JPS62199204A - Hot rolling method to prevent surface cracking of ingot - Google Patents

Abstract

PURPOSE:To shorten the holding time for preventing surface cracking by cooling the transverse ends of an ingot to <=1,050 deg.C, then holding the ingot for 2-10min at 1,300-950 deg.C and starting rolling at >=1 prescribed shape ratio and Ar3 point or above. CONSTITUTION:The transverse ends of the ingot are once cooled down to <=1,050 deg.C and is then held for 2-10min in a 1,300-950 deg.C region in the cooling process in succession to solidification from a melt. The shape ratio (m) given by the equation (1) when the radius of a rolling roll is designated as R, the thickness of the material on the inlet side of the roll as h1 and the thickness on the outlet side of the roll as h2 is made >=1 and the rolling is started at the temp. of the Ar3 point or above. Then the precipitation of sulfide is nearly completed and the greater part of the causes for the cracking are eliminated. Since the shape of the ingot is taken into consideration, the holding time for preventing the cracking is shortened to 2-10min and a large contribution is made to the economization of energy and the reduction of the initial cost.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is applicable to steel plates for automobiles such as aluminum killed steel, aluminum semi-quilted steel, or aluminum silicon killed steel, steel plates for general construction, steel plates for shipbuilding, steel plates for machine structures, etc. This invention relates to a hot rolling method that prevents surface cracking during hot rolling of carbon steel and low alloy steel containing Nb, V, etc., which are used in the hot rolling process. To prevent the occurrence of cracks on the surface of a steel billet during hot rolling, during hot rolling, or in the process of hot rolling after continuous casting and charging the billet directly into an insulating furnace or heating furnace. Regarding the method.

(Prior art) As is already well known in the industry, hot rolling is a method of directly hot rolling an as-solidified slab using its residual heat without heating it midway (hereinafter simply referred to as "direct rolling"). From the point of view of energy conservation, hot rolling (hereinafter simply referred to as "direct rolling") involves charging slabs that still have a surface temperature above the Ar + transformation point into a heating furnace, insulating furnace, etc. and then hot rolling them (hereinafter simply referred to as "direct rolling"). Although this is the most desirable operating mode, there were various problems in realizing it, such as the surface properties of the slab and the layout of the equipment. However, in recent years, as improvements in these technologies have progressed, studies on direct rolling or direct delivery rolling have become active.

As a result, in direct rolling or direct rolling, metallurgical phenomena different from those observed in the conventional method, that is, a method in which after continuous casting, the method is cooled to below the Arl transformation point, near room temperature, and then reheated and rolled. were found in large numbers. In particular, when direct hot rolling is performed, the hot workability of the material is significantly reduced.In other words, even for steel types that did not cause any problems in the conventional method, direct rolling or direct conveyance rolling reduces the strength of the material during hot rolling. It was found that cracks occurred on one surface.

Generally, the hot workability of steel is strongly influenced by the austenite grain size (hereinafter referred to as "γ grain size") and the state of precipitation of sulfides, carbonitrides, etc.; γ
The less precipitation of sulfides, carbonitrides, etc. at grain boundaries, the better the hot workability.

In the conventional method, the material is repeatedly cooled and reheated to undergo T (austenite)-α (ferrite) transformation, thereby refining the γ grains and fixing most of the precipitates within the grains. Hot workability was improved by reducing the amount of precipitation in the interface.

On the other hand, in the case of the direct rolling method or the direct rolling method, it is not possible to make maximum use of the heat retained in the slab.
Since rolling is performed without undergoing transformation, the γ grain size is very large and there is a lot of precipitation at the γ grain boundaries, resulting in a decrease in hot workability. Such thermal history is said to be the cause of cracks during hot rolling.

Several proposals have already been made regarding the prevention of cracks during hot rolling that occur in direct rolling or direct rolling, but the common idea among these is the
As typified by No. 5-84201 or JP-A No. 55-84203, the cooling rate of the slab after solidification is slowed down, or the temperature is maintained at a predetermined temperature for a certain period of time or longer, for example, over 10 minutes, during cooling. In order to change the shape or coarsen the precipitate,
This attempts to prevent cracking by increasing the distance between precipitates at the γ grain boundaries.

