JP2005313175A - Method for producing steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing steel where the generation in the defect of microcavities can be effectively reduced even in the case of a thick plate product, further, the generation in central segregation is prevented, and the generation of the defect in a UT (Ultrasonic Test) can be eliminated as much as possible. <P>SOLUTION: When steel satisfying the relation in the inequality (1); 0.2≤(W/W<SB>0</SB>)≤0.5, wherein W<SB>0</SB>(mm) is the thickness of the cast slab before the rolling-down and W(mm) is the product thickness of the steel after the hot rolling, is produced by a method where a cast slab in which a nonsolidified part is present at the inside is continuously cast as being rolled-down by rollers, and is further hot-rolled, on the rolling-down at the time of the continuous casting, it is started from a position at which the solid phase ratio at the central part of the cast slab reaches 0.4 to 0.6, and further, the rolling-down gradient is controlled, so as to be 0.7 to <4.0 mm/min on the rolling-down. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for reducing the center segregation and the formation of minute cavities in a steel material as much as possible when producing the steel material by continuous casting and hot rolling.

  When steel is continuously cast, there is a problem that components such as carbon, sulfur, soot and manganese are segregated and concentrated in the center of the slab. It is known that such central segregation is caused by the steel flow due to solidification shrinkage at the end of solidification of the slab.

  As a technique for solving such a problem, various methods have been proposed for continuously casting a slab slab at the end of solidification in which an unsolidified portion is present while being rolled by a roll (for example, Patent Documents 1 to 3). In these technologies, the center solid solution ratio of the slab is reduced from the position of 0.1 to 0.3 to the flow limit solid phase ratio, and the reduction amount (reduction speed and reduction amount) is appropriately specified. Prevents the center segregation.

  The technique as described above can be said to be an extremely effective technique for reducing the center segregation. However, such a reduction technique is not sufficient for preventing the occurrence of minute cavity defects (hereinafter referred to as “zaku defects”) in the center of the slab.

Such Zaku defects can be reduced to some extent by reduction in the subsequent hot rolling process. However, in the case of a slab cast slab where the thickness of the final steel plate product is particularly large (for example, 60 to 140 mm), a sufficient reduction ratio cannot be obtained. Zaku defects remain and appear as product defects (hereinafter sometimes abbreviated as “UT defects”) when an ultrasonic flaw detection test is performed, which is a cause that greatly hinders commercialization. For this reason, in order to obtain a product free from UT defects, it is necessary to reduce the zack defects as much as possible at the stage of continuous casting.
JP, 05-212517, A Claims etc. JP, 06-126405, A Claims etc. JP, 07-60424, A Claims etc.

  The present invention has been made in order to solve such problems in the prior art, and its purpose is to effectively reduce the occurrence of zaku defects even in thick plate products, and to generate center segregation. It is another object of the present invention to provide a method for manufacturing a steel material that can prevent occurrence of UT defects as much as possible.

The method of the present invention that has been able to solve the above-mentioned problems is the relationship of the following formula (1) by a method of continuously casting a slab slab having an unsolidified portion therein while being reduced by a roll, and further hot rolling. In the production of a steel material that satisfies the above requirements, when rolling down during continuous casting, the rolling starts at a position where the solid phase ratio at the center of the slab becomes 0.4 to 0.6, and the rolling gradient during rolling is 0. It has a gist in that it is controlled to be 7 mm / min or more and less than 4.0 mm / min.
0.2 ≦ (W / W ₀ ) ≦ 0.5 (1)
However, W ₀ (mm): Thickness of slab slab before rolling W (mm): Steel product thickness after hot rolling

  In the method of the present invention, the reduction may be completed at a position where the solid phase ratio at the center of the slab is 1.0, and more preferably, the casting is further performed from the position where the solid phase ratio at the center of the slab is 1.0. It is preferable to complete the reduction at a position corresponding to a length of 2 to 3 m.

Further, when the rolling gradient is Y (mm / min), it is preferable to operate such that Y satisfies the relationship of the following formula (2) in the relationship of (W / W ₀ ).
Y ≧ 1.8 × (W / W ₀ ) +0.17 (2)

  The slab slab targeted in the present invention is preferably a medium carbon steel containing 0.08 to 0.2% by mass of C. When such a slab is targeted, the effect of the present invention is maximized. Demonstrated.

  In the present invention, when producing a steel material satisfying the relationship of the above expression (1), a slab slab in which the occurrence of UT defects is minimized by appropriately controlling the rolling start time and rolling gradient during continuous casting. Can now be obtained.

