KR900004844B1 - Process for reducing the edge cracks of steel billet in hot rolled processing - Google Patents

Abstract

The maing process of steel sheet to reduce edge crack is characterised by forced-cooling of steel slab to a cooling rate between air cooling rate and critical cooling rate of martensite transformation from Ar, point to room temperature, heating at 1000- 1250 deg. C, finish hot-rolling at 800-950 deg. C, and coiling at 500-700 deg. C. The steel slab is composed of (in wt.%) 0.10-0.70% C, up to 0.40% Si, 0.30-1.50% Mn, up to 0.1% Al, up to 0.03% P, up to 0.03% S and balance Fe with inevitable impurites.

Description

Method for Reducing Crack Edge Cracking in Hot Rolling

1 is a schematic diagram of temperature and processing history according to a conventional rolling method.

Figure 2 is a schematic diagram of the forced cooling of the hot steel piece according to the present invention.

3 is a graph showing the temperature distribution in the width direction of the hot-rolled steel forcibly cooled according to the present invention.

4 is a schematic of the temperature and processing history of the method of the present invention.

5 is a graph showing the volume change of the steel according to the temperature of the present invention.

6 is a CCT curve of a steel according to the present invention.

7 is a schematic diagram of temperature and processing history for an embodiment of the present invention.

The present invention is a material for carbon steel and line pipe or oil pipe of silicon (Si), manganese (Mn) main components mainly used for general building materials, shipbuilding, machine structure, wire rods, etc. Micro alloy element-added steel containing niobium (Nb), titanium (Ti), vanadium (B), etc. used as a hot rolling is performed immediately after continuous casting (hereinafter referred to as "direct rolling") or after continuous casting In a process in which a slab and a billet (hereinafter, referred to as "steel strips" without being distinguished from each other) are subjected to hot rolling after charging in a furnace (hereinafter referred to as "rolling sheet loading rolling") before they are cooled to room temperature, The present invention relates to a method for reducing cracking of a steel edge at the time of hot rolling.

In the conventional steel material manufacturing process, molten steel that has been melted in a converter in the following manner is passed through a process as shown below.

Continuous casting → Cold import → Heating → Hot rolling

However, in order to improve productivity due to the omission of the manufacturing process recently and to save energy due to the reduction of the unit of heat energy source, the hot rolled steel and the cold import process, which omit the cold import and heating process, are omitted, and the high-temperature steel pieces are put into the furnace. A new process has been developed, which is a hot-roll charging technology. However, in order to implement the new process, product quality assurance should be performed, and the real problem is to prevent cracks occurring on the edges and surfaces of the products.

In the former method of reheat-rolling cold flakes, the metallurgical composite which cracks frequently in the direct rolling and hot-rolling insertion rolling, the crushing of the cast structure, the grain boundary segregation and precipitation reduction during the solidification during the reheating after cooling to room temperature, and austenite Elements such as sulfur (S), phosphorus (P), and oxygen (O) are fixed in the mouth as emulsions, phosphides, and oxides, which are harmful to nitrite microparticles and hot workability, and thus silicon (Si) and manganese (Mb). In the reheat-rolling of cold flakes, not only carbon steels containing NiOb and vanadium (B), but crack grooves caused by hot working are not a problem. In the process, segregation and precipitation of the elements as described above occurs on the dendrite interface or austenite grain boundary during the melt-solidification-cooling process. When stress is applied to the specimen, grain boundary cracks are generated on the surface and edges. These cracks are prominent at the edge of the steel sheet, and the cracks generated at this time are called edge cracks.

The latter method, developed to prevent cracking of the surface and edges, lowers the charging temperature during the hot-rolling loading to be lower than the transformation temperature (Ar ₃ or Ar ₁ ) to completely transform the high temperature austenite structure into ferrite and perlide structure. After reheating and rolling, both methods of reheat rolling of the cold piece have almost obtained similar effects using almost the same principle.

However, the above methods, first, cannot increase the energy efficiency of the thermal element because it is not possible to increase the thermal element charging temperature above the Ar ₃ point (approximately 700-800 ° C.), and second, below the Ar ₃ point of the high temperature piece. Due to the time required to cool the furnace, the process loss is large. Third, this technique has a problem that is difficult to apply in the case of direct rolling.