In addition, rMetalurg4cal Transac
tions AJ Vow.

68, Sep, (1975), pp, 1727-17
In 35), regarding ``The influence of thermal history and composition on the hot ductility of low carbon steels'', in the cooling process following melting and solidification, when allowed to cool naturally, heat is released in the temperature range of 1200 to 800℃. It is stated that the ductility decreases and that isothermal holding is effective as a countermeasure against this problem. The above is a fact stated not only in the above-mentioned document but also in the aforementioned Japanese Patent Application Laid-Open No. 55-84201, and in both cases, the holding time (isothermal holding) required to prevent cracking is set to exceed 10 minutes.

(Problems to be Solved by the Invention) The problems to be solved by the present invention are the time required for retention or slow cooling in the prior art, for example,
= 84201, which aims to further shorten the time required for holding, which is more than 10 minutes. It is true that the heating time of over 10 minutes is revolutionary compared to the several hours that was required prior to the prior art, and the energy saving effect is extremely large. Nevertheless, the present invention aims to further shorten this time for the following reasons.

In other words, continuous casting, which appeared as a method to replace the ingot-blowing method, has recently entered its mature stage, and the next development theme is the development of a continuous casting method for thin slabs, which has become an important issue in the industry, and will continue to grow in the near future. We are now at a stage where we can see that it will be realized. This development task includes not only the idea of simply casting thin slabs, but also the idea of immediately connecting the slabs to a rolling mill while keeping them as hot as possible. In this case, the casting machine and the rolling mill can be connected on the same line, that is, the slab is fed into the rolling mill without being cut before the rolling mill (direct connection type), or the slab is first cut and then coiled, for example, into a coil. Two methods are being considered: one method is to feed the material into a rolling mill after forming it into a shape (non-direct type). First, in the case of a direct connection type,
In order to prevent cracking during rolling, the cast slab from the casting machine is
If you try to maintain the casting time for more than 0 minutes, the casting speed of thin slabs will generally exceed 10 m/min.
The distance between the casting machine and the rolling mill is over 100m, and a holding facility is required. This word means increasing the length of the entire facility including the building, and means investing in equipment! This poses a risk not only in terms of cost, but also in terms of equipment simplification, which is one of the objectives of manufacturing thin slabs. This also applies to non-direct connection types. Furthermore, the idea behind the non-directly coupled type is that since the capacity of the casting machine is smaller than that of the rolling mill, one rolling mill can cover the slabs coming out of several casting machines.

Even in this case, it is clear that the equipment such as a holding furnace necessary for holding the slabs sent one after another from several casting machines can be significantly simplified by shortening the holding time.

Furthermore, the above applies not only to machines intended for the production of thin cast slabs, but also to the case where an existing continuous casting machine is modified or newly installed for the purpose of direct rolling or direct rolling.

As described above, the present invention aims not only at saving energy but also at simplifying the capital investment required to prevent cracks on the surface of a steel billet in direct rolling or direct rolling, which is expected to further increase in the future.

(Means for solving the problem) By the way, the present inventor has also conducted various basic studies in order to similarly prevent cracking during direct rolling or direct rolling, and as a result, it has been found that It is very important to consider the rolling conditions in the evaluation of I learned that it is difficult to establish countermeasures against cracks. Therefore, as a result of conducting a basic study using laboratory direct pressure experiments on the conventional technology mentioned above, namely the retention or slow cooling method as a countermeasure against cracking, we obtained important knowledge regarding the technical issues necessary to shorten the retention time, which is the objective. This has led to the present invention.

The first consideration is in what form the retention should be carried out. Naturally, the slab is at a very high temperature when it leaves the casting machine. Then, by the time rolling is started, it is cooled by several 100 degrees Celsius, although it varies depending on the use, purpose, etc. Therefore, it is important to consider which temperature range and what kind of thermal history is more advantageous for holding during this period.

In order to investigate the above items, we conducted a basic study using laboratory direct pressure experiments. The main experimental conditions are 1. T
Equivalent to IS-5PIC thickness 40mm x width 0On+
m x length 300 m, using m slab, 850 mmφ
・40→20→10→5IIl111 by 2H1 mill
A study was conducted based on the pass schedule.

