WO2009087944A1

WO2009087944A1 - Steel plate exhibiting excellent bendability by line heating and process for production of the plate

Info

Publication number: WO2009087944A1
Application number: PCT/JP2008/073928
Authority: WO
Inventors: Kiyotaka Nakashima; Masanori Minagawa; Naoki Oda; Kunitaka Masuda
Original assignee: Nippon Steel Corporation
Priority date: 2008-01-08
Filing date: 2008-12-25
Publication date: 2009-07-16

Abstract

For the purpose of improving the operating efficiency in the plate bending by line heating, the invention provides a steel plate which exhibits large bending deformation even under the conditions of enhanced heating rate and shortened heating time and a process for production of the plate. The steel plate has both a chemical composition which contains by mass C:0.01 to 0.08%, P: ≤0.05%, S: ≤0.05%, Al: 0.002 to 0.1%, and N: 0.001 to 0.008% with the balance being iron and unavoidable impurities and a microstructure which comprises a non-deformed ferrite phase at an area fraction of 90% or above and in which the mean grain diameter of theferrite phase is 15 to 45μm and cementite particles having circle-equivalent diameters of 0.5μm or below are present in the ferrite grains at a number density of 100000 particles/mm2 or above. Further, the plate exhibits a yield strength of 235MPa or above at room temperature, a yield strength of 180MPa or below at 400°C, and an average Charpy absorbed energy of 100J or above at 0°C.

Description

MEXICO BOOKS Steel plate with excellent bending workability by linear heating and its manufacturing method Technical Field

The present invention relates to a steel plate surface that is deformed and formed by linear heating of a steel plate frequently used in the shipbuilding field, in particular, in the field of welded steel structures such as shipbuilding, construction, bridges, and offshore structures, that is, a gas burner. In addition, the present invention relates to a thick steel plate and a method of manufacturing the same that can greatly improve the work efficiency in the heat processing operation of a steel plate in which the back surface is linearly heated and the heated portion is subsequently cooled with water to bend and deform the steel plate. . Background art

Ship structures such as hulls in the shipbuilding field need to have a smooth curvature surface with a continuous outer surface in order to reduce water flow resistance during voyage.

. Therefore, a thick steel plate having a thickness of 10 to 30 mm is bent into a predetermined shape in advance, and then the end surfaces of the steel plates are welded to form a welded structure having a continuous smooth curvature surface.

Such bending of steel sheets needs to be processed into a complicated and delicate curvature depending on the part of the ship structure, so it cannot be dealt with by simple and uniform press work alone. Therefore, usually, after roughing the press, bending is performed by linear heating, that is, a steel plate is locally heated linearly using a gas burner or the like, and then water-cooled immediately after heating.

This bending process by linear heating takes a long time to obtain a predetermined shape, and is therefore one of the bottlenecks in the shipbuilding process, which increases costs. For this reason, it will contribute to improving work efficiency. A steel sheet is desired.

The thermal deformation of a steel sheet due to linear heating is a phenomenon in which when the heated part contracts due to cooling after thermal expansion, the heated part of the steel sheet yields due to restraint from the surrounding non-heated region and plastically deforms. For this reason, various techniques related to steel sheets have been proposed that aim to increase the amount of deformation caused by linear heating by controlling the yield strength of the steel sheet.

These technologies can be broadly classified into technologies related to steel plates with high yield strength at high temperatures, technologies related to steel plates with low yield strength at high temperatures, and technologies related to steel plates with low yield strength at room temperature.

A technique related to a steel sheet having a high yield strength at high temperature is described in Japanese Patent Application Laid-Open No. 7-1 3 8 7 15. In the technique described in Japanese Patent Laid-Open No. 7-1 3 8 7 1 5, Nb, Mo, etc. are added in combination, and appropriate hot rolling conditions are applied, so that the heat history of the linear heating operation is achieved. The present invention relates to a steel sheet having high yield strength at high temperatures by precipitating Nb and Mo-containing carbonitrides.

A technique related to a steel sheet having a low yield strength at a high temperature is described in Japanese Patent Application Laid-Open No. 20 07-5 6 3 48. The technology described in Japanese Patent Laid-Open No. 2007-0 5 6 3 4 8 contains 20 to 95% of a ferrite phase in which dislocations are introduced into the microstructure by processing or transformation strain. Yield strength at high temperature with yield stress at 0,0 ° C not more than 0.75 times the yield stress at room temperature, and yield stress at 600 ° C not more than 0,5 times the yield stress at room temperature This is related to a steel plate with a low slab. In order to obtain a ferrite phase with dislocations introduced, the steel plate manufacturing process requires two-phase rolling or accelerated cooling from the two-phase zone.

A technique related to a steel sheet having a low yield strength at room temperature is a technique described in Japanese Patent Application Laid-Open No. 2 0 06 1 2 0 5 1 8 1. The technique described in Japanese Patent Laid-Open No. 20 0 6-2 0 5 1 8 1 has a ferrite fraction of 20% or less. The present invention relates to a steel sheet in which the yield strength at room temperature is reduced by straightening the above steel sheet at a reduction rate of 0.1% to 0.5% at a temperature at which no aging occurs. Disclosure of the invention

In bending of a steel sheet by linear heating, the amount of deformation usually tends to increase as the maximum temperature reached by the linear heating part increases. This is because the region that undergoes thermal expansion and contraction widens as the maximum temperature reached by the linear heating section increases.

However, in order to increase the maximum temperature reached in the linear heating section, it is necessary to lengthen the heating time, and the working efficiency when bending is reduced.

