KR960007424B1 - Making method of hot rolling steel sheet - Google Patents

Abstract

The steel sheet(4) for pressure bottle is prepared by (a) hot rolling steel slab that comprises C:0.10-0.14wt%, Mn:0.70-0.90wt%, t-Al:0.005-0.06%, N;below 0.01% with or without the addition of Si:0.01-0.3wt%, balance Fe, and inevitable impurity; (b) cooling with air to give transformation ratio of ferrite of 50-70% after hot rolling; (c) cooling with water down to 600deg.C to give finally polygonal ferrite and micro perlite or beinite morphology, and then rewinding.

Description

Manufacturing method of hot rolled steel sheet for pressure vessel with excellent workability

1 is a conceptual diagram of a cooling method in manufacturing a hot rolled steel sheet for a pressure vessel according to a conventional method.

2 is a conceptual diagram of a cooling method in manufacturing a hot rolled steel sheet for pressure vessel according to the present invention.

3 is a graph showing a comparison between vertical crack resistance in the present invention method and the conventional method.

* Explanation of symbols for main parts of the drawings

1: hot rolling mill 2: water-cooled bank

3: winding machine 4: hot rolled steel sheet

5: hot rolling finish thermometer 6: winding thermometer

7: Plunger 8: run out table

F: Ferrite P: Pearlite

B: bainite

The present invention relates to a method for manufacturing a hot rolled steel sheet by a hot rolling facility, and more particularly, to deep drawing a high pressure gas strip to form upper and lower containers, and then to joining the upper and lower containers by welding. The present invention relates to a method of manufacturing a hot rolled steel sheet (hereinafter also referred to as a "hot rolled steel sheet") having excellent workability when producing a gas storage container (hereinafter also referred to as a "two-piece pressure vessel").

Recently, in the manufacture of high-pressure gas storage containers for home or industry, in order to satisfy the strength of the container and the airtightness of the welded part, the blank of the appropriate size is cut from the hot-rolled steel sheet and then deep-drawn to trim the end of the container. After removal, the two-piece container manufacturing method of joining the upper and lower containers by welding is used. When a container drawn in this two-piece pressure vessel manufacturing process is taken out from the die or when an impact force is applied from the outside after the drawing, the brittle crack is formed along the container wall in parallel to the drawing direction. This is a common phenomenon in drawing products. Factors affecting the occurrence of vertical cracks can be classified into the following three categories. First, as the internal factors of the material, the grain boundary strength can be relatively lowered due to the increase in the intragranular strength due to processing, which is fundamentally influenced by the ductility of the matrix structure and the predetermined nonuniformity between phases of the matrix structure. Second, the processing aggregate structure is formed during deep drawing process, and in particular, the greater the development degree of the aggregate structure (100), the higher the probability of occurrence of vertical cracks. Third, after forming, the container wall has a residual stress that bows like a bow in the circumferential direction and a bow that bows as a bow in the drawing direction, and this stress becomes larger as the workability of the material increases. When the above three factors interact with each other and the residual stress or impact force applied to the container wall becomes more than the breaking strength of the drawing container wall, the vertical crack is generated, and the direction in which the vertical crack propagates is the processing set generated during the deep drawing process. It is known to propagate under the influence of tissues, especially in the direction of grain boundary destruction.

In general, the vertical crack resistance tends to be improved as the strength of the material is low and the ductility is high, and the frequency occurs in a direction parallel to the rolling direction of the material. In addition, the relationship between the notched elongation of the original disc and the notch elongation of the container wall after deep drawing is proportional to each other, and the notch elongation of the container wall increases the limit blank diameter, that is, the deep drawing ratio, to evaluate the vertical crack resistance. It has a linear relationship between the blank diameter at which vertical cracks occur and good. Vertical cracks, on the other hand, have a mode of brittle fracture. Therefore, the relationship between the Charpy wavefront and brittle transition temperature (vTrs) of the vessel wall was investigated. It is known that there is a good correlation between diameters. In addition, since the soft-brittle transition temperature of the vessel wall is closely related to the shelf energy of the disc, the higher the self-energy material, the higher the transition temperature after processing is, so that the vertical crack resistance is excellent.

