JP2011058038A - Hot rolled shape steel for vessel having excellent corrosion resistance, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot rolled shape steel for a vessel having excellent corrosion resistance without depending on the existing state of a coating film which used in a corrosive environment such as the blast tank of a vessel, and to provide an advantageous method therefor. <P>SOLUTION: A steel stock having a composition comprising, by mass, 0.05 to 0.20% C, 0.05 to 0.50% Si, 0.1 to 2.0% Mn, ≤0.025% P, ≤0.01% S, 0.005 to 0.10% Al, 0.01 to 1.0% W, 0.01 to <0.20% Cr, 0.001 to 0.03% Nb and 0.001 to 0.008% N is heated at 1,000 to 1,350°C, thereafter subjected to hot rolling in which rolling finish temperature is controlled to >800°C, and thereafter let to cool so as to obtain a hot rolled shape steel for a vessel having a microstructure composed of a ferrite-pearlite structure free from worked ferrite. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention includes coal and ore carrier, crude oil tanker, LPG and LNG carrier, chemical tanker, container ship, bulk carrier, wood carrier, chip carrier, refrigerated carrier, automobile carrier, heavy ship, RORO ship, Hot-rolled form for ships used for longitudinal members (longi materials) of ballast tanks in corrosive environments with severe seawater, especially for ship shaped steels used in various ships such as limestone ships and cement ships The present invention relates to steel and its manufacturing method. Here, the hot-rolled section steel is specifically hot-rolled such as equilateral chevron, non-equal chevron, uneven unequal thick chevron, channel steel, spherical flat steel, T-shape steel, etc. Says what was manufactured by.

  Since the ship's ballast tank plays a role of injecting seawater to enable stable navigation of the ship when there is no cargo, it is placed in an environment subject to severe corrosion by seawater. Therefore, the steel material used for the ballast tank is usually subjected to anticorrosion by forming an epoxy resin coating and cathodic protection for corrosion prevention.

  However, even if these anticorrosion measures are taken, the corrosive environment of the ballast tank is still severe. For example, when seawater is being injected into the ballast tank, the portion that is completely immersed in the seawater functions as an anticorrosion, so that corrosion can be effectively suppressed. However, the vicinity of the uppermost part of the ballast tank, particularly the backside portion of the upper deck, is not soaked in seawater and is in a state where it is exposed to only seawater splashes. Furthermore, since the steel plate temperature is increased by sunlight, the corrosive environment becomes more severe in this part. On the other hand, when seawater is not injected into the ballast tank, since the anti-corrosion does not work at all, it will be 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, the repair coating is not a preferable working environment because it is performed in a narrow space. 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.

  Against this background, 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 in which Cu: 0.05 mass% or more and less than 0.15 mass% and W: 0.05 to 0.5 mass% are added to steel of C: 0.15 mass% or less. It is disclosed.

  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 1 to 1.0 mass% is added and is resin-coated. 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. Steel for ships having corrosion resistance is disclosed even in use environments such as an outer plate, a ballast tank, a cargo oil tank, and a coal ore cargo hold. Furthermore, Patent Document 10 discloses a crude oil tank by adding Mo, W and Cu in a steel of C: 0.001 to 0.2 mass% and reducing the contents of impurities P and S. Steel that suppresses general corrosion and local corrosion that occur in JIS is disclosed.

  However, in the above Patent Documents 1 to 3, sufficient investigation has not been made on the corrosion resistance in the presence of a coating film such as a zinc epoxy resin paint that is normally applied to a steel material constituting a ballast tank or the like. Moreover, in order to improve the corrosion resistance of the base metal, the steel material of Patent Document 4 has a relatively large amount of P added, from 0.03 to 0.10 mass%, which is problematic in terms of weldability and weld toughness. is there. 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 for a crude oil tank used in an environment where H ₂ S exists, and the corrosion resistance in a ballast tank without H ₂ S is unknown. Furthermore, the corrosion resistance in a state where a zinc primer coating generally used for steel for ballast tanks is applied has not been studied.
Therefore, there is still room for further improvement in the steel materials of the prior art used for the ballast tank, including the corrosion resistance in the presence of the coating film.

