KR101241935B1 - Hot-rolled shape steel for ships and process for manufacturing the same - Google Patents

Abstract

In mass%, C: 0.03 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.1 to 2.0%, P: 0.025% or less, S: 0.01% or less, Al: 0.005 to 0.10%, W: 0.01 to 1.0% , Cr: 0.01% or more and less than 0.20%, after heating the steel material containing N: 0.001% to 0.008% at 1000 to 1350 ° C, the cumulative reduction ratio at the Ar ₃ temperature or less is 10 to 80% and the rolling finish temperature ( Ar ₃ -30 ℃) ~ subjected to hot rolling to a (Ar ₃ -180 ℃), then, by the vessel, hot-rolled section steel having a micro-tissue made of a ferrite and pearlite structure containing ferrite processed by cooling, the ship It provides inexpensive hot-rolled steel for ships used for longitudinal agents (loan materials) and the like, which has excellent corrosion resistance under severe corrosive environments by seawater such as ballast tanks and has a strength of 315 MPa or more.

Description

Hot rolled section steel for ships and its manufacturing method {HOT-ROLLED SHAPE STEEL FOR SHIPS AND PROCESS FOR MANUFACTURING THE SAME}

The present invention is a coal ship or ore carrier (ore carrier), ore coal carrier (ore coal carrier), crude oil tanker (crude oil tanker), LPG carrier (LPG carrier), LNG carrier (LNG carrier), chemical Chemical tankers, container ships, bulk carriers, log carriers, chip carriers, refrigerated cargo ships, pure car carriers, The present invention relates to a marine section steel used in heavy load carriers, roll-on / roll-off ships, limestone carriers, cement carriers, and the like. TECHNICAL FIELD This invention relates especially to the hot-rolled steel for ships used for longitudinal materials (longi material) etc. of a ballast tank under severe corrosion environment by seawater, and its manufacturing method.

Here, the hot rolled section steel refers to a section steel formed into a final shape by hot rolling. (On the other hand, a section steel made of a thick steel sheet is obtained by cutting and welding the thick steel sheet to a predetermined dimension to obtain a final shape of the section steel. ). In addition, the ship's hot-rolled steel used for longitudinal materials (long paper), etc. are specifically, the equal leg angle (AB), the unequal leg angle (ABS), which were shape | molded by hot rolling, Unequal leg and thickness angle (NAB), channel beam (CB), bulb plate (BP), T-bar and the like.

When there is no dripping, the ballast tank of a ship plays the role which injects seawater and enables stable navigation of a ship. For that reason, the ballast tanks are placed under very severe corrosion environment. Therefore, in the anticorrosion of steel used for a ballast tank, the formation of the anti-corrosion paint film by epoxy paint and cathodic protection are used together.

However, even if these anticorrosion measures are taken, the corrosion environment of the ballast tank is still in a severe condition. That is, when seawater is injected into a ballast tank, since the electric system functions in the part completely submerged in seawater, corrosion can be suppressed. However, near the top of the ballast tank, in particular the back deck of the upper deck, is in a state where only the splash of seawater is hit without being submerged in seawater. Therefore, the electric system does not function at this site. Moreover, since this steel plate temperature rises with sunlight, it becomes a more severe corrosion environment. On the other hand, when seawater is not injected into the ballast tank, since the electric system does not work at all, severe corrosion is caused by the remaining salt attached.

Therefore, the lifespan of the anticorrosive coating film of the ballast tank in the severe corrosive environment mentioned above is generally called about 10 years, and is about half of the life of a ship (about 20 years). Therefore, it is a fact that corrosion resistance is maintained by performing repair painting etc. for the remaining 10 years. However, since the corrosive environment of a 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 maintenance coating becomes work in a narrow space, it is not preferable as a work environment. Therefore, development of steel materials excellent in corrosion resistance which can extend the period until repair coating as much as possible, and can reduce a work load is desired.

Therefore, some techniques for improving the corrosion resistance of steel materials used under severe corrosive environments such as ballast tanks have been proposed.

For example, Japanese Patent Application Laid-Open No. 48-050921 (Patent Document 1) discloses Cu: 0.05 to 0.50 mass% as an element that improves corrosion resistance in steel of C: 0.20 mass% or less. W: 0.01 to 0.05% by mass or less, and further, 0.01 to 0.2 mass% of one or two or more of Ge, Sn, Pb, As, Sb, Bi, Te, and Be. -corrosion low alloy steel). In addition, Japanese Unexamined Patent Publication No. 48-050922 (Patent Document 2) adds Cu: 0.05 to 0.50 mass% and W: 0.05 to 0.5 mass% as a corrosion resistance improving element to C: 0.20 mass% or less. Furthermore, the corrosion-resistant low alloy steel which added 0.01-0.2 mass% of 1 type, or 2 or more types of Ge, Sn, Pb, As, Sb, Bi, Te, and Be is disclosed. In addition, Japanese Unexamined Patent Publication No. 48-050924 (Patent Document 3) discloses a corrosion-resistant low alloy steel obtained by adding C: 0.15 mass% or less to Cu: 0.05 to 0.15 mass% and W: 0.05 to 0.5 mass%. Is disclosed.

In addition, Japanese Patent Application Laid-Open No. 07-034197 (Patent Document 4) discloses P: 0.03-0.10 mass%, Cu: 0.1-1.0 mass%, Ni as a corrosion resistance improving element in C: 0.15 mass% or less. : Corrosion-resistant paints, such as tar epoxy paint, pure epoxy paint, epoxy paint without solvent, and urethane paint, for low alloy corrosion resistant steels having 0.2 to 1.0 mass% added thereto. And the resin-coated ballast tank is disclosed. This technique aims to extend the life of anticorrosive coating by improving the corrosion resistance of the steel itself and to achieve maintenance-free over a period of 20 to 30 years, which is the life of the ship.

