JP2008274379A - Steel sheet having excellent pit resistance, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet having excellent pit resistance utilizable even without being subjected to coating and electrolytic protection, and capable of exhibiting the pit resistance even when being applied to a crude oil tank. <P>SOLUTION: The steel sheet has a composition comprising 0.03 to 0.2% C, 0.05 to 0.5% Si, 0.4 to 1.8% Mn, ≤0.04% (not including 0%) P, ≤0.040% (not including 0%) S, 0.01 to 0.10% Al, 0.002 to 0.0080% N, 0.1 to 0.5% Cu and 0.1 to 0.50% Ni, and the balance Fe with inevitable impurities, and in which a concentrated region in which the total content of Cu and Ni is ≥1.2% is present on the old austenite grain boundaries from the steel sheet surface to a depth of 10 μm, and further, the area ratio in the cross-section in the sheet thickness direction of the concentrated region is ≥5%. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a steel plate used as a structural material such as a tank for transporting crude oil or a tank for storing oil, and in particular, effectively generates local corrosion (pitting corrosion or pits) generated in a tank bottom plate of a crude oil tanker or the like. The present invention relates to a steel plate that can be prevented and has excellent pit resistance that is useful as a material for crude oil tanks.

  Steel plates used as raw materials for the crude oil tank and the like are often corroded because they are exposed to salt from seawater and high temperature and humidity. Since such corrosion may cause marine accidents such as inundation and sinking, it is necessary to apply some anticorrosion means to the steel. Conventionally, (a) coating, (b) cathodic protection, and the like are well known as anticorrosion means used so far.

  Of these, in coatings represented by heavy coating, there is a high possibility that coating film defects exist, and the coating film may be damaged due to a collision or the like in the manufacturing process, so that the base steel material is often exposed. In such a steel exposed portion, the steel material corrodes locally and intensively, leading to early leakage of the petroleum-based liquid fuel contained therein.

  On the other hand, in the anti-corrosion, it is very effective for the part completely immersed in the seawater. However, in the part that receives the seawater splash in the atmosphere, the electric circuit necessary for the anticorrosion is not formed, and the anticorrosion. The effect may not be fully demonstrated. In addition, when the galvanic anode for anticorrosion disappears due to abnormal consumption or dropping, severe corrosion may immediately proceed.

  In addition to the above technique, for example, a technique such as Patent Document 1 has been proposed as a means for improving the corrosion resistance of the steel material itself. This technology discloses a corrosion-resistant steel for shipbuilding that has excellent corrosion resistance by appropriately adjusting the chemical composition of the steel material and can be used even without coating. However, in this technique, since the content of Mg is relatively large, the production stability of the steel is hindered (for example, the occurrence of clogging of the immersion nozzle at the time of casting), or when adding an alloy element There is a problem of an increase in manufacturing cost.

  Moreover, in this technique, when reducing the amount of corrosion by forming an anticorrosion film such as primer coating which is described to be used in combination, there is a problem that construction costs are required. In addition, micro-defects at the time of anti-corrosion coating are generated, and local corrosion inevitably occurs and develops mainly in areas where the coating tends to be thin, such as welded joints, so normal use Then, corrosion may progress so that it is not much different from naked use in 5 to 10 years. Furthermore, after the anticorrosion film is deteriorated, there is a problem that the rate of progress of the pit depth is not much different from that of bare use due to local corrosion (pitting corrosion: pit corrosion).

  Patent Document 2 contains Ni, Cu, and Mo as essential components, and the inner oxide layer in the vicinity of the steel sheet surface is 2 μm or less, and the concentration of Ni, Cu, and Mo is 2 μm or more on the inner oxide layer. A steel sheet having improved weather resistance and fatigue properties by forming a layer is disclosed.

  However, in this technique, in order to form the internal oxide layer or the concentrated layer as described above, the furnace temperature needs to be raised to a high temperature (for example, 1300 ° C.), and for a long time (for example, 4 to 5 hours). ) Must be maintained, a batch-type heating furnace must be used, and there is a problem that a continuous heating furnace excellent in productivity and economy cannot be applied. Moreover, even steel sheets obtained by such treatment do not necessarily exhibit excellent pit corrosion characteristics (hereinafter referred to as “pit resistance”), and further improvement in corrosion resistance can be achieved. Required.

As a material for a crude oil tank, for example, a technique as disclosed in Patent Document 3 has been proposed as a material having improved corrosion resistance. In this technique, the corrosion resistance of the raw material of the tank for storing crude oil is improved by appropriately adjusting the chemical composition. In this technique, local corrosion such as “crevice corrosion” as well as overall corrosion is taken into consideration, but it cannot always be said that good pit resistance is exhibited.
Japanese Patent Laid-Open No. 2000-17371 JP 2000-54066 A JP 2001-214236 A

  The present invention has been made paying attention to the circumstances as described above, and its purpose is excellent in pit resistance that can be put into practical use without painting or cathodic protection, and also when applied to a crude oil tank. An object of the present invention is to provide a steel plate that can exhibit high pit resistance.

