JPWO2019176112A1 - Steel plate for hold of coal and ore carrier - Google Patents

Abstract

Microstructure of a ferrite pearlite steel sheet having a predetermined composition, wherein the microstructure is such that the 1/4 thick part has a ferrite area fraction of 80 to 95%, and the 1/4 thick part has a pearlite area fraction of 5 to 20%. Then, the average aspect ratio of the 1/4 thick part ferrite particles is 1.0 to 1.5, the average particle size of the 1/4 thick part ferrite particles is 5 to 20 μm, and the average in the 1/4 thick part ferrite. In the Vickers hardness test with a dislocation density of 7 × 10 12 / m 2 or less and a pitch of 1 mm, the average Vickers hardness from the front surface of the steel plate to the 1/4 thick portion or from the 3/4 thickness portion to the back surface is 1 / The maximum Sn concentration is 0.01 in the range of 80 to 105% of the average value of Vickers hardness from the 4th thickness portion to the 3 / 4th thickness portion and 1/2 thickness ± (of the thickness) 10% in the thickness direction of the thickness. ~ 5.0%, steel plate for coal and ore carrier hold.

Description

  The present invention relates to a steel plate for holding a coal or ore carrier. For example, the present invention relates to a steel sheet for holding a coal or ore carrier having a plate thickness of 5 mm or more, which can suppress the breakage of the side surface of the ship even if a ship collision occurs.

  In recent years, in the recent field of shipbuilding, even in the unlikely event of a collision between ships, the destruction (breakage) is minimized, the damage such as flooding from the damaged part is minimized, and human life and cargo are protected. Techniques for doing so are being studied.

  Among them, as an approach from the aspect of steel materials for hulls, it has been proposed to suppress the destruction of the hull by absorbing a large amount of energy at the time of collision in the steel materials themselves.

  For example, as a method for improving the energy absorption capacity at the time of collision, Patent Document 1 proposes a technique in which the structure of the steel sheet is mainly ferrite (α) and α is strengthened. This technique is characterized in that the α fraction F is 80% or more, and the hardness H of α defines a lower limit value (H ≧ 400−2.6 × F).

  Further, Patent Document 2 proposes a technique of including retained austenite (γ) in the front and back layers of a steel sheet. This technique contains C, Si, Mn, and Al, and further contains a strengthening element as necessary, and has an area ratio of 1.0 to 20% in the front and back layers of at least ⅛ of the plate thickness of the steel plate. The residual γ is included.

  In addition to these, in Patent Document 3, the fraction of ferrite (α) in the steel sheet metallographic structure is 70% or more in the central portion of the sheet thickness and 50% or more in the surface layer portion of the sheet thickness, thereby increasing the uniform elongation. , A technique for improving the collision resistance is disclosed.

  Further, in Patent Document 4, the area fraction of α occupying the entire metallographic structure of the steel sheet is 90% or more, the average α grain size is 3 to 12 μm, the maximum α grain size is 40 μm or less, and the average circle equivalent diameter of the second phase. Is set to 0.8 μm or less and the product of uniform elongation and rupture stress is increased to improve collision absorbability.

  The above-mentioned Patent Documents 1 and 2 disclose means for increasing the product of elongation and strength (EL × (YP + TS) / 2) as an index (impact absorption energy) indicating impact resistance. However, from the viewpoint of suppressing the breakage of ships when they collide with each other, it is becoming clear from the large-scale collision simulation that the growth value itself has a greater effect than the above index. In the technique of Patent Document 1, since the α particle size is 5 μm or less and the hardness of α is as high as Hv160 to 190, the elongation itself is not necessarily high, and the effect of suppressing the breakage at the time of collision cannot be expected so much.

  Further, in the technique of Patent Document 2, alloy elements are added in large amounts in order to contain residual γ in the structure, and the steel disclosed in the examples has a high carbon equivalent (Ceq) or Si. Is a high grade steel. Therefore, it is difficult to secure weldability and joint toughness, and application to actual ships is considered to be limited.

  On the other hand, in the technique of Patent Document 3, the amount of alloying elements added is kept low, and the uniform elongation is improved by controlling the α fraction, hardness, and grain size, especially in the center of the plate thickness, by two-stage cooling. However, when manufacturing a wide and long steel sheet for shipbuilding, material variations occur, and it is difficult to say that this is a practical manufacturing method.

  Patent Document 4 discloses information on the chemical composition and metal structure of steel materials, but there are many practical uncertainties in the manufacturing method. In other words, the manufacturing method described in the detailed description recommends hot rolling, reheating after cooling, but for steel plates for shipbuilding that are inexpensive and mass production is essential, processes such as reheating are not costly. Therefore, there is concern that it will be put to practical use from the viewpoint of the manufacturing period.

  Furthermore, when the hull steel material is used as a steel plate for holding a coal or ore carrier, which is placed in an environment where special corrosion resistance is required, corrosion may occur during use and the thickness may be reduced. If a steel material having a significantly reduced thickness is used, in the unlikely event that a collision occurs, there is a greater possibility that a breakage will occur as compared with the material without the reduced thickness.

  Although Patent Document 5 shows the collision safety, it does not consider the thinning due to corrosion.

  On the contrary, in Patent Document 6, although suppression of corrosion is shown, collision safety performance is not considered.

  In view of the above, with a high-strength and high-ductility thick steel plate for a coal or ore carrier hold, for example, strength and ductility that can suppress the breakage of the side surface of the ship when the ship collides, and also corrosion resistance The fact is that the technology for holding steel sheets for coal and ore carriers has not been established yet.

JP, 10-306340, A JP, 11-246935, A JP-A-2003-98841 JP, 2007-162101, A JP, 2016-125077, A JP, 2008-174768, A

  When used as a steel plate for a coal or ore carrier hold, the corrosive environment becomes severe due to the following reasons.

  In the hold of a coal or ore carrier, since the solid materials such as coal and ore are stored and transported, the coating film is easily mechanically damaged. If it is peeled off on the entire surface, the steel material is directly exposed to the corrosive environment.

Further, the corrosive environment in the hold of the coal or ore carrier is a wet environment containing SO ₄ ²⁻ and Cl ⁻ or a repeated dry-wet environment, and the pH value of dew condensation water is reduced to about 2 to 3 by SO ₄ ^2−. . And, immediately below the scratched portion of the coating film is a rich chloride environment, and the pH value is significantly lowered. Therefore, in the hold of the coal / ore carrier, the coating film is not only mechanically damaged, but also susceptible to both SO ₄ ²⁻ and Cl ⁻ .

  In this way, the coal or ore carrier hold is in a severe corrosive environment, and even if the steel plate at the start of use has excellent collision safety, if the corrosion thinning is significant during the course of long-term use, collision safety Sex decreases.

In view of the above circumstances, the present invention provides a steel sheet for holding a coal or ore carrier, for example, a steel sheet for holding a coal or ore carrier that has strength and ductility capable of suppressing breakage of a side surface of a ship during a collision. In addition to that, even in a corrosive environment in which the coating film is easily mechanically damaged and is susceptible to both SO ₄ ²⁻ and Cl ⁻ , the life of the coating film is extended and the coating film is peeled off. An object of the present invention is to provide a steel sheet for holding a coal or ore carrier, which can prevent corrosion and can prevent rolling cracks from occurring in a usual manufacturing method.

