JPH0627287B2 - Hot rolling method that prevents surface cracking of billets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is applied to steel plates for automobiles such as aluminum killed steel, aluminum semi-killed steel or aluminum silicon killed steel, steel plates for general construction, steel plates for shipbuilding, and steel plates for machine structures. The present invention relates to a hot rolling method in which surface cracks of carbon steels and low alloy steels containing Nb, V, etc., which are contained in hot rolling are prevented during hot rolling, and in particular, slabs immediately after continuous casting of those steels are immediately heated. In the process of hot rolling or hot rolling after continuous casting or charging the slab into a heat insulation furnace or a heating furnace after continuous casting,
The present invention relates to a method for preventing cracks from being generated on the surface of a steel slab during hot rolling.

(Prior Art) As is well known in the art, directly hot-rolling a slab that has been solidified without directly heating it by using its heat (hereinafter referred to as “direct It is energy-saving to temporarily charge a slab having a surface temperature higher than the Ar ₁ transformation point into a heating furnace or a heat-retaining furnace and then hot-roll it (hereinafter simply referred to as “direct rolling”). From the point of view, the most desirable form of operation,
In realizing this, there were various problems regarding the surface property of the slab or the layout of equipment. However, in recent years, as the technical improvement for them has progressed, studies on direct rolling or direct feed rolling have become active.

As a result, in direct rolling or direct rolling, it was different from the metallurgical phenomenon found in the conventional method, that is, the method in which after continuous casting, the material was once cooled to below the Ar ₁ transformation point and cooled to near room temperature and then reheated and rolled. Many phenomena were found. In particular, the hot workability of the material is significantly reduced during direct hot rolling, that is, even in the case of steel grades that did not cause any problems in the conventional method, during hot rolling during direct rolling or direct feed rolling. It was found that cracking occurred on one surface.

In general, the hot workability of steel is strongly affected by the austenite grain size (hereinafter referred to as “γ grain size”) and the precipitation state of sulfides, carbonitrides, etc. The less precipitation of sulfides, carbonitrides, etc. at the γ grain boundaries,
Hot workability is improved.

In the conventional method, the material is repeatedly cooled and reheated to experience the γ (austenite) α (ferrite) transformation, and the γ grains are refined, and most of the precipitates are fixed within the grains to form the γ grain boundary. The hot workability was improved by reducing the amount of precipitation in the.

On the other hand, in the case of the direct rolling method or the direct feed rolling method, the retained heat of the slab is utilized to the maximum.
Since the rolling is performed without undergoing the α transformation, the γ grain size is very large and a large amount of precipitation occurs at the γ grain boundary, so that the hot workability is deteriorated. Such heat history is a cause of cracking during hot rolling.

Several proposals have already been made to prevent the occurrence of cracks during hot rolling in such direct rolling or direct feed rolling, but the common idea to these is JP-A-55.
-84201 or Japanese Patent Laid-Open No. 55-84203, slow the cooling rate of the slab after solidification, or keep it at a predetermined temperature during cooling for a certain time or more, for example, for 10 minutes or more,
It is intended to prevent cracks by changing the form or coarsening of the precipitates and increasing the distance between the precipitates at the γ grain boundaries.

In addition, `` Metallurgical Transactions A '' Vol.6A, Sep.
(1975) pp. 1727 to 1735, regarding the "effect of thermal history and composition on the hot ductility of low carbon steel,"
1200 if naturally cooled in the cooling process following solidification
It states that the hot ductility decreases in the temperature range of ~ 800 ℃, and that isothermal holding is effective as a countermeasure.
The above is not limited to the above-mentioned documents, but the above-mentioned JP-A-55-8420.
This is also the case described in No. 1, and both have a retention (isothermal holding) time of more than 10 minutes required to prevent cracking.

(Problems to be Solved by the Invention) The problem to be solved by the present invention is the time required for retaining or gradual cooling, which is a conventional technique, that is, for example, JP-A-55-
It is intended to further reduce the time required for holding of more than 10 minutes disclosed in 84201. Certainly, compared with the heating time of several hours that was performed before the above-mentioned prior art, 10
The time exceeding minutes is epoch-making, and the energy saving effect by this is very large. Nevertheless, the present invention intends to further reduce this time for the following reason.