  By the way, in general, ships are constructed by welding various steel materials such as thick steel plates, thin steel plates, shaped steels, and bar steels. However, these steel materials, especially thick steel plates and shaped steels, can be reduced in cost due to reduced usage. Higher strength is being promoted from the viewpoint of ensuring safety. Therefore, in the production of thick steel plates by hot rolling, a steel material having a low carbon equivalent and imparted high weldability is adopted by adopting TMCP (Thermo-mechanical control process) that combines controlled rolling and controlled cooling. By introducing a hard bainite structure as two phases, the yield strength YS is increased to 315 MPa or more.

  On the other hand, steel materials used for ballast tanks such as long steel, especially steels manufactured by hot rolling such as unequal unequal thick angle steels and T-shaped steels, are compared with thick steel plates used in the same ship. As a result, the cross-sectional shape and dimensions are complicated, and there is a problem that the product is likely to be distorted and uneven in strength. Therefore, in rolling the shape steel, it is necessary to manufacture while considering distortion such as bending and warping in the middle of rolling. For example, the yield stress YS is 235 MPa or more and the tensile strength TS is 400 MPa or more. There is a problem that it is difficult to adopt TMCP which performs a controlled rolling / accelerated cooling process similar to that of a thick steel plate as means for building a high-strength material.

  To solve this problem, for example, as disclosed in Patent Document 11, when manufacturing a steel material by hot rolling, the surface temperature of the flange (short side) portion becomes 800 to 600 ° C. Thus, finish rolling is performed. However, in this method, during finish rolling, a waiting time for temperature adjustment occurs, which causes a reduction in rolling efficiency and an increase in manufacturing cost. Therefore, development of a manufacturing method that does not generate the waiting time is desired. .

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 JP 2007-216251 A

The surface of steel used for ships is usually coated with anticorrosion. In the anticorrosion coating, a zinc primer is generally applied as a primary rust prevention, and after a small assembly or a large assembly, an epoxy resin coating is generally applied as a secondary coating (main coating). Accordingly, most of the steel surface of the ship is subjected to a two-layer anticorrosion coating of a zinc primer and an epoxy resin. However, since the ging primer is burned down by the heat during welding, the steel material welded part may be subjected to repair coating (touch-up) with a zinc primer as a rust prevention measure between welding and main coating. However, if the period until this painting is short, repair painting may not be performed. In addition, in ships that have been used for many years after construction, the above-mentioned coating film has deteriorated, there are parts that have not fulfilled the rust prevention function, or the coating film has peeled off and the steel surface has become bare There is a part to do.
In other words, on the surface of the steel material of the ship in service, there are three states: a part where a two-layer coating film of zinc primer and epoxy resin is present, a part where only an epoxy resin coating film is present, and a bare part Exists. 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.

  Furthermore, as described above, high strength is achieved with steel materials used for ballast tanks, such as long steels, and in particular, shape steels manufactured by hot rolling such as unequal side unequal thick angle steel and T-shape steel. As a means, it is difficult to adopt the same TMCP as the thick steel plate. Therefore, in order to manufacture a high-strength steel having a yield stress YS of 235 MPa or more and a tensile strength TS of 400 MPa or more by hot rolling, it is necessary to study a unique manufacturing method.

  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 provide marine hot-rolled section steel with excellent corrosion resistance that can reduce the work of repair coating, and to produce marine heat that can be manufactured with high productivity without causing distortion and uneven strength in hot rolling. The purpose of this invention is to propose a method for producing hot rolled steel.

  The inventors exhibit excellent corrosion resistance without being influenced by the surface state (existing state of the coating film) even in an environment subject to severe corrosion by seawater, and have a high strength with a yield stress YS of 235 MPa or more. Intensive research for the development of. As a result, by adding W, Cr and Nb as essential elements for improving corrosion resistance, not only in the presence of a two-layer coating film of zinc primer and epoxy resin, but also in the presence or absence of an epoxy resin coating film Ship shape steel that exhibits excellent corrosion resistance in any state of the bare state can be obtained, and by adding an appropriate amount of corrosion resistance improving elements such as Sb and Sn to this, further excellent corrosion resistance can be obtained. The obtained steel shape steel can be obtained, and furthermore, by adding Nb as an essential element as described above, the steel structure after the rolling has processed ferrite even if the finishing temperature in hot rolling exceeds 800 ° C. Although it is a structure consisting of ferrite and pearlite not included, it can achieve the expected high strength, so without interfering with productivity and weldability, It found that it is possible to manufacture the marine section steel to the valence, and completed the present invention.

