Manufacturing method for continuous casting of B-containing steel

Abstract

To provide a method of producing a continuously cast slab of B-containing steel, which prevents swelling occurring on a slab surface after continuous casting.SOLUTION: A method of producing a continuously cast slab of steel having a predetermined composition includes controlling an in-mold molten steel hydrogen concentration at 2.0 ppm or lower by providing a hydrogen concentration of molten steel immediately after vacuum degasification treatment at lower than 2.0 ppm in RH degasification treatment and a tundish refractory temperature meeting a relation of the following expressions (1) and (2): (1) tundish refractory temperature≥150(°C) in the case a hydrogen concentration in molten steel immediately after vacuum degasification treatment is lower than 1.0 ppm; and (2) tundish refractory temperature≥150×hydrogen concentration after vacuum degasification (ppm) (°C) in the case a hydrogen concentration of molten steel immediately after vacuum degasification treatment is 1.0 ppm or higher and lower than 2.0 ppm (2).SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a technique for continuously casting B-containing steel without causing blister formation on the slab surface. [Background technology]

[0002] It is known that the addition of C, Si, Mn, etc. is effective in ensuring the strength of steel when manufacturing high-tensile steel plates for automotive applications. Furthermore, adding just a few tens of ppm of B to steel lowers the transformation temperature and improves the hardenability of grain boundaries. Therefore, it is one of the useful elements in steel design, as it can control the steel structure and increase the strength of the steel.

[0003] To efficiently produce such steel, steel with the specified composition is generally cast using a curved or vertical continuous casting machine. Blisters as shown in Figure 1 often occur on the surface of the slab immediately after continuous casting of B-containing steel.

[0004] If the bulging is severe, it is of course impossible to use the steel in the rolling process that follows the continuous casting, but even if the bulging is minor, it can result in serious defects such as double plates or holes.

[0005] Patent Documents 1 and 2 disclose a technique for preventing slab bulging, which is intended for grain-oriented silicon steel plate and involves heating a slab that has been continuously cast while applying electromagnetic stirring to molten steel with a liquidus temperature of 25°C or higher. Furthermore, Patent Document 3 discloses a technique for preventing slab bulging, intended for grain-oriented silicon steel plate, that specifies the unsolidified thickness in the center of the slab when applying electromagnetic stirring during continuous casting and specifies the appropriate heating temperature for the continuously cast slab.

[0006] These disclosed techniques are aimed at a Si content of 2.5 to 4.5% by mass, and are not aimed at the Si concentration of 1% or less that is commonly seen in general-purpose steels, and furthermore, do not suggest the effect of B that is the subject of the present application. Furthermore, they focus on the fact that the cause of bulging is derived from excessive central segregation of the cast slab, and disclose a continuous casting method that specifies the degree of superheat of molten steel and electromagnetic stirring.

[0007] However, there is no suggestion of a workaround based on the provision for "secondary refining conditions and other continuous casting conditions" that does not require measures against center segregation. In addition, the conventional techniques mainly target slab expansion after high-temperature heating, and it is not necessarily easy to derive a method to prevent slab expansion that has already occurred immediately after casting without heating. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 3133868 [Patent Document 2] Patent No. 3538855 [Patent Document 3] Patent No. 3612717 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made in view of the above problems, and has an object to provide a method for producing a continuously cast slab of B-containing steel that prevents blistering from occurring on the slab surface after continuous casting. [Means for solving the problem]

[0010] From the viewpoint of productivity, vertical bending type or curved type continuous casting machines are mainstream in the continuous casting of steel. Blisters often occur on the surface of the slab immediately after it is cast by these continuous casting machines. If the slab is severe, it cannot be used for the rolling process following the continuous casting, but even if the slab is only slightly swollen, it can cause serious defects such as double plates or holes.

[0011] Therefore, the inventors investigated various causes of slab swelling, and as a result, discovered the inclusion of B and its correlation with hydrogen in steel. From a viewpoint not disclosed in the prior art, the inventors discovered a continuous casting method for steel that prevents slab surface swelling immediately after continuous casting from a new approach of managing hydrogen in molten steel for B-containing steel.

