JPH038863B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention is aimed at preventing the segregation of impurity elements found in the center of the thickness of continuously cast slabs, such as sulfur, phosphorus, and manganese in the case of steel slabs, and obtaining a homogeneous metal. This relates to a continuous casting method that can be used. (Prior art) In recent years, requirements for material properties for offshore structures, storage tanks, steel pipes for oil and gas transportation, high-tensile wire rods, etc. have become more severe, and providing homogeneous steel materials has become an important issue. There is. Originally, steel should be homogeneous in its cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, and these segregate and become partially concentrated during the casting process. becomes vulnerable. Particularly in recent years, continuous casting methods have become popular for purposes such as improving productivity and yield and saving energy, but noticeable component segregation is usually observed in the center of the thickness of slabs obtained by continuous casting. . Such component segregation significantly impairs the homogeneity of the final product and causes serious defects such as cracking due to stress acting on the steel during the product usage process and wire drawing process, so there is an urgent need to reduce it. . Such component segregation is caused by the residual molten steel flowing at the final stage of solidification due to solidification contraction force, etc., washing out the concentrated molten steel near the solid-liquid interface, and the residual molten steel progressively evolving. Therefore, in order to prevent component segregation, it is important to eliminate the cause of the flow of residual molten steel. Causes of such molten steel flow include flow caused by solidification shrinkage, slab bulging between rolls, and flow caused by roll misalignment, but the most important cause of these is solidification shrinkage, and segregation is To prevent this, it is necessary to reduce the slab by an amount that compensates for this. Attempts to improve segregation by rolling down slabs have been made for a long time; for example,
As described in Publication No. 16862, during the continuous casting process, the slab is heated at a constant rate greater than the amount that compensates for solidification shrinkage while the temperature at the center of the slab reaches from the liquidus temperature to the solidus temperature. A method of rolling down is known. However, in this case, depending on the conditions, there is a problem that almost no segregation improvement effect is observed, and in some cases, segregation may even worsen, making it difficult to sufficiently improve component segregation. . The present inventors conducted various investigations into the causes of such problems in the conventional method, and found that the reason why the conventional method does not have an effect on improving segregation or causes segregation to worsen is basically due to pressure. It was found that this was due to the inappropriate coagulation timing range, and that the following three facts were not taken into consideration. One of these is that segregation is worsened by mechanical factors such as roll misalignment and roll bending, and the negative effects thereof become more pronounced as the reduction amount increases. The segregation improvement effect of rolling the slab is obtained as the difference between the segregation improvement effect due to solidification shrinkage compensation and the adverse effect due to segregation worsening due to mechanical factors.If the mechanical factor is large, the negative effect is due to solidification shrinkage compensation. This may outweigh the segregation improvement effect and actually worsen the segregation. The second fact is the amount to be reduced. The amount of reduction must be an amount that justly compensates for solidification shrinkage, and if a reduction exceeding this value is applied, segregation will worsen again. Another fact concerns linear segregation. Linear segregation is a type of segregation in which the high-concentration part at the center of the slab in the thickness direction is narrow and continuous in the casting direction when the slab is viewed in a cross section cut parallel to the casting direction. When observed in a plane parallel to the plane, the segregated areas are connected in a mesh pattern. Linear segregation remains in the product after rolling, making the product brittle because continuous high-concentration areas serve as preferential propagation paths for cracks. Linear segregation is a form of segregation that occurs when slabs are reduced excessively at the final stage of solidification.In order to achieve the segregation improvement effect of light reduction, the form of segregation should be avoided to become linear, and dispersed spot-like forms should be avoided. It must be in the form. (Problems to be Solved by the Invention) An object of the present invention is to solve the problems of the conventional method and provide a continuous casting method for obtaining a homogeneous steel material. (Means for solving the problems) The gist of the present invention is as follows. (1) In a continuous casting method for molten metal in which slabs are continuously drawn, roll heat warpage occurs in the area from the point where the solid phase ratio at the center of the slab thickness reaches 0.1 to 0.3 to the point where the solid phase rate reaches the flow limit. A continuous casting method characterized by continuously rolling down the slab at a rate of 0.5 mm/min to 2.5 mm/min using rolls with a volume of less than 0.5 mm. (2) In a continuous casting method for molten metal in which slabs are continuously drawn, roll heat warpage occurs in the area from the point where the solid phase ratio at the center of the slab thickness reaches 0.1 to 0.3 to the point where the solid phase rate reaches the flow limit. A continuous casting method characterized by continuously rolling down slabs at a rate of 0.5 mm/min to 2.5 mm/min using rolls with a rolling weight of less than 0.5 mm and a roll wear amount of less than 0.5 mm. The present invention will be explained in more detail below. Although the light reduction method disclosed in the above-mentioned Japanese Patent Publication No. 59-16862 is an effective method for obtaining slabs without center segregation, the light reduction method is effective according to the findings of the present inventors. What is extremely important in the law is this area to be reduced. That is,