  The steel sheet of the present invention that has achieved the above object is C: 0.03 to 0.2% (meaning “mass%”, the same applies to the chemical composition), Si: 0.05 to 0.5 %, Mn: 0.4 to 1.8%, P: 0.04% or less (not including 0%), S: 0.040% or less (not including 0%), Al: 0.01 to 0 .10%, N: 0.002 to 0.0080%, Cu: 0.1 to 0.5% and Ni: 0.1 to 0.50%, with the balance being Fe and inevitable impurities, and In the prior austenite grain boundary from the steel sheet surface to a depth of 10 μm, there is a concentrated region where the total content of Cu and Ni is 1.2% or more, and the area ratio in the cross section in the thickness direction of the concentrated region is 50 It has a gist in that it is at least%.

  In the steel sheet of the present invention, if necessary, (a) Ti: 0.005 to 0.05%, (b) Sn: 0.05% or less (not including 0%), Bi: 0.06% or less (Not including 0%), Mg: not more than 0.004% (not including 0%) and Co: not less than 0.5% (not including 0%), (c) Sb: 0.04% or less (excluding 0%), (d) Ca: 0.005% or less (not including 0%) and / or Zr: 0.006% or less (not including 0%), (E) Mo: 0.5% or less (not including 0%), Cr: 0.5% or less (not including 0%), W: 0.50% or less (not including 0%), Nb: 0.05% or less (not including 0%), V: 0.10% or less (not including 0%), and B: 0.005% or less (not including 0%) Least one member selected from Li Cheng group, etc. is also effective to contain, so that the characteristics of marine steel according to the type of component to be contained is further improved.

  Even when the steel plate of the present invention is used as a raw material for a crude oil transport tank or a crude oil storage tank, it exhibits excellent pit corrosion resistance in the corrosive environment.

  In manufacturing the steel sheet of the present invention, the steel sheet is held for 80 minutes or more in a heating furnace having an oxygen temperature controlled to 0.5 to 3.0% by volume and having an atmosphere temperature of 1000 ° C. or more, and the surface of the steel sheet What is necessary is just to make it take out from a heating furnace in the state whose temperature is 1000 degreeC or more.

  In the steel plate of the present invention, the chemical component composition is appropriately adjusted, and by forming an area where Cu and Ni are concentrated on the steel plate surface, the pit resistance can be put into practical use without applying coating and cathodic protection. An excellent steel plate can be realized, and such a steel plate is useful as a material for tanks for transporting and storing crude oil.

  Cu and Ni are known to be effective elements for improving the corrosion resistance. However, when added in a large amount, not only the weldability is deteriorated, but also Ni is expensive and increases the production cost. Results in. Under these circumstances, the present inventors have studied from various angles with the aim of realizing a steel sheet having excellent corrosion resistance (particularly pit resistance).

  As a result, it has been found that Cu and Ni should not be contained in a large amount in the steel sheet, but should be concentrated only in the surface layer portion of the steel sheet. Moreover, as a means for concentrating Cu and Ni in the steel sheet surface layer portion, if an appropriate temperature range and oxygen concentration at the time of heating the steel sheet are specified, the secondary scale generated during rolling can be formed in a very short time. The inventors have found that a region where Cu and Ni are well concentrated (hereinafter, sometimes referred to as “enriched region”) is formed, and the present invention has been completed.

  When heated under appropriate conditions, Cu and Ni that originally existed in the steel sheet are hardly dissolved in the scale, so as the oxidation during heating proceeds, it concentrates (concentrates) on the steel sheet surface (metal part). ) Will be. When the present inventor examined, at the time of secondary scale generation during rolling, Cu and Ni are immediately below the interface between the secondary scale part and the metal part, and the grain boundary interface of the austenite grain located on the outermost surface in the metal part It was found to concentrate. Although the concentration region is very small, it has been confirmed that it is extremely effective in remarkably improving the pit resistance.

  Since the concentration region in the present invention uses concentration at the time of secondary scale generation, it is estimated that Cu and Ni are concentrated by grain boundary diffusion with a high diffusion rate. Instead of being formed in a layer shape having a uniform thickness from the surface of the base metal part, the form of the thickening exhibits a form in which the concentrated region exists in a mesh shape. Further, since the contents of Cu and Ni are very small, they do not always exist like a film, and a portion where the continuity is interrupted is also observed. However, even in the case where Cu and Ni enriched regions are discontinuously present, since they exhibit good pit resistance, it is not particularly necessary to be present in a film form (layer form) The existence of the “concentration region” of Cu and Ni as described above is an important requirement.

  According to a study by the present inventor, even in the case of using a sample collected from the same steel plate, the maximum pit depth in the pit resistance in the sample including the surface layer part and the sample whose surface is ground (described later) The measurement method in the example) was clearly different. FIG. 1 is a bar graph showing a comparison of the influence of the form of the steel sheet on the maximum pit depth using the normal steel (steel type J) and the steel of the present invention (steel type A) shown in Table 1 below.