In addition, steel sheets with a large amount of bending deformation are required under conditions where the maximum temperature reached by the linear heating section is low. Under such conditions, it is possible to lower the yield strength of the steel sheet as described in Japanese Patent Application Laid-Open No. 2 0 0 7-5 6 3 4 8 or Japanese Patent Application Laid-Open No. 2 0 0 6-2 0 5 1 8 1. It is advantageous to increase the amount of bending deformation due to linear heating.

This is because, in the case of low-temperature heating, a steel sheet with a low yield stress has a smaller reverse deformation due to yielding more easily due to restraint from the non-heated part when the heated part is thermally expanded. is doing.

Since the amount of deformation due to subsequent thermal contraction of cooling hardly depends on the yield strength, the final amount of deformation is greater for steel sheets with low reverse strength and low yield strength.

On the other hand, in a steel plate with a high yield stress, it is difficult for the heated part to yield, and the stress required for deformation increases, so the amount of reverse deformation due to thermal expansion increases, and the final amount of deformation decreases.

Therefore, the technique described in Japanese Patent Laid-Open No. 7-1 3 8 7 1 5 is Since this is a technique for increasing the yield strength of steel sheets at high temperatures, it is not suitable as a steel sheet with a large amount of bending deformation under conditions where the maximum ultimate temperature of the linear heating section is low.

Further, the technique described in Japanese Patent Application Laid-Open No. 2 0 0 7-5 6 3 4 8 is a useful technique for reducing the yield strength at 5 0 0 ° C and 6 0 0 ° C, Since the ferrite phase with dislocations is used, dislocation recovery is unlikely to occur under linear heating conditions where the temperature is lower than 500 and the heating time is short, and dislocation strengthening remains, resulting in high temperatures. It cannot be said that it is a technique for sufficiently lowering the yield strength at.

Furthermore, in the ferrite phase in which dislocations are introduced, the interface with the ferrite phase in which dislocations are not introduced tends to be the starting point of brittle fracture, which causes a decrease in toughness. In addition, when a ferrite phase with dislocations introduced by two-phase rolling is used, segregation is more likely to occur due to the development of the texture, so the Charpy fracture surface transition temperature can be lowered. It is difficult to increase the Charpy average absorbed energy.

In addition, since the anisotropy of the steel sheet is increased, the amount of bending deformation becomes anisotropic, and it becomes difficult to process the surface with a smooth curvature by linear heating.

In addition, the technique described in Japanese Patent Laid-Open No. 2 0 06-2 0 5 1 8 1 can reduce the yield stress at room temperature due to the movable dislocation introduced by the rolling reduction, but in the low temperature range. When heated, it cannot be said to be a technique for sufficiently reducing the yield strength at low temperatures by so-called age hardening by fixing solid solution carbon to dislocations or depositing carbides on the dislocations.

The present invention has been made in consideration of the above circumstances. The purpose is to improve the bending work efficiency by linear heating, that is, to shorten the heating time and to reduce the amount of bending deformation even at the low maximum temperature of the linear heating part. Is to obtain a thick steel plate (mainly 10 to 30 mm thick). For this purpose, a steel plate with a low yield strength at low temperatures and a method for manufacturing the same, and a steel plate with excellent bending workability by linear heating that has sufficient yield strength and toughness as shipbuilding steel and its It is to provide a manufacturing method.

The present invention has been made as a result of intensive studies in order to solve the above-described problems, and the means thereof are as follows.

(1)

C: 0 • 0 1 to 0.08%,

P 0 .05%,

S: Ku 0.55,

A 1: 0 0 0 2 0.1%,

N: 0 0 0 1 to 0. 0 0 8

The balance is a steel plate whose chemical composition is composed of iron and inevitable impurities. Ferrite with an unprocessed microstructure is 90% or more in area ratio. The average grain size of the ferrite phase is 1%. In addition, cementite particles with an equivalent circle diameter of 0.5 xm or less are present in the ferrite grains in a number density of 1 000 000 mm ² or more, and at room temperature. Yield strength at 2 35 MPa or higher, yield strength at 400 ° C or lower at 180 ° C, and Charpy average absorbed energy at 0 ° C or higher at 100 J or higher A thick steel plate with excellent bending workability by linear heating.

(2) In addition, the amount purchased is

S i: 0.05 to 0.5%,

M n: 0.05 to 0.5%,

C u: 0.05 to 0.5%,

N i: 0.05 to 0. C r 0. 0 5 0 • 3%,

M o 0. 0 0 5 0 1%,

N b 0. 0 0 5 0 • 0 1 Zo,

V 0. 0 0 5 to 0 • 0 2 / o,

T i 0. 0 0 5 0 • 0 2%,

B 0. 0 0 0 5 to 0 0 0 3

The thickness excellent in bending workability by linear heating according to claim 1, characterized in that it contains at least one kind of chemical component as a chemical component, and C eq is 0.2% by mass or less. steel sheet.

However, C eq = C + S i / ^/ 2 4 + M n / 6 + (C u + N i) / 1 5 + (C r + M o + V) / 5

C, Si, Mn, Cu, Ni, Cr, Mo, V: Content of each element (% by mass)

(3) Furthermore, in mass%,

C a: 0. 0 0 0 3 to 0.0. 0 5%,

M g: 0. 0 0 0 3 to 0.0. 0 5%,

R E M: 0. 0 0 0 3 to 0 • 0 0 5%

A thick steel plate excellent in bending workability by linear heating as described in fij (1) or (2), characterized by containing at least one or more of these as chemical components.

,

(4) A steel slab having the chemical composition described in any one of (1) to (3) above is heated to 10:00 to 1300 ° C, and is heated above the Ar3 transformation point. Stennai 圧延 Rolling at a cumulative reduction rate of 30% or more in the single-phase region to obtain the product sheet thickness, then the average sheet thickness from 7500 ° C to a cooling rate of 5 to 50 / s, 400 ° C A method for producing a thick steel plate excellent in bending workability by linear heating, characterized by performing accelerated cooling to a temperature below C.