In other words, continuous casting, which has emerged as an alternative to the ingot-casting method, has recently entered the maturity stage, and the development of continuous casting of thin slabs has become an important issue in the industry as the next development theme, and in the near future. It is at the stage where the perspective of realization is obtained. This development task includes not only the idea of casting a thin slab but also the idea of immediately connecting the slab to a rolling mill while keeping the slab at a temperature as high as possible. In this case, as a method of connecting the casting machine and the rolling mill, the casting line and the rolling mill can be connected to each other on the same line, that is, by feeding the slab into the rolling mill without cutting it before the rolling mill (direct connection type) or by cutting the slab once, Two methods are considered: a method of forming into a shape and then sending it to the rolling mill (non-direct connection type). First, in the case of direct connection type,
In order to prevent cracks during rolling, 10
When attempting to hold for more than a minute, the casting speed of thin slab generally becomes a high speed exceeding 10 m / min, and therefore a holding facility of more than 100 m is required between the casting machine and the rolling mill. This means that the entire facility including the building will be lengthened, and it will jeopardize its realization not only in terms of capital investment costs, but also in terms of facility simplification, which is one of the purposes of thin slab production. is there. This also applies to the non-direct connection type. In addition, the non-direct-coupling type idea is that the casting machine has a smaller capacity than the rolling machine as described above, and therefore one rolling machine can cover the slabs coming out from several casting machines. Even in this case, it is obvious that the equipment such as a holding furnace required to hold the cast pieces successively sent from several casting machines can be remarkably simplified by shortening the holding time.

Further, the above is not limited to the case of producing thin slabs, but is also applicable to the case where the current continuous casting machine is modified or newly installed for the purpose of direct feed rolling or direct rolling.

As described above, the present invention aims at not only energy saving but also simplification of the capital investment necessary for the countermeasures against the surface crack of the billet in the straight-forward rolling or the direct rolling which is expected to further expand in the future.

(Means for Solving Problems) By the way, the present inventor has also conducted various basic studies in order to prevent cracking during direct feed rolling or direct rolling. As a result, the hot workability It is very important to consider the rolling conditions in the evaluation of the above, that is, it is not only difficult to quantitatively evaluate with the conventional torsion test or tensile test method, but as long as it depends on such an evaluation method, it is sufficient. I found it difficult to establish a countermeasure against cracking. Therefore, as a result of conducting a basic study by a laboratory direct pressure experiment on the previous conventional technology, that is, the retention or gradual cooling method as a countermeasure against cracking, we obtained important knowledge about the technical issues required to shorten the retention time, which is the purpose. The present invention has been accomplished.

The first consideration is the form of holding. Naturally, the slab is very hot when it leaves the casting machine. By the time the rolling is started, it is cooled by several hundreds of degrees Celsius, though it depends on the use and purpose. Therefore, it is an important subject to consider in which temperature range and during which heat history it is more advantageous to perform holding.

In order to investigate the above items, a basic examination was conducted by a laboratory direct pressure experiment. The main experimental conditions are JIS
・ Using a slab of thickness 40 mm × width 200 mm × length 300 mm, which is equivalent to SPHC, an examination was conducted with a pass schedule of 40 → 20 → 10 → 5 mm using an 850 mm φ ・ 2Hi mill.

Figure 1 shows the relationship between the holding conditions and the occurrence of cracks when rolling is started immediately after holding. In this case, 950
It was found that cracking can be prevented by holding for 2 minutes or more in the temperature range of ℃ or more. On the other hand, in actual operation, the rolling start temperature is naturally determined from the viewpoint of product quality, etc., and a predetermined time is required to convey the slab between holding and rolling, and the slab is cooled during that period. Considering the above, it is difficult to satisfy the conditions shown in FIG. 1 in actual operation.

Therefore, next, the rolling start temperature is fixed at 1000 ° C (however, 1000 ° C
In the following holding, the case where rolling was started immediately after holding was examined. As shown in FIG. 2, in the case of holding at a temperature higher than the rolling start temperature, it is air-cooled after holding and cooled to a predetermined rolling start temperature. In the case of such a heat history, the higher the holding temperature, It became clear that the holding time required to prevent cracking was long.

The above results presume that, when the cast slab is air-cooled after being held, a phenomenon that deteriorates the hot workability occurs again in the cooling process.

Therefore, in order to clarify this point, the following study was conducted, that is, holding at an arbitrary temperature of 1200 to 950 ° C for 2 minutes, then air-cooling the slab once and then rolling. By changing the air cooling time from the start to rolling
The relationship with cracks was investigated. As a result, it was found that there are two types depending on the holding temperature, as shown in Fig. 3.