In addition, Japanese Laid-Open Patent Publication No. 07-034196 (Patent Document 5) adds Cr: 0.2-5 mass% as a corrosion resistance improving element to C: 0.15 mass% or less to improve corrosion resistance, A proposal has been made to realize maintenance freeization.

In addition, Japanese Patent Application Laid-Open No. 07-034270 (Patent Document 6) uses a steel material containing Cr: 0.2 to 5 mass% as a constituent material as a corrosion resistance improving element in C: 0.15 mass% or less. In addition, an anticorrosive method of a ballast tank has been proposed, wherein the oxygen gas concentration inside the ballast tank is set at a ratio of 0.5 or less with respect to the value in the atmosphere.

In addition, Japanese Laid-Open Patent Publication No. 07-310141 (Patent Document 7) improves corrosion resistance by adding Cr: 0.5 to 3.5 mass% to steel of C: 0.1 mass% or less, thereby realizing maintenance maintenance of the vessel. The proposal is made.

In addition, Japanese Laid-Open Patent Publication No. 2002-266052 (Patent Document 8) adds Ni: 0.1 to 4.0 mass% to C: 0.001 to 0.025 mass% of steel, thereby providing paint-film damage resistance. Is disclosed, and steel materials for ships that reduce maintenance costs such as maintenance coating are disclosed.

In addition, Japanese Unexamined Patent Publication No. 2000-017381 (Patent Document 9) adds Cu: 0.01-2.00 mass% and Mg: 0.0002-0.0150 mass% to C: 0.01-0.25 mass% of steel, thereby providing a ship shell. Marine steels having corrosion resistance in use environments such as ballast tanks, cargo oil tanks, ore and coal hold are disclosed.

Furthermore, in Japanese Laid-Open Patent Publication No. 2004-204344 (Patent Document 10), in a steel of C: 0.001 to 0.2 mass%, Mo, W and Cu are added in combination to limit the addition amount of P and S as impurities. The steel which suppressed the full surface corrosion and local corrosion which generate | occur | produces in a crude oil tank is disclosed.

However, a zinc primer, an epoxy resin paint, or the like is usually applied to the steel constituting the ballast tank or the like. In the technique disclosed in the said patent documents 1-3, sufficient examination is not made about corrosion resistance in the presence of these coating films, and there exists room for further examination.

In addition, in order to improve the corrosion resistance of the base metal, the steel materials disclosed in Patent Document 4 are added with P in a relatively large amount at 0.03 to 0.10 mass%, and thus weldability and toughness of weld joints There is a problem.

In addition, since the steel materials disclosed by patent document 5 and patent document 6 contain 0.2-5 mass% of Cr, and the steel material disclosed by patent document 7 contains relatively large amount of Cr at 0.5-3.5 mass%, all There is a problem in weldability and weld toughness. Moreover, the steel materials with many Cr contents have a problem that manufacturing cost becomes high.

Furthermore, since the steel material disclosed by patent document 8 has low C content and high Ni content, there exists a problem that manufacturing cost becomes high.

Moreover, since the steel material disclosed by patent document 9 makes Mg addition essential, there exists a problem that a steelmaking yield is not stabilized and the mechanical property of steel materials is also not stable.

In addition, the steel disclosed in Patent Document 10 is a corrosion resistant steel developed for use in an environment where H ₂ S called a crude oil tank exists, and corrosion resistance in ballast tanks in which H ₂ S does not exist is unknown. Do. In addition, the corrosion resistance in the state in which the said zinc primer was apply | coated is not examined. Therefore, there is room for further corrosion resistance examination for use in ballast tanks.

In general, ships are constructed by welding steel materials such as thick plates, thin plates, section steels, bars, and the like, and anticorrosive coating is performed on the surfaces of the steel materials. The anticorrosive coating is coated with a zinc primer as primary rust prevention, after subassembly or after large assembly, as a secondary rust prevention. It is common that epoxy resin coating is performed. Therefore, most of the steel surface of the ship is subjected to anticorrosive coating of a two-layer structure of zinc primer and epoxy resin coating. In addition, since the weld primer burned out by the heat at the time of welding, repair coating (touch-up) by the zinc primer was performed as a rust prevention measure between after welding and this painting. All. However, when the period until this coating is short, repair coating may not be performed. Moreover, in the ship used for a long time after drying, there exists a part which the said coating film deteriorates and does not fully perform the function as an rustproof coating film, or the part in which the coating film peels and the steel plate is exposed.

That is, three states of the part in which the coating of two layers of a zinc primer and an epoxy resin coating are performed, the part only of the epoxy resin coating, and the part of the exposed state exist on the surface of the steel material of the ship in service. Therefore, in order to achieve the objective of improving the corrosion resistance of a ship, it is necessary to be a ship steel material which shows the outstanding corrosion resistance in any of those states.

However, thick steel plates used in ships are undergoing high strength from the viewpoint of cost reduction and securing safety by reducing the amount of steel used, and yield stress is 315 MPa or more, and tensile strength (TS) high strength materials with a tensile strength of 440 MPa or more are used. In the case of a thick steel sheet, control of strength and toughness is generally achieved by adjusting the conditions of a controlled rolling / accelerated cooling process (TMCP: Thermo-Mechanical Control Process).

