KR20120099516A - Steel plate exhibiting little welding deformation and excellent corrosion resistance - Google Patents

Abstract

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 to 0.3%, Al: 0.003-0.1%, N: 0.01% or less, B: 0.0005-0.004%, and Sn: 0.03-0.50%, consisting of residual Fe and impurities, and having a Cu / Sn ratio of 1 or less A steel sheet having a composition, wherein the metal structure contains 80% or more of bainite structure, and the bainite hardness is 150 to 250 Vickers hardness, and has small weld deformation and excellent corrosion resistance. Moreover, you may contain 1 type, or 2 or more types of Ti, Ni, Cr, Mo, V, Zr, Ca, Mg, and REM.

Description

Steel plate with small welding deformation and excellent corrosion resistance {STEEL PLATE EXHIBITING LITTLE WELDING DEFORMATION AND EXCELLENT CORROSION RESISTANCE}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having a small welding deformation and excellent corrosion resistance, which is used in fields such as shipbuilding, offshore structures, building structures, bridges, and civil engineering. In particular, it is related with the thick steel plate with small welding deformation which arises at the time of the operation of fillet welding.

In general, in the production of various welded steel structures, deformation occurs due to solidification shrinkage of the weld metal and subsequent shrinkage and expansion due to subsequent cooling and phase transformation. Typical examples of the welding deformation include angular deformation of the T-shaped fillet welded portion. When the structure is manufactured with the angular deformation remaining, the buckling strength is greatly reduced due to the deformation of the member, or the fracture characteristics are degraded, so that the structure intended by the designer does not become a target. In order to prevent such a situation, preventive measures are taken by various designs.

If the welding deformation prevention measures applied to the present situation are largely divided, it becomes three of following (i)-(iii).

(i) design of the design (how to increase the rigidity of the deformed member);

The reason why the welding strain remains is that the vicinity of the weld end of the weld metal or the base metal receives plastic deformation. The portion subjected to plastic deformation attempts to elastically deform the outer portion thereof, but when the rigidity is high, that is, when the cross-sectional area is large, the deformation amount is small. Therefore, a design change to increase the cross-sectional area may be one preventive measure. However, the design change to increase the cross-sectional area is large in terms of cost increase, weight increase, and prolonged construction period of the steel used.

(ii) devising during welding

At the time of welding, it is possible to prevent welding deformation by devising any design. There are several ways to do this, but first, bend in the reverse direction before welding. After welding, angular deformation occurs, which may be finished in a desired shape by bending it in the reverse direction in advance. There is also a method in which the end is constrained during welding so as not to allow deformation. Moreover, the method of returning in reverse by providing a trailing torch and reheating a suitable position after welding may be employ | adopted. However, both of them involve a significant increase in the number of processes, which leads to a cost increase.

(iii) straightening after welding

As a method of correcting after welding, there are mechanical correction and linear heating correction. However, these methods also require a significant increase in the number of processes, and also require a high level of skill.

Although the countermeasure of said (i)-(iii) is all devised on manufacture, it is proposed by patent document 1 to aim at reduction of a welding deformation by devising a welding material, for example. However, there are many problems, such as an increase in the cost of the welding material, which impairs economic efficiency or insufficient effect, and in reality, the application is not progressing.

On the other hand, there exists an example which tried to suppress welding deformation by devising the steel material used as a base material, and several are proposed as follows.

Patent Literature 2 discloses a method of increasing the yield stress by promoting precipitation in the history of welding heat by complex addition of Nb and Mo. However, in particular, the addition of Mo causes a significant increase in cost, and thus lacks versatility.

Patent Documents 3 and 4 disclose that the welding strain is suppressed by increasing the yield stress by controlling the fraction of bainite and / or martensite of the steel as the base metal to 20% or more and defining the dispersed state of carbonitride. There is a description. However, it did not necessarily reach until a practically sufficient welding distortion reduction effect was obtained.

Patent Document 5 also describes that welding deformation is suppressed by setting the bainite rate of the steel serving as the base material to 70% or more and securing the solid solution Nb amount to 0.0040% or more. However, when the bainite ratio is 70% or more, not only the strength of the base metal may deviate from the general range but also the inhibition of weld cracking property by Nb may be a problem.

On the other hand, welded steel structures are often used in environments with high amounts of adjuvant salts, such as in seaside areas and areas where snow melt is sprayed. Moreover, in the shipbuilding field, it is often used under the seawater splash environment.

