JPH11277105A - Manufacture of thick steel plate excellent in resistance to scale peeling - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the manufacturing method of thick steel plates which is excellent in resistance to scale peeling. SOLUTION: In this manufacturing method, after heating the steel to which 0.03-0.25% C, 0.05-less than 0.50% Si, 0.05-1.60% Mn and 0.005-0.10% Al are added at 1050-1300 deg.C and, after executing rolling under specified conditions under which temp. and draft are limited, the steel plates are gradually cooled by putting one plate on another so that the cooling rate at <=350 deg.C becomes $0.1 deg.C/sec. Moreover, at least one kind of element which is selected from the 2nd element group consisting of Nb, V, Ti, Cu, Ni, Cr, Mo and B and the 3rd element group consisting of Ca and REM may be made to contain in this steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a thick steel plate having excellent scale peel resistance.

[0002]

2. Description of the Related Art In the fields of construction machinery, concrete formwork, shipbuilding, and bridges, the major issues for the entire industry are reduction of manufacturing costs (automation of manufacturing lines, thoroughness of processing operations), improvement of product appearance, workplaces. (Particularly in response to the seriousness of young people leaving the manufacturing industry and countermeasures against this). On a surface of a thick steel plate (mainly a plate thickness of 6 to 25 mm) used in a large amount in these fields, a large amount of a red fine scale called a red scale adheres. This red scale easily peels off from the steel sheet during the processing of the steel sheet and rises up as a fine powder. When used as a concrete formwork as a countermeasure, the appearance of the product is deteriorated, which is a major problem. For this reason, in the field described above, there is a very strong demand for a thick steel sheet having excellent scale peeling resistance, in which there is almost no red scale that easily peels off on the surface of the steel sheet.

[0003] Furthermore, in the above-mentioned fields, mainly for construction machines, laser cutting is rapidly spreading in cutting thick steel plates, but red scale thick steel plates cannot be stably laser cut. This is because red scale thick steel plates have a large number of scale peeled parts when viewed microscopically, causing self-burning during laser cutting. If the steel plate is excellent in quality, stable laser cutting can be expected.

However, in the manufacturing process of steel plates, the residence time at a high temperature during rolling is long, and the scale and ground iron are easily oxidized during cooling in order to cool without winding after rolling. Characteristically, the surface of the steel sheet is very easily red-scaled.

As a conventional main countermeasure against the red scale, improvement of the capacity of a descaling device such as increasing the pressure of the descaling water at the time of rolling has been studied. Since capital investment is required, manufacturing costs increase. As a prior art other than the improvement of the capacity of the descaling device, Japanese Patent Application Laid-Open No. 9-8779
No. 9 discloses a technique in which the rolling temperature is limited to 880 ° C. or more and the rolling reduction is limited to 21% or more to prevent scale crushing during rolling and suppress the generation of red scale. Thick steel plates can be manufactured stably at low cost. However, as a problem at the time of applying this technology, there is a problem that partial scale peeling occurs when a process of correcting the shape of a steel sheet cold (cold leveler process) is performed. Although this is not as large as the red scale, it poses a problem because it reduces user workability. For this reason, it is necessary to further increase the adhesion of the scale while suppressing the generation of the red scale.

As a prior art for improving the scale adhesion of a thick steel plate, Japanese Patent Application Laid-Open No. 9-209036 discloses a method in which a predetermined plate is formed by hot rolling with a rolling finish temperature in the range of 700 to 850 ° C. after limiting the components. A method of manufacturing a thick steel plate having excellent scale adhesion, characterized in that a slow cooling treatment is performed from 650 to 570 ° C. during cooling after the thickness is increased. JP-A-9-272918 discloses that high-pressure water descaling is performed at least three times during rolling after limiting the components, rolling is completed at 760 to 800 ° C., air-cooled, and the temperature of the steel sheet is further increased to 500 ° C. 80 ° C to 300 ° C
A method for producing a thick steel plate having good scale adhesion, characterized by keeping heat for at least min and then allowing it to cool. Any of the techniques disclosed in these publications cannot be a practical means because the rolling finish temperature is low and red scale is easily generated.

