JP5396758B2 - Hot-rolled section steel for ship ballast tank and manufacturing method thereof - Google Patents

Description

The present invention includes a coal ship, an ore ship, a coal mine ship, a crude oil tanker, an LPG ship, an LNG ship, a chemical tanker, a container ship, a bulk carrier, a timber ship, a chip ship, a refrigeration carrier ship, an automobile ship, a heavy article ship, RORO vessels, limestone dedicated ships relates marine section steel used in the cement dedicated ships, etc., particularly, ballast tanks of ships used for stringer ballast tanks under severe corrosive environment due to seawater (longitudinals material) etc. The present invention relates to a hot-rolled section steel and a manufacturing method thereof. Here, the hot-rolled shape steel for ship ballast tanks used for longitudinal materials (longi materials), etc., specifically, equilateral mountain shape steel, unequal side angle shape steel, unequal sides formed by hot rolling Non-uniform thick angle steel, grooved steel, spherical flat steel, T-shaped steel, etc.

  The ship's ballast tank is in a very severe corrosive environment because it plays the role of injecting seawater to enable stable navigation of the ship when there is no cargo. For this reason, formation of an anticorrosion coating film using an epoxy resin paint and cathodic protection are usually used in combination for corrosion protection of steel materials used in ballast tanks.

  However, even if these anticorrosion measures are taken, the corrosive environment of the ballast tank is still severe. That is, when seawater is injected into the ballast tank, the portion that is completely immersed in the seawater functions as anticorrosion, and therefore corrosion can be suppressed. However, the vicinity of the uppermost part of the ballast tank, particularly the back side of the upper deck, is not immersed in seawater and is in a state where it is exposed to only seawater splashes. Therefore, cathodic protection does not function at this site. Furthermore, this part becomes a more severe corrosive environment because the steel plate temperature is increased by sunlight. On the other hand, when seawater is not injected into the ballast tank, since the anticorrosion does not work at all, it is severely corroded by residual adhered salt.

  Therefore, it is generally said that the anticorrosive coating film of the ballast tank under the severe corrosive environment as described above has a life of about 10 years, which is about half the life of the ship (about 20 years). Therefore, in the remaining 10 years, the actual situation is that the corrosion resistance is maintained by performing a treatment such as repair painting. However, since the corrosive environment of the ballast tank is very severe, it is difficult to maintain the effect for a long time even if repair coating is performed. In addition, since repair painting is performed in a narrow space, it is not preferable as a work environment. Therefore, it is desired to develop a steel material with excellent corrosion resistance that can extend the period until repair coating as much as possible and reduce the work load.

  Thus, several techniques for improving the corrosion resistance of steel materials themselves used in severe corrosive environments such as ballast tanks have been proposed. For example, in Patent Document 1, Cu: 0.05 to 0.50 mass%, W: 0.01 to less than 0.05 mass%, as a corrosion resistance improving element, is added to steel of C: 0.20 mass% or less, Furthermore, a corrosion-resistant low alloy steel is disclosed in which one or more of Ge, Sn, Pb, As, Sb, Bi, Te and Be are added in an amount of 0.01 to 0.2 mass%. Further, in Patent Document 2, Cu: 0.05 to 0.50 mass%, W: 0.05 to 0.5 mass%, as a corrosion resistance improving element, is added to a steel material having C: 0.20 mass% or less, and , Ge, Sn, Pb, As, Sb, Bi, Te, and Be are disclosed corrosion resistant low alloy steel added with 0.01 to 0.2 mass% of one or more of them. Patent Document 3 discloses a corrosion-resistant low alloy steel obtained by adding Cu: less than 0.05 to 0.15 mass% and W: 0.05 to 0.5 mass% to steel having C: 0.15 mass% or less. Has been.

  In Patent Document 4, C: 0.15 mass% or less of steel, P: 0.03-0.10 mass%, Cu: 0.1-1.0 mass%, Ni: 0.0. A ballast tank is disclosed in which an anticorrosion paint such as a tar epoxy paint, a pure epoxy paint, a solventless epoxy paint, and a urethane paint is applied to a low alloy corrosion resistant steel material to which 2 to 1.0 mass% is added, and is coated with a resin. This technology intends to extend the life of the anticorrosion coating by improving the corrosion resistance of the steel material itself, and to realize maintenance-free over 20 to 30 years, which is the use period of the ship.

  In Patent Document 5, an attempt is made to improve the corrosion resistance by adding 0.2% to 5% by mass as a corrosion resistance improving element to C: 0.15 mass% or less steel, thereby realizing a maintenance-free ship. Proposals have been made. Further, Patent Document 6 uses a steel material in which Cr: 0.2 to 5 mass% is added as a corrosion resistance improving element to steel of C: 0.15 mass% or less as a constituent material, and oxygen gas inside the ballast tank. An anticorrosion method for a ballast tank has been proposed in which the concentration is a ratio of 0.5 or less with respect to the value in the atmosphere.

  Further, Patent Document 7 proposes to improve corrosion resistance by adding Cr: 0.5 to 3.5 mass% to steel of C: 0.1 mass% or less, and to realize a maintenance-free ship. Has been made. Furthermore, in Patent Document 8, by adding Ni: 0.1-4.0 mass% to C: 0.001-0.025 mass% steel, coating film damage resistance is improved, repair coating, etc. Marine steel materials that reduce maintenance costs are disclosed.