(5) After completing the accelerated cooling, at 3 0 0 and less than 4 0 0 ° C The method for producing a thick steel plate having excellent bending workability by linear heating as described in (4) above, characterized by tempering.

The unprocessed ferrite phase in the present invention refers to a ferrite phase that has not been subjected to rolling by two-phase rolling below the Ar 3 transformation point.

The room temperature is the temperature range of 10 to 35 ° C, which is the test temperature range specified in the “Metal Material Tensile Test Method” of JISZ 2 2 4 1.

The following effects can be obtained by the present invention.

First, it has a sufficient yield strength and toughness, mainly as a steel plate for shipbuilding, and the amount of bending deformation can be increased under conditions where the maximum temperature reached is low, dramatically improving the efficiency of bending work by linear heating. Can be improved. And it contributes to the industrial contribution by shortening the shipbuilding period, reducing costs, and reducing the environmental burden associated with reducing energy consumption. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described.

Generally, a thick steel plate (hereinafter, simply referred to as a steel plate) used for manufacturing a welded marine structure is bent by linear heating, as described above, using a heating source such as a gas burner. Alternatively, a predetermined region on the back surface is locally heated linearly, and when the heated region contracts by cooling after thermal expansion, the steel plate undergoes plastic deformation due to restraint from the surrounding non-heated region. The steel sheet is processed into the desired processing shape.

In this way, bending by linear heating uses plastic deformation of the steel sheet, so the yield strength of the steel sheet has a large effect on the deformation. In particular In order to improve the efficiency of bending work by linear heating, under conditions where the heating temperature during linear heating is lowered, specifically, under the conditions where the maximum temperature reached is a low temperature of 400 to 600, The amount of bending deformation has a good correlation with the yield strength at 400, and it was found that the amount of bending deformation increases as the yield strength at 400 ° C decreases.

The reason why the highest temperature during linear heating is set to 400 to 60 Ot is that if it is less than 400 ° C, the amount of thermal expansion and contraction is small and the amount of bending deformation is insufficient. This is because the processing time is required due to the increase in the number of times of linear heating up to. In other words, even if the heating time is short, the work efficiency is reduced. Also, if it exceeds 60: 0, the heating time becomes longer, which leads to an increase in machining time and lowers work efficiency.

Next, the reason why the steel sheet yield strength at room temperature and 400 ° C and the Charpy average absorbed energy at 0 ° C were limited will be described.

The reason why the lower limit of yield strength at room temperature is 2 3 5 MPa is that the minimum yield strength for shipbuilding structural steel is 2 3 5 to prevent buckling, plastic deformation, fatigue failure, etc. This is because it is MP a.

However, if it exceeds 35 5 MPa, it is not easy to reduce the yield strength at 400 ° C described below, so the upper limit is preferably 3 55 MPa.

In order to efficiently perform the linear heating work, as described above, it is necessary to perform the heating under the condition that the maximum temperature reaches 400 to 600 ° C. In order to increase the amount of bending deformation under these conditions, the yield strength at 40 0 must be 18 OMPa or less, and this is the upper limit. Considering temperature variations, the yield strength at 400 ° C. is preferably 16 OMPa or less. Also, the lower the yield strength at 400 ° C, the more the bending deformation increases. Since it is not easy to ensure the yield strength of temperature, the lower limit is preferably 80 MPa.

In addition, the Charpy-average absorbed energy at 0 ° C is more than 100 J because the risk of brittle smashing increases if the temperature is less than 100 J.

In order to prevent this and make steel plates with improved safety, the lower limit was set to 100 J.

The reason for limiting the mouth tissue in the present invention is described below.

The reason why the microstructure is mainly composed of an unprocessed ferrite phase is to reduce the yield strength at 40 ° C. by utilizing the softest of the steel sheet structures.

In addition, if the ferrite phase is processed by two-phase rolling or the like, and the transition ¹ ^ ³ # enters, dislocation recovery at 400 ° C is unlikely to occur, so dislocation strengthening remains, and at 400 ° C Since it was difficult to make the yield strength of 18 OMPa or less, it was considered as an unprocessed Ferai phase.

Furthermore, the processed ferrite phase causes the steel sheet anisotropy to decrease the Charpy average absorbed energy, and in order to avoid this, the non-processed ferrite phase is mainly used.

In addition, the area ratio of the unprocessed ferrite phase was set to 90% or more. When the area ratio was less than 90%, hard low-temperature transformation structures other than ferrite phase, such as particulates, paynite, and martensite, were 1 This is because it is difficult to make the yield strength at more than 0% and 400 ° C less than 18 OMP a '. Desirably, the area ratio of the unprocessed ferrite phase is preferably 9 397%.

Furthermore, the reason for setting the average crystal grain size of the ferrite phase to 15 45 5 m is that if it is less than 15 m, it is difficult to make the yield strength at 400 0 m or less due to fine grain strengthening. However, if it exceeds 45 ^ m, the toughness deteriorates and it is difficult to increase the Charpy average absorbed energy to 100 J or more. Because it is difficult.

In addition, when the particle size is less than 15, C can easily diffuse to the grain boundary, which makes it difficult to precipitate cementite particles in the ferrite particles as described below. This is one of the reasons why the lower limit is 15 m. Preferably, the average crystal grain size of the ferrite phase is 20 to 40 m.

Next, it is one of the important requirements in the present invention that the cementite particles having an equivalent circle diameter of 0.5 / m or less are present in the ferritite grains in a number density of 1 000 000 mm ² or more. One. The reason for this will be explained below.