  Let us consider the mechanism by which a fracture occurs on the side surface of a ship when the ship collides. For example, when the bow of another ship collides with the side wall of the ship, the entire bow of the ship is embedded in the flat steel plate of the side wall of the ship, so the steel plate of the side wall of the ship undergoes a large bending deformation and It is stretched to and greatly pulled. When the steel plate is broken, the steel plate on the side wall of the ship is broken.

  Therefore, in order to prevent breakage of the steel plate on the side surface of the ship when the ship collides, the steel plate must be able to withstand bending when it is largely bent at the initial stage of the collision, and The unstretched portion is greatly stretched and causes tensile deformation, but it is necessary that the portion does not stretch and break.

  It is essentially important to increase the elongation of the steel plate in order to suppress the breakage of the side surface of the ship when the ship collides, but generally, if the strength of the steel plate is improved, the elongation of the steel plate deteriorates. There is a demand for a high-strength and high-ductility thick steel plate that achieves both high elongation and high elongation.

  The present inventor, in order to obtain a collision-resistant steel plate for a coal or ore carrier hold, particularly a coal or ore carrier hold steel plate capable of suppressing the breakage of the side surface of the ship at the time of collision, the composition of the steel plate and As a result of conducting research focusing on the microstructure, it is a ferrite + pearlite steel that easily suppresses fluctuations in strength and elongation in the steel sheet, and ductility due to the presence of ferrite (hereinafter also referred to as “α”) as a microstructure. Coal and ore having both strength, ductility and corrosion resistance by improving the strength and strength of the second phase, pearlite, and controlling the maximum Sn concentration in the plate thickness and the hardness in the plate thickness direction. The present invention has been completed by finding that a steel sheet for holding a carrier can be obtained.

The gist of the present invention is as follows.
[1] In mass%,
C: 0.05 to 0.20%,
Si: 0.2 to 1.0%,
Mn: 0.5-2.0%,
Nb: 0.003 to 0.030%,
Ti: 0.003 to 0.020%,
Al: 0.002-0.050%,
Sn: 0.010 to 0.30%,
N: 0.0010 to 0.0050%,
O: 0.0005 to 0.0050%,
Ca: 0 to 0.0080%,
Mg: 0 to 0.0080%,
REM: 0 to 0.0080%,
Ca + Mg + REM: 0.0005 to 0.0080%,
P: 0.008% or less,
S: 0.003% or less,
Cu: 0 to 0.05%,
Ni: 0 to 1.0%,
Cr: 0 to less than 0.10%,
Mo: 0-0.5%,
V: 0 to 0.050%,
Co: 0 to 1.0%,
B: 0 to 0.0030%,
Ti / N: 0.5-4.0,
Remainder: Fe and impurities,
And
When observing the cross section in the rolling direction, the microstructure
Ferrite area fraction of 1/4 thick part: 80 to 95%,
Perlite area fraction of 1/4 thick part: 5 to 20%,
Bainite area fraction of 1/4 thick part: 0 to less than 10%,
Average aspect ratio of 1/4 thick part ferrite grains: 1.0 to 1.5,
Average particle size of ferrite particles in 1/4 thick part: 5 to 20 μm,
Average dislocation density in 1/4 thick part of ferrite: 7 × 10 ¹² / m ² or less,
And
In the Vickers hardness test of 1 mm pitch,
The average Vickers hardness from the front surface of the steel sheet to the 1/4 thick portion and from the 3/4 thick portion to the back surface is 80 to 105% of the average Vickers hardness value from the 1/4 thick portion to the 3/4 thick portion. ,
The maximum Sn concentration is 0.01 to 5.0% within a range of ½ thickness ± 10% (of the plate thickness) of the plate thickness;
Steel plate for hold of coal or ore carrier.
[2] The maximum concentration of P is 0.02 to 0.20% within a range of ½ thickness ± (of plate thickness) 10% in the thickness direction of the plate thickness;
The steel plate for holding a coal or ore carrier according to the present invention, characterized in that
[3] Ar ₃ represented by the following formula (1) is 760 to 820 ° C., the steel plate for holding a coal or ore carrier according to the present invention.
_{Ar 3 = 910-310 [C] +65} [Si] -80 [Mn] -20 [Cu]
-55 [Ni] -15 [Cr] -80 [Mo] (1)
However, the element symbol represents the content (mass%) of each element. The elements not contained are 0%.
[4] The steel plate for holding a coal or ore carrier according to the present invention, which has a plate thickness of 5 to 50 mm.
[5] The steel sheet for holding a coal or ore carrier of the present invention, which has a tensile strength (TS) of 400 to 650 N / mm ² .

  By using the steel plate for coal / ore carrier hold of the present invention for the coal / ore carrier hold, even if a collision accident occurs between ships, the possibility that the steel plate of the ship will break and break Therefore, it is possible to reduce the amount of water flooded from the collision damage part in the event of a ship accident, and by avoiding sinking, it is possible to protect human life and cargo and reduce the possibility of marine pollution due to spillage of fuel oil. Also, it has a remarkable effect from the viewpoint of safety.

  Further, the steel sheet for hold of a coal or ore carrier of the present invention is also excellent in corrosion resistance, and therefore, when used for a hold of a coal or ore carrier, it has a particularly remarkable effect that good collision safety can be obtained even after long-term use. Play.

  In addition, the method for manufacturing a steel sheet for holding a coal or ore carrier of the present invention does not require significant modification to the conventional manufacturing equipment, and can prevent the occurrence of rolling cracks only by adding a device to a normal manufacturing process. Has a remarkable effect.

It is a flowchart of the cycle test which simulated the corrosion in the ore carrier hold.

  The present invention will be described in detail below.

  In order to suppress the breakage of the side surface of the ship when the ship collides, it is essentially important to increase the elongation of the steel plate on the side surface of the ship. Elongation can be divided into uniform elongation and local elongation, but these governing factors are different and it is usually difficult to achieve both at the same time. That is, the uniform elongation can be increased by increasing the hardness of the second phase in addition to improving the ductility of α itself, and it is generally advantageous to form a composite structure. On the other hand, it is advantageous for the local elongation to have a uniform structure such as uniform hardness distribution and fine dispersion of the second phase and inclusions. From the viewpoint of preventing the destruction when the structures collide, it is desirable to improve both of them in a well-balanced manner rather than to improve the elongation of either one.

Since the elongation of the steel plate is about 1.4 times that of general steel, the impact absorption energy until a hole is opened is about 3 times that of a collision from the side of a ship, which is more than that of conventional steel materials. Since it is known that the hull is less likely to have holes, the elongation target in the present invention is 24% or more, which is about 1.4 times that of general steel. However, the tensile test piece was a No. 1B test piece (width (W): 25 mm, original gauge length (GL): 20 mm) of JIS Z2241: 2011. Further, as other characteristics, the yield stress (YP) was 355 to 500 N / mm ² , the tensile strength (TS) was 490 to 620 N / mm ² , and the steel plate thickness (t) was 5 to 50 mm as the target values. .

