JPH06339760A - Method for continuously casting cast slab suitable for production of heat treated high tensile strength steel plate - Google Patents

Abstract

PURPOSE:To reduce center segregation inevitable in continuous casting and to evade the deterioration of toughness at the center part of plate thickness by applying the rolling reduction in a specific condition to the center part of the thickness of a cast slab in the process of drawing. CONSTITUTION:A molten steel containing 0.03-0.20% C, 0.05-0.50% Si, 0.30-2.50 Mn, <=0.020% P, <=0.010% S, 0.01-0.10% Al, <=0.007% N and at least one kind of element selected among 0.05-1.3% Cu, 0.05-1.5% Cr, 0.03-0.50% Mo, 0.01-0.15% V, 0.005-0.06% Nb and 0.0003-0.0020% B and the balance Fe with inevitable impurities is subjected to continuous casting to obtain the cast slab 1. At this time, the rolling reduction 6 having >=1.2 times of the unsolidified thickness is applied to the area being >=0.6 solid phase ratio at the center part of the thickness of the cast slab 1 in the process of drawing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous cast slab that is excellent in toughness at the center of plate thickness and is suitable for the production of thick-walled tempered high-strength steel plates used for members such as bridges, buildings and storage tanks. The present invention relates to a casting method.

[0002]

2. Description of the Related Art Generally, as a tempered high-tensile steel sheet increases in thickness, the cooling rate at the time of quenching differs between the plate surface layer part and the plate thickness center part. When toughness is sought, the central part of the plate thickness often has a low toughness structure, and various kinds of proposals have been made so far for such a problem.

For example, JP-A-61-56268 and JP-A-2-
No. 205629 discloses a method of controlling the hardenability in the subsequent quenching treatment by making a difference in the structure between the surface layer of the plate and the center of the plate thickness at the time of the completion of rolling in performing the direct quenching after rolling, and JP-A-1 Japanese Patent Laid-Open No. -116029 discloses a method of providing a temperature difference between the surface layer of the plate and the center of the plate thickness immediately before the quenching when performing reheating quenching, and controlling the quenchability of the inside and outside in the subsequent quenching treatment.

By the way, in the case of producing a thick tempered high-strength steel sheet by using a cast slab obtained by applying continuous casting, the toughness of the central portion of the sheet thickness can be obtained even if the above-mentioned technique is applied. Deterioration was unavoidable in some cases, and there was a need for a solution.

In the 86th and 87th Nishiyama Memorial Lecture "Advances in Plate Manufacturing Technology and Materials", "Increase in continuous casting ratio and problems in materials" refers to the ingot casting method with the same plate thickness and the continuous method. A comparison of tempered 60 kg steel produced using cast slabs obtained by the casting method, especially the latter one, when compared with 1/4 thickness
It has been reported that the toughness deterioration in the 2nd part is remarkable. This is because P or S is concentrated in the part corresponding to the segregation of the plate thickness center, which is peculiar to continuous casting, and S type inclusions are formed in it. (For the former one, a martensite structure with low toughness is formed in the quenching and tempering treatment after rolling).

As a method for reducing the peculiar center segregation (at the center of the plate thickness) that occurs in the continuous casting method, there is disclosed in JP-A-63-1.
Reference is made to 83765 and JP-A-3-281050,
Nothing is said about cast slabs involved in the production of thick-walled, tempered, high-strength steel sheets having a composition as targeted by the present invention.

[0007]

SUMMARY OF THE INVENTION The object of the present invention has occurred when a thick tempered high-strength steel sheet is manufactured (hot rolling) using a cast slab obtained by a continuous casting method. This is to propose a new continuous casting method that can solve the conventional problems.

[0008]

The present invention provides C: 0.03 to
0.20%, Si: 0.05 to 0.50%, Mn: 0.30 to 2.50%, P: 0.
020% or less, S: 0.010% or less, Al: 0.01 to 0.10%,
N: 0.007% or less, further Cu: 0.05 to 1.3%, C
r: 0.05 to 1.5%, Mo: 0.03 to 0.50%, V: 0.01 to 0.15
%, Nb: 0.005 to 0.06%, B: 0.0003 to 0.0020%, containing at least one element selected from the group consisting of Fe and unavoidable impurities, and continuously casting molten steel to form cast slabs. In the process of drawing, in the production of tempered high-strength steel sheet characterized by applying a reduction of 1.2 times or more of the unsolidified thickness to the region where the solid fraction in the thickness center of the slab is 0.6 or more. It is a suitable continuous casting method for cast slabs (first invention).