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.

By the way, not only the beach area but also the inland area where abundant anti-inflammatory salts are applied, such as the area where the snow spray is applied to the snow melt and cryoprotectant, it is difficult to form a protective rust layer on the surface of the weathering steel, so that the effect of suppressing corrosion is It is hard to be exercised. Therefore, in such a region, the weather resistant steel as it is cannot be used, and the use of the coating of the ordinary steel which coats and uses a normal 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 therefore the cost required for maintenance is enormous.

In recent years, weather resistant steels (JIS G 3114: weather resistant hot rolled steels for welded structures), which have been standardized to Japanese Industrial Standards (JIS), have an area of not less than 0.05 mg / dm ² / day (0.05mdd) in terms of nasal salt content, for example, beaches. In area, corrosion loss is caused by scale rust or layer rust, and cannot use without coating (Construction Civil Engineering Research Institute, Steel Club, Japan Bridge Construction Association: Weatherability Joint Research Report on the Application of Steel Bridges (XX)-Design and Construction Tips for Martial Weather-Resistant Bridges (Rev. -1993.3).

As described above, in a salty environment such as a beach area, the steel is usually coated and coped. However, in the present situation, bridges built on roads such as beach areas near estuaries and mountainous areas that spray snow melt are highly corrosive and must be repainted. Since such repainting requires a large number of steps, there is a strong demand for steel that can be used without painting.

Recently, Ni-based highly weather resistant steels with Ni added in amounts of about 1 to 3% have been developed. However, in the region where the amount of adjuvant salt exceeds 0.3 to 0.4mdd, it has been found that application of Ni alone is difficult to apply to steel materials that can be used without coating.

Since corrosion of steel material becomes severe as the amount of adjuvant salt increases, the weather resistant steel according to amount of adjuvant salt is needed from a corrosion resistance and economical viewpoint. 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 torii, it is exposed to rainfall, dew condensation, and sunshine. On the other hand, inside a torii, it is exposed to condensation water, but it does not meet rain. In general, in an environment with a large amount of adjuvant salt, it is known that the inside of the torii is more severe than the outside of the torii.

In addition, in an environment in which a snow melting salt or a cryoprotectant is sprayed on the road, the salt is splashed by the car while traveling and adheres to a bridge supporting the road, which results in a severe corrosive environment. It is also exposed to harsh salty environments, such as underneath eaves slightly off the coast, and in these areas, it is a harsh corrosive environment with abundant aerobic salts greater than 1 mdd.

In order to cope with such a problem, the development of the steel which prevents corrosion in the environment with a large amount of adjuvant salts is progressing conventionally.

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

However, weather resistant steels having an increased content of chromium (Cr) proposed in Patent Document 6 can improve weather resistance in an area of the amount of adjuvant salt below a certain level, but inversely, in a severe salt environment exceeding that, the weather resistance is reversed. Deteriorates.

In addition, in the case of a weather resistant steel material in which the nickel (Ni) content proposed in Patent Document 7 is increased, the weather resistance is improved to some extent, but the cost of the steel material itself becomes high, and the material used for applications such as bridges is expensive. In order to avoid this, when Ni content is reduced, weather resistance does not improve so much, and when there is much amount of adjuvant salt, layered peeling rust will generate | occur | produce on the surface of steel materials, corrosion will be remarkable, and a problem that it will not endure use for a long time will arise.

Japanese Patent Laid-Open No. 7-9191 Japanese Patent Laid-Open No. 7-138715 Japanese Patent Publication No. 2003-268484 Japanese Patent Publication No. 2006-2211 Japanese Patent Publication No. 2006-2198 Japanese Patent Laid-Open No. 9-176790 Japanese Patent Laid-Open No. 5-118011

As described above, the conventional methods have difficulties in terms of economical efficiency and practical reproducibility, respectively, and have a great room for improvement in practical use.

In particular, in a welded structure manufactured using a thick steel sheet having a thickness of 15 mm or more, a large amount of deformation may occur in the entire welded structure even though the amount of deformation at an individual welding location is small. Therefore, it is necessary to reduce the amount of weld deformation as small as possible. In addition, although the upper limit of thickness is not specifically limited, It is preferable to handle the thing to 50 mm.

