JP2006341297A - Continuous casting method, and continuously cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for continuously casting a steel for an extra-thick steel plate such as a bridge and a construction member, and a continuously cast slab produced using the continuous casting method by which the internal quality of an extra-thick steel plate subjected to rolling can be surely assured. <P>SOLUTION: In the continuous casting method, a slab is subjected to bulging, and thereafter, the slab including a unsolidified part 10 is subjected to rolling reduction. The bulging amount is controlled to 2 to 20 mm, and the slab in the unsolidified part in which the thickness of the equiaxed crystals on the upper half face side from the central part of the slab thickness is ≥5 mm and the average central solid phase ratio in the width direction of the slab is 0.85 to <1.0 is subjected to the rolling reduction using at least a pair of rolling reduction rolls 7 under the condition where the draft in the thickness direction of the slab reaches ≥2%. According to this invention, the production of a slab having a remarkably reduced porosity is made possible, and, by using the slab, an extra-thick steel plate of high quality even at a low rolling ratio can be produced. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method of continuously casting steel for an extremely thick steel plate such as a bridge or a building member, and a continuous cast slab manufactured using this continuous casting method. In the present specification, the extremely thick steel plate refers to a steel plate having a thickness of 70 mm or more and 120 mm or less.

For example, in the production of extra-thick steel plates, when rolling continuously cast slab slabs, the rolling ratio cannot be increased, so that the porosity that is a casting defect remains uncompressed enough to cause a product defect.
Japanese Patent Laid-Open No. 2-156022

  In order to solve this problem, assuming continuous casting of large-section slabs in order to increase the reduction ratio, the efficiency is very poor because casting must be performed at a low speed due to the length limit of the continuous casting machine. Become. Further, it is conceivable to cast a large-diameter ingot by a normal ingot forming method without continuous casting, but it goes without saying that the efficiency is further deteriorated as compared with the continuous casting method.

  Therefore, in order to solve the above-mentioned problem, the inventors of the present invention disclosed in Japanese Patent Application No. 2004-127054 that the slab is bulged by 2 mm to 20 mm before the slab is crushed in the continuous casting method, and then a solid phase centered on the thickness of the slab. A continuous casting method was proposed in which when the rate is 0.80 or more, the width central portion of the slab is reduced by 3 mm to 15 mm.

  However, in the proposed continuous casting method, a satisfactory result may not be obtained in the ultrasonic flaw detection test (hereinafter referred to as UST) result in the final steel plate even at the casting speed described in the examples. It has been found.

  As a result of detailed investigation of the slab, it was confirmed that there was a coarse part of the porosity in the entire width direction of the slab, and this was reflected in the result of UST.

  The problem to be solved by the present invention is that conventional continuous casting may not provide satisfactory results for the UST results in the final steel sheet.

The continuous casting method of the present invention comprises:
To reduce the porosity at the center of the thickness over the entire width of the slab, regardless of the casting conditions, and to ensure the internal quality of the extra-thick steel sheet that has been rolled,
After bulging the slab, a continuous casting method of rolling down the slab including the unsolidified portion,
The bulging amount is 2mm ~ 20mm,
The equiaxed crystal thickness from the center of the slab thickness to the upper half is 5mm or more.
The slab of the unsolidified part whose average center solid phase ratio in the slab width direction is 0.85 or more and less than 1.0,
The most important feature is that at least one pair of rolling rolls is used and rolling is performed under the condition that the rolling reduction in the slab thickness direction is 2% or more.

Moreover, the continuous cast slab of the steel of the present invention is
A slab obtained using the continuous casting method of the present invention,
The main feature is that the porosity volume at the center of the slab thickness is 2 × 10 ⁻⁴ (cm ³ / g) or less in the entire width direction of the slab.

  According to the present invention, it is possible to manufacture a slab slab having a significantly reduced porosity. Therefore, by using this slab, it is possible to manufacture a high quality extra heavy steel plate even at a low rolling ratio.

Hereinafter, the best mode for carrying out the present invention will be described in detail together with the process up to the establishment of the invention.
As a result of the inventors' research, knowledge about the following three points was obtained.

(1) When reducing the porosity by applying reduction at the end of solidification of the slab slab, it was found that the degree of reduction differs depending on the structure of the center of the slab immediately before reduction. That is, it has been found that the porosity reduction effect is improved when the center part of the slab is filled with equiaxed crystals immediately before the reduction.