In Type I, hot workability is deteriorated by air cooling for a short time after holding, whereas in Type II, hot workability is not deteriorated by air cooling for a while after holding. As a result of this investigation, it was found that the tendency of type II was exhibited in the holding temperature range of 1050 to 950 ° C., and the tendency of type I was exhibited when holding was performed at a higher temperature.

This means that morphology change or coarsening of sulfide due to holding progresses promptly, so if rolling is performed in that state, cracking does not occur, but if it is air cooled after holding, it precipitates before and during holding. It is shown that cracks are likely to occur because unsaturated sulfide precipitates in the γ grain boundary in a form that deteriorates hot workability again in the cooling process. Moreover, the higher the holding temperature is, the less sulfides are precipitated before the start of holding. Therefore, in order to prevent deterioration of hot workability due to subsequent cooling, it is necessary to lengthen the holding time. On the other hand, when the material is cooled to 1050 ° C or less and held, the sulfide precipitation is almost completed at the holding time. Hot workability is improved by coarsening,
Moreover, even if it is subsequently air-cooled, the precipitation of new sulfide no longer occurs, and therefore the hot workability does not deteriorate again.

Therefore, the condition showing the tendency of type II, that is, once
By holding at a temperature of 50 ° C. or lower for holding, even if the cast piece undergoes a cooling process after holding, cracking can be prevented by holding for a significantly shorter time than in the conventional method.

Under the conditions shown in FIG. 1, cracking does not occur even if held at a temperature of 1200 ° C. or higher, whereas under the conditions shown in FIG. 2, cracking occurs when the holding temperature is higher than 1200 ° C. This is because the initiation temperature of sulfide precipitation, which causes cracking, is 1200 ° C, and in the case of the conditions shown in Fig. 2, no sulfide has precipitated yet, no matter how much the temperature is kept higher than 1200 ° C. Therefore, this is meaningless, and sulfides are precipitated in the subsequent cooling process. On the other hand, in the case of rolling immediately after holding as shown in FIG. 1, at 1200 ° C. or higher, there is no meaning for holding, but cracks do not occur because no sulfide that causes cracks is deposited. Therefore, the type shown in Fig. 3
In the case of holding after cooling to 1050 ° C. or less as in II, the effective range of the holding temperature was similarly investigated. Fig. 4 shows the maximum after the slab has been cooled below 1050 ℃
The figure shows the state of cracking when the material was held for 10 minutes and 30 minutes at each temperature up to 1300 ° C., then air-cooled to 1100 and rolled. From the results of this investigation, it was found that cracks do not occur when held at 1300 ° C for up to 10 minutes, but cracks occur when held at 1300 ° C for 30 minutes. this is
This is because the sulfide that has been once precipitated re-dissolves during holding at 1300 ° C. for 30 minutes and then re-precipitates at the γ grain boundary during the subsequent cooling process. Based on the above, cracking can be prevented by holding for a shorter time compared to the conventional method by holding once after almost complete precipitation of sulfide, but do not re-dissolve sulfide when selecting the holding temperature. Such consideration is necessary. That is, once the slab is cooled to 1050 ° C or lower and the holding time is set to 10 minutes or less, the effective range of the holding temperature, particularly the upper limit, can be expanded to 1300 ° C as compared with the conventional method. This is of great significance in terms of actual operation in terms of giving flexibility to the transportation time, in other words, the transportation method, when considering cooling of the slab during transportation from holding to rolling. Even in such a case, the lower limit temperature of holding is the same as the result of FIG. 1 and FIG.
It is 950 ℃.

Furthermore, in Type II, when the time from holding to rolling start exceeds a certain value, the hot workability deteriorates sharply, but this is because the slab surface temperature at which the cracks occur is Ar ₃
This is because the number of points is less than the point, and proeutectoid ferrite is deposited in a band shape at the old γ grain boundary, and as a result, the tensile stress generated during rolling is concentrated in this portion and cracks are easily generated.

Therefore, in order to effectively utilize the crack prevention measures by holding, it is necessary to set the rolling start temperature to Ar ₃ point or higher.

As is clear from the above, in the prior art, only part of the holding characteristics was confirmed, and in order to reliably prevent cracking by holding in actual operation,
Many essences of the retention process to be clarified still remain.

The present inventor has clarified what kind of holding is performed by the above basic examination, and as a result, clarified the technical items necessary for shortening the holding time, which is the purpose of the present invention, and completed the present invention. It is a thing.

The present invention has been made in order to supplement and improve the problems of the prior art, and in order to clarify the 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, but in actual operation, the slab undergoes a cooling process once from normal holding to the start of rolling, so in this case the holding time is Need to be longer.

