JP5018436B2 - High strength steel - Google Patents

Description

  The present invention relates to a high-strength steel material produced by using a slab in which fine inclusions are dispersed and formed by adding metal elements in the continuous casting process of steel, and continuous casting for casting the slab for the steel material. Regarding the method.

  High-strength steel materials are mainly used as structural steel materials for construction, civil engineering, construction machinery, shipbuilding, pipes, tanks, marine structures and the like. These structures are combined by fixing steel materials by bolting or welding. Among these, in fixing by welding, heat is applied to the steel material itself, which may cause deterioration of mechanical properties such as strength and toughness of the base material. In particular, recently, large heat input welding has been performed to increase the thickness of the steel sheet and to omit the process. Therefore, the heat affected zone (hereinafter referred to as “HAZ”), which is one of mechanical characteristics, is also described. ) Countermeasures against toughness reduction are a major issue.

  There have been many proposals for the technology that pays attention to the toughness of the steel material HAZ at the time of high heat input welding.

  For example, in Patent Document 1, after heating to a temperature range of 1250 to 1400 ° C. so that TiN is dissolved, rolling or forging is performed, or after the rolling or forging, the temperature is not higher than 1150 ° C. A method for producing a steel material for high heat input welding is disclosed in which solid-solution TiN is dispersed and reprecipitated as fine TiN by heating, and austenite grains of HAZ are refined to improve toughness. However, Ti nitride has a problem that the effect of improving toughness is reduced because it is almost dissolved in the vicinity of the boundary with the weld metal having a maximum ultimate temperature exceeding 1400 ° C in HAZ, and is required in high heat input welding. It is difficult to ensure toughness.

As a technique for improving the toughness in the vicinity of such a welded portion, for example, as disclosed in Patent Document 2, an oxide system containing one or two composite crystal phases of TiO and Ti ₂ O ₃ is used. The method of including inclusions is effective in improving the toughness of the HAZ during high heat input welding of steel. However, since Ti oxide tends to cause coarsening and aggregation and coalescence, Ti oxide-based coarse inclusions are generated, and when such coarse inclusions are formed, the toughness of HAZ is reduced. Problems arise.

As a technique for solving this problem, for example, Patent Document 3 discloses a technique for dispersing a Ti—Mg-based oxide. That is, there are 30 oxides / mm ² or more of oxides having a size of 0.5 to 5 μm and a total content of Ti and Mg of 15% by weight or more, and at the same time 0.05 to 0.5 μm in size. It is a steel plate with excellent weld heat-affected zone toughness containing 5,000 oxides / mm ² or more. According to Patent Document 3, it is said that Ti—Mg-based oxides can be dispersed, and the HAZ toughness can be improved during high heat input welding. However, in recent ultra-high heat input welding, the temperature of the HAZ becomes even higher, so it is difficult to maintain a fine steel structure. Can not.

  The technique mentioned above aims at improving the toughness of the HAZ by suppressing the coarsening of the austenite grains of the steel material reheated by adding a metal element to the molten steel. And in the future, it is in the direction which aims at the improvement of the toughness of HAZ by suppressing the coarsening of an austenite particle size also with respect to the further increase in heat input.

  By the way, the influence of hydrogen (H) existing in steel is considered as a factor governing the mechanical properties of steel materials, but examples of examining the influence of hydrogen on the toughness improvement of HAZ, which is one of the mechanical properties, are as follows. I can't find it. If excessive hydrogen is present in the steel, voids are formed due to the reaction of carbon and hydrogen in the steel and the production of methane gas, resulting in the formation and cracking of the decarburized layer, resulting in reduced strength. Embrittlement occurs. In the ultra-high heat input welding, such a reaction proceeds remarkably, so it is necessary to consider the influence of hydrogen.

  When adding an alloy element in a continuous casting process, depending on the type of alloy element, it is often difficult to add the element so as to be uniformly dispersed in the slab. As a method of adding a metal element to the molten steel, a method in which a massive metal element is put on the surface of the molten steel, a wire made of a simple metal element, a wire in which these metal elements are coated with aluminum or steel, or the like A method of adding a wire made of an alloy containing any of the above metal elements is employed. However, using these methods, it is difficult to stably and accurately add a metal element having a high vapor pressure and a low melting point such as Mg. When these high vapor pressure metal elements are added to the molten steel, the metal elements are vaporized and diffused into the atmosphere near the molten steel surface, making it difficult to control the amount added to the molten steel. This is because the addition yield decreases and it is difficult to add uniformly.