[0007] In addition, there is no conventional technology considered to be able to stably produce a thick steel plate excellent in scale peeling resistance at low cost. That is, it is necessary to newly develop a technology capable of inexpensively and stably producing a thick steel sheet having no red scale and excellent in scale peeling resistance without scale peeling even during cold leveler treatment.

[0008]

The problem to be solved by the present invention is that red scale, which can greatly contribute to the reduction of manufacturing costs, which is a major problem in the fields of construction machinery, concrete formwork, shipbuilding, and bridges. It is an object of the present invention to provide a concrete method for stably producing a thick steel plate which does not peel off even during the cold leveler treatment and does not peel off during the cold leveler treatment.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and the gist thereof is as follows. (1) In weight%, C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10% Remainder Is heated to 1050 to 1300 ° C., the roll biting temperature in the final pass of rolling is T _F (° C.), and the final 3 in finish rolling is performed.
The average rolling reduction path when the _{R F (%), 880 ≦} T F ≦ 1020 (℃) 21 ≦ R F ≦ 40 (%) R F ≧ 155-3T F / 20 3 Expressions (%) at the same time 350 ° C after rolling under satisfactory conditions
A method for producing a thick steel sheet having excellent scale peeling resistance, comprising stacking steel sheets and gradually cooling them so that the cooling rate below is 0.1 ° C./sec or less.

(2) By weight%: (a) C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 (B) Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1.50% Ni: 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: 0.0003 to 0.003% A steel slab or a slab containing at least one element, the balance being Fe and unavoidable impurities, is heated to 1050 to 1300 ° C., and the roll bite temperature in the final pass of rolling is T _F (° C.) the average reduction rate of the final three passes of the rolling when the _{R F (%), 880 ≦} T F ≦ 1020 (℃) After rolling with _{1 ≦ R F ≦ 40 (%} ) R F ≧ 155-3T F / 20 at the same time satisfying the condition 3 Expressions (%), 350 ℃
A method for producing a thick steel sheet having excellent scale peeling resistance, comprising stacking steel sheets and gradually cooling them so that the cooling rate below is 0.1 ° C./sec or less.

(3) By weight%: (a) C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 (B) Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1.50% Ni: 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: 0.0003 to 0.003% At least one element and (c) at least one element selected from the group consisting of Ca: 0.0003 to 0.010% REM: 0.001 to 0.030%, with the balance being Fe and A slab or slab consisting of unavoidable impurities is heated to 1050 to 1300 ° C. Assuming that the roll biting temperature is T _F (° C.) and the average draft of the final three passes in finish rolling is R _F (%), 880 ≦ T _F ≦ 1020 (° C.) 21 ≦ R _F ≦ 40 (%) After rolling under conditions that simultaneously satisfy the three equations of R _F ≧ 155-3T _F / 20 (%), 350 ° C.
A method for producing a thick steel sheet having excellent scale peeling resistance, comprising stacking steel sheets and gradually cooling them so that the cooling rate below is 0.1 ° C./sec or less.

The present inventors have solved the above-mentioned problems by controlling the rolling conditions so that the rolling temperature is 880 ° C. or more and the rolling reduction is 2
We have previously developed a technology to prevent scale crushing during rolling and suppress red scale generation by limiting it to 1% or more. In addition to this technology, by performing rolling slow cooling after rolling, scale The present inventors have found that a thick steel plate having not only no red scale but also excellent scale peeling resistance at the time of cold leveler treatment can be stably obtained at low cost and stably, because the adhesiveness of the steel is remarkably improved. It is what made.

[0013] There is a lot of knowledge about the technique of improving the adhesion of the scale in hot-rolled steel sheets, and the winding process after hot rolling transforms the scale composition from wustite with poor adhesion to magnetite with good adhesion. The hot rolled steel sheet has better scale adhesion than the thick steel sheet scale. For this reason, thick steel plates are being studied to apply this scale composition control technology, but it is not easy to secure the temperature required for transformation, and even if the temperature can be secured, the scale adhesion is good. Since it was difficult to obtain a thick steel plate, it had not been put to practical use. In order to secure the temperature required for the transformation, gradual cooling immediately after rolling, heat treatment, holding in a hot bath, etc. are conceivable. However, in terms of manufacturing costs, the gradual cooling immediately after rolling is practical.
However, even if the conventional rolled steel sheet, that is, the thick steel plate in which a large amount of crushed scale powder is present on the steel sheet surface is stacked and gradually cooled, the scale that has already been crushed is subjected to the stacked slow cooling. Since the effect of improving the adhesion between the scale and the base steel does not reach the scale, the scale is easily peeled. In addition, the parts where the steel sheets are not in contact with each other and are oxidized when exposed to air when stacked are red scale, whereas the parts where the steel sheets are in contact and oxidation is suppressed are black scale, so the steel sheet surface is There was a problem that the pattern became mottled and the appearance of the thick steel plate deteriorated. For this reason, conventionally, even if stacking slow cooling is performed immediately after rolling, not only a steel sheet with remarkably improved scale adhesion is not obtained, but also the appearance of the steel sheet deteriorates, so that stacking slow cooling has not been put to practical use.

