JP5343433B2 - Continuous cast slab for high-strength steel sheet and its continuous casting method - Google Patents

Description

  The present invention relates to a continuous cast slab which is a material for a high strength steel sheet such as a high strength alloyed hot dip galvanized steel sheet and a continuous casting method thereof.

  Steel sheets used as materials for automobiles have been increased in strength for the purpose of reducing the environmental burden due to weight reduction, and in particular, there is an increasing need for steel sheets having a tensile strength of 540 MPa or more. Hot-rolled steel sheets for automobiles differ in required properties depending on the parts used, and have high strength and toughness, as well as workability such as good deep drawability, stretchability, hole expansibility or bendability. It is required to do.

  In the molding process, which is the final stage of product processing, the frequency of bending molding is the highest, and since it is processed into parts of various shapes depending on the combination thereof, it is necessary to have good bending moldability. Furthermore, since it is used as a component, it is necessary that the surface properties after bending be good.

  About the improvement of the bendability of a high-strength steel plate, the approach is made from the surface of control of steel structure conventionally. For example, in Patent Document 1, among the structures constituting the steel sheet, it is preferable to reduce the hardness of the low-temperature transformation generation phase and reduce the hardness difference from the ferrite phase.

  On the other hand, Patent Document 2 and Patent Document 3 disclose that when the ferrite crystal grains are refined, both stretch flangeability and local strength that require local deformability as well as bendability can be achieved.

  However, when manufacturing a high-strength steel sheet containing a large amount of Mn for the purpose of increasing the strength, it is necessary to consider the solidification process of the continuous cast slab. It has not been.

  In the solidification process of molten steel to which a large amount of Mn has been added, the equilibrium distribution coefficient of Mn is small, so segregation increases the Mn concentration between the dendritic microstructure trees found in the continuous cast slab after solidification. In the tree core, the Mn concentration decreases. For this reason, in the continuous cast slab, regions where the Mn concentration of the dendrite between the trees and the tree core are different are formed in layers.

  Thus, segregation in the solidification process causes local fluctuations in the composition within the solidified structure, and in particular, if the Mn concentration, which greatly affects the formation of the low-temperature transformation phase, periodically fluctuates, In a hot-rolled steel sheet that has been cold-rolled, if the Mn concentration is not sufficiently uniform by diffusion heat treatment, the structure becomes non-uniform in response to fluctuations in the Mn concentration.

  Therefore, the technique disclosed in Patent Document 1 not only makes it extremely difficult to precisely control the hardness of the ferrite phase and the low-temperature transformation phase in the entire steel sheet, but also a problem corresponding to local composition fluctuations. Due to the uniform structure, remarkable irregularities appear on the surface of the processed part, which are also observed visually. The unevenness further promotes non-uniform deformation, induces cracking, and degrades the bendability itself. Moreover, even if it does not lead to a crack, if unevenness exists in the processed part, not only the appearance is deteriorated, but also the collision performance as a part is deteriorated. The impact performance as a part means the ability to absorb energy by deforming a part obtained by press-molding a steel plate while securing strength at the time of collision.

  Further, the phase transformation phenomenon locally changes due to segregation in the solidification process, and the crystal grain size becomes non-uniform, so that the bendability is improved even by using the techniques disclosed in Patent Document 2 and Patent Document 3. I can't. In particular, in the techniques described in these documents, a large amount of Mn and Ni that easily segregate in steel is added, so that it is easy to be inferior in collision performance as a part as well as bendability during molding as described above. is expected.

  From the viewpoint of homogenization of the structure, there is an ultimate approach of single-phase organization. For example, Patent Document 4 discloses a martensite single-phase structure, which is the ultimate uniform structure, to improve stretch flangeability and bendability. It is described that it can be improved. However, as in the technique disclosed in Patent Document 4, if the steel structure is a single martensite phase, the flatness of the steel sheet is impaired by the undulations that occur during the phase transformation, and the dimensional accuracy is required for automobile parts. It becomes difficult to apply.