  Moreover, in patent document 9, it is a ship by adding Cu: 0.01-2.00 mass% and Mg: 0.0002-0.0150 mass% to C: 0.01-0.25 mass% steel. Steels for ships having corrosion resistance in use environments such as an outer plate, a ballast tank, a cargo oil tank, and a coal ore cargo hold are disclosed. Further, Patent Document 10 discloses a crude oil tank by adding Mo, W and Cu in C: 0.001 to 0.2 mass% steel and limiting the amount of P and S which are impurities. Steel that suppresses general corrosion and local corrosion that occur in JIS is disclosed.

  However, in Patent Documents 1 to 3 above, sufficient examination has not been made on the corrosion resistance in the presence of a coating film such as a zinc epoxy resin paint that is usually applied to a steel material constituting a ballast tank or the like. Further investigation is necessary for the corrosion resistance in the presence of the coating film as described above.

  Further, in the steel material of Patent Document 4, P is added in a relatively large amount of 0.03 to 0.10 mass% in order to improve the corrosion resistance of the base metal, which is problematic from the viewpoint of weldability and weld toughness. There is. Moreover, the steel materials of Patent Literature 5 and Patent Literature 6 contain 0.2 to 5 mass% of Cr, and the steel material of Patent Literature 7 contains relatively large amount of Cr to 0.5 to 3.5 mass%. Therefore, both have problems in weldability and weld zone toughness, as well as the problem of increased manufacturing costs. Moreover, since the steel material of patent document 8 has low C content and high Ni content, there exists a problem that manufacturing cost becomes high.

Furthermore, since the steel material of Patent Document 9 requires the addition of Mg, there is a problem that the steel making yield is not stable and the mechanical properties of the steel material are not stable. Furthermore, the steel material of patent document 10 is a corrosion resistant steel used in an environment where H ₂ S exists in a crude oil tank, and the corrosion resistance in a ballast tank without H ₂ S is unknown. Furthermore, the corrosion resistance in the state in which the zinc primer generally used for the steel for ballast tanks is applied has not been studied. Therefore, further examination of corrosion resistance is necessary for use in a ballast tank.
JP-A-48-050921 JP 48-050922 A JP-A-48-050924 Japanese Patent Application Laid-Open No. 07-034197 Japanese Patent Application Laid-Open No. 07-034196 Japanese Patent Application Laid-Open No. 07-034270 JP 07-310141 A JP 2002-266052 A JP 2000-017341 A JP 2004-204344 A

  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 subjected to anticorrosion coating. The anticorrosion coating is usually applied with a zinc primer as a primary rust prevention, and after a small or large assembly, an epoxy resin coating is applied as a secondary coating (main coating). Therefore, most of the steel surface of the ship is subjected to a two-layer anticorrosion coating of zinc primer and epoxy resin coating. In addition, since the zinc primer is burned off by the heat during welding, the welded portion is subjected to repair coating (touch-up) with the zinc primer as a rust prevention measure from after welding to the main coating. However, if the period until the final coating is short, repair coating may not be performed. In addition, in ships that have been used for many years after construction, there are parts where the above-mentioned coating film deteriorates and the function as a rust-proof coating film is not sufficiently achieved, and the coating film peels off and the steel sheet is in a bare state .

  In other words, on the surface of the steel material of the ship in service, there are three states: a part with two layers of zinc primer and epoxy resin coating, a part with only epoxy resin coating, and a bare part Will exist. Therefore, in order to achieve the object of improving the corrosion resistance of the ship, it is necessary to be a marine steel material that exhibits excellent corrosion resistance in any of these states.

By the way, thick steel plates used in ships are being increased in strength from the viewpoints of cost reduction and safety ensuring by reducing the amount of steel used, yield stress YP is 315 MPa or more, and preferably tensile strength TS. However, high-strength materials of 440 MPa or higher have been used. In the case of a thick steel plate, strength and toughness are generally controlled by adjusting the conditions of a controlled rolling / accelerated cooling process (TMCP).
On the other hand, steel materials used for ballast tanks such as long steel, especially hot-rolled steel shapes such as unequal unequal thick angle steel and T-shaped steel, have a cross-sectional shape compared to thick steel plates used in the same ship. -Since the dimensions are complicated, it is difficult to adopt TMCP similar to that of thick steel plates as a method for controlling strength and toughness. In particular, considering the bending and warping in the middle of rolling, it is necessary to build the material. Therefore, in order to obtain a high-strength shape steel with a yield stress YP of 315 MPa or more, consider a unique method for producing the shape steel. There is a need to.

Therefore, the object of the present invention is to exhibit excellent corrosion resistance without being affected by the presence state of the coating film in a severe corrosive environment such as a ballast tank of a ship, and it is possible to extend the period until repair coating, As a result, it is possible to reduce the work of repair coating. Hot rolled steel for ship ballast tanks (hereinafter referred to simply as “for ship use” ), which is used as a longitudinal material (longi material) with a strength of 315 MPa or more that has excellent corrosion resistance . It is also provided at a low cost.