In addition, in the welded steel structure that is used under an environment where the amount of adjuvant salt is high, coating peeling resistance becomes 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 the corrosion progresses even if the coating is performed. It is necessary to repaint the coating every few decades. Moreover, when repainting, it is necessary to provide a scaffold on the once corroded bridge and perform reblasting as a preliminary step, which is expensive. Further, even if the reblasting is performed, it is difficult to completely remove the rust, and even if the rust is completely removed on the steel, the coating life is remarkably shortened. The coating peel resistance is largely dependent on the properties including the corrosion resistance of the steel which is the base material.

Therefore, it is strongly desired to extend the life of painting and to greatly increase the interval between repair paintings. 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 when considering the life cycle management of bridges.

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 sheet with small welding deformation. In particular, in fillet welding, it aims at providing the steel plate with small welding deformation. In addition, the target value of the deformation amount of a welding shall be 1/2 of the conventional steel.

In addition, the present invention includes corrosion resistance in a high chloride environment (the coating is not peeled off and corrosion at the coating defect is suppressed to maintain corrosion resistance (paint resistance) and weather resistance when not coated). It is an object to provide excellent steels.

MEANS TO SOLVE THE PROBLEM In order to solve such a subject, the present inventors prescribed | regulated the chemical composition of the steel plate as a result of various examination, and also prescribed | regulated the metal structure. It shows in FIG. 1 which showed the independent influence of the material property value obtained by the hot-ductility FEM analysis performed with 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), and intensity TS (square plot), and the vertical axis represents angular strain amount. It can be seen from FIG. 1 that even if the thermal conductivity of the steel sheet is increased, the angular strain does not change, and when the transformation point rises, the angular strain increases, and when the strength increases, the angular strain decreases. Therefore, the welding deformation is largely dependent on the strength and the transformation point, and when the target value of the welding deformation amount (angular deformation amount) is 1/2 of conventional steel (angular deformation amount is about 0.8 mm), that is, 0.4 mm, the strength becomes too high and general strength Depart from the class. Deviations from the general-purpose strength class are not only subject to general commerce, but also may have problems in structural design and weldability, which is not preferable.

Therefore, the present inventors aimed at developing a kind of steel obtained by increasing the high temperature strength while maintaining the room temperature strength suitable for the general-purpose strength class.

In addition, Mo, which is known to have an effect on high temperature strength in the past, becomes a factor of cost increase due to an increase in alloy cost, which is not practical. Therefore, the present inventors carried out various tests, paying attention to the fact that Nb and B were mixed and contained while reducing the content of C. As a result, the knowledge shown to the following (a)-(d) was obtained.

(a) In order to make the strength fit within the general-purpose strength range, it is possible to stably obtain 570 MPa-class strength by lowering C than the normal level and by adding B in combination.

(b) By containing Nb and B in combination, high-temperature strength can be increased. If it does not contain Nb, securing high temperature strength will become inadequate. However, a small amount of Nb may be sufficient, and if it is 0.02% or more, welding deformation can be suppressed through securing high temperature strength.

(c) Although the manufacturing method of a steel plate may be general conditions, compared with normal steel, since steel formed by combining Nb and B tends to have high hardenability, it is preferable to devise in order to make it suitable for general-purpose strength levels. .

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

On the other hand, the present inventors, flying objects yeombunryang is reviewed with respect to corrosion in many environments, under these circumstances, is an essential condition of the wet and dry repetition of the FeCl ₃ solution to corrosion, pH is lowered by hydrolysis of the Fe ^{3 +} to the state, and by the Fe ^{3 +} acts as an oxidizing agent and it found that the corrosion is accelerated.

The corrosion reaction at this time is as showing below.

As a cathode reaction, the following reaction mainly occurs.

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

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, for the reduction reaction of the Fe ^{3 +,} it takes place, the following anode reaction.

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

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

2Fe ^{3 ++} Fe → 3Fe ^{2 +} ... ... ... (1) expression

(1) reaction of Fe ^{2 +} generated by the formula is, to be oxidized to Fe ^{+ 3} by the air oxidation, the resulting Fe ^{3 +} is again acts as an oxidizing agent, accelerate corrosion. At this time, the reaction rate is low pH environment of the Fe ^{2 +} air oxidation of only generally slow, the enriched in chloride solutions is accelerated, is apt to Fe ^{+ 3} is generated. Because of this cyclic reaction, the flying object yeombunryang so many environments, Fe ^{+ 3} are always supplied continuously, it was found that Corrosion is accelerated, the corrosion resistance is remarkably deteriorated.