  In addition, since these metal elements have a large volume expansion when vaporized, when vaporized in the vicinity of the molten steel surface, the molten steel is severely scattered, making it difficult to ensure operational safety. Furthermore, when the melting point of the additive metal element is low, it becomes difficult to add a predetermined amount because it is softened or melted by the radiant heat of the molten steel before reaching the molten steel. 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. On the other hand, when a metal element having a high density is added, the added metal only settles from the addition position to the bottom of the molten steel, and it is difficult to uniformly mix the entire molten steel. In addition, these metals are likely to adhere to the refractory used during continuous casting and block the immersion nozzle, and it is difficult to perform stable operation of continuous casting using molten steel containing these elements.

  Patent Document 4 discloses a Bi addition method in which Bi is added to a molten steel flow that is moving out of the ladle and moving to the molten steel bath surface in the tundish. Bi has a low boiling point like Mg and reacts explosively when it comes into contact with the molten steel stream, so that it becomes a metal vapor and scatters in the atmosphere, so the addition yield is low, and it is difficult to add uniformly into the molten steel. It is difficult to disperse uniformly in the continuous cast slab.

Japanese Patent Publication No. 55-26164 (claims and column 8, line 3 to column 14, line 12) JP 61-79745 A (claims and page 4, upper left column, line 2 to lower left column, line 17) JP-A-11-124652 (Claims and paragraph [0006]) JP 2001-1116 A (claims, paragraphs [0013] and [0014]) JP 2004-249315 A (claims and paragraphs [0011] to [0015]) Japanese Patent Laying-Open No. 2005-169404 (Claims and paragraphs [0011] to [0015]) Japanese Patent Laying-Open No. 2005-219072 (Claims and paragraphs [0013] to [0017]) Sugaya Sugaya, Yu Fuwa: Iron and Steel, vol. 60 (1974), pages 1299 to 1309

  The present invention has been made in view of the above problems, and the first problem is that a fine oxide is dispersed in the steel material, and the oxide can fix hydrogen in the steel. It is to provide a strength steel material. The second problem is to provide a continuous casting method in which Mg necessary for obtaining the above high-strength steel material can be efficiently added to molten steel and Mg oxide can be uniformly dispersed in the continuous casting slab. There is to do.

  When excess hydrogen is dispersed in the steel, carbon and hydrogen in the steel react to generate methane gas to form voids with volume expansion, forming a decarburized layer by consuming carbon, Cracks occur, reducing the strength and toughness of the steel sheet. In particular, since the heat-affected zone during super-high heat input welding is in a high-temperature state, carbides in the steel decompose and carbon becomes free and the reactivity with hydrogen becomes extremely high. .

  Conventionally, in order to prevent the occurrence of such a phenomenon, a method has been employed in which carbide is reformed by annealing a steel material. This method requires annealing, which not only increases man-hours and costs, but also limits the amount of processing and the size of the object that can be processed, making it difficult to process large structures. There was also. Furthermore, since hydrogen atoms are small in size, even if the annealing treatment as described above is performed, they easily enter the inside from the surface of the steel material, and the diffusion coefficient is extremely large, so in a short time. There was also a problem that the hydrogen concentration of the entire steel material increased.

  If the steel does not contain any hydrogen, cracks and toughness due to methane gas will not occur, but since the steel is used in an air atmosphere, hydrogen in the atmosphere penetrates into the steel and diffuses in the steel. As a result, hydrogen is distributed in the steel material. Moreover, it is technically difficult to make the hydrogen concentration in the steel material zero.

  In the continuous casting process, the molten steel whose components have been adjusted is poured from the converter into the ladle and then into the mold through the tundish. During this time, hydrogen is introduced into the molten steel from hydrogen in the atmosphere or moisture in the refractory. Absorbed. It is known from Non-Patent Document 1 that the amount of hydrogen absorbed in the molten steel is affected by the components of the molten steel. For example, in general, when Ti is added for the purpose of improving toughness, the chemical affinity between Ti and hydrogen is large in the molten steel, so the solubility of hydrogen in the molten steel increases. As described above, the amount of dissolved hydrogen changes depending on the metal element added to the molten steel, and the amount of hydrogen remaining in the steel also changes. Therefore, the required reduction amount of the hydrogen concentration differs depending on the steel type. . Therefore, it is necessary to manage the hydrogen concentration according to the steel type. In order to manage this hydrogen concentration, it is necessary to keep track of the amount of dehydrogenation for each steel type, which requires a large amount of man-hours for management. It is.