On the other hand, the steel used in the ballistic tank of the ballast tank and the like, and especially, the hot rolled steel such as the post-isotropic acid-shaped steel or T-shaped steel is more complicated in cross-sectional shape and dimensions compared to the thick steel plate used in the same ship. As a control method of toughness, it is difficult to adopt TMCP similar to a thick steel plate. In particular, in the shape steel, it is necessary to produce the material while considering the bending and the bending during the rolling. Therefore, in order to obtain a high-strength shape steel having a yield stress of 315 MPa or more, it is necessary to examine a method for manufacturing the original steel.

Therefore, an object of the present invention is to exhibit excellent corrosion resistance without being influenced by the existence state of the coating film in severe corrosive environments such as ballast tanks of ships, and to extend the period until repair painting, and furthermore, repair painting. The present invention provides an inexpensive hot-rolled steel for ships having a strength of 315 MPa or more, which is excellent in corrosion resistance for reducing work.

The inventors have intensively studied for the development of a section steel having a high strength while exhibiting excellent corrosion resistance even in a severe corrosive environment by seawater without being influenced by the surface state (existing state of the coating film). As a result,

By adding W and Cr as essential elements and further including corrosion resistance improving elements such as Sb and Sn in an appropriate range, a two-layer coating film state of an epoxy primer and an epoxy resin coating, an epoxy resin coating film state, and an exposed state To obtain hot-rolled steel for ships exhibiting excellent corrosion resistance in any of the following conditions, and

In order to increase the strength of the shaped steel without degrading productivity or weldability, it has been found that it is effective to introduce strain hardening ferrite by (α + γ) hot rolling during (γ + α) region. The present invention has been completed.

That is, in the present invention, 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: 0.005-0.10 mass %, W: 0.01 to 1.0 mass%, Cr: 0.01 mass% or more, less than 0.20 mass%, N: 0.001 to 0.008 mass%, the balance has a component composition composed of Fe and unavoidable impurities, and includes processed ferrite It is a ship's hot-rolled section steel excellent in corrosion resistance which has a microstructure which consists of ferrite and pearlite.

It is preferable that the hot-rolled steel for ships of this invention contain the component which belongs to at least 1 group among the following A-E groups in addition to the said component composition.

Group A; 1 type or 2 types chosen from Sb: 0.001-0.3 mass% and Sn: 0.001-0.3 mass%

Group B; 1 type or 2 or more types chosen from 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%

Group C; Nb: 0.001-0.1 mass%, Ti: 0.001-0.1 mass%, Zr: 0.001-0.1 mass%, and V: 0.002-0.2 mass%

Group D; B: 0.0002 to 0.003 mass%

E group; 1 or 2 or more types selected from Ca: 0.0002 to 0.01 mass%, REM: 0.0002 to 0.015 mass%, and Y: 0.0001 to 0.1 mass%

In addition, the hot-rolled steel for ships of the present invention, on its surface,

Epoxy resin coating film,

Zinc primer coating film,

It is preferable to have either a zinc primer coating film and an epoxy resin coating film.

In addition, the present invention is in the manufacture of the vessel, hot-rolled section steel, heating the steel material having the above composition in the 1000~1350 ℃, then, Ar ₃ temperature of (Ar ₃ transformation point) 10 to the cumulative rolling reduction below 80%, the rolling finishing temperature _{(finishing temperature) (Ar 3 -30} ℃) ~ (Ar 3 -180 ℃) subjected to hot rolling which, after which the vessel was allowed to cool hot-rolled section steel, characterized in that (air cooling) It is a manufacturing method.

The production method of the present invention, it is preferable to perform the hot rolling at the Ar ₃ temperature or less, and the temperature difference in the beams in cross-section less than 50 ℃.

Best Mode for Carrying Out the Invention [

(Furtherance)

The inventors have three states that exist in steel used in a ship in service, that is, a state having a two-layer coating film of a zinc primer and an epoxy resin coating film, a state of only an epoxy resin coating film, and an exposed state. In order to develop the hot rolled section steel for ship which has the outstanding corrosion resistance also in the following experiment, it carried out.

Steels to which various alloying elements were added were experimentally melted and hot rolled to obtain a hot rolled sheet having a sheet thickness of 5 mm. A 5 mmt x 100 mmW x 200 mmL or 5 mmt x 50 mmW x 150 mmL test piece is taken from these hot rolled sheets, and shot blasting is performed on the surface of the test piece to scale the surface. : After removing oxide film and oil, the test piece for exposure test which produced the following three types of surface treatments was produced.

Condition A: A two-layer film of zinc primer (film thickness of about 15 µm) and tar epoxy resin paint (film thickness of approximately 100 µm) was formed on the surface of the test piece.

Condition B: A single layer film of tar epoxy resin coating material (film thickness of about 100 μm) was formed on the surface of the test piece.

Condition C: The exposed state with shot blasting on the surface of the specimen (no corrosion coating)

Then, these test pieces were used for the corrosion test on the condition which simulated the corrosion environment on the back of the upper deck of an actual ship ballast tank. Specifically, (35 ° C., 5 mass% NaCl solution spray × 2hr) → (60 ° C, RH (relative humidity) 5 mass% × 4hr) → (50 ° C, RH 95 mass% × 2hr) as 1 cycle, This was provided to the salt spray dry-wet repeated corrosion test which carries out 132 cycles.

About the test piece of the conditions A and B which have a coating film, before the test, the scratch scratch of 80 mm length reaching from the top of a coating film to a branch iron surface with a cutter knife is provided in the form of a straight line, and after a test, the scratch Corrosion resistance was evaluated by measuring the area of paint swelling that occurred around the scratch. In addition, about the test piece of the condition C which does not have a coating film, it derusts with hydrochloric acid after a test, and calculates an average plate | board thickness decrease from the difference (reduction amount) of the weight of the dehydrated test piece and the weight of the test piece before a corrosion test, and shows corrosion resistance. Evaluated.