Based on the mechanism of corrosion in such a salty environment, the present inventors studied the influence on the weather resistance of various alloying elements and obtained the findings shown in the following (e) to (g).

(e) Sn is dissolved as Sn ^{2 +} , and 2Fe ^{3 +} + Sn ^{2 +} → 2Fe ^{2 +} + Sn ^{4 +} is added The reaction of Formula (1) is suppressed by decreasing the concentration of Fe ³⁺ by the reaction. Sn also has the effect of suppressing anode dissolution.

(f) Cu is an element which has conventionally been the basis of the corrosion resistance improvement effect in an environment having a large amount of adjuvant salt, and the corrosion resistance improvement effect is seen in an environment with a relatively wet time. However, in an environment where chloride concentrations are higher, where the pH is locally lowered, e.g., salts are attached, humidity is changed, and humidity is repeated, and in a relatively dry environment where β-FeOOH is produced, Cu rather corrodes. It turned out to be facilitating.

(g) Thus, the steel material which contains Sn actively and suppressed Cu content can expect high corrosion resistance. In addition, since corrosion resistance is high, even when coating 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 more. Therefore, in addition to corrosion resistance, the service life of a coating can be extended and it also has an effect which extends the maintenance coating interval significantly. In particular, it is effective in improving the coating peelability in the ship field or the bridge field.

This invention is completed based on said knowledge, The summary is the steel plate excellent in corrosion resistance with small welding deformation shown to following (1)-(4).

(1) A steel sheet comprising, 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% : 0.02 to 0.3%, Al: 0.003 to 0.1%, N: 0.01% or less, B: 0.0005 to 0.004% and Sn: 0.03 to 0.50%, the balance being Fe and impurities, 1 or less, wherein the metal structure contains 80% or more of bainite structure, and the bainite hardness is 150 to 250 in terms of Vickers hardness.

(2) A steel sheet having a small welding strain of (1) and excellent corrosion resistance, further containing Ti: 0.1% or less by mass%.

(3) by mass%, further comprising one or two or more of 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 Steel sheet which is small in welding deformation of said (1) or (2), and excellent in corrosion resistance.

(4) Any one of the above items (1) to (3), which further contains one or two or more of Ca: 0.004% or less, Mg: 0.002% or less and REM: 0.002% or less by mass%. Steel plate with small welding deformation and excellent corrosion resistance.

Further, when the present invention is considered in view of the small welding method of welding deformation in the steel sheet, the welding deformation in the steel sheet is substantially the welding deformation in the welding heat affected zone (HAZ), and therefore, the predetermined deformation in the weld heat affected zone. When welding is performed after satisfying the requirements, it is considered that the welding deformation suppression ability is improved.

Therefore, the present invention, from the viewpoint of the welding method,

"In mass%, C: 0.0005% or more and less than 0.02%, Si: 0.01-0.7%, Mn: 0.1-5.0%, P: 0.05% or less, Cu: less than 0.2%, S: 0.008% or less, Nb: 0.02 0.3%, Al: 0.003-0.1%, N: 0.01% or less, B: 0.0005-0.004% and Sn: 0.03-0.50%, and is a welding method of a steel sheet having a chemical composition consisting of balance Fe and impurities, and welding The metal structure of the site | part which becomes a welding heat affected zone in the previous steel plate contains the bainite structure 80% or more, and the bainite hardness is 150-250 by Vickers hardness. "

Of course, the steel sheet, in 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 further contain one kind or two or more kinds.

Here, as described later, the entire steel sheet to be the base material may be welded after it is manufactured so as to satisfy the above requirements, and only the portion to be welded (the portion to be welded heat affected portion) And may be welded after satisfying the above requirements.

And this welding method is applicable also at the time of fillet welding with large welding deformation | transformation. In addition, fillet welding is performed in a lamination joint, a T joint, a cross joint, etc., but this welding method is especially effective for the fillet welding in T joint and cross joint which generate especially large welding deformation in the relative positional relationship of a joint base material.

According to the present invention, it is possible to provide a steel sheet which can reliably suppress welding deformation at low cost and is also excellent in corrosion resistance.

1 is a FEM calculation result showing the effect of each material property value on the weld angular strain.
It is a schematic diagram which shows the calculation conditions of FEM analysis.
3 is a view showing a test piece used to evaluate the weld angular strain amount.
4 is a view showing the definition of the welding angle deformation amount.

