JP5392397B2 - Steel sheet with small welding deformation and excellent corrosion resistance - Google Patents

Description

  The present invention relates to a steel plate having a small weld deformation and excellent corrosion resistance, which is used in the fields of shipbuilding, offshore structures, building structures, bridges, civil engineering, and the like. In particular, the present invention relates to a thick steel plate with small welding deformation that occurs during fillet welding.

  In general, during the production of various welded steel structures, deformation occurs due to solidification shrinkage of the weld metal and subsequent shrinkage and expansion due to cooling and phase transformation. As a typical welding deformation, there is an angular deformation of a T-type fillet weld. If a structure is manufactured with the angular deformation left, the buckling strength is greatly reduced due to the deformation of the member, or the fracture characteristics are deteriorated. Therefore, the structure is not aimed by the designer. In order to prevent such a situation, various measures have been taken to prevent it.

  The welding deformation prevention measures currently applied are roughly divided into the following three (i) to (iii).

(i) Design ingenuity (method to increase the rigidity of the deformed member)
The reason why the welding deformation remains is that the vicinity of the weld toe of the weld metal or the base metal is subjected to plastic deformation. The part that has undergone plastic deformation tends to elastically deform its outer part, but when the rigidity is high, that is, when the cross-sectional area is large, the amount of deformation is small. Therefore, changing the design to increase the cross-sectional area can be one preventive measure. However, the design change to increase the cross-sectional area has a lot of loss in terms of cost increase, weight increase and construction period extension of the steel material used.

(ii) Device for welding Welding deformation can be prevented by making some device for welding. There are several methods, but the first is to bend in the opposite direction before welding. Although angular deformation occurs after welding, there is a possibility that it will be finished in a desired shape by bending it in the opposite direction in advance. There is also a method in which the end is constrained during welding and deformation is not allowed. Further, there is a case where a backward torch is installed and bent back by reheating an appropriate position after welding. However, all of them are accompanied by a significant increase in man-hours, which causes a cost increase.

(iii) Straightening after welding There are mechanical straightening and linear heating straightening as methods for straightening after welding. However, these methods also require a significant increase in man-hours and require highly skilled skills.

  All of the measures (i) to (iii) above are contrivances in production. For example, Patent Document 1 proposes reducing welding deformation by contriving with welding materials. However, there are many problems that the increase in the cost of the welding material hinders the economic efficiency and the effect is insufficient, and the application is not progressing in reality.

  On the other hand, there is an example of trying to suppress welding deformation by devising a steel material as a base material, and some have been proposed as follows.

  Patent Document 2 discloses a method of increasing yield stress by promoting precipitation in welding heat history by adding Nb and Mo in combination. However, since addition of Mo brings about a significant cost increase, it is poor in versatility.

  In Patent Documents 3 and 4, by controlling the fraction of bainite and / or martensite of the steel material as the base material to 20% or more and further defining the dispersion state of carbonitride, the yield stress is increased, and There is a description of suppressing welding deformation. However, it has not yet reached a practically sufficient weld deformation reduction effect.

  And patent document 5 has description of suppressing welding deformation by ensuring the bainite rate of the steel materials used as a base material to 70% or more, and also ensuring the amount of solute Nb 0.0040% or more. However, when the bainite ratio is 70% or more, not only does the strength of the base material deviate from the general-purpose range, but there is a concern that inhibition of weld cracking by Nb may become a problem.

  On the other hand, a welded steel structure is often used in an environment where there is a large amount of incoming salt, such as in a beach area or an area where snow melting salt is scattered. In the shipbuilding field, it is often used in a seawater splash environment.

  Generally, when a weather-resistant steel material is exposed to an atmospheric corrosive environment, a protective rust layer is formed on the surface, and subsequent steel material corrosion is suppressed. For this reason, weathering steel is used for structures such as bridges as a minimum maintenance steel that can be used as it is without being painted.

  However, a protective rust layer is formed on the surface of weathering steel not only in the beach area but also in inland areas where there is a large amount of incoming salt, such as areas where snowmelt salt and antifreeze are sprayed. Since it is hard to be done, the effect which suppresses corrosion is hard to be exhibited. Therefore, in these regions, it is not possible to use bare weatherproof steel, and ordinary steel is used by painting on ordinary steel. However, in the case of using such ordinary steel for coating, it is necessary to repaint every 10 years because of coating deterioration due to corrosion, and therefore the cost required for maintenance becomes enormous.