  The present inventors have empirically found that when fine Mg oxide is dispersed in a steel material, hydrogen is accumulated and fixed around the oxide, and as a result, hydrogen is dispersed. As a result of analyzing the region containing Mg oxide by secondary ion mass spectrometry, hydrogen (H) was detected. When the oxide is fine, not only does the amount of hydrogen accumulated around it increase, but if the oxide is evenly dispersed in the steel, hydrogen is also dispersed. As a result, the amount of hydrogen that can be contained in the steel material can be increased, the reaction with the carbon in the steel material can be suppressed, and the reduction in toughness due to hydrogen, which has been a problem in the past, has been suppressed. can do. Moreover, since the amount of hydrogen that can be contained in the steel material can be increased, the appropriate range of the hydrogen concentration can be expanded, and the restrictions on the casting operation can be relaxed.

  By the way, when a metal element having a high vapor pressure or a metal element having a low melting point, such as Mg, is added to the molten steel, the added metal is melted or vaporized by contact with the molten steel or radiant heat from the molten steel. 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. Therefore, 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 in the continuous cast mold is optimal.

  The present inventors have already proposed a method of adding a vapor of a metal element or a compound of a metal element into molten steel in a tundish or a continuous casting mold in Patent Document 5, Patent Document 6, and Patent Document 7. By these methods, it has become possible to add a metal element or a compound of a metal element to molten steel uniformly and with good yield.

  The present inventors further studied a continuous casting method for efficiently and uniformly adding Mg into a continuous casting slab in producing a high-strength steel sheet, and the following (a) to (c): Knowledge was obtained and the present invention was completed.

  (A) In order to improve the mechanical properties of high-strength steel materials, the mechanical properties due to hydrogen are reduced by dispersing fine oxides in the steel materials and collecting and fixing hydrogen around these oxides. It is effective to suppress this.

  (B) In order to further enhance the effect of (a), it is effective to generate fine Mg oxide in the steel material and to collect and fix hydrogen around it.

  (C) As described in (a) and (b) above, when Mg having a high vapor pressure and a low boiling point is added to the molten steel, the Mg contacts with the molten steel or receives radiant heat from the molten steel. To melt or vaporize. If Mg is melted or vaporized before it is added to the molten steel or at the moment when it is added, it is difficult to add Mg to the molten steel uniformly and with a good yield. To solve this problem and add Mg uniformly into the continuous casting slab, the best method is to add Mg vapor to the molten steel in the tundish near the continuous casting mold or in the continuous casting mold. It is.

The present invention has been completed based on the above findings and has as its gist lies in shown to high-strength steel material below.

A high-strength steel obtained by continuous cast slab as a material, in mass%, C: 0.03~0.20%, Si : 0.005~0.5%, Mn: 0.1 -3.0%, P: 0.02% or less, S: 0.005% or less, Ti: 0.003-0.03%, N: 0.002-0.008%, Al: 0.001- 0.01%, O: 0.001~0.008%, H: 0.00001~0.0002%, Mg: containing from 0.0001 to 0.005%, the balance being Fe and impurities, non Exists per unit volume (1 mm ³ ) when measured at each position of 1/2 width, 3/4 width, and 1/4 width in the slab width direction by a constant potential electrolytic extraction method using an aqueous solvent solution. the number particle size 1μm or less of Mg oxide exceeds 1.0 × 10 ⁷ cells / mm ^3, not containing Mg of the above component composition The impact absorption energy by Charpy test specified in JIS Z 2242 of test piece as a reference (1.0), high-strength steel material in which the index and this and wherein 1.3 or more.

  In the present invention, “Mg oxide is finely dispersed” means that a sample collected from a hot-rolled steel sheet is present per unit volume by conducting a constant potential electrolytic extraction test in a non-aqueous solvent shown below. The number of fine Mg oxides having a certain particle size or less was investigated and defined as the case where a certain number or more of fine Mg oxides existed.