  From the above, in the steel plate obtained by rolling the continuous cast slab, while maintaining flatness, in order to achieve both bendability and high strength, while containing a large amount of Mn for high strength, It can be seen that the apparently contradictory properties of obtaining a uniform structure must be compatible.

  As a method for obtaining a uniform structure, an approach is adopted in which component segregation during solidification, which is the origin of the heterogeneous structure, is eliminated by diffusion. For example, Patent Document 5 describes that component segregation is reduced and the steel material is homogenized by performing a solution treatment for holding the steel material at a high temperature of 1250 ° C. or higher for a long time of 10 hours or longer. However, the process of holding at high temperature for a long time as described in Patent Document 5 is not practical because it causes a significant increase in cost and a decrease in productivity.

JP-A-62-153333 (Claims and upper right column on page 4) JP 2004-211126 A (claims, paragraphs [0014], [0047] and [0049]) Japanese Patent Laying-Open No. 2004-250774 (Claims and paragraph [0043]) JP 2002-161336 A (claims, paragraphs [0042], [0058] and [0062]) JP-A-4-191322 (Claims and second page) Mizukami, two others, “High Temperature Deformation Behavior of Peritectic Carbon Steel during Solidification”, ISIJ International, Japan Iron and Steel Institute, Vol. 42 (2002) No. 9, p. 964-97338

  The present invention has been made in view of the above problems, and the problem is that it has flatness and has a uniform structure even when it contains a large amount of Mn necessary for increasing the strength, and during bending. An object of the present invention is to provide a continuous cast slab capable of obtaining a high-strength steel sheet having excellent surface properties, and a casting method thereof.

  The solidification structure of the continuous cast slab usually has a dendrite form. This dendrite is formed due to diffusion of a solute element in the solidification process, and the solute element is concentrated in the dendrite between trees or the tree core depending on its equilibrium partition coefficient. Since Mn contained in the high-strength steel plate has a small equilibrium partition coefficient, it is concentrated in the portion between the trees, and the concentration in the tree core portion is reduced.

  When producing a high-strength steel sheet using this continuously cast slab as a raw material, the slab after continuous casting is kept in a heating furnace at about 1200 to 1300 ° C. for several hours, and then rolled in a hot rolling process and a cold rolling process. To do.

  Through detailed experiments and analyses, the inventors have found that segregation of Mn formed in the dendrite structure of continuous cast slabs can be eliminated by diffusion within the range of temperature and time in normal furnace operation. In addition, it was clarified that segregation remains even after the subsequent hot rolling process and cold rolling process.

  In particular, the streak pattern generated on the surface of the steel sheet corresponds to the primary arm interval of the dendrite formed in the surface layer part of the continuous cast slab, and hard pearlite or bainite is formed in the Mn-concentrated part between dendrite trees. It has been clarified that soft ferrite is formed in the core portion having a low Mn concentration. When such hard pearlite or bainite and soft ferrite are formed in layers in the steel sheet, a streak-like pattern is generated during bending of the steel sheet.

  One method for reducing the segregation of Mn, which is the cause of the streak pattern, is to reduce the Mn concentration. However, from the viewpoint of ensuring the strength of the steel sheet, the Mn concentration cannot be lowered to the extent that segregation is eliminated.

  As another method for reducing the segregation of Mn, there is a method for promoting the diffusion of Mn.

The Fourier law is known for heat conduction, and the Fourier number F ₀ = α · t / x ² is derived from the theoretical analysis result of heat conduction in a semi-infinite solid. Here, α: thermal diffusion coefficient (m ² / s), t: time (s), x: heat transfer distance (m).

By applying this Fourier number F ₀ to the solidification structure and element diffusion in the solidification process of steel, the Fourier number Fr = D · t / λ ² generally used as an index representing the effect of diffusion can be obtained. Here, D: diffusion coefficient of solute (cm ² / s), t: diffusion time (s), λ: diffusion distance (cm). This λ is equal to the dendrite primary arm spacing in the solidified tissue. The diffusion coefficient D is a function of the temperature T, and generally increases as the temperature increases.

  Using the Fourier number Fr, the following examination was performed on the effect of changing the operation factor in actual operation.