In recent years, weathering steel standardized by Japanese Industrial Standards (JIS) (JIS G 3114: weathering hot rolled steel for welded structures) has an incoming salt content of 0.05 mg / dm ² / day (0. 05mdd) and higher areas, for example, beach areas, where the loss of corrosion is large due to the occurrence of scale-like rust, layered rust, etc., so it cannot be used without painting (Ministry of Construction, Public Works Research Institute, Steel Club) Japan Bridge Construction Association: Joint Research Report on the Application of Weatherproof Steel to Bridges (XX)-Design and Construction Guidelines for Uncoated Weatherproof Bridges (Ref. Rev. 1993.3)).

  In this way, in a salty environment such as a beach area, ordinary steel materials are usually coated. However, the current situation is that bridges constructed in the coastal area near the river mouth and roads such as mountainous areas where snow melt is salted are extremely corroded and must be repainted. Since these repainting takes a lot of man-hours, there is a strong demand for steel materials that can be used without painting.

  Recently, a Ni-based high weathering steel to which about 1 to 3% of Ni has been added has been developed. However, it has been found that in regions where the amount of incoming salt exceeds 0.3 to 0.4 mdd, it is difficult to apply to steel materials that can be used without coating with such addition of Ni alone.

  Since corrosion of steel materials increases as the amount of flying salt increases, weathering steel according 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 is not the same by the place and site | part used. For example, outside the girders, they are exposed to rainfall, condensed water and sunlight. On the other hand, inside the girder, it is exposed to condensed water, but there is no rain. In general, in an environment where the amount of incoming salt is large, it is said that the inside of the girders is more corrosive than the outside of the girders.

  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.

  In order to cope with such problems, the development of steel materials that prevent corrosion in an environment with a large amount of incoming salt has been underway.

  For example, Patent Document 6 proposes a weather-resistant steel material having an increased chromium (Cr) content, and Patent Document 7 proposes a weather-resistant steel material having an increased nickel (Ni) content. .

  However, although the weathering steel material with the increased chromium (Cr) content proposed in Patent Document 6 can improve the weathering resistance in a region where the amount of incoming salt is below a certain level, it is severer than that. In a salt environment, the weather resistance is deteriorated.

  Further, in the case of the weathering steel material with the increased nickel (Ni) content proposed in Patent Document 7, the weather resistance is improved to some extent, but the cost of the steel material itself is increased, and it is used for applications such as bridges. As an expensive material, it becomes 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.

Japanese Unexamined Patent Publication No. 7-9191 Japanese Laid-Open Patent Publication No.7-138715 JP 2003-268484 A JP 2006-2211 A Japanese Unexamined Patent Publication No. 2006-2198 Japanese Patent Laid-Open No. 9-1779090 Japanese Patent Laid-Open No. 5-118011

  Thus, the conventional methods have difficulty from the viewpoints of economy and practical reproducibility, and there is much room for improvement in practical use.

  In particular, in a welded structure manufactured using a thick steel plate having a thickness of 15 mm or more, even if the amount of deformation at each weld location is small, large deformation can occur as a whole welded structure. It is necessary to do. In addition, although the upper limit of thickness is not specifically limited, It is preferable to handle a thing to 50 mm.

  Furthermore, in a welded steel structure that is used in an environment where the amount of incoming salt is large, paint peeling resistance is a major problem. That is, as shown above, in a coastal environment where a large amount of chloride comes in or an environment where a snow melting agent or an antifreezing agent is sprayed, the coating peels off early and corrosion progresses. Therefore, it is necessary to repaint the paint every few to a few dozen years. In addition, when repainting is performed, it is necessary to assemble a scaffold on a once-corroded bridge and perform a reblasting process as a previous process, which is very expensive. And even if it re-blasts, it is difficult to remove rust completely, but even if it paints again on the steel material which has not removed rust completely, the coating life will become remarkably short. The paint peel resistance is largely due to the characteristics including the corrosion resistance of the steel material as the base.

  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, in the marine field and the bridge field where painting is required, there is a high demand for minimizing the life cycle cost, and extending the coating life is very important in considering the life cycle management of the bridge.

  In view of the above circumstances, an object of the present invention is to establish a technique for reliably suppressing welding deformation at low cost, and to provide a steel plate having small welding deformation. In particular, in fillet welding, an object is to provide a steel plate with small welding deformation. Note that the target value of the amount of welding deformation is ½ that of conventional steel.

  Furthermore, the present invention provides corrosion resistance in a high chloride environment (including that the coating does not peel off and that corrosion at the coating defect is suppressed and corrosion resistance is maintained (including coating peeling resistance) and weather resistance when no coating is applied). It aims to provide an excellent steel material.