  The inventors have shown excellent corrosion resistance without being influenced by the surface state (existing state of the coating film) even in a severe corrosive environment by seawater, and repeated earnest research toward the development of a shape steel having high strength. . As a result, by adding W and Cr as essential elements, and further adding corrosion resistance improving elements such as Sb and Sn in an appropriate range, a two-layer coating state of zinc primer and epoxy resin coating, epoxy resin coating To obtain a hot-rolled section steel for marine vessels that exhibits excellent corrosion resistance in both the film state and the bare state, and to increase the strength of the shape steel without impairing productivity, weldability, etc. The introduction of processed ferrite by (α + γ) two-phase rolling was found to be effective, and the present invention was completed.

That is, the present invention is C: 0.03-0.25 mass%, Si: 0.05-0.50 mass%, Mn: 0.1-2.0 mass%, P: 0.025 mass% or less, S: 0 .01 mass% or less, Al: 0.005 to 0.10 mass%, W: 0.01 to 1.0 mass%, Cr: 0.01 mass% or more and less than 0.20 mass%, N: 0.001 to 0.008 mass% Is a hot-rolled section steel for ballast tanks of ships , which has a component composition consisting of Fe and inevitable impurities, and has a microstructure composed of ferrite containing processed ferrite and a pearlite structure, and is excellent in corrosion resistance.

In addition to the above component composition, the hot rolled steel bar for ship ballast tank of the present invention preferably further contains a component belonging to at least one of the following groups A to E.
Group A; Sb: 0.001 to 0.3 mass% and Sn: one or two types selected from 0.001 to 0.3 mass% ; Group B ; Mo: 0.01 to 0.5 mass% and Co: 1 or two or C group selected from among 0.01~1.0mass%; Nb: 0.001~0.1mass%, Ti: 0.001~0.1mass%, Zr: 0.001 -0.1 mass% and V: 1 type or 2 or more types chosen from 0.002-0.2 mass%; B: 0.0002-0.003mass%
Group E; Ca: 0.0002 to 0.01 mass%, REM: 0.0002 to 0.015 mass%, and Y: 0.0001 to 0.1 mass%, or one or more selected from the group

Moreover, it is preferable that the hot rolled shape steel for ship ballast tanks of the present invention has any one of an epoxy resin coating, a zinc primer coating, a zinc primer coating and an epoxy resin coating on the surface of the shape steel.

Further, in the present invention, in manufacturing the hot rolled steel for ballast tanks of the ship , the steel material having the above component composition is heated to 1000 to 1350 ° C., and the cumulative reduction ratio at Ar ₃ temperature or less is 10 to 10%. Hot-rolled section steel for ballast tanks of ships , characterized by being hot-rolled with 80% rolling finishing temperature of (Ar ₃ -30 ° C.) to (Ar ₃ -180 ° C.) and then left to cool. It is a manufacturing method.

In the production method of the present invention, it is preferable to perform hot rolling at an Ar ₃ temperature or less with a temperature difference in the cross section of the shaped steel within 50 ° C.

  According to the present invention, it is possible to provide a marine hot-rolled section steel having high strength and excellent corrosion resistance even in a severe corrosive environment with seawater at a low cost. In addition, since the shape steel of the present invention is excellent in corrosion resistance, it can greatly contribute to the extension of the period until the repair coating of the ship and the reduction of the work load of the repair coating.

  The inventors of the present invention have three states existing in a steel material used in a ship in service, that is, a state having a two-layer coating of a zinc primer and an epoxy resin coating, a state of only an epoxy resin coating, and a bare state. In any of the states, the following experiment was conducted to develop a marine hot-rolled section steel having excellent corrosion resistance.

Steels added with various alloying elements are melted in the laboratory and hot rolled to form hot rolled sheets having a thickness of 5 mm. From these hot rolled sheets, 5 mmt × 100 mmW × 200 mmL or 5 mmt × 50 mmW × 150 mmL A test piece was collected and shot blasted on the surface of the test piece to remove the scale and oil on the surface, and then a test piece for an exposure test subjected to the following three types of surface treatment was prepared.
Condition A: Two-layer coating of zinc primer (film thickness of about 15 μm) and tar epoxy resin paint (film thickness of about 100 μm) is formed on the surface of the test piece Condition B: Tar epoxy resin paint (film thickness of about 100 μm) on the surface of the test piece ) Condition C: Bare state with shot blasting on the specimen surface (no anticorrosion coating)

  Then, these test pieces were simulated the corrosive environment on the upper deck side of the actual ballast tank, (35 ° C., 5 mass% NaCl solution spray × 2 hr) → (60 ° C., RH 5 mass% × 4 hr) → (50 ° C., RH95 mass% × 2 hr) was taken as one cycle, and this was subjected to a salt spray wet and dry repeated corrosion test for 132 cycles. And, for the corrosion resistance, for the test pieces of the conditions A and B having a coating film, before the test, an 80 mm length scratch scissor that reaches the surface of the iron core with a cutter knife from the top of the coating film is given in a single character, After the test, by measuring the area of the swollen swelled around the scratching flaw, and for the test piece of the condition C having no coating film, after the test, derusting, Evaluation was made by calculating the average thickness reduction amount from the difference (decrease amount) in the weight of the specimen before the corrosion test.

