JP2015157969A - Corrosion resistant steel for holding coal ship and coal and ore ship - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a corrosion resistant steel for holding a coal ship and a coal and ore ship exhibiting excellent corrosion resistance under dry and wet repeating and low pH environment in holding the coal ship and the coal and ore ship as well as excellent in weld zone toughness when high heat input welding is applied.SOLUTION: A component composition of a steel material has a composition of, by mass%, C:0.01 to 0.25%, Si:0.01 to 0.50%, Mn:0.1 to 2.0%, P:0.035% or less, S:0.035% or less, Al:0.003 to 0.10%, Cu:0.05 to 0.35%, Ni:0.02 to 0.40%, Sb:0.01 to 0.2%, W:0.005 to 0.5%, Nb:0.003 to 0.025%, Cr:0.1% or less, Ti:0.005 to 0.030% and N:0.0015 to 0.0070% and the balance Fe with inevitable impurities and 5×10/cmor more of TiN particles having a circle equivalent diameter of 10 to 50 nm exist in the steel.

Description

The present invention relates to a steel material having excellent weld resistance and toughness when applied to a coal ship and a coal / ore combined-use ship hold when applying high heat input welding.
In addition, the steel materials of this invention shall include a thin steel plate, a shape steel, and a bar steel including a thick steel plate. In the present invention, high heat input welding refers to welding with a heat input of 60 kJ / cm or more.

  Merchant ships are often used to transport energy resources, of which bulk cargo ships account for 30% of the volume. In this bulk carrier, marine accidents occurred one after another in the early 1990s, which became an international issue. In particular, many accidents have been reported on coal ships and coal / ore combined ships, most of which were caused by damage in the hold (hereinafter also referred to as “hold”).

  Bulk cargo ships are loaded directly on the hold, so they are easily affected by corrosive loads. It is considered that the strength is locally reduced by eating. In fact, there have been reports of cases in which this pitting corrosion has remarkably progressed and cases in which the plate thickness of the rib portion that ensures the strength of the ship has been extremely reduced.

  As described above, dew condensation water is likely to occur in the side wall portion and rib portion of the bulk cargo ship where pitting corrosion occurs. Since the sulfur component of coal dissolves in the place where the condensed water is generated and reacts with the condensed water to produce sulfuric acid, the inside of the hold is in a low pH environment where sulfuric acid corrosion is likely to occur.

  As a countermeasure against such corrosion in the hold, a modified epoxy coating is applied in the hold with a coating thickness of about 150 to 200 μm. However, since the paint often peels off due to mechanical damage caused by coal or iron ore, and scratches and wear caused by heavy machinery during loading and unloading, it was difficult to obtain a sufficient anticorrosion effect.

  Therefore, as a further countermeasure against corrosion, methods such as periodic repainting and partial repairs have been taken, but such methods are extremely expensive, so life costs including ship maintenance costs can be reduced. In order to reduce cycle costs, the development of new corrosion-resistant steel has become an issue.

  By the way, steel developed for cargo oil tanks and ballast tanks is known as a corrosion resistant steel for ships. However, the holding environment of coal ships and coal / ore combined ships is the environment where cargo oil tanks and ballast tanks are used in terms of corrosive environments (temperature, humidity, corrosive substances, etc.) and mechanical damage caused by the contents. It is completely different. For this reason, original material design and characteristic evaluation are required for steel for holding coal ships and coal / ore combined ships.

  Patent Documents 1 to 3 are known as conventional techniques referring to a coal ship and a coal / ore combined ship holding application. Patent Literature 1 discloses a steel material containing Mg as an essential component, and Patent Literatures 2 and 3 disclose steel materials containing Sn as an essential component.

  On the other hand, in steel structures in the shipbuilding field, steel materials are generally joined by welding and often assembled into a desired shape. Steel materials used for such welded structures are required to have excellent weld toughness as well as base metal toughness from the viewpoint of ensuring safety.