  As is clear from this result, the maximum pit depth is reduced to about 2/3 when the surface layer portion is present, compared to the case without the surface layer portion. When the Cu and Ni concentration of this sample (steel type A) was measured by EPMA (Electron Probe Microanalyzer), the Cu and Ni contents of the steel plate (base material) were 0.3% and 0.35%, respectively. On the other hand, the surface layer portion [excluding the outermost black skin portion (secondary scale portion)] has a region where Cu and Ni are concentrated, although the depth is only about 5 to 10 μm from the surface. It was found to exist. Further, it has been found that the Cu and Ni contents in this region are about 2 to 3 times the content of the steel sheet and about 0.7 to 1.0% in terms of concentration.

  Such a phenomenon corresponds very much to the maximum pit depth when the pit resistance is examined by increasing the Cu and Ni contents of the steel sheet, and although it is a small region of the surface layer portion, it is concentrated in the surface layer portion. It can be considered that the converted Cu and Ni greatly improve the pit resistance. As a result of measuring the composition of the black skin part (secondary scale part) with EPMA, it is mainly composed only of Fe and O and contains almost no other elements. Judging from the fact that the pit property is inferior, the secondary scale layer itself of the surface layer portion has no effect of improving the pit resistance, and the improvement factor of the pit resistance is the concentration of Cu and Ni. I was able to judge that there was.

  There are two types of scale layers formed on the steel plate surface. One of them is a primary scale formed during heating of a steel slab, and the other is called a secondary scale formed during rolling. However, since the secondary scale is generated in a very short time of a minute unit during rolling, there is a problem that it is difficult to secure a diffusion time necessary for concentrating the element. As a result of the study by the present inventors, it is possible to efficiently concentrate the element to the austenite grain boundaries in advance during heating when the primary scale is formed, so that the secondary scale is efficiently generated even in a very short time. It has been found that Cu and Ni can be concentrated.

  Usually, reheating for rolling is performed in a high-temperature heating furnace of about 1000 to 1250 ° C. for about 1 to 3 hours so that the entire steel slab is heated uniformly. At this time, if the oxygen concentration in the heating furnace is high, the formation of primary scale increases, and as a result, it causes surface flaws in the rolled product. Therefore, the oxygen concentration should be reduced to 0% for operation. However, it is said that it suppresses the generation of the primary scale and is effective in reducing surface wrinkles due to the primary scale.

  On the other hand, in the present invention, the concentration of the effective elements (Cu and Ni) discharged to the austenite crystal boundary on the bare metal side of the steel slab by the primary scale is utilized, and at the time of secondary scale generation that is generated in a very short time. To achieve effective element enrichment, primary scales must be actively formed. Since the formation of the primary scale is greatly related to the oxygen concentration in the heating furnace, as a result of detailed examination of the oxygen concentration in the heating furnace, which is effective for the concentration of effective elements and hardly generates surface defects, It has been found that by appropriately setting the heating conditions, it is possible to apply a region having a higher oxygen concentration than before (specific manufacturing conditions will be described later).

  In the steel sheet of the present invention, the presence of the concentrated region is important, but in order to effectively exhibit the effect, the contents of Cu and Ni in the concentrated region are also an important requirement. In order to exhibit good pit resistance in the steel sheet of the present invention, the total content of Cu and Ni in the concentrated region [hereinafter referred to as (Cu + Ni) content] needs to be at least 1.2% or more. There is. The (Cu + Ni) content is preferably 1.3% or more, and more preferably 1.4% or more. Regarding the (Cu + Ni) content, the pit resistance improves as the amount increases, so the upper limit is not particularly limited, but there is a limit naturally depending on the content in steel and the production conditions (Examples described later) reference).

  In addition, it is preferable that the content ratio of Cu and Ni in a concentration area | region is 2.5 or less by (Cu / Ni: mass ratio). Originally, from the viewpoint of preventing pitting corrosion in the presence of solute S, it is conceivable that Cu is concentrated alone, but when Cu is concentrated alone, hot cracking occurs. It is easy to use, and Ni also has an effect of improving corrosion resistance.

  Even if there is a concentrated region where the (Cu + Ni) content is 1.2% or more, the effect of the present invention cannot be exhibited unless the amount of formation is small. FIG. 2 shows the relationship between the area ratio of the concentrated region (the region where the Cu and Ni contents are 1.2% or more) and the maximum pit depth. As is clear from this result, it is understood that the maximum pit depth is reduced to 150 μm or less by setting the area ratio of the concentrated region to 5% or more. This area ratio is preferably 6% or more.

  In the steel plate of the present invention, basic components such as C, Si, Mn, P, S, and Al need to be appropriately adjusted in order to satisfy the basic characteristics of the steel plate. The reasons for limiting the ranges of these components will be described below together with the effects of Cu and Ni.