In order to reduce center segregation, the center of the thickness of the slab should solidify in the region from the time when the solid fraction reaches the flow limit solid fraction of 0.1 to 0.3 (hereinafter, this region is referred to as stage-2). It is important to continuously reduce the slab so as to compensate for shrinkage in just the right amount. Here, the flow limit solid fraction is the upper limit solid fraction at which molten steel can flow, and is a value of 0.6 to 0.8. Center segregation occurs due to the flow of molten steel within the solid-liquid coexistence region, that is, the region between the time when the center of the slab reaches the liquidus temperature and the time when the center reaches the solidus temperature. According to their findings, the segregation improvement effect of applying reduction to slabs is large in the downstream region where the central solid fraction is high, and small in the upstream region. This is because the molten steel supplied from the upstream side to compensate for solidification shrinkage on the downstream side is mainly molten steel near the center of the thickness where the flow resistance is the smallest in the thickness direction of the slab, but the concentration of molten steel near the center of the thickness is This is because as the central solid fraction increases, the higher the concentration of molten steel becomes, the lower the downstream region, the more concentrated molten steel is attracted to the final solidification zone, which has a greater adverse effect on center segregation. On the other hand, in the upstream region, the concentration of molten steel in the center is low, so the influence of molten steel flow on center segregation is small, in other words, the effect of reduction on segregation improvement is small. By the way, the present inventors discovered the following fact from numerous experiments. That is, in general, the distance between the upper and lower rolls of a continuous casting machine that forms a pair with each other often deviates from a set value during casting (this deviation is hereinafter referred to as dynamic misalignment). This dynamic misalignment is caused by bearing play, differences in reaction force in the axial direction of the slab, deflection of the rolls, thermal warping of the rolls, etc. The larger the amount of reduction, the larger the amount, which generates new flow,
worsen segregation. The effect of improving segregation by rolling down the slab is obtained as the difference between the effect of improving segregation due to solidification shrinkage compensation and the reverse effect of worsening segregation due to increasing dynamic misalignment. The former's segregation improvement effect is large in the downstream region and small in the upstream region, so if the reduction is applied in the upstream region, the adverse effect of worsening segregation due to dynamic misalignment will exceed the segregation improvement effect due to solidification shrinkage compensation, and the segregation will worsen instead. things happen. The inventors have determined from numerous experiments that the boundary is
This is the point at which the solid phase ratio in the center is 0.1 to 0.3,
It has been found that in a normal industrial-scale continuous casting machine, center segregation may be worsened by rolling down the slab upstream from this point. The degree of deterioration becomes more significant as the continuous casting machine is poorly maintained, the degree of dynamic misalignment is significant, and the reduction amount is large. In other words, in the region upstream from the point at which the solid fraction in the center reaches 0.1 to 0.3 and downstream from the point at which the center reaches the liquidus line (hereinafter this region is referred to as stage-1), center segregation due to light pressure reduction occurs. If the improvement effect is small and the dynamic misalignment is not managed to be extremely small,
Since center segregation may worsen, it is basically better not to carry out the reduction, and if it worsens, it is desirable to reduce the amount of reduction to less than 0.5 mm/min. In addition, in the normal rolling area, it is necessary to have a roll support structure that can withstand the rolling reaction force, which increases equipment costs, so not rolling down the above area has the economic effect of reducing equipment costs. Become. In the area downstream from the point at which the center of the thickness of the slab reaches the flow limit solid phase ratio and upstream from the point at which the center becomes solid (hereinafter referred to as the stage), the unsolidified molten steel at the center of the thickness becomes solid. Since they are isolated from each other and blocked by the phase, molten steel flow due to solidification and compression cannot occur, and therefore there is no need to reduce the steel. On the other hand, if a temperature reduction is applied to the slab in this region, the form of center segregation becomes linear segregation, which is harmful to the product properties. In order to obtain a dispersed, fine, spot-like segregation morphology that is most advantageous for product properties, it is basically preferable not to reduce the material in this region, and if rolling is performed, the amount of reduction should be less than 0.5 mm/min. It is desirable to do so. From the above, in the present invention, the region to be rolled is defined as the region from the time when the solid fraction in the central part of the slab reaches 0.1 to 0.3 to the time when the solid fraction reaches the flow limit. However, if the dynamic misalignment is extremely small and the adverse effect of reduction can be almost ignored, or if the amount of reduction is 0.5
If it is within a range of less than mm/min, it is permissible to reduce the pressure on the upstream side of the region (stage-1) as well. Further, if the linear segregation form is not harmful in terms of product characteristics, or if the reduction amount is within a range of less than 0.5 mm/min, the downstream stage may also be reduced. Stage-1 according to the present invention,
Fig. 1 shows the relationship between the rolled state and the solidified state in each region of -2. Next, the amount to be reduced will be explained. In addition to segregation in the center, continuous cast slabs usually exhibit V-shaped segregation (V-segregation) as shown in FIG. 2. Since this V segregation is caused by solidification shrinkage, by observing the number of occurrences, it can be determined whether the reduction amount is sufficient for the solidification shrinkage amount. The present inventors discovered the following two facts by observing such phenomena.