(2) This is a form in which, when the slab is air-cooled after holding, the sulfide that does not completely precipitate until the start of holding and during holding again reduces the hot workability during the cooling process. This is because precipitation occurs at the γ grain boundary, and therefore the slab is held for a short time, and in order to prevent cracking even when the cooling process is performed thereafter, in order to prevent cracking, precipitation of sulfide is newly added in the cooling process after holding. By preventing the occurrence of the sulfide, in other words, by substantially completing the precipitation of the sulfide by the start of holding, and holding the sulfide in a state where it does not re-dissolve, the effect is sufficiently utilized. In this way, even if the cast piece undergoes the cooling process after holding, in order to prevent cracking by holding for a short time,
It is effective to maintain the temperature at 1050 to 950 ° C, but in some cases it is necessary to set the rolling start temperature to the above-mentioned holding temperature or higher due to the requirements of product quality and the like. In that case, the temperature may be raised to the required temperature again after holding it in the temperature range of 1050 to 950 ° C for 2 minutes or more, but once the slab is cooled to 1050 ° C or less, it should be held at the required temperature of 1200 to 950. However, it is desirable from the viewpoint of energy saving. As will be described later, it is the surface layer portion of the slab that becomes the surface crack generation position that requires the temperature history, so in the latter case, only the slab surface temperature that becomes the crack generation position is cooled to 1050 ° C or less. By doing so, the other parts, especially the inside of the slab, are still at high temperature, so that even if the temperature is raised to the required temperature thereafter, the energy required by using the heat retained in the slab can be reduced. On the other hand, in the former case, since the entire slab is soaked during holding, much energy is required to raise the temperature of the entire slab to the required temperature again. In any case, one of the requirements required in the present invention is that the slab is once cooled to 1050 ° C. or lower (in this case, the slab surface temperature, and only the part at which the cracks are generated) is then used. Hold for 2-10 minutes in the temperature range of ~ 950 ° C.

(3) Since the sulfide undergoes a morphological change or coarsening due to holding, hot workability is improved, but if the slab surface at the position where cracking occurs after holding is less than Ar ₃ points, causes other than sulfide,
That is, the stress concentration on the α band along the old γ grain boundary makes the bending angle holding process meaningless. Therefore, hold time is 2
By ensuring only ~ 10 minutes, the deterioration of hot workability in the subsequent cooling process can be prevented, but it is necessary to start rolling at an Ar point of ₃ or higher.

As described above, the above knowledge is that not only is the original purpose of direct feed rolling or direct rolling to be energy saving,
This is significant because it also aims to simplify the equipment required for holding the slab.

The present invention has the above findings as its constituent features, and the gist thereof is that in a method of directly feeding or directly rolling a continuously cast slab, after casting, a cooling process following solidification of the melt. Then, the entire surface of the cast slab is once cooled to below 1050 ° C. and above 900 ° C. to precipitate sulfides at grain boundaries, and then held in the temperature range of 1300 to 950 ° C. for 2 minutes or more and less than 10 minutes, and It is a hot rolling method that prevents surface cracking of steel slabs, characterized by starting rolling at an Ar point of ₃ or more.

Here, "holding" includes not only holding at a constant temperature but also raising the temperature.

In the present invention, first cooling to 1050 ° C. or lower means that once the slab is once cooled to 1050 ° C. or lower as described above, precipitation of sulfide is completely completed, and sulfide is newly added during cooling after holding. This is because it does not precipitate. This is because unprecipitated sulfides may precipitate as fine sulfides during the subsequent holding process only by cooling to a temperature exceeding 1050 ° C, which may cause cracking.

In addition, as described above, the points that are once cooled to 1050 ° C or lower are
Since only the surface of the slab where the crack occurs is sufficient, it is effective to forcibly cool only that part with water, gas, etc. in the sense that the slab retained heat is utilized more during the subsequent temperature rise. .

Thus, the present invention, in the actual operation, even if the slab after holding undergoes the cooling process without cracking in holding for a short time, by setting the rolling start temperature to Ar ₃ points or more,
The effect of retention is effectively used to prevent cracking.

Further, in the present invention, since the essence of the surface cracking during direct feed rolling or direct rolling is the precipitation state of sulfides, it is held as a countermeasure, and further, 2
In order to prevent cracking by holding for a short time of up to 10 minutes, the temperature history of the slab is specified as described above.

The temperature described here indicates the surface temperature of the slab at the position where cracking occurs. This is defined as above because the slab temperature is generally not uniform in the thickness direction and the width direction, and the crack generation position also differs depending on the rolling conditions.