Figure 1 shows the relationship between the holding conditions and the occurrence of cracks when rolling is started immediately after holding.
It has been found that cracking can be prevented by holding the sample for 2 minutes or more in a temperature range of 0°C or higher. On the other hand, in actual operations, the rolling start temperature is determined naturally from the viewpoint of product quality, etc., and furthermore, a certain amount of time is required from holding to conveying the slab between rolling stages, and When considering cooling, it is difficult to satisfy the conditions shown in FIG. 1 in actual operation.

Therefore, next, the rolling start temperature is kept constant at 1000°C (but 1
When holding the temperature at 000°C or lower, a study was conducted on the case where rolling was started immediately after holding. As shown in Fig. 2, when holding at a temperature higher than the rolling start temperature, air cooling is carried out after maintenance.
Although cooling is performed to a predetermined rolling start temperature, it has become clear that in the case of such a thermal history, the higher the holding temperature, the longer the holding time required to prevent cracking.

The above results suggest that when the slab is air-cooled after being held, a phenomenon occurs that reduces hot workability again during the cooling process.

Therefore, in order to clarify this point, we conducted the following study. Namely, we held the slab at an arbitrary temperature between 1200 and 950°C for 2 minutes, and then air-cooled the slab. The air cooling time from the start of rolling to the start of rolling was varied, and the relationship with cracking was investigated. As a result, as shown in FIG. 3, it was found that there were two types depending on the holding temperature.

In type (2), hot workability deteriorates when air-cooled for a period of time after holding, whereas in type (2), hot workability does not deteriorate even after air-cooling for a while after holding. As a result of this investigation, type ■
It was found that holding at a higher temperature resulted in a type (■) tendency.

This means that the morphological change or coarsening of sulfides progresses rapidly due to holding, so if rolling is carried out in that state, cracks will not occur, but if the sulfide is air cooled after holding, precipitation will occur before and during holding. This shows that cracks are likely to occur because unrefined sulfides precipitate at the γ grain boundaries in a form that reduces hot workability again during the cooling process. Moreover, the higher the holding temperature, the less sulfide precipitates before the start of holding, so in order to prevent the deterioration of hot workability due to subsequent cooling, it is necessary to make the holding time longer. On the other hand, when cooling and holding to 1050°C or lower, 1. Precipitation of sulfides is almost complete at the time of holding, so by holding at this stage, hot workability is improved due to the shape change or coarsening of sulfides as described above, and furthermore, after air cooling. However, since new sulfide precipitation no longer occurs, hot workability does not deteriorate again.

Therefore, the conditions showing the tendency of type ■, that is, once 1
By holding after cooling to 050°C or less, even if the slab goes through a cooling process after being held, cracking can be prevented with a significantly shorter holding time than in conventional methods.

Note that under the conditions shown in FIG. 1, no cracking occurs even if the temperature is held at 1200° C. or higher, whereas under the conditions shown in FIG. 2, cracking occurs when the holding temperature is higher than 1200° C. This means that the temperature at which sulfide precipitation, which causes cracks, starts is 1.
200°C, and in the case of the conditions shown in Figure 2, no matter how much you hold ψ in a temperature range higher than 1200°C, it is meaningless because sulfides have not precipitated yet, and the sulfides will be removed in the subsequent cooling process. This is because precipitation occurs. On the other hand, when rolling is carried out immediately after holding as shown in Fig. 1, there is no point in holding at temperatures above 1200°C, but no cracks occur because sulfides that cause cracks are not precipitated.

Therefore, we similarly investigated the effective range of holding temperature in the case where holding is carried out after cooling to 1050° C. or below, as in type (3) in FIG. Figure 4 shows that after the slab has been cooled to below 1050°C, it is maintained at each temperature up to a maximum of 1300°C for 10 minutes and 30 minutes, and then heated to 110°C.
This figure shows the occurrence of cracks during rolling after air cooling to 0°C. The results of this investigation revealed that cracks did not occur when held at 1300°C for up to 10 minutes, but cracks occurred when held at 1300°C for 30 minutes. This is because, when held at 1300°C for 30 minutes, sulfides that have precipitated once become solid solution again during the holding, and then re-precipitated at the γ grain boundaries during the subsequent cooling process. From the above, by holding after the sulfide precipitation is almost complete, cracking can be prevented with a shorter holding time compared to conventional methods, but when selecting the holding temperature, it is important to ensure that the sulfide is not dissolved Care must be taken to prevent this from happening. That is, by once cooling the slab to 1050° C. or lower and further holding it for 10 minutes or less, the effective range of the holding temperature, especially the upper limit, can be expanded to 1300° C. compared to the conventional method. When considering the cooling of the slab during transportation from holding to rolling, this has great significance in terms of actual operation in that it provides flexibility in the transportation time, or in other words, in the transportation method. In addition,
Even in such a case, the lower limit temperature for holding is determined by the first
The temperature was 950°C, similar to the results shown in Figures and Figure 2.