  That is, the present invention is C: 0.05-0.20 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%, Nb: 0.001 to 0.03 mass% , N: 0.001 to 0.008 mass%, the remainder having a component composition composed of Fe and inevitable impurities, and having a microstructure composed of ferrite and pearlite structure not containing processed ferrite Rolled section steel.

Moreover, in addition to the said component composition, it is preferable that the hot rolled shape steel for ships of this invention contains the component of at least 1 group of the following AE group further.
Group A; Sb: 0.001 to 0.3 mass% and Sn: 0.001 to 0.3 mass% or one selected from Group B; 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%, one or more C groups; Ti: 0.001 ~ 0.1 mass%, Zr: 0.001 to 0.1 mass%, and V: 0.002 to 0.2 mass%, one or more selected from group D; B: 0.0002 to 0.003 mass %
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

  The marine hot-rolled section steel of the present invention forms an epoxy resin coating, a zinc primer coating, and any one of a zinc primer coating and an epoxy resin coating on the surface of the shape steel. It is characterized by.

  In addition, the present invention is a method in which a steel material having any of the above-described component compositions is heated to 1000 to 1350 ° C. and then hot-rolled with a rolling finishing temperature exceeding 800 ° C. We propose a method for manufacturing a hot-rolled section steel for marine vessels, characterized by having a microstructure comprising no ferrite and a pearlite structure.

  ADVANTAGE OF THE INVENTION According to this invention, the hot rolled shape steel for ships which exhibits the high corrosion resistance in the environment which receives high intensity | strength and severe corrosion by seawater can be manufactured with low productivity and 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 repair painting of a ship and reduction of the work load of repair painting.

The inventors have three states existing in the steel materials of ships in service, namely, a state having a two-layer coating film of a zinc primer and an epoxy resin, a state having only an epoxy resin coating film, and a bare state without a protective coating film In order to develop a marine hot-rolled section steel exhibiting excellent corrosion resistance in any of these conditions, the following experiment was conducted.
Steels added with various alloy elements are melted in the laboratory to form steel ingots, and then hot rolled to form hot rolled sheets with a thickness of 5 mm. From these hot rolled sheets, 5 mmt x 100 mmW x 200 mmL Or the test piece of 5mmtx50mmWx150mmL was extract | collected, the surface of the test piece was shot-blasted, and the scale and oil component of the surface were removed.

Next, the following three types of surface treatments were applied to the test piece to prepare a test piece for corrosion resistance test.
・ Condition A: Zinc primer (film thickness of about 15 μm) and tar epoxy resin (film thickness of about 100 μm) are applied to the surface of the test piece to form a two-layer film. ・ Condition B: Tar epoxy resin (film) on the surface of the test piece (Approx. 100 μm thick) to form a single layer coating ・ Condition C: Bare state with shot blasting on test piece surface (no anticorrosion coating)
In addition, about the test piece of the said conditions A and B in which the coating film was formed, 80 mm length scratch scissors which reach | attain to a ground-iron surface with a cutter knife from the top of the coating film were provided to the character shape before the test.