Furthermore, in recent years, with the increase in size of welded structures, improvement in welding efficiency has been demanded from the viewpoint of improving the construction efficiency of the structure and reducing the construction cost, and an increase in welding heat input has been directed. At that time, the most serious problem is the toughness of the weld bond portion and the weld heat affected zone in the vicinity of the weld portion.
That is, since the weld bond is exposed to a high temperature just below the melting point during welding, the crystal grains tend to coarsen, and the cooling rate decreases as the welding heat input increases, so a fragile upper bainite structure is formed. It becomes easy to be done. Furthermore, an embrittlement structure such as a Widmann-Stätten structure or an island-like martensite is easily generated in the weld bond part and the weld heat affected part in the vicinity thereof. For this reason, there is a problem that the toughness tends to become brittle particularly in the weld bond portion and the weld heat affected zone in the vicinity thereof.

  In order to solve such a problem, the following proposals have been made as means for improving the toughness of the welded portion, in particular, the welded bond portion and the welded heat affected zone in the vicinity thereof.

That is, in Patent Documents 4 and 5, the Ti amount, the N amount, and the Ti / N ratio of the Ti amount and the N amount are defined, and TiN particles and MnS are combined to suppress the austenite grain coarsening. A technique for improving weld toughness is disclosed.
Patent Document 6 discloses a technique for improving weld toughness by defining Ti amount and N amount and combining TiN particles and REM oxysulfide to suppress coarsening of austenite grains. Has been.

Patent Document 7 discloses a technique in which weld toughness is improved by adding more Al than usual in addition to Ti and REM to suppress austenite grain growth.
Patent Documents 8 and 9 disclose techniques for improving weld toughness by finely dispersing Ti oxide and using this as a nucleation site for ferrite transformation.
Patent Document 10 discloses a technique for improving weld toughness by using BN precipitated on REM, Ca oxysulfide or TiN as a nucleation site for ferrite transformation during the cooling process during welding. Has been.
Patent Documents 11 and 12 disclose techniques for improving weld toughness by controlling the morphology of sulfides by adding Ca and REM.

JP 2000-17381 A JP 2007-262555 JP 2008-174768 A JP-A-2-50917 JP-A-2-254118 Japanese Patent Publication No. 3-53367 JP-A-60-184663 JP-A-60-245768 JP 61-79745 JP 61-253344 JP JP 60-204863 A Japanese Patent Publication No. 4-14180

  However, the steel material disclosed in Patent Document 1 is intended to improve the corrosion resistance in common use environments such as ship outer plates, ballast tanks, cargo oil tanks, ore ship cargo hold, etc. Although it is mentioned that the results of the corrosion test of the cargo oil tank and the ballast tank are good, the test results in consideration of the holding use environment of coal ships and coal / ore combined ships are not shown.

  In Patent Documents 2 and 3, although corrosion resistance under a coating film simulating the use environment of a coal ship or a coal / ore combined ship is evaluated, it can be said that it is unavoidable in a hold use environment. An evaluation test assuming a situation in which coating film peeling easily occurs due to damage has not been performed.

  As described above, the development of steel materials with excellent corrosion resistance for use in coal ships and coal / ore combined ships holds, taking into account the corrosive environment peculiar to coal ships and coal / ore combined ships, and at the same time, the coating film peels off. In spite of the importance of evaluating the corrosion of steel in the absence of a coating film, these points have not been considered in the past.

  In addition, although the steel materials of Patent Documents 1 to 3 are used for holding coal ships and coal / ore combined ships, the point of securing weld toughness, which is an extremely important characteristic as steel materials in the shipbuilding field, is completely different. It was not considered.