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

(A) chemical composition of steel sheet

The effect of each component of a steel plate, and preferable content of each component are as follows. In addition, "%" regarding content means "mass%."

C: 0.0005% or more and less than 0.02%

C is the most effective element for improving strength, and is an inexpensive element. However, if it is less than 0.0005%, strength assurance by the use of other elements is required, resulting in a cost increase factor. Moreover, when it contains 0.02% or more, intensity | strength will increase too much and versatility will be lost. Therefore, content of C is made into 0.0005% or more and less than 0.02%. Preferably, it is 0.0005 to 0.02%. Moreover, the upper limit of content of C preferable from a viewpoint of strength versatility is 0.002%.

Si: 0.01 ~ 0.7%

Si is an element which contributes to strength improvement. However, if it is less than 0.01%, the required strength cannot be secured. If the content exceeds 0.7%, the base metal toughness and weldability toughness are significantly degraded. Therefore, content of Si is made into 0.01 to 0.7%.

Mn: 0.1 to 5.0%

Mn is an element necessary for securing strength. However, if it is less than 0.1%, the required strength cannot be secured. Moreover, when it contains exceeding 5.0%, weldability will deteriorate. Therefore, content of Mn is made into 0.1 to 5.0%. Since excessive addition of Mn may degrade corrosion resistance, Preferably it is 4.0% or less, More preferably, you may be 2.0% or less.

P: 0.05% or less

P is an element which exists in steel as an impurity. When P content exceeds 0.05%, it will segregate in a grain boundary and will not only reduce toughness but will cause high temperature crack at the time of welding, Therefore, content of P shall be 0.05% or less.

S: 0.008% or less

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

Cu: less than 0.2%

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

Nb: 0.02-0.3%

Nb expresses precipitation behavior in high temperature, and causes an increase in high temperature strength. However, if the content is less than 0.02%, the effect cannot be obtained. If it exceeds 0.3%, the toughness of the weld heat affected zone is remarkably impaired. Therefore, content of Nb is made into 0.02 to 0.3%. Moreover, preferable content is 0.02 to 0.18%.

Al: 0.003-0.1%

Al is an essential element for deoxidation. In order to perform deoxidation stably, content of 0.003% or more is required. However, when it exceeds 0.1%, especially toughness will deteriorate easily in a weld heat affected zone. This is considered to be because coarse cluster-shaped alumina inclusion particles easily form. Therefore, content of Al is made into 0.003 to 0.1%.

N: 0.01% or less

N is an element which exists in steel as an impurity. When content of N exceeds 0.01%, it will cause deterioration of base material toughness and toughness of a weld heat affected zone. Therefore, the upper limit of content of N is made into 0.01%.

B: 0.0005-0.004%

B has the effect of improving the hardenability and increasing the strength. In order to acquire this effect stably, content of B 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 toughness deterioration is remarkable in both the base material and the weld heat affected zone. Therefore, content of B is made into 0.0005 to 0.004%. Moreover, preferable content is 0.0005 to 0.0025%.

Sn: 0.03 to 0.50%

Sn is dissolved is an Sn ^{+ 2,} has the effect of inhibiting the corrosion inhibitor by the action of an acidic chloride solution. Further, by quickly reducing the Fe ^{3 +,} so by having the effect of reducing the Fe ^{3 +} concentration as the oxidizing agent, suppressing the corrosion promotion action of the Fe ^{3 +,} high boiling below to improve the weather resistance in saline environments. In addition, Sn has an effect of suppressing the anode dissolution reaction of steel to improve corrosion resistance. This effect is obtained by containing 0.03% or more of Sn, and when it exceeds 0.50%, it is saturated. Therefore, content of Sn is made into 0.03 to 0.50%. Preferable content of Sn is 0.03 to 0.20%.

Cu / Sn ratio: below 1

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 is also 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, 1.0 or less.

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

In addition to the above components, the steel sheet according to the present invention may, if necessary, contain at least one component selected from at least one of the following first to third groups. Hereinafter, the component which belongs to these groups is described.

Ingredients of First Group: Ti

Ti: 0.1% or less

Since Ti mainly acts as a deoxidation element, it can be contained as needed. However, since deoxidation is possible also by Al, it does not necessarily need to be contained. However, when Ti content is large, since Ti oxide or Ti-Al oxide is formed, the ability to refine | miniaturize the structure in the weld heat affected zone especially of small heat input welding loses. For this reason, Ti content in the case of making it contain is made into 0.1% or less. In addition, in order to acquire the deoxidation effect by containing Ti stably, it is preferable to make the content into 0.01% or more.