[C: 0.03-0.2%]
C is an element necessary for ensuring the strength of the steel sheet. In order to obtain the minimum strength (for example, yield point: 355 MPa or more, tensile strength TS: 490 MPa or more) as a structural member such as a ship, it is necessary to contain 0.03% or more. However, if the content exceeds 0.2%, the weldability, which is a characteristic required as a structural member, is deteriorated. For these reasons, the C content range was 0.03 to 0.2%. In addition, the minimum with preferable C content is 0.05%, It is good to set it as 0.07% or more more preferably. Moreover, the upper limit with preferable C content is 0.16%, It is good to set it as 0.12% or less more preferably.

[Si: 0.05 to 0.5%]
Si is a necessary element for deoxidation, and in order to exhibit a sufficient deoxidation effect, it is necessary to contain 0.05% or more. However, if the content exceeds 0.5%, the toughness deteriorates. In addition, the minimum with preferable Si content is 0.1%, It is good to set it as 0.15% or more more preferably. Moreover, the upper limit with preferable Si content is 0.45%, More preferably, it is good to set it as 0.4% or less.

[Mn: 0.4 to 1.8%]
Mn is an element that exhibits the effect of increasing the strength of the steel sheet at a low cost. In order to exhibit such an effect, it is necessary to contain 0.4% or more. However, if the content exceeds 1.8%, weldability deteriorates. In addition, the minimum with preferable Mn content is 0.5%, It is good to set it as 0.7% or more more preferably. Moreover, the upper limit with preferable Mn content is 1.6%, It is good to set it as 1.4% or less more preferably.

[P: 0.04% or less (excluding 0%)]
P is an impurity element inevitably contained in the steel and reduces weldability. In particular, when the content exceeds 0.04% and becomes excessive, the weldability deteriorates remarkably. Therefore, the P content needs to be 0.04% or less, preferably 0.03% or less, and more preferably 0.02% or less. However, since P exhibits the effect of increasing the overall corrosion resistance while lowering the weldability, it is useful to contain P at 0.005% or more.

[S: 0.040% or less (excluding 0%)]
S is an impurity element inevitably contained in the steel and needs to be reduced as much as possible. If the S content exceeds 0.040%, the weldability is lowered. For these reasons, it is necessary to suppress the S content to at least 0.040% or less, preferably 0.02% or less, more preferably 0.01% or less.

[Al: 0.01 to 0.10%]
Al is an element necessary as a deoxidizing agent, and the deoxidizing effect is not exhibited unless it is less than 0.01%. However, since an excessive content deteriorates the toughness of the steel material, the amount of Al added needs to be 0.10% or less. In addition, the minimum with preferable Al content is 0.02%, More preferably, it is good to set it as 0.03% or more. Moreover, the upper limit with preferable Al content is 0.06%, More preferably, it is good to set it as 0.05% or less.

[N: 0.002 to 0.0080%]
N is a gas component contained in the steel and is inevitably mixed. However, since N exhibits the effect of improving the corrosion resistance, it is effective to contain a small amount. In order to exhibit such an effect, it is necessary to contain 0.002% or more. However, if the N content is excessive, the toughness of the weld heat affected zone (HAZ) deteriorates, so it is necessary to make it 0.0080% or less. In addition, the minimum with preferable N content is 0.003%, It is good to set it as 0.004% or more more preferably. Moreover, the upper limit with preferable N content is 0.007%, It is good to set it as 0.006% or less more preferably.

[Cu: 0.1 to 0.5%]
Cu exhibits the effect of significantly improving the general corrosion resistance in the presence of hydrogen sulfide, and is also effective in suppressing the occurrence of pitting corrosion in the presence of S. In order to increase the pit resistance of a steel sheet applied to a crude oil tank, it is necessary to contain Cu at least 0.1% or more, and the pit corrosion resistance improves as the content increases. Cu also exhibits the effect of increasing the adhesion of the paint to the steel plate. However, when the Cu content is excessive, the steel sheet is easily embrittled (hot brittleness during rolling), so from the viewpoint of preventing embrittlement of the steel, the Cu content needs to be 0.5% or less. is there.

[Ni: 0.1 to 0.50%]
Ni has an effect of improving the overall corrosion resistance by forming a corrosion-resistant sulfide film in a wet hydrogen sulfide environment and an effect of improving the pitting resistance. Moreover, the melting point is raised by forming a complete solid solution with Cu, and the effect of preventing hot cracking which becomes a problem when Cu alone is added is also exhibited. Among these, in order to exert the effect of improving the pit resistance of the crude oil tank, Ni needs to be contained in an amount of 0.1% or more, and the pit resistance improves as the content increases. Furthermore, with such Ni content, the effect of improving the adhesiveness of the coating material of a steel plate is exhibited like Cu. However, when the Ni content is excessive, the pit resistance is further improved, but the economic efficiency is deteriorated. Therefore, the Ni content needs to be 0.50% or less.

  The basic components in the steel sheet of the present invention are as described above, and the balance is composed of iron and unavoidable impurities (for example, H, O, etc.). For example, rare earth elements are also acceptable. However, these allowable components should be suppressed to about 0.1% or less because their toughness deteriorates when the amount is excessive.