Although various means have been proposed in Patent Documents 4 to 12 regarding the improvement of the weld zone toughness, there are the following problems.
That is, in the means using TiN particles as in Patent Documents 4 to 6, attention is required for dispersion control. In other words, when the amount of fine TiN particles produced is small, there is a problem that the grain refinement effect is lost, weld toughness is not improved, and good weld toughness cannot be stably obtained. .
Further, in the means utilizing Al or Ca as in Patent Documents 7 and 11, oxides are clustered, and this may become a starting point of fracture, resulting in a decrease in toughness.
Furthermore, the means using the Ti oxide as in Patent Documents 8 and 9 has a problem that variation in weld toughness becomes large because it is difficult to disperse the oxide uniformly and finely.
In addition, in the technique described in Patent Document 10, BN is formed on REM or Ca oxysulfide or TiN, but it is difficult to increase the number of REM or Ca oxysulfide and TiN. Has a problem that BN does not sufficiently act as a ferrite-forming nucleus and the effect cannot be exerted greatly.
In addition, in the case of the sulfide form control method using conventional Ca and REM as in Patent Documents 11 and 12, it is difficult to ensure high toughness in high heat input welding with a heat input of 60 kJ / cm or more. There was a problem.

  The present invention has been developed in view of the above situation, and exhibits excellent corrosion resistance in a dry and wet repeated and low pH environment, which is a corrosive environment in a coal ship and a coal / ore combined-use ship hold, and performs high heat input welding. It is an object of the present invention to provide a corrosion resistant steel for holding a coal ship and a coal / ore combined ship excellent in weld toughness when applied.

Generally, a ship is constructed by welding steel materials such as thick steel plates, thin steel plates, shaped steels, and steel bars, and the surface of the steel materials is used with anticorrosion coating. However, in the coal ship and coal / ore combined ship hold environment, the coating is easily peeled off due to mechanical damage caused by the coal / ore, and the steel material is repeatedly exposed to dry and wet conditions and exposed to a low pH environment.
Therefore, the inventors first tried to develop a steel material that can exhibit corrosion resistance even after peeling off the anticorrosion coating on the surface of the steel material. That is, the inventors developed a test method that simulates the environment in a coal ship and a coal / ore combined-use ship hold, and examined the influence of each alloy element using the test method.
As a result, it has been found that Cu, Ni, Sb, W and Nb effectively contribute to the improvement of the corrosion resistance of the steel material.

On the other hand, various factors that affect toughness are also studied and studied for high-level stabilization of weld toughness (hereinafter also referred to as “high heat input weld toughness”) when high heat input welding is applied. As a result, the following knowledge was obtained.
(1) The toughness of the welded part, particularly the welded bond part and the welded heat-affected part in the vicinity when high heat input welding is applied, is greatly influenced by the presence or absence of formation of an embrittled structure.
(2) Formation of the embrittlement structure can be prevented by suppressing the coarsening of austenite grains in a region heated to a high temperature and by finely dispersing ferrite-forming nuclei that promote ferrite transformation during cooling.
However, in the past, these were insufficient, and it is considered that high level stabilization of the high heat input weld zone could not be realized.

(3) In other words, fine dispersion of TiN particles is effective for suppressing coarsening of austenite grains and promoting ferrite transformation during cooling. Only the definition of Ti / N, which is the ratio of N amount, does not provide sufficient refinement and dispersion density of TiN particles, and as a result, the desired toughness cannot be obtained in the weld bond part and the weld heat affected zone in the vicinity thereof. There was a case.

(4) Therefore, the inventors conducted further studies in order to achieve high-level stabilization of the high heat input weld toughness, and as a result, the Ti content and the N content were within a predetermined range, specifically, Ti: 0.005 to 0.030. %, N: Controlled within the range of 0.0015 to 0.0070%, and by making fine TiN particles with an equivalent circle diameter of 50 nm or less in the steel present at least 5 × 10 ⁷ particles / cm ² per unit area, It has been found that even when heat welding is applied, it is possible to prevent toughness deterioration of the weld bond portion and the weld heat affected zone in the vicinity thereof.