[0012] In order to solve the above problems, (1) A method for producing a continuous cast slab of steel having a composition, by mass%, of C: 0.01% to 0.25%, Si: 0.01% to 1.20%, Mn: 0.05% to 2.40%, P: 0.020% or less, S: 0.018% or less, Al: 0.005% to 0.040%, N: 0.0015% to 0.0060%, B: 0.0014% to 0.0030%, with the balance being Fe and impurities, characterized in that the hydrogen concentration in the molten steel immediately after vacuum degassing is controlled to less than 2.0 ppm by the RH degassing treatment, and the refractory temperature of the tundish satisfies the relationship of the following formulas (1) and (2), thereby controlling the hydrogen concentration in the molten steel in the mold to 2.0 ppm or less. When the hydrogen concentration in the molten steel immediately after vacuum degassing is less than 1.0 ppm, Tundish refractory temperature ≧150(℃) (1) If the hydrogen concentration in the molten steel immediately after vacuum degassing is 1.0 ppm or more but less than 2.0 ppm, Tundish refractory temperature ≧ 150 × hydrogen concentration after vacuum degassing (ppm) (℃) (2)

[0013] moreover, (2) It is also preferable to contain, by mass%, one or more of the following: Nb: 0.005% or more and 0.050% or less, V: 0.005% or more and 0.050% or less, Cr: 0.010% or more and 0.500% or less, Mo: 0.01% or more and 0.50% or less, Ca: 0.0001% or more and 0.0010% or less, Ni: 0.005% or more and 0.05% or less, Cu: 0.005% or more and 0.030% or less, and Ti: 0.010% or more and 0.040% or less. Effect of the Invention

[0014] INDUSTRIAL APPLICABILITY The present invention provides a method for preventing the occurrence of blistering on the surface of a cast slab and improving the productivity and yield in a continuous casting process, and is therefore of great technical significance. [Brief description of the drawings]

[0015] [Figure 1] FIG. 2 is a diagram showing the state of blisters occurring on the surface of a slab immediately after it is obtained by continuous casting of B-containing steel. [Diagram 2] FIG. 1 is a diagram showing that the blisters that occur on the surface of a slab after continuous casting are strongly influenced by hydrogen and B in the steel. [Diagram 3] FIG. 1 is a diagram showing the interaction forces (thermodynamic property values) between solute elements in molten steel and hydrogen in molten steel. [Figure 4] FIG. 1 is a graph showing the correlation between the hydrogen concentration in the molten steel in the mold (or the presence or absence of slab bulging), the refractory temperature in contact with the steel shell of the tundish at a position 300 mm above the bottom of the tundish, and the hydrogen concentration after RH treatment. [Diagram 5] FIG. 1 is a graph showing the distribution of hydrogen concentration in molten steel and tundish refractory temperature immediately after vacuum degassing treatment, depending on whether or not blistering occurs in a slab after continuous casting. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The following describes an embodiment of the invention. First, the components of the B-added steel according to the present invention will be described. In the following description, all percentages and ppm are mass percent and mass ppm.

[0017] C: 0.01 or more and 0.25% or less C is the most basic element that controls the hardenability and strength of steel, and is also an essential element for ensuring the presence of retained austenite. In particular, C is an important element for dissolving sufficient C in the austenite phase and allowing the desired austenite phase to remain even at room temperature, and is useful for improving the balance between strength and stretch flangeability. If C is less than 0.01%, it is difficult to ensure the retained austenite structure required for a structure-strengthened steel plate. On the other hand, if C exceeds 0.25%, not only does the effect saturate, but weldability also decreases.

[0018] Si: 0.01 or more and 1.20% or less Silicon is an element effective for deoxidization and strength increase, and in order to obtain this effect, it is necessary to add 0.01% or more. On the other hand, if the Si content exceeds 1.20%, it may cause a deterioration in toughness.

[0019] Mn: 0.05 or more and 2.40% or less Mn is the second most commonly used element after C because it increases the strength of the base metal and is inexpensive. If the Mn content is less than 0.05%, the effect of increasing strength cannot be obtained. On the other hand, if the Mn content exceeds 2.40%, the slab becomes more susceptible to cracking and the spot weldability deteriorates.