The target value of this strength is, for example, “Material Code KA36, KD36, KE36, KF36” (YP36 standard: Yield point or proof stress 355 N / mm ² (MPa) or more in “Steel Vessel Regulations, K Edition Materials” established by the Japan Maritime Association It corresponds to a tensile strength of 490 to 620 N / mm ² (MPa)).

  As a steel sheet for holding a coal or ore carrier that can achieve such a target value, the present inventors have assumed that ferrite + pearlite steel, which easily suppresses fluctuations in strength and elongation in the steel sheet, as a prerequisite for improving ductility of ferrite and Based on the guideline of improving the strength by the two-phase pearlite, a detailed investigation was conducted on the influence of the chemical composition of the steel sheet and the manufacturing conditions, and the following was found.

  In order to improve the ductility of α, it is necessary to increase the cleanliness of α as much as possible. However, since it is necessary to secure the strength of the steel sheet, C forming pearlite and Si, Mn, etc., which are substitutional solid solution elements, have to be added in a certain amount.

The present inventor has found out the following points.
Elements such as Nb and Ti that form precipitates in α are added to the minimum necessary amount, and N, which is an interstitial solid solution and significantly increases the yield stress, and P and S, which are impurity elements, are added as much as possible. It is effective to reduce.

  Effective addition of Ca, Mg, and REM (rare earth elements such as La and Ce) or a combination thereof to form sulfides containing them and suppress the formation of coarse inclusions (stretched MnS, etc.) is effective in improving elongation. Is.

If the dislocation density in α becomes high, it easily proliferates due to plastic deformation to harden α and reduce elongation, so the dislocation density is reduced.

Similarly, if Sn, which is necessary for improving the corrosion resistance, is segregated in the center of the plate thickness, an embrittlement zone is formed to cause cracks and the elongation is deteriorated. Therefore, it is necessary to build Sn to reduce the maximum Sn concentration. We also found that it was necessary.
Although the strength can be improved by dispersing pearlite, which is the second phase, in order to suppress the breakage of the side surface of the ship when the ship collides, the structure in the steel plate thickness direction is made uniform and the steel plate It was found that there is an effect in making the hardness distribution in the thickness direction uniform.

In the present invention, based on these findings, the steel composition and microstructure of the collision-resistant steel plate for hold of a coal / ore carrier are determined.
First, the reasons for limiting the steel components of the steel sheet of the present invention will be described. In addition, all "%" regarding a component means the mass%.

(C: 0.05 to 0.20%)
C is an element indispensable for forming pearlite and increasing the strength, so C is contained in an amount of 0.05% or more. On the other hand, if the C content increases, it becomes difficult to secure weldability and joint toughness, so 0.20% is made the upper limit. The C content is preferably 0.10% or more and 0.16% or less.

(Si: 0.2-1.0%)
Si is an inexpensive deoxidizing element, is effective in solid solution strengthening, and raises the transformation point to contribute to the reduction of dislocation density in α, so Si is contained by 0.2% or more. On the other hand, if the Si content exceeds 1.0%, the weldability and joint toughness deteriorate, so the upper limit is made 1.0%. The amount of Si is preferably 0.3% or more and 0.5% or less.

(Mn: 0.5-2.0%)
Mn is effective as an element that improves the strength and toughness of the base material, and is therefore contained in an amount of 0.5% or more. On the other hand, if Mn is excessively contained, the joint toughness and the weld cracking property are deteriorated, so 2.0% is made the upper limit. The amount of Mn is preferably 0.8% or more and 1.6% or less, and more preferably 0.9% or more and 1.5% or less.

(Nb: 0.003 to 0.030%)
Nb contributes to the refinement of the structure by the addition of a trace amount, and is an element particularly effective for improving the ductility of the high strength steel such as YP36 and ensuring the strength of the base metal, so Nb is contained at 0.003% or more. If Nb is contained in excess of 0.030%, the weld zone is hardened and the toughness is significantly deteriorated, so 0.030% is made the upper limit.

(Ti: 0.003 to 0.020%)
Since Ti contributes to the improvement of ductility and toughness through the refinement of the structure of the base material and the welded portion by the addition of a trace amount, it is contained in 0.003% or more. On the other hand, if added excessively, the welded part is hardened and the toughness is remarkably deteriorated, so 0.020% is made the upper limit. The amount of Ti is preferably 0.006 to 0.013%.

(Al: 0.002-0.050%)
Since Al is an important deoxidizing element, it is contained in an amount of 0.002% or more. On the other hand, when Al is contained excessively, the surface quality of the steel slab is impaired and inclusions harmful to toughness are formed, so the upper limit is 0.050%. The amount of Al is preferably 0.002 to 0.040%, more preferably 0.010 to 0.040%.

(Sn: 0.010 to 0.30%)
When Sn is contained as an alloying element, not only the corrosion resistance of the coated part is significantly improved, but also the coating film is mechanically damaged to contain and transport solid substances such as coal and ore, and the coating film is peeled off to leave it naked. The corrosion resistance after becoming steel is also significantly improved. This is because Sn dissolves and precipitates on the steel material in an environment where the pH inside the coal or ore carrier hold decreases, but since Sn is an element with a large hydrogen overvoltage, the part where Sn precipitates is in a low pH environment. The hydrogen generation reaction, which is a cathode reaction, is significantly suppressed, and as a result, the corrosion resistance is improved. Further, even when Sn is present as an ion, it has an effect of suppressing an anode reaction which is a dissolution reaction of a steel material. This is because the action of Sn ions suppresses the adsorption of OH ⁻ and Cl ^{− on} the iron surface, which serves as a dissolution route of iron, and suppresses the dissolution itself of iron. In order to obtain these effects, the content of 0.010% or more is necessary. However, if the content exceeds 0.30%, the above effects are not only saturated, but also elongation and toughness are significantly deteriorated. Inspire. Therefore, the content is set to 0.010 to 0.30%. It is preferably 0.02 to 0.25%.

(N: 0.0010 to 0.0050%)
N forms a nitride together with Al and improves the joint toughness, so the lower limit of the content is made 0.0010% or more, preferably 0.002% or more. On the other hand, if the N content is excessive, embrittlement and elongation decrease due to solid solution N occur, so the upper limit is made 0.0050%. Preferably, it is 0.0040% or less.

(O: 0.0005 to 0.0050%)
O forms an oxide together with Mg, Ca and REM. If it exceeds 0.0050%, the oxide becomes coarse and the elongation and toughness decrease, so the content is made 0.0050% or less. On the other hand, the smaller the amount of O, the better, but in order to reduce the amount of O, for example, the reflux work in the RH vacuum degassing apparatus takes a long time and is not realistic, so the content is made 0.0005% or more. Here, O is total oxygen (TO).