Furthermore, in Figure 3, hot workability rapidly decreases when the time from holding to the start of rolling exceeds a certain value in Type ■, but this is because the surface temperature of the slab where cracks occur at this point increases. This is because pro-eutectoid ferrite precipitates in a band shape at the prior γ grain boundaries, and as a result, the tensile stress generated during rolling is concentrated in this area, making cracks more likely to occur.

Therefore, in order to effectively utilize the crack prevention measures by holding, it is necessary to raise the rolling start temperature by a point or more above the Ar+ point.

As is clear from the above, the conventional technology has only confirmed a part of the retention characteristics, and in order to reliably prevent cracking through retention in actual operation, it is necessary to
Furthermore, much of the essence of the retention process remained to be clarified.

The inventor of the present invention has clarified what retention is through the above basic studies, and as a result, has clarified the technical items necessary for shortening the retention time, which is the purpose of the present invention, and has completed the present invention. It is something.

In this way, the present invention was made to compensate for and improve the problems faced by the prior art, and in order to clarify its requirements, the knowledge obtained from the previous study is summarized below. .

(1) When rolling immediately after holding, the holding time required to prevent cracking is very short; however, in actual operations, the slab normally undergoes a cooling process from holding to rolling, so in this case the holding time is very short. Needs to be longer.

(2) This means that when the slab is air cooled after holding, sulfides that do not completely precipitate before and during holding start.
This is because during the cooling process, it precipitates at the γ grain boundaries in a form that reduces hot workability again, and therefore prevents cracking even if the slab is held for a short time and then undergoes a cooling process. In order to prevent new sulfide precipitation from occurring during the cooling process after holding, in other words, by holding in such a state that sulfide precipitation is almost completed before the start of holding and is not dissolved again. Its effects will be fully utilized. Even when the slab undergoes a cooling process after being held, it is effective to hold it at 1050 to 950°C in order to prevent cracking within a short period of time.
In some cases, the rolling start temperature must be higher than the holding temperature due to product quality requirements. In that case, 1050
After holding the temperature in the range of ~950℃ for 2 minutes or more, the temperature may be raised again to the required temperature, but in this case, the slab will be soaked during the holding, and the temperature of the entire slab will be raised to the required temperature again. requires a lot of energy. Generally, the inside of a cast slab after casting is higher in temperature than the surface, and even on the surface, the temperature is higher in the center than in the width direction ends. Therefore, if the crack generation position can be limited to the end faces in the width direction, the precipitation of sulfides can be almost completed by temporarily lowering only the end faces, which would normally have the lowest temperature, to 050°C or less, and then the required holding temperature can be reached. Even when the temperature of the other parts is raised, the temperature of other parts is still high, so by utilizing the retained heat, it is possible to raise the temperature without external heating or with a small amount of heating.

For example, in Japanese Patent Application No. 60-15'6247, the present inventor discovered that surface cracks that occur during direct rolling or direct rolling are cracks that occur on the long sides of steel pieces (surfaces in contact with the rolls) (face cracks). and cracks that occur on the end face in the sheet width direction (edge cracks), and which type of crack occurs is determined by the shape ratio (m) shown by the +11 formula from the roll diameter, sheet thickness, and reduction amount. said. Note that the shape ratio is the ratio of the contact arc length of the roll and the material to the average plate thickness, and its meaning indicates the penetration degree of rolling reduction, that is, the uniformity of deformation.

h, ten ri.