From the results of the corrosion test, Table 1 shows the effect of improving the corrosion resistance of each alloy element for each coating film condition on the surface of the test piece. In short, the result is:
1) In the case of condition A (zinc primer + tar epoxy resin coated two-layer coating film); the most effective element for improving corrosion resistance is Cr, then W, and then Sb.
2) In the case of condition B (only one layer of the tar epoxy resin coating); W is the most effective element for improving corrosion resistance, and then Sb and Sn.
3) In the case of condition C (bare state); the most effective element for improving the corrosion resistance is W, and then Sb and Sn.
4) When W and Cr are added in combination, the corrosion resistance under the condition A is improved as compared with the case of containing alone, and when Sb and Sn are additionally added, not only the condition A but also the conditions B and C have a remarkable improvement effect.
5) Addition of Mo slightly improves the corrosion resistance in both conditions A, B, and C, and Cu, Ni, and Co slightly improve the corrosion resistance in conditions A and C.

  Based on the results of the above test, in the present invention, a component system in which W and Cr are added together as a basic element for improving corrosion resistance is adopted, and when corrosion resistance is required, 1 is selected from Sb and Sn. It was decided to adopt a component design in which seeds or two kinds were added in addition. When more excellent corrosion resistance is required, one or more selected from Cu, Ni, Mo, and Co are added.

Next, the component composition that the marine hot-rolled section steel excellent in corrosion resistance of the present invention should have will be described.
C: 0.03-0.25 mass%
C is an element effective for increasing the strength of steel, and in the present invention, it is necessary to contain 0.03 mass% or more in order to obtain a desired strength. On the other hand, addition exceeding 0.25 mass% lowers the toughness of the weld heat affected zone (HAZ). Therefore, the C content is in the range of 0.03 to 0.25 mass%. In addition, C is preferably in the range of 0.05 to 0.20 mass% from the viewpoint of achieving both strength and toughness by the processed ferrite described later.

Si: 0.05-0.50 mass%
Si is an element added as a deoxidizer and to increase the strength of steel. In the present invention, it is added in an amount of 0.05 mass% or more. However, since addition exceeding 0.50 mass% lowers the toughness of steel, the upper limit of Si is made 0.50 mass%.

Mn: 0.1 to 2.0 mass%
Mn is an element that has the effect of preventing hot brittleness and increasing the strength of steel, and is added in an amount of 0.1 mass% or more. However, the addition of Mn exceeding 2.0 mass% lowers the toughness and weldability of the steel, so the upper limit is made 2.0 mass%. Preferably, it is in the range of 0.5 to 1.6 mass%.

P: 0.025 mass% or less P is a harmful element that lowers the base metal toughness, weldability and weld zone toughness of steel, and is preferably reduced as much as possible. In particular, when the P content exceeds 0.025 mass%, the deterioration of the base metal toughness and the welded portion toughness increases. Therefore, P is set to 0.025 mass% or less. Preferably, it is 0.014 mass% or less.

S: 0.01 mass% or less Since S is a harmful element that lowers the toughness and weldability of steel, it is preferably reduced as much as possible. In the present invention, it is 0.01 mass% or less.

Al: 0.005-0.10 mass%
Al is an element added as a deoxidizer, and it is necessary to add 0.005 mass% or more. However, if it is added in excess of 0.10 mass%, the pH of the surface of the iron core is lowered due to Al ³⁺ eluted by the corrosion of the iron and the corrosion resistance is lowered, so the upper limit of the Al content is 0.10 mass%. .

W: 0.01-1.0 mass%
As described above, W improves the corrosion resistance of steel in the presence of the zinc primer and the epoxy resin coating film, but has the effect of significantly improving the corrosion resistance in the presence of the epoxy resin coating film and in the bare state. Therefore, in the present invention, it is one of the most important elements as an element for improving corrosion resistance. The above effect is manifested by adding W: 0.01 mass% or more. However, if the addition amount exceeds 1.0 mass%, the above effect is saturated. Therefore, the W content is in the range of 0.01 to 1.0 mass%. Preferably it is the range of 0.02-0.3 mass%. More preferably, it is the range of 0.02-0.2 mass%.

The reason why W has the above-described effect of improving corrosion resistance is that WO ₄ ²⁻ is generated in the rust generated along with corrosion of the steel sheet, and the presence of this WO ₄ ²⁻ causes chloride ions to enter the steel sheet surface. In addition to being suppressed, poorly soluble FeWO ₄ is generated at sites where the pH is lowered, such as the anode portion on the steel sheet surface, and the presence of this FeWO ₄ also suppresses the penetration of chloride ions into the steel sheet surface. As a result, corrosion of steel is effectively suppressed. Moreover, corrosion of steel is also suppressed by the inhibitor action of WO ₄ ²⁻ .

Cr: 0.01 mass% or more and less than 0.20 mass% Cr is a component that exhibits excellent corrosion resistance in the presence of a zinc primer and an epoxy resin coating film. One of the important elements. When the zinc primer is present, the corrosion resistance improving effect is as follows. Zn in the zinc primer is eluted, and Zn-based corrosion products such as ZnO and ZnCl ₂ · 4Zn (OH) ₂ are generated. It is presumed that it acts on this Zn-based corrosion product and has a function of further improving the anticorrosion property of the base iron by the Zn-based corrosion product. Such an effect of improving the corrosion resistance of Cr in the presence of a zinc primer is manifested when the content is 0.01 mass% or more. However, when it contains 0.20 mass% or more, the weld zone toughness is lowered. Therefore, the Cr content is set to a range of 0.01 mass% or more and less than 0.20 mass%. Preferably it is the range of 0.02-0.15 mass%. As described above, when both Cr and W in the above range are added, extremely good corrosion resistance can be obtained regardless of the type and presence of the coating film.

