JP2010274300A - Method for treating slab in cooling of slab whose ductile-brittle transition temperature reaches >=160°c - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique for preventing the cracks of a slab with a stage where a slab whose ductile-brittle transition temperature reaches ≥160°C is cooled to ordinary temperature included as an assumption. <P>SOLUTION: When the slab whose ductile-brittle transition temperature reaches ≥160°C and cast by continuous casting is cooled to ordinary temperature, before the surface temperature of the slab lowers the ductile-brittle transition temperature, the slab is subjected to rolling reduction at a draft R≥0.1 in such a manner that the thickness D2 [mm] of the slab after the rolling reduction is not lower than 140, and the cooling velocity Vco [°C/hr] of the slab after the rolling reduction is controlled to ≤70. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a slab handling method during cooling of a slab slab.

  As this type of technology, Patent Document 1 discloses a method for preventing cracking of a continuous cast piece of high-tensile steel. According to this document 1, even if it is a high-strength steel that transitions from ductility to brittleness as soon as it falls below 150 ° C. during cooling, it can be cooled to room temperature under appropriate temperature control. It is said that cracking can be prevented.

JP 2007-83274 A

  Certainly, when the ductile-brittle transition temperature was around 150 ° C. as in the above-mentioned document 1, cracks could be avoided only by the appropriate temperature control described above, but for example, components such as Cu, Cr, Ti, etc. As a result of addition, when the ductile-brittle transition temperature was 160 ° C. or higher, cracking could not be avoided by the above temperature control alone. This is presumably because the newly generated thermal stress from the time when the ductile state is changed to the brittle state until it reaches room temperature becomes larger than the former.

  The present invention has been made in view of these points, and its main purpose is to include a step of cooling a slab slab having a ductile-brittle transition temperature of 160 ° C. or higher to room temperature, It is providing the technique which prevents the crack of a slab slab.

Means and effects for solving the problems

  The problems to be solved by the present invention are as described above. Next, means for solving the problems and the effects thereof will be described.

  According to the aspect of the present invention, the carbon content C [wt%] is set to 0.07 to 0.60, the silicon content Si [wt%] is set to 0.1 to 3.0, and the manganese content Mn [wt %] Is 0.5 to 6.0, the phosphorus content P [wt%] is 0.15 or less (however, 0 is not included), and the sulfur content S [wt%] is 0.02 or less ( However, 0 is not included.) The molten aluminum sol-Al [wt%] is 0.01 to 3, the copper content Cu [wt%] is 0.5 or less (however, 0 is included), The nickel content Ni [wt%] is 0.5 or less (provided that 0 is included), the chromium content Cr [wt%] is 1.0 or less (provided that 0 is included), and molybdenum Mo [wt %] Is 1.0 or less (including 0), and niobium Nb [wt%] is 0.1 or less (however, 0 And titanium Ti [wt%] is 0.1 or less (excluding 0), the balance is made of iron Fe and inevitable impurities, and the ductile-brittle transition temperature is 160 ° C. or higher. When the slab slab cast by casting is cooled to room temperature, before the surface temperature of the slab slab falls below the ductile-brittle transition temperature, the slab steel slab after being reduced with respect to the slab slab In order to prevent the thickness D2 [mm] of the steel sheet from falling below 140, the reduction is applied at a reduction ratio R = 0.1 or more, and the cooling rate Vco [° C./hr] of the slab steel slab after the reduction is 70 or less. According to the above method, cracking of the slab cast can be prevented.