As a method for preventing such vertical cracks, there is a method of ironing in which the container wall is compressed again after deep drawing in terms of processing, and in the material side, the grain is refined to improve the structure so that cracks do not propagate well. How to do is known.

In the present invention, in the manufacture of hot rolled steel sheet for home or industrial gas storage container, the front side of the winder is cooled directly in the water cooling stand immediately after hot rolling (hereinafter, referred to as "shear cooling"). By controlling the cooling method, winding temperature, and constituents in the steel in a proper manner, the raw ferrite and fine pearlite or bainite structure can be released and thus the vertical crack resistance, that is, compared to the conventional ferrite and pearlite or acicular ferrite and bainite structure, , To manufacture a hot rolled steel sheet for pressure vessel excellent in workability, the purpose is.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention contains, by weight, C: 0.10 to 0.14%, Mn: 0.70 to 0.90%, t-Al: 0.005 to 0.06%, N: 0.01% or less, or 0.3% or less of Si is added thereto. , Hot-rolled steel slab composed of remnant Fe and other unavoidable impurities, and then cooled to the target winding temperature in the water cooling zone in front of the winder equipped with a plurality of water-cooled banks, and then wound to produce a hot rolled steel sheet for pressure vessels. In the method, air-cooled to obtain a ferrite transformation rate of 50 to 70% after hot rolling, and then water-cooled to obtain raw ferrite and fine ferrite or bainite structure after the final tissue is unwound, followed by winding or hot rolling. Immediately after cooling to a temperature at which the nose on the continuous cooling transformation curve is formed, it is maintained to obtain a ferrite transformation rate of 50 to 70%, and then the final structure is released to obtain the ferrite and fine pearlite or bainite structure. Processability, characterized in that a water-cooled, and then taken volumes to a method of manufacturing a hot rolled steel plate for high pressure vessels.

In the present invention, the steel component is limited in order to satisfy the tensile strength of 45kg / mm ² or more in the production of hot-rolled steel sheet for pressure vessel for gas storage, the preferred composition of the present invention by weight, C: 0.10 ~ 0.14 %, Mn: 0.7% to 0.9%, t-Al: 0.005% to 0.006%, N: 0.01% or less, or Si added 0.3% or less of Si in the present invention. The addition amount is preferably 0.3% or less, because it acts to promote production, because the vertical crack resistance decreases when the amount is 0.3% or more, and the Si addition amount is more preferably 0.01 to 0.3%.

Usually, in performing continuous cooling on the run-out table 8 after hot rolling, the temperature change of a hot-rolled steel sheet is the same as that of FIG. After hot rolling, the hot rolled steel sheet 4 exiting the filament rolling mill 1 is forcedly cooled by passing through the water pump 7 of the water-cooled bank 2 while reaching the winding machine 3, whereby The temperature drop except the temperature drop is achieved by water cooling. The air cooling drop from the water cooling start temperature T1 to the target winding temperature T5 is cooled to the water cooling end temperature T4 which is considered, and then wound up after the air cooling process. Such a conventional cooling method is called a shear cooling method, and the final structure becomes ferrite (F1) and pearlite (P) structure or acicular ferrite and bainite (B) structure and mechanical properties are determined by the height of the winding temperature.

In Fig. 1, reference numeral "5" denotes "hot rolling finishing thermometer" and "6" denotes "winding thermometer".

In the present invention, unlike the conventional method of pouring directly after hot rolling, that is, shear cooling (FIG. 1), as in FIG. 2, the temperature of T1 to T3 is obtained so that a ferrite transformation rate of 50-70% is obtained after hot rolling. Air cooled to and then cooled to temperature T4 to obtain the final ferrite and fine ferrite or bainite structure, followed by winding at temperature T5, or continuous cooling transformation at temperature T1 at which water cooling begins after hot rolling. After cooling to the temperature T2 near the ferrite transformation start point, the air cooling unit is set to allow ferrite transformation to occur, and water cooling is subsequently performed again from the temperature T3 to T4 where air cooling ends, where the secondary cold start temperature T3 is about 50 ~. The ferrite transformation of 70% should be set at an advanced temperature, which is determined by the air cooling time, the plate speed and the thickness of the material in the water cooler after hot rolling. The water cooling end temperature T4 and the coiling temperature T5 are set so that the retained austenite is transformed into a second phase, i.e., fine pearlite or bainite, and finally, unlike the conventional shear cooling application material, it is applied to a soft polygonal ferrite matrix. This is how the two phases are created. In the case where the second phase is a bainite phase, the coiling temperature should be set to 600 ° C or lower.

