JP4502950B2 - Marine steel with excellent corrosion resistance and fatigue crack growth resistance - Google Patents

Description

  The present invention relates to corrosion-resistant and fatigue-resistant steel for ships used as main structural materials in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships, and the like, and particularly in environments exposed to seawater and high temperature and humidity. The present invention relates to a marine steel material that is excellent in corrosion resistance and has excellent fatigue crack growth characteristics under repeated stress.

  Steel materials used as main structural materials (for example, outer plates, ballast tanks, crude oil tanks, etc.) in the above various ships are often corroded because they are exposed to salt from seawater and high temperature and humidity. Since such corrosion may cause marine accidents such as inundation and sinking, it is necessary to apply some anticorrosion means to the steel. Conventionally, (a) coating, (b) cathodic protection, and the like are well known as anticorrosion means used so far.

  Of these, in coatings represented by heavy coating, there is a high possibility that coating film defects exist, and the coating film may be damaged due to a collision or the like in the manufacturing process, so that the base steel material is often exposed. In such a steel exposed portion, the steel material corrodes locally and intensively, leading to early leakage of the petroleum oil liquid contained therein.

  On the other hand, in the anti-corrosion, it is very effective for the part completely immersed in the seawater. However, in the part that receives the seawater splash in the atmosphere, the electric circuit necessary for the anticorrosion is not formed, and the anticorrosion. The effect may not be fully demonstrated. In addition, when the galvanic anode for anticorrosion disappears due to abnormal consumption or dropping, severe corrosion may immediately proceed.

  In addition to the above technique, for example, a technique as disclosed in Patent Document 1 has been proposed as a means for improving the corrosion resistance of the steel material itself. This technology discloses a corrosion-resistant steel for shipbuilding that can be used even without coating as having excellent corrosion resistance by appropriately adjusting the chemical composition of the steel material. Patent Document 2 discloses a marine steel material having an improved coating film life by making the chemical composition of the steel material appropriate. With these technologies, it can be said that a certain degree of corrosion resistance can be ensured as compared with the prior art.

  However, the corrosion resistance in a more severe corrosive environment is still not sufficient, and further improvement in corrosion resistance is required. In particular, corrosion (so-called crevice corrosion) in the “clearance” portion formed at the contact portion between the foreign material and the steel material, the structural reason, or the damaged portion of the anticorrosion coating film, etc., becomes prominent and may reduce the service life. So far, the proposed techniques have insufficient corrosion resistance in these areas.

  By the way, since there are many cases where the above various structural materials are subjected to repeated stress, in order to ensure the safety of the structural materials, it is extremely important in design that not only corrosion resistance but also steel materials have good fatigue characteristics. is important.

  The fatigue process of steel materials can be broadly divided into two processes, namely, the generation of cracks in stress-concentrated portions and the progress of cracks once generated. In general machine parts, the occurrence of macroscopic cracks is considered as a use limit, and there is almost no design that allows the cracks to propagate. However, in a welded structure, even if a fatigue crack occurs, it does not immediately break, and before this crack reaches the final stage, it is discovered by periodic inspection etc., and the cracked part is repaired, Or if the crack does not grow to the length that will lead to final failure within the period of use, the structure will be able to withstand sufficient use even if there is a crack.

  In welded structures, there are many weld toes as stress-concentrated parts, and it is almost impossible technically and economically to completely prevent the occurrence of fatigue cracks. Absent. That is, in order to improve the fatigue life of the welded structure, it is effective to significantly extend the crack propagation life from the state in which the crack already exists, rather than preventing the occurrence of the crack itself. For that purpose, the design which makes the progress rate of the crack of steel materials as slow as possible becomes an important matter.

Various techniques have been proposed as techniques for suppressing the rate of fatigue crack growth. For example, Patent Document 3 discloses a two-phase structure of a hard phase and a soft phase, and bending of a crack at the soft phase / hard phase boundary. A technique for suppressing the crack growth rate by stopping and branching has been proposed. However, this technique does not take into consideration the reduction in thickness due to corrosion of steel materials and corrosion damage, and it is necessary to satisfy both fatigue resistance and corrosion resistance characteristics in order to further improve safety.
JP, 2000-17381, A Claims etc. JP, 2002-266052, A Claims etc. Japanese Patent No. 3298544 Patent Claims etc.

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to suppress fatigue crack growth rate under repeated stress to increase fatigue crack growth resistance and to perform coating and cathodic protection. Steel for marine vessels with excellent corrosion resistance that can be put to practical use without any problem, especially for marine vessels that show improved durability against crevice corrosion and also against salt adhesion caused by seawater and corrosion due to wet environments To provide steel.

The marine steel material of the present invention that has achieved the above object is C: 0.01 to 0.2% (meaning of mass%, the same shall apply hereinafter), Si: 0.01 to 1%, Mn: 0.00. In addition to containing 01 to 2%, Al: 0.005 to 0.1%, Co: 0.01 to 1% and Mg: 0.0005 to 0.02%, the balance being Fe and inevitable It is a composite structure consisting of impurities, consisting of a soft phase and a hard phase, and the ratio of the hard phase Vickers hardness Hv ₁ to the soft phase Vickers hardness Hv ₂ (Hv ₁ / Hv ₂ ) is 1.5 to 5 0.0 and the diameter of the soft phase is 20 μm or less in terms of the equivalent circle diameter.