Ingredients of Group II: Ni, Cr, Mo, V, Zr

Ni: 3.5% or less

Ni is an element which improves the base metal toughness and also contributes to the strength improvement by improving the hardenability, and can be included as necessary. However, since Ni is an expensive element, excessively high content of Ni causes a large cost increase. Moreover, when coexist with Sn, corrosion resistance in chloride presence will deteriorate. For this reason, the upper limit of content of Ni in the case of making it contain is made into 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 to make the content into 0.02% or more.

Cr: 2.0% or less

Cr is an effective element for increasing the strength through improvement of the hardenability, so that it can be contained as needed. However, when it exceeds 2.0%, toughness will deteriorate. Therefore, content of Cr in the case of making it contain is made into 2.0% or less. Cr is an element that degrades corrosion resistance in a salty environment, but when coexisted with Sn, its adverse effects are remarkably suppressed. Moreover, in order to acquire the strength improvement effect by containing Cr stably, it is preferable to make the content into 0.02% or more.

Mo: 0.5% or less

Since Mo is an effective element for increasing the strength, it can be contained as necessary. However, when Mo is contained in excess of 0.5%, a significant increase in cost is caused, and the improvement in strength is also saturated. Therefore, content of Mo in the case of making it contain is made into 0.5% or less. In addition, in order to acquire the strength improvement effect by containing Mo stably, it is preferable to make the content into 0.06% or more.

V: 0.1% or less

Since V is an element effective for strength improvement, it can be contained as needed. However, when V content exceeds 0.1%, toughness will deteriorate largely, Therefore, V content in the case of making it contain is made into 0.1% or less. In addition, in order to acquire the strength improvement effect by containing V stably, it is preferable to make the content into 0.005% or more.

Zr: 0.02% or less

Zr has an effect of finely dispersing nitride in steel and improving strength, and therefore Zr can be contained as necessary. However, when it contains exceeding 0.02%, coarse precipitate will be formed and toughness will deteriorate. Therefore, content of Zr in the case of making it contain is made into 0.02% or less. In addition, in order to acquire the strength improvement effect by containing Zr stably, it is preferable to make content of Zr into 0.0003% or more.

Ingredients in Group 3: Ca, Mg, REM

Ca: 0.004% or less

Ca reacts with S in the steel to form an oxysulfide (an oxysulfide) in molten steel. Since the oxysulfide is different from elongated inclusions such as MnS and is spherical even after rolling without being stretched in the rolling direction in rolling, welding cracks or hydrogen-organic cracks that use the tip of the elongated inclusions as the starting point of the cracks. Since there is an effect of suppressing the content, it can be contained as necessary. However, when the content exceeds 0.004%, deterioration of toughness may be caused. Therefore, content of Ca in the case of making it contain is made into 0.004% or less. Moreover, in order to acquire the effect which suppresses welding crack and hydrogen organic cracking stably, it is preferable to make content of Ca into 0.0003% or more.

Mg: 0.002% or less

Mg forms an Mg-containing oxide, becomes a nucleus of TiN, and has an effect of finely dispersing TiN, so that it can be contained as necessary. However, when the content is more than 0.002%, the amount of oxide is too large to cause ductility decrease. Therefore, the upper limit of content of Mg in the case of making it contain is made into 0.002%. In addition, in order to acquire the effect of micro-dispersing TiN stably, it is preferable to make content of Mg into 0.0003% or more.

REM: 0.002% or less

Since REM contributes to the refinement | miniaturization of the structure of the weld heat affected zone, and fixation of S, it can be contained as needed. However, if the content exceeds 0.002%, the REM becomes an inclusion which adversely affects the toughness of the base material, so the content of the REM in the case of containing it is made 0.002% or less. Moreover, in order to acquire the structure refinement | miniaturization and the S fixed effect stably, it is preferable to make content of REM into 0.0003% or more. In addition, REM is a general term of 17 elements which combined Y and Sc with 15 elements of lanthanide, and can contain 1 type, or 2 or more types of these elements. In addition, content of REM means sum total content of these elements.