(Ca: 0 to 0.0080%, Mg: 0 to 0.0080%, REM: 0 to 0.0080%, Ca + Mg + REM: 0.0005 to 0.0080%,)
Ca, Mg, and REM are all important elements that suppress the formation of coarse inclusions (stretched MnS, etc.) by forming sulfides. Since these elements have the same effect, the individual contents are not limited, but the total of the Ca content, the Mg content and the REM content needs to be 0.0005 to 0.0080%. If the total of these contents, that is, Ca + Mg + REM is less than 0.0005%, the effect of improving elongation cannot be stably obtained. On the other hand, if the content exceeds 0.0080% and is excessively contained, the effect is saturated, and coarse acids and sulfides are formed to deteriorate toughness and elongation. Therefore, the total content of these is set to 0.0005 to 0.0080%, preferably 0.0010 to 0.0060%, and more preferably 0.0015 to 0.0040%. Note that Ca, Mg, and REM each have an individual content of 0 to 0.008% (5 to 80 ppm), but the content of at least one element is 0.0005 to 0.008% ( 5-80 ppm) is preferable.

(P: 0.008% or less, S: 0.003% or less)
P and S are unavoidable impurities, and here, in particular, Sn, which is an alloy undesired in terms of elongation and toughness, is intentionally contained. Therefore, in order to secure these characteristics, P The lower the content of S and the lower the content of S, the better. Therefore, P is 0.008% and S is 0.003%.

(Ti / N is 0.5 to 4.0)
The Ti / N ratio of 0.5 to 4.0 is for fixing Ti with N to suppress the generation of TiC that causes deterioration of elongation. In this case, the amount of N increases, which causes solid solution N to deteriorate elongation and further causes surface defects of the slab. On the other hand, when it exceeds 4.0, TiC is generated and the elongation is deteriorated. Therefore, Ti / N is set to 0.5 to 4.0.

The above elements are essential components or components that are inevitably contained. Next, the optional additional element will be described.
Further, in order to secure strength, as selective elements, Cu: 0 to 0.05%, Ni: 0 to 1.0%, Cr: 0 to less than 0.10%, Mo: 0 to 0.5%, You may contain 1 type (s) or 2 or more types in the group of V: 0-0.050%, Co: 0-1.0%, B: 0-0.0030%.

  Cu is effective in improving hardenability and strengthening, but a steel material containing excessive Cu is likely to be cracked during rolling in the manufacturing process thereof, and particularly, if it coexists with Sn, has a higher cracking susceptibility. Therefore, the upper limit is set to 0.05%. Preferably, the Cu content is less than 0.01%.

  In particular, Ni is an element that is effective in securing strength and improving toughness, and also improves corrosion resistance in an acidic environment, and has the effect of suppressing corrosion by both the corrosion resistance of the base material and the corrosion resistance of rust. However, when Ni is contained by more than 1.0%, the precipitation of Sn is suppressed, so that the effect of improving corrosion resistance by Sn is reduced. Therefore, the Ni content is 1.0% or less. It is preferably 0.01 to 1.0%, more preferably 0.05% or more, but even if it is less than that, the effect of the present invention is not impaired.

  Cr is an element that improves hardenability and enhances strength, but is an element that lowers corrosion resistance in a low pH environment, so Cr is set to less than 0.10%. It is more preferably less than 0.04%, and even more preferably less than 0.02%.

  Mo improves the hardenability and is effective for increasing the strength, and also has the effect of improving the corrosion resistance of bare steel and the corrosion resistance of the painted part, but if it is contained excessively, the hardness of the joint increases and the toughness decreases. Therefore, the content is preferably 0.5% or less. In order to obtain the effect, it is preferable to contain 0.01% or more, but even if it is less than that, the effect of the present invention is not impaired.

  V contributes to an increase in strength due to precipitation strengthening, so V is preferably contained in an amount of 0.050% or less. If the content of V exceeds 0.050%, the joint toughness may be impaired, so 0.050% is the upper limit. In order to obtain the effect of V addition, it is preferable to contain 0.010% or more, but even if it is less than that, the effect of the present invention is not impaired.

  In the present invention, Sn is contained to improve the corrosion resistance, but Co may be contained in addition to Sn to further improve the corrosion resistance. Co is an element that improves corrosion resistance in an acidic environment, and has an effect of suppressing corrosion by both the corrosion resistance of the base material and the corrosion resistance of rust. However, when Co is contained more than 1.0%, the precipitation of Sn is suppressed, so that the corrosion resistance improving effect of Sn is reduced. Further, the ferrite is hardened to reduce the elongation. It is preferably 0.01 to 1.0%.

Addition of a small amount of B enhances the hardenability and contributes to the improvement of the base metal strength, so B is preferably contained in an amount of 0.0030% or less. If added in excess of 0.0030%, the elongation and joint toughness deteriorate. In order to obtain the effect of B addition, it is preferable to contain 0.0003% or more, but even if it is less than that, the effect of the present invention is not impaired.
The lower limit of these selective elements may be 0%.
The balance of the chemical composition described above is Fe and inevitable impurities.

  Next, the reasons for limiting the microstructure of the steel sheet of the present invention will be described. The following microstructure is a numerical value when observing the cross section perpendicular to the rolling direction. The cross section perpendicular to the rolling direction is a plane that is perpendicular to the rolling direction and is also perpendicular to the steel sheet surface.

(Ferrite area fraction of 1/4 thick portion is 80 to 95%, pearlite area fraction of 1/4 thick portion is 5 to 20%, bainite area fraction of 1/4 thick portion: less than 0 to 10%)
As the ferrite (α) area fraction becomes higher, the uniform elongation property is improved, and when the α area fraction is 80% or more, the elongation property is rapidly improved. The structure changes somewhat in the plate thickness direction, but the ferrite area fraction of the 1/4 thick portion is required to be 80% or more in order to secure sufficient elongation. On the other hand, if it exceeds 95%, the strength cannot be secured, so the ferrite area fraction of the 1/4 thick portion is set to 80 to 95%. This 1/4 thick part is a region in which the cooling rate is relatively faster than the central part of the plate thickness during cooling, a hard phase is likely to be generated, and uniform elongation is likely to deteriorate. When considering the entire plate thickness, it is necessary to consider the characteristic difference from the central part of the plate thickness, so the ferrite area fraction of the 1/4 thick part was limited to 80 to 95%, but 85 to 90% is more preferable. preferable.

  In addition, the yield point or yield strength (YP) and tensile strength (TS), which are strength characteristics, generally have a trade-off relationship with the elongation characteristic EL, and it is generally difficult to improve both at the same time. Although the elongation property is improved by increasing the ferrite area fraction, the tensile strength decreases as the elongation increases, so there is a limit to securing the strength property only by increasing the ferrite area fraction.

  Therefore, in the present invention, the pearlite area fraction of the 1/4 thick portion is 5% or more in order to secure the elongation property and the yield point or proof stress (YP) and tensile strength (TS) which are strength properties. And However, if it exceeds 20%, the elongation cannot be secured, so the upper limit was made 20%. Preferably, it is 10 to 15%.