Next, the present invention will be described in more detail by way of examples, but they are merely provided as exemplifications of the present invention and do not limit the present invention in any way.

Example 1 C: ≤0.06%, Si: 0.04%, Mn: 0.15 to 0.30%, P:
≤0.030%, S: ≤0.030%, sol.Al: 0.020 to 0.050
% Composition (Ar ₃ points = 850 ° C), thickness 40 mm x width 600
After cooling the slab with mm shape by air cooling to T ₁ ℃, hold it at T ₂ for t minutes, and then directly feed it for 3 consecutive passes with a rolling ratio of 50% for each pass by a rolling machine with a roll diameter of 800 mm. It was subjected to rolling or direct rolling.

As shown in Table 1, cracking is prevented by satisfying the constitutional requirements of the present invention. The cracks generated under the rolling conditions of the present example are cracks on the end surface in the width direction of the steel slab, and therefore T ₁ shown in Table ₁ is the surface temperature of the end surface of the slab.

Example 2 C: 0.13 to 0.17%, Si: 0.25 to 0.45%, Mn: 1.25 to
1.50%, P: ≤0.030%, S: ≤0.030%, Nb: 0.020
~ 0.040%, V: 0.030-0.050%, sol.Al: 0.020-0.
Thickness 264mm x width with 050% composition (Ar ₃ points = 750 ° C)
A slab with a shape of 1240 mm was once cooled to T ₁ ℃, held at T ₂ for t minutes, and then rolled with a roll having a roll diameter of 1300 mm at a rolling reduction of 15% for each pass. Direct rolling or direct rolling was carried out under the condition of continuous 5 passes.

The results are summarized in Table 2, and cracking is effectively prevented by satisfying the constituent requirements of the invention. The cracks generated under the rolling conditions of the present example are cracks on the long side surface of the steel slab that come into contact with the roll, and therefore T ₁ shown in Table 2 is the surface temperature of the long side surface of the slab.

In the examples shown in Examples 1 and 2, the holding method is, for example, the case where 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 Fig. 4, the end surface of the cast slab was cooled to 950 ° C and then charged into a furnace kept at 1200 ° C.
And the holding time at that time is 2 minutes is the time from the time of charging into the furnace to the extraction, the temperature of the cast slab end face at the time of extraction is not raised to 1200 ℃, but the constituent requirements of the present invention As a result of being held in the temperature range of 1300 to 950 ° C for 2 minutes, cracking is prevented.

Example 3 Same composition, cast shape and roll diameter as in Example 1, pass
Direct-feed rolling or direct rolling was performed depending on the schedule conditions. At this time, in the cooling process subsequent to the solidification from the melt, when the steel was rolled from 1100 ° C without any holding, cracks occurred on the end faces of the steel pieces. Therefore, after cooling the slab end face, that is, the plate width end portion to 1000 ° C., the slab end face is subjected to an edge heater such as a gas burner method or an induction heating method
After heating to 1150 ° C, rolling was started from 1100 ° C. At this time, the time from the start of heating by the edge heater to the end is 5
It was a minute, and no cracking occurred. As described above, the holding by the edge heater is effective in that the holding can be performed locally and efficiently.

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

As described above, the term "holding" in the present invention includes not only the isothermal holding but also the isothermal process as described above, and is distinguished from the "retaining" in the conventional method, that is, the isothermal holding only. It is used as a meaning.

(Effects of the Invention) As described above, the present invention relates to retention as a countermeasure for surface cracking during direct rolling or direct rolling, by clarifying the essence of retention that has not been sufficiently clarified in the past, Elucidation of the technical items necessary for shortening the holding time, which is the purpose, and as a result not only further energy saving can be achieved, but also remarkable simplification of equipment is possible, which has a great effect on the actual operation of direct feed rolling or direct rolling It is a thing.

[Brief description of drawings]

1 to 4 are graphs in which the results of preliminary tests in the present invention are arranged according to holding conditions.

Claims (1)

[Claims] 1. A method of directly feeding or directly rolling a continuously cast slab, wherein the entire surface of the slab is once cooled to below 1050 ° C. and above 900 ° C. in a cooling process subsequent to solidification from a melt. After precipitation at the grain boundary, hold the material in the temperature range of 1300 to 950 ° C for 2 minutes or more and less than 10 minutes, and
A hot rolling method that prevents surface cracking of steel slabs, characterized by starting rolling at ₃ or more points of Ar.