Photograph showing an example of cracking in a slab slab Photograph showing the fracture surface of the slab slab shown in FIG. Graph to explain the ductile-brittle transition temperature Explanatory drawing of measuring method of surface temperature of slab steel slab (or slab slab) Explanatory drawing of measuring method of surface temperature of slab steel slab (or slab slab) Photograph of broken slab slab (or slab slab) Photograph of opening crack of slab slab (or slab slab) Photograph of fine surface defects of slab slab (or slab slab) Photograph of cracks on the four circumferences of a slab slab (or slab slab) Photograph of perforation during sheet rolling The graph which shows the relationship between the weld crack sensitivity index Pcm and the ductile-brittle transition temperature by a Charpy test. The graph which shows the relationship between reheat cracking sensitivity index Psr and the ductile-brittle transition temperature by a Charpy test The graph which shows the relationship between HAZ coarse grain area | region ductility index | index CE and the ductility-brittle transition temperature by a Charpy test. Graph showing the relationship between the ductility-brittle transition temperature (estimated value) and the ductility-brittle transition temperature (measured value) by Charpy test

(Steel grades covered by this application)
In general, when a slab slab continuously cast by a continuous casting facility is cooled to room temperature, the slab slab may be broken due to thermal stress, for example, as shown in FIG. FIG. 2 shows a fracture surface that appears as a result of the fracture. As shown in FIG. 2, there are cases where a brittle fracture surface and a ductile fracture surface are mixed in this fracture surface, and the ratio of this mixture is known to transition according to the temperature as shown in FIG. . The graph of FIG. 3 shows the test result of the Charpy test standardized by JIS Z 2422, the horizontal axis shows the surface temperature of the test piece during the Charpy test, and the vertical axis shows the fracture surface obtained by the Charpy test. The area ratio of the ductile fracture surface is shown. Both black circle plots and white triangle plots in the figure are alloy steels, and so-called high-tensile steels were used as test pieces. As shown in FIG. 3, since the temperature at which the ductile fracture surface transition varies depending on the steel type, a ductile-brittle transition temperature is generally used to index this characteristic. This ductile-brittle transition temperature means the surface temperature of the test piece when the ductile fracture surface ratio becomes 50% in the Charpy test standardized by JIS Z 2422. Accordingly, the ductility-brittle transition temperature of the steel type shown by the black circle plot in FIG. 3 is 160 ° C., and the ductility-brittle transition temperature of the steel type shown by the white triangle plot is 325 ° C. The steel types targeted by the present application are limited to those having a ductile-brittle transition temperature of 160 ° C. or higher. This is because, for steel types whose ductile-brittle transition temperature is lower than 160 ° C., a countermeasure against cracking can be achieved by a known technique described in Japanese Patent Application Laid-Open No. 2007-83274. In addition, although the ductile-brittle transition temperature mentioned above exists also in 600-800 degreeC vicinity, for example, generally the ductile-brittle transition temperature which this application is for exists between 0-400 degreeC.

(Operations covered by this application)
As is well known, when paying attention to the casting path of the continuous casting equipment, there are a curved continuous casting equipment and a vertical bending continuous casting equipment. The former has an arc path section, a correction path section, and a horizontal path section along the casting path from the mold, and the latter has a vertical path section upstream of the arc path section, and floats inclusions in the molten steel. Is intended. Focusing on the cross-sectional shape of the slab cast by the continuous casting equipment, there are slabs having a cross-sectional aspect ratio of 2 or more, blooms of 2 or less, and billets having a square cross-sectional shape. The operations targeted by the present application are both curved continuous casting equipment and vertical bending continuous casting equipment in terms of the casting path, and are limited to slabs in terms of the cross-sectional shape of the slab.

  In addition, since this application is intended for slab slabs that need to be cooled to room temperature, ingots that are carried out until final rolling after casting are excluded from the scope of this application. is there. Similarly, so-called HDR (Hot Direct Roll) and HCR (Hot Charge Roll), which are not cooled to room temperature once, are out of the scope of the present application.