  The steel plate of the present invention may further include (a) Ti: 0.005 to 0.05%, (b) Sn: 0.05% or less (excluding 0%), Bi, depending on the necessity in addition to the above components. : 1 selected from the group consisting of 0.06% or less (not including 0%), Mg: 0.004% or less (not including 0%), and Co: 0.5% or less (not including 0%) Seeds or more, (c) Sb: 0.04% or less (excluding 0%), (d) Ca: 0.005% or less (excluding 0%) and / or Zr: 0.006% or less (0 %), (E) Mo: 0.5% or less (not including 0%), Cr: 0.5% or less (not including 0%), W: 0.50% or less (0% Nb: 0.05% or less (not including 0%), V: 0.10% or less (not including 0%), and B: 0.005% or less (0%) At least one member selected from the group consisting included not), or the like is also effective to contain, so that the characteristics of the steel sheet according to the type of component to be contained is further improved. The reasons for limiting the range when these components are contained are as follows.

[Ti: 0.005 to 0.05%]
Ti is an element effective for densifying the structure of the secondary scale (surface rust film) film formed on the surface layer portion of the steel sheet and improving the corrosion resistance. In order to achieve densification of the rust film, the Ti content is preferably 0.005% or more, but the effect is saturated even if it is excessively contained in excess of 0.05%. Therefore, the upper limit is made 0.05% or less. A more preferable range for achieving both rust film and steel sheet toughness is 0.01 to 0.045%, and more preferably 0.015 to 0.04%.

[Sn: 0.05% or less (not including 0%), Bi: 0.06% or less (not including 0%), Mg: 0.004% or less (not including 0%), and Co: 0.0. 1 or more selected from the group consisting of 5% or less (excluding 0%)]
Sn, Bi, Mg, and Co are all known as elements that improve the corrosion resistance. However, when contained in a large amount, the toughness of the steel sheet and the welded HAZ toughness are deteriorated. It is preferable. More preferable upper limit when these elements are contained is Sn: 0.045% or less (more preferably 0.040% or less), Bi: 0.055% or less (more preferably 0.05% or less), Mg : 0.0035% or less (more preferably 0.003% or less) and Co: 0.45% or less (more preferably 0.40% or less).

[Sb: 0.04% or less (excluding 0%)]
Sb is an element having a large effect of improving pit resistance. However, Sb is an element belonging to the same group as P, and when contained in a large amount, it improves pit resistance while deteriorating HAZ toughness and steel plate toughness. For these reasons, when Sb is contained, its content is preferably set to 0.04% or less. A more preferable upper limit is 0.035%, and further preferably 0.03% or less.

[Ca: 0.005% or less (not including 0%) and / or Zr: 0.006% or less (not including 0%)]
Ca and Zr are elements that exhibit the effect of suppressing the growth of corrosion pits by shifting the pH of the bottom of the corrosion pits, which is said to be a cause of the growth of corrosion pits, from alkaline to alkaline. These effects increase as the content increases, but if contained excessively, the toughness of the steel sheet is deteriorated. Therefore, it is preferable to make Ca 0.005% or less and Zr 0.006% or less. More preferably, it is 0.0045% or less (more preferably 0.004% or less) for Ca, and 0.0055% or less (more preferably 0.005% or less) for Zr. In order to exhibit the above effects of these elements, the preferable lower limit is 0.0005% or more for Ca, more preferably 0.001% or more (more preferably 0.0015% or more), and 0.0005% for Zr. Thus, more preferably 0.001% or more (more preferably 0.0015% or more).

[Mo: 0.5% or less (not including 0%), Cr: 0.5% or less (not including 0%), W: 0.50% or less (not including 0%), Nb: 0.0. 05% or less (excluding 0%), V: 0.10% or less (not including 0%), and B: 0.005% or less (not including 0%)]
Mo, Cr, W, Nb, V, and B are all elements that are basically effective in increasing the strength of the steel sheet, and are contained as necessary. Among these, Mo is an element effective in supplementing the lack of strength by increasing the strength of the steel sheet. However, when the Mo content is excessive, the toughness of the steel sheet and the welded HAZ toughness are deteriorated. More preferably, it is 0.45% or less (more preferably 0.3% or less).

Although Cr is known as an element that is effective in increasing strength and contains Cr, in severe corrosive environment of a crude oil tank bottoms, Cr ^- with dissolution of ions, derived from seawater present in minor crude oil tank bottom As a result of the interaction with Cl ⁻ ions, the pH of the bottom of the corrosion pit is lowered and the corrosion is further promoted. For these reasons, when Cr is contained, the upper limit is preferably 0.5% or less. More preferably, it is 0.45% or less, and further preferably 0.3% or less.

  W is also an element effective in making up for insufficient strength by increasing the strength of the steel sheet. However, if the W content is excessive, the toughness of the steel sheet and the welded HAZ toughness are deteriorated. More preferably, it is 0.45% or less (more preferably 0.4% or less).

  Nb is an element that is effective in increasing the strength of the steel sheet by precipitation of carbonitrides, and is an element that is very effective in refining ferrite crystal grains by increasing the non-recrystallization temperature. However, if the Nb content exceeds 0.05% and becomes excessive, the toughness of the steel sheet and HAZ will be deteriorated. More preferably, it is 0.04% or less (more preferably 0.03% or less).