(B) metallographic

The bainite fraction of the metal structure is 80% or more. Although bainite is excellent in deformation resistance, in low carbon content steel, there is a lack of carbon in bainite, resulting in a low carbon bainite structure, which is slightly inferior in high carbon bainite structure. Therefore, in order to secure the deformation resistance in low carbon steel, it is necessary to secure a certain amount of bainite structure and prevent excessive ferrite. If the bainite ratio is 80% or more after the inclusion of Nb and B, sufficient weld deformation performance is obtained, so that the proportion of bainite structure is defined as 80% or more. The balance is considered to be mainly composed of ferrite, but is not particularly specified.

Moreover, even if the hardness (Vickers hardness) of bainite is too low, the deformation resistance cannot be ensured, so the lower limit thereof is set to 150. In addition, when too hard, the generality of strength is impaired, so the upper limit is made into 250.

Next, the conditions of rolling, heat treatment, etc. which can obtain the steel plate concerning this invention are demonstrated.

Prior to hot rolling, the steel ingot is first heated, but if the heating temperature at this time is at least Ac ₃ point, it can be completely austenite and homogenized in the absence of untransformed portion, so the heating temperature is at least Ac ₃ point. It is preferable to set it as. It is preferable to heat at 900-1200 degreeC specifically ,. When the rolling finish temperature of the thin end is 900 ° C or lower during hot rolling, the crystal grains become an appropriate size, and fracture toughness of the raw material is sufficient, so it is preferable to set it to 900 ° C or lower. The lower limit of the rolling finish temperature is not particularly determined and may be any condition as long as the strength can be adapted to the general-purpose strength range. In addition, when rolling finish temperature is 700 degreeC or more, since the anisotropy by a two-phase process will become inconspicuous, it is preferable to make rolling finish temperature into 700 degreeC or more. After rolling, accelerated cooling may be performed. When performing accelerated cooling, it is preferable to control the cooling rate of the center part at 0.5-40 degreeC / s immediately after rolling, or after some leaving time. The cooling stop temperature is preferably controlled based on 150 to 500 占 폚. Moreover, you may heat-process suitably after rolling. When heat processing is performed, it is preferable to perform a soaking process and a tempering process, and it is preferable to select the temperature range of 800-1100 degreeC and 300-700 degreeC, respectively.

<Examples>

An example of the steel plate which concerns on this invention is shown. The ingot of the composition component shown in Table 1 was manufactured on each heating temperature, finishing temperature, acceleration cooling, and heat processing 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, Vickers hardness, weld angular deformation amount, sheet thickness reduction amount and peeling area ratio of the steel sheet thus obtained, respectively.

In addition, in order to measure the tensile characteristic of the obtained steel plate, the test piece was extract | collected according to the test method of JIS-Z-2201. The sampling position was made into 1/4 vicinity of a plate | board thickness direction, and L direction (parallel with a rolling direction). In addition, a yield point was calculated | required at the test speed of 10 N / mm * s, and when a clear yield point does not appear, it was set as 0.2% yield strength. The target value of the tensile characteristics, the yield point YP is 350N / mm ² or more, and the tensile strength TS was 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 of JIS-Z-2202. The sampling position was made into a 2 mmV notch Charpy test piece in the quarter vicinity of a plate | board thickness direction, and L direction (parallel to a rolling direction), and measured the brittle fracture rate at various temperature, and calculated | required the transition temperature. The target value of the Charpy characteristic was that the transition temperature was 0 ° C or lower. Tissue observation was performed with an optical microscope. The image obtained by observation was image analyzed. The bainite fraction of a metal structure was calculated | required by calculating the area ratio of bainite with respect to the area of 100 field observation minutes obtained by said observation method.

In addition, the welding angle deformation amount by fillet welding was evaluated with the following method.

As shown in FIG. 3, the steel plate produced the T-shaped welding test piece, constrained one side to the high rigid steel plate, and performed the fillet welding of 1 pass on the opposite side. The used welding material was a common 50-kilogram melt-containing wire, and welding conditions were 10.4 kJ / cm (200A-26V-30cm / min). After sufficient time after welding, the test piece is placed on the platen, and the angular deformation amount θ defined in FIG. 4 is measured by a gap gauge at three places of the welding start position, the center position, and the end position, and their average values are measured. It was set as welding angle deformation amount. In addition, the weld angular deformation amount of the general-purpose 50 kilo steel measured by this method is about 1 degree, and the welding angular deformation level of the objective of this invention is 0.5 degree.

