JP2011042859A - Continuous casting method for low alloy steel for corrosion resistant thick plate, and continuously cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the generation of surface flaws upon continuous casting for a low alloy plate for a corrosion resistant thick plate to which, by mass, 0.09 to 0.20% C and 0.05 to 0.50% Sn are added. <P>SOLUTION: A low alloy steel for a corrosion resistant thick plate comprising, by mass, 0.09 to 0.20% C, 0.05 to 1.00% Si, 0.20 to 2.50% Mn, &le;0.05% P, &le;0.02% S, 0.003 to 0.1% Al, 0.05 to 0.50% Sn, 0.01 to 0.20% Cu and 0.01 to 0.20% Ni, and the balance Fe with impurities is cast in such a manner that the value of componential ratios in mass%, Cu/Sn is controlled to 0.02 to 0.5, and the value of (Cu+Ni)/Sn is controlled to 0.04 to 0.7. In this way, the low alloy steel for a corrosion resistant thick plate to which 0.09 to 0.20% C and 0.05 to 0.50% Sn are added can be continuously cast with the generation of surface flaws prevented. Further, even in hot rolling in the post-step, a continuously cast slab having low crack sensitivity can be provided. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention is a low alloy for corrosion-resistant thick plates containing Sn with good surface quality, which does not generate surface flaws during continuous casting and is a base material for hot rolling with low cracking sensitivity even in the subsequent hot rolling. The present invention relates to a method for continuously casting steel, and a continuously cast slab produced by this method.

  Like Cu, Sn is a trump element in scrap iron, but it is also known to improve the corrosion resistance of steel. However, since Sn is also an embrittlement element that causes cracking during hot working of steel, prevention of surface flaws is the biggest issue when producing corrosion-resistant steel containing Sn.

  Patent Document 1 discloses a steel material for bridges that uses the effect of improving the weather resistance by Sn. In this steel for bridges, the upper limit of Cu and Ni is regulated from the viewpoint of corrosion resistance.

  However, this Patent Document 1 does not describe hot surface embrittlement or surface flaw prevention during continuous casting.

  Patent Document 2 discloses a hot rolled steel containing Cu and Sn that can be produced without generating surface defects during hot working. This Cu and Sn containing hot rolled steel is in mass%, C: 1.0% or less, Si: 1.0% or less, Mn: 0.1-1.5%, Al: 0.001-0.1 %, Cu: 0.1 to 0.5%, Sn: 0.2 to 1.0%, and the Sn concentration is 2 × Cu% or more.

  However, the Cu and Sn-containing hot rolled steel disclosed in Patent Document 2 is limited to prevention of surface flaws generated due to the influence of the composition ratio of Cu and Sn melts caused by selective oxidation of Fe. Further, when Sn is contained in addition to Cu, it is only described that the effect of preventing cracking of Ni is lowered, and there is no sufficient description about the effect when Ni coexists. Furthermore, in the C region where the sensitivity to vertical cracks such as the C content of 0.09 to 0.20% by mass is increased, it is necessary to prevent casting cracks, but there is no mention of the means.

  Non-Patent Document 1 describes a method for suppressing surface red hot brittleness caused by Cu or Sn. In this Non-Patent Document 1, the influence of Cu and Sn on hot work cracking due to surface red hot embrittlement (liquid embrittlement) is explained as follows.

  The steel material heated to 1000 ° C. or more generates a scale on the surface by atmospheric oxidation. In a Cu-containing steel of about 0.3% by mass, the main component Fe is selectively oxidized, and Cu is concentrated in the surface layer portion. At that time, Cu having a low melting point precipitates as a liquid phase on the surface layer portion, and this penetrates into the crystal grain boundary and causes embrittlement of the liquid film.

  In addition to Cu, Sn and Ni are noble metal elements that are less susceptible to oxidation than Fe, the main component of the matrix. Cu-Sn containing steel (0.3 mass% Cu-0.04 mass% Sn) containing Cu and Sn among metals nobler than Fe contains Cu alone containing only Cu among metals nobler than Fe Surface cracking becomes more prominent than steel (0.3 mass% Cu). However, surface cracking does not occur in the case of the Sn-only steel containing only Sn among the metals nobler than Fe. For example, surface cracking does not occur even when a steel material to which 0.04 mass% Sn or 0.3 mass% Sn is added alone is oxidized in the atmosphere.