  In order to increase the Fourier number Fr, it is necessary to reduce the diffusion distance λ or increase the diffusion coefficient D or the diffusion time t.

  As described above, the diffusion distance λ corresponds to the primary arm interval of the dendrite found in the continuous cast slab, and usually decreases in inverse proportion to the 1/2 power of the cooling rate of the slab. However, since the power index is ½, the change in λ is small even if the cooling rate changes greatly, and it is difficult to change the cooling rate greatly in a normal continuous casting method. Therefore, it is difficult to reduce λ in actual operation.

  The diffusion coefficient D can be increased by increasing the temperature T. In operation, the temperature of the heating furnace is increased. However, since the normal operating temperature is about 1200 to 1300 ° C., raising the temperature beyond this will not only significantly increase the cost, but also increase the amount of scale generated during heating, and increase the yield. The surface property of the steel sheet after rolling due to deterioration of the surface property of the slab is deteriorated. Therefore, it is practically difficult to increase the temperature T, that is, increase the diffusion coefficient D in actual operation.

  Increasing the diffusion time t extends the charging time into the heating furnace in operation. Assuming that the normal heating time is several hours, it is estimated that several times as much is required to eliminate the segregation of Mn. If the charging time is extended in the current operation, the production efficiency is greatly reduced, so it is practically difficult to increase the time t.

  From the above, it can be seen that increasing the Fourier number Fr in order to reduce the segregation of Mn is not realistic.

  By the way, when the solidification mode of high-strength steel is simply examined from the equilibrium diagram, the following three solidification modes can be considered. In the first solidification mode, the δ phase of the primary crystal is crystallized from the liquid phase and becomes a two-phase coexistence state of the liquid phase and the δ phase. When solidification at the solidus temperature is completed, the δ phase becomes a single phase. The δ / γ transformation occurs and shifts to a two-phase coexistence state of δ phase and γ phase. In the second solidification mode, the δ phase of the primary crystal is crystallized from the liquid phase to be in a two-phase coexistence state of the liquid phase and the δ phase, and then the γ phase is crystallized or precipitated to be in the three-phase coexistence state. When solidification at the line temperature is completed, the state shifts to a two-phase coexistence state of δ phase and γ phase. In the third solidification mode, the δ phase of the primary crystal is crystallized from the liquid phase and becomes a two-phase coexistence state of the liquid phase and the δ phase, and then the γ phase crystallizes or precipitates to become the three-phase coexistence state. After that, the δ phase disappears and the liquid phase and the γ phase coexist, and when the solidification at the solidus temperature is completed, the phase shifts to the γ phase single phase.

  As described above, since the dendrite structure is formed while the solute is redistributed, it is understood that the diffusion coefficient dominates the formation of the structure. Mn, which is a problem in high-strength steel sheets, has a different diffusion coefficient in the δ phase and in the γ phase, and the diffusion coefficient in the δ phase is larger than the diffusion coefficient in the γ phase. Increasing the proportion of the δ phase at the end of solidification when the formation of Mn is completed facilitates the diffusion of Mn and reduces the segregation of Mn.

  Therefore, the present inventors focused on the relationship between the solidification mode of the continuous cast slab and the streak pattern of the steel plate as the final product, and performed a basic study.

  First, the thermodynamic equilibrium calculation software Thermo-Calc was used to calculate the relationship between the abundance ratios of δ phase and γ phase and temperature of Fe-0.1 mass% C-2.5 mass% Mn steel. As a result, it was predicted that the proportion of the δ phase in the solidified tissue at the time of completion of solidification (solidus temperature) was as small as about 30%. However, this is a calculated value for an equilibrium state, and the influence of diffusion of Mn, which is a solute element, is not taken into consideration, and therefore it is assumed that evaluation regarding the occurrence of streak patterns related to the diffusion phenomenon cannot be performed.