On the other hand, as a result of many experiments, the present inventors have found that control of the scale state of the steel sheet surface prior to the stack slow cooling is very important for improving the scale adhesion by the slow stack cooling. It was found that. That is, the scale is not crushed by rolling and is controlled to be in close contact with the base iron, and by performing slow cooling by stacking, the scale adhesiveness is remarkably improved, and furthermore, crushed which is a cause of the generation of red scale. The present invention has been found that the surface of the steel sheet is uniformly black because there is no scale, and that the appearance of the steel sheet becomes good without generating red and black mottled patterns.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the components in the present invention will be described. C is required to be added in an amount of 0.03% or more in order to secure the strength. A large amount of C deteriorates the base metal toughness and hardens the weld heat affected zone to deteriorate the weldability, so the upper limit is 0.25%. I do. Si is 0.05% for deoxidizing steel
The above addition is necessary, and a large amount of addition deteriorates the descaling property at the time of rolling, easily causes scale indentation flaws, and raises the cost. Therefore, the upper limit is set to less than 0.50%. Mn needs to be added in an amount of 0.05% or more in order to fix S contained in steel and improve toughness. A large amount of Mn causes an increase in manufacturing cost and also deteriorates toughness. 1.60%. Al needs to be added in an amount of 0.005% or more to deoxidize steel, and a large amount of Al significantly deteriorates toughness. Therefore, the upper limit is set to 0.10%.

In the present invention, in addition to the above-mentioned elements, the steel used mainly contains Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1.50% Ni: 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: At least one element selected from the group consisting of 0.0003 to 0.003% can be contained.

Nb, V, and Ti are precipitated as carbonitrides in steel to increase the strength. In addition, by reducing the microstructure of the steel, the strength, base metal toughness, and toughness of the weld heat affected zone can be improved. It has the effect of improving. In order to obtain these effects, 0.001% or more of each element is required for each element, and the addition of a large amount significantly deteriorates the toughness of the heat affected zone.
The upper limits are 0.20% for Nb, 0.30% for V, and 0.20% for Ti. Cu has an effect of improving strength and corrosion resistance. To obtain these effects, addition of 0.05% or more is necessary. Since addition of a large amount causes hot cracking of the slab, its upper limit is set to 1.50%. Ni
Has the effect of improving both strength and toughness. To obtain this effect, it is necessary to add 0.05% or more. Since addition of a large amount significantly impairs the economic efficiency, the upper limit is set to 1.50%.
And Cr has an effect of increasing quenching properties, an effect of increasing temper softening resistance, and an effect of improving corrosion resistance.
To obtain these effects, addition of 0.05% or more is necessary. Since addition of a large amount deteriorates the toughness of the weld heat affected zone, the upper limit is made 1.00%. Mo has an effect of increasing the hardenability and an effect of increasing the tempering softening resistance. In order to obtain these effects, addition of 0.05% or more is necessary. Since addition of a large amount deteriorates the toughness of the heat affected zone, the upper limit is made 1.00%. B has the effect of significantly improving hardenability and increasing strength. In order to obtain this effect, 0.0003% or more of addition is necessary, and the addition of a large amount hardens the heat affected zone by welding to greatly deteriorate the weldability, so the upper limit is 0.003%.

Further, in the present invention, in addition to the above-mentioned elements or separately from the above-mentioned elements, steel is mainly used for improving toughness. It can contain at least one element selected from the group consisting of 001 to 0.030%.