  The total of the ferrite area fraction and the pearlite area fraction of the 1/4 thick portion is preferably 90% or more, and even if less than 10% bainite is present, the effect of the present invention is not impaired. Absent. That is, the lower limit of the bainite area fraction is 0% and the upper limit thereof is less than 10%. The ferrite area fraction and the pearlite area fraction are obtained by photographing the microstructure with an optical microscope at a magnification of 500 times and determining the area fraction of each phase by image analysis. All the remaining structures other than ferrite and pearlite are bainite, and there is no structure other than ferrite, pearlite, and bainite. The term "absent" as used herein means that its presence cannot be confirmed as long as it is observed with an optical microscope.

(Average aspect ratio of 1/4 thick ferrite grains is 1.0 to 1.5)
The smaller the average aspect ratio of the ferrite grains in the 1/4 thick part is, the better. If it exceeds 1.5, the dislocation density becomes high and the elongation deteriorates, so the upper limit was made 1.5. Further, the lower limit was set to 1.0 at which the ferrite grains became spherical.

(Average grain size of ferrite grains in 1/4 thick part is 5 to 20 μm)
If the average grain size of the ferrite grains in the 1/4 thick portion exceeds 20 μm, the strength cannot be ensured, so the upper limit was made 20 μm. Further, finer ferrite grains are more preferable, but it is difficult to industrially realize ferrite grains having a grain size of less than 5 μm, so the lower limit was made 5 μm. The average grain size limited here can be extracted from, for example, a photograph of an optical microscope structure of 250 μm × 200 μm × 5 fields of view taken at a magnification of 500. The ferrite grain size can be obtained as an average circle equivalent diameter of crystal grains obtained by simply averaging the circle equivalent diameters converted from the area of each crystal grain from such a structure photograph.

(The average dislocation density in the ferrite of 1/4 thick part is 7 × 10 ¹² / m ² or less)
In order to secure the elongation, the average dislocation density in ferrite (α) needs to be 7 × 10 ¹² / m ² or less. When the dislocation density is more than 7 × 10 ¹² / m ² , dislocations are prominently proliferated due to plastic deformation of the steel sheet and the ferrite (α) becomes hard, and sufficient total elongation (T.EL%) cannot be obtained. The lower the dislocation density, the better, but it is rarely lower than 1 × 10 ¹² / m ² . The preferable upper limit of the average dislocation density is 6 × 10 ¹² / m ² .

(In the 1 mm pitch Vickers hardness test, the average Vickers hardness values from the front surface of the steel plate to the 1/4 thick portion and from the 3/4 thickness portion to the back surface are 1/4 thick portion to 3/4 thick portion. Up to 80-105% of the average Vickers hardness)
When a thick steel plate is cooled, the front and back layer portions of the plate thickness have a relatively higher cooling rate than the central portion of the plate thickness and are easily hardened. If the hardness near the surface layer portion is too large, the elongation is deteriorated. Considering the elongation characteristics of the entire plate thickness, the effect of hardening the front and back layers of the plate thickness can be tolerated to some extent, but the effect cannot be ignored if the difference in hardness between the front and back layers of the plate thickness and the center of the plate thickness becomes large. Come on. Therefore, in the 1 mm pitch Vickers hardness test, the average value of the Vickers hardness (Hv) of the plate thickness front and back layer portions (from the front surface of the steel plate to the 1/4 thickness portion and from the 3/4 thickness portion to the back surface) is calculated. It is necessary that the average value of the Vickers hardness (Hv) at the center part of the plate thickness (from the 1/4 thick part to the 3/4 thick part of the plate thickness) is 80 to 105%. In order to secure the elongation, it is better to suppress the hardness of the front and back layers of the plate thickness, and the industrially possible lower limit of the Vickers hardness of the front and back layers of the plate thickness is the average Vickers hardness at the center of the plate thickness. 80% of the value. If it exceeds 105%, it becomes difficult to secure the elongation. Therefore, the average value of Vickers hardness of the front and back layer portions of the plate thickness / (the average value of Vickers hardness of the central portion of the plate thickness) is set to 80 to 105%. Note that the Vickers hardness is HV10 of JIS Z 2244 (that is, the Vickers hardness of the test force of 98.07N).

(1/2 thickness of the plate thickness in the thickness direction ±, the maximum concentration of Sn is 0.01 to 5.0% within a range of 10% (of the plate thickness))
Since Sn segregates in the center during continuous casting to form an embrittlement region in the center of the plate thickness and causes cracking to deteriorate local elongation, it is preferable that the maximum concentration of Sn is small. The upper limit of the maximum concentration of Sn is to secure the elongation, the maximum concentration of Sn in the central part of the plate thickness (meaning a range of ½ thickness ± 10% of the plate thickness in the thickness direction of the plate thickness) 5.0% or less is required. More preferably, it is 0.01 to 1.0%. Since the lower limit of the Sn addition concentration is 0.01%, the lower limit of the Sn concentration at the center of the plate thickness is naturally 0.01%.

  Sn improves corrosion resistance and suppresses metal thinning even after long-term use, resulting in improved collision safety performance. However, simply adding Sn causes a large amount of segregation particularly in the central portion of the plate thickness and is harmful to the elongation. Therefore, it is not easy to use Sn in view of the collision safety performance as in the present case. By suppressing the segregation, Sn is relatively uniformly distributed at an appropriate concentration over the entire plate thickness, so that the corrosion resistance on the surface of the steel plate is further improved and the reduction in plate thickness due to the corrosion reaction during long-term use is further suppressed. . Desirably, the value obtained by dividing the maximum Sn concentration at the center of the plate thickness by the average Sn concentration 1 mm below the plate surface is 60 or less.

  The maximum concentration of Sn is in the range of ± 10% (of the plate thickness) of the center part of the plate thickness where center segregation easily occurs, for example, if the plate thickness is 10 mm, the central part of the plate thickness is 20% (± 10%) angle, that is, For a 2 mm (± 1 mm) square, an accelerating voltage: 15 kV, a beam diameter: 20 μm, an irradiation time: 20 ms, a measuring pitch: 20 μm, and a measuring range of the above 2 mm square are measured by an EPMA (Electron Probe MicroAnalyzer). It is the maximum value of the Sn concentration when measured.

(Maximum concentration of P in the thickness direction ½ thickness ± (% of plate thickness) 10% range is 0.02 to 0.20%)
Since P segregates at the center during continuous casting to form an embrittlement region at the center of the plate thickness and causes cracking to deteriorate local elongation, it is preferable that the maximum concentration of P is small. The upper limit of the maximum concentration of P is not particularly specified, but in order to secure the elongation, it means the central portion of the plate thickness (1/2 thickness of the plate thickness in the thickness direction ± (of the plate thickness) 10% range). It is preferable that the maximum concentration of P in (1) is 0.20% or less. Further, since it is practically difficult to set the maximum concentration of P to less than 0.02%, the lower limit is 0.02%, and the preferable range is 0.02 to 0.20%.