  In the marine steel material of the present invention, the soft phase is at least one selected from the group consisting of ferrite, tempered bainite and tempered martensite, and the hard phase includes bainite and / or martensite (including island martensite). Is mentioned. Moreover, it is preferable to adjust the value ([Co] / [Mg]) of the Co content [Co] and the Mg content [Mg] to a range of 2 to 350.

  In the marine steel material of the present invention, (1) Cu: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), Ni: 2% or less (0) as necessary. %) And Ti: at least one selected from the group consisting of 0.1% or less (not including 0%), (2) Ca: 0.02% or less (not including 0%), (3 ) Mo: 0.5% or less (not including 0%) and / or W: 0.3% or less (not including 0%), (4) B: 0.01% or less (not including 0%) , V: 0.1% or less (not including 0%) and Nb: 0.05% or less (not including 0%) are also effective. The characteristics of the marine steel will be further improved depending on the type of component to be contained.

In the steel for shipbuilding of the present invention, a predetermined amount of Co and Mg are contained in combination, and by appropriately adjusting the chemical composition, the corrosion resistance that can be put into practical use without coating and cathodic protection is excellent. A marine steel material can be realized, and in particular, a marine steel material that can improve the durability against crevice corrosion and exhibits excellent durability against salt adhesion caused by seawater and corrosion caused by a moist environment has been realized. Further, the steel structure is a composite structure composed of a soft phase and a hard phase, and the ratio (Hv ₁ / Hv ₂ ) of the Vickers hardness Hv _{1 of the} hard phase and the Vickers hardness Hv ₂ of the soft phase is within a predetermined range. In addition to reducing the particle size of the soft phase, the steel material was also excellent in fatigue crack growth resistance. Such marine steel materials are useful as raw materials for outer plates, ballast tanks, crude oil tanks and the like in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships and the like.

The inventors of the present invention have made extensive studies to solve the above problems. As a result, when a predetermined amount of Co and Mg are contained in combination and the chemical composition is appropriately adjusted, durability against crevice corrosion is improved, and salt adhesion caused by seawater and corrosion due to wet environments are prevented. It was also found that marine steels exhibiting excellent durability can be realized. Further, the fatigue crack propagation resistance of the steel material is a composite structure in which the ratio (Hv ₁ / Hv ₂ ) of the Vickers hardness Hv _{1 of the} hard phase and the Vickers hardness Hv ₂ of the soft phase is controlled within a predetermined range. At the same time, the inventors have found that the characteristics can be improved by reducing the particle size of the soft phase, thereby completing the present invention.

  In the steel material of the present invention, it is important to contain Co and Mg in combination, and the object of the present invention cannot be achieved without any of these components. Although each effect of these components will be described later, the reason why the corrosion resistance is improved by using these components in combination can be considered as follows.

  Mg exerts the action of suppressing the corrosion drop by suppressing the pH drop at the corroded portion and improving the corrosion resistance. In such a component system of a normal steel material (for example, Si-Mn steel material), such an action is porous, so that dissolved Mg immediately diffuses outside (for example, in seawater) without staying in the vicinity of the steel plate surface. Will end up. Therefore, when Mg is contained alone, the effect of improving the corrosion resistance is small. However, by including Co together with Mg, a fine surface rust film is formed, and diffusion of Mg to the outside is suppressed. In addition, it was considered that the corrosion resistance can be greatly improved by a synergistic effect with the hydrolysis equilibrium reaction of dissolved Co.

  Such an effect is exhibited by controlling to an appropriate amount described later, but the value of the ratio of these contents ([Co] / [Mg]: mass ratio) should also be appropriately controlled. Is preferred. That is, if this value ([Co] / [Mg]) is less than 2, local corrosion is likely to be insufficiently suppressed, and if it exceeds 350, overall corrosion is insufficiently suppressed. The value of [Co] / [Mg] is more preferably 10 or more, and further preferably 20 or more. On the other hand, the upper limit of the value of [Co] / [Mg] is more preferably 100 or less, further preferably 95 or less, further preferably 80 or less, and preferably 60 or less.

On the other hand, fatigue cracks proceed in a direction perpendicular to the stress in a normal stable growth region. In order to increase the resistance to crack propagation in consideration of such a fatigue crack propagation mechanism, the steel structure is made into a composite structure, and by detouring (bending) the crack at the boundary between the soft phase and the hard phase, The idea is that the crack growth rate can be reduced and the fatigue life can be extended. A certain hardness difference is required for bending of cracks in the hard phase (hereinafter sometimes referred to as “second phase”). However, if the difference in hardness becomes too large, brittle fracture occurs and the crack propagates in the hard phase, so that the effect is reduced. From such a viewpoint, in the marine steel material of the present invention, the ratio (Hv ₁ / Hv ₂ ) of the Vickers hardness Hv _{1 of the} hard phase and the Vickers hardness Hv ₂ of the soft phase is in the range of 1.5 to 5.0. Need to control.

That is, by setting the ratio (Hv ₁ / Hv ₂ ) to 1.5 or more, the plastic zone at the interface crack tip between the soft phase and the hard phase changes during the movement of the dislocation at the crack tip, and the bending and retention Since branching occurs, the crack growth rate is reduced. However, if the hardness of the hard phase becomes too high, as described above, the hard phase will cause brittle fracture due to the stress at the crack tip, and the effect of suppressing crack propagation will be reduced, so the ratio (Hv ₁ / The value of Hv ₂ ) needs to be 5.0 or less. The preferable lower limit of the value of this ratio is 1.7, more preferably 2.0 or more, and the preferable upper limit is 4.5, more preferably 4.0 or less. In order to control the value of the ratio (Hv ₁ / Hv ₂ ) as described above, it is preferable to appropriately control the ratio of the hard phase and the soft phase. From this viewpoint, the ratio of the soft phase is 20 to 90% by area. It is preferable to do this. Hereinafter, the ratio (Hv ₁ / Hv ₂ ) may be referred to as “hardness ratio”.

