JPH11290902A - Method for Preventing Surface Cracking During Hot Width Rolling of Continuously Cast Slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for preventing a crack defect on a slab surface that occurs when a steel containing N and Al or Nb is manufactured by continuous casting and then hot-rolled down. . SOLUTION: Within 10 mm below the surface layer of the continuous cast slab, A
After cooling to r ₁ or less and reheating to 1000-1150 ° C,
Hot width rolling is completed with Ar ₃ or more, or 10 mm or less under the surface layer of the continuous cast slab is cooled to Ar ₁ -50 ° C. or less and reheated to 1000 to 1150 ° C., followed by hot width rolling. This makes it possible to simultaneously prevent cracks due to carbonitride precipitation and cracks due to the film α within the slab surface layer of 10 mm or less, and to perform width reduction rolling without causing cracks on the slab surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing the occurrence of surface cracks in a slab when continuously casting molten steel containing N and Al or Nb and performing hot width reduction rolling.

[0002]

2. Description of the Related Art Conventionally, in order to improve the productivity and stabilize the operation of continuous casting, a process of casting with a constant width and a constant thickness, performing hot width reduction rolling, and adjusting the dimensions according to the product width has been adopted. ing. Here, the hot width reduction rolling, for example, using a rolling mill composed of a vertical roll and a horizontal roll having a groove shape, width reduction is performed by a vertical roll, the resulting near the slab short side. A method of repeatedly rolling and smoothing the dog bone shape using horizontal rolls, adjusting the slab width to a predetermined size, and in some cases, reducing the thickness to a predetermined thickness to improve the productivity of the rolling process. Is shown.

[0003] In this hot width reduction rolling, when a dog bone produced by reducing the width by a vertical roll is leveled and rolled by a horizontal roll, tensile stress acts in the rolling direction at the center of the width of the slab, and the casting stress is reduced. It is known that crack defects easily occur on one surface. If hot rolling is performed as it is in the presence of the cracks, a surface defect called a dent is generated in the product. For this reason, it has been necessary to once cool the high-temperature slab after the width reduction rolling to room temperature, and to inspect and care for cracks.

[0004] In particular, it has been clarified that cracks tend to occur particularly in steels containing N and Al or Nb, and that carbonitrides precipitated at austenite (hereinafter referred to as γ) grain boundaries cause cracks. The following measures have been proposed to prevent such surface cracking.
For example, as typified by JP-A-7-290101, a slab is cooled and temporarily changed from γ to ferrite (hereinafter referred to as α).
) By reheating or reheating.
This is a method of preventing cracking by repeating the reverse transformation treatment to refine the γ grains and fixing most of the precipitates in the γ grains to reduce the amount of precipitation at the γ grain boundaries.

[0005]

However, Japanese Patent Application Laid-Open No.
By applying the method disclosed in -290101, surface cracks during hot width reduction can be reduced, but not enough, and surface cracks occur depending on conditions of continuous casting or hot width reduction rolling, If the cracks could not be removed, or the cracks could not be removed by the care, they had to be turned into waste. Accordingly, an object of the present invention is to provide a method for reliably preventing surface cracks during continuous width reduction rolling of a continuously cast slab of a steel type containing N and Al or Nb.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to achieve the above-mentioned object, and (1) by weight%,
N: 0.0020% to 0.0100%, and further A
l: 0.020% to 0.100%, Nb: 0.020%
In the method of continuously casting molten steel containing any one or two of 0.060%, charging into a heating furnace without cooling to room temperature, and reheating to perform hot width reduction rolling, At least 10 m of the slab surface layer after casting
Cool down to a temperature within ₁ m
A method of preventing surface cracks during continuous width reduction of continuous cast slabs, wherein the width reduction is completed at a temperature of Ar ₃ or higher after reheating to a temperature of 1 to 1150 ° C.

(2) N: 0.0020% by weight%
0.0100%, Al: 0.020% ~
0.100%, Nb: 0.020% to 0.060%, and continuously casting molten steel containing one or two kinds thereof,
In a method in which the material is charged into a heating furnace without cooling to room temperature and reheated to perform hot width reduction rolling, the temperature of at least 10 mm or less of the surface layer of the slab during continuous casting or after casting is adjusted to Ar ₁
Cool down to -50 ° C or less, 1000 ° C to 115 ° C.
After reheating to 0 ° C., a width reduction rolling is performed, and a method for preventing surface cracks during continuous width reduction rolling of a continuously cast slab was found.