N: 0.001 to 0.008 mass%
N is a harmful component for the toughness of steel. Therefore, in order to improve toughness, N is desirably reduced as much as possible, and is set to 0.008 mass% or less. However, industrially, it is difficult to reduce N to less than 0.001 mass%. Therefore, in the present invention, the N content is in the range of 0.001 to 0.008 mass%.

In addition to the said component, the following component can be further added to the hot rolled shape steel for ships of this invention in order to further improve corrosion resistance.
One or two of Sb: 0.001 to 0.3 mass% and Sn: 0.001 to 0.3 mass% Sb is present in the presence of a zinc primer and an epoxy resin coating, in the presence of an epoxy resin coating and bare There is an effect of improving the corrosion resistance in the state. Sn has an effect of improving the corrosion resistance both in the presence of the epoxy resin coating film and in the bare state. The above effect of Sb and Sn is considered to suppress corrosion of a portion where the pH is lowered, such as the anode portion on the steel sheet surface. These effects are manifested when both Sn and Sb are contained in an amount of 0.001 mass% or more. However, if added over 0.3 mass%, the base material toughness and the HAZ part toughness are lowered, so it is preferable to add each in the range of 0.001 to 0.3 mass%. It is more preferable to add both Sb and Sn.

One or two of Cu: 0.005-0.5 mass%, Ni: 0.005-0.25 mass%, Mo: 0.01-0.5 mass% and Co: 0.01-1.0 mass% More than seeds Cu, Ni, Mo and Co improve the corrosion resistance of steel in the presence of zinc primer and epoxy coating and in the bare state, and Mo has the effect of improving the corrosion resistance even in the presence of epoxy coating. is there. Therefore, these elements can be supplementarily contained when it is desired to further improve the corrosion resistance. The above effect of Cu, Ni, Mo, Co is considered to be due to the refinement action of rust particles. Furthermore, in the case of Mo, chloride ions are formed on the surface of the steel sheet by generating MoO ₄ ²⁻ during rust. It is thought that it has also contributed to suppressing intrusion into the water. These effects are manifested by containing 0.005 mass% or more in Cu, Ni, 0.01 mass% or more in Mo, and 0.01 mass% or more in Co. However, even if Cu: more than 0.5 mass%, Ni: more than 0.25 mass%, Mo: more than 0.5 mass%, Co: more than 1.0 mass%, the effect is saturated and economically disadvantageous. Become. Therefore, Cu, Ni, Mo, and Co are preferably added in the above ranges.

Furthermore, in order to increase the strength and improve the toughness, the hot rolled steel of the present invention can further contain the following components in addition to the above components.
One or two of Nb: 0.001 to 0.1 mass%, Ti: 0.001 to 0.1 mass%, Zr: 0.001 to 0.1 mass%, and V: 0.002 to 0.2 mass% More than seeds Nb, Ti, Zr and V are all elements that increase the strength of steel, and can be selected and added according to the required strength. In order to obtain such an effect, it is preferable to add Nb, Ti, and Zr in an amount of 0.001 mass% or more, and V in an amount of 0.002 mass% or more. However, if Nb, Ti, Zr is added in an amount of 0.1 mass% and V exceeds 0.2 mass%, the toughness is lowered. On the other hand, Nb, Ti, Zr, V should be added up to the above value. preferable. A preferable upper limit is 0.04 mass%. Among these elements, Ti is the most preferable from the viewpoint of weld toughness, and Nb is the next most preferable.

B: 0.0002 to 0.003 mass%
B is an element that increases the strength of the steel, and can be contained as necessary. In order to acquire the said effect, adding 0.0002 mass% or more is preferable. However, if added over 0.003 mass%, the toughness is reduced instead. Therefore, it is preferable to add B in the range of 0.0002 to 0.003 mass%.

Ca: 0.0002 to 0.01 mass%, REM: 0.0002 to 0.015 mass%, and Y: 0.0001 to 0.1 mass%, or two or more of Ca, REM, and Y It is an element effective in improving the toughness of the weld heat affected zone, and can be selected and added as necessary. This effect can be obtained by adding Ca: 0.0002 mass% or more, REM: 0.0002 mass% or more, and Y: 0.0001 mass% or more. However, since Ca: 0.01 mass%, REM: 0.015 mass%, and Y: addition exceeding 0.1 mass% will cause a decrease in toughness, Ca, REM and Y are added with the above values as the upper limit. It is preferable to do this.

  In the hot-rolled steel section for ships of the present invention, components other than the 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.

Next, the microstructure of the marine hot-rolled section steel having high strength and excellent corrosion resistance according to the present invention will be described.
In marine steel plates, especially high strength thick steel plates with a yield stress YP of 315 MPa or more, generally, a steel material imparted with a high weldability as a low carbon equivalent is employed with TMCP combining controlled rolling and controlled cooling. High strength is achieved by introducing a hard bainite structure as two phases. And the case where low temperature toughness is calculated | required and the request | requirement to thickening respond | correspond by optimizing the conditions of the said controlled rolling and controlled cooling. Therefore, in this case, the microstructure of the steel sheet is usually a ferrite + bainite structure.