(Extraction condition)
Nonaqueous solvent electrolyte: 1000 ml of methanol, 20 ml of triethanolamine and 10 g of tetramethylammonium chloride,
Electrolytic potential: -150 mV,
Electrolytic quantity: 100 coulombs
Sample size: width 20 mm x length 20 mm x thickness 5 mm,
Filter for extraction filtration: pore size 0.05 μm,

The filter obtained by the extraction experiment was subjected to vapor deposition of a platinum-palladium alloy, and observed using a field emission SEM at a magnification of 500 to 10,000 times, and the number of Mg oxides was measured. The particle size distribution of the oxide per unit volume was calculated from the difference in mass of the sample before and after the electrolytic extraction test. A case where the number of Mg oxides having a particle diameter of 1 μm or less present per unit volume (1 mm ³ ) was 10 ⁴ or more was defined as “a finely dispersed Mg oxide” state.

  “Mg vapor and / or particles” are formed by condensation of Mg vapor and / or Mg particles present as liquid or solid particles due to insufficient evaporation, or Mg vapor. Particle. “Mg” includes both pure metal Mg and Mg alloys.

“High strength steel material” means a steel material having a tensile strength of 590 MPa or more.
In the following description, “mass%” for the component composition of steel is also simply expressed as “%”.

  In the steel material of the present invention, Mg oxide is uniformly and finely dispersed in the steel material, and hydrogen is accumulated around the steel material. Therefore, the steel material is excellent in mechanical strength such as strength and toughness. It is suitable as a material for structural steel materials such as line pipes and offshore structures. In addition, the continuous casting method of the present invention is an optimum method that can efficiently add an appropriate amount of metal Mg necessary for obtaining the above-mentioned high-strength steel material to the molten steel and uniformly disperse it in the continuous cast slab. It is a continuous casting method.

  The high-strength steel material of the present invention and the continuous casting method of the steel material production slab will be described in more detail below.

(1) Reason for limitation of component composition of steel and preferred range C: 0.03 to 0.20%
C is an element effective for ensuring the strength and toughness of the steel material. If the content is less than 0.03%, the above effects cannot be obtained sufficiently. On the other hand, if the content exceeds 0.20%, the toughness of the base material and the HAZ decreases. Therefore, the appropriate range of the C content is set to 0.03 to 0.20%.

Si: 0.005 to 0.5%
Si is an element necessary for ensuring strength. If the content is less than 0.005%, the strength of the base material cannot be secured, so 0.005% or more must be contained. On the other hand, if the Si content exceeds 0.5%, the weldability decreases, so the content needs to be 0.5% or less. For the above reason, the appropriate range of the Si content is set to 0.005 to 0.5%.

Mn: 0.1 to 3.0%
Mn is an element effective for increasing the strength of steel and ensuring toughness. In order to obtain these effects, the content must be 0.1% or more. However, if the Mn content exceeds 3.0%, the toughness is impaired. For this reason, the appropriate range of the Mn content is set to 0.1 to 3.0%.

P: 0.02% or less P is an impurity element in the present invention, and its content is preferably as low as possible. It is an element that deteriorates the ductility and toughness and workability of steel. For this reason, the content rate was limited to 0.02% or less.

S: 0.005% or less S is an impurity element, and the lower the content, the better. MnS inclusions are formed in the steel material to reduce the ductility of the steel material. For this reason, the content rate was made into 0.005% or less.

Ti: 0.003 to 0.03%
Ti is an element that mainly precipitates carbonitride and contributes to the improvement of the strength of the base metal by its precipitation strengthening action. If the Ti content is less than 0.003%, the above effects cannot be obtained sufficiently. On the other hand, if the content exceeds 0.03%, coarse precipitates and inclusions are formed in the steel, and the toughness of the steel is reduced. For the above reason, the appropriate range of Ti content is set to 0.003 to 0.03%.

N: 0.002 to 0.008%
N is an element necessary for reacting with Ti to precipitate TiN. However, when the content rate exceeds 0.008% and the toughness decreases, the upper limit of the content rate is set to 0.008%. On the other hand, in order to precipitate TiN in steel, N needs to be contained by 0.002% or more.

Al: 0.001 to 0.01%
Since Al is a deoxidizing element of molten steel and suppresses the formation of Ti-containing oxides, its content is preferably low. And when the content rate exceeds 0.01%, the generation amount of Ti-containing oxide and Mg-containing oxide decreases, so the content rate was set to 0.01% or less. On the other hand, in order to ensure toughness while leaving some dissolved oxygen in the molten steel, the Al content needs to be 0.001% or more. For the above reason, the appropriate range of the Al content is set to 0.001 to 0.01%.