The present inventors have studied from various angles in order to achieve the above object. as a result,
When casting a slab in which the ratio (W / W ₀ ) of the steel product thickness W after hot rolling and the slab slab thickness W ₀ before reduction is 0.2 to 0.5, If a strong reduction is performed with a predetermined reduction gradient from the position where the solid phase ratio (hereinafter abbreviated as “central solid ratio fs”) at the center of the slab slab becomes a predetermined value, The inventors have found that the occurrence of defects can be prevented and completed the present invention. That is, in order to eliminate zaku defects, the conventional reduction gradient that compensates for the solidification shrinkage is not sufficient, and if the reduction is performed with a larger reduction gradient, the zaku defects can be reduced. It is.

  In order to reduce the zaku defects, it is expected that there is an effect even if the rolling (that is, hot rolling) is performed after the solidification is completed (after the central solid phase ratio fs is 1.0). It is most effective to apply the reduction in the production stage. Moreover, by applying the reduction at such a stage, the effect can be sufficiently exhibited with a small amount of reduction. For example, when the rolling shape ratio is the same, when the shape is realized at the casting stage and when the shape is realized at the rolling stage, the temperature of the surface portion of the slab becomes lower at the time of casting. Because of its rigidity, it is easy for penetration from outside to penetrate inside. On the other hand, since the surface temperature has to be high in the rolling stage, the reduction permeation is not sufficient. In addition, it is much less than the necessary reduction amount to reduce and press the zaku once generated by applying the reduction to the end of the solidification phase, which is the zack generation stage. It is considered more efficient to apply the reduction.

  In carrying out the method of the present invention, it is most effective to start the reduction from a position where the central solid fraction fs is 0.4 to 0.6. Conventionally, in order to improve the center segregation, it is common to start the reduction from the stage where the center solid phase ratio fs is 0.1 to 0.3. Since the phase ratio fs is on the higher side (that is, the stage where fs = 0.4 at which the solid phase cannot freely move) and thereafter, at the start of the reduction, at least the position where the central solid phase ratio fs is 0.4. This is effective (both the slab width central portion and the solidification delay portion). However, if the reduction starts after the position where the central solid fraction fs becomes 0.6, there is a zaku that has been formed, and a large reduction amount is required to crimp this, so the reduction start position is the center. It is preferable that the solid phase ratio fs be set to a position where the solid phase ratio fs becomes 0.6.

  Considering only the center segregation, it is preferable to start the reduction (light reduction) from the stage where the central solid fraction fs is from 0.1 to 0.3. Since it moves freely, it does not contribute much to the generation of microporosity even if it is reduced at this site.

  In the present invention, since the solid phase ratio fs that affects the formation of microporosity that causes zaku defects is added under high pressure, the central solid ratio (fs = 0.1 to 0.1) that is not related to microporosity formation. From the viewpoint of segregation formation, it is preferable not to reduce the material until 0.3) because there is no squeezing of the concentrated molten steel.

  In the present invention, it is necessary to cast with a rolling gradient that compensates for solidification shrinkage in order to reduce the zaku defects. From this viewpoint, the rolling gradient during rolling needs to be 0.7 mm / mim or more. Further, if the rolling gradient is made too high, the concentrated molten steel is squeezed out and reverse V segregation is likely to occur during rolling, so even if it is considered to use a steel type that does not easily segregate (described later), the rolling gradient Needs to be less than 4.0 mm / min.

In the method of the present invention, when the thickness of the slab slab before rolling is W ₀ (mm) and the thickness of the steel product after hot rolling is W (mm), these satisfy the relationship of the following formula (1). Therefore, the effect of reducing the microporosity of such a steel material is achieved on the premise of manufacturing such a steel material.
0.2 ≦ (W / W ₀ ) ≦ 0.5 (1)

If the steel material is such that the value of (W / W ₀ ) is greater than 0.5, the effect of reducing sack defects due to high pressure will not be achieved, and if it is less than 0.2, sag will be present in the hot rolling stage. It becomes possible to crimp the defect.

The value of (W / W ₀ ) should satisfy the relationship of the above formula (1) finally (after hot rolling), and the value of this ratio also takes into account the rolling rate after hot rolling. It is what I put in. Therefore, it is not necessary to satisfy the relationship of the above formula (1) only by the continuous casting reduction, but in order to achieve the object of the present invention, it is necessary to strongly reduce the above conditions under the continuous casting reduction. is there.