From the result of the said corrosion test, the corrosion resistance improvement result by each alloying element was put together for the coating film conditions of the test piece surface, and is shown in Table 1. To briefly describe this result,

1) in the case of condition A (two-layer coating film of zinc primer + tar epoxy resin coating); The most effective element for improving the corrosion resistance is Cr, followed by W and then Sb.

2) in the case of Condition B (only one layer of the tar epoxy resin coating film); The most effective element for improving the corrosion resistance is W, followed by Sb and Sn.

3) for condition C (exposed state); The most effective element for improving the corrosion resistance is W, followed by Sb and Sn.

4) When W and Cr are added in combination, the corrosion resistance under the condition A is improved compared with the case where it is contained alone, and when Sb and Sn are added additionally, not only condition A but also conditions B and C bring a remarkable improvement effect.

5) Addition of Mo slightly improves the corrosion resistance under conditions A, B, and C, and Cu, Ni, and Co slightly improves the corrosion resistance under conditions A and C.

[Table 1]

Based on the result of the said test, in this invention, the component system which combines W and Cr as a basic element which improves corrosion resistance is employ | adopted, and when corrosion resistance is requested | required, 1 or 2 types chosen from Sb and Sn is required. The component design which adds and adds was decided. And when more excellent corrosion resistance is calculated | required, 1 type (s) or 2 or more types chosen from Ni, Mo, Co, and Cu were added.

Next, the component composition which the hot-rolled steel for ship which is excellent in corrosion resistance of this invention should have is demonstrated.

C: 0.03-0.25 mass%

C is an effective element for increasing the strength of the steel, and in the present invention, it is necessary to contain C at least 0.03 mass% in order to obtain a desired strength. On the other hand, addition exceeding 0.25 mass% reduces the toughness of a weld heat affected zone (HAZ: Heat Affected Zone). Therefore, C content is taken as the range of 0.03-0.25 mass%. Moreover, the range of C is 0.05-0.20 mass% from a viewpoint of making intensity and toughness compatible with the process ferrite mentioned later.

Si: 0.05 to 0.50 mass%

Si is an element added in order to increase the strength of the steel as a deoxidizing agent and, in the present invention, is added at 0.05 mass% or more. However, since the addition exceeding 0.50 mass% reduces the toughness of steel, the upper limit of Si is made into 0.50 mass%.

Mn: 0.1-2.0 mass%

Mn is an element having the effect of preventing hot shortness and increasing the strength of the steel, and is added in an amount of 0.1 mass% or more. However, since the addition of more than 2.0 mass% of Mn reduces the toughness and weldability of steel, an upper limit shall be 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 which lowers the base material toughness, weldability, and weld part toughness of steel, and it is preferable to reduce it as much as possible. In particular, when the content of P exceeds 0.025 mass%, the lowering of the base metal toughness and the welded part toughness increases. Therefore, P is made into 0.025 mass% or less. Preferably it is 0.014 mass% or less. Although P may be free of addition, the practical lower limit is about 0.005 mass% in industrial production.

S: 0.01 mass% or less

Since S is a harmful element that lowers the toughness and weldability of steel, it is preferable to reduce S as much as possible. In the present invention, S is made 0.01 mass% or less. Although S may be free of addition, the practical lower limit in industrial production is about 0.001 mass%.

Al: 0.005 to 0.10 mass%

Al is an element added as a deoxidizer and needs to be added at 0.005 mass% or more. However, over the addition of 0.10 mass%, and the pH of the metal part surface decreases due to the Al ^{3 +} eluted by corrosion of a metal part, because the corrosion resistance is lowered, the upper limit of the Al content is set to 0.10 mass%.

W: 0.01-1.0 mass%

As described above, W also improves the corrosion resistance of the steel in the presence of the zinc primer and the epoxy resin coating film, but particularly has the effect of significantly improving the corrosion resistance in the presence and exposure of the epoxy resin coating film. Therefore, in this invention, it is one of the most important elements as a corrosion resistance improvement element. The said effect is expressed by addition of W: 0.01 mass% or more. However, when the addition amount exceeds 1.0 mass%, the above effect is saturated. Therefore, content of W is made into the range of 0.01-1.0 mass%. Preferably it is the range of 0.02-0.3 mass%. More preferably, it is 0.2 mass% or less.

The reason why W has the above corrosion resistance improving effect is

WO ₄ ^{2 −} is formed in the rust generated by corrosion of the steel sheet, and the presence of this WO ₄ ^{2 −} suppresses the penetration of chloride ions into the surface of the steel sheet,

Poorly soluble FeWO ₄ is formed in a portion of the surface of the steel sheet where the pH is lowered, and the presence of FeWO ₄ inhibits the penetration of chloride ions into the steel sheet surface.

This is because corrosion of the steel is effectively suppressed. In addition, corrosion of steel is also suppressed by the inhibitory effect of WO ₄ ^{2 −} .

Cr: 0.01% by 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, and is one of important elements in the hot rolled steel for ships of the present invention.

The said corrosion resistance improvement effect is estimated by the following reason. In the case where a zinc primer is present, Zn in the zinc primer elutes on the surface to form a zinc-based corrosion product such as ZnO or ZnCl ₂ · 4Zn (OH) ₂ . Cr acts on this Zn type corrosion product, and it is estimated that there exists an effect which improves the antiferrous corrosion resistance by Zn type corrosion product more.

