JP4656088B2 - Continuous casting slab of steel having equiaxed dendrite on surface layer and method for continuous casting - Google Patents

Description

  The present invention relates to a continuous cast slab in which a solidified structure of a surface layer portion is made into a steel structure having an equiaxed dendrite structure by adding a metal element during a continuous casting process of steel, 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 by reducing the weight. Hot-rolled steel sheets for automobiles differ in required properties depending on the parts used, and have high strength and high toughness, as well as good deep drawability, stretchability, hole expansibility, and bendability. It is required to have. In order to improve these properties, it is necessary to optimize the metal structure and precipitates of the hot-rolled steel sheet in accordance with the required level of properties.

  For example, there is a technique disclosed in Patent Document 1 as a method for improving hole expandability in a high-strength steel sheet used as an undercarriage part of an automobile. This document consists of a ferrite and a second phase mainly composed of bainite. In addition, the total amount of inclusions in the steel sheet is 0.05% or less, and the total amount of A-based + B-based inclusions is 0.01% or less. A high-strength steel sheet excellent in hole expansibility and a method for producing the same are disclosed.

  In order to respond to the increasingly thinner and lighter steel sheets in the future, and to expand the application of thin steel sheets to automotive interior and exterior materials, it is necessary to further increase the strength and improve the surface properties of the steel sheets. However, it has been difficult to achieve both of them with the conventional technology. This is because, even if the strength of the steel sheet is improved by the control of the metal structure after hot rolling as described above, the control of the metal structure does not contribute to the improvement of the surface properties of the steel sheet. Thus, in order to achieve both the strength improvement of the steel sheet and the improvement of the surface properties, it is necessary to control not only the control of the metal structure after hot rolling, but also the solidification structure of the slab surface layer that governs the surface properties. is there.

  The solidified structure of the surface layer of the continuous cast slab is formed in the mold. The molten steel in the mold is removed with a water-cooled mold, and generally the structure continuously grows in the direction opposite to the direction of heat removal to form a columnar dendrite structure. During the solidification process, the solute component elements segregate in the gaps between adjacent columnar dendrites, so that the compositions at the center and the gaps of the columnar dendrites are different and the component composition becomes non-uniform. In the process of rolling from a continuous cast slab to a steel plate, it is not easy to eliminate the non-uniformity of the composition, so the hardness and ductility depending on the above component composition become non-uniform, and cracks and spots on the surface of the steel plate As a result, the surface properties are lowered.

  In order to eliminate the uneven composition due to the formation of the columnar dendrite, the gap between adjacent columnar dendrites may be reduced to promote the diffusion of segregated elements in the process after continuous casting. The interval between adjacent columnar dendrites varies depending on the cooling rate, and decreases as the cooling rate increases. However, since it is difficult to greatly change the heat removal ability of an actual continuous casting mold, it is difficult to change the interval between the columnar dendrites.

  By the way, the form of dendrite changes depending not only on the cooling rate but also on the composition of steel and the solid-liquid interface energy during solidification. However, the influence of the steel composition on the change of the dendrite form is small, and the dendrite form cannot be changed greatly so as to meet the required characteristics of the steel material.

  Therefore, it is considered effective to change the solid-liquid interface energy during solidification in order to greatly change the dendrite morphology in the normal continuous casting method. It is thought that the method of adding in is appropriate.

  To add metal elements to the molten steel, put the bulk metal elements on the surface of the molten steel, or a wire or rod made of a simple metal element, and cover these metal elements with aluminum or steel. For example, a method of adding a wire or a rod, a wire or a rod made of an alloy containing these metal elements, or the like is employed. However, using these methods, metal elements having a high vapor pressure and a low melting point such as magnesium (Mg), bismuth (Bi), calcium (Ca), and rare earth elements (REM) are accurately added to the molten steel. It is difficult. The reason is that when a metal element with a high vapor pressure is added to the molten steel, the metal element is vaporized and diffused into the atmosphere in the vicinity of the molten steel surface. This is because it is difficult to add uniformly and the addition yield also decreases.

  Moreover, since the volume expansion at the time of vaporization of a metal element is large, when vaporization is carried out in the vicinity of the molten steel surface, the molten steel is severely scattered, making it difficult to ensure the safety of the casting operation. Furthermore, when the melting point of the added metal element is low, it becomes difficult to add a predetermined amount by softening or melting by the radiant heat of the molten steel before the addition. When a metal element having a density lower than that of the molten steel is added, the added metal is unevenly distributed only in the surface layer portion of the molten steel and does not penetrate into the molten steel. In the case of adding a metal element having a high density, it only settles into the molten steel from the addition position, and it is difficult to uniformly mix the entire molten steel.