Ca and REM have the effect of improving the anisotropy of the mechanical properties of the steel and improving the lamella tear resistance. To obtain these effects, Ca must be added in an amount of 0.0003% or more, and REM must be added in an amount of 0.001% or more. For both elements, the addition of a large amount deteriorates the toughness. 010% and REM are 0.030%.

Next, the heating condition and the rolling condition of the steel slab or the cast slab will be described. Rolling at a high temperature of 880 ° C. or higher is required to prevent scale crushing during rolling and to suppress the generation of red scale. In order to enable this, 1050 ° C.
Heating at the above temperature is required. The upper limit of the heating temperature is set to 1300 ° C. in order to suppress the occurrence of scale indentation flaws. The heating time of the slab at the time of heating requires about 120 minutes in order to make the slab temperature uniformly 1050 ° C. or more, and since heating for a long time causes an increase in manufacturing cost, it is limited to about 300 minutes. It is desirable.

Regarding the rolling conditions, in order to prevent scale crushing during rolling, the roll biting temperature (hereinafter referred to as a rolling finish temperature) in the final pass of rolling is T _F (° C.), and the final three passes in finish rolling are performed. When the average reduction ratio (hereinafter referred to as the finish reduction ratio) of R is defined as R _F (%), 880 ≦ T _F ≦ 1020 (° C.) 21 ≦ R _F ≦ 40 (%) R _F ≧ 155-3T _F / 20 (%) It is necessary to perform rolling under conditions that simultaneously satisfy the three equations.
When rolling under the conditions that do not simultaneously satisfy the above three equations,
Even if the stack is gradually cooled at the time of cooling after rolling, the adhesiveness of the already crushed scale is not improved and the scale is easily peeled. Rolling finish temperature stably 102
Since it is difficult to make the temperature higher than 0 ° C., the upper limit of the rolling finish temperature is set to 1020 ° C. If the rolling finish temperature exceeds 980 ° C., fine blisters may be generated on the scale during air cooling after rolling and the surface appearance of the steel sheet may be deteriorated.
Accelerated cooling is started within 5 seconds at a cooling rate of 5 ° C / sec or more, and accelerated cooling is stopped when the surface temperature of the steel sheet is 500 ° C or more and 750 ° C or less. It is possible.

The condition of the slow cooling in stacking is that the cooling rate at a temperature of 350 ° C. or less at a temperature of 350 ° C. or less is 0.1 ° C./sec or less at a temperature of the steel sheet surface in order to sufficiently enable transformation from wustite to magnetite and improve the scale adhesion. The steel sheets are stacked and gradually cooled so that Regarding the temperature at the time of stacking steel sheets and the number of steel sheets to be stacked, the surface temperature of each stacked steel sheet is 3
There is no particular limitation except that the cooling rate at 50 ° C. or lower is controlled to be 0.1 ° C./sec or lower. The temperature at which the stacking is completed is set to 150 ° C. or less in order to sufficiently enable the transformation from wustite to magnetite. 350
With respect to the cooling rate below 0.1 ° C., if the cooling rate is greater than 0.1 ° C./sec, the transformation from wustite to magnetite does not occur sufficiently and the adhesion of the scale becomes insufficient, so the upper limit is 0.1 ° C./sec. I do. More preferably 0.05
A more stable scale adhesion can be obtained by stacking at a temperature of at most ° C / sec. The lower limit of the cooling rate does not need to be particularly limited, but may be 0.0 to avoid lowering the productivity.
It is desirable that the temperature be not lower than 01 ° C./sec.

The present invention can be realized only by controlling rolling conditions and stacking steel sheets after rolling. For this reason, the addition of special alloying elements, expensive high-pressure ultra-high-pressure water descaling equipment during rolling, or heat treatment after rolling, etc., does not cause an increase in production cost or a decrease in productivity, and enables a stable, low-cost scale. It is possible to manufacture thick steel plates with excellent peelability.

The method of the present invention is effective irrespective of the strength of the steel. For example, even if the yield strength is 200 to 700 MPa, for example, the microstructure of the steel is ferrite, pearlite, bainite, martensite, retained austenite. And a single phase or a mixed structure of the tempered structures thereof.