(2) On the other hand, in the continuous casting of a normal slab that has a curved part such as a curved type or a vertical bending type and is cast in an unsolidified and straightened state, when the casting speed is increased, the center part of the slab It has been found that the equiaxed crystal may not be sufficiently filled in the width direction of the slab. In this case, the porosity in the full width direction of the slab was improved by squeezing the slab at the end of solidification to sufficiently fill the equiaxed crystal at the center of the slab.

(3) The inner quality of the slab center can be indexed by the porosity volume to which density measurement described later is applied, and this index agrees well with the UST results of the rolled steel sheet. Therefore, a good steel plate can be supplied by supplying a slab that defines the limit value of this index over the entire width.

The present invention has been made based on the above findings,
After bulging the slab, a continuous casting method of rolling down the slab including the unsolidified portion,
The bulging amount is 2mm ~ 20mm,
The equiaxed crystal thickness from the center of the slab thickness to the upper half is 5mm or more.
The slab of the unsolidified part whose average center solid phase ratio in the slab width direction is 0.85 or more and less than 1.0,
This is a continuous casting method of steel that uses at least a pair of rolling rolls and rolls the steel under a condition that the rolling reduction in the slab thickness direction is 2% or more.

  In the continuous casting method of the steel of the present invention, the reason why the slab is bulged once by 2 mm to 20 mm is to reduce the slab by avoiding the reduction of the slab end which is completely solidified and has high deformation resistance at a low temperature. This is because the part closer to the center part is strongly reduced from the end in the width direction.

  Also, the lower limit of the bulging amount is set to 2 mm. 2 mm avoids the reduction of the short side portion that is the end portion in the width direction of the slab, and the reduction of the portion near the center in the width direction proceeds (at least the reduction rate is 2). Because it is the lowest value. Moreover, the upper limit of the bulging amount is set to 20 mm because if it exceeds 20 mm, the progressive effect of reduction is saturated.

  In the present invention, the reason why the equiaxed crystal thickness from the center portion of the slab thickness to the upper half surface side is set to 5 mm or more is based on the knowledge of the porosity reduction effect as described above.

  In general, equiaxed crystals have a higher specific gravity than molten steel, so they settle slowly in the unsolidified molten steel being cast. In the case of a continuous casting machine having a curved portion, if the casting speed is increased, the solidification time in the horizontal portion becomes longer, and the equiaxed crystals tend to accumulate on the lower surface side. At that time, when a slab having a large width is cast as in slab casting, the number of equiaxed crystals in the molten steel in the width direction varies, so that the equiaxed crystal filling is not necessarily uniform in the width direction. Further, when the absolute amount of equiaxed crystals is small, not all equiaxed crystals are filled at the center of the slab thickness.

  Therefore, even if these slabs are squeezed, variations occur in the reduction of porosity in the width direction of the slabs. In addition, the amount of equiaxed crystals at the center of the slab thickness is also required to some extent, and according to the study by the inventors, it has been found that the lower limit is at least 5 mm above the center of the slab thickness. In order to secure a large number of equiaxed crystals, means such as low-temperature casting and electromagnetic stirring (hereinafter referred to as EMS) can be considered.

  Next, when the average center solid phase ratio in the slab width direction at the end of solidification of the slab is 0.85 or more and less than 1.0, at least one pair of reduction rolls is used and 2% in the slab thickness direction. The reason for applying the above reduction will be described.

  In order to efficiently reduce the inside of the slab, it is desirable that the temperature difference between the center of the slab and the surface is as large as possible, and the reduction is applied to the porosity formation time (center fs = 0.6 to 1) at the end of solidification. Is a good idea. Therefore, it is effective to apply the reduction while the inside is not solidified.

  On the other hand, when the central solid phase ratio at the time of rolling is small, depending on the type of steel containing many elements that easily segregate such as C, P, S, etc., the concentrated molten steel due to segregation flows backward in the direction opposite to the casting direction due to rolling. Then, it becomes a lump of concentrated molten steel and stays upstream of the reduction roll.

  This lump of concentrated molten steel reaches the reduction roll again by casting. At this time, there is no problem if it is completely discharged to the upstream side by the rolling roll, but due to the non-uniformity of solidification due to the presence of this lump, the internal effect varies in the width direction of the slab during rolling.

  That is, if there is a lump of concentrated molten steel in a large unsolidified portion in the width direction of the slab, pressing by the upper and lower solidified shells becomes insufficient, and the concentrated molten steel remains in the slab as it is without being completely discharged. May be defective.