[0008] The present inventors have, first, in order to clarify whether the surface cracks in any case such occurs, the surface temperature of the slab continuous casting using a prone steels cracking is Ar ₃
After cooling as described below, a test was performed in which it was inserted into a heating furnace, reheated, and hot width reduction was performed. At this time, the cooling temperature and the width reduction rolling temperature were changed, and the presence or absence of surface cracks in the slab, the cracks in the slab section, and the structure investigation were carried out. Here, the steel type subjected to the test is expressed by weight%, N: 0.0020.
% To 0.0100%, and further, Al: 0.020%
０．１0.100%, Nb: 0.020% to 0.060% steel containing any one or two of them, and a test of several tens of charges was carried out for various steel types in this component range.

As a result of the test, as shown in FIG. 2, when the cooling temperature during casting was high and the rolling reduction temperature in width was low, there was a tendency that surface cracks tended to occur. As a result of investigating the microstructure and cracks of the cross-section of the slab and the slab that did not have surface cracks, the surface layer portion of the slab that did not have surface cracks had a fine structure and the inside had a coarse structure. Although cracks occurred inside, no cracks occurred in the region of fine structure on the surface layer.

On the other hand, in the slab having a surface crack, the structure of the surface layer is large and the surface has a crack, and the crack is also generated at the surface despite the fine structure of the surface layer. There were things. A more detailed investigation was conducted to clarify the cause of the cracking in spite of the fine structure of the surface layer.
Since this crack has occurred when it is less than r ₃ and the crack part has a dimple-like form,
It was found that the formation of the film-like α had an effect.

Conventionally, it was thought that if the γ grains were made finer, cracks caused by the film-like α would hardly occur, but it was newly found that the effect of the finer grains was not so large. For this reason, it turned out that cracking could not be completely prevented by the conventional technique. Therefore, in order to clarify the conditions for simultaneously preventing cracks caused by carbonitride precipitation and film-like α in the slab surface layer, tests were conducted based on the γ → α transformation temperature of each steel type. As a result, it was found that there is a clear relationship between the transformation temperature and the presence or absence of surface cracks as shown in FIG.

That is, when the cooling temperature during casting is Ar ₁ or less (0 ° C. or less on the vertical axis), the width reduction completion temperature is equal to or higher than the Ar ₃ temperature (0 ° C. or more on the horizontal axis) and when casting. Cooling temperature is A
In the case of r ₁ −50 ° C. or less (−50 ° C. or less on the vertical axis), it was found that surface cracking did not occur irrespective of the completion temperature of the width reduction. Under both conditions, cracks due to carbonitride precipitation can be prevented by the finer γ grains in the surface layer, and in the former, cracks due to film-like α can be prevented in order to complete width reduction before film-like α is generated. is there.

In the latter case, as a result of examining the reason why cracking can be prevented despite the formation of film-like α, cracking caused by film-like α is promoted by embrittlement due to precipitation of carbonitride in film-like α. When the cooling temperature during casting is Ar ₁ -50 ° C., most of the carbonitrides precipitate during cooling, and the amount of precipitation in the film α decreases. It was found that cracking hardly occurred. A test in which the number of times of cooling and reheating was increased was also performed, but the effect was not recognized, and a sufficient effect was obtained by performing the predetermined cooling once.

From the results of various rolling tests, if the thickness of the region where the surface layer is not cracked is 10 mm or more,
It is known that the inherent cracks are pressed during hot rolling and are not exposed on the surface, and in order to prevent surface defects of the hot coil after hot rolling, a predetermined temperature condition within 10 mm is sufficient. It is. However, the reheating temperature is 115
If the temperature exceeds 0 ° C., coarsening of γ grains occurs, and the effect of fine graining by cooling and reheating is lost, so 1150 ° C. was set as the upper limit of the reheating temperature. Further, if the temperature is too low, the rolling reaction force at the time of hot width reduction rolling becomes too large to perform rolling.

Next, the reasons for limiting the components will be described. N is Al
Has the effect of forming fine grains with Ti and Ti, and has the effect of refining the crystal grains. However, when the content is large and the amount of nitride is large, cracks are likely to occur during width reduction rolling. Even in the invention, the upper limit is set because surface cracks cannot be prevented. On the other hand, the lower limit of 0.0020% or less is 0.0020% in order to prevent surface cracks without using the present invention.