JP 2003-3240 A (Claims and paragraph [0009]) JP 2004-249315 A (claims and paragraphs [0011] to [0017]) JP 2005-169404 A (Claims and paragraphs [0011] to [0016]) JP 2005-219072 (Claims and paragraphs [0013] to [0020])

  The present invention has been made in view of the above problems, and the first of the problems can be used as a raw material for a high-strength cold-rolled steel sheet, and the continuous casting in which the surface layer structure is a structure having equiaxed dendrites. It is to provide a slab. The second problem is a continuous casting method in which an appropriate amount of a metal element necessary to obtain a slab having the above solidified structure can be efficiently added to the molten steel and uniformly dispersed in the slab. Is to provide.

  The form of the solidified tissue is determined by the temperature gradient at the solid-liquid interface during the solidification process and the growth rate of the solidified tissue. When the temperature gradient is constant, the tissue morphology changes depending on the growth rate of the solidified tissue. When the growth rate of the solidified tissue is small, a columnar dendrite structure is formed. It is formed. As the temperature gradient increases, the growth rate required to change from columnar dendrites to equiaxed dendrites increases. However, in reality, the surface layer structure of continuous cast slabs is columnar dendrites, and it is practically difficult to change the cooling conditions during continuous casting. Therefore, there is a correlation between the temperature gradient and the growth rate of the solidified structure. It is difficult to form an equiaxed dendrite by controlling.

  Therefore, the present inventors pay attention to the solid-liquid interfacial energy in the physical property values, and experimentally show that it is possible to change the form of the solidified structure by adding a small amount of a metal element having a large interfacial activity effect. In addition to the confirmation, the present inventors have studied a continuous casting method for efficiently and uniformly adding an appropriate amount of the above metal element into a continuous casting slab, and obtained the following knowledge (a) to (d). Was completed.

  (A) High-strength cold-rolled steel sheets such as automobile steel sheets have high strength, and surface properties need to be good for expanding their applications. The solidification structure of the surface layer portion of the continuous cast slab also affects the surface properties of the steel sheet after hot rolling and cold rolling.

  (B) The solidified structure of the slab surface layer that affects the surface properties of the steel sheet as in (a) above is usually a columnar dendrite, and solute component elements segregate in the gaps between adjacent columnar dendrites. In order to eliminate this segregation by diffusion, it is necessary to heat the slab to a high temperature and hold it for a long time, which significantly reduces the operation efficiency.

  (C) If the columnar dendrite structure can be changed to a fine equiaxed dendrite structure, the diffusion distance of segregated component elements can be reduced, and as a result, the surface properties of the steel sheet can be improved. As described above, it is possible to change the dendrite from a columnar structure to an equiaxed structure. For this purpose, metal elements such as bismuth (Bi), tellurium (Te), and tin (Sn) having a high surface-active effect are used. It is effective to add.

  (D) The metal element having a high surface-active effect as in (c) above has a high vapor pressure or a low melting point, and when these metal elements are added to the molten steel, these added metals are Melt or vaporize by contact or radiant heat from molten steel. If the metal elements are melted or vaporized before or when they are added to the molten steel, it becomes difficult to add these metal elements to the molten steel uniformly and with high yield. In order to solve such problems and to uniformly add the metal element into the continuous cast slab, the vapor of metal element and / or the metal in the tundish near the continuous cast mold or in the molten steel in the continuous cast mold The method of adding particles is optimal.

This invention is completed based on said knowledge, The summary exists in the continuous cast slab shown to following (1 ) .

(1) Metal vapor and / or metal particles are generated in the lance by inserting a metal wire or rod containing bismuth into the immersion lance immersed in molten steel in the tundish or molten steel in the mold, A continuous cast slab of steel produced by a continuous casting method in which metal vapor and / or metal particles are added to molten steel together with a carrier gas, wherein the bismuth content in the steel is 0.005 to 0.1% by mass The solid cast structure of the slab surface layer part is a steel structure having equiaxed dendrite, and the equiaxed dendrite ratio in the slab surface layer part is 0.7 to 1.0. Piece.