  If the above-mentioned requirements can be satisfied, the object of the present invention will be basically achieved, and the timing for terminating the reduction is not limited, but the solid phase ratio at the center of the slab is 1.0. It is sufficient to finish the reduction at the position where A slab having no UT defect can be obtained by finishing the reduction at such a time, but more preferably from the position where the solid phase ratio at the center of the slab becomes 1.0, the slab length corresponds to 2-3 m. It is preferable to end the reduction at the position where the movement is performed. However, although the reduction may continue until after this position, the temperature of the slab shaft center portion is lowered, and there is a concern that the pressure-bonding effect of the zaku will be lowered.

  In the method of the present invention, since the reduction starts from a position where the central solid fraction fs becomes 0.4 to 0.6, the center segregation is deteriorated even if the Zaku defects can be reduced in the steel type in which the center segregation is likely to occur. There is a fear. From such a viewpoint, examples of the steel types that are the subject of the present invention include low carbon steel, ultra-low carbon clean steel, stainless steel, and the like that have a low C, P, and S content in the steel. In addition, steel grades with large solidification shrinkage (steel grades with δ → γ transformation) and steel grades that do not become so large as segregation even if they contain impurity elements (P and S) to some extent can be preferably applied. Examples include medium carbon steel having a content of 0.08 to 0.2 mass%. In these steel types, alloy elements such as Cu, Cr, Mo, Ni, Ti, Nb, V and the like are about 1.0 to 3.0% by mass (1.0% each for Cu, Cr, Mo, Ni). (About 2.0% by mass of Ti, Nb, and V, respectively) (the balance is substantially Fe).

Note that even if the rolling gradient is in the range of 0.7 mm / min or more and less than 4.0 mm / min, a tendency to easily generate zaku defects as the value of (W / W ₀ ) increases (shows). See Examples below). Considering these points, it is preferable to control the rolling gradient so as to satisfy the expression (2) according to the value of (W / W ₀ ).

  Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are not intended to limit the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are technical aspects of the present invention. It is included in the range.

Example 1
The slab slab made of steel of the following chemical composition is subjected to various conditions shown in Tables 1 to 3 below [steel product thickness / slab thickness (W / W ₀ ), rolling start center solid fraction fs, rolling end center. Continuous casting was performed at a solid phase ratio fs]. In addition, the slab slab thickness at this time is 280 mm (width: 2100 mm).

(Chemical composition of slab slab)
C: 0.08 to 0.2 mass%, Si: 0.1 mass%, Mn: 1.45 mass%, P: 0.01 mass%, S: 0.004 mass%, Al: 0.017 mass% %
About each obtained slab slab, it investigated about the zack generation | occurrence | production situation, the center segregation generation | occurrence | production situation, and the UT defect occurrence situation, and evaluated the slab slab quality. These evaluation methods and evaluation criteria are as follows. The results are shown in Tables 1 to 3 below together with the casting conditions.

[Zaku defect occurrence status]
The number of zaku having a diameter of 3 mm or more present in the cross section of the axial center portion of the slab slab [within 840 cm ² (4 mm × 210 mm)] was observed by the X-ray transmission method, and converted into the number per 1 cm ² for evaluation. The evaluation criteria at this time are as follows.
○: The number of zaku with a diameter of 3 mm or more is less than 0.1 / cm ² Δ: The number of zaku with a diameter of 3 mm or more is 0.1 / cm ² or more and less than 0.15 ×: The number of zaku with a diameter of 3 mm or more Is 0.15 / cm ² or more

[Center segregation occurrence]
The maximum value (Cmax) of the C content in the axial center section of the slab slab was measured, and the occurrence of center segregation was evaluated based on the ratio (Cmax / C ₀ ) between this and the average C content (C ₀ ). . The evaluation criteria at this time are as follows.
○: (Cmax / C ₀ ) is less than 1.3 Δ: (Cmax / C ₀ ) is 1.3 or more and less than 1.35 ×: (Cmax / C ₀ ) is 1.35 or more

[UT test]
An ultrasonic flaw detection test was performed according to a criterion (defect echo <25%) four times that of the JIS ultrasonic flaw detection standard (JIS B0901), and the occurrence of UT defects was evaluated based on the size of the measured defect echo height. The evaluation criteria at this time are as follows. The “defect echo height” indicates the ratio (%) of the defect echo height to the bottom surface echo height, and the smaller this value is, the more the Zaku defect is not generated.
○: Defect echo height is less than 10% Δ: Defect echo height is 10% or more and less than 20% ×: Defect echo height is 20% or more