[0020] P:0.020% or less P is one of the impurity elements that is inevitably contained in steel, so the lower the content, the better. P has a small equilibrium distribution coefficient at the solid-liquid interface during solidification, so it segregates significantly. For this reason, there are concerns that it may have a negative effect on various product characteristics. Since the melting point is also significantly lowered in the segregated areas, the concentrated areas may melt during rolling, leading to product defects. For this reason, the upper limit of the P content is set at 0.020%. To prevent various problems in the segregated areas, a content of less than 0.016% is preferable.

[0021] S: 0.018% or less S is also one of the impurity elements that is inevitably contained in steel, and it is preferable that the content is as low as possible. Not only is S an element that segregates significantly due to its small equilibrium distribution coefficient at the solid-liquid interface after solidification, but in the segregated areas, it lowers the melting point like P, and is a cause of surface defects, especially during rolling. For this reason, the upper limit of the S content is set at 0.018%. Under more stringent conditions such as high-strength steel, it is preferable to set the upper limit of the S content at 0.015%.

[0022] Al: 0.005 or more 0.040% Al is also an effective element for reducing the oxygen concentration in steel as a deoxidizing element. The content required for deoxidization is 0.005% or more. If the content is less than that, sufficient desulfurization in the smelting process becomes difficult. If added in excess, AlN is likely to be generated, which causes cracks on the surface of the slab, which is contrary to the object of the present invention, so it is preferable to keep the content at 0.040% or less.

[0023] B: 0.0014% or more and 0.0030% or less B is added as an ingredient to improve the hardenability of grain boundaries, control the structure of steel, and increase the strength of steel, and 0.0014% or more is required to realize this effect. On the other hand, even if 0.0030% or more is added, not only does the effect saturate, but it also generates BN in the steel, which causes defects on the surface of the cast slab.

[0024] N: 0.0015% or more and 0.0060% or less N is an element that inevitably penetrates into steel when it is melted in an air atmosphere such as in a converter. In steel, N forms nitrides with Ti and other elements, and these nitrides have the effect of refining crystal grains as pinning particles during hot working, thus affecting the mechanical properties of the steel. For this reason, the N concentration must be 0.0015% or more. On the other hand, as mentioned above, these nitrides dynamically precipitate at the austenite grain boundaries during continuous casting, causing cracks on the surface of the slab, so the upper limit is set to 0.0060%. From the viewpoint of ensuring the pinning effect of the structure and preventing a decrease in toughness due to the formation of coarse carbonitrides in the center of the slab, it is preferable to set the N concentration to 0.0020% or more and 0.0040% or less.

[0025] In order to impart other properties such as weather resistance, one or more of the following elements may be added.

[0026] Nb: 0.005% or more and 0.050% or less Nb is an element that forms carbonitrides in steel to increase the strength of steel and is also effective in improving toughness. To achieve this, it is necessary to add 0.005% or more. It is also used to control the microstructure of steel plates by controlling solid solution and precipitation, especially in the Thermo-Mechanical Control Process (TMCP). To achieve this effect, it is necessary to add 0.005% or more. However, if it is contained at 0.050% or more, it will not dissolve even when heated, making it impossible to control the structure. Furthermore, if it is added in excess, it will precipitate as NbC inside the slab, causing cracks on the surface of the slab. For this reason, the Nb concentration is specified to be 0.005 to 0.050%.

[0027] V: 0.005% or more and 0.050% or less V is an effective element for increasing the strength of steel by dissolving in ferrite and forming carbonitrides. To achieve this, it is necessary to add 0.005% or more. However, if the V content exceeds 0.050%, it has a negative effect on toughness. Furthermore, if added in excess, it precipitates as VN inside the slab, causing cracks on the slab surface.

[0028] Cr:0.010% or more and 0.500% or less Cr has the effect of increasing the strength and toughness of steel. To achieve this, the addition of 0.010% or more is necessary. On the other hand, even if the addition exceeds 0.500%, the effect becomes saturated.