  As a result of various studies, the present inventors have prescribed the chemical composition of the steel sheet and the metal structure thereof in order to solve such problems. FIG. 1 shows the independent influence of the physical property values of each material obtained by the thermal coupled FEM analysis conducted in conjunction with the experiment. Moreover, the calculation conditions of FEM analysis are shown in FIG.

In FIG. 1, the horizontal axis represents thermal conductivity (white circle plot), transformation point Ac ₁ (black circle plot), strength TS (square plot), and the vertical axis represents the amount of angular deformation. From FIG. 1, it can be seen that the amount of angular deformation does not change even when the thermal conductivity of the steel sheet is increased, the amount of angular deformation increases as the transformation point increases, and the amount of angular deformation decreases as the strength increases. Therefore, the welding deformation particularly depends largely on the strength and transformation point, and the target value of the welding deformation amount (angular deformation amount) is ½ that of conventional steel (angular deformation amount is about 0.8 mm), that is, 0.4 mm. The strength becomes extremely high and deviates from the general-purpose strength class. Deviations from the general strength class are not desirable because they are not only subject to general commercial transactions, but may also cause structural design problems and weldability problems.

  Therefore, the present inventors have aimed to develop a steel type in which the high-temperature strength is increased while maintaining the normal temperature strength suitable for the general-purpose strength class.

  Note that Mo, which has been conventionally said to be effective in high-temperature strength, is not realistic because it causes a cost increase due to a rise in alloy costs. Accordingly, the present inventors conducted various tests focusing on reducing the C content and containing Nb and B in combination. As a result, the following findings (a) to (d) were obtained.

  (a) In order to adapt the strength to the general-purpose strength range, it is possible to stably obtain a strength of 570 MPa class by reducing C below the normal level and adding B in combination.

  (b) By containing Nb and B in combination, the high temperature strength can be increased. If Nb is not contained, securing of high temperature strength is insufficient. However, the Nb content may be small, and if it is 0.02% or more, welding deformation can be suppressed through securing high temperature strength.

  (c) The steel plate manufacturing method may be under general conditions, but steels containing Nb and B in combination tend to have higher hardenability than ordinary steels, so they conform to general-purpose strength levels. It is preferable to devise for this purpose.

  (d) In order to meet the general-purpose strength level, it is essential to have a low carbon bainite structure as a main component. On the other hand, the low carbon bainite structure has a lower hardness than the high carbon bainite structure, and the deformation resistance tends to be slightly inferior. For this reason, it is necessary to reduce as much as possible the ferrite structure that causes the hardness to decrease. In addition, the hardness of the bainite structure, which is the main component of the steel structure, also affects the strength and deformation resistance. For this reason, it is necessary to adjust the hardness of the bainite structure.

On the other hand, the present inventors examined 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 became an essential condition of corrosion, and due to hydrolysis of Fe ³⁺ It has been found that corrosion is accelerated by lowering the pH and by Fe ³⁺ acting 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.

  As a result of examining the influence of various alloy elements on the weather resistance based on the mechanism of corrosion in such a salt environment, the present inventors have obtained the findings shown in the following (e) to (g).

(e) Sn is dissolved as Sn ²⁺ , and the concentration of Fe ³⁺ is reduced by a reaction of 2Fe ³⁺ + Sn ²⁺ → 2Fe ²⁺ + Sn ⁴⁺ to suppress the reaction of formula (1). Sn also has an effect of suppressing anodic dissolution.

  (f) Cu is an element that has conventionally been regarded as the basis for the effect of improving the corrosion resistance in an environment with a large amount of incoming salt, and the effect of improving the corrosion resistance is seen in an environment with a relatively long wetting 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.

  (g) Thus, the steel material which contains Sn actively and suppresses the Cu content can be expected to have high corrosion resistance. In addition, since the corrosion resistance is high, even if the steel material is painted, there is little peeling of the paint due to the corrosion of the steel material, and the corrosion of the coating defect part is suppressed, but the anticorrosive effect by the coating film can also be expected. In addition, a further effect of corrosion resistance can be expected. Therefore, in addition to the corrosion resistance, the service life of the coating can be extended and the repair coating interval can be greatly extended. In particular, it is effective in improving paint peeling resistance in the marine and bridge fields.

  The present invention has been completed on the basis of the above knowledge, and the gist thereof lies in a steel sheet having small corrosion deformation and excellent corrosion resistance as shown in the following (1) to (4).