Next, in order to examine the influence of diffusion, the analysis method described in Non-Patent Document 1 was used to calculate the relationship between the existence ratio of the δ phase and the γ phase and the temperature in the solidification process considering diffusion. The diffusion coefficient of Mn is 7.6 × 10 ⁻⁵ × exp (−53640 / RT) m ² / s in the δ phase, and 5.5 × 10 ⁻⁶ × exp (−59600 / RT) m ² / s in the γ phase. In the liquid phase, it was 4.4 × 10 ⁻⁹ m ² / s.

  As a result, the proportion of the δ phase at the completion of solidification was about 50%, which was different from the result of simple equilibrium calculation. This also indicates that the phenomenon of the solidification process cannot be accurately evaluated by equilibrium calculation. Therefore, an actual coagulated tissue was selected for evaluation.

  In general, when evaluating a solidified structure, the surface of a sample is mirror-polished, and then the structure is revealed by etching and observed with an optical microscope. A picric acid solution is usually used to reveal a dendrite-shaped solidified structure of carbon steel. In etching using a picric acid solution, the γ phase of the Mn-concentrated portion between dendritic trees is etched, and when viewed with an optical microscope, it appears black and the δ phase that is not etched appears white. Therefore, it is possible to determine the δ phase and the γ phase and to determine the ratio, and when the etching conditions are the same, it is possible to compare the ratio of the δ phase and the γ phase among a plurality of samples.

  Under the conditions shown in Table 1, the solidified structure of the continuously cast slab was revealed, photographed with a CCD camera through an optical microscope with a magnification of 50 times, and the ratio of δ phase and γ phase in the slab was calculated by image analysis.

  The inventors of the present invention manufactured steel sheets using continuous cast slabs with different component compositions, and the Mn segregation ratio at which streaks occur during bending, and the δ phase in the slabs calculated by the above analysis. The ratio of the δ phase was clarified, and the ratio of the δ phase capable of suppressing the stripe pattern was obtained. As a result, it was found that when the proportion of the δ phase at the time of solidification was 50% or more, the Mn segregation ratio was 1.12 or less, and no streak pattern was generated during bending of the steel sheet.

  Here, the Mn segregation ratio is defined by (Mn concentration between dendrite trees (mass%)) / (Mn concentration in molten steel (mass%) in the initial molten steel). Using. The beam diameter at the time of measurement by EPMA was 1 μm, a line analysis of a region of 50 mm was performed in a direction perpendicular to the dendrite growth direction, and the maximum value was defined as the Mn concentration between dendrite trees.

  As described above, the research focusing on the fact that the streak pattern of the steel sheet is related to the dendrite structure of the continuous cast slab and the phase transformation of the slab is the first research in the present invention. There is no literature reported.

  This invention is made | formed based on said knowledge, The summary exists in the continuous casting slab shown to following (1), and the continuous casting method shown in (2).

(1) By mass%, C: 0.03 to 0.15%, Si: 0.005 to 0.5%, Mn: 3.1 to 4.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01-1.0%, N: 0.01% or less, Mg: 0.0001-0.0015% and Ti: 0.003-0.50%, Furthermore, it contains one or more of Nb: 0.50% or less, Ca: 0.01% or less, and REM: 0.01% or less, with the balance being composed of Fe and impurities, A continuous cast slab for a high-strength steel sheet, wherein the proportion of the δ phase in the solidified structure at a line temperature is 50% or more.

(2) A continuous casting method for producing the slab according to (1) above, wherein Mg and Ca and / or REM are immersed in a dipping lance immersed in molten steel in a tundish or molten steel in a mold. Characterized in that a metal vapor and / or metal particles are generated in a lance by inserting a metal wire or rod containing the metal, and the metal vapor and / or metal particles are added to the molten steel together with a carrier gas. Continuous casting method.

  In the present invention, “metal vapor and / or metal particles” means metal vapor and / or metal particles present as liquid or solid particles due to insufficient evaporation, or metal formed by condensation of metal vapor. Means particles. The “metal” includes both pure metals and alloys.

  In the following description, “mass%” regarding the composition of steel is also simply expressed as “%”.

  The slab of the present invention is suitable as a material for a steel sheet having a tensile strength of 540 MPa or more, and by using this slab as a material, it is possible to suppress the generation of surface streaks during bending of the steel sheet. Also, it can be expected to exhibit good performance in terms of hole expansibility, ductility, and material stability.