R: Milling roll radius hI: Material thickness on the roll entry side h2: Material thickness on the roll exit side, that is, as shown in Figure 4, when m is 1 or more, there is a tendency for edge cracking, and when m is less than 1, there is a tendency for surface cracking. It has been found that by setting m to 1 or more, cracks occur only at the end faces.

Therefore, by setting m to 1 or more, after cooling only the end face of the slab to 1050°C or less,
Cracking can be prevented by holding at a temperature of 50°C for 2 to 10 minutes. Preferably, m is 1.2 or more.

(3) Holding causes sulfidation to change shape or become coarse, improving hot workability, but if the surface of the slab, where cracks occur after holding, becomes less than 3 points Ar, causes other than sulfide, i.e. Due to stress concentration in the α band along the γ grain boundaries, the long-awaited retention process becomes meaningless. Therefore, by ensuring a holding time of 2 to 10 minutes, deterioration in hot workability during the subsequent cooling process can be prevented, but it is further necessary to start rolling at an Ar point of 3 or more.

The present invention has the above findings as its constituent elements, and its gist is that in a method of direct rolling or direct rolling of continuously cast slabs, the cooling process following the solidification of the molten material after casting is After cooling the plate width end of the slab to 1050°C or less,
℃ temperature range for 2 to 10 minutes, rolling is started under rolling conditions such that the shape ratio (m) shown by the following formula (1) is 1 or more, and at an Ar point of 3 or more. This is a hot rolling method that prevents surface cracking of steel slabs.

h Tsuta + h2 R: Roll radius hI - Material thickness on the roll entry side h2: Material thickness on the roll exit side Here, "holding" includes not only maintaining a constant temperature but also raising the temperature. do.

As described above, in actual operation, even when the slab is cooled after being held, cracks do not occur even after holding for a short time, and by setting the rolling start temperature to 113 points or higher, the holding effect is maintained. The aim is to use it effectively to prevent cracking.

In addition, in the present invention, since the essence of the cause of surface cracking during direct rolling or direct rolling is the state of sulfide precipitation, maintenance is carried out as a countermeasure against this problem.
In order to prevent cracking during holding for a short time of 10 minutes, the temperature history of the slab was specified as described above. On the other hand, conventional methods (for example, JP-A No. 60-115305) require heating of the edges of the strip because edge cracks or non-uniform quality in the strip width direction are caused by temperature non-uniformity in the strip width direction. It is simply used as a temperature equalization means, and there is no mention of the temperature history of the slab, which is one of the constituent elements of the present invention.

Note that the temperature described here indicates the surface temperature of the slab where cracks occur. This is because the temperature of the slab is generally not uniform in both the thickness and width directions, and the location of crack occurrence also varies depending on the rolling conditions, so this is defined as above.

Next, the present invention will be explained in more detail with reference to Examples, but these are merely illustrative of the present invention and are not intended to limit the present invention in any way.

Example I C: 50.06%, Si: 60.04%, Mn: 0.
15-0.30%, P: ≦0.030%
, S: ≦0.030%, sol, 8Q: 0°020~0.050% composition (Ar3 points = 85
After cooling the end face of the slab, that is, the plate width end, having a shape of 40 mm thick and 6001 mm width, having a temperature of 0°C), by air cooling to T1°C, and holding it at 12°C for t minutes, it became 800 mm in diameter.
Using a rolling mill having a roll diameter of , the material was subjected to three continuous passes of direct rolling or direct rolling at a rolling reduction of 50% in each bath.

The results are summarized in Table 1 together with each rolling condition. The shape ratio (m) was 2.98 to 5.96. As shown in Table 1, cracking can be prevented by satisfying the constituent requirements of the present invention.

Table 1 (Note) *2 Example 2 outside the scope of the present invention C': Q, 13 ~ 0.1T%, Si: 0.25 ~
0.45%, Mn; 1.25-1.50%, P: 50
．． 030%, S: 50.030%, Nb: 0.020~
0.040%, V: 0.030-0.050%, so
l.

AQ: 0.020-0.050% composition (Ar3 points-
750℃) Thickness 40IIIl x Width 600m
After cooling the end face of the slab of m to 1000°C,
After being held at 1150°C for 3 minutes, it was subjected to direct rolling or direct rolling from 1050°C under the conditions shown in Table 2.