  The soft phase in the steel material of the present invention includes at least one selected from the group consisting of ferrite, tempered bainite and tempered martensite, and the hard phase includes bainite and / or martensite (including island martensite). Can be mentioned. Moreover, the structure of the steel material of the present invention may be any structure as long as it includes a soft phase as the first phase and a hard phase as the second phase, but does not necessarily have a two-phase structure. Or the composite structure containing 4 or more types may be sufficient. However, pearlite is a microscopic structure in which soft ferrite and hard cementite that easily brittlely breaks are present in stripes, and the above effect is difficult to obtain, so it is not included in any phase.

  In addition to the hard phase / soft phase boundary described above, crack growth causes bending, retention, and branching at grain boundaries, thereby reducing the crack growth rate. When the particle size of the soft phase becomes coarse, the frequency of collision with the grain boundary that becomes the resistance to crack propagation decreases, so the crack growth rate does not decrease. In the steel material of the present invention, for example, by carrying out supercooling, the nucleation sites increase and the hard phase is finely dispersed as the ferrite becomes finer. As a result, the probability of encountering the hard phase when the crack progresses is averaged, and the frequency of encounter is increased, so that the effect of decreasing the crack growth rate is obtained. From such a viewpoint, in the steel material of the present invention, it is also necessary that the soft phase has a diameter equivalent to a circle of 20 μm or less (a method for measuring the particle size will be described later). The particle size of the soft phase is preferably 15 μm or less.

  In the steel material of the present invention, basic components such as C, Si, Mn, and Al need to be appropriately adjusted in order to satisfy the basic characteristics as the steel material. The reasons for limiting the ranges of these components will be described below together with the effects of the above Co and Mg elements.

[C: 0.01 to 0.2%]
C is an element necessary for ensuring the strength of the material. In order to obtain the minimum strength as a structural member of a ship, that is, about 400 MPa (depending on the thickness of the steel material used), it is necessary to contain 0.01% or more. However, if the content exceeds 0.2%, the toughness and weldability deteriorate. For these reasons, the C content range was set to 0.01 to 0.2%. In addition, the minimum with preferable C content is 0.02%, More preferably, it is good to set it as 0.04% or more. Moreover, the upper limit with preferable C content is 0.18%, It is good to set it as 0.16% or less more preferably.

[Si: 0.01 to 1%]
Si is an element necessary for deoxidation and securing strength, and if it is less than 0.01%, the minimum strength as a structural member cannot be secured. However, if the content exceeds 1%, weldability and HAZ (welding heat affected zone) toughness deteriorate. The preferable lower limit of the Si content is 0.02%, more preferably 0.05% or more, and the preferable upper limit is 0.8%, more preferably 0.6% or less. .

[Mn: 0.01-2%]
Mn is also necessary for deoxidation and securing strength in the same manner as Si, and if it is less than 0.01%, the minimum strength as a structural member cannot be secured. However, if the content exceeds 2%, the toughness deteriorates. In addition, the minimum with preferable Mn content is 0.05%, It is good to set it as 0.10% or more more preferably. Moreover, the upper limit with preferable Mn content is 1.8%, It is good to set it as 1.6% or less more preferably.

[Al: 0.005 to 0.1%]
Al is also necessary for deoxidation and securing of strength in the same manner as Si and Mn, and if less than 0.005%, there is no effect on deoxidation. However, if added over 0.1%, weldability is impaired, so the range of Al addition amount is set to 0.005 to 0.1%. In addition, the minimum with preferable Al content is 0.01%, It is good to set it as 0.015% or more more preferably. Moreover, the upper limit with preferable Al content is 0.09%, More preferably, it is good to set it as 0.08% or less.

[Co: 0.01 to 1%]
Co is an indispensable element for forming a dense surface rust film that greatly contributes to improving the corrosion resistance of steel in a high salinity environment. In order to exert such an effect, the Co content needs to be 0.01% or more. However, if the content exceeds 1%, weldability and HAZ toughness deteriorate. For these reasons, the Co content is set to 0.01 to 1%. In addition, the minimum with preferable Co content is 0.015%, It is good to set it as 0.02% or more more preferably. Moreover, the upper limit with preferable Co content is 0.8%, More preferably, it is good to set it as 0.6% or less.

[Mg: 0.0005 to 0.02%]
Since Mg exhibits a pH raising action by being dissolved, it has an action of suppressing a pH reduction due to a hydrolysis reaction in a local anode where iron is dissolved, thereby suppressing a corrosion reaction and improving corrosion resistance. In order to exert such effects, Mg needs to be contained in an amount of 0.0005% or more. However, if it exceeds 0.02%, workability and weldability are deteriorated. For these reasons, the Mg content is suitably in the range of 0.0005 to 0.02%. A preferable lower limit of the Mg content is 0.0007%, and more preferably 0.001% or more. Moreover, the upper limit with preferable Mg content is 0.018%, It is good to set it as 0.015% or less more preferably.

