JPS6149763A - Production of continuously cast ingot - Google Patents

Abstract

PURPOSE:To prevent the time-crack of a steel ingot and the time-crack of the ingot in a so-called direct rolling process, etc. by subjecting the surface layer part of the ingot to working under specific conditions applicable in practicable use during cooling of the ingot in the stage of continuous casting. CONSTITUTION:The steel ingot is subjected to >=5% working strain at >=2mm. depth in the surface layer part under the conditions (within the hatched region shown in the figure) of the strain speed epsilon(S<-1>) at epsilon>=a exp(bT) (where a=4X 10<-5>, b=4.6X10<-3>, T is the surface temp. of the ingot and 700 deg.C<=T<=1,200 deg.C) during cooling of the ingot in the stage of continuous casting. The ingot after the above-described treatment is passed through drawing rolls. The above-mentioned working strain is applied to the ingot by a method for using rolls formed by providing projections to the surfaces of, for example, guide rolls. The hot crack of the ingot during production of particularly the steel ingot is prevented by the above-mentioned method.

Description

Detailed Description of the Invention (Industrial Application Branch) The present invention provides a method for producing continuously cast slabs and a method for hot working thereof, particularly for preventing hot cracking of slabs during the production of continuously cast slabs. The present invention relates to a method and a method for preventing hot cracking of a steel billet in a so-called direct rolling process or a hot charge rolling process.

More specifically, the present invention is directed to medium-low coal HQ or A(!,
A method of continuously casting low-alloy steel containing 1% or less of alloying elements such as Nb, Ti, Ta5V, B, etc., and a direct rolling process in which the steel is rolled immediately without reheating immediately after continuous casting, or it is cooled to room temperature. The present invention relates to a method for preventing cracking of continuously cast slabs and steel slabs during hot rolling in a pot charge rolling process in which hot rolling is performed after reheating without any damage.

(Prior art) When producing medium-low carbon steel or low alloy steel as described above by a continuous casting method using a b-x curved continuous mirror casting machine, the continuously cast slab is mainly subjected to slab straightening. Surface cracks may occur due to bending stress applied at times or thermal stress caused by cooling (often, especially in Nb-containing steel). Since a pretreatment process is required, it is necessary to cool the product to near room temperature.Even in the case of a normal rolling process using cold slabs, the necessity of a treatment process complicates the operation. On the other hand, the occurrence of such cracks is a significant impediment to the practical application of direct rolling and hot charge rolling processes that aim to reduce costs through energy and labor savings.

Moreover, even if no flaws occur in the slab, cracks may occur during the direct rolling or hot charge rolling process, which also poses a significant obstacle to the practical application of these processes. Incidentally, the occurrence of such cracking is particularly remarkable in materials containing a high content of S as an impurity.

Therefore, in order to manufacture products stably and inexpensively by the direct rolling or real charge rolling process, it is necessary to avoid the occurrence of defects in slabs during continuous casting and the surface defects during the subsequent process of direct rolling or bot charge rolling. It is desired to establish a method to completely prevent the occurrence of each of these. On the other hand, even when continuous casting slabs are once cooled, reheated, and hot-worked, if there are no defects in the continuous casting slabs themselves, the flaw removal process becomes unnecessary, and the practical benefit is extremely large. Even in such cases, it is desired to establish a method that completely prevents the occurrence of defects during the production of continuously cast slabs.

First, as a method for preventing surface flaws occurring in such continuously cast slabs, Japanese Patent Application Laid-Open No. 128255/1983 discloses a method for continuous collision of shipped balls. however,
As stated on page 290.3 to 7 and below of the publication, this method is aimed at crimping cracks directly under the mold, removing trapped foreign matter, and preventing oxidation on the surface of the slab. Moreover, as is clear from FIG. 4 of the publication, this is just a process immediately below the mold and before entering the guide roll. Cracks will continue to occur after that, and as will be described later, this is not a complete preventive measure against the occurrence of cracks.