Such an effect of improving the corrosion resistance of Cr in the presence of a zinc primer is expressed by containing 0.01 mass% or more. However, when it contains 0.20 mass% or more, weld part toughness will fall. Therefore, Cr content is taken as the range of 0.01 mass% or more and less than 0.20 mass%. Preferably, it is in the range of 0.02 to 0.15 mass%.

As described above, when Cr and W in the above ranges are added together, a synergistic effect can be obtained, and very good corrosion resistance can be obtained regardless of the type or presence of the coating film.

N: 0.001-0.008 mass%

N is a harmful component with respect to the toughness of steel. Therefore, in order to improve toughness, it is preferable to reduce N as much as possible, and to make it 0.008 mass% or less. However, industrially, it is difficult to reduce N to less than 0.001 mass%. Therefore, in this invention, N content is taken as 0.001 to 0.008 mass%.

The hot rolled section steel for ships of this invention can add the following component in addition to the said component for the purpose of the further improvement of corrosion resistance.

One or two of Sb: 0.001-0.3 mass% and Sn: 0.001-0.3 mass%

Sb has the effect of improving the corrosion resistance in the presence of the zinc primer and the epoxy resin coating film, in the presence of the epoxy resin coating film, and in the exposed state. In addition, Sn has an effect of improving corrosion resistance in the presence and presence of an epoxy resin coating film. The said effect of Sb and Sn is considered to be because it suppresses corrosion of the site | part which pH reduced, such as an anode part of the steel plate surface. These effects are expressed by containing 0.001 mass% or more of both Sn and Sb. However, when the content exceeds 0.3 mass%, the toughness of the base metal and the toughness of the HAZ portion decrease. Therefore, it is preferable to add in the range of 0.001 to 0.3 mass%. In addition, it is more preferable to add both Sb and Sn.

Cu: 0.005 to 0.5 mass%, Ni: 0.005 to 0.25 mass%, Mo: 0.01 to 0.5 mass%, and Co: 0.01 to 1.0 mass%

Cu, Ni, Mo, and Co improve the corrosion resistance of steel in the presence and presence of a zinc primer and an epoxy coating film, and Mo has the effect of improving corrosion resistance also in presence of an epoxy coating film. Therefore, these elements can be contained auxiliaryly, in order to improve corrosion resistance more. The said effect of Cu, Ni, Mo, and Co is considered to be due to the effect | miniaturization of rust particle. In addition, in the case of Mo, it is thought that the formation of MoO ₄ ^{2 −} in the rust contributes to suppressing the penetration of chloride ions into the surface of the steel sheet.

These effects are expressed 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 more than 0.5 mass% of Cu, more than 0.25 mass% of Ni, more than 0.5 mass% of Mo, and more than 1.0 mass% of Co, the effect is saturated and disadvantageously economically disadvantageous. Therefore, it is preferable to add Cu, Ni, Mo, and Co in the said range, respectively.

The hot rolled section steel of the present invention may further contain the following components in addition to the above components in order to increase the strength or improve the toughness.

Nb: 0.001-0.1 mass%, Ti: 0.001-0.1 mass%, Zr: 0.001-0.1 mass%, and V: 0.002-0.2 mass%

Nb, Ti, Zr, and V are all elements which raise the strength of steel, and can be selected and added according to the strength required. In order to acquire such an effect, it is preferable to add Nb, Ti, Zr 0.001 mass% or more, and V2 0.002 mass% or more, respectively. However, when Nb, Ti, Zr exceeds 0.1 mass% or V exceeds 0.2 mass%, the toughness decreases. Therefore, it is recommended that Nb, Ti, Zr, V be added as the upper limit. desirable. The upper limit is preferably 0.04 mass%. Among these elements, Ti is most preferred from the viewpoint of weld toughness, and Nb is subsequently preferred.

B: 0.0002 to 0.003 mass%

B is an element which raises the intensity | strength of steel, and can contain it as needed. In order to acquire the said effect, it is preferable to add 0.0002 mass% or more. However, when it exceeds 0.003 mass%, toughness will fall rather. Therefore, it is preferable to add B in 0.0002 to 0.003 mass%.

One or two or more of Ca: 0.0002 to 0.01 mass%, REM: 0.0002 to 0.015 mass%, and Y: 0.0001 to 0.1 mass%

Ca, REM and Y are all the elements which are effective in the toughness improvement of a weld heat influence part, and can select and add as needed. This effect can be obtained by addition of Ca: 0.0002 mass% or more, REM: 0.0002 mass% or more, and Y: 0.0001 mass% or more. However, if Ca: 0.01 mass%, REM: 0.015 mass%, and Y: 0.1 mass% are added, the toughness is rather deteriorated. Therefore, Ca, REM, and Y should be added with the above values as the upper limit, respectively. desirable.

In the ship hot rolled section steel of this invention, components other than the above are Fe and an unavoidable impurity. However, as long as it is in the range which does not impair the effect of this invention, containing of the component of that excepting the above is not rejected.

(Microstructure)

Next, the microstructure of the hot-rolled section steel for ships with high strength and excellent corrosion resistance which concerns on this invention is demonstrated.

In marine steel sheets, particularly high strength thick steel sheets having a yield stress of 315 MPa or more, generally, a steel material having a reduced carbon equivalent and giving high weldability is adopted as a second phase by adopting TMCP combining control rolling and control cooling. The bainite structure of bainite is used to achieve high strength. And when low-temperature toughness is requested | required and the request | requirement of a thickening furnace are respond | corresponded 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 hot-rolled steel for ships, the width and thickness of the short side and the long side are often different (for example, a cross section is not rectangular, but an acid trap after inequality, etc.). Temperature nonuniformity occurs at the time. In particular, in the case of using the strength adjustment to which the control cooling (acceleration cooling) is applied, the residual stress becomes nonuniform, causing twisting, bending and warping, resulting in a decrease in dimensional accuracy. For this reason, the load of shape correction after rolling increases. Therefore, it is difficult to apply the method of introducing a hard bainite structure as a 2nd phase and making it high strength to a hot rolled section steel. This can be said throughout the ship's hot rolled section steels, such as a rolled T section steel.