Al is effective for deoxidation and is added to improve the toughness of the steel material. However, when the content is increased and the amount of nitride is increased, cracks are liable to occur at the time of width reduction rolling. %, The upper limit is set because surface cracking cannot be prevented even in the present invention. On the other hand, in the case of 0.020% or less, 0.020% is set as the lower limit in order to prevent surface cracking without using the present invention. .

Nb is effective for refining crystal grains.
If the content is large, nitrides are formed from high temperatures and cracks are likely to occur during width reduction rolling. If the content exceeds 0.060%, surface cracks cannot be prevented even in the present invention, so the upper limit is set. % Or less, the lower limit is set to 0.020% in order to prevent surface cracking without using the present invention. The present invention has been made based on the above new findings.

[0018]

FIG. 3 shows an example of a process for carrying out the present invention. The outline of each facility is as follows. Type of continuous casting machine: Vertical / bending type continuous casting machine Continuous casting slab dimensions: 280 mm thick x 1600-1 wide
800mm Cooling equipment: Air-water spray cooling Heating furnace: Walking beam continuous heating furnace Hot width reduction rolling mill: Composed of two pairs of vertical rolls (V) for width reduction and one pair of horizontal rolls (H) for thickness reduction V
-HV structure Note that only cooling equipment for carrying out the present invention is described as cooling equipment, and secondary cooling equipment used in ordinary continuous casting is omitted.

An example of an operation method using the above process will be described. The required amount of cooling water is determined based on the Ar ₁ temperature and the casting speed obtained from the molten steel component, and cooling is performed so that 10 mm below the surface layer of the slab becomes Ar ₁ or less. The slab temperature at the time of charging the heating furnace is measured, and based on the result, the furnace temperature of the heating furnace and the furnace time are controlled, and the slab surface temperature is extracted so as to be 1000 to 1150 ° C. Based on the measurement results of the slab temperature after extraction, control the hot width reduction rolling pass schedule as necessary,
Rolling is completed at a temperature of Ar ₃ or higher. By performing the above operations, it is possible to prevent surface cracks of the slab during hot width reduction rolling.

[0020]

EXAMPLES Examples of the present invention and comparative examples will be specifically described below. Tables 1 and 2 show the components of the test steel and the γ → α transformation temperature. The molten steel of these components was subjected to continuous casting and hot width reduction rolling under the conditions shown in Tables 3 and 4, and then the presence or absence of cracks on the slab surface was examined. Hot slabs with no surface cracks were subjected to hot rolling to investigate the surface quality of the hot coil.
Tables 3 and 4 show the results together with the production conditions. The cooling temperature of 10 mm below the surface layer in Tables 3 and 4 is the slab temperature at the outlet of the cooling equipment, and is a calculated value based on the molten steel temperature, the casting speed, and the amount of cooling water.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

[Table 4]

No. 1 to No. 15 is an example of the present invention,
No. 16-No. 31 is a comparative example. No. 1 to N
o. 8 is a cooling temperature at a position 10 mm below the surface layer of the slab at the time of casting is Ar ₁ or less, and after reheating to 1005 to 1145 ° C,
This is an example in which width reduction rolling was completed at an Ar of ₃ or more. No surface cracks occurred in the slab, and no surface defects occurred even when rolling was performed on a hot coil. No. 9-No. Reference numeral 15 indicates that the cooling temperature at a position 10 mm below the surface layer of the slab at the time of casting is Ar ₁ -50.
It is an example of performing width reduction rolling after reheating at 1000 ° C. or lower to 1000 to 1150 ° C., and no surface cracks occurred in the slab, and no surface defects occurred even when rolled into a hot coil. .

No. 16-No. 18 is an example in which the cooling temperature at the position 10 mm below the surface layer of the slab at the time of casting is Ar ₁ or higher,
When the structure of the surface layer portion of the slab was not fine, the slab after width reduction rolling had a surface crack. Even when the surface layer had a fine structure, the thickness was less than 10 mm. Therefore, even if no surface cracks were found in the cast slab, a surface defect occurred when it was rolled into a hot coil. No. 19-No. 2
1 indicates that the cooling temperature at a position 10 mm below the surface layer of the slab during casting is A
This is an example in which the width reduction rolling is completed at less than Ar ₃ after reheating at a temperature of r _{1 to} Ar ₁ -50 ° C. within a range of 1000 to 1150 ° C., and the structure of the surface layer is fine, but a film-like ferrite is formed. Since the width reduction rolling was performed in this state, surface cracks occurred.

