JPH08134537A - Hot-rolling of steel cast slab - Google Patents

Abstract

PURPOSE: To provide a hot-rolling method of a steel cast slab capable of preventing the development of surface flaw, in the method for rolling a bloom or a product without cooling a Al-deoxidized steel to the room temp. after continuous casting. CONSTITUTION: In the steel having a compsn. containing the specific quantities of C, Si, Mn and Al and one or more than two kinds selected from among Cr, Mo, Ni and V, the surface temp. of the cast slab after continuous casting is cooled to <=800 deg.C. Thereafter, the cast slab is heated or recuperated to 850-1050 deg.C, and then, cooled at <=600 deg.C for >=30sec. Thereafter, the cast slab is heated to 1050-1200 deg.C to execute the hor-rolling. By this method, the surface flaw of crack, etc., is eliminated because of the fining of γ grain before blooming or rolling and the harmlessness of AlN.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to the continuous casting of steel, especially low alloy steel, which has been deoxidized with Al, and charged into a heat-retaining furnace or a heating furnace without heating to room temperature. The present invention relates to a hot rolling method for preventing the occurrence of surface defects in hot charge rolling for hot rolling.

[0002]

2. Description of the Related Art In steel, it is necessary to reduce nonmetallic inclusions in order to improve the properties of strength and toughness. For that purpose, deoxidation by Al is performed as an extremely effective means.
However, when the steel deoxidized with Al is subjected to direct rolling (hereinafter abbreviated as DR) or hot charge rolling (hereinafter abbreviated as HCR) after casting, cracks occur on the surface and surface defects occur in the final product. There is a drawback.

Low alloy steels containing Cr, Mo and the like have high strength and toughness and good hardenability, so that they are mainly used for strength and abrasion resistant parts of automobiles and industrial machines. However, these low alloy steels also have a drawback that when they are rolled in a DR or HCR process after casting, cracks are generated on the surface and surface defects occur. Various surface cracking phenomena have been recognized in the conventional slab-rolling process of slabs. Among them, continuously cast slabs of carbon steel and low alloy steel that have undergone Al deoxidation are DR and HCR.
It is recognized that fine cracks are generated on the surface when hot rolling is performed in the process.

It is considered that the cause of these cracks is that the precipitation of AlN in the austenite grain boundaries reduces the strength of the grain boundaries and causes embrittlement. However, stopping the addition of Al in order to prevent precipitation of AlN not only impairs the cleanliness of the steel, but also deteriorates the workability and the strength / toughness, which is impractical. A for N
In the deoxidized steel, it is technically impossible to reduce the content to a level that can prevent the precipitation of AlN.

As a method for preventing surface defects caused by AlN, Japanese Patent Publication No. Sho 49-7771 and JP-A-6-73445 are proposed. In these methods, the ferrite + pearlite or bainite transformation is performed once after casting, and then the crystal grains are made fine by charging in a heating furnace and heating. However, although these methods have some effect of preventing surface defects, they have not completely prevented surface defects.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides a hot rolling method for steel slabs capable of reliably preventing surface cracks in hot rolling such as slabbing after continuous casting due to precipitation of AlN. With the goal.

[0007]

The present inventors have studied in detail the precipitation behavior of AlN, which causes surface cracking. As a result, it was found that AlN did not precipitate in the steel during the cooling process after casting and AlN precipitated during the temperature rising process. That is, until now, it was said that AlN was precipitated in the austenite grain boundaries during the cooling process after continuous casting, but it was found that it was actually precipitated during the temperature rising process in the heating furnace after the continuous casting, not during the cooling process. .

That is, after the continuous casting, the surface layer structure of the slab is transformed into ferrite + pearlite or bainite,
Even if the austenite in the surface layer is made finer by the subsequent temperature rise, AlN precipitates at the grain boundaries, so it is impossible to render AlN harmless. Although the generation of surface defects is suppressed to some extent by the effect of grain refinement, Al precipitated at grain boundaries
Since N cannot be prevented, surface defects cannot be completely prevented.

In order to prevent the precipitation of AlN grain boundaries, it is effective to prevent surface defects by precipitating AlN almost completely before cooling the surface layer structure to a ferrite + pearlite or bainite structure. It is thought that there is.

Based on the above idea, the present inventors precipitate AlN before the surface layer structure is made to be ferrite + pearlite or bainite structure, and the surface layer structure is made to be ferrite +.
When the prevention of precipitation of AlN in the austenite grain boundaries retransformed in the temperature rising process after being transformed into pearlite or bainite was examined, it was found that the temperature control of the slab in the cooling process after casting is important. The present invention has been devised based on this finding.