  The test piece for the corrosion resistance test prepared as described above simulated the corrosive environment on the back side of the upper deck of the ballast tank of the actual ship. (35 ° C., 5 mass% NaCl solution spray × 2 hr) → (60 ° C., relative Humidity (RH) 5% × 4 hr) → (50 ° C., RH 95% × 2 hr) was taken as one cycle, and this was subjected to a salt spray dry and wet repeated corrosion test repeated 132 cycles to evaluate the corrosion resistance. Corrosion resistance is determined by measuring the area of the swollen coating film generated around the scratch ridge after the test for the test pieces of conditions A and B on which the coating film was formed. About the test piece of condition C, after the test, it evaluated by calculating an average board thickness reduction | decrease amount from the difference (reduction | decrease amount) of the test piece mass derusted and the test piece mass before a 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 based on the results of the corrosion test. The result is briefly described as follows.
1) In the case of condition A (zinc primer + tar epoxy resin two-layer coating); the most effective element for improving corrosion resistance is Cr, then W, Nb, and then Sb.
2) In the case of condition B (one-layer coating film of tar epoxy resin); W is the most effective element for improving corrosion resistance, followed by Nb, then Sb, Sn.
3) In the case of condition C (bare state); the most effective element for improving the corrosion resistance is W, then Nb, then Sb, Sn.
4) The combined addition of W, Cr and Nb improves the corrosion resistance under condition A over the addition of a single addition, and by adding Sb and Sn further, the corrosion resistance significantly improves not only under condition A but also under conditions B and C. An effect is obtained.
5) 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.

  Therefore, in the present invention, based on the results shown in Table 1 above, W, Cr and Nb are added in combination as essential basic elements for improving the corrosion resistance, and when corrosion resistance is required, Sb and Sn are used. It was decided to adopt a component design in which one or two selected ones were additionally added. When more excellent corrosion resistance is required, one or more selected from Cu, Ni, Mo and Co are further added.

Next, the component composition that the marine structural steel excellent in corrosion resistance of the present invention should have will be described.
C: 0.05-0.20 mass%
C is an element effective for increasing the strength of steel, and in the present invention, it is necessary to contain 0.05 mass% or more in order to obtain a desired strength. On the other hand, addition exceeding 0.20 mass% lowers the toughness of the weld heat affected zone (HAZ). Therefore, C is set to a range of 0.05 to 0.20 mass%.

Si: 0.05-0.50 mass%
Si is an element added as a deoxidizer for steel and to increase the strength of steel. In the present invention, Si is contained 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 due to S and increasing the strength of steel, and is contained in an amount of 0.1 mass% or more. However, since addition exceeding 2.0 mass% reduces the toughness and weldability of steel, the upper limit of Mn is set to 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 toughness of steel, and is desirably 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 S is a harmful element that lowers the toughness and weldability of steel, and is desirably reduced as much as possible. In the present invention, it is limited to 0.01 mass% or less.

Al: 0.005-0.10 mass%
Al is an element added as a deoxidizer for steel, 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 ground iron is lowered by A1 ³⁺ eluted by corrosion of the ground iron, and the corrosion resistance is lowered. Therefore, the upper limit of Al needs to be 0.10 mass%. .

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

The reason why W has the above-described effect of improving the corrosion resistance is that WO ₄ ²⁻ is generated in the rust generated by the corrosion of the steel plate, and the presence of this WO ₄ ²⁻ causes chloride ions from the steel plate surface. Intrusion is prevented, and in addition, poorly soluble FeWO ₄ is generated at sites where the pH is lowered, such as the anode portion of the steel sheet surface, and the presence of this FeWO ₄ also prevents the penetration of chloride ions into the steel sheet surface. As a result, it is considered that corrosion of steel is effectively suppressed. Moreover, it is thought that 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 an element that greatly improves the corrosion resistance in the presence of a two-layer coating film of a zinc primer and an epoxy resin. Is one of the important elements. The reason why the corrosion resistance improvement effect is obtained is that, in the corrosion in the presence of the zinc primer, Zn in the zinc primer is eluted and a Zn-based corrosion product such as ZnO or ZnCl ₂ · 4Zn (OH) ₂ is generated. However, it is presumed that Cr acts on this Zn-based corrosion product to further improve the anticorrosion effect of the ground iron by the Zn-based corrosion product. Such an effect of improving the corrosion resistance of Cr in the presence of the zinc primer can be obtained by addition of 0.01 mass% or more. However, when 0.20 mass% or more is added, the toughness of the welded portion decreases. Therefore, Cr is in 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%.