  In addition, the continuous casting method of the present invention efficiently adds an appropriate amount of a metal element having a high vapor pressure and a low melting point, which is necessary for producing a slab to be a raw material for the steel sheet, into the molten steel. This is an optimum continuous casting method for uniformly dispersing in the slab.

  As described above, the continuous cast slab of the present invention is in mass%, C: 0.03 to 0.15%, Si: 0.005 to 0.5%, Mn: 2.0 to 4.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 1.0%, N: 0.01% or less, Mg: 0.0001 to 0.005%, and Ti: 0.00. 003 to 0.50%, Nb: 0.50% or less, Ca: 0.01% or less and REM: 0.01% or less, and the balance is Fe and A continuous cast slab for a high-strength steel sheet having a composition composed of impurities and having a δ phase ratio in a solidified structure at a solidus temperature of 50% or more. Hereinafter, the contents of the present invention will be described in more detail.

1. Steel composition range and reasons for limitation C: 0.03 to 0.15%
C is an element necessary for ensuring the strength of the steel, and in order to obtain the effect, the content needs to be 0.03% or more. However, when the content exceeds 0.15%, the proportion of the δ phase in the continuous cast slab decreases. Therefore, the appropriate range of C is set to 0.03 to 0.15%.

Si: 0.005 to 0.5%
Si is an element that improves strength by solid solution strengthening and promotes ferrite transformation to improve ductility and the like, and its content needs to be 0.005% or more in order to obtain the effect. However, when the content is higher than 0.5%, the wettability and adhesion of the plating on the steel sheet surface decrease. Therefore, the appropriate range of Si is set to 0.005 to 0.5%.

Mn: 2.0 to 4.0%
Mn is an element that has the effect of improving the strength by solid solution strengthening, lowering the Ac ₃ transformation temperature of the steel, and widening the suitable annealing temperature range. To obtain the effect, the content is 2.0%. It is necessary to do it above. However, when the content is higher than 4.0%, not only the generation of streak patterns due to segregation becomes remarkable, but also ferrite transformation is suppressed and ductility deteriorates. Therefore, the appropriate range of Mn is set to 2.0 to 4.0%.

P: 0.1% or less P is an element having an action effective for increasing the strength of a steel sheet. To obtain the effect, the content is preferably 0.0005% or more. However, if the content exceeds 0.1%, the weldability of the steel sheet decreases. Therefore, the appropriate range of P is limited to 0.1% or less.

S: 0.01% or less S is an element that forms MnS inclusions and the like and decreases the ductility and hole expansibility of the steel sheet. For this reason, the content was limited to 0.01% or less.

Al: 0.01 to 1.0%
Al is an element having a deoxidizing action of molten steel, and has an action of improving the yield of carbonitride-forming elements such as Ti. In order to achieve both of these effects, the content needs to be 0.01% or more. However, if the content exceeds 1.0%, the amount of coarse oxide inclusions in the steel increases, which adversely affects the surface properties and weldability of the steel sheet. For the above reason, the appropriate range of the content is set to 0.01 to 1.0%.

N: 0.01 or less N is an element that is inevitably contained in general, but in the present invention, a Ti-based, Nb-based, or Ti-Nb composite-based nitride or carbonitride is contained in the steel sheet. It is an element effective to increase the strength of the steel sheet when formed. In order to obtain the effect, the content is preferably 0.0005% or more. On the other hand, if the content is excessively large, coarse TiN is formed and the bendability and hole expansibility deteriorate, so the content needs to be 0.01% or less.

Mg: 0.0001 to 0.005%
Mg reacts with oxygen in the molten steel to produce Mg oxide. The Mg oxides in the molten steel, which contains one or more of the other MgO alone such as MgO and _{Al 2 O 3, SiO 2,} Ti 2 O 3 is generated. These oxides are finely dispersed in the steel and improve the workability of the steel sheet. Moreover, it reacts with S in steel and produces | generates a MgS compound, and there also exists an effect which suppresses the production | generation of MnS which reduces the mechanical characteristic of a steel plate. In order to obtain these effects, the content needs to be 0.0001% or more. However, if the content exceeds 0.005%, the amount of coarse oxide inclusions in the steel increases, which adversely affects the strength of the steel sheet. For the above reason, the appropriate range of the content is set to 0.0001 to 0.005%.