Further, according to the present invention, C: 0.03 to 0.20%, S
i: 0.05 to 0.50%, Mn: 0.30 to 2.50%, P: 0.020% or less, S: 0.010% or less, Al: 0.01 to 0.10%, N: 0.007
% Or less, Ni: 0.10 to 10.0%, Ti: 0.005 to
0.06%, Ca: 0.0005 to 0.0040%, REM: 0.001 to 0.02%
Continuous casting of molten steel containing at least one element selected from among the above, the balance consisting of Fe and unavoidable impurities to form cast slabs, and in the drawing process, solidification at the center of the thickness of the slabs. A continuous casting method (second invention) for casting slabs suitable for manufacturing tempered high-strength steel sheets, characterized by applying a reduction of 1.2 times or more of the unsolidified thickness to a region where the phase ratio is 0.6 or more. is there.

Further, according to the present invention, C: 0.03 to 0.20%, S
i: 0.05 to 0.50%, Mn: 0.30 to 2.50%, P: 0.020% or less, S: 0.010% or less, Al: 0.01 to 0.10%, N: 0.007
% Or less, Cu: 0.05 to 1.3%, Cr: 0.05 to 1.
5%, Mo: 0.03 to 0.50%, V: 0.01 to 0.15%, Nb: 0.00
At least one element selected from 5 to 0.06%, B: 0.0005 to 0.0020%, Ni: 0.10 to 10.0%, Ti: 0.005
~ 0.06%, Ca: 0.0005 ~ 0.0040%, REM: 0.001 ~ 0.02%
Containing at least one element selected from
Continuous casting of molten steel consisting of Fe and unavoidable impurities was performed to form cast slabs, and in the drawing process, the unsolidified thickness of 1.2% for the region where the solid fraction in the thickness center of the slab was 0.6 or more. It is a continuous casting method (third invention) of a cast slab suitable for producing a heat-treated high-strength steel sheet, which is characterized by applying a reduction of more than twice.

[0011]

First, the reasons for limiting the composition of the steel specified in the present invention will be explained.

C: 0.03 to 0.20% (“%” means “mass%”) C is an element necessary for improving the hardenability of steel and ensuring strength, and at least 0.03% is required for that purpose. Need to be added. However, if it exceeds 0.20%, the toughness of the base material and the toughness of the HAZ are deteriorated, so the C content is limited to the range of 0.03 to 0.20% in the present invention.

Si: 0.05 to 0.50% Si is an element necessary to secure the deoxidation and strength of steel, and at least 0.05% must be added for that purpose. However, if it exceeds 0.50%, the toughness and H of the base metal
It deteriorates the toughness of AZ. Therefore, in the present invention,
The Si content was limited to the range of 0.05 to 0.50%.

Mn: 0.30 to 2.50% Mn is an element effective in increasing the strength without impairing the toughness of steel, and for this purpose, addition of at least 0.30% is required. However, addition of more than 2.50% deteriorates workability. Therefore, in the present invention, the Mn content is limited to the range of 0.30 to 2.50%.

P: 0.020% or less P is an element that forms an unavoidable plate thickness center segregation in continuous casting of steel. Since the center segregation remarkably deteriorates the toughness of the base material and the HAZ, the P content is usually small. 0.
It is necessary to limit it to 010% or less. However, in the present invention, the center segregation is extremely reduced, so the upper limit is limited to 0.020%. If P exceeds the upper limit, the toughness of the base material and the HAZ toughness will be deteriorated irrespective of center segregation. Therefore, it is important to set the upper limit to 0.020%.

S: 0.010% or less S is an element that forms inevitable plate thickness center segregation when the continuous casting method is applied, and deteriorates the toughness of the base material and the HAZ toughness through this. For this reason, S is usually 0.
Although it is necessary to limit the content to 003% or less, in the present invention, the occurrence of center segregation due to such elements is extremely reduced, so the upper limit was limited to 0.010%. If this element exceeds the value specified here, the toughness of the base material and HAZ will be deteriorated regardless of the center segregation, so the upper limit is 0.003%.
It is essential to

Al: 0.01 to 0.10% Al is at least 0.01% for deoxidizing steel and refining the structure.
Must be added. However, if over 0.10% is added, a large amount of oxide inclusions are generated in the steel and the toughness of the steel is significantly deteriorated. Therefore, in the present invention, the Al content is limited to the range of 0.01 to 0.10%.

