an environment with a large amount of incoming salt, so bridges, etc. in the beach area, especially in areas where snowmelt salt and antifreeze are sprayed It can be used as a minimum maintenance material that does not require painting on structures such as bridges. Furthermore, when coating is performed, it is possible to remarkably improve the peel resistance and scratch resistance of the coated portion, which contributes to a reduction in life cycle cost.

  In the actual production process of bolts, after hot rolling, the bolts are formed and formed into bolts, followed by quenching and tempering heat treatment. Therefore, originally, the evaluation of properties must be performed on steel that has been subjected to forming processing after hot rolling, heat treatment such as quenching and tempering. However, the properties of steel that has been subjected to heat treatment of quenching and tempering without forming after hot rolling are almost the same as the properties of steel that has been subjected to quenching and tempering after forming, so here As shown in Fig. 4, the properties were evaluated by a steel plate that had been subjected to heat treatment such as quenching and tempering after hot rolling.

  180 kg of steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace. The oxygen content contained as impurities in the steel produced in this example was in the range of 0.0001 to 0.005%.

  The obtained ingot was heated to 1250 ° C. and subjected to ingot rolling and hot rolling to obtain a steel plate having a thickness of 20 mm. Then, after hold | maintaining at 750 degreeC for 7 hours, it cooled over 650 degreeC over 7 hours, and then air-cooled, and the hot-rolled steel material was obtained. A part of the hot-rolled steel material was cold-rolled to a thickness of 12 mm, held again at 750 ° C. for 7 hours, then cooled to 650 ° C. over 7 hours, and then air-cooled to obtain a cold-rolled steel material.

  From this cold-rolled steel material, as shown in FIG. 1, it was processed into a test piece for an upsetting test for evaluating the cold forgeability. The upsetting test piece has a cylindrical shape with an outer diameter φ = 10 mm × a length L = 15 mm, and has an angle of 30 °, a depth d = 0.8 mm, and a deepest radius R = 0.15 mm in the length direction of the outer surface. It has a notch section (No. 2 notch).

  The upsetting test was performed in accordance with the method described in “Plasticity and processing” vol.22 (1981-2) p.139, and then compressed 6 mm at a time. = 5 were compressed and checked for cracks. As the limit upsetting rate, the value of the limit compression rate when n / 2 is broken (cracking rate 50%) is adopted. The test results are shown in Table 2. If the limit upsetting rate exceeds 55%, there is no problem in manufacturing the bolt. Therefore, when the limit upsetting rate shows a value of 55% or more, all are described as “> 55”.

  Next, the front and back surfaces of the hot-rolled steel material were mechanically ground, and a test piece of blast steel material having a thickness of 3 mm × width of 60 mm × length of 100 mm was cut out. In order to evaluate the coating film peel resistance of the part where the bolt coating part is thin and the scratch part, the steel material No. 1 to 29, sprayed with a general-purpose epoxy resin paint (trade name “Banno 200” manufactured by China Paint Co., Ltd.) to a coating thickness of 150 μm, and a vinyl chloride cutter until the steel surface is scratched Then, the cloth was cut and SAE (Society of Automotive Engineers) J2334 test was performed 160 cycles.

  Separately, 10 cycles of SAE J2334 tests were conducted in advance, and after forming rust on the surface, general-purpose epoxy resin paint (trade name “Banno 200” China Paint Co., Ltd.) The product was spray-coated with a coating thickness of 150 μm, and the cloth was cut with a vinyl chloride cutter until the steel surface was scratched, and the SAE J2334 test was performed 160 cycles.

  After performing these tests, the maximum corrosion depth was measured using a point micrometer. The test results are shown in Table 2.

  The SAE J2334 test is an accelerated test under the following conditions, and the corrosion form is said to be similar to the atmospheric exposure test (Hiroo Nagano, Masato Yamashita, Hitoshi Uchida: Environmental Materials Science, Kyoritsu Publishing ( 2004), p.74). This test simulates a severe corrosive environment in which the amount of incoming salt exceeds 1 mdd.