  The basic components in the marine steel of the present invention are as described above, and the balance is composed of iron and inevitable impurities (for example, P, S, O, etc.), but does not impair the properties of the steel other than these. Some components (eg, Zr, N, etc.) are acceptable. However, these allowable components should be suppressed to about 0.1% or less because their toughness deteriorates when the amount is excessive.

  Further, in the marine steel of the present invention, one or more selected from the group consisting of (1) Cu, Ni, Ti and Cr, (2) Ca, (3) Mo and / or, depending on the necessity other than the above components. It is also effective to contain one or more selected from the group consisting of W, (4) B, V and Nb, etc., and the characteristics of the steel for shipbuilding will be further improved according to the type of component to be contained. Become.

[Cu: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), Ni: 2% or less (not including 0%), and Ti: 0.1% or less ( 1 or more selected from the group consisting of:
Cu, Cr, Ni and Ti are all effective elements for improving corrosion resistance. Among these, Cu and Cr are elements that are effective for forming a dense surface rust film that contributes greatly to the improvement of corrosion resistance like Co. These effects increase as the content increases, but if excessively contained, weldability, hot workability and HAZ toughness deteriorate, so that Cu is 1.5% or less, and Cr is 1% or less. It is preferable that The preferable lower limit when Cu and Cr are contained is 0.01%, more preferably 0.05% or more. Moreover, a more preferable upper limit is Cu: 1.0% or less and Cr: 0.8% or less.

  Ni is an element effective for stabilizing a dense surface rust film that greatly contributes to the improvement of corrosion resistance. Such an effect increases as the content increases, but if the Ni content becomes excessive, weldability and heat The inter-workability deteriorates and further leads to a significant cost increase. A preferable lower limit when Ni is contained is 0.01%, more preferably 0.05%, and a more preferable upper limit is 1.5%.

  Ti is an element that densifies the surface rust coating, which greatly contributes to the improvement of corrosion resistance, improves its environmental barrier properties, suppresses corrosion inside the crevice, and improves crevice corrosion resistance. Such an effect increases as its content increases, but if it exceeds 0.1%, the workability, weldability and HAZ toughness are deteriorated. In order to ensure the corrosion resistance required in such an environment, the content is preferably 0.005% or more, more preferably 0.008% or more, and a more preferable upper limit is 0.05%. .

[Ca: 0.02% or less (excluding 0%)]
Ca, like Mg, exhibits an effect of increasing pH when dissolved, and is an element effective in improving corrosion resistance by suppressing pH reduction due to hydrolysis reaction in the local anode where iron is dissolved and suppressing corrosion reaction. is there. Although such an effect by Ca increases as the content increases, if it exceeds 0.02% and is contained excessively, workability and weldability are deteriorated. A preferable lower limit when Ca is contained is 0.0005%, more preferably 0.001% or more, and a more preferable upper limit is 0.015%.

[Mo: 0.5% or less (not including 0%) and / or W: 0.3% or less (not including 0%)]
Mo and W have the effect of increasing the uniformity of corrosion and suppressing perforations due to local corrosion. In particular, when it is contained simultaneously with Co, a remarkable effect of improving uniform corrosion is exhibited. Such an effect increases as the content thereof increases, but if excessively contained, weldability deteriorates. Therefore, it is preferable to set Mo to 0.5% or less and W to 0.3% or less. A preferable lower limit when Mo is contained is 0.01%, more preferably 0.02% or more, and a more preferable upper limit is 0.3%. Moreover, a preferable lower limit when W is contained is 0.01%, more preferably 0.02%, and a more preferable upper limit is 0.2%.

[B: selected from the group consisting of 0.01% or less (not including 0%), V: 0.1% or less (not including 0%) and Nb: 0.05% or less (not including 0%) One or more
In marine steel materials, higher strength may be required depending on the site to be applied, but these elements are elements necessary for strength improvement. Among these, B is effective in improving the hardenability and improving the strength, but if contained in excess of 0.01%, the base material toughness and the HAZ toughness deteriorate, which is not preferable. V is effective in improving the strength, but if it exceeds 0.1%, it is not preferable because it causes deterioration of the toughness of the steel and HAZ toughness. Nb is effective for improving the strength, but if it exceeds 0.05%, Nb causes toughness deterioration of the steel and HAZ toughness. The preferable lower limit of these elements is 0.0001% for B, more preferably 0.003% or more, 0.003% for V, more preferably 0.005% or more, and 0.003% for Nb. %, More preferably 0.005% or more. The more preferable upper limit is 0.009% for B, 0.07% for V, and 0.03% for Nb.

  The marine steel material of the present invention basically exhibits excellent corrosion resistance even if it is not coated, but if necessary, the tar epoxy resin paint shown in the examples below, or other than that It can be used in combination with other anticorrosion methods such as heavy duty anticorrosion coating, zinc rich paint, shop primer, and anticorrosion. When such anticorrosion coating is applied, the corrosion resistance of the coating film itself (coating corrosion resistance) is also good as shown in the examples described later. Moreover, it can be set as the steel material excellent also in fatigue crack progress resistance by controlling a manufacturing method appropriately as follows and setting it as the above structures.

  In order to manufacture the steel material of the present invention having the above-described structure, for example, by the methods (1) to (3) shown below, the hard phase and the soft phase are appropriately controlled to suppress the progress of fatigue cracks. be able to.