  The maximum concentration of P is within the range of ± 10% (of the plate thickness) of the center part of the plate thickness where center segregation easily occurs, for example, if the plate thickness is 10 mm, the center part of the plate thickness is 20% (± 10%) angle, that is, For a 2 mm (± 1 mm) square, an accelerating voltage: 15 kV, a beam diameter: 20 μm, an irradiation time: 20 ms, a measuring pitch: 20 μm, and a measuring range of the above 2 mm square are measured by an EPMA (Electron Probe MicroAnalyzer). It is the maximum value of the concentration of P when measured.

(Ferrite transformation starting temperature Ar ₃ during cooling is 760 to 820 ° C.)
Regarding the ferrite transformation start temperature Ar ₃ when cooling the steel, the higher the Ar ₃ as the steel composition, the higher the temperature of the ferrite transformation, so that the dislocation density in the ferrite grains decreases and the elongation improves. Therefore, it is preferable that the Ar _{3 of} the steel is large, but if it exceeds 820 ° C. and is too large, ferrite coarsens and the strength decreases, so the upper limit is preferably 820 ° C. On the other hand, if Ar ₃ is too low, bainite is formed and elongation is deteriorated, so 760 ° C. is preferable as the lower limit.
The ferrite transformation start temperature Ar _{3 at} the time of cooling is represented by the following known formula (1).

_{Ar 3 = 910-310 [C] +65} [Si] -80 [Mn] -20 [Cu]
-55 [Ni] -15 [Cr] -80 [Mo] (1)
However, the element symbol represents the content (mass%) of each element. The elements not contained are 0%.

  Next, suitable manufacturing conditions for the steel sheet according to the present invention will be described.

  First, as a casting pretreatment, after performing primary refining to remove carbon from molten steel, when adjusting the composition of molten steel, the dissolved oxygen content of molten steel is preferably 65 ppm or less, more preferably 40 ppm or less by vacuum degassing treatment. adjust. In order to adjust the dissolved oxygen amount of the molten steel to 40 ppm or less, for example, the degree of vacuum of the RH vacuum degassing device is 1 to 5 torr (133 to 667 Pa), and the molten steel is refluxed for 1 to 3 minutes for adjustment. It is preferable to add Al to the molten steel so that the final content of Al is 0.002 to 0.050% after the dissolved oxygen amount of the molten steel becomes 40 ppm or less. If the dissolved oxygen amount in the molten steel is 40 ppm or less, by adding Al as a deoxidizing agent and performing the reflux operation in the RH vacuum degassing device, the final dissolved oxygen amount in the molten steel is 16 ppm or less, particularly 10 ppm or less. Can be adjusted to. Further, the smaller the dissolved oxygen amount, the better, and it is not necessary to set the lower limit of the dissolved oxygen amount of the molten steel.

  Then, after adjusting the dissolved oxygen content of the molten steel to 10 ppm or less, one or two or more of Ca, Mg, and REM have a total final content of one or more of Ca, Mg, and REM of 0.0005. It is preferable that the MnS formation is suppressed by preferentially sulfiding by adding so as to be 0.0080%.

When the dissolved oxygen amount is 10 ppm or less, sulfide control can be sufficiently performed even if Ca, Mg, and REM are added.
To adjust the amount of dissolved oxygen of molten steel to 10 ppm or less, for example, after adding Al as a deoxidizer, the degree of vacuum of the RH vacuum degassing device is 1 to 5 torr (133 to 667 Pa), and the molten steel is used for 10 to 60 minutes. Reflux to adjust the dissolved oxygen content of the molten steel to 10 ppm or less. If the degree of vacuum is 1 to 5 torr (133 to 667 Pa) and the molten steel is not refluxed for 10 to 60 minutes, the amount of dissolved oxygen cannot be reduced to 10 ppm or less. Further, the smaller the dissolved oxygen amount, the better, and it is not necessary to set the lower limit of the dissolved oxygen amount of the molten steel.

  When continuously manufacturing the molten steel with adjusted composition, to produce a slab, when the central solid fraction of the slab, which is the final stage of solidification of the slab, is 0.2 to 0.7, the gap of the casting roll is cast. 0.2 mm to 3.0 mm per 1 m of advancing direction, preferably 0.5 to 2.0 mm per 1 m of advancing direction of casting, more preferably 0.7 to 1.5 mm per 1 m of advancing direction of casting, and casting with light pressure reduction. However, it is preferable to discharge concentrated molten steel such as Sn and P to the upstream side. This makes it possible to reduce harmful center segregation. It is known that the central solid fraction here can be defined as the solid fraction of the molten portion in the slab width direction and the central portion in the slab thickness direction, and can be obtained by heat transfer, solidification calculation, etc. ing. It is preferable to carry out a light reduction, but in the case of a chemical composition having a low Sn and P content, a light reduction may not be carried out.

Then, the cast slab (steel slab) is hot-rolled.
In hot rolling, first, before rolling, the slab is heated at 1200 to 1300 ° C. for 4 to 48 hours and then cooled to room temperature. Although necessary for corrosion resistance, segregation diffuses Sn segregation, which is not necessarily favorable for toughness, so that the maximum Sn concentration is 0% within the range of ½ thickness ± 10% (of plate thickness) in the thickness direction of the plate thickness. It was introduced by finding that this heat treatment (SP treatment) is particularly effective in order to achieve 0.01 to 5.0%. Desirably, it heats at 1200-1300 degreeC for 24 hours-48 hours.

  After that, the cast steel piece is heated at a low temperature in the range of 950 to 1300 ° C, preferably 1000 to 1100 ° C, more preferably 1000 to 1050 ° C. This low temperature heating is called second heating because it is performed after the SP processing heating. When the second heating temperature is set to 1300 ° C. or lower, the austenite (sometimes called γ) grains are made finer, the ferrite is made finer, and the γ → α transformation temperature can be increased to reduce the dislocation density. Therefore, the upper limit was 1300 ° C. Further, if the temperature is lower than 950 ° C, γ conversion is insufficient and the toughness deteriorates, so 950 ° C was set as the lower limit.

  After roughly rolling the heated steel slab, finish rolling with a cumulative reduction of 50 to 75% is performed. When the cumulative rolling reduction exceeds 50%, the α nucleation sites in γ increase, and α can be made finer and the γ → α transformation temperature can be increased, but if it exceeds 75%, the productivity decreases, so The cumulative rolling reduction is 50 to 75%, preferably 55 to 65%.