  The non-uniform solidification in the slab width direction is originally caused by non-uniform cooling by spraying the slab surface, non-uniform contact state of the solidified shell in the mold, and non-uniform flow in the molten steel. However, as described above, the uniformity of the equiaxed crystal deposition is not necessarily guaranteed.

Therefore, even if the central solid fraction is 0.8 at a certain point in the slab width direction, solidification is delayed in other portions, and the central solid fraction may be about 0.5.
Generally, when the central solid fraction is 0.8 or more, the molten steel is said to lose its fluidity in a state of being confined between dendrite trees. When considering sex, it is extremely dangerous.

  As a result of repeating various tests by the inventors within the scope of the present invention, if the average of the central solid phase ratio of the unsolidified portion excluding the completely solidified portion at the end in the width direction of the slab is 0.85 as a reference, It was found that segregation defects, which are thought to be caused by the backflow of concentrated molten steel, can be prevented at all positions in the slab width direction.

  The reason why the central solid phase ratio is less than 1 is that a state in which the solidification latent heat is not sufficiently solid and the central temperature is sufficiently high and the center temperature is sufficiently high is extremely effective in reducing the penetration into the inside. Of course, considering the non-uniformity of the solidification in the width direction, it can be said that there is a solidification completion part in part, but the reduction amount may be set larger as described later.

The central solid phase ratio fs can be obtained by fs = (TL−T) / (TL−Ts) from the liquidus temperature TL of the molten steel, the solidus temperature Ts, and the thickness center temperature T of the slab.
When the thickness center temperature T of the slab is equal to or higher than the liquidus temperature TL of the molten steel, fs = 0, and when the thickness center temperature T is lower than the solidus temperature Ts of the molten steel, fs = 1.0. is there. Further, the thickness center temperature T of the slab can be obtained by unsteady heat transfer analysis calculation considering the casting speed, the surface cooling of the slab, the physical properties of the cast steel type, and the like.

  The center solid phase ratio in the slab width direction is obtained by two-dimensional unsteady heat transfer analysis in the slab thickness direction and width direction. The accuracy of this analysis was confirmed by a method usually performed in a hammering test or surface temperature measurement on an unsolidified slab. From these tests and heat transfer analysis, the average center solid phase ratio in the slab width direction is the sum of the center of the slab width and ¼ of the width, the distance corresponding to the cast slab thickness + 10 cm from both ends. It was found that the average of 5 points was almost equal to the average of the entire width.

  Next, regarding the amount of reduction, a solid-liquid coexisting phase exists at a certain width in the thickness direction of the slab even in the central solid phase ratio range at the center of the slab. Although the porosity is formed in this, the size of the solid-liquid coexistence phase width in the thickness direction has a correlation with the thickness of the cast slab to be cast. The width also increases.

  In addition, as described above, the width is distributed in the slab width direction due to non-uniform solidification, and in order to reduce the porosity at all positions in the slab width direction, the minimum value depends on the thickness of the slab. It is necessary to set a limit reduction ratio. The reduction ratio referred to here is {(cast slab thickness before reduction-slab thickness after reduction) / slab thickness after reduction} × 100 (%).

  As a result of repeating various tests by the inventors, it was found that the minimum rolling reduction should be 2%. In order to obtain the reduction ratio, when there is no deformation of the roll, for example, as the slab thickness before the reduction, the upper and lower roll intervals upstream of the reduction roll, and the upper and lower roll interval of the reduction roll as the slab thickness after reduction .

When the slab cast as described above is examined in detail, the porosity volume at the center of the slab thickness is 2 × 10 ⁻⁴ (in the full width direction of the slab excluding both ends where solidification has been completed at the start of slab reduction. cm ³ / g) or less, and when this slab-rolled extra heavy steel UST was performed, no unbonded porosity defect was found. This is the continuous cast slab of steel of the present invention.

Here, the porosity volume (hereinafter referred to as Pv) is Pv = (1 / ρ), where ρ0 is the density of the representative sample of the 1/4 thickness part of the same slab and ρ is the sample density of the center part. -(1 / ρ0) (cm ³ / g)

  In this case, the size of the sample is preferably within 30 mm × 30 mm × 30 mm. This is because if the sample is made too large, it will enter the part of the slab where there is almost no porosity from the center of the thickness, and the sensitivity for detecting the porosity at the center will be reduced.