Ti: 0.003-0.5%
Ti mainly precipitates carbides, nitrides or carbonitrides and promotes the enhancement of the base metal strength and the refinement of the crystal grain size by its precipitation strengthening action, and has an effective action for improving the bendability of the steel sheet. It is an element. In order to sufficiently obtain the effect, the content needs to be 0.003% or more. On the other hand, when the content exceeds 0.5%, coarse precipitates and inclusions are formed in the steel, and the ductility, toughness and workability of the steel are reduced. For the above reason, the appropriate range of the content is set to 0.003 to 0.5%.

  About Nb, Ca, and REM, 1 type (s) or 2 or more types are included in the range of the following content among these component elements.

Nb: 0.50% or less Nb is an element that mainly forms carbides, nitrides, or carbonitrides and has an action effective for increasing the strength of steel sheets. In addition, it is an element effective for improving the bendability of a steel sheet because it is effective in making crystal grains finer. Furthermore, it has the effect | action which suppresses the production | generation of a hard transformation phase while remarkably accelerating the ferrite transformation at the time of cooling after annealing. However, even if contained excessively, the above effect is saturated and the cost increases, so the content is made 0.50% or less. Moreover, the preferable minimum in the case of making it contain is 0.001%.

Ca: 0.01% or less Ca reacts with S in steel to produce CaS, and has the action of suppressing the production of MnS, which reduces the ductility and hole expansibility of the steel sheet. It is effective to improve. However, even if contained excessively, the above effect is saturated and coarse inclusions are generated, so that the mechanical properties are lowered. Therefore, the content was made 0.01% or less. Moreover, the preferable minimum in the case of making it contain is 0.0005%.

REM: 0.01% or less REM has an effect of suppressing the generation of MnS which reacts with S in steel to generate sulfides and lowers the mechanical properties of the steel sheet. However, even if it contains too much, said effect will be saturated. Therefore, the content was made 0.01% or less. Here, REM means a compound containing one or more of Nd, Ce, Pr, Dy, Sm and Y. Moreover, the preferable minimum in the case of making it contain is 0.0001%.

  By setting the steel composition of the continuous cast slab in the above range, the ratio of the δ phase of the solidified structure at the solidus temperature can be set to 50% or more, so that the segregation of Mn can be reduced. Generation | occurrence | production of the stripe pattern at the time of the bending process of the steel plate manufactured using the slab can be suppressed.

2. Continuous casting method As described above, the continuous casting method of the present invention inserts a metal wire or rod containing Mg and Ca and / or REM into a dipping lance immersed in molten steel in a tundish or molten steel in a mold. Thus, metal vapor and / or metal particles are generated in the lance, and the metal vapor and / or metal particles are added to the molten steel together with the carrier gas. By adding Mg and Ca and / or REM by such a method, an appropriate amount of these elements can be efficiently added to the molten steel and uniformly dispersed in the continuous cast slab.

  As an apparatus for carrying out this continuous casting method, for example, as described in the examples described later, a tundish, an immersion nozzle provided at the lower part of the tundish and for supplying molten steel in the tundish to the mold, A mold located below the tundish, a dipping lance for supplying a wire or rod to the molten steel in the tundish, or a dipping lance for supplying a wire or rod to the molten steel in the mold, and a hole in the dipping lance A continuous casting apparatus having a wire or rod supply device for supplying a wire or a rod to the steel and a gas supply device for supplying a carrier gas into the immersion lance is suitable.

  In order to confirm the effects of the continuous cast slab and the continuous casting method of the present invention, the following tests were conducted and the results were evaluated.