In the present invention, in order to set the yield strength at 400 ° C. to 1800 MPa or less, and preferably 1 6 OMPa or less, it is possible to use perlite, bainite, martensite, etc. Strengthening with a hard low temperature transformation structure and addition of alloying elements other than C are minimized, so solid solution strengthening and precipitation strengthening with alloying elements cannot be used. Therefore, it becomes extremely difficult to increase the yield strength at room temperature to 2 35 MPa or more. Thus, among the many steel precipitates, cementite particles, which are thermally unstable, were used to increase the yield strength at room temperature.

The cementite is relatively stable at room temperature and contributes to an increase in yield strength. However, at 400 t: or more, it easily agglomerates and coarsens in a short time, and hardly contributes to the increase in yield strength. In other words, if the cementite particles are appropriately controlled, the yield strength at room temperature is superimposed on the fine grain strengthening and particle dispersion strengthening, which contributes significantly to the increase in yield strength. Above ° C, the increase in yield strength hardly contributes to grain dispersion strengthening, and only the crystal grain size can be used as a controlling factor for strengthening.

Such dispersion strengthening by fine cementite particles within the grains is This is remarkable when the ferrite fraction is large, when the crystal grain size of the ferrite is relatively large, and when the cooling rate is large. In other words, this is because the addition of alloying elements increases the hardenability and the ferritic fraction is small and the second phase fraction is large. This is because it is difficult to secure the amount of cementite precipitation. Also, if the crystal grain size becomes extremely small, C diffuses easily to the grain boundary, making it difficult to disperse the cementite within the grain. If the cooling rate is further reduced,

Similar to the above, C easily diffuses to the grain boundary, making it difficult to disperse the cementite within the grains, and consolidating and coarsening the cementite, contributing to enhanced particle dispersion. This is because it becomes difficult to control the size and the number density as possible.

Here, the reason why the equivalent particle diameter of cementite particles is 0.5 m or less and the number density is 10 0 0 0 0 0 pieces / mm ² or more is more than 0.5 m, or 10 If it is less than Zmm ² , grain dispersion strengthening does not contribute, and it is difficult to make the yield strength at room temperature 2 3 5 MPa or more. The lower limit of the equivalent circle diameter and the upper limit of the number density of the cementite particles are those that can tolerate a decrease in toughness due to enhanced cementite particle dispersion. The lower limit of the equivalent circle diameter is 20 nm, and the upper limit of the number density is 10 Preferably, the number is 0 0 0 0 0 0 pieces / mm ² .

Hereinafter, the reason for limiting the amount of each element will be described. “%” Below means “% by mass” unless otherwise specified.

C is the most important element in the present invention. In order to secure the precipitation amount of cementite particles and to obtain a yield strength of 2 35 MPa or more at room temperature, 0.0 1% or more is necessary. However, if it exceeds 0.08%, for example, the second phase fraction such as perlite increases, resulting in an increase of 4 0 0 ° C. Since it is difficult to make the yield strength at 1800 MPa or less at 0.08%, the upper limit was set to 0.08%, but preferably 0.02 to 0.05%

P is an impurity element, and the yield strength at a high temperature is increased by solid solution strengthening, leading to deterioration of toughness. Therefore, P must be reduced as much as possible. However, at 0.05% or less, those adverse effects can be tolerated, so 0.05% is the upper limit.

S is also an impurity element, and it is desirable to reduce it as much as possible in order to degrade the toughness and ductility of steel. However, since the adverse effect is acceptable at 0.05% or less, the upper limit is set to 0.05%. .

A 1 is an important element in the present invention. Add mainly for deoxidation. For that purpose, 0.02% or more is necessary. However, if it exceeds 0.1%, alumina-based coarse oxides and their clusters are formed and the toughness is impaired, so the upper limit is 0.1%. Preferably, A 1: 0.0 1 to 0.0 7%.

N, in a trace amount, forms fine nitrides when the steel slab is heated and refines the heated austenite grains, contributing to improved toughness. For that purpose, 0.0 0 1% or more is necessary. On the other hand, if it exceeds 0.08%, the toughness due to the coarsening of the nitride tends to deteriorate, and the yield strength at 400 ° C is increased by increasing the solid solution N content and strengthening the solid solution. Since it is difficult to make it OMPa or less, the force having an upper limit of 0.0 0 8%, preferably 0.0 0 1 to 0.0 0 5%.

S i and M n less than 0.05%, Cu, Ni and Cr less than 0.05%, Mo, Nb, V and T i less than 0.05%, 0. 0 0 0 Less than 5% B, 0. 0 0 0 Less than 3%. &, Mg and REM may be mixed as inevitable impurities from raw materials and refractories. However, if it is within these ranges, it will have no adverse effect. In the present invention, it is acceptable as an inevitable impurity.

The above are the basic components of the steel sheet of the present invention, and it can be a steel sheet excellent in bending workability by linear heating, strength and toughness as shipbuilding steel, which is the object of the present invention.

Furthermore, one or more of Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, and B can be contained for the purpose of adjusting strength and toughness.

However, these selective elements increase the hardenability of steel even when added in a small amount and contribute to strength and toughness improvement by grain refinement, solid solution strengthening, precipitation strengthening, etc. Since it is difficult to set the yield strength at 0 0 ° C to 1800 MPa or less, it is necessary to set an upper limit for each.

This upper limit is 0.5% for S i and M n respectively, and Cu, Ni and Cr are each

0.3%, Mo is 0.1%, Nb is 0.01%, V and Ti are 0 respectively.