  (1) By mass%, C: 0.0005% or more and less than 0.02%, Si: 0.01 to 0.7%, Mn: 0.1 to 5.0%, P: 0.05% or less , S: 0.008% or less, Cu: less than 0.2%, Nb: 0.02-0.3%, Al: 0.003-0.1%, N: 0.01% or less, B: 0 .0005 to 0.004% and Sn: 0.03 to 0.50%, the balance is made of Fe and impurities, and the Cu / Sn ratio is 1 or less, and the metal structure is a bainite structure. , And a bainite hardness of 150 to 250 in terms of Vickers hardness, having a small weld deformation and excellent corrosion resistance.

  (2) The steel sheet having a small weld deformation and excellent corrosion resistance according to the above (1), characterized by containing, by mass%, Ti: 0.1% or less.

  (3) 1% by mass, further, Ni: 3.5% or less, Cr: 2% or less, Mo: 0.5% or less, V: 0.1% or less, and Zr: 0.02% or less A steel plate having small corrosion deformation and excellent corrosion resistance, characterized by containing seeds or two or more kinds.

  (4) The above-mentioned, characterized by further containing one or more of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less in mass%. A steel sheet having small corrosion deformation and excellent corrosion resistance according to any one of (1) to (3).

  Considering the present invention from the viewpoint of a welding method with small welding deformation in a steel plate, the welding deformation in the steel plate is substantially a welding deformation in the weld heat affected zone (HAZ). If welding is performed after satisfying the requirements, it is considered that the ability to suppress welding deformation is improved.

Therefore, from the viewpoint of the welding method, the present invention
“In mass%, C: 0.0005% or more and less than 0.02%, Si: 0.01 to 0.7%, Mn: 0.1 to 5.0%, P: 0.05% or less, Cu : Less than 0.2%, S: 0.008% or less, Nb: 0.02 to 0.3%, Al: 0.003 to 0.1%, N: 0.01% or less, B: 0.0005 -0.004% and Sn: 0.03-0.50%, a welding method of a steel sheet having a chemical composition consisting of the balance Fe and impurities, the portion of the steel heat-affected zone in the steel sheet before welding A welding method characterized in that the metal structure contains 80% or more of a bainite structure, and the bainite hardness is 150 to 250 in terms of Vickers hardness.
It can also be grasped.

  Of course, the steel sheet is in% by mass, Ti: 0.1% or less, Ni: 3.5% or less, Cr: 2% or less, Mo: 0.5% or less, V: 0.1% or less, Zr : 0.02% or less, Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less may be contained.

  Here, as will be described later, the steel plate as a base material may be welded after being manufactured so as to satisfy the above requirements, or the part to be welded (welding heat effect in the steel plate as a base material). It is also possible to weld only after satisfying the above requirements for the part by processing only the part to be a part.

  And this welding method is applicable also in the case of fillet welding with a large welding deformation. Fillet welding is performed on lap joints, T joints, cruciform joints, etc., but this welding method involves fillet welding on T joints and cruciform joints, which cause particularly large welding deformations due to the relative positional relationship of the joint base material. Is particularly effective.

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate which can suppress welding deformation | transformation reliably at low cost and was excellent also in corrosion resistance can be provided.

It is a FEM calculation result which shows the influence of each material physical property value on the amount of welding angle deformation. It is a schematic diagram which shows the calculation conditions of FEM analysis. It is a figure which shows the test piece used in evaluating the welding angle deformation. It is a figure which shows the definition of the welding angle deformation amount.

  In the present invention, the reason for limiting the chemical composition and metal structure of the steel sheet is as follows.

(A) Chemical composition of steel sheet The effects of each component of the steel sheet and the preferred contents of each component are as follows. In addition, "%" regarding content means "mass%".

C: 0.0005% or more and less than 0.02% C is an element most effective for improving the strength and is an inexpensive element. However, if it is less than 0.0005%, it is necessary to guarantee strength by using other elements in combination, resulting in an increase in cost. Moreover, when it contains 0.02% or more, intensity | strength will raise too much and versatility will be lost. Therefore, the C content is 0.0005% or more and less than 0.02%. Preferably, it is 0.0005 to 0.02%. In addition, the upper limit of content of C preferable in terms of strength versatility is 0.002%.

Si: 0.01 to 0.7%
Si is an element contributing to strength improvement. However, if it is less than 0.01%, the required strength cannot be ensured. Moreover, when it contains exceeding 0.7%, base material toughness and weldability toughness will deteriorate remarkably. Therefore, the Si content is set to 0.01 to 0.7%.