One of these concerns the concept of rolling reduction. What is important in compensating for solidification shrinkage is not the rolling reduction per roll (unit: mm), but the range of several meters near the crater end (solidification tip). I learned that this is the average rolling speed (mm/min). The rolling speed here refers to the amount by which a given point on the slab is rolled down per unit time during the process of passing between a plurality of rolls. When setting the roll spacing in actual operation, the value obtained by dividing the above rolling speed by the drawing speed,
That is, the amount of reduction per unit length in the casting direction (that is, the amount of narrowing of the roll interval) can be determined from the reduction gradient (unit: mm/m). Another fact concerns the amount of reduction (hereinafter referred to as the appropriate amount of reduction) to compensate for solidification shrinkage in just the right amount.
If the reduction amount is too small compared to the appropriate reduction amount, V segregation will occur in the casting direction, but if the reduction amount is too large, V segregation will occur in the opposite direction to the casting direction (i.e. in the direction of the meniscus).
V segregation toward (hereinafter referred to as inverse V segregation) occurs.
The appropriate rolling reduction amount is defined as the rolling reduction amount at which neither V segregation nor reverse V segregation occurs. The appropriate amount of reduction varies depending on the thickness, width, and cooling conditions of the slab, but in the present invention, when the amount of reduction is less than 0.5 mm/min, V segregation occurs, and when it exceeds 2.5 mm/min, reverse V segregation occurs. Based on the experimental fact that it causes center segregation and worsens center segregation, it is specified to be 0.5 to 2.5 mm/min. Preferred ranges are usually 0.5 to 1.5 mm/min for slabs and 1.0 to 2.5 for blooms or billets.
mm/min. Next, we will discuss measures to make the center segregation improvement effect of rolling the slab more noticeable. As mentioned above, the segregation improvement effect by rolling the slab is obtained as the difference between the segregation improvement effect due to solidification shrinkage compensation and the reverse effect of worsening segregation due to increasing dynamic alignment irregularities. Therefore, dynamic misalignment must be kept as small as possible. Examples of such alignment irregularities include roll spacing setting errors and bearing play, which have been managed to a sufficiently low level than in the past. In addition to these static misalignments that can be measured before casting, we
It was discovered that during casting, new alignment irregularities occur due to the hot slab passing between rolls. Alignment misalignment in a broad sense including these is referred to as dynamic alignment misalignment. The most important factor among them is roll thermal warpage. The phenomenon in which rolls are deformed and warped by the heat they receive from slabs (also called roll bending) has been known for a long time.For example, Japanese Patent Application Laid-open No. 111557/1983 describes a method for reducing heat warping of rolls by using cooling spray. A method of casting while straightening is disclosed. However, until now, it was unclear the quantitative causal relationship between thermal warpage of the rolls and the center segregation of the slab, the area within the continuous caster that affects it, and the relationship with rolling down the slab. For this reason, no effort has been made to control the thermal warping of the rolls. As a result of investigating these relationships, the present inventors found that the thermal warpage of the roll has a significant effect on center segregation when the solidus temperature reaches the solidus temperature at the center of the thickness of the slab, from the point when the solidus fraction reaches 0.1 to 0.3. (Stage-2 and Stage), and its negative effects become more pronounced as the amount of slab reduction increases. As shown, it was confirmed that it was necessary to suppress the amount of thermal warpage of the roll in the rolling area to less than 0.5 mm. If the amount of thermal warpage of the roll is 0.5 mm or more, no significant segregation improvement effect can be obtained by rolling. Therefore, in the present invention, the amount of thermal warpage of the roll in the rolling region is defined as less than 0.5 mm. Methods for suppressing the amount of thermal warpage of the roll include a method of intermittently cooling the roll, and a method of dividing the roll and providing at least three bearings in the width direction of the slab. Another important dynamic misalignment factor is roll wear. As the number of times that slabs of various widths are cast increases, the roll surface wears unevenly in the lengthwise direction of the roll body and becomes extremely uneven. The depth of these irregularities can sometimes reach 1 mm or more, but conventionally, the amount of wear between the front and rear rolls in the casting direction is relatively small, and the key to keeping roll wear low is the lifespan of the roll (roll modification or replacement with a new one). Strict control of roll wear has not been achieved for several reasons, including the fact that it is not economical as it means a decrease in the amount of time required for roll wear and center segregation, and the cause-and-effect relationship between roll wear and center segregation is unclear. The present inventors investigated the relationship between the actual state of roll wear and center segregation, and found that roll wear causes flow due to uneven pressure in the casting direction and width direction, worsening center segregation, and that roll wear is noticeable in center segregation. It has been found that the stage-2 region has a significant influence, and the adverse effect becomes more pronounced as the slab reduction amount increases. As shown in FIG. 3, in order to make the segregation improvement effect by rolling noticeable, it is necessary to suppress the amount of thermal warpage of the roll to less than 0.5 mm. In addition to this, a further significant improvement in segregation can be achieved by suppressing roll wear to less than 0.5 mm. The rolls whose heat warping and wear should be controlled within the above-mentioned ranges are all rolls within the rolling area. Here, the amount of roll wear is defined by the depth of unevenness of each roll in the lengthwise direction of the roll body. Next, the present invention will be explained by examples. Smelting in a converter using the composition shown in Table 1 as the target component.