  Non-Patent Document 1 also examines the effect of embrittlement suppression by Ni. Embrittlement of 0.3 mass% Cu steel is suppressed by adding 0.15 mass% of Ni, but in 0.3 mass% Cu-0.04 mass% Sn steel, Ni is 0.15 mass%. % Addition is insufficient, and embrittlement can be suppressed by adding 0.3% by mass of Ni.

  Thus, Non-Patent Document 1 clarifies the effect of Sn and Ni on the embrittlement of Cu-containing steel of about 0.3% by mass and that the Sn single steel does not embrittle.

JP2008-163374 JP-A-6-256904

Materials and Processes, Vol.13, 2000, p.1080

  The problem to be solved by the present invention is that, when producing corrosion-resistant steel containing Sn, the biggest problem is prevention of surface flaws. There is no consideration for surface embrittlement and surface flaw prevention during continuous casting.

  Further, the Cu and Sn-containing hot rolled steel disclosed in Patent Document 2 is limited to the prevention of surface flaws generated due to the influence of the composition ratio of Cu and Sn melt caused by selective oxidation of Fe. . Further, this Patent Document 2 does not mention the effect when Ni coexists or the means for preventing casting cracks in the C region where the sensitivity to vertical cracks such as the C content is 0.09 to 0.20% by mass. That is the point.

  In addition, Non-Patent Document 1 only shows that Sn and Ni affect the embrittlement of about 0.3% by mass of Cu-containing steel, and that Sn alone steel does not embrittle. is there.

The continuous casting method of the low alloy steel for corrosion resistant thick plate of the present invention is as follows:
In order to prevent the occurrence of surface flaws during continuous casting of the corrosion-resistant low-alloy steel for thick plates to which C is added by 0.09 to 0.20 mass% and Sn is added to 0.05 to 0.50 mass%,
In mass%, C: 0.09 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.50%, P: 0.05% or less, S: 0.02 %: Al: 0.003 to 0.1%, Sn: 0.05 to 0.50%, Cu: 0.01 to 0.20%, Ni: 0.01 to 0.20%, Low alloy steel for corrosion resistant thick plates with the balance being Fe and impurities,
Casting with a mass% component ratio, Cu / Sn values of 0.02 or more and 0.5 or less, and (Cu + Ni) / Sn values of 0.04 or more and 0.7 or less. Yes.

  And produced by atmospheric oxidation in the high temperature state immediately after continuous casting by the continuous casting method of the present invention, Sn, Cu and Ni in the slab surface layer portion is concentrated, Fe as the parent phase The continuously cast slab of the present invention has a composition of a different phase containing at least 30% by mass or more of Fe.

  The present invention continuously casts a corrosion-resistant low alloy steel for thick plates to which C is added in an amount of 0.09 to 0.20% by mass and Sn is added in an amount of 0.05 to 0.50% by mass while preventing surface flaws Can do. In addition, a continuous cast slab with low cracking sensitivity can be provided even during the subsequent hot rolling, and a steel material with good quality can be manufactured.

In the Cu-Ni-Sn ternary composition diagram, 0.02 ≦ Cu / Sn ≦ 0.5 and a range showing 0.04 ≦ (Cu + Ni) /Sn≦0.7. It is the figure which showed the ternary system conversion composition of the (Cu, Sn, Ni) concentrated phase of the slab surface deposited on the surface of a parent phase on a 1127 degreeC Cu-Ni-Sn ternary phase diagram. On the Fe- (Cu + Ni) -Sn composition diagram, the composition change of (Cu, Sn, Ni) concentration behavior is indicated by arrows, and the (Cu, Ni, Sn) enriched phase Fe formed on the slab surface It is the figure which showed the result of having investigated the included quaternary system conversion composition by SEM / EDS analysis. It is the figure which showed the high Fe side of the Fe- (Cu + Ni) -Sn system phase diagram in 1250 degreeC created by adding an attached database and the thermodynamic data of literature using analysis software. Regarding the (Cu, Ni, Sn) concentrated phase formed on the surface of the continuous cast slab, the results of investigation of the quaternary equivalent composition including Fe by SEM / EDS analysis are Fe- (Cu + Ni) at 1250 ° C. It is the figure shown on the -Sn composition chart.

  According to the present invention, the occurrence of surface flaws is prevented when continuously casting a low alloy steel for corrosion-resistant thick plates to which C is added in an amount of 0.09 to 0.20 mass% and Sn is added in an amount of 0.05 to 0.50 mass%. The purpose was achieved by studying the embrittlement suppression conditions of steel in which Cu and Ni coexist.