That is, the first aspect of the present invention is a method of continuously rolling a steel deoxidized with Al and then hot rolling it without cooling it to room temperature.
After cooling to 00 ° C or lower, and then heating or reheating the slab to 850 ° C to 1050 ° C, the slab is cooled to 600 ° C or lower for 30 seconds or more, and then heated to 1050 ° C to 1200 ° C, and hot rolling is performed. It is a hot rolling method of a steel slab characterized by the above.

Furthermore, the second aspect of the present invention is an embodiment for a low alloy steel containing Cr, Mo, etc., and by limiting the heat history until hot rolling, C: 1% or less, Si:
0.01% or more and 1% or less, Mn: 0.01% or more and 2% or less, Al: 0.001% or more and 0.1% or less, and other Cr: 0.2% or more and 3.5% or less, Mo : 0.1% or more and 1.5% or less, Ni: 0.1% or more and 2.5% or less,
V: In a method of hot-rolling steel containing at least one of 0.05% or more and 0.5% or less, or two or more, without hot cooling to room temperature, the surface of the slab before hot rolling Cool the temperature below 800 ℃, then 850 ℃ ~ 1050 ℃
After heating or reheating the cast slab,
It is a hot rolling method for a steel slab characterized by cooling for at least seconds and then heating to 1050 ° C to 1200 ° C for hot rolling.

[0013]

The basic principle of the present invention will be described below based on the experimental results. C: 0.21%, Si: 0.2
3%, Mn: 0.79%, acid-soluble Al: 0.034%,
Steel containing Cr: 1.20% and Mo: 0.21% is once
After heating and holding at 300 ° C. to completely dissolve AlN in the steel, cooling from 1200 ° C. to 600 ° C. at 1 ° C./s, followed by rapid cooling, the precipitation amount of AlN precipitated in the steel was measured. analyzed.

FIG. 1 shows the relationship of the amount of AlN precipitation at each temperature. 1200 ℃ ~ 6 in the cooling process from 1300 ℃
It can be seen that AlN does not precipitate at all in the temperature range of 00 ° C.

Next, the above steel was heated to 1300 ° C., then air-cooled to room temperature, and further, in the temperature rising process from room temperature, 60
The result of analyzing the precipitation amount of AlN when it is rapidly cooled after reaching 0 ° C to 1200 ° C is shown in Fig. 2. As the temperature rises, the amount of precipitation increases, reaches a peak at 1000 ° C., and when heated to a higher temperature, AlN begins to form a solid solution again.
As is clear from FIGS. 1 and 2, it is understood that AlN does not precipitate in the cooling process but precipitates in the temperature rising process.

Hereinafter, the present invention will be described in detail along with its operation. First, the reasons for limitation of the present invention will be described. A feature of the present invention is that in a hot rolling method such as slab rolling without cooling to room temperature after continuous casting, AlN is precipitated in the steel before the surface layer structure of the slab is transformed into ferrite + pearlite or bainite. . Therefore, in the process of cooling the steel from the temperature at which AlN is not precipitated, in order to precipitate AlN, it is necessary to raise the surface temperature of the slab by recuperating or heating. Since the temperature range in which AlN precipitates in a relatively short time is 850 ° C to 1050 ° C, in order to raise the temperature to that temperature range, the steel was once cooled to 800 ° C or less.

Further, heating is performed in a temperature range in which AlN precipitates in a relatively short time. The heating temperature was set to 850 ° C. or higher to finish AlN in a short time, and the upper limit was set to 1050 ° C. at which AlN was not re-dissolved after precipitation.

After depositing AlN, the surface of the slab was transformed into ferrite + pearlite or bainite, so that the slab was kept at 600 ° C. or lower and the holding temperature was 30 seconds or longer.

Thereafter, the heating temperature for hot rolling such as slabbing is heated to a temperature at which re-dissolution begins, even if the amount of AlN precipitated in the previous step is insufficient, whereby the heating process The lower limit was heating to 1050 ° C. or higher at which AlN precipitated at the grain boundary could be re-dissolved. In addition, from the viewpoint of deterioration of hot ductility due to coarsening of crystal grains by heating at high temperature and heat saving energy, the upper limit is 1200 ° C or lower.