(1) A steel slab having the chemical composition as described above is heated to 950 ° C. or more and 1250 ° C. or less, and rolling is finished in a temperature range from a heating temperature to an Ar ₃ transformation point, and cooling at 20 ° C./second or more. The first accelerated cooling is performed at a speed, and after cooling to 600 to 700 ° C., the temperature is maintained for 10 to 100 seconds (the cooling may be performed at a cooling rate of 0.5 ° C./second or less). Thereafter, the second accelerated cooling is performed at a cooling rate of 5 ° C./second or more to 400 ° C. or less. The reason for setting the range of each condition in this method is as follows.

  When the heating temperature is less than 950 ° C., the rolling temperature becomes too low, and when it exceeds 1250 ° C., the austenite grains become coarse and the base material toughness deteriorates. Therefore, it is necessary to heat at 950 to 1250 ° C.

Rolling temperature: When the rolling temperature is lower than the Ar ₃ transformation point, anisotropy occurs in the structure, and the impact absorption energy may be reduced. In addition, the rolling load is increased in production and the productivity is lowered.

  First accelerated cooling rate: By performing accelerated cooling, γ becomes a supercooled state, and transformation is suppressed to a low temperature. Thereafter, the transformation is performed at a low temperature, so that a ferrite having a high transformation driving force and a uniform and fine structure is generated. If the cooling rate is less than 20 ° C./second, partial transformation occurs during accelerated cooling, and uniform refinement of the structure cannot be achieved.

  Cooling stop temperature: When the stop temperature is lower than 600 ° C., the ferrite becomes acicular or a bainite structure. Although the acicular ferrite has good toughness, the hardness is higher than that of the polygonal ferrite, and the hardness difference from the second phase is reduced. Therefore, the effect of suppressing crack propagation at the phase boundary is small. On the other hand, when the cooling stop temperature exceeds 700 ° C., the transformation is slow at a predetermined holding temperature, and a sufficient ferrite fraction (for example, 20 area% or more) cannot be secured, and the crystal grains become coarse and toughness Will deteriorate.

  Holding time after cooling: If this holding time is less than 10 seconds, the transformation is not sufficient, the ferrite fraction is not sufficient, and there is no room for C to concentrate to unreacted γ. Further, when the holding time exceeds 100 seconds, the productivity is lowered, the equilibrium state is approached, and the generation of pearlite can be seen. This pearlite has a layered structure of ferrite and cementite, but cementite is brittle and causes brittle fracture at the crack tip, so that the effect of suppressing crack propagation is small.

  Second accelerated cooling rate: If this cooling rate is less than 5 ° C./second, ferrite + pearlite is generated from untransformed austenite in the cooling stage, and the hardness of the hard phase is not sufficient.

  Final cooling stop temperature: If the stop temperature at this time exceeds 400 ° C., the hard phase is softened by self-tempering and the hardness cannot be sufficiently secured, so the cooling stop temperature needs to be 400 ° C. or less. Yes, preferably 300 ° C. or lower.

(2) a steel slab having a chemical composition as described above, 950 ° C. or higher, then heated to 1250 ° C. or less, and terminates the rolling in a temperature range of the heating temperature to Ar ₃ transformation point, (Ar ₃ transformation point -30 ° C.) or air-cooled to a temperature range of ~ (Ar ₁ transformation point + 30 ° C.), or 5 ° C. / sec after cooling the following cooling rate, to an accelerated cooling at 5 ° C. / sec or more cooling rate. The reason for setting the range of each condition in this method is as follows.

  When the heating temperature is less than 950 ° C., the rolling temperature becomes too low, and when it exceeds 1250 ° C., the austenite grains become coarse and the base material toughness deteriorates. Therefore, it is necessary to heat at 950 to 1250 ° C.

Rolling temperature: When the rolling temperature is lower than the Ar ₃ transformation point, anisotropy occurs in the structure, and the impact absorption energy may be reduced. In addition, the rolling load is increased in production and the productivity is lowered.

Cooling rate: Air cooling to a temperature range of (Ar ₃ transformation point −30 ° C.) to (Ar ₁ transformation point + 30 ° C.), or by performing cooling accelerated cooling at a cooling rate of 5 ° C./second or less, By forming a hard phase from untransformed γ in which C is concentrated by subsequent accelerated cooling with ferrite (α) + γ, a composite structure of soft phase + hard phase can be obtained. When the cooling rate is higher than 5 ° C./second and the holding time is not taken, the time for concentrating C to the untransformed γ is small, and a sufficient hard phase is obtained even by the subsequent accelerated cooling. Not only can it not be obtained, but the uniformity in the thickness direction will be reduced.

Cooling stop temperature: When the stop temperature exceeds (Ar ₃ transformation point −30 ° C.), almost no ferrite is formed, and when it is less than (Ar ₁ transformation point + 30 ° C.), almost the transformation is completed to ferrite + pearlite. Therefore, a soft phase + hard phase cannot be obtained.

  Second cooling rate: When the cooling rate is less than 5 ° C./second, ferrite + pearlite is generated from untransformed austenite in the cooling stage, and the hardness of the hard phase is not sufficient.

  Final cooling stop temperature: If the stop temperature at this time exceeds 400 ° C., the hard phase is softened by self-tempering and the hardness cannot be sufficiently secured, so the cooling stop temperature needs to be 400 ° C. or less. Yes, preferably 300 ° C. or lower.