  Hereinafter, the best mode for carrying out the present invention will be described together with the process from the idea of the present invention to the solution of the problem.

  The inventors reviewed the influence of Sn on the Cu embrittlement phenomenon caused by the selective oxidation of Fe. In particular, we focused on the relationship between the low-melting-point alloy composition and cracking susceptibility produced by the enrichment of Cu, Ni, and Sn formed during the selective oxidation of Fe, and studied a composition system that suppresses embrittlement.

  Originally, Cu embrittlement is caused by the fact that when Fe is selectively oxidized, Cu that is nobler than Fe is concentrated, and the Cu concentration exceeds the solid solubility limit of the austenite Fe phase and a low melting point Cu liquid phase is precipitated. It is believed that the phase Cu penetrates into the austenite grain boundaries of Fe, the parent phase, and weakens the grain boundaries.

  Ni and Sn are more noble elements than Fe, like Cu. Ni is an element that suppresses Cu embrittlement because it raises the solid solubility limit of Cu and raises the melting point of the Cu liquid phase. On the other hand, Sn is an element that promotes Cu embrittlement because it lowers the solid solubility limit of Cu and lowers the melting point of the Cu liquid phase.

  For this reason, conventionally, Ni has been added as an embrittlement suppression element, and measures to limit the amount of Sn added, that is, measures to prevent the formation of a low melting point liquid phase have often been taken.

  On the other hand, the conventional measures cannot be taken in the case of steel, in which Sn is the main additive element among the three alloy elements of Cu, Ni, and Sn, which are nobler than Fe. The reason is that Sn has a much lower melting point than Cu, and even if Ni is added, the effect of suppressing liquid phase formation is small.

  Therefore, the inventors focused on the liquid phase composition formed by selective oxidation of Fe, and examined it by basic experiments and thermodynamic analysis. As a result, it is not the (Cu, Sn) liquid phase that precipitates from the parent phase Fe as a heterogeneous component that does not dissolve in the parent phase, but the Sn melting in the parent phase Fe to lower the melting point (Fe, Sn) liquid phase It was found that embrittlement can be suppressed by giving priority to the generation of.

  That is, the (Cu, Sn) liquid phase accumulates in the form of a film at the scale / matrix interface and penetrates into the austenite crystal grain boundary of Fe, which is the matrix phase, to cause grain boundary embrittlement. On the other hand, the behavior of the (Fe, Sn) liquid phase is significantly different from the properties of the (Cu, Sn) liquid phase that cause embrittlement. Because the (Fe, Sn) liquid phase contains a large amount of the Fe component of the matrix and is converted to a liquid phase at a relatively high temperature with a high oxidation rate, Sn can be effectively eliminated into the scale formed on the surface of the matrix. is there. Therefore, if the (Fe, Sn) liquid phase is generated preferentially without forming the (Cu, Sn) liquid phase, the problem of surface defects can be solved.

  FIG. 1 shows the range of the composition ratio of Cu, Ni, and Sn that can prevent surface cracking of the corrosion-resistant thick plate presented in the present invention, that is, 0.02 ≦ Cu / Sn ≦ 0.5 and 0 .04 ≦ (Cu + Ni) /Sn≦0.7.

  First, for slabs that were experimentally continuously cast steel containing various Cu, Ni, Sn, check the presence or absence of surface cracks, collected a cross-sectional sample of the slab surface layer portion, scanning electron microscope (hereinafter, Observation by SEM).

  A reflected electron image is used for cross-sectional observation of the slab sample. In the backscattered electron image, the larger the atomic weight, the brighter the image. Therefore, the contrast of the image is shaded by the compositional difference of the material. Therefore, it is possible to discriminate a portion where Cu, Ni, and Sn having a larger atomic weight than Fe and Fe as the parent phase are concentrated.

  The ternary equivalent composition of the (Cu, Ni, Sn) concentrated phase on the slab surface produced by selective oxidation of Fe was investigated by energy dispersive X-ray spectroscopy (hereinafter referred to as EDS).

  FIG. 2 shows the ternary equivalent composition of the (Cu, Sn, Ni) concentrated phase on the surface of the cast slab deposited on the surface of the parent phase on the Cu-Ni-Sn ternary phase diagram at 1127 ° C. It is. The calculation phase diagram calculation was created by adding analysis data Thermo-Calc (product of Thermo-Calc Sotware AB) and adding general thermodynamic data described in the attached database and literature.