Nb: 0.001 to 0.03 mass%
As described above, Nb is an element that enhances corrosion resistance regardless of the presence of steel.
Nb is also an element that refines the grain size of ferrite in the (ferrite + pearlite) structure and increases the strength of the steel. Hereinafter, a basic experiment for investigating the influence of Nb on the strength characteristics will be described.
No. having the component composition shown in Table 2. The steel of 1-5 was melted in a vacuum melting furnace or converter to form a bloom, and this bloom was reheated in a heating furnace, and then hot-rolled under the conditions shown in Table 3, and the cross section shown in Table 3 A non-uniform unequal thickness angle steel (NAB) was produced. Next, from the short side of the unequal side unequal thickness steel, a No. 1A tensile test piece defined in JIS Z2201 was sampled and subjected to a tensile test, and the yield stress YS, tensile strength TS, and elongation El were measured. In addition, the short sides of the unequal unequal thickness chevron are butt multi-layered (GMAW) with a heat input of 20 kJ / cm, and a Charpy impact test piece (2 mmV notch test piece) is taken from the center of the HAZ. The absorbed energy in a Charpy impact test at 20 ° C. was measured. In addition, from the short side of the unequal side unequal thickness angle steel, a sample for observing the structure is separately collected, and the microstructure at the ¼ part thickness is observed at a magnification of 200 times using an optical microscope. It was confirmed that the processed ferrite was not present.

  The results of the above test are shown in Table 4. From this result, the steel material added with 0.001 mass% or more of Nb is hot-rolled at a finishing temperature of over 800 ° C., so that the toughness of the microstructure is composed of ferrite and pearlite structure not containing processed ferrite. It has been newly found that high strength (TS ≧ 400 MPa) can be achieved without a decrease. However, if the amount of Nb added exceeds 0.03 mass%, the strength of the steel increases too much and the toughness is greatly reduced. Therefore, in the present invention, Nb is added in the range of 0.001 to 0.03 mass%. Preferably, it is the range of 0.005-0.03 mass%.

N: 0.001 to 0.008 mass%
N is a harmful component that lowers the toughness of steel. Therefore, in order to improve toughness, it is desirable to reduce N as much as possible. In the present invention, the upper limit is set to 0.008 mass%. However, it is difficult to reduce N to less than 0.001 mass% on an industrial scale. Therefore, N is set to a range of 0.001 to 0.008 mass%.

Moreover, in addition to the said component, the hot-rolled shape steel for ships of this invention is Sb: 0.001-0.3mass% and Sn: 0.001-0 in addition to the said component for the purpose of obtaining the more outstanding corrosion resistance. One or two of 3 mass% can be added.
Sb has an effect of improving the corrosion resistance not only in the presence of a two-layer coating film of a zinc primer and an epoxy resin but also in the presence of an epoxy resin coating film and in a bare state. Moreover, Sn has an effect of improving the corrosion resistance in the presence of an epoxy resin coating film and in a bare state. The above effect of Sb and Sn is thought to be due to the suppression of corrosion at sites where the pH is lowered, such as the anode portion on the steel sheet surface. The above effects are manifested by adding 0.001 mass% or more for both Sn and Sb. However, if each exceeds 0.3 mass%, the base material toughness and the HAZ part toughness are lowered. Therefore, it is preferable to add Sb and Sn in the range of 0.001 to 0.3 mass%, respectively. In order to further enhance the effect of addition, it is preferable to add Sb and Sn in combination.

In addition to the above components, the hot-rolled section steel for marine vessels of the present invention further includes Cu: 0.005-0.5 mass%, Ni: 0.005-0.25 mass%, in addition to the above components. One or two or more selected from Mo: 0.01 to 0.5 mass% and Co: 0.01 to 1.0 mass% can be added.
Cu, Ni, Mo and Co improve the corrosion resistance of steel in the presence of a two-layer coating film of zinc primer and epoxy resin and in the bare state, and Mo improves the corrosion resistance even in the presence of an epoxy resin coating film. effective. Therefore, these elements can be supplementarily added when it is desired to further improve the corrosion resistance. The above-mentioned effects of Cu, Ni, Mo and Co are considered to be due to the refinement of rust particles. Furthermore, Mo generates MoO ₄ ^{2− in the} rust, and chloride ions enter the steel sheet surface. It is thought that the suppression is also contributing.
These effects are manifested by adding 0.005 mass% or more for Cu and Ni, 0.01 mass% or more for Mo, and 0.01 mass% or more for Co. However, addition of 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 above-mentioned corrosion resistance improvement effect is saturated, and the raw material cost is increased. It only invites. Therefore, Cu, Ni, Mo, and Co are preferably added in the above ranges.