(3) The steel slab having the chemical composition as described above is heated to 950 ° C. or more and 1250 ° C. or less, and rolling is finished in the temperature range from the heating temperature to the Ar ₃ transformation point, and cooling at 10 ° C./second or more. The first accelerated cooling is performed at a rate of 400 ° C. or lower, and then reheating to a temperature range of (Ac ₁ transformation point + 30 ° C.) to (Ac ₃ transformation point−30 ° C.), and then a cooling rate of 5 ° C./second or more. Then, the second accelerated cooling is performed. In this method, the structure unit can be made fine by making the structure before reheating a quenching structure, and by reheating above the Ac ₁ transformation point, high temperature tempered bainite or martensite + austenite structure Become. Carbide diffuses from tempered bainite and martensite to reverse-transformed austenite, and the hardness of tempered bainite and martensite is greatly reduced, and then γ enriched with C is transformed into a hard phase by accelerated cooling. It can be a complex structure of phases. The reason for setting the range of each condition in this method is as follows.

  When the heating temperature is less than 950 ° C., the rolling temperature becomes too low, and when it exceeds 1250 ° C., the austenite grains become coarse and the base material toughness deteriorates. Therefore, it is necessary to heat at 950 to 1250 ° C.

Rolling temperature: When the rolling temperature is lower than the Ar ₃ transformation point, anisotropy occurs in the structure, and the impact absorption energy may be reduced. In addition, the rolling load is increased in production and the productivity is lowered.

  Cooling rate, cooling stop temperature: When the cooling rate is less than 10 ° C / second or the cooling stop temperature exceeds 400 ° C, the structure does not become a tempered structure, so the grain size becomes coarse, and fatigue crack growth resistance along with toughness Sex is reduced.

When the reheating temperature is less than (Ac ₁ transformation point + 30 ° C.), the α → γ transformation hardly occurs and a sufficient hard phase cannot be secured. If it exceeds (Ac ₃ transformation point + 30 ° C.), most of them undergo α → γ transformation after reheating, and all of them become a hard phase by subsequent quenching.

  Second accelerated cooling rate: When the cooling rate is less than 5 ° C./second, the hardness of the hard phase is not sufficient.

  Final cooling stop temperature: If the stop temperature at this time exceeds 400 ° C., the hard phase is softened by self-tempering and the hardness cannot be sufficiently secured, so the cooling stop temperature needs to be 400 ° C. or less. Yes, preferably 300 ° C. or lower.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Steel materials having the chemical composition shown in the following Tables 1 and 2 were melted in a converter, and various steel plates were produced by continuous casting and hot rolling. The transformation points (Ar ₃ , Ar ₁ , Ac ₁ , Ac ₃ ) shown in Tables 1 and 2 are values obtained by the following formulas (1) to (4). The production conditions at this time are shown in Tables 3 and 4. The temperature at this time is managed by the temperature at the position of t / 4 (t is the plate thickness), and the detailed temperature management procedure is as follows. The obtained steel plate was cut and subjected to surface grinding to finally produce a test piece having a size of 100 × 100 × 25 (mm) (test piece A). The external shape of the test piece A is shown in FIG.
Ar ₃ = 868-369 ・ [C] +24.6 ・ [Si] -68.1 ・ [Mn] -36.1 ・ [Ni] -20.7 ・ [Cu] -24.8 ・ [Cr]
+29.6 ・ [Mo] +190 ・ [V] (1)
Ar ₁ = 630.5 + 51.6 [C] +122.4 [Si] -64.8 [Mn] -57.5 [Mo] (2)
_{Ac 1 = 723-14 · [Mn]} +22 · [Si] -14.4 · [Ni] +23.3 · [Cr] ... (3)
Ac ₃ = 908-223.7 ・ [C] +30.49 ・ [Si] -34.3 ・ [Mn] +37.92 ・ [V] -23.5 ・ [Ni] (4)
However, [C], [Si], [Mn], [Ni], [Cu], [Cr], [Mo] and [V] are C, Si, Mn, Ni, Cu, Mo and V, respectively. Content (mass%) is shown.

[Temperature management procedure]
1. Using a process computer, the heating temperature at an arbitrary position (for example, t / 4 position) from the front surface to the back surface of the steel slab is calculated based on the atmospheric temperature from the start of heating to the end of heating and the in-furnace time.
2. Using the calculated heating temperature, rolling is performed while calculating the rolling temperature at an arbitrary position in the sheet thickness direction based on the rolling pass schedule during rolling and the data of the cooling method (water cooling or air cooling) between passes.
3. The surface temperature of the steel sheet is measured using a radiation type thermometer installed on the rolling line. However, the theoretical value is also calculated in the process computer.
4). The surface temperature of the steel sheet measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is collated with a calculated temperature calculated from a process computer.
5). If the difference between the calculated temperature and the measured temperature is ± 30 ° C or more, recalculate the calculated surface temperature so that it matches the measured temperature to obtain the calculated temperature on the process computer. The calculated temperature is used as it is.
6). Using the calculated temperature calculated above, the rolling temperature in the region to be controlled is managed.

  Further, as shown in FIG. 2, four small test pieces of 20 × 20 × 5 (mm) are brought into contact with a large test piece of 100 × 100 × 25 (mm) (the same as the test piece A), A test piece B having a clearance was formed. The small test piece and the large test piece for forming the gap were steel materials having the same chemical composition, and the surface finish was the same as that of the test piece A. Then, a hole of 5 mmφ was formed in the center of the small test piece, and a screw hole was made on the base material side (large test piece side), and fixed with an M4 plastic screw.

  Further, a test piece C (FIG. 3) on which an entire thickness of 250 μm thick tar epoxy resin coating (undercoating: zinc rich primer) was applied was used. Then, in order to investigate the degree of corrosion progress when the base steel material is exposed due to scratches on the anticorrosion coating film, the cut surface (length: 100 mm, width: about 0) is reached on one side of the test piece C. 0.5 mm) was formed with a cutter knife.