  The ternary composition of the concentrated phase on the slab surface is roughly classified into four groups shown in FIGS. 2A to 2D. At 1127 ° C., (a) to (c) are in the liquid phase region, ( d) is in the solid phase region.

  Of these, minor slab surface cracks occurred in (b) of the liquid phase region, and slab surface cracks occurred frequently in (c). On the other hand, since (d) is in the solid phase region on the Cu-Ni axis, it is natural that there is no slab surface cracking, but (a) in the high Sn side liquid phase region is also slab surface cracking. There was nothing.

  That is, it was found that even in a composition system in which a liquid phase is generated on the Cu-Ni-Sn ternary phase diagram, a composition range capable of suppressing slab surface cracking is on the high Sn side. However, since this is a ternary compositional study, it is thought that the main mechanism is actually the enrichment in the Fe matrix during the Fe selective oxidation process. Therefore, the inventors examined the behavior of the quaternary system to which Fe was added.

  Fig. 3 shows the composition change of (Cu, Sn, Ni) concentration behavior on the Fe- (Cu + Ni) -Sn composition diagram by arrows, and (Cu, Ni, Sn) concentration formed on the slab surface. It is the figure which showed the result of having investigated the quaternary system conversion composition containing Fe of a phase by SEM / EDS analysis.

  When the (Cu + Ni) / Sn ratio is 0.25 (= 20/80), the composition of the concentrated phase generated in the Fe selective oxidation process contains a relatively large amount of Fe, and Fe ≧ 30 before Fe is completely oxidized and lost. In this case, the surface crack of the slab does not occur (the region marked with dots in FIG. 3).

  On the other hand, when the (Cu + Ni) / Sn ratio is 1.5 (= 60/40), the composition of the concentrated phase generated in the Fe selective oxidation process is relatively small in Fe, and Fe < This is a Cu-Sn low melting point liquid phase produced at 30% by mass, and in this case, it is accompanied by surface cracks in the slab (regions without dots in FIG. 3).

  Furthermore, as a result of investigating steels with various compositions, slab surface cracking has a close relationship with the (Cu + Ni) / Sn ratio, and surface cracking can be suppressed by setting the (Cu + Ni) / Sn ratio to 0.7 or less. I understood.

  FIG. 4 shows the high Fe side of the Fe— (Cu + Ni) —Sn phase diagram at 1250 ° C. created using the analysis software Thermo-Calc and adding the general thermodynamic data described in the attached database and literature. FIG.

It can be seen from the state diagram of FIG. 4 that the liquid phase is separated into two phases. Fe-Sn in the shaft near the high Fe side L1 (Fe, Sn) liquid phase region, at a low Fe and height (Cu + Ni) side, L ₂ (Cu, Sn) liquid phase region exists, respectively.

  The concentrated phase composition shown in (a) of FIG. 3 without surface cracks is a region where the (Fe, Sn) liquid phase is generated in the 1250 ° C. phase diagram of FIG. This Fe-side liquid phase is one in which Sn is concentrated by selective oxidation of Fe, and the Fe phase itself, which is the parent phase, contains 30% by mass or more of Sn, thereby melting and melting.

The Fe side liquid phase in which the matrix phase is dissolved has poor grain boundary permeability and is harmless to grain boundary cracking. That is, when an L ₁ (Fe, Sn) liquid phase containing a large amount of Fe is generated near a relatively high temperature of 1250 ° C., surface cracking can be suppressed.

On the other hand, the concentrated phase composition shown in (b) of FIG. 3 in which surface cracks occurred has no L ₁ liquid phase in the 1250 ° C. state diagram of FIG. 4 and L ₂ ( Cu, Sn) liquid phase is formed.

Whether or not the slab surface cracks occur in this way depends on the composition of the concentrated phase generated in the selective oxidation process of Fe, and by generating a L ₁ (Fe, Sn) liquid phase containing high Fe, It can be seen that surface cracks can be prevented.

  FIG. 5 shows the results of investigating the quaternary equivalent composition including Fe on the (Cu, Ni, Sn) concentrated phase formed on the surface of the continuous cast slab by SEM / EDS analysis. It is the figure shown on the (Cu + Ni) -Sn composition diagram.