Furthermore, JP-A-54-155123 discloses a method of applying plastic strain to a slab, which involves adjusting the amount of plastic strain in the surface layer, slab temperature, and austenite grain size within a certain range. According to the findings of the present inventors, it is not possible to completely prevent the occurrence of defects with these conditions alone. Moreover, roll rolling, shot blasting, and laser pulses, which have been proposed as means for imparting such plastic strain, do not provide sufficient effects. That is, if a slab including an unsolidified portion is rolled down with a normal roll, the entire thickness of the solidified shell will simply be depressed, and strain cannot be imparted to the surface layer of the slab. In addition, the depth at which shot blasting can apply strain is too shallow to be effective, and the shot recovery method has many problems, making it unrealistic.''Furthermore, the method using laser pulses is The idea is to apply heat to micrometers and apply strain due to the temperature difference between the inside and the inside, and it is almost impossible in principle to apply this method to hot slabs because the temperature difference is small.

Furthermore, since there is cooling water on the surface of the slab, the results are even weaker, making it extremely difficult to apply to actual production lines.

In addition, as a means to prevent the occurrence of defects during hot rolling in direct rolling or hot charge rolling processes following continuous casting, cooling rate during continuous casting is Countermeasures have been proposed, such as controlling the cooling rate, but since such methods require an extremely long time to complete cooling as they control the cooling rate to be slow, even though it is theoretically possible, it is difficult to implement it in actual operation. There are many problems in its application.

(Problems to be Solved by the Invention) Thus, an object of the present invention is to completely prevent cracks as surface defects that occur during the production of continuously cast slabs and when they are subjected to direct rolling or bot charge rolling. The objective is to enable stable operation of such a process and to significantly reduce costs.

The present inventors have discovered that cracks as surface defects are caused by thermal stress and stress applied to continuously cast slabs in the low-temperature austenite (r) region during the cooling process, and in some cases in the coexistence region with ferrite (α). This is caused by low strain rate deformation such as external stress applied to the slab during straightening of the slab in such a temperature range (Mat, Sci,
Eng, + 62 (1984) p, 109-119
, and Trans, JIM, 25 (1984)
p-160 to 167), and also discovered and announced that during hot rolling, this occurs due to high strain rate deformation in the relatively low-temperature γ region, and that both are caused by the destruction of γ grain boundaries.

The embrittlement of the material during low strain rate deformation is caused by AQN and NbC.
Alternatively, carbonitrides such as TaC5Tic%VN may continuously precipitate at the γ grain boundaries during deformation, and may also precipitate finely within the grains, or even relatively soft ferrite (α) may be present at the grain boundaries.
precipitates in the form of a film, and the inside of the grain becomes relatively strengthened, and strain concentrates in the precipitate-free zone along the γ grain boundaries and the soft part of the film-like α, resulting in interfacial peeling between the grain boundary precipitates and the matrix and matrix. (Mat, Sci, Eng, +
62 (1984) p, 109-119, Tra
ns, JIM, 25 (1984) p, 16.
0-167).

In addition, the embrittlement during high strain rate deformation observed during hot rolling is due to the continuous precipitation of (Fe, Mn)S at one grain boundary during deformation and the intragranular strengthening due to the precipitation of m-rings within the grain. This is also caused by In this case, if γ-grain boundary continuous precipitation and intragranular precipitation of carbonitrides occur before this high strain rate deformation,
(Fe, Mn) The embrittlement caused by S is significantly accelerated.

Therefore, in order to prevent embrittlement due to γ grain boundary cracking in both processes, the γ grains must be made finer to reduce the susceptibility to grain boundary embrittlement, or precipitates must be removed by the time of problematic deformation (for example, during slab straightening and rolling). γ grain boundary precipitation and intragrain fine ♀ during deformation by coarsening
■Precipitation can be prevented. However, the reality is that sufficient countermeasures are not currently being taken due to equipment and operational constraints. For example, agglomeration and coarsening of precipitates can be achieved by reducing the cooling rate or maintaining constant temperature during cooling [for carbonitrides, see Mat, Sci, E
NGO, Roku (1984) p. 109-119; for sulfides, see JP-A No. 58-52442], but this is not practical because it takes an order of magnitude longer time for cooling and significantly impairs productivity. There is also a proposal to refine grains by recrystallizing one grain (see JP-A-54-155123), but since the original γ grains are extremely coarse, they cannot be used as recrystallization nuclei. Since the area of the field is small, it is necessary to apply a large strain, and as stated in JP-A-54-155123, to obtain fine crystal grains with a grain size of 0.1 mm or less, a plastic strain of at least 40% or more is applied. This is necessary, and it is virtually impossible to apply large strains to the slab including the unsolidified portion.