[0025]

EXAMPLE Using test steels having the chemical components of steel numbers A to H shown in Table 1, air-cooling was performed after heating and rolling under the conditions shown in Table 2, and steel sheets were stacked in the middle of air-cooling and gradually cooled. Regarding the thick steel plate, the color tone of the scale on the steel plate surface and the scale peeling resistance during the cold leveler treatment were investigated. The stacking temperature indicates the temperature of the steel sheet surface when the thick steel sheets are stacked. The cooling rate at 350 ° C. or lower indicates the maximum value of the cooling rate of the steel sheet surface at 350 ° C. or lower.

The color tone of the scale was measured using a colorimeter in accordance with JISZ8722.
The case where the maximum value of the a ^* value at zero (the color display method conforms to JISZ8729) is 0.5 or less is expressed as black, and the case where it exceeds 0.5 is expressed as red. The scale peeling resistance in the cold leveler was evaluated by evaluating the degree of scale peeling by the area ratio of the thick steel plate after the cold leveler treatment.
% When it was less than 5%, and x when it was 5% or more.

Test Steel Nos. 1 to 13 are steels manufactured in accordance with the present invention and have a black scale and excellent scale peeling resistance during cold leveler treatment. On the other hand, the test steel numbers 14 to 18 have rolling conditions outside the condition range of the present invention, and are red scale thick steel plates. Test steel Nos. 19 and 20 were not subjected to slow cooling after rolling, and thus had poor scale peel resistance during cold leveler treatment. The test steel Nos. 21 to 24 have inferior scale peeling resistance during cold leveler treatment because the annealing conditions after rolling deviate from the condition range of the present invention.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

As described above, according to the present invention, the major problems in the fields of construction machinery, concrete formwork, shipbuilding, and bridges are to reduce manufacturing costs and improve product appearance.
It is extremely useful in industry because it is possible to supply inexpensively and stably a thick steel plate having excellent scale peeling resistance, which can greatly contribute to improving the hygiene condition of the workplace.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/58 C22C 38/58

Claims (3)

[Claims] C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10% by weight % A steel slab or a slab consisting of Fe and unavoidable impurities is heated to 1050 to 1300 ° C., and the roll bite temperature in the final pass of rolling is T _F (° C.).
The average rolling reduction path when the _{R F (%), 880 ≦} T F ≦ 1020 (℃) 21 ≦ R F ≦ 40 (%) R F ≧ 155-3T F / 20 3 Expressions (%) at the same time 350 ° C after rolling under satisfactory conditions
A method for producing a thick steel sheet having excellent scale peeling resistance, comprising stacking steel sheets and gradually cooling them so that the cooling rate below is 0.1 ° C./sec or less. 2. In weight%, (a) C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10%, and (b) Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1 .50% Ni: 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: 0.0003 to 0.003% At least selected from the group consisting of: A steel slab or cast slab containing one element and the balance consisting of Fe and inevitable impurities is heated to 1050 to 1300 ° C., and the roll bite temperature of the final pass of rolling is T _F (° C.), and finish rolling Last 3 average rolling reduction path when the _{R F (%), 880 ≦} T F ≦ 1020 (℃) 21 ≦ with After rolling under the condition that simultaneously satisfies the following three formulas: R _F ≦ 40 (%) R _F ≧ 155-3T _F / 20 (%), and then 350 ° C.
A method for producing a thick steel sheet having excellent scale peeling resistance, comprising stacking steel sheets and gradually cooling them so that the cooling rate below is 0.1 ° C./sec or less. 3. In% by weight: (a) C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10%, and (b) Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1 .50% Ni: 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: 0.0003 to 0.003% At least selected from the group consisting of: Contains one element and (c) at least one element selected from the group consisting of: Ca: 0.0003 to 0.010% REM: 0.001 to 0.030%, with the balance being Fe and inevitable Slabs or cast slabs consisting of natural impurities are heated to 1050 to 1300 ° C and rolled in the final pass of rolling. The biting temperature T _F (℃), the average reduction ratio of the final 3 passes in finish rolling when the _{R F (%), 880 ≦} T F ≦ 1020 (℃) 21 ≦ R F ≦ 40 (%) R After rolling under the condition that simultaneously satisfies the three equations of _F ≧ 155-3T _F / 20 (%), 350 ° C.
A method for producing a thick steel sheet having excellent scale peeling resistance, comprising stacking steel sheets and gradually cooling them so that the cooling rate below is 0.1 ° C./sec or less.