Therefore, in hot rolled steel for ships, it is required to achieve high strength of yield stress: 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 aim at high strength in the ferrite + pearlite structure which is a normal hot rolling structure. As a means of achieving high strength in the ferrite + pearlite structure, a method of increasing the pearlite fraction of the second phase, a method of further refinement of the ferrite structure, a method of solidifying or solidifying the ferrite by solid solution strengthening or precipitation strengthening, or ( γ + α) It is possible to consider a method of hot rolling in the biphasic region to form a part of the ferrite into the processed ferrite of high potential density.

Of the above methods, the method of refining the ferrite is advantageous for increasing the yield stress, but since the rise of TS is small, sufficient high strength cannot be achieved only by this method. In addition, the method of increasing the pearlite fraction is required to add a large amount of C, but excessive addition of C causes undesirable weldability. In addition, the method of reinforcing ferrite by adding a solid solution strengthening element or a precipitation strengthening element causes a decrease in weldability or an increase in material cost by adding a large amount of alloying elements.

On the other hand, utilization of processed ferrite can raise yield stress and TS in the state which suppressed addition of C and an alloying element to the minimum, and maintained weldability. That is, since the method using the processed ferrite can achieve high strength without performing controlled cooling (accelerated cooling) after hot rolling, bending, bending, or warping during the rolling, cooling, which are inherent problems in manufacturing hot rolled steel for ships, are caused. It is possible to increase the strength while suppressing this. Therefore, in the present invention, as a means of increasing the strength of the hot rolled steel for ships, a method is adopted in which the microstructure of the steel is used as a ferrite + pearlite structure containing the processed ferrite.

Here, it is preferable that the fraction of the said processed ferrite is 10 to 70% of range of the whole steel structure as area ratio. If the fraction of the processed ferrite is less than 10%, the steel reinforcement cannot be sufficiently obtained. On the other hand, if the fraction of the processed ferrite is less than 70%, the strength increase is saturated, and the roll accompanying the increase in the load at the time of two-phase rolling of (α + γ) This is because the risk of splitting) increases. In addition, the processed ferrite, Ar ₃ of the transformation point or less (α + γ) processing strain refers to the introduction of ferrite, typically, pieces of processing ferrite (trace) traces a peace formed by hot rolling in the two-phase region, and the micro-organization The area occupied in the water can be quantified, and the fraction thereof can be measured. As a measurement position of a micro structure, the plate | board thickness 1/4 part in the site | part where the thickness is thickest is preferable.

Moreover, in the whole ferrite containing a processed ferrite, it is preferable to exist in about 10%-70% of the whole steel structure by area ratio. The remainder is a pearlite structure, but a structure other than ferrite pearlite, that is, bainite or the like may be present in an area ratio of 10% or less.

(Surface treatment)

As already mentioned, the surface of the ship's hot-rolled shaped steel of this invention,

No coating (exposed state)

One layer coating film of epoxy resin coating film

2-layer coating film of zinc primer and epoxy resin coating

It is preferable to make it into any one state. However, surface treatment other than this is not prohibited. In particular, it is free to replace the zinc primer and / or the epoxy resin coating film with a substitute.

In the exposed state, the surface may be hot rolling, but the oxide layer or the oil layer may be removed by shot blasting or the like. Regardless of the type of epoxy resin coating film or zinc primer, those mentioned in the present specification and other known ones can be used. Moreover, tar epoxy coating resin is preferable as an epoxy resin coating film.

(Manufacturing method)

Next, the method of manufacturing the hot rolled steel for ships which has a ferrite + pearlite structure containing the said processed ferrite is demonstrated.

In the production of hot rolled steel for ships of the present invention, first, a steel having the above-described component composition is dissolved in a conventionally known facility such as a converter and an electric furnace, and then commonly known, such as a continuous casting method or an ingot method. It is preferable to make steel materials, such as a slab, a billet, and a bloom, by the method. In addition, after the solvent, treatments such as refining and vacuum degassing may be added.

Subsequently, the steel material is charged into a heating furnace to be reheated, and then hot rolled to obtain hot rolled steel for ships having desired dimensions, structures, and properties. At this time, the reheating temperature of the steel material needs to be in the range of 1000 to 1350 ° C. If heating temperature is less than 1000 degreeC, deformation resistance is large and hot rolling becomes difficult. On the other hand, heating exceeding 1350 ° C. causes surface traces, and scale loss and heat source unit increase. Preferably, it is the range of 1100-1300 degreeC.

In the subsequent hot rolling, it is necessary to set the cumulative reduction ratio at the Ar ₃ temperature or lower to 10 to 80%. If the pre-rolling temperature is higher than the Ar ₃ temperature, the microstructure of the steel does not include the processed ferrite, and the required strength and toughness cannot be secured. Similarly, when the cumulative reduction ratio below the Ar ₃ temperature is less than 10%, since the amount of produced ferrite is small, the toughening effect is small. On the contrary, when the reduction ratio exceeds 80%, the rolling load increases, which makes the rolling difficult, or increases the number of passes of the rolling, leading to a decrease in productivity. Therefore, the cumulative reduction ratio below the Ar ₃ temperature is set to 10 to 80%. Preferably, it is 10 to 60% of range. Further, Ar ₃ is rolled at a temperature below, may performed in at least one or more paths, it does not matter even if multiple passes. Here, the cumulative reduction ratio at or below the Ar ₃ temperature means Ar _3. The cross-sectional reduction rate of the cross-sectional area (B) of the rolling material after the end of rolling with respect to the cross-sectional area (A) of the rolling material at temperature is indicated, and is represented by the following formula.