Also, considering the embrittlement mechanism mentioned above, it is possible to suppress the occurrence of surface defects by adjusting the chemical composition of the steel, but the chemical composition of the steel must be added to the material and to give the desired properties. There are many restrictions as there are some things that cannot be obtained, so it is not a fundamental countermeasure. For example, to prevent precipitation of A2N, AQ,
It is possible to improve ductility by reducing H or by adding Ti to fix N in one grain as TiN, but these reductions come with an increase in cost, and addition of Ti can be harmful, such as impairing the toughness of the weld. many. Further, addition of Nb, etc. is essential for ensuring product quality, and it is impossible to take countermeasures by changing it. Reducing S is also effective, but it is accompanied by an increase in cost and does not necessarily lead to a reduction in total cost.

(Means for Solving the Problems) The present inventors have devised a method for solving the agglomeration and coarsening of carbonitrides and sulfides in a practical and short time without using the above-mentioned ultra-slow cooling of slabs or constant temperature maintenance during cooling. After repeated research into methods to achieve this goal, they discovered that the objective could be achieved if the surface layer of the slab could be processed under specific conditions that were practically applicable while the slab was cooling. As described above, the present invention was made for the first time through this discovery, and is completely different from the conventional method both metallurgically and essentially in terms of technical concept and effect.

Here, the gist of the present invention is to apply a processing strain of 5% or more to the surface depth of the slab during continuous casting when the strain rate 2 (S-1) is ε≧a exp ( bT) (However, a-4X10-5, b = 4.6 Xl0-'
, T is a method for producing a continuously cast slab, characterized in that the slab is subjected to a surface temperature of 700°C≦T≦1200°C) and then passed through a drawing roll. The continuously cast slab thus obtained may be directly hot worked without being reheated, or may be reheated without being cooled to room temperature and then hot worked. Here, hot working refers to all hot working such as forging in addition to normal rolling.

As described above, according to the present invention, a working strain of 5% or more is applied to a surface layer depth of 2 mm or more under the conditions within the shaded area in FIG. Thus, hot cracking of the slab during continuous casting, and furthermore, hot cracking of the steel slab during so-called direct rolling and hot charge rolling can be effectively prevented.

Here, the principle of the present invention will be further explained based on experimental data as follows.

That is, steel having the composition shown in Table 1 was prepared, and tensile test pieces were taken from it to conduct the following experiment.

Table 1 and FIG. 2 are explanatory diagrams showing various processing and heat treatment conditions employed in this experiment.

First, in order to simulate the machining applied to continuously cast slabs during the cooling process, the material solution-treated at 1350°C was heated to 850 °C at the same strain rate (2~1O-3s-) as during slab straightening.
We investigated the influence of the aperture value (RA) when tensile deformation is performed at ℃ and the holding time when isothermally held at 1100 ℃ as a treatment equivalent to slow cooling prior to this tensile deformation (Figure 2 Case ■, (see ■, ■, and ■).

The test results for steel A containing Nb are summarized in a graph in FIG. In case ■ corresponding to the conventional method, I? A is extremely small, and in case ■, it is 11
Ductility does not improve unless the temperature is maintained at 00°C for a long time. However, as in cases ■ and ■, 10
% machining strain at 2=xo-IS-1, the short-time 1
Holding at 100°C greatly improves ductility. When the surface layer of the slab is processed at a low temperature, it is preferable to slow down the cooling and reheat afterward, as in case ①. A similar graph obtained for B steel is shown in Fig. 4. Case ■ is 1
The case with 0% pre-processing is shown.

Therefore, it can be seen that processing the surface layer of the slab is effective in preventing cracking.

Next, we investigated the effects of strain N (ε) and strain rate (ε) in pre-processing. In case ■ in Figure 2; Figure 5 summarizes the results of investigating the effect of g (strain amount) on the RA value when pre-processing at ~10''s-1 and holding at 1100℃ for 10 minutes. It was also found from the results shown in i that if the pre-processing strain was as low as 5%, a reduction of area of 50% or more could be obtained, and slab cracking was extremely unlikely to occur. A trend was observed.