Mn: 0.1 to 5.0%
Mn is an element necessary for ensuring strength. However, if it is less than 0.1%, the required strength cannot be ensured. Moreover, when it contains exceeding 5.0%, weldability will deteriorate. Therefore, the Mn content is 0.1 to 5.0%. Since addition of excessive Mn may deteriorate the corrosion resistance, it is preferably 4.0% or less, more preferably 2.0% or less.

P: 0.05% or less P is an element present in steel as an impurity. If the P content exceeds 0.05%, it not only segregates at the grain boundaries and lowers the toughness, but also causes hot cracking during welding, so the P content is 0.05% or less.

S: 0.008% or less S is an element present in steel as an impurity. If the S content exceeds 0.008%, center segregation is promoted or a large amount of stretched MnS is generated, so that the mechanical properties of the base material and the weld heat affected zone deteriorate. Therefore, the upper limit of the S content is 0.008%.

Cu: Less than 0.2% 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. Rather, it reduces the corrosion resistance. Further, if it coexists with Sn, cracking occurs during rolling. Therefore, it is necessary to reduce the Cu content. Even if contained as an impurity, the Cu content needs to be less than 0.2%. Preferably it is less than 0.1%.

Nb: 0.02-0.3%
Nb expresses the precipitation behavior at high temperature and brings about an increase in high temperature strength. However, if the content is less than 0.02%, the effect cannot be obtained. On the other hand, if it exceeds 0.3%, the toughness of the weld heat affected zone is significantly impaired. Therefore, the Nb content is 0.02 to 0.3%. A preferable content is 0.02 to 0.18%.

Al: 0.003-0.1%
Al is an essential element for deoxidation. In order to stably perform deoxidation, a content of 0.003% or more is necessary. However, if it exceeds 0.1%, the toughness tends to deteriorate particularly in the weld heat affected zone. This is presumably because coarse cluster-like alumina inclusion particles are easily formed. Therefore, the Al content is 0.003 to 0.1%.

N: 0.01% or less N is an element present in steel as an impurity. When the N content exceeds 0.01%, the base material toughness and the weld heat affected zone toughness are deteriorated. Therefore, the upper limit of the N content is 0.01%.

B: 0.0005 to 0.004%
B has the effect of improving the hardenability and increasing the strength. In order to stably obtain this effect, the B content needs to be 0.0005% or more. However, when the content exceeds 0.004%, the effect of increasing the strength is saturated, and the tendency of deterioration of toughness becomes remarkable in both the base material and the weld heat affected zone. Therefore, the B content is 0.0005 to 0.004%. In addition, preferable content is 0.0005 to 0.0025%.

Sn: 0.03-0.50%
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 having an effect of reducing Fe ³⁺ concentration as 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. These effects are obtained by containing 0.03% or more of Sn, and saturate when it exceeds 0.50%. Therefore, the Sn content is 0.03 to 0.50%. The preferable content of Sn is 0.03 to 0.20%.

Cu / Sn ratio: 1 or less In the case of steel containing Sn, the corrosion resistance is significantly reduced by the inclusion of Cu. Moreover, when manufacturing steel materials, it becomes a cause of the rolling crack by inclusion of Cu. For this reason, it is necessary to make Cu / Sn ratio, ie, ratio of Cu content with respect to Sn content 1.0 or less.

  The steel sheet according to the present invention has the chemical composition described above, with the balance being Fe and impurities. Here, the impurity is a component that is mixed due to various factors in the manufacturing process including raw materials such as ore and scrap when industrially manufacturing a steel sheet, and does not adversely affect the present invention. It means what is allowed.

  In addition to the above components, the steel sheet according to the present invention can contain one or more components selected from at least one of the following first group to third group, if necessary. Hereinafter, components belonging to these groups will be described.

Group 1 ingredients: Ti
Ti: 0.1% or less Since Ti mainly acts as a deoxidizing element, it can be contained if necessary. However, since deoxidation can be performed with Al, it is not always necessary to contain it. However, since Ti oxide or Ti—Al oxide is formed when the Ti content is large, the ability to refine the structure particularly in the weld heat affected zone of small heat input welding is lost. For this reason, Ti content in the case of making it contain shall be 0.1% or less. In addition, in order to acquire the deoxidation effect by containing Ti stably, it is preferable that the content shall be 0.01% or more.