  The reason for selecting the standard sample density ρ0 of the 1/4 thickness part of the same slab as the standard is that this part is a part where porosity is hardly detected (for example, by inspection with a microscope, etc.). This is because they can be replaced as equal.

  In addition, the extra-thickness steel plate after rolling evaluated the non-pressure bonding porosity based on JIS G 0801-1993 “Ultrasonic flaw detection inspection method of steel plate for pressure vessels”. The UST apparatus was an A scope display type flaw detector, and a vertical flaw detector having a transducer diameter of 30 mm and a nominal frequency of 2 MHz was used.

Hereinafter, tests conducted for verifying the present invention will be described.
FIG. 1 shows a vertical bending type continuous casting machine used in the test. The mold 3 used in the test had a casting slab thickness of 310 mm, 230 mm, and a width of 2300 mm. The steel type used for the test is 40 kg grade steel with 0.15 mass% C.

  The central solid fraction in each slab thickness (including bulging) before reduction was adjusted by changing the casting speed and the secondary cooling strength. The amount of secondary cooling water was 0.75 to 0.85 L / kg-steel.

  The molten steel 4 injected into the mold 3 from the tundish (not shown) through the immersion nozzle 1 is cooled by spray water sprayed from the mold 3 and a group of secondary cooling spray nozzles (not shown) below it, A solidified shell 5 is formed. The slab 8 from which the unsolidified portion 10 is removed by being strongly squeezed by the rolling roll 7 while the unsolidified portion 10 is held inside the solidified shell 5 is pulled out by the pinch roll 11 through the group of guide rolls 6.

  The reduction roll 7 was installed at a position 21 m below the meniscus 2 of the mold 3. The diameter of the rolling roll 7 was 450 mm, and the rolling force was 600 tons at maximum. Although the continuous casting machine used for the test is a vertical bending type continuous casting machine, it goes without saying that a curved type continuous casting machine may be used.

  The guide rolls 6 group are arranged so that the interval in the thickness direction of the slab can be controlled to a constant value, and when the unsolidified portion 10 is present inside the slab (between B1 and B2), The thickness of the central portion of the width is made larger than the short side length of the slab, and the central portion to the end portion of the width is reduced by the subsequent reduction roll 7.

  The amount of bulging can be adjusted by adjusting the distance in the thickness direction of the pair of guide rolls 6 arranged in the short side length direction of the slab to be larger than the thickness of the slab on the mold exit side. The above-mentioned rolling reduction was obtained from the thickness of the slab bulged and the slab after rolling, that is, from the respective roll intervals as described above.

  The central solid fraction during rolling is mainly determined by the casting speed and the thickness of the center of the slab width, that is, the slab bulging amount. The average central solid fraction fs in the width direction was determined by the above method.

  Moreover, the molten steel temperature in the tundish was made substantially constant between ΔT = 40 ° C. and 50 ° C. ΔT is the difference between the molten steel temperature and the liquidus temperature.

  Further, as shown in FIG. 1, in order to ensure equiaxed crystals at the center, an electromagnetic stirring device 9 was installed in the continuous casting machine, and EMS was performed. The EMS conditions were as follows: alternating current stirring of 1.5 Hz at a current value of 900 A, a magnetic flux density at the center of the slab of 400 gauss, 30 seconds normal rotation, 5 seconds stopped, and 30 seconds reversed. The thickness of the equiaxed crystal of the slab was confirmed by polishing the following cross-sectional sample and acid corrosion, and numerically marking the upper side of the thickness center as + and the lower side as-.

  The obtained slab was sampled from a portion for porosity and structure investigation, heated to 950 to 1170 ° C., and finish-rolled in the range of 1050 to 750 ° C. to produce a very thick material.

  The mold exit side size was a steel plate having a thickness of 100 mm from a 310 mm thick cast, and a steel plate having a thickness of 75 mm from a 230 mm thick cast. The finish rolling mill used has a work roll diameter of 1040 mm and a maximum rolling force of 6300 tons.

  The sample for examining the porosity volume (Pv) was collected from the center part at 15 points evenly in the width direction from the slab cross section block of the stationary part. As for the size of the sample, the plane parallel to the transverse section was 30 mm × 30 mm and the thickness was 20 mm. Similarly, a sample for measuring the reference density (ρ) was taken from the 1/4 thickness position at the center of the slab width with the same size. At the same time, a macro plate was sampled from the cross section of the slab, and after polishing, the polished surface was corroded with a 10% by mass picric acid aqueous solution to investigate the presence or absence of segregation in the discharged concentrated molten steel.