And about corrosion resistance, the test piece obtained from the obtained steel material was evaluated by SAE (Society of Automotive Engineers) J2334 test. SAE J2334 test, wet: 50 ° C., 100% RH, 6 hours, salt deposit: 0.5% NaCl, 0.1% CaCl ₂ , 0.075% NaHCO ₃ aqueous solution immersion, 0.25 hours, drying: 60 ° C., 50% RH, 17.75 hours Is an accelerated test with 1 cycle (24 hours total), and the corrosion pattern is considered to be similar to the atmospheric exposure test (Higashi Nagano, Masato Yamashita, Hitoshi Uchida: Environmental Materials Studies, Public Publications (2004), p. 74). In addition, this test simulates a harsh corrosive environment where the amount of adjuvant salt exceeds 1mdd.

After 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 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 coating peeling resistance, a modified epoxy paint (Benno 200: made in China paint) is coated to a thickness of 150 μm by a dry spray on a test piece having a size of 150 × 70 mm to reach a steel plate. After inserting a crosscut to depth, it evaluated similarly by SAE J2334 test.

As a result, in the steel plate (comparative example) of Mark 1-e, since the heat processing after cooling was set to 1150 degreeC, and was called high temperature, sufficient bainite was not produced | generated. For this reason, the welding angle deformation amount also increased. Since the steel sheet of Mark 1-e has a large weld angular strain, it is an inappropriate steel material for structural steel sheets.

In the steel sheet (comparative example) of Mark 1-f, since the cooling stop temperature was quenched to 120 degreeC and comparatively low temperature, the bainite phase hardness became hard, tensile strength became large, and toughness also worsened. For this reason, it is inappropriate steel as a structural steel plate.

In the steel plate (comparative example) of Mark 12-b, since the water cooling stop temperature was 120 degreeC and quenched to comparatively low temperature, the bainite phase hardness became hard, tensile strength became large, and toughness also deteriorated. For this reason, this steel is also an inappropriate steel as a structural steel sheet.

In the steel plate (comparative example) of Mark 38, Sn did not satisfy the composition prescribed | regulated by this invention, corrosion resistance fell, and peeling of coating also increased. For this reason, it is inappropriate as a structural steel used in a high chloride environment.

In the steel plates (comparative examples) of Mark 39-44, the steel composition prescribed | regulated by this invention was not satisfied, and the toughness of the steel plate itself fell. For this reason, it is inappropriate as a structural steel material. In particular, in the steel plate (comparative example) of Mark 39, since the Cu / Sn ratio exceeded 1, the rolling crack also generate | occur | produced. Moreover, in the steel plate (comparative example) of Mark 41, Sn did not satisfy the composition prescribed | regulated by this invention, corrosion resistance fell, and peeling of coating also increased.

On the other hand, that in the other Mark steel sheet according to the present invention example shown in, both the tensile properties, the yield point YP is 350N / mm ² or more, and the tensile strength TS is a general-purpose steel of 490 ~ 720N / mm ² class, transition It is understood that the temperature vTrs, the bainite fraction, and the Vickers hardness of the bainite phase are also in the appropriate ranges, and the weld angular strain also falls within 0.5 ° of the target, so that it is suitable as a structural steel sheet. Moreover, it also has high corrosion resistance and corrosion was seen in the crosscut part at the time of coating, but since it peels also little in any steel plate, it turns out that the maintenance interval of coating can be extended.

Industrial availability

As described above, according to the present invention, it is possible to provide a steel sheet excellent in corrosion resistance at low cost and excellent in 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 to 0.3%, Al: 0.003-0.1%, N: 0.01% or less, B: 0.0005-0.004%, and Sn: 0.03-0.50%, consisting of residual Fe and impurities, and having a Cu / Sn ratio of 1 or less A steel sheet having a composition, wherein the metal structure contains 80% or more of bainite structure, and the bainite hardness is 150 to 250 Vickers hardness, and has small weld deformation and excellent corrosion resistance. The method according to claim 1,
A steel sheet having a small welding strain and excellent corrosion resistance, further containing Ti: 0.1% or less by mass%. The method according to claim 1 or 2,
Weld deformation characterized in that it further contains one or two or more 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. This small, corrosion-resistant steel sheet. The method according to any one of claims 1 to 3,
A steel sheet having a small welding strain and excellent corrosion resistance, further comprising one or two or more of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less.

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