These results can be considered as follows. First, the ones cast under conditions satisfying the requirements defined in the present invention (Nos. 4 to 10, 13 to 18, 23 to 30, 34, 41 to 44, 47 to 52) have very few zaku defects and center segregation. It turns out that the UT test result is also good. In particular, in the case where the end point of the reduction was reduced from the solidification completion point (central solid fraction fs = 1.0) to 2.5 m (No. 34), the small size zaku defects were further reduced (0.07 pieces). / cm less than ^2), also it has also become more small defect echo height in UT test (5% or less), both had good results (in Table 2, indicated by "◎ mark").

  On the other hand, in what (No.1-3,11,12,19-22,31-33,35-37,45,46) cast on the conditions which deviate from the requirements prescribed | regulated by this invention, it is a Zaku defect or center. It can be seen that at least one defect of segregation occurs.

  In addition, No. which obtained the best result was obtained. In the case of No. 34, a reduction was further applied from the position of the solidification completion point (central solid phase ratio fs = 1.0) to the downstream side of 2.5 m. It was confirmed that the position was lowered by about 25 ° C. from the temperature. That is, if the temperature is suppressed by this level of temperature decrease, even if a reduction is applied at that time, it is possible to press the zaku minutely. Since the casting speed at this time is 1.0 m / min, the casting speed is set to Vc (m / mm) at the position where the temperature of the axial center portion decreases by 25 ° C. from the position of the complete solidification point (fs = 1.0). Sometimes it becomes 2.5 Vc (m). That is, by further reducing to a position of 2.5 Vc after the complete solidification point (1.0 at the central solid phase ratio fs), a more favorable zaku defect reduction effect can be obtained.

Based on the above results, FIG. 1 shows the influence of (W / W ₀ ) and the rolling gradient on the remaining zaku defects and the occurrence of center segregation. From this result, it can be seen that a defect in the slab is generated when the rolling gradient is basically out of the range of 0.7 mm / min or more and less than 4.0 m / min. That is, when the rolling gradient is less than 0.7 mm / min, a zaku defect is generated, and when it is 4.0 mm / min or more, the center segregation is deteriorated.

However, even if the rolling gradient is in the range of 0.7 mm / min or more and less than 4.0 mm / min, it tends to be more likely to cause zaku defects as the value of (W / W ₀ ) increases ( No. 38 to 40 in Table 3). As shown in FIG. 1, the region is a region below the line of Y = 1.8 × (W / W ₀ ) +0.17 (Zaku residual region shown in FIG. 1). ). From these facts, it can be seen that it is preferable to control the rolling-down gradient so as to satisfy the expression (2) according to the value of (W / W ₀ ).

(W / W ₀₎ and reduction gradient is a graph showing the occurrence Influence of zag defects and center segregation.

Claims (5)

When producing a steel material that satisfies the relationship of the following formula (1) by continuously casting a slab slab having an unsolidified portion inside while being reduced by a roll, and further hot rolling, it is reduced during continuous casting. At the time, the reduction starts from the position where the solid phase ratio at the center of the slab becomes 0.4 to 0.6, and the reduction gradient during the reduction is 0.7 mm / min or more and less than 4.0 mm / min. A method of manufacturing a steel material characterized by controlling.
0.2 ≦ (W / W ₀ ) ≦ 0.5 (1)
However, W ₀ (mm): Thickness of slab slab before rolling W (mm): Steel product thickness after hot rolling   The method for producing a steel material according to claim 1, wherein the reduction is finished at a position where the solid phase ratio at the center portion of the slab becomes 1.0.   2. The method for producing a steel material according to claim 1, wherein the reduction is finished at a position corresponding to a slab length of 2 to 3 m from a position where the solid phase ratio of the slab center is 1.0. The operation according to any one of claims 1 to 3, wherein when the rolling gradient is Y (mm / min), the operation is performed so that Y satisfies the relationship of the following expression (2) in the relationship of (W / W ₀ ). The manufacturing method of steel materials as described in 2.
Y ≧ 1.8 × (W / W ₀ ) +0.17 (2) The method for producing a steel material according to any one of claims 1 to 3, wherein the slab slab is a medium carbon steel containing 0.08 to 0.2 mass% of C.

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