[Casting conditions]
Molten steel components: each component of C, Si, Mn, P, S, Al, N, Ti, and Nb is a molten steel prepared in the composition described in Table 2 to be described later, and each component of Mg, Ca, and REM Is added to the composition shown in Table 2 by the following addition method Molten steel temperature: 1600 ° C
Mold size: width 1100mm x thickness 250mm
Casting speed: 1.0 m / min Addition metal: Addition in combination of Mg, Ca and REM as follows Addition method: Supply metal wire with a diameter of 3 mm having the following composition into molten steel When Mg is added: Pure Mg metal Using wire Mg + Ca added: 40% Ca-Mg alloy wire used Mg + REM added: 5% REM-Mg alloy wire used Mg + Ca + REM added: 5% REM-40% Ca-Mg alloy wire used REM In case of addition: REM single wire is used Addition position: in tundish Carrier gas: Argon gas 10 L / min Gas pressure: 0.05 MPa

  FIG. 1 shows a method of continuous casting while supplying a metal wire to molten steel in a tundish through an immersion lance. The molten steel 1 supplied from the ladle 3 to the tundish 2 is poured into the mold 8 through the immersion nozzle 6 and forms a solidified shell 7 while being drawn downward to form a slab. A metal wire 50 containing an additive metal element is inserted into the hole of the immersion lance 4 immersed in the molten steel 1 in the tundish 2 at a predetermined speed.

  The upper end of the immersion lance 4 is connected to the wire feeder 5. A wire reel 51 is loaded in the metal wire feeder 5, and the metal wire 50 is inserted and supplied into the immersion lance 4 by a wire feeding roll 52 whose feeding speed is controlled by a wire feeding speed control device 53. . The metal wire feeder 5 is introduced with a flow rate control valve 56 that operates in accordance with a command from the flow rate pressure control device 57 and a carrier gas 54 whose flow rate and pressure are controlled by the pressure indication control valve 55, and the immersion lance 4 together with the metal wire 50. Supplied in.

  Then, using the continuous cast slab obtained by the continuous casting test as a raw material, hot rolling and cold rolling were performed, and a steel plate was prototyped. In this test, in order to collect a test piece for EPMA analysis, the continuous cast slab was once cooled to room temperature, then charged in a heating furnace and heated to a predetermined temperature, followed by hot rolling. Cold rolling was performed. The rolling conditions were as shown below.

[Rolling conditions]
Rolling start temperature of steel material: 1050-1300 ° C
Finishing temperature: 800-950 ° C
Winding temperature: 450-750 ° C
Hot rolling the total reduction ratio in cold rolling: 97% Annealing temperature: Ac ₃ transformation point to 950 ° C.
Annealing time: 5 to 200 seconds Average cooling rate from 750 ° C to 600 ° C: 1 to 50 ° C / second

  The composition of the molten steel cast in the test of the present invention example is shown in the columns of Invention Examples 1 to 6 in Table 2, and the composition of the molten steel cast in the test of the comparative example in which Mg is not added is shown in Table 2. It showed in the column of the comparative examples 1 and 2 in the inside.

  In this test, continuous casting was performed by changing the molten steel components to produce a continuous cast slab. And the test piece of width 50mm x length 50mm x thickness 8mm was extract | collected in the thickness direction from the surface layer of the obtained slab, and Mn density | concentration distribution of the solidification structure | tissue was analyzed using EPMA. In the analysis by EPMA, the beam diameter at the time of measurement was set to 1 μm, and a line analysis of a region of 25 mm was performed in a direction perpendicular to the dendrite growth direction.

  Moreover, about the steel plate produced using the continuous cast slab as the raw material, a U-bending test with a tip angle of 180 ° was carried out, and the presence or absence of streaks on the surface of the bent portion was investigated. For the bending specimen, a specimen having a width of 40 mm, a length of 100 mm, and a thickness of 2.6 mm is taken so that the longitudinal direction of the specimen is perpendicular to the rolling direction of the steel sheet, and the bending ridge line is the rolling direction. It was made to be bent as follows.

〔Test results〕
Three types of items were evaluated for continuous cast slabs and steel plates produced under the above conditions. The test results are shown in Table 2 together with the composition. The evaluation items were “ferrite ratio”, “Mn segregation ratio”, and “streaks”.