  On the other hand, in the case of marine hot-rolled section steel, the width and thickness of the short side and the long side are often different (for example, unequal side unequal thick angle steel with a non-rectangular cross section), and inevitably rolled. Temperature unevenness occurs during cooling and cooling. In particular, strength adjustment using controlled cooling (accelerated cooling) results in uneven residual stress, inducing twisting, bending, warping, and a reduction in dimensional accuracy, thus increasing the shape correction load after rolling. Therefore, it is difficult to apply this method of increasing the strength by introducing a hard bainite structure as the second phase to the hot rolled shape steel. This is true for all of the marine hot-rolled sections such as rolled T-sections.

  Accordingly, in the marine hot-rolled section steel, it is required to achieve a high strength of yield stress YP: 315 MPa or more and tensile strength TS: 440 MPa or more without performing accelerated cooling after rolling. For this purpose, it is necessary to increase the strength with a ferrite + pearlite structure which is a normal hot rolled structure. As a means to achieve high strength in the ferrite + pearlite structure, a method of increasing the pearlite fraction of the second phase, a method of further reducing the ferrite structure, a method of hardening the ferrite by solid solution strengthening or precipitation strengthening, Alternatively, a method of hot rolling in the (γ + α) two-phase region to make a part of the ferrite a processed ferrite having a high dislocation density can be considered.

  Of the above methods, the method of refining ferrite is advantageous for increasing YP, but since the increase in TS is small, sufficient strength cannot be achieved by this method alone. Moreover, although the method of increasing a pearlite fraction needs to add C in large quantities, since excessive addition of C causes the fall of weldability, it is unpreferable. Moreover, the method of strengthening ferrite by adding a solid solution strengthening element or a precipitation strengthening element causes a decrease in weldability or an increase in material cost due to the addition of a large amount of alloy elements. On the other hand, the utilization of processed ferrite can suppress the addition of C and alloy elements to the minimum, and can increase YP and TS while maintaining weldability. In other words, since the method using the processed ferrite can achieve high strength without hot-rolling and controlled cooling (accelerated cooling), rolling is a problem inherent in the production of hot-rolled section steel for ships. It is possible to increase the strength while suppressing the occurrence of bending and warping during cooling. Therefore, in the present invention, as a means for increasing the strength of the marine hot-rolled section steel, a method is adopted in which the microstructure of the steel is a ferrite containing processed ferrite + pearlite structure.

Here, the fraction of the processed ferrite is preferably in the range of 10 to 70% of the entire steel structure as an area ratio. If the fraction of the processed ferrite is less than 10%, the steel cannot be sufficiently strengthened. On the other hand, if it exceeds 70%, the increase in strength is saturated and a roll accompanying an increase in load during two-phase rolling of (α + γ). This is because the risk of breakage increases. The processed ferrite is a ferrite into which a working strain formed by hot rolling in the (α + γ) two-phase region below the Ar ₃ transformation point is introduced. Usually, the flattened processed ferrite is traced. The area occupied in the microstructure can be quantified and the fraction can be measured. The measurement position of the microstructure is preferably a ¼ part thickness at the thickest part.
In addition, it is preferable that the entire ferrite including the processed ferrite is present in an area ratio of about 10 to 70% of the entire steel structure. The balance is a pearlite structure, but a structure other than ferrite and pearlite, that is, bainite or the like may be present in an area ratio of 10% or less.

Next, a method for producing a marine hot-rolled section steel having a ferrite + pearlite structure containing the processed ferrite will be described.
In producing the marine hot-rolled section steel of the present invention, first, steel having the above-described component composition is melted by a generally known method such as a converter, an electric furnace, etc., and a generally known method such as a continuous casting method or an ingot forming method. It is preferable to use a steel material such as a slab or billet by the above method. In addition, after melting, treatment such as ladle refining or vacuum degassing may be added.

  Next, the steel material is charged into a heating furnace, reheated, and hot-rolled to obtain a hot-rolled steel for ships having desired dimensions, structure, and characteristics. At this time, the reheating temperature of the steel material needs to be in the range of 1000 to 1350 ° C. When the heating temperature is less than 1000 ° C., the deformation resistance is large and hot rolling becomes difficult. On the other hand, heating exceeding 1350 ° C. causes generation of surface marks, or increases scale loss and fuel consumption rate. Preferably, it is the range of 1100-1300 degreeC.