In addition, the influence of ε (strain rate) is also large; for example, when Atl is held at 1100°C for 10 minutes in cases ① and ① in Figure 2, 10% pre-processing is performed at 1100°C and 900°C, respectively. , the strain rate and R at that time
The relationship with the A value is summarized in a graph in Figure 6. From Figure 6, the larger the ε, the greater the effect. At the pre-processing temperature of 1100℃, ε≧10-2s-', and at 900℃. It can be seen that ε≧3×10−3s −,′ is required.

Next, the relationship between cracking during direct rolling after slab straightening was evaluated. The results are shown graphically in FIG. 7 as a relationship between RA value and isothermal holding time. In case ■, which is the conventional method, steels A, B, and C all showed low RA values, while case ■, in which only isothermal holding treatment was performed prior to processing.
It is not always possible to obtain a large improvement effect. On the other hand, in case 2, in which pre-processing with approximately 10% strain was performed, an RA value of 50% or more was obtained with isothermal holding within 4 minutes after pre-processing, indicating that ductility could be greatly improved in both cases. .

Next, regarding the reason for limiting the processing conditions in the present invention, go to 12.
I will clarify.

The reason why the area to which processing strain is applied is limited to 2 mm or more of the surface layer of the slab is based on the knowledge that flaws that occur within 2 mm from the surface remain as cracks or cracks in subsequent processes.

The reason why the amount of machining strain is limited to 5% or more and the strain rate is limited as shown in the above equation is that the amount of machining strain is limited to 5% or more, and the strain rate s (5-'') is; ≧a ex
This is based on the reason that unless p (bT) is present, it is difficult to nucleate the precipitates. In other words, in order to accumulate strain at such high temperatures, precipitation nuclei must be generated before the introduced dislocations recover, and as the temperature increases, ε must be increased to increase the strain. We must aim to accumulate, and the conditional force at that time <
g≧aexp(bT). Also, the lower limit of T is 700°
The reason for choosing C is that if the slab has already been cooled to a lower temperature, it will be difficult to coarsen the precipitates even with subsequent reheating.On the other hand, if the slab temperature is 120
This is because if the temperature exceeds 0°C, the introduced dislocations will recover extremely quickly, making it impossible to form the desired precipitates. The conditions for surface processing of the cast slab in the present invention are shown as the shaded area in FIG.

Furthermore, the optimum conditions for ε and T will be explained using FIG. 8. Figure 8 is a graphical representation of the embrittlement tendency observed in the slab when strain processing performed prior to slab straightening was simulated by tensile deformation under a series of processing conditions. , the shaded area is the area showing RA of 50% or less. The composition of the sample steel at this time was as follows.

CSi M Rainbow -L 2 l-N NbO,
120,200,410,0100,0150,040
, 00500, 045, in region (B), precipitation of carbonitrides occurs at the γ grain boundaries and within the grains, and there is a risk of cracking in some cases during metal machining of the surface layer of the cast slab. region(
In D), there is a risk of cracking due to the precipitation of sulfides, so it is preferable to avoid it. Area (A), (
Regions C) and (E) are safe, but region (A) is not preferable in this case due to the distortion of 1ffff as described in FIG.

FIG. 8 shows the phenomenon when tensile deformation is applied to Nb-containing steel as described above. If the target steel type or processing method is different, the area (D) may disappear. For example, high Mn
Alternatively, in low S steel, the DgJ region is less likely to occur, and the D region also disappears when the stress is compressive reduction.

In the present invention, as a processing method for imparting processing strain as described above, for example, the use of a roll with protrusions on the surface of a guide roll, an air hammer, a special hydraulic press, etc. can be considered, and the required processing strain and strain rate can be considered. Other methods may be adopted in some cases as long as they can be achieved.

The steel types to which the present invention is applied are not particularly limited, but continuous casting slabs include AQN, NbC, TaC, TiC, [IN, V
This is particularly effective for steel types that are prone to surface flaws that are likely to be caused by precipitation of N or the like. On the other hand, in a component system in which carbonitrides are difficult to precipitate, a great effect can be obtained in preventing surface defects mainly caused by sulfide precipitation during direct rolling or hot charge rolling.