0 2%, B was 0.0 0 3%. However, preferably, S i and M n are each 0.3% or less, Cu, N i and Cr are each 0.1% or less, Mo is 0.05% or less, and Nb is 0.0%. 5% or less, V is 0.01% or less, T i is 0.01% or less, and B is 0.00 1% or less. 0. 0

5% or more of S i, M n, Cu, N i or C r, 0.0 5% or more of Mo, N b, V or T i, or 0.0 0 0 5% or more of B is

This value was made the lower limit because it contributes to strength and toughness improvement by crystal grain refinement, solid solution strengthening and precipitation strengthening.

Also, S i, M n, C u, N i, C r, M o, N b, V, T i,

When multiple types of B are included, C e Q obtained by the following formula is 0

. Must be 0 or less. However, if eq exceeds 0.20%, it is difficult to make the yield strength at 400 ° C. less than 180 MPa. C eq = C + S i / 2 4 + M n / 6 + (C u + N i) / 1 5 + (C r + M o + V) / 5

C, Si, Mn, Cu, Ni, Cr, Mo, V: Content of each element (% by mass)

Further, in addition to the above-described contained elements, in the present invention, for the purpose of improving the ductility of the steel sheet and improving the HA Z toughness, C a: 0.00 0 3 to 0.0 0 5%, M g: 0. At least one of 0 0 0 3 to 0.0 0 5%, R EM: 0, 0 0 0 3 to 0.0 0 5% may be contained as a chemical component. Inclusion of these improves ductility and HAZ toughness.

When Ca, ¥ and £ ¼ are less than 0.03%, respectively, the effect of improving the ductility of the steel sheet and the improvement of HAZ toughness cannot be obtained. On the other hand, even if each content exceeds 0.005%, it is effective. Are saturated, so that they are set to 0.0 0 0 3 to 0.0 0 5%, respectively.

Hereinafter, the reason for limiting the production method of the present invention will be described. First, the molten steel adjusted to the appropriate chemical composition described above is melted by a commonly known melting method such as a converter, and is made into a steel material by a generally known forging method such as continuous casting.

Next, the steel material is heated to a temperature of 100 ° C. to 1300 ° C. to make an austenite single phase. If it is less than 100 ot :, the austenite single phase is insufficient, and if it exceeds 1300 ° C, the grain size of the heater becomes extremely coarse and it becomes difficult to obtain a fine structure after rolling. This is because toughness decreases.

The subsequent rolling process is the most important part of the present invention. In other words, it is necessary to perform rolling with a cumulative reduction of 30% or more in the austenite single phase region above the Ar 3 transformation point.

First, rolling was performed in the austenite single-phase region above the Ar 3 transformation point. The reason is that when dislocations are introduced into the ferrite phase by two-phase rolling below the Ar 3 transformation point, the dislocation strengthening remains, so the yield strength at 400 ° C is 18 OMPa. It is difficult to do.

Also, it is necessary to avoid two-phase rolling because the interface between the ferrite phase with dislocations and the ferri phase without dislocations is likely to be the starting point of brittle fracture, leading to a decrease in toughness. .

Furthermore, when two-phase rolling is performed, separation tends to occur due to the development of the texture, and it is difficult to secure Charbi average absorbed energy of 10 J or more.

In addition, since the anisotropy of the steel sheet is increased, anisotropy also occurs in the bending workability due to linear heating, and it becomes difficult to perform processing so as to obtain a smooth curvature surface.

Next, the reason why the cumulative rolling reduction ratio is set to 30% or more is that if it is less than 30%, the austenite is not sufficiently refined by recrystallization, and the crystal grain size is reduced within a predetermined range by subsequent accelerated cooling. This is because it becomes difficult to control it. Preferably, the rolling reduction ratio of rolling should be 50% or more after the above rolling, from 70 ° C or more to 40 ° C at a cooling rate of 5 to 50 ° CZ s in the cross-sectional average in the sheet thickness direction. It is necessary to perform accelerated cooling to a temperature below.

The reason for setting the cooling start temperature to 75 ° C. or higher is that if it is lower than 75 ° C., the ferrite transforms and grows before cooling, and it is difficult to reduce the average crystal grain size to 45 m or less. In addition, it is difficult to secure the precipitation amount of cementite due to a decrease in solid solution C in the ferrite, and further, cementite precipitates, agglomerates and coarsens to ensure yield strength at room temperature. It is difficult.

The cooling rate during accelerated cooling is 5 to 50 ° C The reason for the above is that the average crystal grain size of ferrite is difficult to be 45 m or less at less than 5 ° C / s for the same reason as above, and that C can easily diffuse to the grain boundary. This is because it is difficult to disperse the cementite within the grains, and the cementite precipitates, aggregates and coarsens, making it difficult to secure yield strength at room temperature. In addition, when the temperature exceeds 50 ° CZ s, the ferrite grain size becomes smaller than 15 and the yield strength at 40 ° C is less than 18 OMPa due to fine grain strengthening. Since it is difficult to do this, the upper limit was set to 5 OX: / s. The cooling rate during accelerated cooling is more preferably 10 to 40 ° C. Zs in terms of the cross-sectional average in the thickness direction.

The reason for accelerated cooling to temperatures below 400 ° C is that when cooling is completed at 400 ° C or higher, cementite precipitates, aggregates, and coarsens, making it difficult to secure yield strength at room temperature. It is. In consideration of temperature variation, it is preferable to perform accelerated cooling to a temperature of 300 ° C. or lower.

After accelerated cooling, it can be tempered at a temperature of not less than 300 and not more than 400 ° C as necessary for the purpose of adjusting strength and toughness. In order to obtain this effect, it is necessary to set the temperature to 300 ° C or higher, but at 400 ° C or higher, the cementite aggregates and becomes coarse, and it becomes difficult to ensure the yield strength at room temperature. The temperature should be less than 400 ° C, preferably not more than 35 ° C.