[0029] Mo: 0.01% or more and 0.50% or less Mo: Increases strength through structure control during hot rolling. This effect is significant when added at 0.01% or more, so it is necessary to add at least 0.01%. Also, adding more than 0.50% not only saturates the effect, but also worsens weldability and hot workability.

[0030] Ca: 0.0001% or more and 0.0010% or less Unlike other component elements, Ca does not have a significant effect on the material properties of steel, but it is effective in preventing nozzle clogging during continuous casting, and is sometimes added for this purpose. Also, adding Ca to steel reduces the S concentration and prevents the formation of MnS, so it is sometimes added to control the morphology of sulfides. To achieve the above effect, it is necessary to add 0.0001% or more of Ca. Adding more than 0.0010% of Ca not only saturates the effect and increases manufacturing costs, but may also promote nozzle clogging.

[0031] Ni: 0.005% or more and 0.050% or less Ni improves the strength of steel through solid solution strengthening, and also improves toughness. To obtain these effects, it is necessary to add 0.005% or more of Ni, but adding 0.050% or more of Ni also has the disadvantage of increasing manufacturing costs.

[0032] Cu: 0.005% or more and 0.030% or less Cu improves the hardenability of steel. To achieve this, the addition of 0.005% or more is necessary, but if the content exceeds 0.030%, in addition to the effect, there is also the disadvantage that the hot workability of the steel material decreases.

[0033] Ti: 0.010% or more and 0.040% or less Ti not only improves the strength of steel, but also fixes N in steel as TiN, which helps prevent surface cracks during bending and straightening of continuously cast slabs. To achieve this effect, it is necessary to add 0.010% or more. However, if the content exceeds 0.040%, a large amount of carbides are formed, which reduces the toughness of the material.

[0034] Regarding the blister that occurs on the surface of the slab after continuous casting, the following investigations and analyses (A) to (C) were carried out, and factors preventing the blister on the surface of the slab were identified, and a continuous casting method of steel that can achieve the prevention was derived.

[0035] (A) Identifying the cause of slab bulging In a steel that had been tapped from a converter and had its composition adjusted using a vacuum degassing device at RH, and had a composition of 0.04%C-0.01%Si-0.21%Mn-0.01%P-0.009%S-0.03%Ti-0.015%Al-0.0017%B-0.0015%N, with the remainder of Fe and unavoidable impurities, blistering occurred in the slab after continuous casting. The blister was drilled with a special drill to recover the gas contained within. Analysis of the recovered gas using a gas chromatograph mass spectrometer revealed that it was mainly hydrogen and hydrocarbon gas. Therefore, we focused on the possibility that the increase in hydrogen concentration in the steel promotes the blister blistering.

[0036] (B) Effect of hydrogen and B on slab swelling The effect of hydrogen in steel on the blister bulge was summarized, but no clear correlation was found. Then, a steel containing 0.0003% to 0.0025% B and having a basic composition of 0.04%C-0.01%Si-0.21%Mn-0.01%P-0.009%S-0.03%Ti-0.015%Al-0.0015%N was analyzed by trial and error, and it was found that the blister on the slab surface after continuous casting is strongly influenced by hydrogen in steel and B, as shown in Figure 1. The result is shown in Figure 2. The hydrogen concentration in steel shown on the vertical axis of Figure 2 is the value obtained by analyzing a sample taken by immersing a pin sampler in the molten steel in the mold at a position 300 to 500 mm from the center of the width of the mold and at a depth of about 30 mm from the surface of the molten steel during casting, and the sample was taken when the amount of molten steel in the ladle reached about half. It was found that when the steel contains 14 ppm or more of B and the hydrogen concentration of the molten steel in the mold is 2.0 ppm or more, blisters occur on the surface of the slab after continuous casting.