Hereinafter, embodiments of the present invention will be described.
(1) Continuous casting process of slab slab Using the curved continuous casting equipment or the vertical bending continuous casting equipment, the following conditions (a) to (m) relating to the steel type components are satisfied and the ductile-brittle transition temperature is satisfied. A steel type having a temperature of 160 ° C. or higher is continuously cast, and the continuously cast slab slab is cut into a predetermined length (for example, around 12.5 meters). Examples of the steel types that satisfy the above conditions include alloy steels such as high-tensile steels.
(A) Carbon content C [wt%]: 0.07 to 0.60
(B) Silicon content Si [wt%]: 0.1 to 3.0
(C) Manganese content Mn [wt%]: 0.5 to 6.0
(D) Phosphorus content P [wt%]: 0.15 or less (excluding 0)
(E) Sulfur content S [wt%]: 0.02 or less (however, 0 is not included)
(F) Molten aluminum sol-Al [wt%]: 0.01 to 3
(G) Copper content Cu [wt%]: 0.5 or less (including 0)
(H) Nickel content Ni [wt%]: 0.5 or less (including 0)
(I) Chromium content Cr [wt%]: 1.0 or less (including 0)
(J) Molybdenum Mo [wt%]: 1.0 or less (including 0)
(K) Niobium Nb [wt%]: 0.1 or less (including 0)
(L) Titanium Ti [wt%]: 0.1 or less (excluding 0)
(M) The balance is iron Fe and inevitable impurities The technical significance or detailed description of each of the above steel type components (a) to (m) is attached to the end of the present specification, so please refer to them as necessary.

(2) Slab slab reduction process Next, the slab slab cast by continuous casting is cooled to room temperature. At this time, when the surface temperature of the slab slab falls below the ductile-brittle transition temperature, an embrittled structure is generated on the surface layer of the slab slab, and cracks along the grain boundaries of the embrittled structure are caused by the thermal stress generated during cooling. appear. Therefore, in the present embodiment, before the surface temperature of the slab slab falls below the ductile-brittle transition temperature, the slab steel slab thickness D2 after being reduced by using a lump rolling facility for the slab slab. While taking care that [mm] does not fall below 140, the rolling reduction R = 0.1 or more is applied. Here, the reduction ratio R is obtained by the following formula (1), where D1 [mm] is the thickness of the slab cast before reduction and D2 [mm] is the thickness of the slab steel piece after reduction. Thus, by applying appropriate reduction to the slab cast before the surface temperature of the slab cast falls below the ductile-brittle transition temperature, the surface layer structure of the slab cast is refined. Therefore, even if a new crack is generated due to thermal stress, the propagation of this crack is strongly suppressed, and it can be suppressed to a level of fine surface defects. In addition, if it is a fine surface flaw, it does not protrude toward the axial center of the slab steel piece from the oxide layer having a thickness of about 2 mm generated when the slab steel piece is heated in a hot-rolling heating furnace described later. This oxide layer is always cut and removed anyway, and as a result, cracks newly generated by thermal stress can be made harmless.

  In addition, although the minimum of said reduction ratio R was defined from the viewpoint of refinement | miniaturization of the surface layer of a slab cast piece, on the other hand, the upper limit of reduction was provided from a viewpoint of the downward curvature prevention of the slab steel piece in a heating furnace. This is because the slab steel pieces inserted in the heating furnace are supported by a plurality of support bases called skids, and the slab steel pieces are warped by their own weight between the skids. This is because there is a possibility that the heated slab steel slab cannot be taken out from the heating furnace, and it is preferable to secure a certain amount of secondary moment of section of the slab steel slab after the reduction so as not to cause such a situation. Further, when the rolling reduction increases, the thickness of the slab steel piece decreases, and the yield loss due to surface oxidation in the heating furnace increases. From the viewpoint of suppressing the yield loss due to the surface oxidation, it is meaningful to provide an upper limit for the rolling reduction R.