O (oxygen): 0.001 to 0.008%
O means total oxygen including oxygen contained in oxide inclusions and dissolved oxygen in molten steel, and the total oxygen content (T. [O]) is mainly an indicator of the amount of inclusions. Concentration. When the content is higher than 0.008%, the amount of inclusions is excessively increased, the cleanliness of the high-strength steel is lowered, and the improvement in toughness is hindered. On the other hand, in order to reduce the O content to less than 0.001%, a cleaning process for removing inclusions in the steel is required, and the refining cost increases. Therefore, the appropriate range of the O content is set to 0.001 to 0.008%.

H: 0.00001 to 0.0002%
Since hydrogen is an element that reduces the toughness of HAZ, it is necessary to reduce it as much as possible. If excessive hydrogen is present in the steel, the carbon in the steel reacts with hydrogen to generate methane gas, forming voids, forming a decarburized layer and cracking, reducing the strength of the steel and Embrittlement occurs. However, in order to reduce the H content, it is necessary to spend a lot of man-hours and costs in the refining process, and there are restrictions from the viewpoint of economy. In particular, if the H content is less than 0.00001%, a great amount of processing time and cost are generated, and the manufacturing cost is remarkably increased.

  In this regard, when fine Mg oxide is dispersed in the steel material, it is possible to accumulate and fix hydrogen around the fine Mg oxide, so the necessary reduction amount of H content in the steel material Can be mitigated, and it is effective for suppressing a decrease in strength of the steel material and suppressing embrittlement.

  Therefore, the lower limit of the H content is set to 0.00001% in consideration of the processing time and cost required for the reduction of the H content and the effect of fixing hydrogen by the Mg oxide. On the other hand, since the toughness of the steel material decreases when the H content in the steel material exceeds 0.0002%, the upper limit of the H content is set to 0.0002%. From the viewpoint of improving economic efficiency and further improving the toughness of the steel material, the H content is preferably in the range of 0.00005 to 0.00015%.

Mg: 0.0001 to 0.005%
Mg is the most important component element in the present invention. Oxygen in molten steel reacts with Mg to produce Mg oxide. As the Mg oxide, an oxide containing MgO and one or more of Al ₂ O ₃ , SiO ₂ , Ti ₂ O ₃ in addition to MgO alone is generated. In order to produce these, it is necessary to contain Mg 0.0001% or more. However, when the content rate exceeds 0.005%, the amount of coarse oxide inclusions in the steel increases and the toughness of the steel material decreases. For the above reason, the appropriate range of the Mg content is set to 0.0001 to 0.005%. In order to reduce the refining cost and further improve the toughness of the steel material, it is preferable to set the range of the Mg content to 0.0005 to 0.003%.

(2) Continuous casting method As described above, the continuous casting method of the slab for manufacturing steel materials is made of Mg, which is an added metal, through an immersion lance immersed in molten steel in a tundish or an immersion lance immersed in molten steel in a mold. This is a continuous casting method for steel, characterized in that a wire or rod containing vapor and / or metal particles of Mg or a Mg-containing wire or rod is supplied into molten steel together with a carrier gas.

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

  In order to confirm the effect of the continuous casting method of the steel material and the steel material production slab of the present invention, the following tests were conducted and the results were evaluated.

1. Test conditions Molten steel: Molten steel having the composition shown in Table 1 described later Molten steel temperature: 1570 ° C
Mold size: 1200mm width x 250mm thickness
Casting speed: 1.0 m / min Addition metal: Mg
Addition method: Pure Mg metal wire with a diameter of 3 mmφ is used Wire feed rate: 1.5-13.0 m / min Addition position: in tundish Immersion depth of immersion lance: 250 mm
Carrier gas: Argon gas 10 L / min Gas pressure: 0.03 MPa

  In this test, in order to produce continuous cast slabs with varying Mg content in the molten steel, the addition rate of Mg vapor added to the molten steel in the tundish was varied by changing the supply rate of the Mg wire. . Further, the H content in the molten steel was changed by changing the treatment time of the RH vacuum degassing treatment. By rolling the continuous cast slab produced by performing these operations, steel materials for test welding were produced, and their toughness was evaluated using a Charpy tester.

  FIG. 1 shows a method of continuous casting while supplying Mg metal wire to molten steel in a tundish. 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 Mg metal wire 50 containing the additive metal element Mg is inserted into the hole of the immersion lance 4 immersed in the molten metal 1 in the tundish 2 at a predetermined speed, and the additive metal element Mg is finally Mg. The metal vapor is supplied into the molten metal 1 in the tundish 2.