It is also conceivable to further promote the coarsening of precipitates by utilizing recuperation from inside the slab after surface processing is completed, but such a method is of course included in the present invention. This method is particularly effective in cases where the slab must be strongly cooled due to internal cracks or other problems.

Scrap ±1 This example is intended to demonstrate by a simulation method that by applying processing strain in advance, cracks will not occur in the slab even during subsequent rolling processing.

Ifii2 with a thickness of 40 mm and a composition shown in Table 2
A slab having a length of 20+nn+ and a length of 600 m- was cast, and half of the slab surface was subjected to processing strain under the conditions shown in Table 3 using a small electric hammer with a flat tip. The strain rate at this time was approximately 50 s-'. The amount of processing strain was approximately 20% on average for the five surface layers. Next, the thus IMed slab was subjected to hydraulic bending, and the presence or absence of surface cracks was visually inspected. In the case of steel type ■, deep cracks with a length of 20 to 50 mm were found in the parts that were not subjected to processing distortion using an electric hammer.
Further, in the case of steel type (III), horizontal cracks with a length of 5 to 1 degrees occurred, but no surface cracks were observed at all in the areas where processing strain was applied.

FIG. 16 is a photograph showing the occurrence of cracks after bending of steel type 1 (test 1). The left half is the part that was pre-processed with a hammer prior to bending, and the right half is the part that was not pre-processed. A large crack is seen on the right half, but no surface cracks are seen on the left half.

Table 2 Table 3 Yu M Adhesive Using a curved continuous casting machine with a radius of 12.5 m at the N steel factory, slabs with a cross section of 250 mm x 2100 mm were cast under different conditions, and the cast pieces after straightening were cast. The degree of occurrence of surface flaws on the pieces was visually evaluated. Figure 9 shows a slab striking device whose power source is a hydraulic cylinder that moves in the width direction of the slab between rolls on the upper surface of the slab, which was used to apply processing strain to the surface layer of the slab at this time. In the illustrated example shown together with a casting line, machining strain is applied to the slab 1 by the slab striking device 2 at a stage where there is a portion of unsolidified molten steel. The slab striking device consists of an indenter 3 and a hydraulic cylinder 4 connected to it.
These are passed through a hydraulic unit 5 and a hydraulic pump unit 6, and a controller 7 controls the amount of impact and the like.

FIG. 10 shows the state of the surface layer of a slab subjected to processing strain by an indenter attached to the tip of a slab striking device. The indenter spherical surface was 115 am, the depth of reduction was 3 dragons, the number of blows was 180 cycles/min, and the average strain in the 3 mm surface layer of the slab was 7% and the strain rate was 0.3 s-'. . Fourth
The table shows the composition of the steel used in this example, and Table 5 shows the casting conditions and results.

This did not occur.

Using a curved continuous casting machine with a radius of 12.5 m at the Sharp Plate Main Steel Factory, a slab with a cross section of 250 mm x 2100 mm and the chemical composition shown in Table 8 was cast under different conditions as shown in Table 9, and after straightening. The surface defects of the cast slabs were visually evaluated. In addition, at this time, as shown in Fig. 12, the method for applying processing strain to the surface layer of the slab is to replace the upper guide roll between 9 and 11 m from the scene with a roll with protrusions shown in the same figure. The temperature pattern shown in Figure 13 was used. At this time, the protrusion having the shape shown in Fig. 12 is formed on the surface of the solidified shell with a thickness of 0.06 to 79 mm.
It sinks in with the static iron pressure of 0.07 kg/mm2 as a reaction force, and the strain spreads as shown in Figure 14. From the formula calculated by the following formula, at least 7% strain is applied to the depth of 5 mm in the slab surface layer. was completed.

The strain rate was estimated to be 2xlO-'s-'.

! + = (Z +0.5)-1/JΣ×aS =
(1,8-2.2) x a, and in order to give a minimum strain of 5%, a = 7 mm + 1 (= 3 mm) was required.