Of the components of the steel in the second invention, if C exceeds 1.0%, a large amount of pro-eutectoid cementite precipitates and the toughness deteriorates, so it cannot be used in ordinary low alloy steels, so the upper limit is set. And If the Si content is less than 0.01%, deoxidation will be insufficient, so the lower limit is set. If it exceeds 1%, the hot ductility is deteriorated and enormous cracks other than the surface cracks targeted by the present invention occur. And If Mn is less than 0.01%, deoxidation becomes insufficient, so the lower limit is set. If it exceeds 2%, the austenite phase becomes stable and the austenite phase remains even after cooling to room temperature.

If the content of Al is less than 0.001%, the deoxidation is insufficient, so the lower limit is set. If the content of Al exceeds 0.1%, the hot workability is deteriorated, so the upper limit is set. Cr is a low alloy steel with a lower limit of 0.2% to ensure hardenability,
Even if added in a large amount, the effect does not change significantly and only the cost increases, so 3.5% was made the upper limit. Mo is also
As a low alloy steel, 0.1% to ensure hardenability
Was set as the lower limit, and even if added in a large amount, the effect did not change significantly and only the cost increased, so 1.5% was made the upper limit.

Ni is also a low alloy steel with a lower limit of 0.1% in order to ensure hardenability, and even if added in a large amount, the effect does not change significantly and only the cost rises. % Was set as the upper limit. V is a low alloy steel with a lower limit of 0.1% in order to ensure hardenability, and if added in a large amount, a large amount of carbide precipitates, workability toughness deteriorates, and only the cost increases. The upper limit was 0.5%.

The cooling method and the medium thereof are not particularly limited because the temperature is important. Further, the temperature rise after cooling is performed by combining heating or cooling as necessary in addition to the recuperation of itself, but the method is not particularly limited.

[0024]

EXAMPLE A steel having the chemical composition shown in Table 1 was manufactured to have a cross-section size of 35.
After continuous casting to 0 mm x 560 mm, after primary cooling to various temperatures shown in Table 2, reheat or heating is performed and the subsequent 2
As the next cooling, it was immersed in water and cooled to 600 ° C or lower. Then, it was taken out from water, charged into a heating furnace, heated to various temperatures shown in Table 2, and then slab-rolled into 165 mm square steel pieces.

Visually observing the surface of the slab of rolled slab,
The presence or absence of surface cracks was investigated. For comparison, surface cracks were also investigated for those that were subjected to slab rolling with heat history other than the present invention. The results of the surface crack investigation are shown in Table 2 together with the type of steel used, the temperature in the thermal history of the slab, and the time conditions. According to the thermal history of the present invention, it was confirmed that no surface cracking occurred.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

As described above, according to the present invention, A
l The hot workability of continuously cast slabs of deoxidized steel and low alloy steel was improved, and it became possible to adopt the hot charge rolling process. Particularly in the conventional method, since the effect is thin and surface cracks occur in the relatively low cost steel having relatively high cost, the surface of the slab has been cared for, but the present invention eliminates the need for surface servicing of the slab. It is possible to improve the quality and reduce the manufacturing cost.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the precipitation amount of AlN and the temperature when the steel is rapidly cooled at each temperature in the cooling process after being heated to 1300 ° C.

FIG. 2 is a diagram showing the relationship between the precipitation amount of AlN precipitated at each temperature and the temperature when the steel is heated to 1300 ° C., cooled to room temperature, and then rapidly cooled at each temperature during heating from room temperature.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C22C 38/00 301 A 38/06

Claims (2)

[Claims] 1. A method of hot-rolling a steel deoxidized with Al after continuous casting without cooling to room temperature, wherein the surface temperature of the slab is cooled to 800 ° C. or lower before hot-rolling. After heating or reheating the slab to 850 ° C to 1050 ° C, it is cooled to 600 ° C or lower for 30 seconds or more, and then 1
A hot rolling method for a steel slab, comprising heating to 050 ° C to 1200 ° C and hot rolling. 2. C: 1% or less, Si: 0.01% or more 1
% Or less, Mn: 0.01% or more and 2% or less, Al: 0.0
Including 01% to 0.1%, other Cr: 0.2%
Or more and 3.5% or less, Mo: 0.1% or more and 1.5% or less,
Ni: 0.1% or more and 2.5% or less, V: 0.05% or more and 0.5% or less, steel containing at least one type is continuously cast, and then hot rolled without cooling to room temperature. In the method, the surface temperature of the slab is cooled to 800 ° C. or lower before hot rolling, and then the slab is heated or reheated to 850 ° C. to 1050 ° C. and then cooled to 600 ° C. or lower for 30 seconds or more, and then A method for hot rolling a steel slab, which comprises heating to 1050 ° C to 1200 ° C for hot rolling.