JP2015113486A - Continuously cast b-containing steel cast metal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuously cast B-containing steel cast metal in which the occurrence of surface cracks is suppressed.SOLUTION: The continuously cast B-containing steel cast metal contains C: 0.050-0.180%, Si: 0.10-0.40%, Mn: 0.5-2.0%, P: 0.020% or less, S: 0.0030% or less, Ti: 0.005-0.030%, Sol. Al: 0.005-0.060%, B: 0.0005-0.0050%, N: 0.0015-0.0070%, and rare earth metals: 0.001-0.050%. The slab further contains at least one of Cu, Cr, Ni, Mo, V, Nb, and Ca in amounts of Cu: 0.1-0.5%, Cr: 0.2-2.0%, Ni: 0.3-2.5%, Mo: 0.1-0.8%, V: 0.01-0.10%, Nb: 0.005-0.050%, and Ca: 0.0005-0.0060%, and the balance comprising Fe and impurities, and contents of Ti, rare earth metals, B, and N satisfy a relationship: ([%Ti]+[%REM])/([%B]+[%N])≥2.5.

Description

  The present invention relates to a continuous cast slab of B (boron) -containing steel produced using a vertical bending type or curved type continuous casting machine.

  When steel slabs are produced using a vertical bend type or curved type continuous caster, the slabs are subjected to tensile strain as the bends and subsequent bends are corrected. Due to this distortion, cracks may occur on the surface of the slab along the prior austenite grain boundaries. One of the steel types in which such cracks are particularly likely to occur is steel containing B.

  N is inevitably present in the steel, and since B has a very high affinity with N, BN tends to precipitate thermodynamically. Further, B and N are easily diffused in the steel and are easily segregated at the grain boundaries. Therefore, BN tends to precipitate on the grain boundary. Since B-containing steel tends to be embrittled starting from this precipitated BN, it is inferior in high temperature ductility compared to other steel types.

  On the other hand, B in steel has the characteristic of lowering the transformation point of steel and is an element effective for enhancing the hardenability of grain boundaries. Therefore, the B-containing steel can increase the strength of the steel material by controlling the structure of the steel material, and is often used for a high-strength thick steel plate whose demand is increasing. B is an element that is very useful for improving the characteristics of the heat-affected zone of steel.

  Thus, when manufacturing a high strength thick steel plate, B is one of the essential additive elements.

  However, since the B-containing steel is liable to cause surface cracks in the slab and surface flaws of the steel sheet due to the surface cracks as described above, the frequency of surface care tends to increase, and the yield loss tends to increase. Also, due to these, productivity decreases and production costs increase.

  Therefore, in the case of continuous casting of B-containing steel having excellent properties in terms of strength and heat-affected zone for thick steel plates, prevention of surface cracks in the slab is attempted.

  For example, in Patent Document 1, as a method for suppressing cracking in a continuous casting slab containing B, the content ratios of B and N in steel are set so as to satisfy a predetermined relationship with respect to the content ratios of Ti, Zr, and Hf. A method of controlling has been proposed.

  In Patent Document 2, as a method for preventing slab surface cracking when steel containing B and N is continuously cast by a vertical bending type continuous casting machine, an area where the slab is subjected to bending stress and correction in the continuous casting machine A method has been proposed in which the surface temperature of a slab in a region subjected to stress is set to a predetermined temperature range.

  In Patent Document 3, as a method for reducing the occurrence of surface cracks and wrinkles during continuous casting or hot rolling for a continuous cast slab of B-containing steel for a high-strength thick steel plate, the equilibrium precipitation amount of BN is predetermined. A method of making it less than the value is proposed. In the same document, the range of the content ratios of B, N and Ti in the steel is defined.

JP-A-56-80354 Japanese Patent No. 4561755 JP 2010-189712 A

  Patent Documents 1 to 3 propose methods for suppressing surface cracking of a continuous cast slab of B-containing steel. However, even with the methods described in these documents, it is still insufficient to suppress the surface cracking.

  The present invention has been made in view of such a situation, and continuous casting of B-containing steel in which the occurrence of surface cracks is suppressed when manufactured using a vertical bending type or curved type continuous casting machine. The object is to provide a slab.

  As described above, when a slab of B-containing steel is produced using a vertical or curved continuous casting machine, the slab is subjected to tensile strain as the bending and subsequent bending are corrected. Due to this distortion, surface cracks of the slab such as grain boundary cracks may occur.

  It is generally known that this surface crack is likely to occur particularly when the slab surface temperature when tensile strain acts is in the embrittlement temperature region (600 to 900 ° C.) of the third region. This temperature range corresponds to a temperature range in which the steel transforms from austenite to ferrite, and since ferrite forms in a film shape along the austenite grain boundary, embrittlement of the steel occurs.

  Moreover, even when the surface temperature of the slab when tensile strain acts is not lowered to a temperature at which it transforms into ferrite, nitrides and carbides such as AlN and NbC are formed and precipitated in the steel composition. When it is an easy component system, embrittlement due to these precipitates tends to occur.