(3) Cooling process of slab steel slab after reduction Moreover, when the slab steel slab is cooled to room temperature, the surface temperature of the slab steel slab after reduction reaches the ductile-brittle transition temperature from the above completion of the reduction. The cooling rate Vco [° C./hr] of the slab steel slab after the reduction is set to 70 or less until the point of time. Here, “the cooling rate Vco [° C./hr] of the slab steel slab after the reduction” is from the time when the above-described reduction is completed to the time when the surface temperature of the slab steel slab after the reduction reaches the ductile-brittle transition temperature. Means the cooling rate of the surface temperature of the slab steel slab after the reduction, which is obtained by dividing the amount of temperature decrease in the surface temperature of the slab steel slab after the reduction by the elapsed time. In addition, as shown in FIG. 4, the “surface temperature of the slab steel slab after the reduction” is the temperature at the center of the vertical and horizontal surfaces of the upper surface when the slab steel slab after the reduction is placed on the slab steel slab placement site. Is measured using a commercially available thermocouple probe or the like. Thus, by suppressing the cooling rate Vco [° C./hr] of the slab steel slab after the reduction to a predetermined value or less, the thermal strain due to the temperature difference between the surface layer and the inside of the slab steel slab during cooling is alleviated, Contributes to prevention of cracking of slab steel pieces.

  In order to set the cooling rate Vco [° C./hr] of the slab steel pieces after reduction to 70 or less as described above, for example, a method of stacking a plurality of slab steel pieces after reduction is mentioned. Even in the case of this method, as shown in FIG. 5, the “surface temperature of the slab steel slab after the reduction” is the same as the upper surface when the slab steel slab after the reduction is placed on the slab steel slab placement site. The temperature at the vertical and horizontal center of the surface to be measured is measured using a commercially available thermocouple probe or the like. In this case, since the thermocouple probe is sandwiched between stacked slab steel pieces, it is preferable to use a thin probe as much as possible so that an excessive gap is not formed between the slab steel pieces. Further, in the present embodiment, the method for setting the cooling rate Vco [° C./hr] to 70 or less is not particularly limited. In addition, the slab steel piece after the reduction is stored in a heat retaining container. And a method of covering with a heat retaining cover. If the so-called cooling is performed by placing only one slab steel piece after the reduction, the cooling rate Vco [° C./hr] is about 90.

(4) Care process for slab steel slab after rolling After cooling the slab steel slab after rolling down to room temperature, the slab steel slab is then visually checked for surface defects (small cracks, impurity precipitates, etc.). Then, if necessary, perform partial cutting and removal work on the found surface flaws. In addition, the surface flaw of a slab steel piece can be visually confirmed only after cooling a slab steel piece to normal temperature (approximately 100 degreeC).

(5) Heating and rolling process of slab steel slab after maintenance Next, the slab steel slab after the above maintenance is put into a hot-rolling heating furnace (consisting of a heating furnace and a rolling mill), and slab steel slabs are 1200 to 1250. Heat to ° C. And after removing the oxide layer of the surface layer of the slab steel piece which generate | occur | produced in this heating with suitable means, such as a descaler, a slab steel piece is hot-rolled.

<Test>
Hereinafter, a test for confirming the technical effect of the slab handling method during cooling of the slab slab according to the present embodiment will be described. Each numerical range described above is reasonably supported by the following confirmation test.