(Accumulated rolling reduction (%) at or below the Ar ₃ temperature) = (A-B) / A x 100

Further, the hot rolling, finishing rolling temperature: it is necessary in terms of _{(Ar 3 -30 ℃) ~ (} Ar 3 -180 ℃). In the finish rolling temperature, in the (Ar ₃ -30 ℃) is exceeded, can not sufficiently obtain the effect by the two toughened sangyeok rolling, on the other hand, less than (Ar ₃ -180 ℃), the rolling load by the increase in deformation resistance It is because it increases and it becomes difficult to roll.

Moreover, in the said hot rolling, it is preferable to perform rolling below Ar <_3> temperature, making the temperature difference in each site | part within the cross section of the ship's hot-rolled shaped steel into 50 degrees C or less. For example, in the hot-rolled section steel for ships, for an inequality post-angular shaped steel having a difference in thickness at the long side and the short side, the short side having a thickness thicker than the long side at the thin side is cooled with water before and after the rolling mill, and the long side and the short side. It is preferable to suppress the temperature difference on the side within 50 degreeC. When the temperature difference exceeds 50 ° C, not only the nonuniformity of the strength and toughness characteristics of the short side and the long side is increased, but also the bend in the cooling step after rolling increases, and the burden required for calibration increases, thereby lowering productivity.

As means for suppressing the temperature difference between the short side and the long side within 50 ° C., a method of controlling cooling using a cooling facility arranged before and after a rough rolling mill is preferable. Specifically, the above-mentioned cooling apparatus is preferably a method in which the short side of thick thickness is water-cooled to eliminate the temperature difference. At this time, the water cooling may be performed only at the front and back sides of the rolling mill, only at the rear surface, or both at the front and rear sides, and may be performed a plurality of times depending on the size and the required precision of the rolled steel. In addition, the water density at the time of water cooling is preferably 1 m ³ / m · min or more.

The temperature difference in the cross section of the shaped steel is determined by the difference between the highest temperature and the lowest temperature obtained by measuring the surface temperature of the flange and the web (see Example) with a radiation thermometer.

Although cooling following hot rolling does not have a restriction | limiting in particular, It is preferable to set it to room cooling. Thereby, the shape change of the shape steel, such as the bending and curvature which arises from the cooling nonuniformity after rolling, can be reduced, and the burden of the correction with respect to the product after rolling can be reduced. Although the cooling rate at the time of cooling is dependent also on plate | board thickness, it is about 0.4-1.0 degreeC / s. Since the treatment (forced cooling, heat retention, etc.) which accelerates and decelerates cooling within the said cooling rate range is substantially the same as room cooling, it does not exclude this in particular.

(Example)

The steel having the composition shown in Table 2 (Table 2-1 and Table 2-2) was dissolved in a vacuum melting furnace or converter to form a bloom, charged into a heating furnace, and then heated, followed by Table 3 (Table 3-1 and Hot rolling was carried out under the conditions shown in Table 3-2) to produce an anisotropic post-mounted acid steel (NAB) and a rolled T-shaped steel having the cross-sectional dimensions shown in Table 3. In addition, in Table 3, about the trapezoidal shaped steel (NAB), the long side is shown as the web and the short side is used as the flange.

An unbalanced post-unsteady acid steel from a short side and a T-shaped steel from a flange were taken from a JIS1A tensile test piece, and tensile properties (yield stress, tensile strength (TS), and elongation (El)) were measured. In addition, multi-sided welding (GMAW) was performed by shorting the side edge of the trapezoidal steel and the flange of the T-shaped steel with a heat input of 20 kJ / cm, and from the center of the HAZ, the Charpy impact test piece (2). Mm V notch test piece) was taken and the absorbed energy in the Charpy impact test at -20 ° C was measured.

In addition, a sample for tissue observation was taken from the short side for the trapezoidal annealed steel and from the flange for the T-shaped steel, and the tissue having a quarter thickness of the plate was observed at a magnification of 200 times under a microscope. In the observed structure, the flattened processed ferrite produced by biphase rolling was traced, and the area occupied in the microstructure was quantified by image analysis to obtain a fraction of the processed ferrite.

TABLE 2-1

Table 2-2

Table 3-1

Table 3-2

Next, for each hot rolled section steel, 5 mmt x 100 mm W x 200 mm L or 5 mmt x 50 mm W x 150 mm After the L test piece was taken out and the test piece surface was shot blasted, the surface treatment was performed under the following conditions A to C to obtain a corrosion resistance test piece.

<Surface Treatment Condition>

Condition A: A two-layer coating of zinc primer (film thickness of about 15 µm) and tar epoxy resin paint (film thickness of about 200 µm) was formed on the surface of the test piece.

Condition B: A single layer film of a tar epoxy resin coating (film thickness of about 200 μm) is formed on the surface of the test piece.

Condition C: The state which was shot blasted on the test piece surface (there is no anticorrosive film)

Moreover, the scratches of length 80mm which reach the surface of a base steel with a cutter knife from the coating film were provided to the test piece of the said conditions A and B which formed the coating film in linear form.

The test piece produced as mentioned above was then used for the exposure test attached to the back surface of the ballast tank upper deck of a real ship for 2 years. The corrosive environment of this exposure test was, on average, about 20 days in which the seawater was contained in the ballast tank, and about 20 days in which the seawater was not contained, and repeated this cycle.