In the subsequent hot rolling, it is necessary that the cumulative rolling reduction at the Ar ₃ temperature or lower is 10 to 80%. When the rolling temperature is Ar ₃ temperature or higher, the microstructure of the steel does not contain processed ferrite, and the required strength and toughness cannot be ensured. Similarly, if the cumulative rolling reduction at an Ar ₃ temperature or less is less than 10%, the toughening effect is small because the amount of processed ferrite produced is small. Conversely, when the rolling reduction exceeds 80%, the rolling load increases and rolling becomes difficult, or the number of rolling passes increases, leading to a decrease in productivity. Therefore, the cumulative rolling reduction at the Ar ₃ temperature or lower is set to 10 to 80%. Preferably, it is 10 to 60% of range. Note that rolling at an Ar ₃ temperature or lower may be performed at least one pass, or may be a plurality of passes. Here, the cumulative rolling reduction at Ar ₃ temperature or less, refers to a cross-section area reduction rate of the cross-sectional area of the rolled material after rolling completion to the cross-sectional area of the rolled material (A) in the Ar ₃ temperature (B), It is expressed by the following formula.
(Cumulative rolling reduction [%] at Ar ₃ temperature or lower [%]) = 100 × (A−B) / A

Further, between the hot rolling needs to be performed in the conditions of the finish rolling temperature _{(Ar 3 -30 ℃) ~ (} Ar 3 -180 ℃). When the rolling finishing temperature exceeds (Ar ₃ -30 ° C), the toughening effect by the two-phase region rolling cannot be sufficiently obtained. On the other hand, when the rolling finishing temperature is less than (Ar ₃ -180 ° C), the rolling load increases due to the increase in deformation resistance. It is because it increases and it becomes difficult to roll.

Further, in the above hot rolling, it is preferable to perform rolling at an Ar ₃ temperature or less so that the temperature difference at each part in the cross section of the marine hot rolled section steel is within 50 ° C. For example, among the hot-rolled steel shapes for ships, for unequal side unequal thick angle steels with a difference in wall thickness between the long side and the short side, the short side is thicker than the long side where the wall is thin. It is preferable that the side is cooled with water before and after the rolling mill to suppress the temperature difference between the long side and the short side to within 50 ° C. If the temperature difference exceeds 50 ° C., not only the variation in strength and toughness characteristics between the short side and the long side will increase, but also the bending in the cooling process after rolling will increase, and the burden required for correction will increase. Reduce productivity. The temperature difference in the cross section of the section steel is obtained by measuring the surface temperature of the flange and the web with a radiation thermometer and obtaining the difference between the maximum temperature and the minimum temperature obtained.

As a means for suppressing the temperature difference between the short side and the long side to be within 50 ° C., a method of controlling cooling using cooling equipment disposed before and after the rough rolling mill is preferable. Specifically, a method of eliminating the temperature difference by intensively water-cooling the thicker short side with the cooling facility is preferable. The water cooling at this time may be performed only on the front surface before and after the rolling mill, only on the rear surface, or both on the front and back sides, or may be performed a plurality of times depending on the dimensions and required accuracy of the shape steel to be rolled. Absent. The water density at the time of water cooling is preferably 1 m ³ / m · min or more.

  The cooling following the hot rolling is not particularly limited, but it is preferable to cool. Thereby, the shape change of the shape steel, such as bending and warping resulting from non-uniform cooling after rolling, can be reduced, and the correction burden on the product after rolling can be reduced.

  Steel having the composition shown in Table 2 was melted in a vacuum melting furnace or converter to form a bloom, and after this bloom was charged in a heating furnace and heated, it was hot-rolled under the conditions shown in Table 3. Unequal-sided unequal thick angle steel (NAB) and rolled T-shaped steel having the cross-sectional dimensions shown in Table 3 were produced. In Table 3, regarding the unequal side unequal thick angle steel (NAB), the long side is shown as a web and the short side is shown as a flange. JIS1A tensile test specimens were collected from the short side for unequal side unequal thick angle steel and from the flange for T-shaped steel, and the tensile properties (yield stress YP, tensile strength TS, elongation El) were measured. Also, Charpy impact test piece (2mmV notch) from the center of HAZ where butt multi-layer welding (GMAW) was applied with heat input of 20kJ / cm, short side for unequal side unequal thickness angle steel and T-type flange. Test piece) was collected, and the absorbed energy in a Charpy impact test at -20 ° C was measured. In addition, a sample for observing the structure was collected from the short side for the unequal side unequal thickness angle steel, and from the flange for the T shape steel, and the structure having a thickness of 1/4 part was observed with a microscope at a magnification of 200 times. The flattened processed ferrite produced by two-phase rolling was traced, and the area occupied in the microstructure was quantified by image analysis to determine the fraction of processed ferrite.

Next, for each hot-rolled section steel, a test piece of 5 mmt × 100 mmW × 200 mmL or 5 mmt × 50 mmW × 150 mmL was taken from the short side of the unequal side unequal thickness angle steel and from the flange of the T shape steel. The surface of the test piece was shot blasted and then subjected to surface treatment under the following conditions A to C to obtain a corrosion resistance test piece.
<Surface treatment conditions>
Condition A: Two-layer coating of zinc primer (film thickness of about 15 μm) and tar epoxy resin paint (film thickness of about 200 μm) is formed on the surface of the test piece Condition B: Tar epoxy resin paint (film thickness of about 200 μm) on the surface of the test piece ) Condition C: Bare state with shot blasting on test piece surface (no anticorrosion coating)
In addition, the test piece of the said conditions A and B which formed the coating film was provided with the scratch scissors of 80 mm in length which reaches the surface of the iron bar with a cutter knife from the top of the coating film.