[0037] Furthermore, in the region where the B concentration in steel is 14 ppm or higher, the effect of hydrogen in steel is due to the strong repulsive thermodynamic interaction that acts between B in steel and hydrogen. Figure 3 shows the interaction force (thermodynamic property value) between hydrogen in molten steel and solute elements in molten steel, and it can be seen that B has a greater effect on hydrogen than other elements. This suggests that when B coexists, the solubility of hydrogen in steel is likely to decrease, and dissolved hydrogen is likely to gasify, causing blistering.

[0038] However, there remains some doubt as to whether a quantitative difference occurs due to a difference in B content between, for example, 5 ppm and 18 ppm. Therefore, the B concentration on the prior austenite grain boundaries of six types of steel with different B contents of 5, 11, 13, 14, 18, and 22 ppm was analyzed by TOF-SIMS (time-of-flight secondary ion mass spectrometry).

[0039] As a result, it was found that only in steels containing 14, 18, and 22 ppm B, B segregated and concentrated at the prior austenite grain boundaries at about 5 to 30 times the amount in the bulk. On the other hand, when the steels contained 5, 11, or 13 ppm B, the segregation of B at the prior austenite grain boundaries was only observed to be about twice as concentrated as the bulk. Therefore, when the B concentration in the steel was 14 ppm or more, B segregation at the prior austenite grain boundaries was likely to occur, and under this influence, when the hydrogen was 2.0 ppm or more, blisters occurred on the surface of the slab after continuous casting.

[0040] In this way, in order to prevent hydrogen-induced blistering on the slab surface after continuous casting in B-containing steel, strict control of hydrogen in the steel is important. When producing steel using the blast furnace-converter process, the steel tapped from the converter is dehydrogenated under reduced pressure using a vacuum degassing device such as RH (Ruhrstahl-Heraeus, vacuum degassing equipment). There is no process to remove hydrogen after the dehydrogenation process, and hydrogen must be controlled to at least less than 2 ppm after this dehydrogenation process.

[0041] In addition, the treatment process using the vacuum degassing equipment is a process immediately before continuous casting, so it also has the function of adjusting the steel composition to a specified level. The various raw materials for adjusting the composition, such as ferromanganese, ferrosilicon, and metallic silicon, used for adjusting the composition contain moisture such as adhering water, and when they are added to molten steel, the hydrogen concentration in the molten steel may increase. Therefore, from the viewpoint of hydrogen pickup, it is strictly not preferable to adjust the composition by adding raw materials after dehydrogenation treatment with RH, and hydrogen control after RH treatment is also important.

[0042] Furthermore, even if hydrogen dehydrogenation is controlled to less than 2 ppm by RH dehydrogenation, hydrogen pickup into molten steel may occur in the subsequent process. The main source of hydrogen pickup is the refractory material that comes into contact with the steel. In the continuous casting process, molten steel is poured from the ladle into the tundish, and then from the tundish into the mold. The tundish has an outer wall made of a steel shell, and on the inside, three to four layers of refractory material are installed.

[0043] Considering that this refractory contains moisture and that the residence time of molten steel in the tundish is only a few minutes, it was thought that this would be a cause of hydrogen pick-up into the molten steel. Figure 4 shows the correlation between the hydrogen concentration in the molten steel in the mold (or the presence or absence of slab bulging), the refractory temperature in contact with the tundish shell at a position 300 mm above the bottom of the tundish, and the hydrogen concentration after RH treatment.

[0044] Figure 4 shows the distribution of the hydrogen concentration in the molten steel immediately after vacuum degassing and the tundish refractory temperature, depending on whether or not blistering occurs in the slab after continuous casting. To prevent blistering in continuously cast slabs, it is necessary to control the hydrogen concentration in the molten steel in the mold to less than 2.0 ppm, as mentioned above, but to do this, it is necessary to control the hydrogen concentration in the molten steel immediately after vacuum degassing by the RH degassing process to less than 2.0 ppm, as shown in Figure 4, and for the tundish refractory temperature to satisfy the relationship between equations (1) and (2) below. If the hydrogen concentration in the molten steel immediately after the vacuum degassing process is less than 1.0 ppm, the tundish refractory temperature is ≧150 (℃), (1) If the hydrogen concentration in the molten steel immediately after the vacuum degassing process is 1.0 ppm or more but less than 2.0 ppm, the tundish refractory temperature is ≧ 150 × hydrogen concentration after vacuum degassing (ppm) (℃) (2)