  In the present invention, “equiaxial dendrite” means a dendrite structure that grows three-dimensionally and isotropically, that is, spherically, with respect to the dendrite growth start point. The equiaxed dendrite grows in the supercooled molten steel near the slab surface, and the temperature distribution at this time is the highest at the dendrite growth interface, and the temperature decreases three-dimensionally isotropically as the distance from the growth interface increases. Become. Even when the columnar dendrite grows, a supercooled region is formed in the surface layer portion of the slab. However, when a surface active element such as Bi is present, the interfacial tension between the solid and the liquid is reduced. Does not become a columnar dendrite, but grows into an equiaxed dendrite structure.

  Further, the “slab surface layer portion” means a region within 10 mm from the surface of the slab toward the inside.

  “Steel structure having equiaxed dendrite” means a steel structure in which the ratio of the equiaxed dendrite structure in the steel structure (hereinafter also referred to as “equal axis dendrite ratio”) is 0.1 (10%) or more. To do. The method for obtaining the equiaxed dendrite ratio is as described in detail in the examples.

  The term “metal vapor and / or metal particle” means metal vapor and / or metal particles present as liquid or solid particles due to insufficient evaporation or metal particles formed by condensation of metal vapor. To do. The “metal” includes both pure metals and metal alloys.

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

  Since the slab of the present invention contains a metal element having a high surface-active effect such as Bi, the structure of the surface layer part is a structure having equiaxed dendrites, so a steel sheet having high strength and excellent surface properties is manufactured. It is suitable as a material for doing this. Further, 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 obtaining the above-described steel plate having high strength and good surface properties, to the molten steel, This is an optimum continuous casting method that can be uniformly dispersed in a cast slab.

As described above, the present invention provides metal vapor and / or metal particles in a lance by inserting a metal wire or rod containing bismuth into a dipped lance immersed in molten steel in a tundish or molten steel in a mold. A continuous cast slab of steel produced by a continuous casting method in which the metal vapor and / or metal particles are added to the molten steel together with a carrier gas, wherein the bismuth content in the steel is 0.005 to 0.005. Continuous casting of steel with 0.1% by mass, the solidified structure of the slab surface layer part is a steel structure having equiaxed dendrite, and the equiaxed dendrite ratio in the slab surface layer part is 0.7 to 1.0 It is a piece . Below, it will be described in more detail the contents of the present invention.

1. Confirmation of equiaxed dendrite formation by addition of metal element The present inventors focused on the interfacial energy between solid and liquid among the physical properties of molten steel, and solidified structure by adding a small amount of metal element having a large interfacial activity effect The possibility of adjustment was experimentally confirmed.

〔experimental method〕
Using a melting furnace, the molten steel whose component composition was adjusted under the following conditions in an argon gas atmosphere was put out into a copper mold to produce an ingot. The surface layer structure of this ingot was observed to investigate whether the dendrite form was a columnar dendrite or an equiaxed dendrite.

Molten steel composition: C: 0.09%, Si: 1.0%, Mn: 2.5%, P: 0.001%, S: 0.001%
Molten steel temperature: 1600 ° C
Molten steel volume: 5kg
Mold: copper 100 mm long x 100 mm wide x 70 mm high Additive metal element: Bi (content in steel: 0.0001%, 0.0005%, 0.001%, 0.002%, 0.005%, 0 .01%, 0.02%)
Metal element addition method: Bi is wrapped in iron foil and immersed in molten steel in the furnace. Detection of solidified structure: A sample is taken from the surface of the ingot, and the structure is detected using a picric acid saturated solution [Experimental result]
When the metallic element Bi was not added, the solidified structure of the surface layer within 10 mm from the surface of the ingot was a columnar dendrite. And as the Bi content in the steel was increased, the dendrite morphology changed as follows.

  That is, even when the Bi content in the steel was 0.0001%, the solidified structure form of the surface layer portion was columnar dendrite. However, in the case of containing Bi, the interval between the columnar dendrites was smaller than that in the case of not containing Bi, and the interval was further reduced as the Bi content increased. When the Bi content is 0.0002%, 0.0005%, 0.001%, 0.002%, and 0.005%, a mixed structure of columnar dendrites and equiaxed dendrites is exhibited, and Bi content is included. An equiaxed dendrite was obtained when the rate was 0.01% and 0.1%.

  From the above results, it was found that an equiaxed dendrite structure can be formed by containing Bi.

  Bi element has a melting point of 271 ° C. and a boiling point of 1560 ° C., both of which are low. When a metal element having a high vapor pressure or a metal having a low melting point is added to the molten steel, such as Bi, the added metal element is melted by contact with the molten steel or receiving radiant heat from the molten steel. Vaporize. If metal elements are melted or vaporized before or during the addition to the molten steel, it is difficult to uniformly add these metal elements to the molten steel with good yield. Further, in order to uniformly add the metal element into the continuous cast slab, the method of adding to the molten steel in the tundish near the continuous cast mold or the molten steel in the continuous cast mold is optimal.

  So far, the present inventors have efficiently added a vapor of a metal element or a compound of a metal element to molten steel in a tundish or molten steel in a continuous casting mold in Patent Document 2, Patent Document 3 and Patent Document 4. A method was proposed. By these methods, it has become possible to add an appropriate amount of a metal element or a compound of a metal element to molten steel uniformly and with good yield.

  Furthermore, the inventors of the present invention, when producing a slab whose surface layer structure is an equiaxed dendrite, efficiently and uniformly introduces a metal element having an effective effect in forming an equiaxed dendrite structure into the continuous cast slab. The present inventors have completed the present invention by studying the continuous casting method for adding, obtaining the above knowledge (a) to (d).

2. Preferred Mode In the present invention, the preferred range of Bi content for forming an equiaxed dendrite structure in the slab surface layer is 0.0002 to 0.1% from the results of Examples described later. If the Bi content is less than 0.0002%, it is difficult to obtain the effect of improving the surface properties of the steel sheet due to the formation of the equiaxed dendrite structure. On the other hand, if the Bi content exceeds 0.1%, This is because the entire surface layer portion has an equiaxed dendrite structure, and as a result, the effect of improving the surface properties of the steel sheet is saturated and an improvement effect commensurate with the addition cost of the metal element cannot be obtained .

A preferable range of the equiaxed dendrite ratio is 0.2 to 0.7. If the ratio is less than 0.2, the effect of improving the surface properties of the steel sheet is not sufficient, and in order to increase it beyond 0.7, the addition yield decreases. However, as shown in the examples described later, it is possible to obtain a steel sheet having a very good surface texture with a surface texture index of 0.98 or more from a cast slab having an equiaxed dendrite ratio exceeding 0.7. It is.

  In addition, as a metal element having a high surface-active effect that exhibits the same effect as Bi, tellurium (Te), tin (Sn), and the like can be used, and the preferred content is 0.0005 to 0.005%, respectively. Range.

In order to confirm the effects of the continuous casting slab and the continuous casting method of the present invention, the following continuous casting test was performed and the results were evaluated.
[Test conditions and test methods]
Molten steel composition: C: 0.09%, Si: 1.0%, Mn: 2.5%, P: 0.001%, S: 0.001%
Molten steel temperature: 1600 ° C
Mold size: 1200mm width x 250mm thickness
Casting speed: 1.2 m / min Addition metal: 80% Bi-20% Mg alloy Addition method: Supply a rod of 3 mm diameter made of the above metal into molten steel Supply speed of metal rod: 2 m / min Addition position: in tundish Molten steel lance immersion depth: 300mm (molten steel depth 700mm)
Carrier gas: Argon gas 10 L / min Gas pressure: 0.03 MPa
FIG. 1 shows a method of continuous casting while supplying a metal rod 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. The metal rod 50 containing the additive metal element is inserted from the metal rod feeder 5 into the hole of the immersion lance 4 immersed in the molten steel 1 in the tundish 2 at a predetermined speed. Metal vapor and / or metal particles are supplied into the molten steel 1 in the tundish 2.

  An upper end portion of the immersion lance 4 is connected to a metal rod feeder 5, and a metal rod 50 is loaded in the metal rod feeder 5. The metal rod 50 is inserted and supplied into the immersion lance 4 by a rod feeding roll 51 whose feeding speed is controlled by a rod feeding speed control device 52. The carrier gas 54 is introduced into the metal rod feeder 5 with its flow rate and pressure controlled by a flow rate control valve 56 and a pressure indicating control valve 55 that are operated according to a command of the flow rate pressure control device 57, together with the metal wire 50. It is supplied into the immersion lance 4. In the figure, reference numeral 53 denotes a guide for supplying the metal rod 50 to the rod feeding roll 51.

  In this experiment, a metal rod made of an alloy of Bi and Mg was used. However, a metal rod made of only Bi may be used.

  Next, the slab obtained by continuous casting as described above was reheated under the following conditions and hot-rolled to obtain a hot-rolled steel sheet.

Reheating temperature of continuously cast slab: 1250 ° C
Reheating time: 2 hours Hot rolling finishing temperature: 850-900 ° C
Hot rolled steel sheet thickness: 3mm
Winding temperature: 400-500 ° C
〔Test results〕
Table 1 shows the Bi content of the test conditions and the equiaxed dendrite ratio and the surface property index, which are test results.

  The equiaxed dendrite ratio and the surface texture index shown in the same table were determined by the following methods.

1) Equiaxial dendrite ratio In order to investigate whether the solidified structure of the surface layer portion of the slab is a columnar dendrite or an equiaxed dendrite, the structure was detected using a picric acid saturated solution and the form was observed. The observation of the structure was performed by continuously moving the field of view of the microscope at a magnification of 100 using an optical microscope in an area within a depth of 10 mm from the slab surface perpendicular to the slab surface. The area ratio occupied by the equiaxed dendrite was determined, and this was defined as the equiaxed dendrite ratio. As the slab surface goes deeper, the equiaxed dendrite ratio decreased and the columnar dendritic ratio tended to increase.

2) Surface texture index Since the main cause of the stripe pattern appearing on the surface of the steel sheet is segregation of solute component elements, the surface of the obtained hot-rolled steel sheet is subjected to line analysis by EPMA, and the minimum value of Mn content / Mn content The ratio value was calculated as the surface texture index. Here, the EPMA measurement conditions were such that the beam diameter was 1 μm and the length of the line analysis was in the range of 10 mm. In this case, the maximum value of the Mn content is the Mn content in the segregated portion of Mn, and the minimum value of the Mn content is the average value of the Mn content in the slab. Segregation is low, thus meaning that the surface properties are good.

  Test Nos. 1 to 7 are tests on examples of the present invention that contain Bi and the solidified structure of the slab surface layer portion has an equiaxed dendrite structure, and test numbers 8 and 9 are the solidified structures of the slab surface layer portion. This is a test for a comparative example having no equiaxed dendrite structure.

  In Test Nos. 1 to 7, which are examples of the present invention, a good surface texture with a surface texture index of the hot-rolled steel sheet exceeding 0.9 was obtained. In particular, in test numbers 6 and 7 having a Bi content of 0.01 to 0.1%, a steel sheet having a very good surface texture with a surface texture index of 0.98 or more is obtained.

  On the other hand, the test number 8 of the comparative example which does not contain Bi and the solidified structure of the slab surface layer portion does not have an equiaxed dendrite structure, and the solidified structure of the slab surface layer portion is too low because the Bi content is too low. In test number 9 having no axial dendrite structure, the steel sheet was inferior in surface quality with a surface texture index of about 0.7 or less.

  Here, throughout the test numbers 1 to 9, the tensile strength of the steel sheet was in the range of 590 to 780 MPa, which met the requirements for high-strength steel sheets.

  Since the slab of the present invention contains a metal element having a high surface-active effect such as Bi, the structure of the surface layer part is a structure having an equiaxed dendrite. Therefore, a steel plate having high strength and excellent surface properties can be obtained. It is suitable as a material for manufacturing. Further, 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 obtaining a steel plate having high strength and good surface properties, to the continuous steel, This is an optimum continuous casting method that can be uniformly dispersed in a piece. Therefore, the slab of the present invention is a raw material for producing steel sheets having excellent surface properties including hot rolled steel sheets for automobiles, and the casting method of the present invention is a continuous casting method of the above slab for producing steel sheets. It is a widely applicable technology.

It is a figure which shows the method of continuous casting, supplying a metal rod to the molten steel in a tundish through an immersion lance.

Explanation of symbols

1: molten steel, 2: tundish, 3: ladle, 4: immersion lance,
5: Metal rod feeder, 50: Metal rod, 51: Rod feeding roll,
52: Rod feeding speed control device, 53: Guide, 54: Carrier gas,
55: Pressure indicating control valve, 56: Flow control valve, 57: Flow pressure control device,
6: immersion nozzle, 7: solidified shell, 8: continuous casting mold

Claims (1)

Metal vapor and / or metal particles are generated in the lance by inserting a metal wire or rod containing bismuth into the immersion lance immersed in molten steel in the tundish or molten steel in the mold, and the metal vapor and A continuous cast slab of steel produced by a continuous casting method in which metal particles are added to molten steel together with a carrier gas,
The content of bismuth in the steel is 0.005 to 0.1% by mass,
A continuous cast slab of steel, characterized in that the solidified structure of the slab surface layer part is a steel structure having equiaxed dendrite, and the equiaxed dendrite ratio in the slab surface layer part is 0.7 to 1.0 .

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