Furthermore, in order to increase the strength or improve the toughness of the hot rolled steel of the present invention, the following components can be added in addition to the above components.
One or more of Ti: 0.001 to 0.1 mass%, Zr: 0.001 to 0.1 mass% and V: 0.002 to 0.2 mass% Ti, Zr and V are all It is an element that increases the strength of steel and can be selected and added according to the required strength. In order to acquire the said effect, it is preferable to add Ti and Zr 0.001 mass% or more respectively, and V to add 0.002 mass% or more. On the other hand, when Ti and Zr are added in excess of 0.1 mass% and V is added in excess of 0.2 mass%, the toughness is lowered. Therefore, Ti, Zr, and V are preferably added within the above ranges.

B: 0.0002 to 0.003 mass%
B is also an element that increases the strength of the steel and can be added 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, addition exceeding Ca: 0.01 mass%, REM: 0.015 mass%, and Y: 0.1 mass% causes a decrease in toughness. Therefore, Ca, REM, and Y are preferably added in the above ranges.

  In the hot rolled steel of the present invention, the 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.
As described above, in the case of marine steel plates, particularly high strength thick steel plates with a yield stress YS of 315 MPa or more, generally, steel materials with low carbon equivalent and high weldability are subjected to controlled rolling and controlled cooling. High strength is achieved by employing a combined TMCP and introducing a hard bainite structure as the second phase. And when lower temperature toughness and thickening are calculated | required, it respond | corresponds by controlling and optimizing the conditions of the said controlled rolling and controlled cooling. Therefore, when TMCP is adopted, the microstructure of the obtained steel sheet is usually a structure composed of ferrite and bainite.

  On the other hand, in the case of a hot-rolled section steel for ships, for example, in the case of an unequal side unequal thick angle steel with a non-rectangular cross section, the width and thickness of the short side and the long side are different, so that inevitably during rolling. During cooling, temperature non-uniformity occurs between the sections of the shape steel. Therefore, in the strengthening means using controlled cooling (accelerated cooling), the residual stress becomes non-uniform, causing twisting, bending, and warping, leading to a decrease in dimensional accuracy and shape, resulting in increased shape correction load after rolling. To do. Therefore, it is difficult to apply TMCP for increasing strength by introducing a hard bainite structure as the second phase to a hot rolled steel. About this point, the same thing can be said also in other hot rolled steel shapes such as T-shape steel.

  Therefore, for example, in order to produce a high-strength hot-rolled steel for a marine vessel having a yield stress YS: 235 MPa or more and a tensile strength TS: 400 MPa or more, it is not necessary to perform accelerated cooling after rolling. It is necessary to increase the strength of a steel structure composed of ferrite and pearlite, which is a cold rolled structure.

  Here, as means to achieve high strength in the (ferrite + pearlite) structure, a method of increasing the pearlite fraction of the second phase, a method of refining the ferrite structure, solid solution strengthening and precipitation strengthening of the ferrite ground Or a method in which a part of the ferrite is processed into ferrite (high dislocation density) by hot rolling in the (γ + α) two-phase region.

  Of the above methods, the method for increasing the pearlite fraction requires the addition of a large amount of C, but excessive addition of C is not preferable because it causes a decrease in weldability. Moreover, the method of strengthening the ferrite ground by adding a solid solution strengthening element or a precipitation strengthening element causes a decrease in weldability and an increase in raw material cost due to the addition of a large amount of alloy elements. Further, the utilization of processed ferrite is a preferable method in that the addition of C and alloy elements is suppressed to a minimum and the weldability is maintained, so that high strength can be achieved, but it is necessary to reduce the rolling efficiency. Therefore, the manufacturing cost increases.