[0045] When this condition is satisfied, it is possible to prevent the occurrence of blistering on the surface of the slab after continuous casting. The reason for specifying the temperature at 300 mm from the bottom of the tundish is that when temperatures were measured in advance at positions 100 mm, 200 mm, 300 mm, and 400 mm above the bottom of the tundish, the temperature at 300 mm above the bottom of the tundish was a maximum of 40°C lower than the other positions, and in any case it was the lowest temperature. EXAMPLES

[0046] The 270 ton molten steel in the ladle was cast with the composition shown in Table 1 using a vertical bending type continuous casting machine to produce slabs. The size of the mold for the continuous casting machine was 250 mm thick and 1600 mm wide, and the casting speed was 1.2 m / min. The mold oscillation during casting was 2.58 Hz and 2.07 mm in frequency and stroke. Mole powder was used with a viscosity of 1.5 Pa·s (1300°C), a solidification point of 1230°C, and a basicity of 1.3, defined as the mass percent concentration of CaO divided by the mass percent of SiO2. In addition, spray cooling was performed below the mold with a specific water amount of 1.1 liters per kg of slab weight.

[0047] [Table 1]

[0048] A comparative example of the present invention corresponds to a case where the cast slab is swollen and cannot be used in the rolling process following the continuous casting, while an example of the present invention corresponds to a case where the cast slab is not swollen and does not have defects such as double plates or holes even if it is used in the rolling process. FIG. 5 shows a correlation diagram between the hydrogen concentration in the molten steel immediately after the vacuum degassing process and the tundish refractory temperature for the examples and comparative examples listed in Table 1. Comparative Examples 1 to 5 are examples in which the relationship of formula (1) is not satisfied even if the hydrogen after the vacuum degassing process is 2.0 ppm or more and the refractory temperature of the tundish is sufficient. Comparative Examples 6 to 13 are examples in which the hydrogen after the vacuum degassing process can be controlled to less than 2.0 ppm, but hydrogen absorption occurs in the tundish and it cannot be controlled to an appropriate level or lower, causing swollenness.

Claims

1. A method for producing a continuously cast slab of a steel having a composition, in mass%, of C: 0.01% or more and 0.25% or less, Si: 0.01% or more and 1.20% or less, Mn: 0.05% or more and 2.40% or less, P: 0.020% or less, S: 0.018% or less, Al: 0.005% or more and 0.040% or less, N: 0.0015% or more and 0.0060% or less, B: 0.0014% or more and 0.0030% or less, with the balance being Fe and impurities, characterized in that the hydrogen concentration in the molten steel immediately after vacuum degassing treatment is made less than 2.0 ppm by the RH degassing treatment, and the hydrogen concentration in the molten steel in the mold is controlled to 2.0 ppm or less by the refractory temperature of the tundish satisfying the relationship of the following formulas (1) and (2). When the hydrogen concentration in the molten steel immediately after the vacuum degassing process is less than 1.0 ppm, Tundish refractory temperature ≧ 150 (℃) (1) When the hydrogen concentration in the molten steel immediately after the vacuum degassing process is 1.0 ppm or more and less than 2.0 ppm, Tundish refractory temperature ≧ 150 × hydrogen concentration after vacuum degassing (ppm) (℃) (2) Here, the refractory temperature of the tundish is defined as the temperature of the refractory in contact with the iron shell of the tundish at a position 300 mm above the bottom surface of the tundish.

2. 2. The method for producing a continuous cast slab of B-containing steel according to claim 1, further comprising, by mass%, one or more of the following: Nb: 0.005% or more and 0.050% or less; V: 0.005% or more and 0.050% or less; Cr: 0.010% or more and 0.500% or less; Mo: 0.01% or more and 0.50% or less; Ca: 0.0001% or more and 0.0010% or less; Ni: 0.005% or more and 0.05% or less; Cu: 0.005% or more and 0.030% or less; and Ti: 0.010% or more and 0.040% or less.

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