The concentrated phase composition on the surface of the slab with a (Cu + Ni) / Sn ratio of 0.7 or less (circle mark) is a composition containing a relatively large amount of Fe, and the L ₁ (Fe, Sn) liquid phase formation on the phase diagram It is in good agreement with the area. In this case, the surface crack of the slab does not occur.

On the other hand, the concentrated phase composition (□) on the surface of the slab containing Sn and having a (Cu + Ni) / Sn ratio exceeding 0.7 deviates from the L ₁ (Fe, Sn) liquid phase formation on the phase diagram, Only the L ₂ (Cu, Sn) liquid phase is formed.

  FIG. 5 shows only the high Fe concentration side (Fe: 60 mass% or more, Cu + Ni + Sn: 40 mass% or less) region, but as shown by “→ □” in FIG. 5, the concentration outside this range is shown. Many phases (Cu + Ni + Sn: more than 40% by mass) are detected. In this case, surface cracks frequently occur in the slab.

  The alloy composition other than Cu, Ni, and Sn in the steel targeted by the present invention is mass%, C: 0.09 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.2. -2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.003-0.1%.

  This is a general composition system used for general structural materials. However, the steel of the composition system targeted by the present invention has a C amount of 0.09 to 0.20% by mass, and, as will be described later, a peritectic reaction (L + δ → γ) is involved in the solidification process. Extremely high sensitivity to vertical cracks during continuous casting. Therefore, it is important for the steel targeted by the present invention to suppress longitudinal cracks during casting and to prevent intergranular cracking due to Cu and Sn.

  Hereinafter, the reasons for limiting the amount of each alloy composition in the steel targeted by the present invention will be described. In the following, “%” in the blending amount of the alloy composition means “mass%”.

C: 0.09 to 0.20%
C is an element necessary for ensuring strength as a material. The range of 0.09 to 0.20% targeted by the present invention is a composition suitable as a structural material. However, this C range is an area where the cracking sensitivity of continuous casting increases. This is because in this C range, the solidification process from the liquid phase to the solid phase is accompanied by a peritectic reaction (L + δ → γ), and each shrinkage of solidification (L → δ) and transformation (δ → γ) overlaps, so the initial solidification shell This is because non-uniform growth and grain boundary cracking are likely to occur, and vertical cracks are particularly likely to occur.

Si: 0.05-1.00%
Si is an element necessary for deoxidation, and in order to obtain a sufficient deoxidation effect, it is necessary to contain 0.05% or more. However, if the content exceeds 1.00%, the toughness of the base material is impaired. Therefore, in the present invention, it is 0.05 to 1.00%. A preferable range is 0.10 to 0.50%.

Mn: 0.20 to 2.50%
Mn is an element necessary for increasing the steel strength, and a content of 0.20% or more is necessary to obtain this effect. However, if the content exceeds 2.50%, the toughness decreases. Therefore, in the present invention, it is 0.20 to 2.50%. A preferred range is 0.40 to 1.5%.

P: 0.05% or less
P is an impurity element contained in the steel, and it is better that it is less. When the content exceeds 0.05%, weldability is remarkably lowered. Therefore, in the present invention, it is set to 0.05% or less, but it is preferable that the content be as small as possible.

S: 0.02% or less
S is an impurity element contained in the steel, and it is better that it is less. When the content exceeds 0.02%, the amount of MnS inclusions that become the starting point of corrosion increases, and the corrosion resistance is lowered. Therefore, in the present invention, it is set to 0.02% or less, but it is preferable that the content be as small as possible.

Al: 0.003-0.1%
Al is an element necessary for deoxidation and is unavoidably present in steel. If the content is 0.003% or more, the weather resistance is improved, but if it exceeds 0.1%, the steel tends to become brittle. Therefore, in the present invention, it is made 0.003 to 0.1%. A preferred range is 0.005 to 0.06%.

  Sn, Cu, and Ni are “noble” metals that are less susceptible to oxidation than Fe, and are elements added to improve corrosion resistance. In addition, these three elements are concentrated in the high-temperature oxidizing atmosphere of the steel manufacturing process with the selective oxidation of Fe as the parent phase, and when the low melting point liquid phase is precipitated from the surface concentrated composition, red hot embrittlement occurs. Cause. Therefore, these composition ratios are extremely important in determining steel characteristics such as corrosion resistance and high temperature embrittlement of steel, and are defined as follows in the present invention. In the steel targeted by the present invention, Sn is a main additive element.