  About each sample material obtained with the manufacturing method shown in the said Table 3, 4, the test piece A, the test piece B, and five test pieces C were used for the corrosion test, respectively. The corrosion test method at this time is as follows.

[Corrosion test method]
First, a combined cycle corrosion test was conducted by simulating a marine environment and repeating a seawater spray test and a constant temperature and humidity test. In the seawater spray test, the specimen (each test piece A to C) was tilted at an angle of 60 ° from the horizontal, and was placed in a test tank, and 35 ° C artificial seawater (salt water) was sprayed in the form of a mist. Spraying of salt water was continuously performed. At this time, in the test tank, the spray amount was adjusted in advance so that 1.5 ± 0.3 mL of artificial seawater was collected at an arbitrary position per hour on a circular dish having an area of 80 cm ² installed horizontally. The constant temperature and humidity test was carried out by placing the test material at an angle of 60 ° from the horizontal in a test tank adjusted to a temperature of 60 ° C. and a humidity of 95%. Seawater spray test: 4 hours, constant temperature and humidity test: 4 hours as one cycle, these were alternately performed to corrode the specimen. The total test time was 6 months.

(1) For test piece A, the weight change before and after the test is converted into the average thickness reduction D-ave (mm), the average value of the five test pieces is calculated, and the overall corrosivity of each specimen is calculated. Evaluated. Further, the maximum erosion depth D-max (mm) of the test piece A is obtained using a stylus type three-dimensional shape measuring apparatus, and normalized by the average thickness reduction amount [D-ave (mm)] (that is, D-max / D-ave was calculated) and corrosion uniformity was evaluated. In addition, the weight measurement and the plate thickness measurement after the test were performed after removing corrosion products such as iron rust by the cathodic electrolysis method [JIS K8284] in an aqueous solution of diammonium hydrogen citrate.
(2) For test piece B, the crevice portion (contact surface) was visually observed to check for crevice corrosion. If crevice corrosion was observed, the corrosion product was removed by the cathodic electrolysis method, The maximum crevice corrosion depth D-crev (mm) was measured using a stylus type three-dimensional shape measuring apparatus.
(3) About the test piece C (with cut flaws) which performed the coating process, the ratio (bulging area rate) of the coating film swollen area in the surface which formed the cut flaw after a test was measured. The swollen area ratio was determined by a lattice point method (lattice interval 1 mm). That is, an average value of five test pieces was obtained by defining a swelling area ratio by dividing the number of lattice points where swelling was observed by the total number of lattice points. In addition, the swollen width of the coating film in the direction perpendicular to the cut flaw was measured with calipers, and the maximum value of five test pieces was defined as the maximum swollen width.

  The evaluation criteria for the above general corrosion resistance (D-ave), corrosion uniformity (D-max / D-ave), crevice corrosion resistance (D-crev), and coating corrosion resistance (blowing area ratio and maximum swollen width) are as follows: As shown in Table 5.

Regarding each sample, the fatigue crack growth rate was measured by the hardness ratio (hard phase / soft _{phase) (Hv 1 / Hv 2)} , and the particle size of the soft phase of the following methods.

[Fatigue crack growth rate]
The hot-rolled material was cut, and the fatigue crack growth rate was determined by conducting a fatigue crack growth test using a compact test piece in accordance with ASTM E647. At this time, the value in the stable growth region ΔK = 20 (MPa · √m) where the Paris law defined by the following equation (5) is satisfied was evaluated as a representative value. Regarding the evaluation and standard of the fatigue crack growth rate, the normal steel material has a growth rate of about 4 to 6 × 10 ⁻⁵ mm / cycle (when ΔK = 20), so 3.5 × 10 ^−5. The standard was mm / cycle or less.
da / dn = C (ΔK) ^m (5)
Here, a: crack length, n: number of repetitions, C, m: constants determined by matters such as material and load, respectively.

[Hardness ratio of (hard phase / soft phase)]
The Vickers hardness Hv _{1 of the} hard phase and the Vickers hardness Hv _{2 of the} soft phase were measured using a 10 gf micro Vickers hardness meter, and the average value of each of the five points was determined, and the hardness ratio (Hv ₁ / Hv ₂ ) Was calculated.

[Measurement method of particle size of soft phase]
(A) It cut | disconnected in the direction parallel to the rolling direction of steel materials, and the sample containing the front and back surface part of board thickness was prepared.
(B) Polishing was performed using a wet emery polishing paper of # 150 to # 1000 or a polishing method having the same function, and a mirror finish was performed using an abrasive such as diamond slurry.
(C) The polished sample was corroded using a 3% nital solution (corrosive solution) to reveal the crystal grain boundaries of the soft phase.
(D) The exposed tissue was photographed at a magnification of 100 times or 400 times (taken as a photograph of 6 cm × 8 cm) and taken into an image analyzer (600 μm × 800 μm at 100 times, 150 μm × 200 μm at 400 times) Equivalent). In this capture, the number of sheets corresponding to 1 mm × 1 mm (at least 6 fields of view at 100 × and 35 fields of view at 400 ×) was captured at any magnification.
(E) In the image analysis apparatus, the image was converted into a circle having an area equivalent to that of a region surrounded by one grain boundary, and the diameter of the converted circle was defined as the particle diameter of the equivalent-circle soft phase.
(F) The average of the values measured for all visual fields was calculated as the average equivalent-circle soft phase particle size.