1 shows the temperature history received by the steel sheet in the direct-roll rolling (FIG. 1 (a), the hot-piece charging rolling (FIG. 1 (b)) and the cold-sheet heating rolling (FIG. 1 (c)). The process history is shown graphically, in the case of direct-rolling and hot-roll-loading rolling, hot rolling is carried out without cooling the steel slab to room temperature or hot-rolled into a heating furnace, unlike conventional cold-roll reheating rolling. In the case of hot-rolled insertion rolling in which the thermal element is charged at a temperature of ₃ or more Ar, when the continuous multipass rolling is carried out at a temperature range of 1200-900 ° C, edge cracks are formed in the rolling of about 1-5 passes, and the cracks occur in the subsequent continuous rolling. Is enlarged and remains in the product, and in severe cases, there is a problem that it is difficult to use the product, etc. Therefore, the present invention eliminates such a conventional problem and thus affects harmful segregation on hot workability. It can be reduced through repeated phase transformation, and when the austenitic steel is cooled under Ar _! Transformation point, the volume expansion effect due to transformation and shrinkage effect due to cooling cancel each other, and there is almost no volume change. The purpose of the present invention was to provide a method for controlling the cracks of steel strips because the difference in austenite grain size in the city is almost eliminated at the end of rolling due to recrystallization during rolling.

Hereinafter, the present invention will be described in detail.

The present invention, in the direct feed rolling and hot-rolled sheet rolling of the steel strip, in the cooling process carried out after melting and solidification, only the edge portion of the steel sheet is selectively cooled to below ₁ point of Ar, and then subjected to reheat rolling to perform the steel sheet in hot rolling The present invention relates to a method for reducing edge cracking.

The steel strips to be applied to the present invention include niobium, which is used for carbon steel or line pipe or oil pipes of silicon (Si) and manganese (Mn), which are mainly used for general construction materials, shipbuilding, mechanical structures, and wire rods. A trace alloy element-added steel containing titanium (Ti), vanadium (V), and the like, and preferably, in weight percent, C: 0.10-0.70%, Si: 0.40%, or less, Mn: 0.30-1.50% , Al: 0.1%, or less, P: 0.03%, or less, S: 0.03%, or less, remainder: Fe, and a steel strip composed of other unavoidable impurities.

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

In order to prevent edge cracking which occurs during hot-rolling and rolling when the rolling temperature of the steel sheet is higher than or equal to Ar ₃ point, the method of the present invention selectively selects the edge portion of the high-temperature steel sheet only at the Ar ₁ point during the subsequent cooling process after melt coagulation. It is forced cooling (FIG. 2) below and the austenite structure is transformed into a ferrite + perlite structure. As shown in FIG. 3, the edge portion of the hot slab is completely transformed from the austenitic structure to the ferrite + perlite structure after the forced cooling treatment, and then the austenite is re-heated again. In the case of reverse transformation into the tissue, in this case, the temperature and processing history of the edge portion and the central portion of the hot steel strip are shown in FIG. In the process of going through this history, the center of the slab keeps the high temperature as it is in an untransformed state, which has the advantage of increasing the energy energy effect, and the edge portion undergoes the austenite → ferrite + pearlite → austenite transformation. It is possible to effectively suppress the occurrence of edge cracking during rolling due to the effects of segregation reduction, cast structure breakdown, and structure microstructure obtained through the process.

The reason why quality can be guaranteed when only the edge part is forcedly cooled by high temperature steel strips is as follows. First, when the cooling is performed locally because the crystal structure of austenite, which is a metamorphic transfer structure and ferrite, is different from each other, This is because cracks do not occur because there is little negative volume difference. That is, austenite, which is a high temperature structure of Ar ₃ or higher, has a face centered cubic (FCC) structure, while ferrites produced at or below Ar ₃ have a body centered cubic (BCC). The volume is increased by about 8.8%. Therefore, when cooling the steel from high temperature, as shown in FIG. 5, when the volume is contracted while reaching the Ar ₃ point there is to the transformation begins rather volume expansion with temperature becomes lower, the expansion is Ar ₁ It shows a unique pattern that stops reaching a point and then continues to shrink as it cools. In this case, there is almost no volume difference between the edge portion cooled below Ar ₁ (figure 5 point A) and the inside of Ar ₃ or higher point (figure 5 point B), and there is a sudden volume change even if the edge portion is selectively quenched. This will not cause cracks.

Second, although the edge part and the inexperienced inside experienced transformation, the austenite grain size difference is small at the time of extraction of the furnace, the final microstructure is almost similar due to repeated recrystallization during hot rolling. Only the edge of can be forced to cool selectively.

Third, in actual direct rolling and hot-roll insertion, the cracks at the edges are quite serious, but the surface cracks are not serious enough to affect the product quality.

Next, the cooling conditions at the time of forced cooling of the edge portion in detail, the cooling speed must be faster than the natural air cooling rate to obtain the desired effect, there is no upper limit speed limit. However, if the cooling rate is higher than the threshold value ((2) of Fig. 6), the cooling end temperature range shall be from directly below the Ar point ₁ to directly above the Ms point (see Fig. 6). 6 (3) shows that the metamorphic structure becomes ferrite + perlite to obtain the microstructure required in the present invention, but if it is above the critical velocity ((1) in FIG. 6), the martensite transformation occurs to cause cracks during forced cooling. It is likely to occur.