(5) In addition, the inventors have noted that the refinement and dispersion density of TiN particles in steel are influenced not only by the amount of Ti and N, but also by the precipitation process of TiN, particularly the slab cooling rate at the melting-casting stage. (That is, the slab cooling rate changes due to the slab casting speed and slab size, but coarse TiN particles are generated when the slab cooling rate is slow, while fine TiN is generated when the slab cooling rate is fast. In order to make fine TiN particles of 50 nm or less in the steel 5 × 10 ⁷ particles / cm ² or more per unit area in the steel, the amount of Ti and N should be within the above range, and the slab It was also found that it is important to control the cooling rate to 0.05 ° C / s or more in the range of 1400-1250 ° C.
The present invention was completed after further studies based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.01 to 0.25%
Si: 0.01 to 0.50%
Mn: 0.1-2.0%
P: 0.035% or less,
S: 0.035% or less,
Al: 0.003-0.10%,
Cu: 0.05 to 0.35%,
Ni: 0.02-0.40%,
Sb: 0.01-0.2%
W: 0.005-0.5%
Nb: 0.003-0.025%,
Cr: 0.1% or less,
Ti: 0.005-0.030% and N: 0.0015-0.0070%
Ship, coal and ore, characterized in that the balance is Fe and inevitable impurities, and TiN particles with a circle equivalent diameter of 10 to 50 nm are present in steel at 5 × 10 ⁷ particles / cm ² or more Corrosion resistant steel for dual-purpose ship hold.

2. The steel is further mass%,
Zr: 0.001 to 0.030% and V: 0.002 to 0.20%
The corrosion-resistant steel for holding a coal ship and a coal / ore combined ship as described in 1 above, comprising one or two kinds selected from the above.

3. The steel is further mass%,
Ca: 0.0002 to 0.01%
The corrosion-resistant steel for holding a coal ship and a coal / ore combined ship as described in 1 or 2 above.

4). The steel is further mass%,
Mo: 0.01-0.5%
Co: 0.01-0.5% and B: 0.0002-0.0050%
The corrosion-resistant steel for holding a coal ship and a coal / ore combined ship according to any one of the above 1 to 3, which contains one or more selected from among the above.

ADVANTAGE OF THE INVENTION According to this invention, the corrosion after peeling of a coating film can be effectively suppressed in the dry and wet repetition and low pH environment in a coal ship and a coal and ore combined use ship hold | maintenance.
Moreover, since the corrosion-resistant steel of the present invention exhibits excellent large heat input weld toughness, it is possible to increase the efficiency of welding by applying large heat input welding during shipbuilding.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel material is limited to the above range in the present invention will be described. In addition, although the unit of element content in the component composition of steel materials is “mass%”, hereinafter, unless otherwise specified, it is simply indicated by “%”.
C: 0.01-0.25%
C is an element effective for increasing the strength of steel, and in the present invention, it is necessary to contain 0.01% or more in order to ensure the strength. On the other hand, when C is contained exceeding 0.25%, the weldability and the toughness of the heat affected zone are deteriorated. Therefore, the C content is in the range of 0.01 to 0.25%. Preferably it is 0.015 to 0.18% of range.

Si: 0.01-0.50%
Si is added as a deoxidizing material and is an element that enhances the strength of steel. Therefore, in the present invention, 0.01% or more is contained. However, if Si is contained in excess of 0.50%, the toughness of the steel is deteriorated, so the upper limit of Si is 0.50%. Preferably it is 0.05 to 0.40% of range.

Mn: 0.1-2.0%
Mn is an element that increases the strength of steel and is contained by 0.1% or more. However, if Mn is contained in excess of 2.0%, the toughness and weldability of the steel are lowered, so the upper limit of Mn is set to 2.0%. Preferably it is 0.5 to 1.6% of range.

P: 0.035% or less P is a harmful element that deteriorates not only the base metal toughness of steel but also the weldability and weld zone toughness, so it is desirable to reduce it as much as possible. In particular, when the P content exceeds 0.035%, the deterioration of the base metal toughness and the weld zone toughness increases. Therefore, P is set to 0.035% or less. Preferably it is 0.025% or less. However, excessive P removal leads to an increase in production cost, so the lower limit of P is preferably 0.003%.

S: 0.035% or less Since S is a harmful element that deteriorates the toughness and weldability of steel, it is preferably reduced as much as possible. In the present invention, it is 0.035% or less. However, since excessive desulfurization leads to an increase in production cost, the lower limit of S is preferably 0.0001%.

Al: 0.003-0.10%
Al is contained in an amount of 0.003% or more as a deoxidizer, but if it exceeds 0.10%, the toughness of the weld is adversely affected, so the Al content is 0.10% or less.