  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 carrier gas 54 whose flow rate and pressure are 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, and is immersed together with the Mg metal wire 50. 4 is supplied.

   On the other hand, as a test of the comparative example, continuous casting was performed under the condition where the above Mg was not added, and the same tests and investigations were performed as when Mg vapor was added.

A continuous cast slab (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: 4 hours Hot rolling finishing temperature: 850-900 ° C
Hot rolled steel sheet thickness: 10mm
Winding temperature: 400-500 ° C

2. Test results Table 1 shows the results of investigations on the composition of steel used in the test and the toughness of the steel.

  In Table 1, toughness was investigated by evaluating the results of the Charpy test. The Charpy test was performed using a V-notch test piece according to the method described in JIS Z 2242. The test temperature was −40 ° C., and the test results were evaluated by indexing the impact absorption energy in the test number C1 of the comparative example as the standard (1.0).

  Table 2 shows the results of investigation on the fine dispersion state of Mg oxide investigated for test number H1 of the present invention.

  In Table 2, samples were taken from positions of ½ width, ¾ width and ¼ width in the slab width direction, and measured by a constant potential electrolytic extraction method using a nonaqueous solvent liquid as described above. Results are shown.

  The test numbers H1 to H8 are tests for the present invention examples that satisfy the conditions specified in the present invention, and the test numbers C1 to C5 do not contain Mg metal elements or the content ratios are specified in the present invention. It is a test for a comparative example that is high beyond the range.

As can be seen from the examination result of the number of Mg oxides in the test number H1 of the present invention example shown in Table 2 above, the particle size of 1 μm with the number of 10 ⁷ pieces / mm ³ far exceeding 10 ⁴ pieces / mm ³ It was confirmed that the following fine Mg oxide was well dispersed at each position in the slab width direction.

  Moreover, as shown in Table 1, the test numbers H1 to H8, which are examples of the present invention, have a larger Charpy impact absorption energy index than the test numbers C1 to C5 of the comparative example. These results show that even when hydrogen is present in the steel, high toughness is ensured as a result of the hydrogen being fixed around the fine Mg oxide due to the fine dispersion of the Mg oxide. It shows that.

  On the other hand, in the test numbers C1 to C5 which are comparative examples, since the fine dispersion of the Mg oxide does not occur, hydrogen in the steel is not fixed around the fine oxide, and therefore, the Charpy impact absorption energy index The results are also inferior.

  In the steel material of the present invention, Mg oxide is uniformly and finely dispersed in the steel material, and hydrogen is accumulated around the steel material. Therefore, the steel material is excellent in mechanical strength such as strength and toughness. It is suitable as a material for structural steel materials such as line pipes and offshore structures. In addition, the continuous casting method of the present invention is an optimum method that can efficiently add an appropriate amount of metal Mg necessary for obtaining the above-mentioned high-strength steel material to the molten steel and uniformly disperse it in the continuous cast slab. It is a continuous casting method. Therefore, the steel material of the present invention can be widely used as a mechanical structural steel material having excellent strength and toughness, and the casting method of the present invention can be used as a continuous casting method for producing the above-described steel material production slab. Can be widely applied.

It is a figure which shows the method of continuously casting, supplying a metal wire 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 wire feeder, 50: Mg 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 (1)

It is a high-strength steel material obtained using a continuously cast slab as a raw material,
In mass%, C: 0.03 to 0.20%, Si: 0.005 to 0.5%, Mn: 0.1 to 3.0%, P: 0.02% or less, S: 0.005 %: Ti: 0.003-0.03%, N: 0.002-0.008%, Al: 0.001-0.01%, O: 0.001-0.008%, H: 0 0.0001 to 0.0002%, Mg: 0.0001 to 0.005%, with the balance being Fe and impurities,
When measured at each position of 1/2 width, 3/4 width, and 1/4 width in the slab width direction by a constant potential electrolytic extraction method using a non-aqueous solvent liquid, per unit volume (1 mm ³ ) The number of Mg oxides with a particle size of 1 μm or less present exceeds 1.0 × 10 ⁷ / mm ³ ,
Among the above component compositions, a high strength steel material whose index is 1.3 or more, based on the shock absorption energy by the Charpy test specified in JIS Z 2242 of a test piece not containing Mg. .

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