<Test: Indicator>
First, an index used for evaluation of each confirmation test will be described. In the following, faults A to D classified as slab steel slab breakage or surface defects during cooling of a slab cast (or slab steel slab, the same shall apply hereinafter) or in a heating furnace will be described, followed by other operations. The above defects E to G will be described. In this test, the maintenance work after cooling the slab slab is not performed. Therefore, it should be noted that the problems that can be solved by partially cleaning the slab cast are also taken up below.
Failure A: Breakage: See FIG. 6 The breakage of the slab cast as shown in FIG. 6 mainly occurs when the slab cast is cooled or heated in the heating furnace. If the surface temperature of the slab slab falls below the ductile-brittle transition temperature before the above-described reduction, the slab slab may break as shown in FIG. This breakage not only causes a decrease in yield, but also when the slab slab is lifted or transported by a crane, the slab slab piece drops and is very dangerous.
-Problem B: Opening crack: Refer to FIG. 7 When a slab slab having an open crack that can be visually confirmed as shown in FIG. 7 is heated in a heating furnace, there is a high possibility of breakage during the heating. Moreover, when it breaks in a heating furnace, the operation | work which takes out the broken slab cast piece after cooling a heating furnace is needed, and becomes a huge production obstruction.
・ Problem C: Fine surface defect: see FIG. 8 As shown in FIG. 8, surface defect that can be visually confirmed, MT test (magnetic particle test: JIS Z 2319.1991) or PT test (penetration test (color) Check): The soot that can be detected by JIS Z 2343) has a possibility that cracks open during heating in the heating furnace, and perforation occurs in the base material during rolling. Moreover, there is a risk of inducing plate breakage during rolling, and there is a risk that the facility will be forced to stop for a long time.
Failure D: Cracks at the four circumferences: see FIG. 9 As shown in FIG. 9, the cracks are at the four circumferences of the slab slab that can be visually confirmed. The four circumferential portions are particularly susceptible to cracking because cooling is faster than the axis of the slab cast. If rolling is performed while leaving the cracks in the four circumferences as they are, the wrinkles remain in the product and must be removed by care. Further, this crack may be enlarged depending on the care procedure.
・ Problem E: Deformation in the heating furnace If the thickness of the slab slab is too small during charging into the heating furnace, the slab slab after the heating will be taken out of the heating furnace. Can cause problems.
・ Failure F: Perforation during rolling: see FIG. 10 If the slab slab before entering the rolling mill has an opening-like defect, a hole may be formed in the plate during rolling as shown in FIG. Depending on the case, the plate may be broken, and the equipment must be stopped for a long time.
-Failure G: Sheet breakage during rolling When the slab steel piece before entering the rolling mill has an open defect, the sheet may break during rolling depending on the rolling conditions and temperature. When the plate breakage occurs, the rolling material stays in the rolling mill, so that it is necessary to suspend the equipment for a long time.

<Test: Common test method / Common test conditions>
In principle, the procedure as in the above embodiment is executed. However, as described above, the maintenance work is not performed.

<Test: Individual test conditions and test results>
Next, individual test conditions and test results of each confirmation test are shown in Table 1 below. In Table 1 below, “C wt%” to “Ti wt%” indicate the components of the molten steel in each test. In particular, “s-Al wt%” corresponds to sol-Al detailed at the end of the present specification. “Measured transition temperature ° C.” means the ductile-brittle transition temperature actually measured by the Charpy test described above. “Estimated transition temperature ° C.” means the ductile-brittle transition temperature estimated based on the composition of the molten steel. A description of the method for estimating the ductile-brittle transition temperature based on the composition of the molten steel is attached at the end of this specification. The “slab slab temperature during rolling ° C.” means the surface temperature of the slab slab immediately before the continuously cast slab slab is charged into the block rolling facility. In the column of “Cooling means”, “stacked slow cooling” means that the slab steel pieces after the reduction were placed and cooled, and sandwiched between the other two slab steel pieces. The term `` cooling at chilling '' means that the slab steel slab after the reduction was placed and cooled, and the slab steel slab was left alone, and the term `` water cooling '' was applied to the slab steel slab after the reduction. It means that it was cooled strongly by spraying water, and `` Installation of heat insulation cover after stacking '' means that a plurality of stacked slab steel pieces are covered with a heat insulation cover in addition to the above-mentioned stacking slow cooling. Means that. In the column of “slab slab cracking”, the specific situation of cracks occurring in the slab slab is described. In the column of “Operation Abnormality”, other troubles for normal operation are specifically described. In the “comprehensive evaluation”, a case where no crack was generated in the slab slab and a normal operation could be realized was indicated as “◯”, and the other cases were indicated as “X”.