Second group of components: Ni, Cr, Mo, V, Zr
Ni: 3.5% or less Ni is an element that improves the toughness of the base material and contributes to the improvement of the strength by improving the hardenability, and can be contained as necessary. However, since Ni is an expensive element, if Ni is excessively contained, it causes a large cost increase. Moreover, when it coexists with Sn, it will deteriorate the corrosion resistance in the presence of chloride. For this reason, the upper limit of content of Ni in the case of making it contain shall be 3.5% or less. Preferably it is 1.0% or less, More preferably, it is 0.5% or less. In addition, in order to acquire the said effect by containing Ni stably, it is preferable that the content shall be 0.02% or more.

Cr: 2.0% or less Cr is an element effective for increasing the strength through improvement of hardenability, and can be contained as necessary. However, if it exceeds 2.0%, the toughness deteriorates. Therefore, the Cr content when contained is 2.0% or less. Cr is an element that degrades corrosion resistance in a salt environment, but when it coexists with Sn, its adverse effect is remarkably suppressed. In order to stably obtain the strength improvement effect by containing Cr, the content is preferably set to 0.02% or more.

Mo: 0.5% or less Since Mo is an element effective for increasing the strength, it can be contained as required. However, if Mo is contained in excess of 0.5%, the cost is significantly increased, and the improvement in strength is saturated. Therefore, the Mo content in the case of inclusion is 0.5% or less. In addition, in order to acquire the strength improvement effect by containing Mo stably, it is preferable that the content shall be 0.06% or more.

V: 0.1% or less V is an element effective for improving the strength, and can be contained as necessary. However, if the V content exceeds 0.1%, the toughness is greatly deteriorated. In addition, in order to obtain the strength improvement effect by containing V stably, it is preferable to make the content 0.005% or more.

Zr: 0.02% or less Zr has the effect of finely dispersing and precipitating nitrides in steel and improving the strength, and can be contained as required. However, if the content exceeds 0.02%, coarse precipitates are formed and toughness is deteriorated, so the content of Zr in the case of inclusion is 0.02% or less. In order to stably obtain the strength improvement effect by containing Zr, the Zr content is preferably 0.0003% or more.

Group 3 components: Ca, Mg, REM
Ca: 0.004% or less Ca reacts with S in steel to form oxysulfide (oxysulfide) in molten steel. Unlike the elongated shaped inclusions such as MnS, this oxysulfide does not extend in the rolling direction during rolling and is spherical after rolling. Therefore, welding with the tip of the elongated shaped inclusions as the starting point of cracking Since there exists an effect | action which suppresses a crack and a hydrogen induction crack, it can be made to contain as needed. However, if its content exceeds 0.004%, toughness may be deteriorated. Therefore, when Ca is contained, the content of Ca is set to 0.004% or less. In addition, in order to acquire the effect which suppresses a weld crack and a hydrogen induction crack stably, it is preferable that content of Ca shall be 0.0003% or more.

Mg: 0.002% or less Mg forms an Mg-containing oxide, serves as a generation nucleus of TiN, and has an effect of finely dispersing TiN. Therefore, Mg can be contained as necessary. However, when the content exceeds 0.002%, the amount of oxide becomes excessive and ductility is reduced. Therefore, the upper limit of the Mg content in the case of inclusion is set to 0.002%. In order to stably obtain the effect of finely dispersing TiN, the Mg content is preferably 0.0003% or more.

REM: 0.002% or less REM contributes to the refinement of the structure of the weld heat affected zone and the fixation of S, and can be contained as necessary. However, if the content exceeds 0.002%, REM becomes an inclusion that adversely affects the toughness of the base material, so the content of REM when contained is 0.002% or less. In addition, in order to obtain the refinement | miniaturization of a structure | tissue and the fixing effect of S stably, it is preferable that content of REM shall be 0.0003% or more. Note that REM is a generic name for 17 elements in which Y and Sc are combined with 15 elements of lanthanide, and one or more of these elements can be contained. Further, the content of REM means the total content of these elements.

(B) Metal structure The bainite fraction of a metal structure shall be 80% or more. Bainite is excellent in deformation resistance, but steel with a low carbon content lacks carbon in bainite, resulting in a low-carbon bainite structure, which is slightly inferior in deformation resistance compared to a high-carbon bainite structure. Therefore, in order to ensure deformation resistance in low carbon steel, it is necessary to secure a certain amount of bainite structure and not to have excessive ferrite. If the bainite ratio is 80% or more with the inclusion of Nb and B essential, sufficient weld deformation resistance can be obtained, so the ratio of the bainite structure is defined as 80% or more. The balance is thought to be mainly composed of ferrite, but is not particularly specified.

  Further, even when the bainite hardness (Vickers hardness) is too low, the deformation resistance cannot be secured, so the lower limit is set to 150. Moreover, since the versatility of intensity | strength is impaired when too hard, the upper limit is set to 250.