As described above, according to this embodiment, in order to improve the efficiency of bending work by linear heating, the amount of bending deformation is large under the conditions where the heating time is shortened, that is, under the condition where the maximum temperature reached in the linear heating part is low. It is possible to manufacture steel sheets that have sufficient yield strength and toughness as steel for shipbuilding and shipbuilding.

Example

After adjusting the chemical composition of the molten steel in the steelmaking process, A piece was produced. Table 1 shows the chemical composition. In the table, steel types A to P satisfy the chemical component requirements of the present invention, and steel types Q to X do not satisfy the chemical component requirements of the present invention. Steel grades A to P that satisfy the chemical composition requirements of the present invention are S i ≤ 0.0 2%, M n ≤ 0.0 3%, C u ≤ 0.0 3%, N i ≤ 0.0 3%, C r≤ 0. 0 4%, M o≤ 0. 0 0 4%, N b≤ 0. 0 0 2%, V≤ 0. 0 0 2%, T i ≤ 0. 0 0 2% , B≤ 0. 0 0 0 2%, C a≤ 0. 0 0 0 2%, M g≤ 0. 0 0 0 2%, REM≤ 0. 0 0 0 1% These elements are inevitable impurities, and the amount of impurities is shown in Table 1. In addition, the Ar 3 transformation point (° C) in the table is 0.5 after the austenization treatment at 120 ° C. using a formal evening test piece taken from these pieces. This is a value obtained from a thermal expansion curve when a thermal history of cooling at ° C / s is given. Thick steel plates with a thickness of 10 to 30 mm were manufactured using the pieces in Table 1. Table 2 shows the manufacturing method for each steel plate.

table 1

Note 1) Ced = C + Si / 24 + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5

Note 2) () is an analysis value as an inevitable impurity without intentional addition.

Table 2

Table 3 shows the microstructure area ratio (%) of each steel sheet, the average crystal grain size (m) of ferrite phase, the equivalent circle diameter (zzm) and number density (pieces) of cementite grains in ferrite grains. Zmm ² ). Each measured value is the thickness center position excluding the center segregation, and is the representative value of each steel plate.

Microstructure area ratio is 100 times or 500 times optical microscope image Measured by image analysis using true. At this time, a ferrite having a length ratio (aspect ratio) of 1.5 mm or more in the thickness direction of the rolling direction stretched in the rolling direction is a machining ferrite, and a ferrite having an aspect ratio of less than 1.5. Is defined as unprocessed ferrite, and the second phase refers to non-ferrite light, bainite, and martensite.

The average crystal grain size of the ferritic phase was measured in accordance with JISG 0 5 52 “Ferrata of steel 結晶 Grain size test method” using a photomicrograph obtained by measuring the microstructure area ratio. The equivalent-circle diameter and number density of cementite particles in the ferrite particles were measured by image analysis using scanning electron micrographs of 100000 times to 500 00 times.

Table 3

B: H, Ina, M: Martensa Table 4 shows the mechanical properties of each steel plate. Each measured value was measured using a specimen taken from the center of the plate thickness, and was used as a representative value for each steel plate. The yield strength at room temperature and 400 ° C is Using a 0 mm round bar tensile test piece, the tensile test at room temperature conforms to JISZ 2 2 4 1 “Metal Material Tensile Test Method”. The tensile test at 400 is JISG 0 5 6 In accordance with Section 7 “High temperature tensile test method for steel materials and heat-resistant alloys”, two were tested and measured, and the average value was recorded. Charbi average absorbed energy at 0 ° C is 2 mm V notch Charbi impact test piece, in accordance with JISZ 2 2 4 2 “Metallic material impact test method”. The average value was measured.

Furthermore, the result of having evaluated the deformation characteristic after linear heating of each steel plate is shown. The test specimen at this time was the original plate thickness X 500 mm width X 500 mm length. The center of the plate width was linearly heated with a gas burner in the length direction, and then water-cooled using a water-cooled torch placed behind the gas burner. This operation was repeated three times at the same position of the steel sheet, and the amount of steel sheet jumping was measured. The linear heating conditions are as follows: ○ ₂ gas pressure is 5 kg Z cm, flow rate is 50 l Zmin, C ₂ H ₂ gas pressure is 0.5 kg / cm, flow rate is 2 0 1 / min, gas The distance between the burner and the steel sheet was 14 cm, and a cooling torch with a water volume of 61 / min was placed 90 mm away from the rear of the gas burner. The gas burner and water-cooled torch were set on a speed-controllable table, and in the preliminary test, the temperature was measured with a thermocouple at a position 1 mm below the steel sheet surface, and the table speed conditions were determined so that the target temperature was achieved.

The temperature of l mm below the steel sheet surface is assumed to be 400, 50, 60, and the table speeds at that time are 6 40, 48 0, 28 0 cm / min, respectively. The working efficiency was evaluated by determining the heating time to obtain the lapping amount lmm from the measured amount of bounce and table speed. Note that the value at this time is simply the time during linear heating, and does not take into account the setup time or the measurement time of the jump amount. No

Also, the amount of jump (mm) can be measured by placing the test piece on a flat table, fixing one end face of the test piece with a jig, and using a taper gauge on both ends and the center of the opposite end face. The average value was recorded.

Table 4

Steel numbers 1 to 16 are the thick steel plates of the present invention. Since both the chemical composition and the production method satisfied the requirements of the present invention, the mechanical properties and the microstructure also satisfied the present invention requirements. Therefore, the bending deformation characteristics after linear heating are Compared with the comparative example, the amount of jumping was large, and the heating time for obtaining the amount of jumping lmm was shortened, which was extremely efficient.