Table 2 Note that the presence of surface cracks here means that, of course, no edge cracks occur due to holding, but surface cracks do occur.

Under the rolling conditions shown in the above example, only floor 1 has m<1 in all passes, No. 5.6 has m≧1 in all passes, and other examples are in the pass schedule. There are a mixture of cases where m<l and cases where m≧1. Among these mixed examples, there are those in which surface cracking occurs and those in which surface cracks do not occur. As described above, the requirement m≧1, which is one of the constituent requirements of the present invention, does not necessarily require that all paths satisfy this requirement. This is because surface cracking occurs when the cumulative rolling reduction exceeds a certain value, and as a result of the above example and basic study, the cumulative rolling reduction of the passes where m<1 is 20.
It was found that there was no problem of surface cracking when the amount was less than %. Therefore, when m-1 passes are included in the pass schedule, it is desirable that the cumulative reduction rate be less than 20%.

In the examples shown in Examples 1 and 2, the holding method is such that the slab is charged into a furnace kept at a constant temperature, such as a holding furnace. In the case of the example shown in , the end face of the slab was cooled to 1000°C and then charged into a furnace maintained at 1300°C. The holding time of 3 minutes at this time is the time from the time of charging into the furnace until the time of extraction, and the temperature of the end surface of the slab at the time of extraction was not raised to 1300℃,
As a result of being held in the temperature range of 1300 to 950° C. for 3 minutes, which is a constituent feature of the present invention, cracking is prevented.

Example 3 Same composition, slab shape, roll diameter, pass and
Direct rolling or direct rolling was performed depending on schedule conditions. At this time, when the steel piece was rolled from 1100°C without any holding during the cooling process following solidification from the molten body, cracks occurred on the end face of the steel piece. Therefore, after cooling the end face of the slab, that is, the end of the plate width, to 1000°C, the end face of the slab was
After heating to 1150°C using an edge heater such as a gas burner or induction heating type, rolling was started at 1100°C. At this time, the time from the start to the end of heating by the edge heater was 5 minutes, and no cracking occurred. As described above, holding by the edge heater is effective in that holding can be performed locally and efficiently.

Furthermore, it is more effective to install a heat insulating cover or the like to suppress the dissipation of the heat retained in the slab without applying any particular external heating, and to utilize the recuperation of the retained heat.

As mentioned above, "holding" in the present invention is not just constant temperature maintenance, but also includes the above-mentioned heating process, and is different from "holding" in conventional methods, which is only constant temperature maintenance. It was used in the sense of

(Effects of the Invention) The present invention clarifies the essence of retention, which has not been sufficiently elucidated in the past, regarding retention as a countermeasure against surface cracks during direct rolling or direct rolling. By elucidating the technical items necessary to shorten the time, it not only becomes possible to achieve further energy savings, but also to significantly simplify equipment, which has a great effect on the actual operation of direct rolling or direct rolling.

[Brief Description of the Drawings] Figures 1 to 4 are graphs in which the results of preliminary tests in the present invention are arranged according to holding conditions; and Figure 5 is a graph showing the relationship between the shape ratio and surface cracking and edge cracking index. This is a graph showing. Fig. 1 05 + ○ Hosaki expansion (Po・ri Fig. 2 Fig. 3 Fig. 4) 200 1250 1300 I includes 5 碕pu♂L%艷 (002 Fig. 5'""p" ba ￥ 蹟2 2 Whip ink; Meng Ye X/
-1,m ○ 12 Bean soup

Claims (2)

[Claims] (1) In a method of direct rolling or direct rolling of continuously cast slabs, in the cooling process following solidification from the molten body, after the plate width ends of the slab are once cooled to 1050°C or less,
It is characterized by holding in a temperature range of 1300 to 950 ° C for 2 to 10 minutes, under rolling conditions such that the shape ratio (m) shown by the following formula is 1 or more, and starting rolling at Ar_3 point or more. A hot rolling method that prevents surface cracking of steel slabs. m=2√[R(h_1-h_2)]/(h_1+h_2
)...(1) R: Roll radius h_1: Material thickness on roll entry side h_2: Material thickness on roll exit side (2) The method according to claim 1, wherein the cumulative reduction rate of the rolling passes in which the shape ratio (m) is less than 1 is less than 20%.