N: 0.007% or less N combines with Al to form AlN and has the effect of preventing crystal coarsening during heating of the steel slab, but if it exceeds 0.007%, the toughness of the HAZ deteriorates. In the present invention, the upper limit is limited to 0.007% or less.

In the present invention, in addition to the above-mentioned basic components, at least one selected from the group consisting of Cu, Cr, Mo, V, Nb and B and / or selected from the group consisting of Ni, Ti, Ca and REM. The reason for containing the at least one element will be explained. Cu is an element effective in solid solution strengthening and precipitation strengthening, and it is necessary to add 0.05% or more to obtain the former effect, while 0.5% or more is added to obtain the latter effect. You will need it. However, if it exceeds 1.3%, no remarkable effect is observed in any case, which is not economical. Therefore, in the present invention, Cu is limited to the range of 0.05 to 1.3%.

[0020] Cr is an element useful for strengthening steel through improving hardenability. For that purpose, addition of 0.05% or more is necessary, but if it exceeds 1.5%, weldability deteriorates. It was limited to the range of 0.05 to 1.5%.

Mo is an element useful for strengthening steel by improving hardenability and temper softening resistance, and for that purpose 0.03% or more is required, but more than 0.50%. However, it is uneconomical with no significant effect and increased cost, so the range was limited to 0.03-0.50%.

V is an element useful for increasing strength due to precipitation strengthening, and in order to exert its effect, V must be added in an amount of 0.01% or more. However, addition of more than 0.15% deteriorates weldability and HAZ toughness. Therefore, in the present invention, V is limited to the range of 0.01 to 0.15%.

Nb is an element useful for preventing coarsening of austenite grains during heating of a cast slab, for refining during rolling, for strengthening precipitation during tempering, and to exert its effect. Therefore, addition of 0.005% is necessary. However, addition of more than 0.06% deteriorates the toughness of the HAZ. Therefore, in this invention, Nb is 0.005 to 0.06.
It was limited to the range of%.

B is an element useful for improving the hardenability and strengthening the steel. To this end, addition of 0.0003% or more is necessary, but if it exceeds 0.0020%, the toughness due to the formation of the B compound is increased. However, it is limited to 0.0003 to 0.0020%.

Next, Ni has the effect of significantly improving the toughness of steel, and also has the effect of improving the toughness of the center part of the plate thickness by improving the hardenability, and the ductility / brittleness transition temperature at a low temperature. It is a useful element for migrating to the side (advantageous for the production of low temperature steel), and for this purpose 0.10% or more addition is necessary. However, even if it exceeds 10.0%, its remarkable effect cannot be expected. Therefore, in the present invention, for Ni, 0.10 ~
Limited to a range of 10.0%.

By forming TiN finely, Ti makes the structures of the base material and the welded portion fine and contributes to the improvement of toughness. In order to exert such effects, it is necessary to add 0.005% or more. However, if it exceeds 0.06%, coarse TiC is formed, which rather disadvantageously deteriorates the toughness. Therefore, in this invention, Ti is 0.005 to 0.06%.
Limited to the range.

Ca is an element useful for making MnS spherical and improving the toughness of steel. In order to exert such effects, it is necessary to add 0.0005% or more. However, when it exceeds 0.0040%, there is a disadvantage that the oxide inclusions increase and the toughness deteriorates. Therefore, in the present invention, Ca is limited to the range of 0.0005 to 0.0040%.

REM (rare earth element) is an element useful for improving the toughness of steel by a mechanism similar to that of Ca, and for this purpose, addition of 0.001% or more is necessary. However, if it exceeds 0.02%, there is a disadvantage that the oxide-based inclusions increase and the toughness deteriorates. Therefore, in this invention, REM is limited to the range of 0.001 to 0.02%.

The reasons for limiting the component composition of the steel suitable for the present invention are as described above. Next, the conditions for performing continuous casting using molten steel having such a composition will be described.