  V is an element that is effective in increasing the strength due to precipitation of carbonitride, and is also an element that is very effective in refining ferrite crystal grains by increasing the non-recrystallization temperature in the same manner as Nb. However, if the V content exceeds 0.10% and becomes excessive, the toughness of the steel sheet and the HAZ will be deteriorated. More preferably it is 0.08% or less (more preferably 0.06% or less).

  B is an element that is effective in increasing the strength of the steel sheet by improving the hardenability, and is an element that is said to be a ferrite grain generation site of HAZ by forming a nitride, but is contained in excess. If done, the toughness of the steel sheet and HAZ is deteriorated, so 0.005% or less is preferable. More preferably, it is 0.004% or less (more preferably 0.003% or less).

  In the steel sheet of the present invention, Cu and Ni are utilized in the scale (primary scale and secondary scale) formed on the surface of the steel sheet, taking advantage of the fact that Cu and Ni that significantly improve pit resistance are not contained. Is concentrated at the austenite grain boundaries in the surface layer of the steel sheet, thereby improving the corrosion resistance. In order to concentrate these elements, it is necessary to utilize the discharge phenomenon (diffusion phenomenon) of elements accompanying the generation of the primary scale generated during slab heating and the secondary scale generated during rolling.

  The secondary scale generated during rolling is formed at a rolling temperature range of about 800 to 950 ° C. and is a very short time of about 5 minutes in terms of time. It is necessary to increase the initial content (average content in the steel sheet) in order to concentrate the content above a predetermined amount. However, increasing the initial content becomes a problem because it deteriorates the toughness and weldability of the steel sheet and deteriorates the economic efficiency.

In the method of the present invention, the concentration of Cu and Ni in the surface layer portion is preliminarily concentrated from the initial content by utilizing the concentration phenomenon of Cu and Ni accompanying the generation of the primary scale formed during heating. In addition, it succeeded in concentrating a predetermined amount or more of Cu and Ni in a short time when generating the secondary scale. From this point of view, it is necessary to strictly control the slab heating temperature. At the same time, the atmospheric oxygen (O ₂ ) concentration in the heating furnace is required to effectively generate the primary scale while suppressing the generation of surface flaws. Need to be strictly managed.

In the case of reheating that is generally performed, the main purpose is to heat the steel sheet to an Ac ₃ transformation point (about 850 to 910 ° C.) or higher to make the structure into austenite that is a high-temperature structure. In addition, in the present invention, in addition to this, in order to diffuse Cu and Ni, it is necessary that the heating temperature of the region (steel plate surface layer portion) be 1000 ° C. or higher and be held in that temperature region for 80 minutes or more. The higher the heating temperature at this time, the longer the holding time, the more effective the concentration of Cu and Ni. However, excessive heating temperature and holding time will cause surface defects and further production. It becomes a factor that inhibits sex. For these reasons, it is preferable that the heating temperature is 1250 ° C. or less and the holding time is 200 minutes or less. In addition, the heating furnace for heating a steel plate is usually composed of a heating zone for rapidly raising the slab from the viewpoint of productivity and a soaking zone for suppressing temperature deviation in the slab. However, when a heating furnace having such a configuration is employed, the holding time is the total time of the heating zone and the holding time in the soaking zone (see Examples below).

By simply controlling the heating temperature and holding time as described above, the object of the present invention cannot be achieved, and the atmosphere in the heating furnace is also an important requirement. That is, if the oxygen concentration (O ₂ concentration) necessary for forming the primary scale is too low, the primary scale is not effectively formed, and the concentration of Cu and Ni is difficult to proceed. Therefore, in order to effectively generate the primary scale, the O ₂ concentration in the heating furnace needs to be 0.5% by volume or more (the balance is N ₂ , for example). However, if the O ₂ concentration in the heating furnace atmosphere becomes too high, the concentration of Cu and Ni is sufficient, but a large amount of primary scale is generated, which causes manufacturing loss and product surface defects. The O ₂ concentration in the atmosphere needs to be 3.0% by volume or less.

  Although the steel sheet of the present invention basically exhibits excellent corrosion resistance even if it is not coated, it may be represented by tar epoxy resin paints shown in the examples below or other than that if necessary. It can also be used in combination with other anticorrosion methods such as heavy anticorrosion coating, zinc rich paint, shop primer, and electrocorrosion protection. Moreover, even when the steel material of the present invention is used as a raw material for a crude oil transport tank or a crude oil storage tank, it exhibits excellent corrosion resistance without causing local corrosion. The steel plate of the present invention is intended to include both thick steel plates and thin steel plates.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steel materials having the chemical composition shown in Table 1 below were melted in a converter and various slabs (slabs) were produced by continuous casting. Using the obtained slab, various steel plates were produced under the heating conditions, rolling conditions and cooling conditions shown in Table 2 below. The cooling rate shown in Table 2 below indicates the average cooling rate in the thickness direction from the rolling finish temperature to 600 ° C. when the cooling method is “air cooling”, and when the cooling method is “water cooling” The sheet thickness direction average cooling rate from the start temperature to the water cooling stop temperature (about 580 to 600 ° C.) is shown.