As shown in Table 9, according to the continuous casting machine equipped with the roll with projections according to the present invention, impressions of projections remained on the slab surface, but no cracks or defects occurred at all. In the conventional method that does not use a roll with projections, cracks and defects occur frequently, so the effect of the present invention is clear.

Table 8 Table 9 Since it was found that this method could prevent the occurrence of surface flaws in continuously cast slabs, the effect of preventing cracks during direct rolling was further tested. Steel A shown in Table 10, melt the mouth,
In the above method, FIGS. 9 and 10 of the slab are as follows:
A schematic explanatory diagram of a slab striking device as a means for imparting working strain: Figure 11 is a graph showing the temperature pattern of the slab when it is reheated after being worked; Figure 12 is a graph showing the temperature pattern of the slab when it is reheated after being worked; A schematic explanatory diagram when using a roll with protrusions; Figure 13 is a graph showing the temperature pattern of the slab when processing strain is applied with a roll with protrusions; Figure 14 is a graph showing the temperature pattern when using a roll with protrusions. A schematic explanatory diagram of the propagation of processing strain; Fig. 15 is a graph showing the slab surface temperature pattern when the slab is cut after applying processing strain and further hot worked (rolled); and Fig. 16 Figure 2 is Photo 2 showing the occurrence of surface cracks after bending in the inventive example and the conventional example.

1: Slab 2: Slab striking device 3: Indenter 4: Hydraulic cylinder 5 Two hydraulic units 6: Hydraulic pump unit 7: Controller Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent attorney Akira Hirose − Hata / Figure Roe Distortion U, and (S-ri 1r1φ Kite 3 leap Δ Figure 5 Figure 5, -1I''/,) Collection 6 Leap and (s-Ri also 7 concave t (min) Book 8 Concave king road, Shi 1 parking (5-ln#q Figure #IO figure tail 12 Figure hole 11! Menis fI entry Q゛ et al. 1) alil! (m) Parent 13 figure father Nis or stt et al.'s distance turtle (m) for 14 ro sin, 15 Kouichi 1 Gomon-kei O-Stripe Darkness (Association) Procedural Amendment Kido Shiki) March 18, 1985 Manabu Shiga, Commissioner of the Patent Office 2, Name of Invention Method for Manufacturing Continuously Cast Slabs 3, Relationship with the person making the amendment Patent applicant address 5-15 Kitahama, Higashi-ku, Osaka Name (211) Sumitomo Metal Industries Co., Ltd. 4, Agent ＾danshi, Masaori Orokuryo) + $l'!'7 Day 4 -7Jft
Yohake J to 2 city°I (stuttering AJt, l mato・Delete the first line.

(2) On page 27, line 10 of the same book, the words "... explanatory drawing;" are corrected to r... explanatory drawing; and 1.

(3) In the same book, page 27, lines 13-15, the phrase "...; and Figure 16 is..." is replaced by r...
・Correct it as j.

(4) Figure 16 of the attached drawings will be deleted.

that's all

Claims (3)

[Claims] (1) 5% on the surface layer depth of 2mm or more of slab during continuous casting
For the above processing strain, the strain rate ■(s^-^1) is ■≧aex
p(bT) (where a=4×10^-^5, b=4.
6×10^-3, T is the slab surface temperature, 700℃≦T≦1
200° C.) and then passing it through a drawing roll. (2) 5% on the surface layer depth of 2 mm or more during continuous casting
For the above processing strain, the strain rate ■(s^-^1) is ■≧aex
p(bT) (where a=4×10^-^5, b=4.
6×10^-^3, T is the slab surface temperature, 700℃≦T≦
1. A method for hot working continuous cast slabs, which comprises applying the continuous casting slabs under conditions (1200°C) and then passing them through a drawing roll, and directly hot working the obtained continuous cast slabs without reheating them. (3) 5% on the surface layer depth of 2 mm or more during continuous casting
For the above processing strain, the strain rate ■(s^-^1) is ■≧aex
p(bT) (where a=4×10^-^5, b=4.
6×10^-3, T is the slab surface temperature, 700℃≦T≦1
200°C) and then passed through a drawing roll, the obtained continuous cast slab is reheated without cooling to room temperature, and then hot worked. Hot processing method.