  On the other hand, the addition of Nb is effective for the method of increasing the strength by refining the ferrite structure. Fortunately, in the present invention, Nb is added as an essential element for improving corrosion resistance. Therefore, in the present invention, the effect of improving the corrosion resistance by adding Nb and the effect of refining the ferrite structure are effectively utilized, and the ferrite phase is refined and the strength is increased with the ferrite + pearlite structure not including the processed ferrite. did.

Next, a method for producing the marine hot rolled section steel of the present invention having a microstructure composed of ferrite not containing processed ferrite and pearlite will be described.
In producing the marine hot-rolled section steel of the present invention, first, the steel having the above-described component composition is melted by a generally known method such as a converter or an electric furnace, and a continuous casting method or an ingot-making method. It is preferable to use a steel material such as a slab or billet by a generally known method. In addition, in the melting of steel, it goes without saying that treatment such as ladle refining or vacuum degassing may be added.

  Next, the steel material must be charged into a heating furnace, reheated to a temperature of 1000 to 1350 ° C., and hot-rolled to obtain a hot-rolled steel for ships having desired dimensions, structure and characteristics. is there. If heating temperature is less than 1000 degreeC, the deformation resistance in hot rolling will become large and it will become difficult to roll. On the other hand, heating exceeding 1350 ° C. causes surface flaws, increases scale loss, and increases fuel consumption. Preferably, it is the range of 1100-1300 degreeC.

In the subsequent hot rolling, it is necessary to perform finish rolling with the rolling finishing temperature exceeding 800 ° C. When the rolling finishing temperature is 800 ° C. or lower, the microstructure after rolling becomes a structure containing processed ferrite, and the strength of the steel increases, but a waiting time for temperature adjustment occurs during rolling, so the rolling efficiency decreases. And cost increases.
The cooling after hot rolling needs to be allowed to cool. Thereby, the shape defect by the bending and the curvature which arises from the non-uniform cooling accompanying rapid cooling after rolling can be reduced, and the burden required for the shape correction of the product after rolling can be reduced.

No. having the component composition shown in Table 5. Steels 1 to 19 were melted in a vacuum melting furnace or converter to form blooms. These blooms were charged into a heating furnace and heated to the temperatures shown in Table 6 and hot-rolled. Unequal-sided unequal thickness angle steel (NAB) and T-shaped steel with different cross-sectional dimensions were produced. Then, it used for the following tensile test, toughness test, and microstructure observation.
<Tensile test>
JIS 1A tensile test specimen (JIS Z2201) is taken from the short side for unequal side unequal thickness angle steel, and from T flange for the T shape steel, and a tensile test is conducted to obtain a yield stress YS, tensile strength TS and The elongation El was measured.
<Toughness test>
A short side is used for unequal side unequal thick angle steels, and a flange part is used for T-shaped steels by butt multi-layer welding (GMAW) with a heat input of 20 kJ / cm, and Charpy impact test piece (2 mmV) A notch test piece) was collected and the absorbed energy in a Charpy impact test at −20 ° C. was measured, and those having an absorbed energy value of 27 J or more were determined to have good impact characteristics at the weld. In addition, the same impact test was done about the base material of the short side of an unequal side unequal thickness angle steel, and the flange part of T-shaped steel, and the absorbed energy was measured.
<Microstructure observation>
Samples for microstructure observation were taken from the short side for unequal side unequal thickness angle steel, and for T-section steel from the flange part, and the microstructure of the ¼ part thickness was observed at 200 times magnification using an optical microscope. It was confirmed that there was no flattened processed ferrite.

Furthermore, the hot rolled steel was subjected to the following corrosion resistance test.
<Corrosion resistance test>
Samples of 5 mmt x 100 mmW x 200 mmL or 5 mmt x 50 mmW x 150 mmL were sampled from the short side for the unequal side unequal thickness angle steel, and the T-shaped steel from the flange part. A surface treatment under conditions A to C was applied to obtain a corrosion resistance test piece.
・ Condition A: Jing primer (film thickness: about 15 μm) and tar epoxy resin (about 200 μm) are applied to the surface of the test piece to form a two-layer film. • Condition B: Tar epoxy resin (film thickness: about 200 μm) 200 μm) is applied to form a single layer film. Condition C: Bare state with no shot blasting on the surface of the test piece (no anticorrosion film)
In addition, the test piece of said A and B in which the coating film was formed was provided with the scratch scissors of length 80mm which reaches the surface of the iron bar with a cutter knife from the top of the coating film.