In the present invention, the casting slab pulled out from the continuous casting mold is subjected to reduction before being completely solidified, that is, in a non-solidified state. However, by applying such reduction, continuous casting is performed. The center segregation peculiar to the obtained slab has been remarkably reduced, so that the toughness at the center portion of the plate thickness, which has been a problem when manufacturing a tempered high-strength steel plate, can be greatly improved. A method other than the method of applying the reduction to the cast slab cannot be expected to reduce the center segregation. Therefore, the method of applying the reduction is most effective, and it is desirable to use a die for forging during the processing.

The reduction region of the cast slab was set to a region where the solid fraction in the center of the thickness of the slab was 0.6 or more, because the lower the solid fraction is, the lower the solid fraction is. This is because the unsolidified liquid phase in the central portion of the slab moves and is discharged in large amounts, so that the central portion of the slab becomes in a negative segregation state. As a result of studying the strength and toughness of a tempered high-strength steel sheet having a composition similar to that of the subject matter of the present invention, especially when the solid phase ratio during rolling is less than 0.6, it is 1/2 in the thickness direction of the steel sheet.
It is unavoidable that the chemical composition in a part is reduced and the strength of that part is greatly reduced compared to other parts. That is, it is difficult to keep the strength of such a portion within the range defined by the standard in a general commercial production process.
Therefore, in the present invention, the rolling reduction region of the cast slab is set to a region where the solid fraction at the thickness center is 0.6% or more.

The amount of reduction of the cast slab depends on the unsolidified thickness.
It was defined as 1.2 times or more, for the following reason.

In a continuous casting facility for casting slabs such as slabs used in the production of thick tempered high-strength steel sheets, the width of the slabs is relatively wide, so there is a variation in cooling during continuous casting. Inevitably, the unsolidified thickness at the center of the plate thickness of the cast fluctuates in the width direction. And, it is difficult to measure this unsolidified thickness steadily at the current technical level, and in the present situation, it is generally calculated from operating conditions during continuous casting, and such cooling variation and In consideration of the error in the estimated value by calculation, it was decided to reduce the solidified shell of the cast slab by 1.2 times or more of the unsolidified thickness so that the solidified shell can be completely crimped during the reduction.

As a device for applying a rolling reduction to a cast slab, the casting slab is vertically inserted into a frame, and a forging die capable of approaching and separating from each other at a predetermined interval is provided in FIG. The configuration shown in the figure is applicable. In the figure, 1 is a cast slab, 2a and 2b are forging dies, 3 is a body frame, 3a is an inlet, 3b is a link, 3c is a sliding surface, and 4 is a slider. , 4 a is a link, 5
Is a crankshaft and 6 is a hydraulic means.

The cast slab cast according to the present invention is then subjected to hot rolling and heat treatment to finish the heat-treated high-strength steel sheet. The rolling conditions and heat treatment conditions may be in accordance with ordinary methods.

INDUSTRIAL APPLICABILITY The present invention can be applied to the production of all of the heat-treated high-strength steels that are practically used, and the applications of the steel include construction members, steel plates for bridges, and storage tanks used at low temperatures. Steel plates, etc. are advantageously suitable.

[0037]

[Example] Steels having the chemical composition shown in Table 1 were prepared by heating the molten steel.
Thickness of 4 at 15 to 35 ° C and drawing speed of 0.4 to 0.5 m / min
While continuously casting a cast slab having a width of 00 mm and a width of 1900 mm, a device as shown in FIG. 1 was applied to the cast slab to apply reduction, and then the obtained cast slab was subjected to a conventional method. Hot rolling was performed to finish the steel plate to a thickness of 35 to 75 mm. Immediately after quenching, or after quenching after reheating, quenching was performed, and the tensile properties and impact properties of the obtained steel sheet near the surface and in the center of the sheet pressure were investigated. The results are shown in Table 2 together with the rolling reduction conditions during continuous casting.

[0038]

[Table 1]

[0039]

[Table 2]

As is clear from Table 2, the symbols C and F (comparative example) in which the uncoagulated reduction is not performed, or the symbols A and E (comparative example) in which the amount of reduction with respect to the uncoagulated thickness deviates from the specified range, Since segregation remains in the center of the plate thickness, the toughness of such a portion is significantly deteriorated. Further, the symbol B in which the solid fraction of the central portion at the time of reduction is below the range specified in the present invention,
Regarding D (Comparative Example), it was confirmed that significant negative segregation was generated in the center of the plate thickness, and the strength of the center of the plate thickness was significantly reduced.