  The results of the corrosion test and the fatigue test progress rate are shown in Tables 6 and 7 below together with the hardness ratio and the soft phase particle size (equivalent circle diameter).

  These results can be considered as follows. No. containing no Co or Mg No. 3-6, the content of Co or Mg is less than the lower limit specified in the present invention. 8 to 10 have slightly improved overall corrosion resistance compared to conventional steels (Nos. 1 and 2) due to the effect of addition of Co or Mg. However, no. No. 2, 3 and No. with insufficient amount of Co. In the case of 7 and 8, the improvement effect is not recognized by the corrosion uniformity and the swollen area ratio. No. No Mg is contained. Nos. 5 and 6 and No. in which the amount of Mg is insufficient. Nos. 9 and 10 are not sufficient as the corrosion resistance of marine steel materials because no improvement effect is observed in the crevice corrosion resistance and the maximum swollen width.

  On the other hand, those containing a suitable amount of Co and Mg in combination (Nos. 11 to 59) are superior to the conventional steels (Nos. 1 and 2) because of the synergistic effect resulting from the addition of these elements. It can be seen that it is preferable as a marine corrosion resistant steel. In particular, in addition to the combined use of Co and Mg, it can be seen that the corrosion resistance of the steel material is further improved by further containing a corrosion resistance improving element such as Cu, Cr, Ni, Ti, Ca, Mo and W.

  Of these, the specimens added with Cu, Cr, Ni or Ti are particularly effective in reducing the maximum swollen width of the paint specimen, and the rust densification of these elements acts to stabilize the rust in the cut part. Thus, it is presumed that the corrosion progress was suppressed. Further, Ca has an effect of increasing crevice corrosion resistance (No. 22 to 24, 29, 30 and the like), and it is considered that Ca further strengthens the suppression of pH lowering in the crevice and reduces corrosion. Furthermore, it can be seen that the addition of Mo and W is very effective in improving the corrosion uniformity and the paint swellability (No. 48 to 57, etc.). No. As is apparent from the results of 31-59 and the like, it can be seen that by appropriately adjusting the value of ([Co] / [Mg]), various corrosion resistances are greatly improved.

On the other hand, with respect to the fatigue crack growth rate, those that deviated from the preferable manufacturing conditions (No. 12, 15, 17, 19, 21, 24, 28, 30, 32, 34, 36, 38, 40, 42, 45, 47). , 49, 51, 53, 55, 57 and 59), the coarseness of crystal grains occurs, or the hardness ratio (Hv ₁ / Hv ₂ ) is an appropriate value because sufficient hard phase hardness cannot be obtained. It can be seen that the fatigue crack growth rate is increased. Based on this data, the relationship between the hardness ratio (Hv ₁ / Hv ₂ ) and the fatigue crack growth rate is shown in FIG. 4, but by defining the hardness ratio in the range of 1.5 to 5.0, fatigue It can be seen that the crack growth rate is low.

It is explanatory drawing which shows the external appearance shape of the test piece A used for the corrosion resistance test. It is explanatory drawing which shows the external appearance shape of the test piece B used for the corrosion resistance test. It is explanatory drawing which shows the external appearance shape of the test piece C used for the corrosion resistance test. It is a graph which shows the relationship between hardness ratio and fatigue crack growth rate.

Claims (6)

C: 0.01 to 0.2% (meaning of mass%, the same shall apply hereinafter), Si: 0.01 to 1%, Mn: 0.01 to 2%, Al: 0.005 to 0.1%, respectively In addition to Co: 0.01 to 1% and Mg: 0.0005 to 0.02%, the balance consisting of Fe and inevitable impurities, selected from the group consisting of ferrite, tempered bainite and tempered martensite A Viscos hardness Hv _{1 of the} hard phase and a Vickers hardness of the soft phase, which is a composite structure composed of one or more kinds of the soft phase and a hard phase composed of bainite and / or martensite (including island martensite). a Hv ratio of _{2 (Hv} 1 / Hv ₂₎ is 1.5 to 5.0 are, der 20μm or less particle size with a circle equivalent diameter of the soft phases is, the da / dn is the following equation (1) 3.5 × ¹⁰ -5 mm / cycl Corrosion resistance and fatigue crack growth resistance in good marine steel to satisfy the following.
da / dn = C (ΔK) ^m (1)
(Where, a: crack length, n: number of repetitions, C, m: material constant, ΔK: stable growth region, ΔK = 20 (MPa · √m)) 2. The marine steel material according to claim 1, wherein the value ([Co] / [Mg]) of the Co content [Co] and the Mg content [Mg] is 2 to 350. 3. Further, Cu: 1.5% or less (not including 0%), Cr: 1% or less (not including 0%), Ni: 2% or less (not including 0%), and Ti: 0.1% or less The marine steel material of Claim 1 or 2 containing 1 or more types chosen from the group which consists of (it does not contain 0%). Furthermore, the steel materials for ships in any one of Claims 1-3 containing Ca: 0.02% or less (excluding 0%). The marine steel according to any one of claims 1 to 4 , further comprising Mo: 0.5% or less (excluding 0%) and / or W: 0.3% or less (not including 0%). . Further, from the group consisting of B: 0.01% or less (excluding 0%), V: 0.1% or less (not including 0%), and Nb: 0.05% or less (not including 0%) The marine steel material according to any one of claims 1 to 5 , comprising one or more selected.

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