Sn: 0.05 to 0.50%
Sn is a main element that determines the corrosion resistance. If its content is less than 0.05%, the desired corrosion resistance cannot be obtained. In the present invention, since Sn is a main additive element for obtaining corrosion resistance, 0.05% or more of Sn is added. However, even if it exceeds 0.50%, the corrosion resistance is saturated, and excessive addition reduces the toughness of the steel. Therefore, in the present invention, it is 0.05 to 0.50%. A preferable range is 0.08 to 0.30%.

Cu: 0.01-0.20%
Although Cu is a component that improves corrosion resistance, it is not limited to scrap iron, which is a raw material for steel, but at least 0.01% of Cu is unavoidably contained in the blast furnace hot metal and is difficult to remove by ordinary refining. It is. On the other hand, considering only the improvement in corrosion resistance, the higher the Cu content, the better.However, if Cu is excessively present in the steel, the so-called Cu is produced during the steel production process, that is, during continuous casting and hot rolling. Surface flaws occur due to red heat embrittlement and become a problem. Moreover, when Sn is used as a main additive element for improving corrosion resistance, the effect of further improving the corrosion resistance by adding Cu is small. Therefore, in the present invention, the upper limit is set to 0.20%.

Moreover, red heat embrittlement is promoted in the presence of Cu and Sn. In contrast to high Cu, a small amount of Sn shows significant embrittlement. Therefore, in order to control embrittlement by controlling the concentrated phase during selective oxidation of Fe to the L ₁ (Fe, Sn) liquid phase composition, the Cu / Sn ratio is set to 0.5 or less in the present invention. The lower limit of Cu / Sn is 0.02 because Cu may be the lower limit (0.01%) and Sn may be the upper limit (0.50%).

Ni: 0.01-0.20%
Ni has an effect of improving the corrosion resistance, and is inevitably contained in the steel by 0.01% or more. On the other hand, considering only the improvement in corrosion resistance, the higher the Ni content, the better. However, in the case of the present invention in which Sn is a main additive for improving corrosion resistance, the effect of further improving the corrosion resistance is small. In addition, Ni has an effect of suppressing red heat embrittlement, but the effect of suppressing embrittlement by Ni becomes small in the presence of Sn and Cu. Furthermore, Ni is an expensive alloy element, and the addition of a large amount of Ni causes an increase in cost. Therefore, in the present invention, the upper limit of the Ni content is set to 0.20%.

  Further, when Ni and Sn coexist, if the amount of Ni increases, as shown in FIG. 2, the intermetallic compound Ni3Sn2, which becomes the starting point of embrittlement, is not preferable as shown in FIG. Therefore, in the present invention, the (Cu + Ni) / Sn ratio is set to 0.7 or less in order to prevent precipitation of Ni—Sn intermetallic compounds. The lower limit of (Cu + Ni) / Sn is 0.04 because Cu and Ni may be the lower limit (both 0.01%) and Sn may be the upper limit (0.50%).

  In order to prevent vertical cracks in this composition system, for example, continuous casting may be performed under the following conditions.

(Casting conditions)
・ Superheat degree of molten steel: 50 ° C. or less ・ Casting speed: 0.7 to 1.2 m / min
-Specific water content per kg of slab weight for secondary cooling: 2.4 liters or less-Main physical properties of mold flux used: Viscosity 0.3-1.0 poise at solidification temperature 1210-1270 ° C, 1300 ° C, base Degree (CaO / SiO ₂ ) 1.5-2.0

Hereafter, the implementation result performed in order to confirm the effect of this invention is demonstrated.
Example 1
The test was conducted by a vertical continuous casting machine using 2.5 ton molten steel. Molten steel having the composition conditions shown in Table 1 below is melted in a melting furnace, and molten steel having a superheat of 40 to 60 ° C. poured into a tundish through a ladle is supplied to an internal water-cooled copper plate mold that vibrates from an immersion nozzle. Continuous casting was performed at a casting speed of 0.8 m / min. The mold flux used has a solidification temperature of 1235 ° C., a viscosity at 1300 ° C. of 0.04 Pa · s, and a basicity (CaO / SiO ₂ ) of 1.8. Below the mold, spray cooling was performed at a specific water volume of 2.0 liters per kg of slab weight to obtain a slab slab having a thickness of 100 mm × width of 800 mm × length of 3500 mm.