Molten steel whose composition has been adjusted by adding Ca to 240mm thick x 1580mm
The slab cross-section size of mm width was continuously cast and then rolled into thick plates. Samples were taken from slabs immediately after continuous casting, and the center segregation index and the number of V segregation pieces were investigated. In addition, samples were taken from the rolled plates and HIC tests were conducted to investigate the HIC cracking incidence. The results are summarized in Table 2. The central segregation index refers to Mn in steel.
The index shows the thickness of the high concentration area (segregation spot) with a ladle value of 1.3 times or more of this value as a reference, and the larger the value, the greater the segregation of the component.

【table】

[Table] During continuous casting, the casting speed was set to 1.0 m/m so that the point at which the solid fraction in the center reached approximately 0.7 was at the boundary of the roll segments. In addition, the area 2.2 m upstream from the roll segment boundary is defined as stage 2, and in the steels A and B to which the present invention and comparative steel C are applied, the pressure amount at stage 2 is set to 0.85 mm/min before casting. Adjusted roll spacing. The area length of stage-2 is that of stage-1
The boundary between -2 and -2 was determined by heat transfer calculations so that the solid fraction in the center was between 0.1 and 0.3. Inventive steels A and B and comparative steels D and E were cast using three-part rolls in order to keep the amount of roll thermal warpage low. At this time, as a result of measuring roll displacement during casting, the amount of roll thermal warpage was less than 0.5 mm in all cases. In contrast,
Comparative Steel C used a single roll, so the maximum amount of roll heat warpage was 1.2 mm. Comparative steel D is an example in which V segregation occurred due to solidification shrinkage flow, and comparative steel E is an example in which reverse V segregation occurred due to excessive rolling reduction.
High incidence of HIC cracking. In Comparative Steel C, solidification shrinkage flow can be prevented by proper rolling, but uneven rolling occurs due to thermal warping of the rolls, and center segregation due to rolling is not sufficiently improved. On the other hand, in Steel A according to the present invention, center segregation is significantly improved due to the synergistic effect of proper rolling reduction and prevention of roll heat warping. It can be seen that steel A has significantly improved center segregation compared to comparative steel C. Inventive steel B is an example in which the amount of roll wear was suppressed to 0.4 mm by controlling the number of rolls used in addition to the measures taken for steel A.
Segregation has been further improved compared to Steel A, and center segregation has been proven to be further improved by reducing roll heat warpage to less than 0.5 mm and roll wear to less than 0.5 mm. Ta.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between each solidification stage, the amount and range to be reduced according to the present invention, and Fig. 2 is a schematic diagram of center segregation and V segregation seen in continuously cast slabs.
FIG. 3 is a diagram showing the relationship between center segregation, roll thermal warping, and roll wear.

Claims (1)

[Scope of Claims] 1. In a continuous casting method for molten metal in which slabs are continuously drawn, in the region from the point where the solid phase ratio at the center of the thickness of the slab reaches 0.1 to 0.3 to the point where the solid phase rate reaches the flow limit. , a continuous casting method characterized in that a slab is continuously rolled down at a rate of 0.5 mm/min to 2.5 mm/min using rolls with a roll heat warpage of less than 0.5 mm. 2. In a continuous casting method for molten metal in which slabs are continuously drawn, the amount of roll heat warpage is determined in the region from the point where the solid phase ratio at the center of the slab thickness reaches 0.1 to 0.3 to the point where the solid phase rate reaches the flow limit. less than 0.5mm,
and using rolls with roll wear of less than 0.5 mm.
A continuous casting method characterized by continuously rolling down slabs at a rate of 0.5 mm/min to 2.5 mm/min.