  Next, conditions for rolling and heat treatment capable of obtaining the steel sheet according to the present invention will be described.

Prior to hot rolling, the steel ingot is first heated, but if the heating temperature at this time is set to Ac ₃ or higher, it can be completely austenitic phase and homogenized without any untransformed part. It is preferable that the temperature be Ac ₃ point or higher. Specifically, it is preferable to heat to 900 to 1200 ° C. When the rolling finish temperature at the thin end is set to 900 ° C. or lower during hot rolling, the crystal grains become an appropriate size and the fracture toughness of the material becomes sufficient. The lower limit of the rolling finishing temperature is not particularly defined, and any conditions may be used as long as the strength can be adapted to the general-purpose strength range. When the rolling finish temperature is 700 ° C. or higher, the anisotropy due to the two-phase region processing becomes inconspicuous, so that the rolling finish temperature is desirably 700 ° C. or higher. Subsequent to rolling, accelerated cooling may be performed. In the case of performing accelerated cooling, it is preferable to control the cooling rate of the central portion to 0.5 to 40 ° C./s immediately after rolling or after some standing time. The cooling stop temperature is preferably controlled using 150 to 500 ° C. as a guide. Moreover, you may implement heat processing suitably after rolling. When the heat treatment is performed, it is preferable to perform a normalizing process or a tempering process, and it is preferable to select temperature ranges of 800 to 1100 ° C. and 300 to 700 ° C., respectively.

  An example of the steel plate concerning this invention is shown. Steel ingots having the composition components shown in Table 1 were produced under the heating temperature, finishing temperature, accelerated cooling, and heat treatment conditions shown in Table 2. The plate thickness of the steel plate was 16 mm.

  Table 3 shows the yield point YP, tensile strength TS, transition temperature vTrs, bainite fraction, bainite hardness of the bainite phase, welding angle deformation, plate thickness reduction, and peel area ratio of the steel sheet thus obtained. Respectively.

In addition, in order to measure the tensile property of the obtained steel plate, the test piece was extract | collected according to the test method as described in JIS-Z-2201. The sampling position was set to around ¼ of the plate thickness direction and the L direction (parallel to the rolling direction). The yield point was determined as a test speed of 10 N / mm · s, and the yield point was 0.2% proof stress when no clear yield point appeared. The target value of the tensile properties, the yield point YP is 350 N / mm ² or more, and the tensile strength TS has a 490~720N / mm ^2.

  Moreover, in order to measure the impact characteristic of the obtained steel plate, the test piece was extract | collected according to the test method as described in JIS-Z-2202. The sampling position was around 1/4 in the plate thickness direction and the L direction (parallel to the rolling direction), and a 2 mmV notch Charpy test piece was measured. The brittle fracture surface ratio at various temperatures was measured to determine the transition temperature. The target value of the Charpy characteristic is that the transition temperature is 0 ° C. or lower. Tissue observation was performed with an optical microscope. The image obtained by observation was subjected to image analysis. The bainite fraction of the metal structure was obtained by calculating the area ratio of bainite to the area of 100 visual field observations obtained by the above observation method.

  Furthermore, the welding angle deformation amount by fillet welding was evaluated in the following manner.

  As shown in FIG. 3, a T-shaped welding test piece was prepared for the steel plate, one side was constrained with a triangular steel plate having high rigidity, and the other side was subjected to 1-pass fillet welding. The welding material used was a general 50-kilo steel flux cored wire, and the welding conditions were 10.4 kJ / cm (200 A-26 V-30 cm / min). When a sufficient time has elapsed after welding, place the test piece on the surface plate, and measure the angular deformation θ defined in Fig. 4 with three clearance gauges at the welding start position, center position and end position. The average value thereof was taken as the welding angle deformation. In addition, the welding angle deformation amount of ordinary general-purpose 50 kg steel measured by this method is about 1 °, and the target welding angle deformation level of the present invention is 0.5 °.

And about corrosion resistance, the test piece obtained from the obtained steel materials was evaluated by SAE (Society of Automotive Engineers) J2334 test. SAE J2334 test is wet: 50 ° C., 100% RH, 6 hours, salt adhesion: 0.5% NaCl, 0.1% CaCl ₂ , 0.075% NaHCO ₃ aqueous solution, 0.25 hour, dry: 60 It is an accelerated test with 1 cycle (total 24 hours) at ℃, 50% RH, 17.75 hours, and the corrosion form is said to be similar to the atmospheric exposure test (Hiroo Nagano, Masato Yamashita, Hitoshi Uchida) : Environmental Materials Science, Kyoritsu Shuppan (2004), p.74). This test is a test that simulates a severe corrosive environment in which the amount of incoming salt exceeds 1 mdd.