Cu: 0.05-0.35%
Cu densifies corrosion products and suppresses the diffusion of H ₂ O, O ₂ , and SO ₄ ²⁻ into the steel. Thereby, the corrosion resistance of steel improves. This effect is manifested when the Cu content is 0.05% or more, but when it exceeds 0.35% and is contained excessively, weldability and base metal toughness are lowered. Therefore, the amount of Cu is made 0.05 to 0.35% of range. Preferably it is 0.10 to 0.30% of range.

Ni: 0.02-0.40%
Ni, like Cu, densifies corrosion products and suppresses the diffusion of H ₂ O, O ₂ , and SO ₄ ²⁻ into the steel. Thereby, the corrosion resistance of steel improves. This effect is manifested when the Ni content is 0.02% or more. However, if the Ni content exceeds 0.40%, the weldability and the toughness of the base material are lowered. Therefore, the Ni content is in the range of 0.02 to 0.40%. Preferably it is 0.04 to 0.30% of range.

Sb: 0.01-0.2%
When Sb is contained in steel materials in an amount of 0.01% or more as an alloying element, it concentrates in the vicinity of the ground iron in a low pH environment. Since Sb has a large hydrogen overvoltage, the hydrogen generation reaction is suppressed in the portion where Sb is deposited, and the corrosion resistance is improved. Further, by forming the Cu ₂ Sb is Cu intermetallic compound, to further improve the corrosion resistance. On the other hand, if the Sb content exceeds 0.2%, the toughness is lowered. Therefore, Sb is set to a range of 0.01 to 0.2%. Preferably it is 0.02 to 0.15% of range.

W: 0.005-0.5%
W suppresses the diffusion of SO ₄ ^2- into the ground iron by generating WO ₄ ^2- , and also densifies the corrosion products to form H ₂ O, O ₂ , SO ₄ ^2- Suppresses the diffusion of In order to obtain these effects, it is necessary to contain 0.005% or more of W. However, if W is contained in excess of 0.5%, not only is the effect saturated, but the cost also increases, so the W amount is in the range of 0.005 to 0.5%. Preferably it is 0.02 to 0.2% of range.

Nb: 0.003-0.025%
Nb densifies the corrosion products and suppresses the diffusion of H ₂ O, O ₂ , and SO ₄ ²⁻ into the ground iron. In order to acquire this effect, it is necessary to contain Nb 0.003% or more. On the other hand, even if Nb exceeds 0.025%, the effect is saturated. Therefore, the Nb amount is set to 0.003 to 0.025%. Preferably, it is 0.005 to 0.020% of range.

Cr: 0.1% or less
Cr is an element that lowers the corrosion resistance because it causes hydrolysis in a low pH environment, so it is preferable to reduce it as much as possible, but 0.1% or less is acceptable. However, it is difficult to completely remove Cr, and the lower limit is about 0.005%.

Ti: 0.005-0.030%
Ti has a strong affinity for N and precipitates as TiN, thereby suppressing the coarsening of austenite grains in the weld heat affected zone, or contributes to increasing the toughness of the weld heat affected zone as a ferrite nucleus. Such an effect appears when Ti is contained in an amount of 0.005% or more. However, if Ti is contained in an amount exceeding 0.030%, TiN particles become coarse and a desired effect cannot be expected. For this reason, Ti shall be contained in the range of 0.005 to 0.030%. Preferably it is 0.01 to 0.020% of range.

N: 0.0015-0.0070%
N combines with Ti and precipitates as TiN to suppress coarsening of austenite grains in the weld heat affected zone, or contributes to increase the toughness of the weld heat affected zone as a ferrite formation nucleus. In order to secure the necessary amount of TiN having such an effect, N needs to be contained by 0.0015% or more. On the other hand, when N is contained in excess of 0.0070%, the amount of solid solution N increases in a region heated to a temperature at which TiN is dissolved by welding heat, and the toughness is remarkably lowered. For this reason, N shall be contained in the range of 0.0015 to 0.0070%. Preferably it is 0.0030 to 0.0050% of range.