The following is a supplementary explanation for each test.
・ Test No. 1
Continuous casting was performed with the components shown in Table 1 (the ductile-brittle transition temperature is known). And the slab slab cast by this continuous casting was conveyed to the lump rolling equipment. The surface temperature of the slab slab immediately before charging the slab slab into the block rolling facility was 15 ° C. The thickness D1 [mm] of the slab cast before rolling was 280, and rolling was not performed in this test. And the slab slab carried out from the lump rolling equipment tried to insert into a hot-rolling heating furnace, without further cooling. However, in this test, the slab slab was broken before being inserted into the hot-rolling furnace (defect A). In Table 1, it is described as “heating furnace”, which means “heating furnace of the lump rolling equipment”.
・ Test No. 3
Continuous casting was performed with the components shown in Table 1 (the ductile-brittle transition temperature is known). And the slab slab cast by this continuous casting was conveyed to the lump rolling equipment. The surface temperature of the slab slab immediately before charging the slab slab into the block rolling facility was 250 ° C. below the ductile-brittle transition temperature. The thickness D1 [mm] of the slab slab before rolling is 280, and the thickness D2 [mm] of the slab steel slab after rolling is 225. Therefore, the rolling reduction R is 0.20. And apply slab steel slabs to the slab steel slabs carried out from the batch rolling equipment, cool the slab steel slabs to room temperature, and insert the cooled slab steel slabs into the hot rolling furnace. It was. In this test, the opening crack shown in FIG. 7 occurred before charging into the hot rolling furnace (defect B).
・ Test No. 4
This test is the same as the test No. described above. 3, the surface temperature of the slab slab immediately before charging the slab slab into the ingot rolling facility and the reduction ratio R are different. That is, the surface temperature of the slab slab immediately before the slab slab was charged into the block rolling equipment was a temperature that did not fall below the ductile-brittle transition temperature. The rolling reduction R is the above test No. Compared to 3, it is lower. In this test, cracks shown in FIG. 9 occurred in the four peripheral portions of the slab steel piece after cooling. In this test, it was an abnormal operation in which a plate breakage occurred during the above hot rolling (defects D and G).
・ Test No. 5
This test is the same as the test No. described above. Compared to 4, the rolling reduction R is set larger. In this test, the slab slab was not cracked and was able to operate normally.
・ Test No. 6
In this test, the above test no. Compared to 5, the cooling rate Vco [° C./hr] is increased due to the difference in the cooling method. In this test, a fine surface flaw as shown in FIG. 8 occurred in the slab steel piece after cooling (defect C). Moreover, in this test, perforation as shown in FIG. 10 occurred during hot rolling (defect F).
・ Test No. 7
In this test, test no. Compared to 6, the cooling rate Vco [° C./hr] is further increased due to the difference in the cooling method. In this test, breakage as shown in FIG. 6 occurred in the slab steel piece after cooling (defect A).
・ Test No. 11
In this test, test no. Compared with 5, the thickness D2 [mm] of the slab steel piece after the reduction was set small. In this test, it has warped in the hot-rolling furnace (defect E).

(Summary)
As described above, in the embodiment, when the slab slab cast by continuous casting is cooled to room temperature, the slab slab is cooled before the surface temperature of the slab slab falls below the ductile-brittle transition temperature. The slab is pressed at a reduction ratio R = 0.1 or more so that the thickness D2 [mm] of the slab steel slab after the reduction does not fall below 140, and the cooling rate Vco [° C./° C. of the slab steel slab after the reduction. hr] is 70 or less. According to this method, it is possible to prevent cracking of the slab slab as the overall evaluation is “good” in Table 1 above. Moreover, normal operation can be performed.

  The following are reference materials.