For each steel plate obtained, specification of the region where the Cu and Ni are concentrated (concentrated region), measurement of the area ratio of the concentrated region, occurrence of surface flaws, corrosion (pit resistance), mechanical properties of the steel plate Characteristics (yield point YP, tensile strength TS, steel sheet toughness vE _-20 ) and the like were measured by the methods shown below.

[Identify darkened area]
(A) An EPMA apparatus was used for specifying the concentrated region. As a measurement procedure, the area including the interface between the surface scale portion and the metal base portion (steel plate portion) is subjected to surface analysis at a magnification of 300 times, and Cu and Ni concentrations are different from each other at a depth of 10 μm from the steel plate surface. A region that is higher than the portion (the maximum content of the steel sheet is about 0.8% and the required concentration is 1.2%, so the strength ratio of EPMA is at least 1.5 times, and 0.8%. % Area) was identified as the thickened area. In addition, about the form of concentration of Cu and Ni, it confirmed that it existed in the prior austenite grain boundary by contrast with the optical microscope structure of the same site | part.

  (B) About each content of Cu and Ni in a concentration area | region, content of Cu and Ni of a concentration area | region was calculated on the basis of the standard sample by which content of Cu and Ni was known in EPMA. The standard sample is a sample having a small content of Cu or Ni and a sample having a large content [preferably a sample having a content of Cu and Ni of the target steel type less than a chemical analysis value (check analysis value) (0.1% Grade) and many samples (about 2%)] are usually employed. However, even if a standard sample is not used, the contents of Cu and Ni in the concentrated region can be calculated based on the surface analysis value of t / 4 part (t: plate thickness) of the steel plate.

  (C) For example, when there is a standard sample (steel plate), the surface analysis strength of the Cu and Ni contents is investigated with EPMA, and the content is specified by comparison with the standard sample. At this time, the calculation by the interpolation method is preferably performed using a graph of the relationship between the surface analysis intensity and the Cu or Ni content derived from the surface analysis intensity of the standard sample having a low Cu or Ni content and a large standard sample.

  On the other hand, when only the chemical analysis value (check analysis value) of the base material portion is known, for example, the check analysis value is 0.35%, and the area analysis strength of the concentrated region is the base material. If it is about 2.4 times the part, the content is calculated as 0.84% (0.35 × 2.4).

[Measurement of area ratio of concentrated region]
About the concentration area | region specified above, the area ratio of the concentration area | region was measured and evaluated by the image analysis method.

[Occurrence of surface flaws]
Judgment was made based on whether there were hot cracks and scale defects on the product surface.

[Corrosion (pit resistance)]
The mechanism of the generation of corrosion pits is considered to be due to solute S, and in order to verify it, a 30 × 30 (mm) sample having a plate thickness of 5 mm was cut out and evaluated.

(A) Specimen size: 30 x 30 x 5 (mm)
Pretreatment: acetone cleaning Number of times (n): 3 times (Measurement of the mass of each test material before the corrosion test in units of 0.001 g)

(B) Corrosion solution (i) 100% sulfur powder: 500 g and 8% NaCl aqueous solution: 1000 g were mixed to prepare.
(Ii) The test material (three pieces each) was placed upright on the bottom of the thermostat bath in a thermostat controlled to 30 ° C. (filled with the corrosion solution at a depth of 100 mm) (test material) The surface of 5 mm × 30 mm is in contact with the bottom surface of the thermostatic chamber), and after 7 days, the following evaluation items were evaluated for corrosivity by the procedure described below.

(C) Evaluation item (i) Appearance observation after test (ii) Mass change (corrosion rate)
(Iii) Maximum pit depth

(D) Corrosion measurement procedure (i) The mass change before and after the test was calculated, and the average reduction in corrosion on both sides of the test material was measured.
(Ii) Measure the local unevenness on both sides of the test material with a three-dimensional roughness meter, calculate the average value, and detect the pit depth (apparent depth) when the position is 0 did.
(Iii) Average corrosion reduction amount calculated from mass change (corrosion amount thickness based on the reduction amount);
The sum of the pit depths (apparent depth) detected by the roughness measuring instrument was calculated as the maximum pit depth.

[Mechanical properties of steel sheet (yield point YP, tensile strength TS, steel sheet toughness vE- ₂₀ )]
(A) JIS Z 2201 (2007 revised JIS standard) No. 1B test piece was taken in a direction perpendicular to the rolling direction of each steel plate, and subjected to a tensile test in accordance with JIS Z 2241, yield point YP and tensile strength. TS was measured. And the thing of yield point YP: 355 MPa or more and tensile strength: 490 MPa or more was evaluated as a pass.