The basic components have been described above. In the present invention, the following elements can be appropriately contained as necessary.
One or two selected from Zr: 0.001 to 0.030% and V: 0.002 to 0.20%
Both Zr and V are elements that increase the strength of steel, and can be selected and contained according to the required strength. In order to obtain such an effect, it is preferable to contain Zr in an amount of 0.001% or more and V in an amount of 0.002% or more. However, if Zr exceeds 0.030% and V exceeds 0.20%, the toughness decreases. Therefore, when Zr and V are included, each is preferably included in the above range.

Ca: 0.0002 to 0.01%
Ca has the effect of controlling the shape of inclusions, and can increase the ductility and toughness of steel. Such an effect appears when the Ca content is 0.0002% or more. On the other hand, when Ca is contained exceeding 0.01%, coarse inclusions are formed and the toughness of the base material is deteriorated. Therefore, the Ca content is in the range of 0.0002 to 0.01%. Preferably, it is 0.0005 to 0.005% of range.

One or more selected from Mo: 0.01 to 0.5%, Co: 0.01 to 0.5% and B: 0.0002 to 0.0050%
Mo, Co, and B are all elements that increase the strength of steel, and can be selected and contained according to the required strength. Such an effect is manifested when the Mo content and Co content are 0.01% or more and the B content is 0.0002% or more. However, if both Mo and Co exceed 0.5% and B exceeds 0.0050%, the toughness decreases. Therefore, when Mo, Co and B are included, each content is within the above range. It is preferable to make it.

  Among the component compositions in the present invention, components other than those described above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

  Further, in the present invention, the Ti amount and the N amount are within the above ranges, and the slab cooling rate in the temperature range of 1400 ° C. to 1250 ° C. in the solidification stage is set to 0.05 ° C. during the steel manufacturing process as described later. By controlling to more than / s, a sufficient amount of fine TiN particles can be precipitated in the steel, thereby improving the toughness of the weld bond area and the weld heat-affected area in the vicinity of large heat input welding. be able to.

TiN particles in steel: 10 to 50 nm in equivalent circle diameter and 5 × 10 ⁷ particles / cm ² or more Fine TiN particles in the steel with 10 to 50 nm equivalent circle diameter are 5 × 10 ⁷ particles / If there is more than ² cm, the austenite grain coarsening is suppressed in the weld bond part heated to a high temperature during welding and in the vicinity of the heat affected zone, and the ferrite transformation is promoted during cooling. The structure becomes fine and toughness is improved.
On the other hand, if the number of TiN particles having a diameter of more than 50 nm or TiN particles having a diameter of 10 to 50 nm is less than 5 × 10 ⁷ particles / cm ² , the effect of suppressing the austenite grain coarsening is small, and the ferrite transformation Small promotion effect. Therefore, a satisfactory toughness improving effect, in particular, when high heat input welding is applied, a toughness improving effect cannot be obtained at the weld bond portion and the weld heat affected zone in the vicinity thereof.
Therefore, in the present invention, 5 × 10 ⁷ particles / cm ² or more of fine TiN particles having an equivalent circle diameter of 10 to 50 nm are present in the steel. Preferably it is 5 × 10 ⁸ pieces / cm ² or more, more preferably 1 × 10 ⁹ pieces / cm ² or more.
The upper limit of the number of TiN particles having an equivalent circle diameter of 10 to 50 nm is not particularly limited, but is preferably about 1 × 10 ¹² particles / cm ² or less. Further, TiN particles having an equivalent circle diameter of less than 10 nm have a small effect of suppressing the austenite grain coarsening, so the diameter of TiN particles is set to 10 nm as the lower limit.

Next, although the suitable manufacturing method of the corrosion-resistant steel material which concerns on this invention is demonstrated, a manufacturing method is not restricted only to this.
Molten steel having the above component composition is melted by a known method such as a converter or an electric furnace, and is made into a steel material such as a slab or billet by a known method such as a continuous casting method or an ingot-making method. It goes without saying that treatments such as ladle refining and vacuum degassing may be added to the molten steel.