  The density was calculated from each weight and volume. The volume was calculated from the density of water by immersing the sample in water and measuring the weight in water to obtain buoyancy. From these, Pv in the width direction was determined and the maximum value was noted.

The ultra-thick steel plate after rolling was inspected using the above-mentioned UST apparatus. When the measured number of defects, maximum indicated length of one defect, density, space factor, etc. are below the values specified in the JIS, the heavy steel plate is accepted and there is no UST defect. It was judged.
Table 1 below shows a series of test conditions carried out to confirm the effect of the present invention, and Table 2 below shows the results.

Examples 1-7 satisfy | fill all the requirements of this invention, the situation of the porosity of the center part of the slab is very favorable, and a center part porosity volume is 2 * 10 <^-4> (% in the full width position of a slab. It was as follows. This slab was rolled to produce a thick steel plate, and when the UST inspection was conducted, all passed.

On the other hand, although details will be described later, in the comparative examples other than Comparative Examples 7, 8 and 13 in which the segregation due to the discharged concentrated molten steel remained and the rolling was stopped, the situation of porosity was bad and 2 × 10 ⁻⁴ ( %). When the slab was rolled into a thick steel plate, all the USTs failed. Below, each comparative example is demonstrated in detail.

  Comparative Example 1 and Comparative Example 2 are cast from Example 1 and Example 2 with only EMS turned OFF, respectively. As a result, the thickness of the equiaxed crystal on the upper surface side was 4 mm and 2 mm, and the porosity was deteriorated.

  Similarly, Comparative Example 9 and Comparative Example 10 were cast from Example 5 and Example 6 with only EMS turned OFF. Also in this case, the thickness of the equiaxed crystal on the upper surface side became extremely small, -2 mm and 1 mm, and as a result, the porosity deteriorated.

  In Comparative Example 3 and Comparative Example 11, only the bulging amount was reduced to 1 mm from Example 2 and Example 5, but the reduction amount itself was reduced by 1 mm, and as a result, the slab reduction ratio was 2 %, The porosity deteriorated.

  Comparative Example 4, Comparative Example 5 and Comparative Example 12 are those in which only the amount of reduction was reduced from Example 1, Example 4 and Example 7, respectively, and the reduction rate was reduced to 2% or less. After all, porosity became worse.

  Comparative Example 6 and Comparative Example 14 were cast at a slightly lower casting speed than in Examples 3 and 5, respectively. As a result, at the time of the rolling roll, the slab was completely solidified (fs = 1.0), and the effect of improving the porosity was reduced.

In Comparative Example 7, Comparative Example 8, and Comparative Example 13, the casting speed was slightly increased from Example 1, Example 2, and Example 7, respectively. As a result, the central solid fraction at the time of reduction was reduced to a value less than 0.85. When the cross section of the slab was examined, a large number of exhausted concentrated molten steel was segregated.
From the above, the effects of the present invention are clear.

  The present invention is not limited to the above example, and it goes without saying that the embodiment may be appropriately changed within the scope of the technical idea described in each claim.

  The present invention can be applied not only to medium carbon steel slabs as shown in the examples, but also to continuous casting of low carbon steel and high carbon steel.

It is a schematic diagram of the vertical bending type continuous casting machine used for the test.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Immersion nozzle 2 Meniscus 3 Mold 4 Molten steel 5 Solidified shell 6 Guide roll 7 Rolling-down roll 9 Electromagnetic stirrer 10 Unsolidified molten steel 11 Pinch roll B1-B2 Bulging zone

Claims (2)

After bulging the slab, a continuous casting method of rolling down the slab including the unsolidified portion,
The bulging amount is 2mm ~ 20mm,
The equiaxed crystal thickness from the center of the slab thickness to the upper half is 5mm or more.
The slab of the unsolidified part whose average center solid phase ratio in the slab width direction is 0.85 or more and less than 1.0,
A steel continuous casting method characterized in that at least one pair of rolling rolls is used and rolling is performed under a condition that the rolling reduction in the slab thickness direction is 2% or more. A slab obtained using the method according to claim 1,
A continuous cast slab of steel, wherein the porosity volume at the center of the slab thickness is 2 × 10 ⁻⁴ (cm ³ / g) or less in the entire width direction of the slab.

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