  After 120 cycles of the SAE J2334 test, the rust layer on the surface of each test piece was removed, and the thickness reduction was measured. Here, the “plate thickness reduction amount” is an average plate thickness reduction amount of the test piece, and is calculated using the weight reduction before and after the test and the surface area of the test piece.

  In addition, in order to investigate the anti-peeling resistance, a test piece with a size of 150 x 70 mm was coated with a modified epoxy paint (Banno 200: made in China) by air spray to a dry film thickness of 150 μm, and the steel substrate After making a crosscut at a depth reaching, the SAE J2334 test was also evaluated.

  As a result, in the steel sheet of Mark 1-e (comparative example), the heat treatment after cooling was set to 1150 ° C. and normalization was performed at a high temperature, so that a sufficient amount of bainite was not generated. For this reason, the welding angle deformation amount also increased. Mark 1-e steel plate is unsuitable as a structural steel plate because of its large welding angle deformation.

  In the Mark 1-f steel plate (comparative example), the cooling stop temperature was 120 ° C., which was quenched to a relatively low temperature. Therefore, the hardness of the bainite phase became hard, the tensile strength increased, and the toughness deteriorated. For this reason, it is an inappropriate steel material as a structural steel plate.

  In the steel sheet of Mark12-b (comparative example), the water cooling stop temperature was set to 120 ° C., and the steel was quenched to a relatively low temperature. Therefore, the hardness of the bainite phase became hard, the tensile strength increased, and the toughness deteriorated. For this reason, this steel material is also unsuitable as a structural steel plate.

  In the steel plate of Mark 38 (comparative example), Sn did not satisfy the composition defined in the present invention, the corrosion resistance was lowered, and the coating peeled off. For this reason, it is unsuitable as a structural steel material used in a high chloride environment.

  In the steel sheets of Mark 39 to 44 (comparative examples), the steel composition specified in the present invention was not satisfied, and the toughness of the steel sheet itself was lowered. For this reason, it is inappropriate as a structural steel material. In particular, in the steel sheet of Mark 39 (comparative example), since the Cu / Sn ratio exceeded 1, rolling cracks also occurred. Moreover, in the steel plate of Mark 41 (comparative example), Sn did not satisfy the composition defined in the present invention, the corrosion resistance was lowered, and the coating peeled off.

On the other hand, in the steel sheets according to the examples of the present invention indicated by other marks, the tensile properties are all such that the yield point YP is 350 N / mm ² or more and the tensile strength TS is 490 to 720 N / mm ² class. It is a general-purpose steel, and transition temperature vTrs, bainite fraction, Vickers hardness of bainite phase are also in the proper range, and the welding angle deformation is within the target 0.5 °, so it is suitable as a structural steel plate. I understand that. Moreover, although it has high corrosion resistance and corrosion was seen in the crosscut part at the time of painting, it turns out that any steel plate has few peeling, Therefore The repair interval of coating can be extended.

  As described above, according to the present invention, it is possible to provide a steel plate that can reliably suppress welding deformation at low cost and that has excellent corrosion resistance.

Claims (4)

  In mass%, C: 0.0005% or more and less than 0.02%, Si: 0.01 to 0.7%, Mn: 0.1 to 5.0%, P: 0.05% or less, S: 0.008% or less, Cu: less than 0.2%, Nb: 0.02-0.3%, Al: 0.003-0.1%, N: 0.01% or less, B: 0.0005 0.004% and Sn: 0.03 to 0.50%, consisting of the balance Fe and impurities, and having a chemical composition with a Cu / Sn ratio of 1 or less, and the metal structure is 80% of the bainite structure A steel sheet including the above, having a low bainite hardness and having a Vickers hardness of 150 to 250 and having a small weld deformation and excellent corrosion resistance.   The steel sheet having a small weld deformation and excellent corrosion resistance according to claim 1, further comprising, by mass%, Ti: 0.1% or less.   1% by mass of Ni: 3.5% or less, Cr: 2.0% or less, Mo: 0.5% or less, V: 0.1% or less, and Zr: 0.02% or less Alternatively, the steel sheet according to claim 1 or 2, wherein the steel sheet contains two or more kinds and has small weld deformation and excellent corrosion resistance.   2. From mass%, further comprising one or more of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less. 3. A steel sheet having small corrosion deformation and excellent corrosion resistance according to any one of items 3 to 3.

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