Finish rolling is an important step in order to fine the alpha, ferrite transformation starting temperature Ar 3 _-30 ° C. or more when the surface temperature of the steel strip in the middle rolling cools shows a known equation (1) Then, it is performed at a recrystallization start temperature _Trex ° C or lower at which the growth of crystal grains represented by the following formula (2) starts. If the temperature is less than Ar ₃ −30 ° C., the rolling will be in the two-phase region, the stretched ferrite will be formed, and the elongation will be deteriorated. Also, in T _rex than not the non-recrystallization region rolling, ferrite deteriorates the elongation coarsened.
_{Ar 3 = 910-310 [C] +65} [Si] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] ··· (1)
However, the element symbol represents the content (mass%) of each element. The elements not contained are 0%.
^{_{T rex = -91900 [Nb *]}} 2 +9400 [Nb *] + 770 ··· (2)
Here, [Nb *] is obtained by the following equation (3).
_Trex is a temperature (recrystallization limit temperature) required to complete recrystallization generally in the time between passes (about 10 to 15 seconds) of normal thick plate rolling, and is the above-mentioned value using the Nb addition amount. It can be expressed by equation (2).
[Sol. Nb] = (10 ^{(-6770 / (T + 273) +2.26)} ) / ([C] + ^12/14 × [N]) ... (3)
In addition, T of Formula (3) is the second heating temperature of the steel slab, and the unit is Celsius temperature (° C),
[Nb] ≧ [Sol. In the case of Nb], [Nb *] = [Sol. Nb],
[Nb] <[Sol. In the case of Nb], [Nb *] = [Nb],
And Here, as for [Nb], the Nb content (mass%) is [Sol. Nb] is the Sol. It represents Nb (solid solution Nb) (mass%).
_Further, expression of _{T rex} (2) in the empirical formula, because some Nb not dissolved by low-temperature heating, the amount of solute Nb and (Sol.Nb amount), the relationship between the solute Nb and recrystallization temperature This is the formula obtained from

As the cooling step after finish rolling, the finish rolled thick steel plate is air-cooled at a cooling rate of 1 ° C./sec or less, or the surface temperature of the steel plate is cooled to a temperature of Ar ₃ −150 ° C. or more and Ar ₃ −50 ° C. or less. Water cooling at over 1 ° C / sec and at 20 ° C / sec or less and then air cooling. The air cooling end temperature is room temperature.

  Since air cooling at a cooling rate of 1 ° C./sec or less has a low cooling rate, the ferrite transformation temperature rises, so that the dislocation density in the ferrite grains decreases and the elongation can be improved. The lower limit of the cooling rate of air cooling does not need to be particularly limited.

After rolling is completed, air cooling may be performed, but in order to enhance strength, the surface temperature of the steel sheet is set to a temperature of Ar ₃ −150 ° C. or higher and Ar ₃ −50 ° C. or lower at a cooling rate of more than 1 ° C./sec and 20 ° C./sec or less. You may air-cool after water cooling. If the cooling stop temperature is less than Ar ₃ -150 ° C., the transformation temperature becomes low, the dislocation density in ferrite grains increases, bainite is formed, and the elongation deteriorates. On the other hand, if Ar ₃ exceeds 50 ° C, the effect cannot be obtained. If the cooling rate of water cooling exceeds 20 ° C./second, the transformation temperature becomes low and the elongation deteriorates. Therefore, the upper limit of the cooling rate of water cooling was set to 20 ° C./second. Since the water cooling is effective if it is equal to or higher than the cooling rate of the air cooling, the lower limit of the cooling rate of the water cooling is set to more than 1 ° C / sec.

Hereinafter, examples of the present invention will be described with reference to Tables 1 to 4.
Using the steel pieces having the chemical composition shown in Table 1, a steel plate having a plate thickness of 6 to 40 mm was experimentally manufactured under the manufacturing conditions shown in Tables 2 and 3. In addition, the dissolved oxygen amount before Ca, Mg, and REM in Tables 2 and 3 means before the addition of one or more of Ca, Mg, and REM. Dissolved oxygen was measured by inserting an oxygen probe having an oxygen concentration battery using a ZrO ₂ (MgO) solid electrolyte into molten steel. The reflux time is the time from the addition of Al as a deoxidizing agent to the addition of Ca, Mg and REM, and the reduction amount during casting is the reduction amount (mm / m) at the central solidification rate of 0.2 to 0.7, Ar ₃ is the equation (1), _Trex is the equation (2), [Sol. Nb] was calculated from the equation (3). The cooling rate (° C./s) in the column of cooling conditions is the cooling rate in the ½ thick part obtained by the heat conduction analysis by the known difference method from the actually measured surface temperature. "Air cooling" described in the cooling pattern column of Tables 2 and 3 is an example in which air cooling was performed without water cooling (accelerated cooling), and "partial water cooling" was performed partially water cooling after rolling. This is an example of performing air cooling later.

The structural characteristics of each manufactured steel sheet shown in Tables 4 and 5 were measured in the following manner.
First, for the microstructure of a steel sheet, a sample was taken so that the vertical cross section of the steel sheet in the rolling direction could be observed, and the metallographic structure at the center of the plate thickness was 1 mm from the surface with an optical microscope at a magnification of 500 times. I took a picture. Next, after binarizing under appropriate conditions using image analysis software, obtain the total area of α and the second phase (perlite and bainite), and divide by the total area of the imaging unit to obtain each phase. Was calculated (area fraction%). In Tables 4 and 5, fractions below the decimal point are rounded off.
The average aspect ratio of ferrite grains is 500 times, and the ferrite grain size is extracted from an optical micrograph of 250 μm × 200 μm × 5 field of view, and each ferrite grain in the field of view is approximated to an ellipse. It was calculated by obtaining the average value of the ratios. On the other hand, the ferrite grain size is the average equivalent circle diameter of the crystal grains calculated by simply averaging the equivalent circle diameters.
From the front surface of the steel plate (plate thickness t) to the 1/4 thick portion (front surface to t / 4), or from the 3/4 thick portion to the back surface (back surface to 3t / 4), and from the 1/4 thick portion to 3 / Each Vickers hardness average value up to 4 thickness parts (t / 4 to 3t / 4 thickness part central part) is 1 mm pitch Vickers hardness test, HV10 of JIS Z 2244, that is, 98.07N of test force. The measurement was performed under the conditions of and the average value was obtained.
For the average dislocation density in α, thin film samples were taken from each position of the plate thickness, and a bright field observation and photographing were performed using a transmission electron microscope (TEM) at a magnification of 40,000, and the obtained TEM image was arbitrarily selected. The number (N) of intersections between the straight line (length: L) and the dislocation line is measured, and the average dislocation density (ρ) is calculated by the following equation (4) using the value of film thickness: t. did.
ρ = 2N / Lt (4)

Table 6 shows the results of measurement of mechanical properties {yield point or proof stress (YP), tensile strength (TS), total elongation (T.EL)}.
The mechanical properties were evaluated for tensile strength (TS) using a No. 1B tensile test piece of JIS Z 2241 (2011) taken from the center of the plate thickness in a direction perpendicular to the rolling direction. The yield point was the upper yield stress (upper yield point) ReH of JIS Z2241 (2011), and 0.2% proof stress of the offset method was obtained only when there was no exceptional yield phenomenon. The total elongation (T.EL) means the total elongation at break At of JIS Z2241 (2011), and the test piece used was JIS No. 1B.
In Table 6, each steel is manufactured according to the YP36 standard.

  Also, from each steel plate, a test piece having a width of 60 mm, a length of 100 mm and a thickness of 3 mm was sampled, shot-blasted on the entire surface, and then some of the test pieces were dried with a modified epoxy paint. The coating was 200 μm thick. On one of the coated surfaces, a steel material surface having two scratches (marked x) with a width of 1 mm and a length of 10 mm was exposed by a cutter knife was exposed to obtain a corrosion test piece simulating a coating film defect in an ore carrier.