No. 22, no. Reference numeral 23 indicates that the cooling temperature at a position 10 mm below the surface layer of the slab during casting is Ar ₁ or less and 1150
This is an example in which width reduction rolling was completed with Ar ₃ or higher after reheating to at least ₃ ° C., and γ grains were coarsened at the time of reheating, the thickness of the fine structure was reduced, and surface cracks occurred in the slab. Alternatively, even if no surface cracks were found at the slab stage, surface defects occurred when rolling into a hot coil. No. 24, no.
25, the cooling temperature at the position 10 mm below the surface layer of the slab at the time of casting is Ar ₁ -50 ° C. or less, and after reheating to 1150 ° C. or more,
In this example, the γ-grain was coarsened during reheating, the thickness of the fine structure was reduced, and surface cracks occurred in the slab. Alternatively, even if no surface cracks were found at the slab stage, surface defects occurred when rolling into a hot coil.

No. 26, no. 27 indicates that the N content is 0.0
More than 100% and 0.020% of Al or Nb
In the above example, since the amount of nitride precipitation was large, even if the structure 10 mm below the surface layer was fine, surface cracks occurred in the slab. No. 28, no. 29 indicates that the Al content is 0.010%
And N is 0.0020% or more, and the amount of nitride precipitation is large. Therefore, even if the structure 10 mm below the surface layer is fine, a surface crack occurred in the slab. No. 30, N
o. 31 is an example in which the Nb content exceeds 0.060% and the N content is 0.0020% or more. Since the amount of nitride precipitation is large, even if the structure of 10 mm below the surface layer is fine, the surface of the slab remains on the slab. Cracks occurred.

[0029]

As described above, according to the present invention, N:
0.0020% to 0.0100%, further containing Al:
0.020% to 0.100%, Nb: 0.020% to
It is possible to reliably and economically prevent the occurrence of cracks on the slab surface during width reduction rolling after continuously casting molten steel containing one or two kinds of 0.060%,
It is possible to eliminate maintenance and go straight, and the economic effect obtained is extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cooling temperature condition and a width reduction completion temperature condition at the time of casting for preventing surface cracks of a slab.

FIG. 2 is a diagram showing the relationship between the presence or absence of a surface crack of a slab, the cooling temperature during casting, and the completion temperature of width reduction.

FIG. 3 is a diagram showing an example of a process for carrying out the present invention.

[Explanation of symbols]

 1. 1. Mold Cast slabs 3. Cooling equipment 4. Cutter 5. Thermometer 6. Heating furnace 7. 7. Vertical roll for width reduction Horizontal roll for thickness reduction

 ──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Katsushi Kaneko Oita, Oita, Oita, Nishi No. 1, Nippon Steel Co., Ltd. Inside Oita Works of Iron Corporation

Claims (2)

[Claims] 1. N: 0.0020% to 0.0% by weight
100%, and Al: 0.020% to 0.10
0%, Nb: Continuously cast molten steel containing one or two of 0.020% to 0.060%, charged to a heating furnace without cooling to room temperature, and reheated to reduce hot width. In the method of rolling, the temperature of the slab surface layer during continuous casting or after casting is cooled so that the temperature within at least 10 mm of the slab becomes Ar ₁ or less, reheated to 1000 ° C. to 1150 ° C., and the width is reduced to a temperature of Ar ₃ or more. A method for preventing surface cracks during hot width rolling of continuous cast slabs, wherein rolling is completed. 2. N: 0.0020% to 0.0% by weight.
100%, and Al: 0.020% to 0.10
0%, Nb: Continuously cast molten steel containing one or two of 0.020% to 0.060%, charged to a heating furnace without cooling to room temperature, and reheated to reduce hot width. In the method of rolling, the temperature within at least 10 mm of the surface layer of the slab during or after continuous casting is Ar ₁ -50 ° C.
A method for preventing surface cracks at the time of hot width reduction rolling of continuously cast slabs, which is carried out after cooling to 1000 ° C. to 1150 ° C. and then performing width reduction rolling.