  Next, the test piece was subjected to an exposure test that was mounted on the backside of the upper deck of a ballast tank of an actual ship for 2 years. The corrosive environment of this exposure test is, on average, about 20 days when seawater is in the ballast tank and about 20 days when seawater is not in one cycle. It was. And the corrosion resistance evaluation after the exposure test is as follows. For the test pieces of the conditions A and B having the coating film, the film swelling area generated around the scratch ridge is measured, and the condition C without the coating film is measured. For test pieces, calculate the average reduction in thickness from the difference (reduction amount) between the test piece mass after derusting and the test piece mass before the test, and use these values as the corrosion resistance improving element. No. The values of 12 base steel materials were relativized with reference (100), and those with a value exceeding 50% were judged to be inferior.

  Table 7 summarizes the results of the tensile test, impact test, microstructure observation, and corrosion test. From this table, No. 1 satisfying the component composition of the present invention is obtained. The steel materials 1-11 and 17-18 (invention examples) have a ratio of the coating film swelling area and the plate thickness reduction amount to the base steel material (No. 12) in any case where the surface treatment conditions are AC. It can be seen that it is 50% or less, indicating good corrosion resistance. On the other hand, No. which does not satisfy the component composition of the present invention. Even though the steel materials 12 to 16 (comparative example) have improved corrosion resistance compared to the base steel material (No. 12), the ratio of the coating film swelling area and the plate thickness reduction amount to the base steel material exceeds 50%, or welding The toughness of the part is greatly reduced. No. Although 19 steel materials (comparative examples) satisfy the composition of the present invention, the rolling finishing temperature is as low as 700 ° C., so that a waiting time for waiting for the temperature to drop during rolling occurs for about 150 seconds, The rolling efficiency is reduced as compared with the inventive examples.

  The ship shaped steel of the present invention exhibits excellent corrosion resistance in a corrosive environment with seawater regardless of the presence of the anticorrosion coating film. Therefore, the ship shape steel of the present invention is preferably used not only for ship ballast tanks but also as shape steels for other fields used in similar corrosive environments such as offshore structures and chemical plants. Can do.

Claims (10)

C: 0.05-0.20 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 to 1.0 mass%,
Cr: 0.01 mass% or more and less than 0.20 mass%,
Nb: 0.001 to 0.03 mass%,
N: 0.001 to 0.008 mass%,
A marine hot-rolled section steel having a microstructure comprising a ferrite and a pearlite structure, the balance of which is composed of Fe and inevitable impurities, and which does not contain processed ferrite. 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. The hot-rolled section steel for ships described in 1. In addition to the above component composition, 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 marine hot-rolled section steel according to claim 1 or 2, comprising one or more selected from%. In addition to the above component composition, Ti: 0.001 to 0.1 mass%, Zr: 0.001 to 0.1 mass%, and V: 0.002 to 0.2 mass% are selected from one or two types The hot-rolled section steel for marine vessels according to any one of claims 1 to 3, comprising the above. In addition to the said component composition, B: 0.0002-0.003 mass% is contained, The hot rolled shape steel for ships of any one of Claims 1-4 characterized by the above-mentioned. 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 marine vessels according to any one of claims 1 to 5, comprising the above. The hot rolled shape steel for ships according to any one of claims 1 to 6, wherein an epoxy resin coating film is formed on the surface of the shape steel. The hot rolled shape steel for ships according to any one of claims 1 to 6, wherein a zinc primer coating film is formed on the surface of the shape steel. The hot rolled shape steel for ships according to any one of claims 1 to 6, wherein a zinc primer coating and an epoxy resin coating are formed on the surface of the shape steel. The steel material having the composition according to any one of claims 1 to 6 is heated to 1000 to 1350 ° C, then hot-rolled with a rolling finishing temperature of over 800 ° C, allowed to cool, and processed ferrite is obtained. A method for producing a marine hot-rolled section steel, characterized by having a microstructure comprising no ferrite and a pearlite structure.