Here, from the viewpoint of making the TiN particles fine and dispersed and precipitated, the cooling rate in the temperature range of at least 1400-1250 ° C in the solidification process when using a steel material by a continuous casting method or an ingot-making method is 0.05 ° C. Must be at least / s.
This is because when the cooling rate in this temperature range is less than 0.05 ° C./s, 5 × 10 ⁷ particles / cm ² or more of fine TiN particles having an equivalent circle diameter of 10 to 50 nm may be precipitated in the steel described above. It is not possible. A more preferable cooling rate is 0.10 ° C./s or more, and further preferably 0.15 ° C./s or more.

Next, from the viewpoint of preventing grain coarsening, the steel material is preferably heated to a temperature of 1050 to 1250 ° C. and then hot rolled to a desired size or shape, or the temperature of the steel material can be hot rolled. If the temperature is as high as possible, the steel material is immediately hot-rolled into a steel material having a desired size and shape without heating or soaking.
In hot rolling, in order to ensure strength, it is preferable to optimize the hot finish rolling end temperature and the cooling rate at the end of hot finish rolling, and the hot finish rolling end temperature is 700 ° C. or higher, The cooling after the hot finish rolling is preferably performed by air cooling or accelerated cooling at a cooling rate of 150 ° C./s or less. Note that, after cooling, reheating treatment may be performed.

The steel which becomes a component shown in Table 1 was made into a slab by continuous casting after melting in a vacuum melting furnace or in a converter. Subsequently, the slab was charged into a heating furnace and heated to 1150 ° C., and a steel sheet having a thickness of 25 mm was formed by hot rolling at a rolling end temperature of 930 ° C. The slab cooling rates in the temperature range of 1400 to 1250 ° C. in the solidification stage are also shown in Table 1.
For these steel plates, the tensile properties and impact properties of the base material were investigated. In addition, as the toughness of the weld zone, a reproducible thermal cycle test (maximum heating temperature) corresponding to the heat history of the heat affected zone 1mm of the FCB welded joint produced at a heat input of 150kJ / cm (1mm from the fusion line to the base metal side) The absorption energy vE _-20 at -20 ° C was measured by Charpy impact test. In addition, if this vE- ₂₀ is 50J or more, it can be said that it is excellent in the weld zone toughness.

In addition, the observation of TiN particles in the steel and the density measurement were performed according to the following procedure.
(1) A sample for microstructural observation was taken from the position where the thickness of the steel sheet was 1/4 t, and this was embedded in a conductive carbon resin and polished.
(2) Thereafter, electropolishing was performed under the following conditions, and after alcohol washing, drying was performed and Pt coating (about 10 seconds) was performed.
・ Electrolyte: 4% methyl salicylate-1% salicylic acid-1% TMAC-methanol ・ Electrolytic potential: -300mV (vs.SCE)
・ Electropolishing depth: 0.5μm or more
(3) Thereafter, SEM (scanning electron microscope) observation was performed at an acceleration voltage of 30 kV. The magnification is determined by the distribution of TiN, but here the observation was performed at 20000 times. The observation field was 10 fields.
(4) Since the TiN particles appear white in the SEM photograph, the white particles were used as TiN particles, and the particle size distribution and distribution density of the TiN particles were measured for the 10 fields observed using an image analyzer. For the area of 10 fields of view, the number density of TiN particles having a circle equivalent diameter of 10 to 50 nm was calculated.
The number density of TiN particles calculated as described above is also shown in Table 1. Note that the measurement target of this method is fine particles of 10 to 50 nm, and oxides, sulfides, and nitride inclusions on the order of μm are excluded.