On the other hand, in the case of satisfying the requirements defined by the present invention (compliance example), the change in the material at the center of the plate thickness is only the effect of the cooling rate in the quenching treatment, It was confirmed that the change in material at the center of thickness was within 3 kgf / mm ² in tensile strength, and within 20 ° C due to the increase in fracture transition temperature in the Charpy impact test.

[0042]

According to the present invention, it is possible to reduce the center segregation that is unavoidable when performing continuous casting. Therefore, it is possible to obtain a martensite of extremely low toughness in the quenching and tempering process in the production stage of a tempered high-strength steel sheet. No site structure is formed, so it is advantageous to reduce the toughness at the center of the plate thickness, which was a problem when manufacturing a tempered high-strength steel plate using cast slab obtained by continuous casting. It can be avoided.
Further, according to the present invention, since the negative segregation is not generated by the reduction performed in the continuous casting stage, the strength of the central portion of the plate thickness is not deteriorated.

[Brief description of drawings]

FIG. 1 is a diagram showing the configuration of a reduction device suitable for carrying out the present invention.

[Explanation of symbols]

 1 Cast slab 2a Forging die 2b Forging die 3 Main frame 3a Inlet 3b Link 3c Sliding surface 4 Slider 5 Crankshaft 6 Hydraulic means

Claims (3)

[Claims] 1. C: 0.03 to 0.20%, Si: 0.05 to 0.50
%, Mn: 0.30 to 2.50%, P: 0.020% or less, S: 0.01
0% or less, Al: 0.01 to 0.10%, N: 0.007% or less, Cu: 0.05 to 1.3%, Cr: 0.05 to 1.5
%, Mo: 0.03 to 0.50%, V: 0.01 to 0.15%, Nb: 0.00
5 to 0.06%, B: 0.0003 to 0.0020%, containing at least one element selected, the balance being Fe and unavoidable impurities, and continuously casting molten steel to form cast slabs, and in the drawing process The solidification rate of the solidified portion in the central portion of the thickness of the slab is 0.6 or more
A continuous casting method for cast slabs suitable for manufacturing tempered high-strength steel sheets, characterized by applying a reduction of 1.2 times or more. 2. C: 0.03 to 0.20%, Si: 0.05 to 0.50
%, Mn: 0.30 to 2.50%, P: 0.020% or less, S: 0.01
0% or less, Al: 0.01 to 0.10%, N: 0.007% or less, Ni: 0.10 to 10.0%, Ti: 0.005 to 0.06
%, Ca: 0.0005 to 0.0040%, REM: 0.001 to 0.02%, at least one element is selected, and the balance is Fe.
And continuous casting of molten steel consisting of inevitable impurities to form cast slabs, and 1.2 times the unsolidified thickness for the region where the solid fraction in the thickness center of the slab is 0.6 or more during the drawing process. A continuous casting method of a cast slab suitable for manufacturing a heat-treated high-strength steel sheet, characterized by applying the above reduction. 3. C: 0.03 to 0.20%, Si: 0.05 to 0.50
%, Mn: 0.30 to 2.50%, P: 0.020% or less, S: 0.01
0% or less, Al: 0.01 to 0.10%, N: 0.007% or less, Cu: 0.05 to 1.3%, Cr: 0.05 to 1.5
%, Mo: 0.03 to 0.50%, V: 0.01 to 0.15%, Nb: 0.00
At least one element selected from 5 to 0.06%, B: 0.0003 to 0.0020%, Ni: 0.10 to 10.0%, Ti:
0.005 to 0.06%, Ca: 0.0005 to 0.0040% REM: 0.001
To 0.02% and at least one element selected from the range of 0.02%, the remainder being Fe and unavoidable impurities, and the continuous casting of molten steel to form cast slabs, and the thickness of the slabs during the drawing process. The solid fraction in the center is 0.
A continuous casting method for cast slabs suitable for manufacturing tempered high-strength steel sheets, characterized by applying a reduction of 1.2 times or more of the unsolidified thickness to a region of 6 or more.