  “Ferrite ratio” represents the ratio of the δ phase at a solid phase ratio of 1.0. After the mirror sample of the cross section of the collected slab sample is mirror-polished, the microstructure revealed with a picric acid saturated solution is observed with an optical microscope. This is a value evaluated by image analysis.

  As described above, the “Mn segregation ratio” is a value obtained by analyzing a test piece collected from a continuous cast slab by EPMA and dividing the maximum value of the obtained Mn concentration by the Mn concentration in the initial molten steel. means.

  And "streaks generation | occurrence | production" shows the result of having discriminate | determined by visual observation the presence or absence of the generation | occurrence | production of the stripe pattern in the surface of a bending part at the time of implementing a U bending test about the steel plate produced from the continuous casting slab.

  The ferrite ratio, that is, the ratio of the δ phase was 25% and 12% in Comparative Examples 1 and 2 not containing Mg. On the other hand, in Examples 1 to 6 of the present invention containing Mg and further containing one or more of Nb, Ca and REM, good results were obtained in which the proportion of δ phase was 54 to 98%.

  Moreover, although Mn segregation ratio was as large as 1.25 and 1.47 in Comparative Examples 1 and 2, it was 1.03 to 1.10 in Invention Examples 1 to 6, compared with Comparative Examples 1 and 2. Small and good value. This is considered to be because in Examples 1 to 6 of the present invention, the ferrite ratio was 50% or more, and Mn diffusion was easy to proceed.

  In Comparative Examples 1 and 2 having a large Mn segregation ratio, both streaks were generated in the U-bending test, but in Examples 1 to 6 of the present invention having a small Mn segregation ratio, streaks were not generated and the surface properties were good. there were.

  Since the continuous cast slab of the present invention has a larger proportion of δ phase having a larger diffusion coefficient of Mn than that of γ phase, segregation of Mn hardly occurs. Generation of streaks on the surface can be suppressed, and it is suitable as a high-strength, high-workability steel sheet including a high-strength steel sheet for automobiles. In addition, the continuous casting method of the present invention is an optimum continuous casting method for efficiently adding an appropriate amount of a metal element necessary for obtaining the above slab into the molten steel and uniformly dispersing it in the continuous casting slab. It is.

  Therefore, the slab of the present invention is used as a structural or processing steel material excellent in strength, toughness and workability including hot rolled steel sheets for automobiles. As a continuous casting method for casting slabs, it can be widely applied.

It is a figure which shows the method of continuous casting, supplying a metal wire through the immersion lance and supplying the molten steel in a tundish directly.

Explanation of symbols

1: molten steel, 2: tundish, 3: ladle, 4: immersion lance,
5: Metal wire feeder, 50: Metal wire, 51: Wire reel,
52: Wire feeding roll, 53: Wire feeding speed control device,
54: Carrier gas, 55: Pressure indicating control valve, 56: Flow control valve,
57: Flow rate pressure control device, 6: Immersion nozzle, 7: Solidified shell, 8: Continuous casting mold

Claims (2)

In mass%, C: 0.03 to 0.15%, Si: 0.005 to 0.5%, Mn: 3.1 to 4.0%, P: 0.1% or less, S: 0.01 %: Al: 0.01 to 1.0%, N: 0.01% or less, Mg: 0.0001 to 0.0015% and Ti: 0.003 to 0.50%, and Nb : 0.50% or less, Ca: 0.01% or less, and REM: 0.01% or less, containing one or more of them, the balance being composed of Fe and impurities, at the solidus temperature A continuous cast slab for a high-strength steel sheet, wherein the ratio of the δ phase in the solidified structure of the steel is 50% or more.   A continuous casting method for producing a slab according to claim 1, wherein the metal contains Mg and Ca and / or REM in an immersion lance immersed in molten steel in a tundish or molten steel in a mold. A continuous casting method for steel, characterized by generating metal vapor and / or metal particles in a lance by inserting a wire or a rod, and adding the metal vapor and / or metal particles together with a carrier gas into molten steel .

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