(B) In addition, a specimen material was collected from each t / 4 (t: thickness) position of each steel plate in a direction parallel to the rolling direction (based on the surface side), and from this, JIS Z 2242 (2007 revised JIS) Standard) 3 V-notch test pieces defined in FIG. 2 and Table 2 were sampled, and Charpy impact test was conducted in accordance with JIS Z 2242. And the absorbed energy (vE- ₂₀ ) in test temperature: -20 degreeC was measured. And the thing whose average value of this absorbed energy (vE- ₂₀ ) was 100J or more was evaluated as the pass.

When evaluating the toughness of the base metal, if the plate thickness is less than 10 mm, apply the sub-size test piece specified in “2005 Steel Ship Rules K Chapter 2” (issued by the Nippon Kaiji Kyokai). The standard value of the test result (vE _-20 ) was evaluated by multiplying the value specified in Table K2.9.

  These results are collectively shown in Table 3 below, and those satisfying the requirements defined in the present invention (Experiment Nos. 1 to 9) have excellent pit resistance and mechanical properties. I understand. On the other hand, it can be understood that at least one of the characteristics is deteriorated in the requirements (experiment Nos. 10 to 29) that lack any of the requirements defined in the present invention. Experiment No. The product surface flaws in Nos. 14 and 16 are inappropriate as products due to the generation of scale flaws. The product surface defects in 18 are evaluated as “x” because they are inappropriate as products due to the occurrence of hot cracking due to Cu.

  Based on the experimental results similar to the above, the relationship between the (Cu + Ni) content and the maximum pit depth is shown in FIG. 3 (in the figure, “◯” is the content in the steel sheet, “●” is the content in the concentrated region). FIG. 4 shows the effect of heating temperature (holding temperature) and holding time on the concentration region (Cu + Ni) content (“●”, “■”, “▲”, “○” and “△” in FIG. 4). This shows the holding time, however, (Cu + Ni) content in steel sheet: 0.2%, oxygen concentration in heating furnace: 1.0% by volume), and the effect of oxygen concentration in heating furnace on the concentration region (Cu + Ni) content Is shown in FIG. 5 [in the figure, “●”, “■”, “▲” and “◯” represent the average value of (Cu + Ni) content in the steel sheet (a half value)), respectively. As is clear from these results, good pit resistance is exhibited by appropriately controlling the manufacturing conditions (heating temperature, holding time) and controlling the (Cu + Ni) content in the concentrated region to an appropriate range. You can see that.

It is the bar graph which showed the influence which the form of a steel plate has on the maximum pit depth. It is a graph which shows the relationship between the area ratio of a concentration area | region, and the maximum pit depth. It is a graph which shows the relationship between (Cu + Ni) content and the maximum pit depth. It is a graph which shows the influence which heating temperature (holding temperature) and holding time have on concentration area | region (Cu + Ni) content. It is a graph which shows the influence which oxygen concentration in a heating furnace has on concentration area | region (Cu + Ni) content.

Claims (8)

  C: 0.03 to 0.2% (meaning “mass%”, the same applies to the chemical component composition), Si: 0.05 to 0.5%, Mn: 0.4 to 1.8%, P: 0.04% or less (not including 0%), S: 0.040% or less (not including 0%), Al: 0.01 to 0.10%, N: 0.002 to 0.0080%, Cu: 0.1 to 0.5% and Ni: 0.1 to 0.50% are satisfied, the balance is Fe and unavoidable impurities, and Cu is formed at the prior austenite grain boundary from the steel sheet surface to a depth of 10 μm. In addition, there is a concentrated region in which the total content of Ni and Ni is 1.2% or more, and the area ratio in the cross section in the thickness direction of the concentrated region is 50% or more, which is excellent in pit resistance steel sheet.   The steel sheet according to claim 1, further comprising Ti: 0.005 to 0.05%.   Furthermore, Sn: 0.05% or less (not including 0%), Bi: 0.06% or less (not including 0%), Mg: 0.004% or less (not including 0%), and Co: 0 The steel sheet according to claim 1 or 2, which contains at least one selected from the group consisting of 0.5% or less (not including 0%).   Furthermore, Sb: 0.04% or less (0% is not included) The steel plate in any one of Claims 1-3.   Furthermore, it contains Ca: 0.005% or less (not including 0%) and / or Zr: 0.006% or less (not including 0%). steel sheet.   Furthermore, Mo: 0.5% or less (not including 0%), Cr: 0.5% or less (not including 0%), W: 0.50% or less (not including 0%), Nb: 0 0.05% or less (not including 0%), V: 0.10% or less (not including 0%) and B: 0.005% or less (not including 0%) The steel plate according to any one of claims 1 to 5.   The steel plate according to any one of claims 1 to 6, which is used as a raw material for a crude oil transport tank or a crude oil storage tank.   In manufacturing the steel plate according to any one of claims 1 to 7, the steel plate is placed in a heating furnace having an oxygen temperature controlled to 0.5 to 3.0% by volume of 1000 ° C or more for 80 minutes or more. A method for producing a steel sheet having excellent pit resistance, wherein the steel sheet is held and taken out of the heating furnace in a state where the surface temperature of the steel sheet is 1000 ° C. or higher.

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