On the other hand, steel numbers 17-3 are thick steel plates as comparative examples. Among these, Steel Nos. 17 to 24 are comparative examples in which the chemical composition satisfies the requirements of the present invention, but the production method and the mouthpiece structure do not satisfy the requirements of the present invention. Steel numbers 25 to 30 are comparative examples in which the manufacturing method satisfies the requirements of the present invention, but the chemical composition and the mouthpiece structure do not satisfy the requirements of the present invention. Steel numbers 3 1 to 3 3 are comparative examples that do not satisfy the requirements of the present invention in terms of chemical composition, microstructure, and manufacturing method.

The reason why the thick steel plate as a comparative example is inferior to the steel plate of the present invention will be described below.

In the production method, steel No. 17 is air cooled without performing water cooling after rolling, that is, the cooling rate is lower than the lower limit of the present invention. Therefore, since the average crystal grain size of ferrite exceeds the upper limit of the present invention, the Charpy average absorbed energy is also lower than the lower limit of the present invention. In addition, since the equivalent circle diameter of cementite particles exceeded the upper limit of the present invention and the number density fell below the lower limit of the present invention, the yield strength at room temperature was lower than the lower limit of the present invention. Since the yield strength at 400 ° C satisfies the requirements of the present invention, the deformation characteristics after linear heating are excellent, but it does not have the yield strength and toughness as shipbuilding steel.

Steel No. 18 is subjected to two-phase rolling in the production method, and the cooling start temperature is also below the lower limit of the present invention. Therefore, the additive-free ferrite area ratio is below the lower limit of the present invention, and the machining ferrite area ratio is increasing, so the yield strength at 400 ° C exceeds the upper limit of the present invention, and the Charpy average The absorbed energy is below the lower limit of the present invention. Therefore, the deformation characteristics after linear heating are inferior to the steel of the present invention, and it does not have the toughness necessary for shipbuilding steel. Steel No. 19 has a tempering temperature exceeding the upper limit of the present invention in the production method. For this reason, the equivalent circle diameter of cementite particles exceeds the upper limit of the present invention, and the number density falls below the lower limit of the present invention, so that the yield strength at room temperature is lower than the lower limit of the present invention. Since the yield strength at 400 ° C satisfies the requirements of the present invention, the deformation characteristics after linear heating are excellent, but the yield strength necessary for shipbuilding steel is not obtained.

Steel No. 20 has a cooling start temperature lower than the lower limit of the present invention in the production method. Therefore, since the average crystal grain size of the ferrite is above the upper limit of the present invention, the equivalent circle diameter of the cementite particles is higher than the upper limit of the present invention, and the number density is lower than the lower limit of the present invention, The yield strength at room temperature is below the lower limit of the present invention, and the Charpy average absorbed energy is below the lower limit of the present invention. Since the yield strength at 400 ° C satisfies the requirements of the present invention, the deformation characteristics after linear heating are excellent, but it does not have the yield strength and toughness as steel for shipbuilding.

Steel No. 21 has a cooling rate exceeding the upper limit of the present invention in the production method. Therefore, since the average crystal grain size of the ferrite is below the lower limit of the present invention, the yield strength at 400 ° C. exceeds the upper limit of the present invention. Therefore, steel No. 22 whose deformation characteristics after linear heating are inferior to the steel of the present invention has a cooling end temperature exceeding the upper limit of the present invention in the production method. Therefore, since the equivalent circle diameter of the cementite particles exceeds the upper limit of the present invention and the number density falls below the lower limit of the present invention, the yield stress at room temperature is lower than the lower limit of the present invention. Since the yield strength at 400 ° C satisfies the requirements of the present invention, it has excellent deformation characteristics after linear heating, but does not have the yield strength required for shipbuilding steel. Steel No. 23 is subjected to two-phase rolling in the production method, and the cooling start temperature is lower than the lower limit of the present invention. Therefore, no added F Since the area area ratio is below the lower limit of the present invention and the processing ferrite area ratio is increasing, the yield strength at 400 ° C exceeds the upper limit of the present invention, and the Charbi average absorbed energy is It is below the lower limit of the present invention. Therefore, the deformation characteristics after linear heating are inferior to the steel of the present invention, and it does not have the toughness necessary for shipbuilding steel.

Steel No. 24 has a cooling rate below the lower limit of the present invention in the production method. Therefore, since the average crystal grain size of ferrite exceeds the upper limit of the present invention, the Charbi average absorbed energy is also lower than the lower limit of the present invention. Further, since the equivalent circle diameter of the cementite particles exceeds the upper limit of the present invention and the number density falls below the lower limit of the present invention, the yield strength at room temperature is lower than the lower limit of the present invention. Since the yield strength at 400 ° C satisfies the requirements of the present invention, the deformation characteristics after linear heating are excellent, but it does not have the yield strength and toughness as steel for shipbuilding.

Next, in chemical composition, steel number 25 is M n C u N i N b

Steel No. 26 is M n Mo V, Steel No. 2 7 is C n Cr, and Steel No. 28 is S i exceeding the upper limit of the present invention. Steel number 2 9 3

Each chemical component is within the scope of the present invention, but the value of formula (1) exceeds the upper limit of the present invention. Thus, because it is a chemical component with high hardenability, even in the production method that satisfies the requirements of the present invention, the ferrite area ratio is below the lower limit of the present invention, and the average crystal grain size of X Is below the lower limit of the present invention, the yield strength at 400 ° C. is much higher than the upper limit of the present invention. Therefore, the deformation characteristics and efficiency after linear heating are degraded.