(Significance or detailed explanation of each steel type component (a) to (m))
-Carbon is an element necessary for ensuring the strength of steel, and is preferably contained in an amount of 0.06 wt% or more. On the other hand, if carbon is present in the molten steel in excess, the weldability is deteriorated, so that it is preferable to suppress it to 0.6 wt% or less. Refer to Japanese Patent No. 3889768 if necessary.
-Silicon is an element effective for promoting the C concentration to austenite and leaving austenite at room temperature to ensure an excellent strength-ductility balance. On the other hand, if silicon is present in excess, the effect of residual austenite becomes saturated, scale formation by hot rolling becomes remarkable, and the cost for removing wrinkles becomes expensive and economically undesirable. It is preferable to suppress it to 0 wt% or less. If necessary, refer to Japanese Patent No. 3889768 and Japanese Patent Application Laid-Open No. 2006-207019.
Manganese is an element necessary for stabilizing austenite and securing a predetermined amount of residual γ. On the other hand, if manganese is present in excess, both ductility and weldability deteriorate, so 6 wt% or less is preferable. If necessary, refer to Japanese Patent No. 3889768 and Japanese Patent Laid-Open No. 2005-336526.
・ Phosphorus is an element necessary for securing residual γ. On the other hand, when it exceeds 0.15 wt%, secondary workability will deteriorate. If necessary, refer to Japanese Patent Application Laid-Open No. 2005-336526.
Sulfur is an element that forms sulfide-based oxides such as MnS and causes cracking to deteriorate workability, and is preferably reduced as much as possible, that is, 0.02% or less. If necessary, refer to Japanese Patent Application Laid-Open No. 2005-336526.
-Molten aluminum is an element having a deoxidizing action, and is effective in preventing excessive oxygen from forming an oxide and causing local deterioration of workability, and promotes C concentration to austenite. It is an element effective for ensuring a superior strength-ductility balance by allowing austenite to remain at room temperature. On the other hand, if the amount of molten aluminum is excessive, not only the effect of securing retained austenite is saturated, but also steel embrittlement and cost increase are caused. Refer to Japanese Patent No. 3889768 if necessary. Note that “molten aluminum sol-Al” means aluminum that is melted alone in molten steel, not as an oxide or the like.
Copper is a useful element for ensuring the strength of steel and for stabilizing the residual γ and ensuring a predetermined amount. On the other hand, even if copper is added over 0.5%, the above effect is saturated, which is economically useless. If necessary, refer to Japanese Patent Application Laid-Open No. 2005-336526.
-Nickel is an element useful for ensuring the strength of the steel and for stabilizing the residual γ to ensure a predetermined amount. On the other hand, even if nickel is added in excess of 0.5%, the above effect is saturated, which is economically useless. If necessary, refer to Japanese Patent Application Laid-Open No. 2005-336526.
・ Chromium is an element useful for securing and securing the strength of steel. On the other hand, if there is an excess of chromium, both ductility and weldability deteriorate, so it is desirable to keep it to 1.0% or less. Refer to Japanese Patent No. 3889768 if necessary.
・ Molybdenum is a useful element to ensure and secure the strength of steel. On the other hand, since ductility deteriorates when molybdenum is present in excess, it is desirable to keep it at 1.0% or less. Refer to Japanese Patent No. 3889768 if necessary.
・ Niobium is a useful element for securing and securing the strength of steel. On the other hand, if niobium is present excessively, ductility deteriorates, so it is desirable to keep it at 0.1% or less. Refer to Japanese Patent No. 3889768 if necessary.
・ Titanium is a useful element for securing and securing the strength of steel. On the other hand, since ductility deteriorates when titanium is present in excess, it is desirable to keep it at 0.1% or less. Refer to Japanese Patent No. 3889768 if necessary.
-Nitrogen etc. are mentioned as an inevitable impurity.

(Method for estimating ductile-brittle transition temperature)
Welding susceptibility index (hereinafter referred to as Pcm) and reheat cracking susceptibility index (hereinafter referred to as Pcm), which are generally used as indices for evaluating cracks in welded parts, for evaluating the ease of breakage of slab slabs or slab steel slabs , Psr), HAZ coarse grain region ductility index (hereinafter, CE) and the like are also considered effective. The method of obtaining Pcm, Psr, and CE is as shown in the following formulas (2) to (4). However, in the following formulas (2) to (4), each element symbol means a weight percentage of the element.