As shown in FIG. 1, the test pieces of the bare material and the coated material were wetted, dipped, and dried in the order of 1 cycle, and a cycle test was repeated to simulate corrosion in the ore carrier hold. Here, 100% RH and 50% RH mean relative humidity which is% of the saturated water vapor amount at each temperature. The immersion liquid used in the immersion is 0.5% by mass NaCl + 0.1% by mass CaCl ₂ + 0.5% by mass Na ₂ SO ₄ aqueous solution. Moreover, the mass% of this aqueous solution is% with respect to the solution.

  The above test was carried out for 40 cycles (40 days), the coating film and the corrosion product were removed from each test piece after the test, and the bare steel was subjected to the weight loss after the test to calculate the corrosion amount (thickness reduction). . For the coating material, the ratio of the area in which corrosion was observed to the test area was determined as the corrosion area ratio. The maximum corrosion depth in the corroded part was also measured at the same time. These evaluations relating to corrosion resistance are appropriate as evaluations of thinning in use over time. Table 6 shows the above test results.

Nos. 1 to 24 of the examples of the present invention have a microstructure within the scope of the present invention due to appropriate chemical components and manufacturing conditions. Therefore, the total elongation (T.EL) is 24% or more, the yield point or the proof stress. (YP) 355N / mm ² or more, was able to secure a tensile strength (TS) 490N / mm ² or more.

  Further, as can be seen from Table 6, in the invention examples Nos. 1 to 24, rolling cracks did not occur, and both the bare material and the coated material had good corrosion resistance.

  On the other hand, in Comparative Examples Nos. 25 to 46, either the chemical composition or the manufacturing conditions deviated from the scope of the present invention, and therefore the microstructure (ferrite etc.) of the present invention was not obtained, and the machine intended by the present invention was used. Property (YP, TS, T.EL) or corrosion resistance was not obtained.

  That is, in No. 25, the temperature of the steel slab during the finish rolling of 50 to 75% was too low, so that the average dislocation density in the ferrite in the 1/4 thick part was high and the total elongation (T.EL) was low. . In Nos. 26 and 27, since the second heating temperature was too high, γ could not be fine-grained by finish rolling, and ferrite was one of the requirements (fraction, aspect ratio, grain size, average dislocation density 1 or more of the present invention). ) Was not satisfied, and the total elongation (T.EL) was low. Nos. 28, 29, and 30 lacked the cumulative rolling reduction of finish rolling, and the ferrite did not satisfy the requirements (any one or more of fraction, aspect ratio, grain size, and average dislocation density) of the present invention. The strength (YP, TS) or the total elongation (T.EL) was low because the requirement of No. was not satisfied. In Nos. 31 and 32, the finish temperature of finish rolling was too high, and therefore the ferrite (grain size, average dislocation density) did not satisfy the requirements of the present invention, and in No. 31, the total elongation (T.EL) was low, No. 32 had low intensity (YP, TS).

  In No. 33, the total elongation (T.EL) was low because the amount of Si was insufficient. In No. 34, the total elongation (T.EL) was low because the S content was excessive. In No. 35, since the amount of P was excessive, the maximum concentration of P was high and the total elongation (T.EL) was low in the ½ thickness ± (plate thickness) 10% range (central portion). In No. 36, the total elongation (T.EL) was low because the amount of Nb was excessive.

In No. 37, the ferrite grain size was large because the amount of Ca + Mg + REM was excessive, and the average dislocation density was large, so the elongation (T.EL) was low. No. 38 had a high Ti / N value, TiC was produced, and the total elongation (T.EL) was low. No. 39 had a low Sn content, and good corrosion resistance could not be obtained. No. 40 had a high Sn content, had cracks during rolling, and had a low total elongation (T.EL). No. 41 had a high Co content and a low total elongation (T.EL). No. 42 had no SP treatment, so the Sn concentration in the central portion of the plate thickness was high and the total elongation (T.EL) was low. Since No. 43 does not contain Sn, the mechanical properties were good, but the corrosion resistance was poor. No. 44 had a low Nb content and had low strength (YP, TS) and elongation. No. 45 had a low Ti content and thus had a low total elongation (T.EL). No. 46 had a low total content (T.EL) because the content of Ca + Mg + REM was low.

Claims (5)

In mass%,
C: 0.05 to 0.20%,
Si: 0.2 to 1.0%,
Mn: 0.5-2.0%,
Nb: 0.003 to 0.030%,
Ti: 0.003 to 0.020%,
Al: 0.002-0.050%,
Sn: 0.010 to 0.30%,
N: 0.0010 to 0.0050%,
O: 0.0005 to 0.0050%,
Ca: 0 to 0.0080%,
Mg: 0 to 0.0080%,
REM: 0 to 0.0080%,
Ca + Mg + REM: 0.0005 to 0.0080%,
P: 0.008% or less,
S: 0.003% or less,
Cu: 0 to 0.05%,
Ni: 0 to 1.0%,
Cr: 0 to less than 0.10%,
Mo: 0-0.5%,
V: 0 to 0.050%,
Co: 0 to 1.0%,
B: 0 to 0.0030%,
Ti / N: 0.5-4.0,
Remainder: Fe and impurities,
And
When observing the cross section in the rolling direction, the microstructure
Ferrite area fraction of 1/4 thick part: 80 to 95%,
Perlite area fraction of 1/4 thick part: 5 to 20%,
Bainite area fraction of 1/4 thick part: 0 to less than 10%,
Average aspect ratio of 1/4 thick part ferrite grains: 1.0 to 1.5,
Average particle size of ferrite particles in 1/4 thick part: 5 to 20 μm,
Average dislocation density in 1/4 thick part of ferrite: 7 × 10 ¹² / m ² or less,
And
In the Vickers hardness test of 1 mm pitch,
The average value of Vickers hardness from the front surface to the 1/4 thick portion and from the 3/4 thickness portion to the back surface of the steel sheet is 80 to 105% of the average Vickers hardness value from the 1/4 thick portion to the 3/4 thick portion. ,
The maximum Sn concentration is 0.01 to 5.0% within a range of ½ thickness ± 10% (of the plate thickness) of the plate thickness;
Steel plate for hold of coal or ore carrier. The maximum concentration of P is 0.02 to 0.20% in the range of ½ thickness ± (of the plate thickness) 10% in the thickness direction of the plate thickness;
The steel sheet for holding a coal or ore carrier according to claim 1, wherein Ar ₃ represented by the following formula (1) is 760 to 820 ° C., The steel sheet for holding a coal or ore carrier, according to claim 1 or 2.
_{Ar 3 = 910-310 [C] +65} [Si] -80 [Mn] -20 [Cu]
-55 [Ni] -15 [Cr] -80 [Mo] (1)
However, the element symbol represents the content (mass%) of each element. The elements not contained are 0%.   The plate thickness is 5 to 50 mm, and the steel plate for holding a coal or ore carrier according to any one of claims 1 to 3, Tensile strength (TS) is 490-620 N / mm < ² >, The steel plate for hold | maintenance vessels of a coal and ore carrier as described in any one of Claims 1-4 characterized by the above-mentioned.

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