Therefore, if the cooling rate is faster than the threshold, the martensite transformation should be suppressed by setting the cooling end temperature to Ms point or higher ((1) in FIG. 6). and Ar ₁ in the first to point directly under the 6 degrees 3) to room temperature, and if the cooling rate is higher than a threshold value (FIG. 6 (1)) is in direct point to the Ar ₁ to Ms point immediately above. The reason for determining the temperature range as described above is to avoid the possibility of cracking during forced cooling by avoiding martensite transformation as mentioned above. And the deeper the depth in the width direction to be cooled below Ar ₁ point, the more effective there is no upper limit to prevent edge cracking, and the lower limit is that all parts on the side of the steel piece should be cooled to below Ar ₁ point.

As such, if the hot slabs are selectively cooled only at the edge portion, the average temperature of the slabs is higher than that of the overall cooling, thereby avoiding the energy saving effect in the subsequent reheating process. The reheating process is carried out in a heating furnace or an edge temperature compensating furnace. In the case of a heating furnace, not only the hot slabs are heated by the burner heat, but also heat conduction from the inside to the edge portions due to the heat gradient between the inside of the slabs and the edge portions. Since the edge portion is easily heated, it is possible to shorten the time compared to the case where the entire steel sheet is cooled. On the other hand, the preferred hot rolling conditions applied to the present invention is to be heated to 1000-1250 ℃ hot rolled to a finish rolling temperature of 800-950 ℃, then wound at a temperature of 500-700 ℃.

As described above, the present invention exhibits the effect of minimizing process loss and maximizing sex energy effect on the basis of crystallography and phase transformation properties.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

Compositions such as carbon (C): 0.15%, silicon (Si): 0.01%, manganese (Mn): 0.68%, phosphorus (P): 0.016%, sulfur (S): 0.011%, aluminum (Al): 0.054% An ingot having a size of 100 × 100 × 250 mm ³ was dissolved using vacuum induction solubility. After the solidification was completed, some of the ingots were air cooled to room temperature ((3) of FIG. 7), and some of them were air cooled until the surface temperature became 900 ° C ((1) of FIG. 7). After air-cooling until it became 800 degreeC, only the side part of the ingot was water-cooled to 600 degreeC at this temperature (FIG. 7 (2)). In the cooling method, water at room temperature was evenly sprayed on the ingot side surface, and the cooling rate was about 10 ° C / sec. The ingot cooled to the three temperatures as described above was heated to 1250 ℃ at 8 ℃ / min using the same furnace (유지) and maintained at this temperature for 30 minutes and 60 minutes and then extracted and hot rolled. The hot rolling start temperature was 1050 ° C. and the finishing temperature was 850 ° C., and each material was rolled to 15 mm by 6 pass rolling of a 100 mm thick material with the same pass schedule. The finished material was air-cooled and the edge crack of the rolled material was observed at room temperature.

The temperature and the processing history of this example are shown schematically in FIG. The rolled material produced by the present invention showed a very good edge state almost similar to the cold reheated rolled material, whereas the comparative material with the hot-plate insertion temperature of 900 ° C. was very coarse. It shows the edge state.

The treatment conditions were tabulated and shown in Table 1 together with the macro photography results (degree of cracking).

TABLE 1

Claims (2)

By weight%, C: 0.10-0.70%, Si: 0.40%, or less, Mn: 0.30-1.50%, Al: 0.1%, or less, P: 0.03%, or less, S: 0.03%, or less, balance: Fe and In the hot rolling of steel pieces composed of other unavoidable impurities by direct rolling or hot piece loading rolling method, in the cooling process performed after melting and casting, all parts of the edge portion of the steel pieces which are in high temperature are below the Ar ₁ transformation temperature so that Martensitic transformation Forced cooling by cooling rate below the critical cooling rate (Fig. 6 (2)) and cooling end temperature from Ar ₁ to room temperature, and then heating to 1000-1250 ℃ to finish rolling 800-950 ℃ Hot rolled at a temperature, and then wound at a temperature of 500-700 ℃ a slab edge crack reduction method in hot rolling. 2. The forced cooling process according to claim 1, wherein the forced cooling step is directly above Ms at a cooling rate higher than the martensite transformation critical cooling rate ((2) in FIG. 6) and a temperature directly below Ar ₁ such that the entire portion of the edge portion of the steel sheet is equal to or lower than the Ar ₁ transformation temperature. A method for reducing the edge cracks of hot rolls in hot rolling, characterized in that the cooling is carried out under cooling end temperature conditions.

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