  Next, refer to FIG. 11 to FIG. 13 showing the relationship between these indexes or indices and the ductile-brittle transition temperature. In FIG. 11, the horizontal axis represents the weld cracking sensitivity index Pcm, and the vertical axis represents the ductile-brittle transition temperature. In FIG. 12, the horizontal axis represents the reheat cracking sensitivity index Psr, and the vertical axis represents the ductile-brittle transition temperature. In FIG. 13, the horizontal axis represents the HAZ coarse grain region ductility index CE, and the vertical axis represents the ductile-brittle transition temperature. In each figure, data surrounded by a broken line is data for general carbon steel, and data surrounded by a solid line is data for alloy steel. In addition, black circles in each figure mean that no cracks occurred during cooling of the slab slab or slab steel slab, and x means that this breakage occurred. In the test corresponding to each plot, all the cooling uses stacked slow cooling, and no reduction as a countermeasure against cracking is performed. According to each figure, it turns out that the crack at the time of cooling of a slab cast piece or a slab steel piece can be divided into layers using a ductile-brittle transition temperature. With this in mind, in general carbon steel, the ease of cracking can be grasped only with the above index or index, but in order to evaluate the crackability of not only carbon steel but also alloy steel, In short, the ductile-brittle transition temperature should be the only indicator. Therefore, by regression from the test data shown in FIGS. 11 to 13, a new estimation formula for the ductile-brittle transition temperature that comprehensively considers molten elements such as Si, Ti, and P is as shown in the following formula (5). Propose to. However, in the following formula (5), each element symbol means the weight percent of the element, and the unit of the ductile-brittle transition temperature on the left side is ° C.

  In FIG. 14, the horizontal axis is an estimated value of the ductile-brittle transition temperature based on the above formula (5), and the vertical axis is the ductile-brittle transition temperature actually measured through the Charpy test. According to the figure, it can be seen that the ductility-brittle transition temperature can be accurately estimated from the molten steel components even if the steel type has an unknown ductility-brittle transition temperature according to the above equation (5). Test No. in Table 1 above. In Nos. 27 to 35, since the ductile-brittle transition temperature was unknown at the time of the test, the ductile-brittle transition temperature was estimated by the above formula (5). In addition, since the steel type which this application makes object has a ductile-brittle transition temperature of 160 degreeC or more, this limitation matter was imaged by the thick line arrow in FIG.

Claims (1)

The carbon content C [wt%] is 0.07 to 0.60,
The silicon content Si [wt%] is 0.1 to 3.0,
Manganese content Mn [wt%] 0.5-6.0,
The phosphorus content P [wt%] is 0.15 or less (however, 0 is not included),
Sulfur content S [wt%] is 0.02 or less (however, 0 is not included),
Molten aluminum sol-Al [wt%] is set to 0.01 to 3,
The copper content Cu [wt%] is 0.5 or less (including 0),
The nickel content Ni [wt%] is 0.5 or less (including 0),
Chromium content Cr [wt%] is 1.0 or less (however, including 0),
Molybdenum Mo [wt%] is 1.0 or less (including 0),
Niobium Nb [wt%] is 0.1 or less (including 0),
Titanium Ti [wt%] is 0.1 or less (however, 0 is not included),
The balance consists of iron Fe and inevitable impurities,
The ductile-brittle transition temperature is 160 ° C. or higher.
When cooling the slab slab cast by continuous casting to room temperature,
Before the surface temperature of the slab slab falls below the ductile-brittle transition temperature, the reduction ratio R = so that the thickness D2 [mm] of the slab steel slab after the reduction does not fall below 140 with respect to the slab slab. Apply a reduction above 0.1,
The cooling rate Vco [° C./hr] of the slab steel slab after the reduction is 70 or less,
The slab handling method at the time of cooling of the slab slab characterized by the above-mentioned.