Evaluation of the corrosion resistance in an exposure test was performed as follows. About the test piece of conditions A and B which have a coating film, the coating film expansion area which generate | occur | produced around the scratch damage was measured. In addition, about the test piece of the condition C which does not have a coating film, it degreased after a test and the average plate | board thickness reduction amount was computed from the difference (reduced amount) of the test piece mass after the demineralization and the test piece mass before a test. Based on these results, the steel of No. 14 which does not specifically contain a corrosion resistance improvement element was made into the base 100, the ratio of each test piece with respect to it was evaluated, and corrosion resistance was evaluated.

Table 4 shows the results of the tensile test, impact test, microstructure investigation, and corrosion resistance test. From the results of the corrosion resistance test, the steels of Nos. 1 to 13, which are examples of the invention which satisfy the component composition of the present invention, also exhibit 50% of the coating film expansion area and the sheet thickness reduction with respect to the base steel (No. 14) even under the conditions A to C. It turns out that it has favorable corrosion resistance below. In contrast, steel Nos. 14 to 17 that did not satisfy the component composition of the present invention had experimental conditions such that the ratio to the base steel was more than 50% even if the corrosion resistance was improved from the base steel (No. 14), The toughness of the weld portion may be greatly reduced. In addition, in the ferrite + pearlite structure (other than the rolling code Q) in which the microstructure contains the processed ferrite, sufficient strength sought in the present invention is obtained, and the shape change such as bending and warping is also slight, and the productivity is also very good. did.

In the shaped steel of rolling code a (when the temperature difference in the cross section of the shaped steel exceeded 50 ° C. in hot rolling at an Ar ₃ temperature or lower), the characteristic value reached the target, but the bending and warping were large.

[Table 4]

According to the present invention, it is possible to provide inexpensive hot rolled steel for ships having high strength and excellent corrosion resistance even under severe corrosive environment by seawater. Moreover, since the shape steel of this invention is excellent in corrosion resistance, it can contribute greatly to the extension of the period until repair painting of a ship, and reduction of the work load of repair painting.

In addition, since the hot rolled steel for ships of the present invention exhibits excellent corrosion resistance, particularly in a corrosive environment by seawater, it is effective for extending the service life of the ship itself through an extension of the ship's maintenance period. It can also be used in hot rolled section steels used in other applications used in similar corrosive environments.

Claims (28)

C: 0.03-0.25 mass%, Si: 0.05-0.50 mass%,
Mn: 0.1 to 2.0 mass%, P: 0.025 mass% or less,
S: 0.01 mass% or less, Al: 0.005 to 0.10 mass%,
W: 0.01-1.0 mass%,
Cr: 0.01 mass% or more but less than 0.20 mass%,
For ships containing N: 0.001 to 0.008 mass%, the remainder having a component composition consisting of Fe and unavoidable impurities, and having a microstructure consisting of ferrite and pearlite structures containing 10 to 70% of the entire steel structure in an area ratio of processed ferrite As a method of manufacturing hot rolled section steel,
Heat the steel material to 1000 ~ 1350 ℃,
Thereafter, Ar ₃ the cumulative rolling reduction at a temperature of 10 to 80% or less, the rolling finishing temperature of (Ar ₃ -30 ℃) ~ haenghadoe the hot rolling to a (Ar ₃ -180 ℃), at a temperature below the Ar ₃ Hot rolling of the steel sheet is performed with a temperature difference within 50 ° C of the section steel
Then, the manufacturing method of the hot rolled section steel for ships to cool. The method of claim 1,
In addition to the said component composition, the manufacturing method of the hot rolled steel for ships containing 1 type or 2 types chosen from Sb: 0.001-0.3 mass% and Sn: 0.001-0.3 mass%. The method of claim 1,
In addition to the above component composition, one or two or more selected from Cu: 0.005 to 0.5 mass%, Ni: 0.005 to 0.25 mass%, Mo: 0.01 to 0.5 mass%, and Co: 0.01 to 1.0 mass% The manufacturing method of the hot rolled section steel for ships containing these. The method of claim 2,
In addition to the above component composition, one or two or more selected from Cu: 0.005 to 0.5 mass%, Ni: 0.005 to 0.25 mass%, Mo: 0.01 to 0.5 mass%, and Co: 0.01 to 1.0 mass% The manufacturing method of the hot rolled section steel for ships containing these. The method of claim 1,
In addition to the above component composition, one or two or more selected from 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 manufacturing method of the hot rolled section steel for ships containing these. The method of claim 2,
In addition to the above component composition, one or two or more selected from 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 manufacturing method of the hot rolled section steel for ships containing these. The method of claim 3,
In addition to the above component composition, one or two or more selected from 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 manufacturing method of the hot rolled section steel for ships containing these. 5. The method of claim 4,
In addition to the above component composition, one or two or more selected from 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 manufacturing method of the hot rolled section steel for ships containing these. The method according to any one of claims 1 to 8,
In addition to the said component composition, the manufacturing method of the hot rolled section steel for ships containing B: 0.0002-0.003 mass%. The method according to any one of claims 1 to 8,
In addition to the above-mentioned ingredient composition, the hot rolled steel for ships containing one or two or more selected from Ca: 0.0002 to 0.01 mass%, REM: 0.0002 to 0.015 mass% and Y: 0.0001 to 0.1 mass% Manufacturing method. 10. The method of claim 9,
In addition to the above-mentioned ingredient composition, the hot rolled steel for ships containing one or two or more selected from Ca: 0.0002 to 0.01 mass%, REM: 0.0002 to 0.015 mass% and Y: 0.0001 to 0.1 mass% Manufacturing method. delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete