JP4569458B2 - Continuous casting method of steel material with finely dispersed precipitates and slab for steel material - Google Patents

Description

  The present invention relates to a steel material produced by using a slab in which fine precipitates are dispersed and formed by adding a metal element in the continuous casting process of steel, and the growth of austenite grain size during reheating is suppressed, and a steel material The present invention relates to a continuous casting method for casting a casting slab.

  Hot rolled 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. Among these, it is important to make the size of the precipitate fine and to disperse it uniformly in the steel sheet.

For example, as a method for improving hole expandability in a high-strength hot-rolled steel sheet used as an undercarriage part of an automobile, there is a technique disclosed in Patent Document 1. 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 hot-rolled steel sheet excellent in hole expansibility and HAZ fatigue characteristics and a method for producing the same are disclosed. Patent Document 2 discloses that MgO having a particle diameter in the range of 0.005 to 0.5 μm, or MgO-containing Al ₂ O ₃ , SiO ₂ , MnO, Ti ₂ O ₃ or one or more of them. Hole expandability and ductility characterized in that complex oxide is included in the range of 1.0 × 10 ³ to 1.0 × 10 ⁷ pieces / mm ² and the steel structure is mainly composed of ferrite structure and residual bainite structure. A high-strength hot-rolled steel sheet excellent in the above is disclosed. In any of these techniques, a method of generating magnesium compound by adding magnesium (Mg) to molten steel is employed.

  Further, steel plates for automobiles are often joined by welding, and as a result, when the steel plate is thinned, the amount of heat input during welding per unit volume of the steel plate significantly increases. As the heat input during welding increases, the mechanical properties of the heat-affected zone (hereinafter also referred to as “HAZ”) due to large heat input welding, as seen in the case of large heat input welding of thick plates, are reduced. Provoke. In order to prevent the deterioration of the mechanical properties, it is necessary to suppress the coarsening of the crystal grains. For the suppression of the coarsening of the crystal grains, the fine Mg compound particles generated by the above-mentioned Mg addition The pinning effect by has contributed greatly.

  However, in order to reduce the thickness and weight of hot-rolled steel sheets, which will be further promoted in the future, further enhancement of strength is required, and as described above, it is not possible to adequately cope with crystal grain coarsening suppression technology by simply adding Mg. Therefore, even in the case of the same Mg addition, how finely and evenly dispersed Mg added in the molten steel can be dispersed in the continuous cast slab is important for increasing the strength of the hot rolled steel sheet. is there.

  Since the boiling point of Mg is significantly lower than the molten steel temperature, even if it tries to add Mg ingot into the molten steel, it burns and wears until it reaches the molten steel surface, or even if it enters the molten steel, it is abrupt. Due to the volume expansion, the molten steel scatters violently, making continuous casting operation difficult. Mg can be alloyed with Si, Ni, etc. to increase the boiling point, but Si and Ni, which are not intended to be added, will be mixed in the molten steel, and the mechanical properties of the hot rolled steel sheet will be reduced. Damaged. Even if Mg is added to the molten steel, since the amount of Mg dissolved is small and the vapor pressure is high, the Mg concentration rapidly decreases simultaneously with the addition, and it is difficult to maintain the effect of adding Mg.

JP 2003-3240 A (Claims and paragraph [0009]) JP 2001-342543 A (Claims and paragraphs [0006] to [0015]) JP-A-2004-249315 (Claims etc.) Japanese Patent Laying-Open No. 2005-169404 (claims, etc.) Japanese Patent Application No. 2004-27514 (claims, etc.)

  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 hot-rolled steel sheet, and the austenite grains during reheating can be obtained by fine dispersion of precipitates. An object of the present invention is to provide a steel material having a structure in which growth is suppressed. The second of the problems is to provide a continuous casting method capable of efficiently adding a metal element necessary for obtaining the above steel material into molten steel and finely and uniformly dispersing it in the continuous casting slab. is there.

  In order to solve the above-mentioned problems, the present inventors efficiently added metal elements for suppressing the growth of austenite crystal grains during reheating of the steel material, and those metal elements in the continuous casting slab, And the continuous casting method for adding uniformly was examined, the knowledge of following (a)-(d) was obtained, and this invention was completed.

  (A) In order to suppress the growth of austenite crystal grains at the time of reheating the steel material, it is effective to add Ag and Mg, or Bi in addition thereto, and form fine oxides in a dispersed manner in the steel. is there. Furthermore, said effect can be heightened by adding 1 or more types of Ca, Nd, and Sn. In order to add these metal elements uniformly and efficiently into the continuous cast slab, it is effective to add metal element vapor to the molten steel in the tundish or the molten steel in the mold.

  Here, “reheating” means heating or soaking performed prior to the block rolling or hot rolling, or heating by welding. In addition, “the growth of austenite crystal grains during reheating was suppressed” means that the relative growth of crystal grains in the case of not containing any of Ag, Mg, Bi, Ca, Nd, and Sn This means that the growth of crystal grains is small.

  (B) The grain growth suppression effect of (a) is based on the following action. That is, Ag, Bi, and Mg react with oxygen in the molten steel to crystallize fine Ag oxide, Bi oxide, and Mg oxide, respectively. Coarseness is suppressed. In addition, since solute Bi has a surface-active effect, the effect promotes heterogeneous nucleation and suppresses coarsening of crystal grains. When these precipitates are miniaturized, for example, stress concentration on the precipitates is less likely to occur during processing such as hole expansion processing, so that the generation of cracks is suppressed, and therefore workability such as hole expandability is reduced. Remarkably improved.

  (C) Furthermore, Ca, Nd, and Sn each have a “drawing effect at the crystal grain boundary” described later in the solid solution state, and this action suppresses the coarsening of the crystal grains. Moreover, since Nd crystallizes an oxide, the coarsening of a crystal grain is suppressed by the pinning effect by the crystallized substance. And since solid solution Sn has a surface active effect, the heterogeneous nucleation is accelerated | stimulated by the effect and a grain growth is suppressed.

  (D) When a metal element having a high vapor pressure or a metal element having a low melting point as described in the above (a) to (c) is added to the molten steel, the added metal is in contact with the molten steel or molten steel. It melts or vaporizes in response to radiant heat. If the metal elements are melted or vaporized before or at the moment of addition to the molten steel, it is difficult to add these metal elements to the molten steel uniformly and with a high yield. In order to solve such problems and to uniformly add a metal element into a continuous cast slab, a method of adding a vapor of metal element to a molten steel in a tundish near a continuous cast mold or in a continuous cast mold Is the best.

The present invention has been completed based on the above findings, and the gist of the present invention is the steel materials shown in the following (1) to (3) , and the continuous casting method of steel shown in ( 4 ) and ( 5 ). is there.

(1) A steel material obtained by hot rolling using a continuously cast slab as a raw material, and the steel material is in mass%, C: 0.03-0.08%, Si: 0.1-2. 0%, Mn: 0.5 to 3.0%, P: 0.02% or less, S: 0.005% or less, Ti: 0.003 to 0.25%, N: 0.01% or less, Al : comprises 0.002 to 0.2 percent, and et to, Ag and whether each containing 0.00005 to 0.001% Mg, or Ag and Mg, respectively 0.00005 to 0.001% and Bi: containing from 0.00005 to 0.001%, the remainder Ri is Do Fe and impurities, the samples taken from the hot-rolled steel sheet were observed at a magnification of 500 to 2000 times by SEM, 200 per observed dispersoids this precipitates such as the mean value of is 1μm or less are dispersed Steel material shall be the features a.

(2) The steel material according to (1), further containing at least one of Ca: 0.005% or less, Nd: 0.001% or less, and Sn: 0.01% or less in mass%.
(3) Further, Mo: 0.5% or less, Cu: 1.5% or less, Nb: 0.04% or less, V: 0.04% or less, Ni: 5.0% or less, Cr: 2.5 % Or less and B: One or more of 0.003% or less are contained, The steel material as described in said (1) or (2) characterized by the above-mentioned.

( 4 ) A continuous casting method for casting a slab as a hot rolling material for producing the steel material according to any one of (1) to (3), wherein the slab is immersed in molten steel in a tundish. Supplying the Ag and Mg, or the metal vapor and / or metal particles of Ag, Mg and Bi together with the carrier gas into the molten steel through the immersion lance or the immersion lance immersed in the molten steel in the mold, or A continuous casting method of steel in which one or more metal vapors and / or metal particles of Ca, Nd and Sn are supplied into the molten steel together with a carrier gas.

( 5 ) A continuous casting method for casting a slab as a raw material for hot rolling for producing the steel material according to any one of (1) to (3), wherein the slab is immersed in molten steel in a tundish. A wire or rod containing Ag and Mg, or Ag, Mg and Bi is supplied into the molten steel together with a carrier gas through the immersion lance or immersion lance immersed in the molten steel in the mold, or further, Ca, A steel continuous casting method in which a wire or rod containing one or more of Nd and Sn is supplied into the molten steel together with a carrier gas.

  In the present invention, “precipitate is finely dispersed” means that a sample collected from a hot-rolled steel sheet is observed with a SEM at a magnification of 500 to 2000 times, and the average value per 200 precipitate particles observed is 1 μm or less. Means a steel in which precipitates are dispersed.

  “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 particles formed by condensation of metal vapor. . The “metal” includes pure metals and metal alloys.

  In the following description, unless otherwise specified, “%” in the steel component composition display means “mass%”.

  In the steel material of the present invention, since precipitates are finely dispersed, the growth of austenite grains during reheating is suppressed, including HAZ in super high heat input welding, in addition to properties such as strength and toughness, It is suitable as a steel material excellent in workability such as hole expansibility. Further, the continuous casting method of the present invention is an optimum continuous casting method for efficiently adding an appropriate amount of metal elements necessary for obtaining the above steel material to the molten steel and uniformly dispersing it in the continuous casting slab. is there.

  As described above, the steel material of the present invention is a steel material obtained by hot rolling using a continuously cast slab as a raw material, and the steel material includes C, Si, Mn, P, S, Ti, N, and Al. And further containing one or more of Mo, Cu, Nb, V, Ni, Cr and B, and further containing 0.00005 to 0.001% of Ag and Mg, respectively, or Ag and Mg Is a steel material in which precipitates are finely dispersed, each containing 0.00005 to 0.001% and Bi: 0.00005 to 0.001%, the balance being Fe and impurities.

  Further, the method of the present invention is a continuous casting method for casting a slab as a hot rolling material for producing the steel material, wherein the lance is immersed in molten steel in a tundish or in a mold. The above-described Ag and Mg, or Ag, Mg and Bi metal vapor and / or metal particles are supplied into the molten steel together with a carrier gas through a dipping lance immersed in the molten steel, or a wire or rod containing the above metal element Is a continuous casting method of steel in which molten steel is supplied into molten steel together with a carrier gas. Below, the steel material of this invention and the continuous casting method of steel are demonstrated in detail.

(1) Suppression of grain coarsening due to addition of metal element and casting method In order to improve the mechanical properties of steel materials, it is necessary to refine crystal grains. Conventionally, methods such as controlled cooling and controlled rolling have been adopted, Management of the heating temperature, the cooling start temperature, the cooling rate and the rolling reduction of the steel has been performed. In addition to these, techniques for enhancing the above-described effects have been developed by including alloy elements in steel materials. Furthermore, when steel materials are reheated like HAZ at the time of welding, the steel materials are exposed to high temperatures, so the crystal grains initially possessed by the steel materials are coarsened, including strength, toughness, and hole expandability. There is a problem that mechanical properties such as workability are reduced.

  In order to suppress the coarsening of the crystal grains of the steel material, in addition to the method of using the “pinning effect” by the crystallized product or precipitate, it is also possible to use the “drag effect at the crystal grain boundary” of the solute element. . Here, “the drag effect at the grain boundary” means that the moving speed of the solute element taken into the grain boundary moving in the direction opposite to the moving direction of the solvent element (iron element) is higher than the moving speed of the solvent element. Because it is slow, the moving grain boundary drags the solute element taken into the grain boundary layer in the grain boundary moving direction, which acts as a movement resistance of the grain boundary, and coarsens the crystal grain. The effect of suppressing.

  Moreover, crystal grain growth can be suppressed by finely dispersing a compound having a melting point lower than the solidus temperature of steel. Further, even when the pinning effect is used, it is effective to increase the crystallization or precipitation frequency in order to enhance the effect.

  The steel also contains solute elements such as Mn, S, and Ti, and these reaction products such as MnS and TiN are present in the steel. When these precipitates are also coarsened, the mechanical properties are significantly reduced. Therefore, if these precipitates are finely dispersed, the effect of suppressing the coarsening of crystal grains can be exhibited, and rather the strength, toughness and workability can be improved. For this purpose, inhomogeneous nucleation such as MnS and TiN may be generated on finely dispersed Mg oxide and Nd oxide, and in order to promote this, surface active elements Bi and Sn are added. It ’s fine.

  When a metal element having a high vapor pressure or a metal element having a low melting point, such as Ag, Bi, Mg, Nd, or Sn described above, is added to the molten steel, the added metal contacts the molten steel or radiates heat from the molten steel. Upon receiving, it melts or vaporizes. If metal elements are melted or vaporized before or during addition to the molten steel, it is difficult to add these metal elements to the molten steel uniformly and with good yield. In order to solve such problems and to uniformly add the metal element into the continuous cast slab, the method of adding to the molten steel in the tundish near the mold or in the mold is optimal.

  Accordingly, the present inventors have further advanced research and development on a crystal grain coarsening suppression mechanism and a suppression method, and a continuous casting method for achieving the mechanism. The present inventors previously proposed a method of adding a vapor of a metal element or a compound of a metal element to molten steel in a tundish or a continuous casting mold in Patent Document 3, Patent Document 4 and Patent Document 5. In these methods, it has been confirmed that metal elements can be added uniformly to molten steel with good yield. As a result of repeated investigations based on these results, the following (a) and (b) show the crystal grain coarsening suppression mechanism and the knowledge about the method, and (c) and (d) continuous casting. Knowledge of the method and apparatus was obtained to complete the present invention.

(A) Effect of containing Ag, Bi and Mg A test piece is prepared from a molten steel material by adding various metal elements to molten steel, and each test piece is heated to 1400 ° C. and held for 60 seconds. went. Observation of the test piece with an optical microscope revealed that among the metal elements, the growth of the austenite grain size of the test piece to which Ag, Bi and Mg were added was significantly suppressed. Furthermore, as a result of compositional analysis of the inside and grain boundaries of these test pieces using an EDX (energy dispersive X-ray analyzer), it was found that Ag, Bi, and Mg elements were concentrated in the grain boundaries. . It was also confirmed that fine Ag oxide and Mg oxide were dispersed.

  Since Ag and Bi have low solubility in steel, they are concentrated on the liquid phase side of the solidification interface in the solidification process of steel, and this reacts with oxygen in the molten steel to produce fine Ag oxide and Bi oxide. Put out. Since the melting points of these oxides are lower than the solidus temperature of the steel, these grains are not formed while the steel is in the molten state. For this reason, even if the steel is solidified to form steel crystal grains, the grain boundaries of the steel are interrupted by the oxide until the Ag oxide or Bi oxide is completely solidified.

  Only when the temperature of the steel is further lowered and the solidification of the oxide is completed, the crystal grain boundaries are bonded, and crystal grains grow. At the time when the solidification of the oxide is completed, the temperature of the steel is sufficiently lowered, and the growth rate of the crystal grains becomes small. As a result, it was newly found by this test that the growth of crystal grains is suppressed and a so-called “pinning effect” is exhibited.

  Thereby, it became possible to suppress the coarsening of crystal grains more than before due to the synergistic effect of Mg oxide having a high melting point and Ag oxide and Bi oxide having a low melting point.

  Next, an appropriate range of Ag, Bi and Mg contents will be described.

  It is appropriate that the Ag content, Bi content, and Mg content are all in the range of 0.00005 to 0.001%. If the content of Ag, Bi and Mg is less than 0.00005%, the effect of suppressing the coarsening of crystal grains cannot be obtained.

On the other hand, when the Ag content exceeds 0.001%, the manufacturing cost of the steel increases, which is economically disadvantageous. Therefore, the upper limit of the Ag content is set to 0.001%. If the Bi content exceeds 0.001%, the concentration of Bi at the crystal grain boundary becomes remarkable and embrittles, and conversely the mechanical properties deteriorate. Therefore, the upper limit of the content is 0.001%. It was. Further, even if the Mg content exceeds 0.001%, the effect of suppressing the coarsening of the crystal grain size is saturated and the production cost of the steel only rises, so the upper limit of the content is 0.001. %.
In many cases, steel contains solute elements such as Ti, Mn, and S. When these reaction products, TiN and MnS, are coarsely precipitated, mechanical properties such as strength, workability including HAZ toughness and hole expansibility are lowered.

  In addition, when Bi or Sn, which are surface active elements, is contained, TiN, MnS, and the like are likely to precipitate on the oxide after the Mg oxide is crystallized. When the Mg oxide is finely dispersed, TiN and MnS that form heterogeneous nuclei on the Mg oxide are also finely dispersed as a result, which contributes to improvement of mechanical properties.

(B) Effect of containing Ca, Nd and Sn When one or more of Ca, Nd and Sn are contained in the steel material of the above (a), it exerts a drag effect on the movement of the crystal grain boundary and is reheated. It has been found that it has an effect of suppressing the growth of austenite grains at the time. These elements may or may not be contained, but when it is desired to obtain the effects of these elements, Ca is within a range of 0.00005 to 0.005%, and Nd is 0.00005 to In the range of 0.001%, and Sn in the range of 0.00005 to 0.01%, one or more are included.

  For Ca, Nd and Sn, if their content is less than 0.00005%, those effects cannot be obtained. On the other hand, when the Ca content exceeds 0.005%, Ca sulfide is generated, and conversely, the mechanical properties are degraded. Further, when the Nd content exceeds 0.001%, Nd oxide crystallizes and a pinning effect also appears, and the crystal grain growth suppressing effect increases. This is not preferable because it causes troubles in the casting operation.

  Furthermore, since MnS, TiN, and the like are heterogeneously nucleated on the crystallized Nd oxide, heterogeneous nucleation is likely to occur when the surface active elements Bi and Sn are contained. When the Nd oxide is finely dispersed, MnS and TiN that generate heterogeneous nuclei are also finely dispersed thereon, and the mechanical characteristics are improved.

  When the Sn content exceeds 0.01%, Sn is remarkably concentrated at the grain boundaries, so that the steel becomes brittle. Therefore, the content is preferably 0.01% or less.

(C) Continuous casting method capable of avoiding clogging of immersion nozzle When the rare earth element Nd was added to molten steel, the immersion nozzle was easily clogged, and operation for a long time was difficult. Conventionally, a method of adding a single element of metal element Nd or a lump of Nd alloy in a ladle is generally used, but in this method, floating separation, sedimentation in molten steel, or flow of molten steel The mixture is agitated and collided and agglomerated to coarsen, for example, coarse oxides are suspended in the molten steel. And this reaction product adheres to the inner wall of an immersion nozzle, and causes the obstruction | occlusion of an immersion nozzle.

  On the other hand, Nd is supplied into the molten steel together with the carrier gas through the immersion lance immersed in the molten steel in the tundish or the molten steel in the mold, or contains the metal element Nd. By supplying the wire or the rod together with the carrier gas into the molten steel and performing continuous casting, the wire or rod is supplied into the continuous casting mold before being agglomerated and enlarged, so that the problem of clogging of the immersion nozzle does not occur. Furthermore, it can disperse | distribute in a continuous casting slab with a fine product.

(D) Apparatus for carrying out the above continuous casting method As described in the examples described later, a tundish, an immersion nozzle provided at the lower part of the tundish for supplying molten steel in the tundish to the mold, A mold located below the tundish, an immersion lance for supplying a wire or a rod to the molten steel in the tundish, or an immersion lance for supplying a wire or a rod to the molten steel in the mold; A continuous casting apparatus for molten steel having a wire or rod supply device for supplying the wire or rod into the hole and a gas supply device for supplying a carrier gas into the immersion lance is preferable.

(2) Component composition range of steel Among the component compositions of steel according to the present invention, the reasons for limiting the component composition excluding the additive metal elements already described will be described below.

C: 0.03-0.08%
C is an element effective for ensuring the strength and toughness of the base material and the weld. If the content is less than 0.03%, the above effects cannot be obtained sufficiently. On the other hand, if the content exceeds 0.08%, carbides are generated, and the hole expansion required for the hot-rolled steel sheet is required. As a result, the toughness of the welded portion also decreases. Therefore, the appropriate range of C is set to 0.03 to 0.08%.

Si: 0.1 to 2.0%
Si is an element necessary for suppressing the formation of carbides and ensuring strength and ductility. In order to improve strength and ductility, the content needs to be 0.1% or more. On the other hand, when the content exceeds 2.0%, the toughness of the welded portion deteriorates. For the above reason, the appropriate range is set to 0.1 to 2.0%.

Mn: 0.5 to 3.0%
Mn is an effective element for increasing the strength of the steel sheet, ensuring toughness, and ensuring the hardenability of HAZ. In order to acquire said effect, the content needs to be 0.5% or more. On the other hand, when the content is higher than 3.0%, toughness is impaired and weldability is lowered. For this reason, the appropriate range of Mn content was 0.5 to 3.0%.

P: 0.02% or less P is an element that deteriorates the ductility and toughness of the steel sheet, as well as the weldability and workability, so its content is limited to 0.02% or less. Moreover, since P is an element having an effective action for increasing the strength of the steel sheet, it is preferable to contain 0.001% or more in order to obtain the effect.
S: 0.005% or less S is an element that forms MnS inclusions and the like to reduce the ductility and hole expansibility of the steel sheet. For this reason, the content was made 0.005% or less. For steel types that allow an increase in steelmaking cost, it is preferable to reduce the steelmaking cost to 0.0005% or less.

Ti: 0.003-0.25%
Ti is an effective element that mainly precipitates carbonitrides and contributes to improvement of the strength of the base metal by its precipitation strengthening action. When the Ti content is less than 0.003%, the above effects are not sufficient, while when the content is higher than 0.25%, coarse precipitates and inclusions are formed in the steel, Reduces the ductility, toughness and workability of steel. For the above reason, the appropriate range of Ti content is set to 0.003 to 0.25%.

N: 0.01% or less When N is contained in steel, ductility, toughness, and workability deteriorate, so N is a harmful element. When the content is higher than 0.01%, the above characteristics are significantly deteriorated. Therefore, the upper limit of the content is set to 0.01%. However, since it is impossible to remove N completely industrially, the lower limit is preferably set to 0.001% in consideration of a range that can be reduced in actual operation.

Al: 0.002 to 0.2%
Al is a deoxidizing element of molten steel, and in order to obtain the effect, it is necessary to contain 0.002% or more. However, when the content exceeds 0.2%, the amount of coarse oxide inclusions in the steel increases, which adversely affects the strength of the base material. For the above reason, the appropriate range of the content is set to 0.002 to 0.2%. In the present invention, Al means acid-soluble Al (sol. Al).

  Next, an optional additive element of steel will be described.

Mo: 0.5% or less, Cu: 1.5% or less, Nb: 0.04% or less, V: 0.04% or less, Ni: 5.0% or less, Cr: 2.5% or less, and B: Contains one or more of 0.003% or less Mo, Cu, Nb, V, Ni, Cr and B may or may not contain,
When it is desired to obtain the effects of these elements, one or more of these elements can be contained within the following content range.

Mo: 0.5% or less Mo, if contained, is an element that exhibits an effective action for improving hardenability and strength. In order to obtain the clear effect, it is preferable to contain 0.1% or more. However, when the content exceeds 0.5%, the toughness and ductility of the steel and the weldability deteriorate. Therefore, the content range when Mo is contained is set to 0.5% or less.

Cu: 1.5% or less If Cu is contained, it is an element having an effect effective in improving hardenability and precipitation strengthening. To obtain the effect, it is preferable to contain 0.01% or more. On the other hand, when the content exceeds 1.5%, the hot workability decreases. For the above reason, the content range when Cu is contained is set to 1.5% or less.

Nb: 0.04% or less Nb is an element having an effect of improving low-temperature toughness when contained. In order to acquire the effect, it is preferable to contain 0.005% or more. However, when the content is higher than 0.04%, coarse carbides and nitrides are formed in the steel, so that workability such as hole expansibility of the steel sheet is lowered. For the above reasons, the content range when Nb is contained is set to 0.04% or less.

V: 0.04% or less When V is contained, it precipitates as carbonitride in the hot-rolled steel sheet, and exerts an effect of improving strength and toughness by the precipitation effect. In order to acquire the effect, it is preferable to contain 0.003% or more. On the other hand, when the content exceeds 0.04%, the workability and toughness of the steel sheet deteriorate. For the above reasons, the content range when V is contained is set to 0.04% or less.

Ni: 5.0% or less Ni is an element having an action of improving the toughness of the base material when contained. In order to acquire the effect, it is preferable to contain 0.5% or more. On the other hand, if the content exceeds 5.0%, the hardenability becomes excessive and adversely affects toughness. Therefore, the content range when Ni is contained is set to 5.0% or less.

Cr: 2.5% or less When Cr is contained, Cr is an element that exhibits an effective action for improving hardenability and improving base material strength by precipitation strengthening. In order to acquire the effect, it is preferable to contain 0.05% or more. On the other hand, when the content exceeds 2.5%, a tendency to deteriorate toughness and weldability is recognized. Therefore, the content range when Cr is contained is set to 2.5% or less.

B: 0.003% or less B is an element that exhibits an effect of improving the strength of a steel sheet when contained. In order to acquire the effect, it is preferable to contain 0.0005% or more. However, if its content exceeds 0.003%, coarse boride precipitates in the steel, thereby deteriorating the toughness of the steel. For the above reason, the content range in the case of containing B is set to 0.003% or less.

In order to confirm the effect of refinement of inclusions in the steel in the present invention, the workability improvement effect based on the growth suppression of crystal grains during reheating, and the effect of the continuous casting method of steel, the following tests were performed, The results were evaluated.
〔Test conditions〕
Molten steel: Steel having the composition shown in Tables 1 to 4 to be described later
Molten steel amount: Continuous casting at 5 tons / min. Added metal: Metal elements shown in Tables 2 and 4 to be described later
Addition method: Metal wire (wire diameter is 3mmφ)
Addition position: In tundish Carrier gas: Argon gas 10 L / min FIG. 1 is a view showing a method of continuous casting while supplying a metal wire to molten metal in a tundish through an immersion lance.

  The molten steel 1 supplied to the tundish 2 from the ladle 3 is poured into the continuous casting mold 8 through the immersion nozzle 6 and further formed as a slab by forming a solidified shell 7 while being drawn downward. The added metal was finally supplied into the molten steel 1 in the tundish 2 through the immersion lance 4 immersed in the molten steel 1 in the tundish 2 as metal vapor.

  One end of the immersion lance 4 is connected to a metal wire feeder 5. The wire reel 51 is loaded in the metal wire feeder 5, and the metal wire 50 is inserted into the immersion lance 4 by the wire feeding roll 52 whose feeding speed is controlled by the wire feeding speed control device 53. A carrier gas 54 was introduced into the metal wire feeder and supplied into the immersion lance 4 together with the metal wire 50.

  On the other hand, as a comparative example, continuous casting was performed under the condition where the metal element was not added, and the same tests and investigations as those in the case where the metal element was added were performed.

  Tables 1 to 4 show the component compositions of steel used in the test.

  A continuously cast slab obtained by continuous casting was hot-rolled after reheating under the following conditions to obtain a hot-rolled steel sheet.

Continuous heating slab reheating temperature: 1250 ° C
Reheating time: 2 hours Hot rolling finish temperature: 850-900 ° C
Hot rolled steel sheet thickness: 3mm
Winding temperature: 400-500 ° C
The results obtained from the tests are shown in Tables 2 and 4. In the same table, the precipitate particle size index is expressed as an index by a relative value with respect to 1.0 as the precipitate particle size in Test No. C1, which is a comparative example. Here, the grain size of the precipitates was observed according to the method for measuring nonmetallic inclusions defined in JIS G 0555 by observing a sample collected from a hot-rolled steel sheet at a magnification of 500 to 2000 times by SEM. The average value per 200 precipitate particles was determined and adopted.

Moreover, the hole expansibility index of a hot-rolled steel sheet is a comparative example in which a hole-expansion test is performed in accordance with a method defined in the Japan Iron and Steel Federation Standard JFST 1001-1996 using a test piece collected from a hot-rolled steel sheet. The hole expansion test result in test number C1 was set to 1.0, and the result was indexed by the relative value with respect to this. The larger the index value, the better the hole expansibility.
〔Test results〕
Test numbers H1 to H26 are tests for the present invention examples that satisfy the conditions defined in the present invention, and test numbers C1 to C7 are all metal elements such as Ag, Bi, Mg, Ca, Nd, and Sn. It is a test for a comparative example which does not contain or whose content exceeds the range specified in the present invention. In any of the tests of the present invention example and the comparative example, according to the continuous casting method of the present invention, the metal element having the control target content was uniformly added to the slab at a high yield.

  In test number H1 of the present invention example containing Ag and Mg as metal elements, and in test numbers H2 and H7 of the present invention example further containing Bi in addition to Ag and Mg, precipitation in test number C1 of the comparative example The precipitate particle size index based on the product particle size (1.0) is suppressed to 0.4 to 0.5 lower than the test number C1, and thus the precipitate particle size is refined. It can be seen that they are dispersed. Moreover, the hole expansibility index on the basis of the hole expansion test result in the test number C1 of the comparative example (1.0) is 0.3 to 0.5 higher than that of the comparative example, and the hole expandability is high. It turns out that it is improving favorable. Therefore, these facts are mainly due to the pinning effect of Ag, Bi, or Mg oxide, which suppresses the coarsening of the austenite crystal grain size during reheating, resulting in improved workability of the steel sheet such as hole expandability. Is determined.

  Further, in the test numbers H3 to H6 and H8 to H10 containing one or more of Ca, Nd and Sn in addition to the above metals, the effect of refining precipitates equivalent to or higher than the above As a result, the hole expansibility index is further improved. This is considered to be due to the fact that the grain boundary drag effect was added in addition to the pinning effect due to the inclusion of Ca, Nd or Sn.

  On the other hand, in the test numbers C1 to C4 of comparative examples that do not contain any of the metal elements such as Ag, Bi, Mg, Ca, Nd, and Sn, the precipitate particle size index is a value near 1.0. The precipitates are not refined and, therefore, the austenite grain size during reheating is not suppressed, so that the hole expandability is hardly improved as compared with the comparative example.

  Moreover, in the test numbers C5 to C7 in which the content of Ag, Bi, or Mg exceeds the content range defined in the present invention, the effect of containing the above elements is saturated, and the manufacturing cost of the steel material is excessive. The result is rising.

  Since the precipitates of the steel of the present invention are refined and dispersed, the growth of austenite crystal grains during reheating is suppressed, including HAZ in super high heat input welding, in addition to strength and toughness, It is suitable as a steel material excellent in workability such as hole expansibility. Further, the continuous casting method of the present invention is an optimum continuous casting method for efficiently adding an appropriate amount of metal elements necessary for obtaining the above steel material to the molten steel and uniformly dispersing it in the continuous casting slab. is there. Therefore, the steel material of the present invention is a structural or processing steel material excellent in strength, toughness and workability including hot rolled steel sheets for automobiles, and the method of the present invention is for producing the above steel material. As a continuous casting method, it can be widely applied.

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

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 gauge, 56: Flow control valve, 6: Immersion nozzle,
7: Solidified shell 8: Continuous casting mold

Claims (5)

It is a steel material obtained by hot rolling using a continuously cast slab as a raw material, and the steel material is in mass%, C: 0.03 to 0.08%, Si: 0.1 to 2.0%, Mn: 0.5 to 3.0%, P: 0.02% or less, S: 0.005% or less, Ti: 0.003 to 0.25%, N: 0.01% or less, Al: 0. comprises from 002 to 0.2%, in the et, each containing 0.00005 to 0.001% of Ag and Mg, or Ag and Mg, respectively 0.00005 to 0.001% and Bi: 0.00005 containing 0.001%, the remainder Ri is Do Fe and impurities, the samples taken from the hot-rolled steel sheet was observed with a 500 to 2000-fold magnification by SEM, observed precipitate particles 200 per average value Japanese to but that precipitates such that 1μm or less are dispersed Steel material shall be the.   The steel material according to claim 1, further comprising at least one of Ca: 0.005% or less, Nd: 0.001% or less, and Sn: 0.01% or less by mass%. . Furthermore, Mo: 0.5% or less, Cu: 1.5% or less, Nb: 0.04% or less, V: 0.04% or less, Ni: 5.0% or less, Cr: 2.5% or less and B: One or more of 0.003% or less are contained, The steel material of Claim 1 or 2 characterized by the above-mentioned. It is a continuous casting method which casts the slab as a raw material for hot rolling for manufacturing the steel material in any one of Claims 1-3, Comprising: In the immersion lance or the mold immersed in the molten steel in a tundish The Ag and Mg, or the metal vapor and / or metal particles of Ag, Mg and Bi are supplied into the molten steel together with a carrier gas through a dipping lance immersed in the molten steel of Ca, Nd and Sn. One or more metal vapors and / or metal particles are supplied into the molten steel together with a carrier gas. It is a continuous casting method which casts the slab as a raw material for hot rolling for manufacturing the steel material in any one of Claims 1-3, Comprising: In the immersion lance or the mold immersed in the molten steel in a tundish A wire or rod containing Ag and Mg, or Ag, Mg and Bi is supplied into the molten steel with a carrier gas through a dipping lance immersed in the molten steel, or further out of Ca, Nd and Sn. A continuous casting method of steel, characterized in that a wire or rod containing one or more kinds is supplied into the molten steel together with a carrier gas.

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