Wet: 50 ° C., 100% RH, 6 hours,
Adherence of salt: 0.5 mass% NaCl, 0.1 mass% CaCl ₂ , 0.075 mass% NaHCO ₃ aqueous solution immersion, 0.25 hours,
Drying: One cycle of 60 ° C., 50% RH, 17.75 hours (24 hours in total).

Furthermore, as shown in FIG. 2, a salt-attached wet and dry cycle hydrogen permeation test simulating a salt-borne salt environment (T. Omura, T. Kudo and S. Fujimoto, Materials Transactions, vol. 47, No. 12 (2006), p.2956-2962) was used to measure the hydrogen permeation coefficient. The experiment was conducted at a chloride adhesion amount of 0.3 mg / cm ² and a temperature of 60 ° C. The test results are shown in Table 2.

  As is clear from the results in Table 2, all of the inventive examples (test numbers 1 to 24) are 55% or more in the upsetting test and can be manufactured for bolts. Moreover, since it is very excellent in the corrosion resistance of the coated part and has a small hydrogen permeation coefficient, it is excellent in corrosion resistance even in a salt-attached wet and dry environment.

  On the other hand, in the steel materials of the comparative examples, it can be observed that the maximum corrosion depth of the painted portion is greatly reduced in the test numbers 25 and 26 because the Sn content is insufficient. In Test Nos. 27 and 28, although the Sn content is within the specified range, the Cu content is 0.3% or more, and Cu / Sn exceeds 1, so the corrosion resistance of the painted part is Although it is excellent, it can be observed that the upsetting property is lowered. Furthermore, in the test number 29, since the Sn content exceeds the specified range, it can be observed that the corrosion resistance is saturated and the upsetting property is lowered.

  The bolt steel of the present invention has sufficient weather resistance even in an environment where the amount of incoming salt is large. As a minimum maintenance material used for structures such as bridges in beach areas and areas where snow melting salt and antifreezing agents are sprayed, it can be widely applied in the civil engineering and construction fields. Furthermore, when coated and used for steel structures such as ships and bridges, it can be widely applied as a material that contributes to maintenance minimization because it significantly suppresses corrosion from defects and the like.

It is an example of the test piece for an upsetting test for evaluating cold forgeability. It is a schematic diagram of a salt adhesion wet-dry cycle hydrogen permeation test.

Claims (8)

  In mass%, C: 0.15-0.6%, Si: 0.05-0.5%, Mn and Cr: 0.5-3.5% in total, P: 0.05% or less, S : 0.03% or less, Cu: less than 0.3%, Ni: less than 1%, O: 0.01% or less, and Sn: 0.05 to 0.50%, with the balance being Fe and impurities A bolt steel having a composition with a Cu / Sn ratio of 1 or less.   Furthermore, the steel for bolts of Claim 1 characterized by containing Al: 0.3% or less by the mass%.   Furthermore, it is characterized by containing one or more selected from the group consisting of Mo: 3.0% or less, W: 3.0% or less, and V: 0.3% or less by mass%. The steel for bolts according to claim 1 or 2.   Furthermore, it is characterized by containing one or more selected from the group consisting of Nb: 0.1% or less, Ti: 0.1% or less, and Zr: 0.1% or less in mass%. The steel for bolts according to any one of claims 1 to 3.   Furthermore, by mass%, B: 0.0030% or less, N: 0.03% or less, Ca: 0.01% or less, and Mg: 0.01% or less The steel for bolts according to any one of claims 1 to 4 characterized by containing.   The bolt steel according to any one of claims 1 to 5, further comprising 0.02% or less of REM in mass%.   The steel for bolts according to any one of claims 1 to 6, wherein the surface is coated with an anticorrosion film.   The bridge using the volt | bolt produced from the steel for bolts in any one of Claim 1-7.

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