In addition, the corrosion resistance test was conducted under the following conditions to simulate a temperature / humidity environment and condensation conditions that greatly affect the corrosion in the hold of coal ships and coal / ore combined ships.
That is, 5 mmt × 50 mmW × 75 mmL test pieces were sampled from the steel sheet, and the surface of the test piece was shot blasted to remove surface scale and oil. Using this surface as a test surface, the corrosion resistance of the steel material after peeling the coating film was evaluated. After coating the back and end faces with silicone seals, fit them in an acrylic jig, spread 5 g of coal on them, and use a constant temperature and humidity chamber to create an atmosphere A (temperature 60 ° C, relative humidity 95%, 20 hours) A temperature and humidity cycle with atmosphere B (temperature 30 ° C., relative humidity 95%, 3 hours), transition time 0.5 hours was applied for 84 days. Here, the symbol “⇔” means repetition. In addition, 5 g of coal was weighed and immersed in 100 ml of distilled water at room temperature for 2 hours, followed by filtration, and the coal leachate diluted to 200 ml had a pH of 3.0.
In this example, the test under such conditions simulates a temperature / humidity environment and a dew condensation environment that greatly affect the corrosion in the hold of coal ships and coal / ore combined ships. After the test, using a rust remover, the rust of each test piece was peeled off, and the weight loss of the steel material was measured to obtain the amount of corrosion. Further, the maximum pitting corrosion depth was measured using a depth meter.
Table 2 shows the results of the mechanical property investigation and the corrosion resistance test.

As shown in Table 2, both the inventive examples and the comparative examples showed good base metal mechanical properties, but there were significant differences in the weld impact characteristics and corrosion resistance.
That is, in Invention Examples Nos. 1 to 22, the absorbed energy vE- ₂₀ at -20 ° C was 56J or more, exceeding the target vE- ₂₀ , and good weld toughness was obtained. In contrast, in Comparative Example Nanba23～30, below the vE _-20 vE _-20 is obtained by the both 31J below the target, good weld toughness was not obtained. This is because, as shown in Table 1, the number density of the TiN particles of Comparative Examples No. 23 to 30 is less than the appropriate range.

  As for corrosion resistance, the weight loss and maximum pitting corrosion depth of Invention Examples Nos. 1 to 22 are 70% or less of Comparative Example No. 23, which is a base steel, while showing good corrosion resistance. The weight loss and the maximum pitting corrosion depth of No. 24-29 as examples were 80% or more of the base steel No. 23, and good corrosion resistance was not obtained. Comparative Example No. 30 has good corrosion resistance, but is insufficient from the viewpoint of the weld zone toughness described above.

When the steel material according to the present invention is used as a constituent member of a coal ship and a coal / ore combined ship hold, even in a situation where the coating film is peeled off due to mechanical damage of coal or ore, it can exhibit excellent corrosion resistance. There are effects such as reduction of steel material switching due to corrosion.
Moreover, since the high heat input welding part toughness is shown, the high heat input welding is applied at the time of shipbuilding, and the efficiency of welding construction can be improved.

Claims (4)

% By mass
C: 0.01 to 0.25%
Si: 0.01 to 0.50%
Mn: 0.1-2.0%
P: 0.035% or less,
S: 0.035% or less,
Al: 0.003-0.10%,
Cu: 0.05 to 0.35%,
Ni: 0.02-0.40%,
Sb: 0.01-0.2%
W: 0.005-0.5%
Nb: 0.003-0.025%,
Cr: 0.1% or less,
Ti: 0.005-0.030% and N: 0.0015-0.0070%
Ship, coal and ore, characterized in that the balance is Fe and inevitable impurities, and TiN particles with a circle equivalent diameter of 10 to 50 nm are present in steel at 5 × 10 ⁷ particles / cm ² or more Corrosion resistant steel for dual-purpose ship hold. The steel is further mass%,
Zr: 0.001 to 0.030% and V: 0.002 to 0.20%
The corrosion-resistant steel for holding a coal ship and a coal / ore combined ship according to claim 1, comprising one or two selected from among them. The steel is further mass%,
Ca: 0.0002 to 0.01%
The corrosion-resistant steel for holding a coal ship and a coal / ore combined ship according to claim 1 or 2. The steel is further mass%,
Mo: 0.01-0.5%
Co: 0.01-0.5% and B: 0.0002-0.0050%
The corrosion-resistant steel for holding a coal ship and a coal / ore combined ship according to any one of claims 1 to 3, comprising one or more selected from among the above.