  After cooling the slab to room temperature, a part of the slab is cut out to investigate the presence of cracks on the slab surface and to investigate the composition of the concentrated layer on the slab surface, and to collect a sample for the hot rolling test did. The hot rolling test was performed at 1100 ° C. and then under the condition of a rolling reduction of 75%, and the presence or absence of occurrence of steel surface cracks was investigated to evaluate the hot workability.

  In addition, the confirmation method of the surface crack was performed by die-check (dye penetrant inspection) for the presence or absence of grain boundary cracks in the cast slab and the steel slab after hot rolling for the presence or absence of slab vertical cracks. The results are shown in Table 2.

  No. of the example of the present invention shown in Table 1. In Tables 1 to 6, as shown in Table 2, there were no vertical cracks and intergranular cracks in the slab, and there were no cracks during the hot rolling test. In addition, No. of the present invention example. In the compositions of 1 to 6, the target corrosion resistance was sufficiently obtained.

  On the other hand, No. of the comparative example shown in Table 1. In 7-10, as shown in Table 2, all the slab grain boundary cracks occurred. In particular, no. In No. 7, longitudinal cracks peculiar to hypoperitectic steel also occurred, and the crack sensitivity was promoted when Sn coexisted in a high Cu content steel of 0.3% or more. In these comparative examples, surface flaws were also generated even after the hot rolling test was conducted after cleaning the wrinkles.

  Moreover, No. of the comparative example. In Nos. 11 and 12, although there were no vertical cracks and grain boundary cracks during casting, no. In No. 11, since the Sn amount is excessive, the toughness of the steel material deteriorates. No. 12 did not contain Sn, so sufficient corrosion resistance could not be obtained.

(Example 2)
After 250 tons of molten steel decarburized and scoured in a converter was taken out into a ladle, vacuum scouring was performed, and 525 kg of Sn was added at the end of the ladle to obtain a molten steel composition as shown in Table 3 below.

  The ladle molten steel is supplied to the tundish, and the molten steel temperature is maintained in the range of 1540 to 1550 ° C., and at a casting speed of 0.9 m / min with a vertical bending type continuous casting machine, the thickness is 250 mm × width is 2300 mm. Cast into a slab shape with a rectangular cross section.

  Secondary cooling was performed with a specific water amount of 1.2 liters per kg of slab weight. At this time, there was no slab surface cracking, and the hot-rolled base metal was made without maintenance. After reheating to 1100 ° C., hot rolling was performed to obtain a product with a thickness of 20 mm. However, there was no surface flaw and no problem at all.

(Example 3)
After removing 300 tons of molten steel decarburized and smelted in a converter into a ladle, vacuum scouring was performed, and 330 kg of Sn was added at the final stage to obtain a molten steel composition as shown in Table 4 below.

  The ladle molten steel is supplied to the tundish, and the molten steel temperature is maintained at a superheat degree of 35 ° C. or less in the range of 1540 to 1550 ° C. It was cast into a slab shape having a rectangular cross section of 250 mm in width and 2300 mm in width.

  Secondary cooling was performed with a specific water volume of 1.3 liters per kg of slab weight. At this time, there was no slab surface cracking, and the hot-rolled base metal was made without maintenance. After reheating to 1100 ° C., hot rolling was performed, but there was no surface flaws and no problem at all.

The mold flux used in Example 2 and Example 3 had a solidification temperature of 1250 ° C., a viscosity at 1300 ° C. of 0.07 Pa · s, and a basicity (CaO / SiO ₂ ) of 1.7.

  Needless to say, the present invention is not limited to the above-described examples, and the embodiments may be appropriately changed within the scope of the technical idea described in the claims.

Claims (2)

In mass%, C: 0.09 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.50%, P: 0.05% or less, S: 0.02 %: Al: 0.003 to 0.1%, Sn: 0.05 to 0.50%, Cu: 0.01 to 0.20%, Ni: 0.01 to 0.20%, Low alloy steel for corrosion resistant thick plates with the balance being Fe and impurities,
Corrosion resistance characterized by casting at a component ratio of mass%, Cu / Sn values of 0.02 or more and 0.5 or less, and (Cu + Ni) / Sn values of 0.04 or more and 0.7 or less. Continuous casting method of low alloy steel for thick plates.   At least 30% by mass or more of the composition different from Fe, which is the parent phase, in which Sn, Cu, and Ni in the surface part of the slab are concentrated, formed by oxidation in the air at high temperatures during or immediately after continuous casting. The continuous cast slab cast by using the continuous casting method according to claim 1, wherein Fe is contained.

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