Next, steel No. 31 has a tempering temperature exceeding the upper limit of the present invention in the production method, so that the equivalent circle diameter of the cementite particles exceeds the upper limit of the present invention, and the number density is the lower limit of the present invention. Because it is less than Although the particle dispersion strengthening of Nintendo is not contributing, the yield strength at room temperature is sufficiently high. This is because, in the chemical composition, as in Steel No. 25, ¾11, Cu, Ni, Nb exceeds the upper limit of the present invention and is a chemical component with high hardenability. This is because the area ratio is lower than the lower limit of the present invention, and the average crystal grain size of the ferrite is lower than the lower limit of the present invention. Therefore, since the yield stress at 400 is much higher than the upper limit of the present invention, the deformation characteristics and efficiency after linear heating are deteriorated.

Steel No. 32 is air-cooled without rolling after rolling in the production method, that is, the cooling rate is lower than the lower limit of the present invention, so the equivalent circle diameter of cementitious particles is that of the present invention. Although the upper limit is exceeded and the number density is lower than the lower limit of the present invention, cement dispersion does not contribute to strengthening the particle dispersion, but the yield strength at room temperature is sufficiently high. This is because, for the same reason as steel No. 31, in the chemical composition, Mn, Ni, and Nb exceed the upper limit of the present invention, and the chemical composition has high hardenability. This is because the rate is below the lower limit of the present invention. Therefore, since the yield stress at 400 ° C. exceeds the upper limit of the present invention, the deformation characteristics and efficiency after linear heating are deteriorated.

Steel No. 3 3 is subjected to two-phase rolling in the production method, and the cooling start temperature is also lower than the lower limit of the present invention. Therefore, the additive-free ferrite area ratio is below the lower limit of the present invention, and the machining ferrite area ratio is increasing. In addition, in the chemical component, (:, M n exceeds the upper limit of the present invention, and because it is a chemical component with high hardenability, the yield stress at room temperature and 400 ° C is The Charpy average absorbed energy at 0 ° C is much lower than the lower limit of the present invention, which is much higher than the upper limit, so that it should not have toughness as steel for shipbuilding, and its deformation characteristics and efficiency after linear heating Is significantly inferior to the steel of the present invention. Yes.

From the above examples, by applying the present invention, in order to improve the bending work efficiency by linear heating, the heating time is shortened, that is, the maximum ultimate temperature of the linear heating part is low. In order to make steel plates with a large amount of bending deformation, we provide steel plates with low yield strength at low temperatures and their manufacturing methods, as well as steel plates with sufficient yield strength and toughness as steel for shipbuilding and their manufacturing methods. It was confirmed that it was possible.

Note that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. Industrial applicability

As described above, the present invention has sufficient yield strength and toughness mainly as a steel plate for shipbuilding, and can increase the amount of bending deformation even at a low maximum temperature, so that bending work by linear heating can be performed. Efficiency can be improved dramatically. And that brings about shortening the shipbuilding period, reducing costs, and reducing the environmental impact associated with reducing energy consumption.

Claims

The scope of the claims

1. Stomach volume%

c: 0.01 to 0.08%,

P: ≤ 0.05%,

s: ≤ 0.05,

A 1: 0.0 0 2 to 0 · 1%,

N: 0.01 to 0.00 8%

The balance is a steel plate with the chemical composition composed of iron and inevitable impurities in the balance, and the ferrite phase with an unprocessed microstructure is 90% in area ratio.

The average crystal grain size of the Ferai phase is 15 to 45 m

In addition, cementite particles with a circle-equivalent diameter of 0.5 m or less are present in the ferrite grains in a number density of more than 1 000 000 Z mm ² and the yield strength at room temperature is 2 3 5 MP a or higher, yield strength at 400 ° C is 1

0 0 Pa or less, 0. Charpy average absorbed energy at c is 1 0 0

Excellent bending workability by linear heating, characterized by J or higher

2. In addition, by mass%

S i 0 .0 5 to 0 • 5,

M n 0 .0 5 0 • 5,

C u 0 .0 5 to 0 5,

N i 0 .0 5 to 0-3,

C r 0. 0 5 0 • 3%,

M o 0 .0 0 5 0 • 1%

N b 0. 0 0 5 0 • 0 1,

V 0 .0 0 5 to 0-0 2,

T i 0. 0 0 5 0 0 2% B • 0. 0 0 0 5 to 0. 0 0 3%

The composition is excellent in bending workability by linear heating according to claim 1, characterized in that it contains at least one kind of chemical component as a chemical component and has a C eq force of S 0.20% or less. Thick steel plate.

C e Q = C + S i 2 4 + M n / 6 + (C u + N i) /

1 5 + (C r + Mo + V) / 5

Where C, Si, Mn, Cu, Ni, Cr, Mo, V: Content of each element (mass%)

3. In addition, stomach volume%

C a: 0.0 0 0 3 to 0.0 0 5%,

M g: 0.00 0 3 to 0.05%,

R E M: 0 0 0 0 3 0. 0 0 5%

3. A thick steel plate excellent in bending workability by linear heating according to claim 1 or 2, characterized by containing <1 and at least one kind as a chemical component.

4. A steel slab having the chemical composition according to any one of claims 1 to 3 is heated to 10 00 to 13 300 ° C, and austenite 卜 single phase region at or above the Ar 3 transformation point After rolling to a cumulative reduction rate of 30% or more to obtain a product sheet thickness, the average cross-section in the sheet thickness direction from 75 ° C or higher to a cooling rate of 5 to 50 ° C / s is 40 ( A method for producing a thick steel plate excellent in bending workability by linear heating, characterized by performing accelerated cooling to a temperature below TC.

5. Thickness excellent in bending workability by linear heating according to claim 4, wherein after accelerating cooling is finished, tempering is performed at 300 ° C. or more and less than 400 ° C. A method of manufacturing a steel sheet.