  The test piece produced as described above was then subjected to an exposure test that was mounted on the back side of the upper deck of the ballast tank of the actual ship for 2 years. On average, the corrosive environment of this exposure test was a cycle in which the period in which the seawater was in the ballast tank was about 20 days and the period in which the seawater was not contained was about 20 days. Corrosion resistance evaluation in the exposure test is as follows. For test pieces of conditions A and B having a coating film, the swollen area of the coating film generated around the scratch ridge is measured, and for a test piece of condition C having no coating film. After the test, derust, calculate the average reduction in thickness from the difference (decrease) in the test piece mass after the derusting and the test piece mass before the test, and use these results as the corrosion resistance improving element. No. Using 12 steels as the base (100), the ratio of each test piece was calculated and evaluated.

Table 4 shows the results of the tensile test, impact test, microstructure investigation, and corrosion resistance test. From the results of the corrosion test, No. of the invention example satisfying the component composition of the present invention. Steels Nos. 1 to 13 (Nos. 4 and 9 are reference examples) have a bulge area and a reduction in sheet thickness of 50% or less with respect to the base steel (No. 14) in any of the conditions AC It can be seen that it has good corrosion resistance. On the other hand, No. which does not satisfy the component composition of the present invention. Even though the steels of Nos. 14 to 17 have higher corrosion resistance than the base steel (No. 14), there are conditions where the ratio to the base steel exceeds 50%, and the toughness of the welded portion is greatly reduced. In addition, when the microstructure was a ferrite including a processed ferrite + pearlite structure, the sufficient strength as expected in the present invention was obtained, the shape change such as bending and warping was slight, and the productivity was extremely good. .
In addition, in the shape steel of the rolling code a (when the temperature difference in the cross section of the shape steel exceeds 50 ° C. in the hot rolling at a temperature of Ar ₃ or lower), the characteristic value has reached the target, but it is bent and warped. Was big.

  Since the hot-rolled steel for ships of the present invention exhibits excellent corrosion resistance in a corrosive environment caused by seawater, it is effective for extending the life of the ship itself by extending the repair period of the ship. It can also be used for hot rolled section steel used in other fields.

Claims (11)

C: 0.03-0.25 mass%, Si: 0.05-0.50 mass%, Mn: 0.1-2.0 mass%, P: 0.025 mass% or less, S: 0.01 mass% or less, Al : 0.005 to 0.10 mass%, W: 0.01 to 1.0 mass%, Cr: 0.01 mass% or more and less than 0.20 mass%, N: 0.001 to 0.008 mass%, the balance being A hot-rolled shape steel for ship ballast tanks having a component composition composed of Fe and inevitable impurities and having a microstructure composed of ferrite containing processed ferrite and a pearlite structure and excellent in corrosion resistance. 2. In addition to the said component composition, 1 type or 2 types chosen from Sb: 0.001-0.3mass% and Sn: 0.001-0.3mass% are contained, It is characterized by the above-mentioned. Hot-rolled section steel for ship ballast tanks with excellent corrosion resistance. In addition to the above component composition, the composition further comprises one or two selected from Mo: 0.01 to 0.5 mass% and Co: 0.01 to 1.0 mass%. The hot-rolled section steel for ballast tanks of a ship excellent in corrosion resistance according to 1 or 2. In addition to the above component composition, Nb: 0.001 to 0.1 mass%, Ti: 0.001 to 0.1 mass%, Zr: 0.001 to 0.1 mass%, and V: 0.002 to 0.2 mass The hot-rolled section steel for ballast tanks of a ship excellent in corrosion resistance according to any one of claims 1 to 3, comprising one or more selected from the group%. In addition to the said component composition, B: 0.0002-0.003 mass% is contained, Hot rolling for the ballast tank of the ship excellent in corrosion resistance of any one of Claims 1-4 characterized by the above-mentioned. Shape steel. In addition to the above component composition, one or two selected from Ca: 0.0002 to 0.01 mass%, REM: 0.0002 to 0.015 mass%, and Y: 0.0001 to 0.1 mass% The hot-rolled section steel for a ballast tank of a ship excellent in corrosion resistance according to any one of claims 1 to 5, comprising the above. The hot-rolled shape steel for ballast tanks of ships having excellent corrosion resistance according to any one of claims 1 to 6, wherein the shape steel has an epoxy resin coating on the surface thereof. The hot rolled steel for ballast tanks for ships having excellent corrosion resistance according to any one of claims 1 to 6, wherein the steel has a zinc primer coating on the surface of the steel. The hot-rolled section steel for ballast tanks of ships excellent in corrosion resistance according to any one of claims 1 to 6, wherein the surface of the section steel has a zinc primer coating and an epoxy resin coating. In manufacturing the hot-rolled shape steel for a ballast tank of a ship according to claim 1 to 6, after rolling the steel material to 1000 to 1350 ° C., the cumulative rolling reduction at an Ar ₃ temperature or lower is 10 to 80%, and rolling A method for producing a hot-rolled section steel for a ballast tank of a ship , characterized by performing hot rolling with a finishing temperature of (Ar ₃ -30 ° C) to (Ar ₃ -180 ° C) and then allowing to cool. The hot rolling steel for ballast tanks of a ship according to claim 10, wherein the manufacturing method performs hot rolling at an Ar ₃ temperature or less with a temperature difference within a section of the steel shape within 50 ° C. Manufacturing method.