As described above, in the present invention, while controlling the cooling method (control cooling method) after hot rolling, addition of 3% or less of Si to accelerate the generation of soft polygonal ferrite phase during continuous cooling, thereby preventing the vertical cracking of the deep drawing container. A hot rolled steel sheet having a structure effective for improving the sex is produced.

Steel grades to which the present invention can be more preferably applied include SG26, SG30, and SG33. The steel grades are based on those corresponding to SG30.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

As shown in Table 1, the steel slab with 0.12 wt% C-0.8 wt% Mn as the basic composition was changed to Si from 0.01 to 0.3 wt%, and the hot rolling was performed after hot rolling by fixing the hot rolling finish temperature at 870 ° C. As shown in FIG. 3, a hot rolled steel sheet was prepared by changing the temperature from 640 to 560 ° C., performing the conventional shear cooling method and the controlled cooling method of the present invention, and then examining the mechanical properties and the vertical crack resistance, and calculating the results. 3 is shown.

The vertical crack resistance of the hot rolled material was evaluated as follows.

The hot rolled steel sheet was collected by punching to obtain a blank having a diameter of 110 mm, and then subjected to three-stage deep drawing to have a final deep drawing ratio of 2.44, and to evaluate the vertical crack resistance of the container manufactured by deep drawing. The vertical crack resistance of the deep-drawn container was evaluated by measuring the stroke until the crack occurred by pressing the cone at a 45 ° angle to the inlet of. The larger the stroke, the better the vertical crack resistance.

TABLE 1

As shown in FIG. 3, it can be seen that the control coolant of the present invention has superior vertical crack resistance compared to the shear coolant of the conventional method, and among them, the steel containing 0.17 wt% Si is the most excellent in the vertical crack resistance. Able to know.

As described above, the present invention considers that the path of transformation from austenite to ferrite is changed according to the cooling method in the water cooling zone in front of the winder after hot rolling, and the final microstructure is changed, and the ferrite transformation is about 70 after hot rolling. By adding 0.01 ~ 0.3wt% Si at the same time as the cooling control method to cool the water to the target winding temperature after progressing, the soft polygonal ferrite is prepared in comparison with the hot-rolled steel sheet composed of acicular ferrite structure and bainite structure during conventional shear cooling. The hot rolled steel sheet for pressure vessels, which is secured by the ductility and the second phase, the fine pearlite or bainite phase, and the processing structure formed by deep drawing processing is more uniform than the conventional materials, and also has excellent vertical crack resistance. There is an effect that can be provided.

Claims (2)

By weight%, C: 0.10 to 0.14%, Mn: 070 to 0.90%, t-Al: 0.005 to 0.06%, N: 0.01% or less, or 0.01 to 0.3% of Si is added thereto, the balance Fe And hot rolling a steel slab composed of other unavoidable impurities, and then winding the front of the winder having a plurality of water-cooled banks to the target winding temperature in a water-cooling stand and winding the steel slab. Air-cooled to obtain a ferrite transformation rate of 50 to 70% after hot rolling, followed by water-cooling to 600 ° C. or lower so as to obtain a final ferrite and a fine ferrite and fine pearlite or bainite structure. Method for manufacturing hot rolled steel sheet for pressure vessel. By weight%, C: 0.10 to 0.14%, Mn: 070 to 0.90%, t-Al: 0.005 to 0.06%, N: 0.01% or less, or 0.01 to 0.3% of Si is added thereto, the balance Fe And hot rolling a steel slab composed of other unavoidable impurities, followed by water cooling at a target winding temperature in a water cooling zone in front of a winder equipped with a plurality of water cooling banks, and winding the steel slab for pressure vessels. After the hot rolling, the liquid was cooled to a temperature at which the nose on the continuous cooling transformation curve was formed, and then maintained to obtain a ferrite transformation rate of 50 to 70%, and then the final structure was polygonal ferrite and fine pearlite or bainite structure. A method for producing a hot rolled steel sheet for pressure vessels having excellent workability, which is cooled to 600 ° C. or lower so as to be obtained and then wound up.