  In the case of B-containing steel, B in the steel is likely to segregate at the grain boundaries, so that BN that becomes the starting point of embrittlement is easily generated along the austenite grain boundaries. Furthermore, since BN begins to be generated from a temperature range higher than that of the region III, surface cracking occurs in the slab even when bending or correcting the slab is performed in a region higher than the region III. It's easy to do. From the above, it is known that B-containing steel is originally a hardly castable steel type.

  Since the occurrence of surface cracks occurring in the continuous cast slab of B-containing steel is greatly affected by BN precipitated at the austenite grain boundaries, the present inventors have adopted embrittlement as a method for suppressing the occurrence of surface cracks. Focusing on the suppression of the precipitation of BN on the grain boundary that is the starting point of the above, preliminary experiments and the like to be described later were conducted.

  As a result, if the composition of the steel is controlled within a predetermined range, the precipitation of BN on the grain boundary where embrittlement starts will be suppressed even if BN is thermodynamically precipitated in the steel. And the occurrence of surface cracks in the slab can be suppressed.

  This invention is made | formed based on this knowledge, The summary exists in the continuous cast slab of the following B containing steel.

In mass%, C: 0.050 to 0.180%, Si: 0.10 to 0.40%, Mn: 0.5 to 2.0%, P: 0.020% or less, S: 0.0030 % Or less, Ti: 0.005 to 0.030%, Sol. Al: 0.005-0.060%, B: 0.0005-0.0050%, N: 0.0015-0.0070%, and rare earth elements: 0.001-0.050%, Further, at least one of Cu, Cr, Ni, Mo, V, Nb and Ca is Cu: 0.1 to 0.5%, Cr: 0.2 to 2.0%, Ni: 0.3 to 2 0.5%, Mo: 0.1-0.8%, V: 0.01-0.10%, Nb: 0.005-0.050% and Ca: 0.0005-0.0060% A continuous cast slab of B-containing steel, wherein the balance is made of Fe and impurities, and the contents of Ti, rare earth elements, B and N in the steel satisfy the relationship of the following formula (a).
([% Ti] + [% REM]) / ([% B] + [% N]) ≧ 2.5 (a)
Here, [% Ti], [% REM], [% B], and [% N] mean the contents (mass%) of Ti, rare earth elements, B, and N in the steel, respectively.

The continuous casting slab of the B-containing steel of the present invention is obtained from Sol. It is preferable that the content of Al and the rare earth element satisfies the relationship of the following formula (b), and the content of the rare earth element in the oxide inclusions present in the steel is 45% by mass or more.
[% Sol. Al] / [% REM] ≦ 7 (b)
Here, [% Sol. [Al] and [% REM] are obtained from Sol. It is the content rate (mass%) of Al and rare earth elements.

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

  When the continuous cast slab of B-containing steel of the present invention is produced using a vertical bending type or curved type continuous casting machine, the occurrence of surface cracks is suppressed. Furthermore, it has high hot ductility and has excellent strength and surface properties as a material for thick steel plates. In addition, since the occurrence of surface cracks is suppressed, the surface cracks can be removed by unnecessary or light care, and therefore, it is possible to produce with high production efficiency. The steel plate obtained by using the continuous cast slab of the B-containing steel of the present invention as a raw material and hot rolling it has excellent strength and surface properties.

It is a schematic diagram of BN observed in the grain of steel B. It is a figure which shows the relationship between the ([% Ti] + [% REM]) and ([% B] + [% N]) of a slab, and a hull index.

  Below, the content of the examination performed in order to complete the continuous casting slab of the B containing steel of this invention is demonstrated first, and the reason which prescribed | regulated the component composition of steel as mentioned above is demonstrated.

1. Contents of study for completing the present invention 1-1. First Preliminary Experiment In the first preliminary experiment, the form of BN present in the B-containing steel solidified after melting was examined.

1-1-1. Experimental Method B-containing steel having the component composition shown in Table 1 was used as a sample.

  The difference between steel A and steel B is that steel A does not contain Nd, which is a rare earth element, whereas steel B contains Nd. 80 g of each of Steel A and Steel B was heated to 30 ° C. higher than the liquidus temperature and melted in a crucible. The crucible was made of MgO having an inner diameter of 20 mm and a depth of 50 mm. The melted sample was cooled and solidified at a cooling rate of 5 ° C./min, held at 950 ° C. for 2 min, and then rapidly cooled.

  A cylindrical sample obtained by quenching was cut in the longitudinal direction. The longitudinal section of the sample was corroded with a nital solution to reveal austenite grain boundaries. The surface on which the austenite grain boundary appeared was observed with a scanning electron microscope, and the presence or absence of BN and the form of BN that existed were investigated.

1-1-2. Experimental Results As a result of observation with an electron microscope, the austenite grain size was about 0.5 to 2.0 mm for both Steel A and Steel B, and no significant difference was observed between the two. However, the precipitation form of BN was greatly different between Steel A and Steel B.

  In the steel A, BN having a size of several hundred nm was precipitated in a row on the austenite grain boundary. On the other hand, in the steel B, BN precipitated in a row on the austenite grain boundary was hardly observed, and BN was mainly precipitated in the grains.

FIG. 1 is a schematic view of BN observed in the grain of steel B. FIG. As shown in the figure, BN observed in the steel B is precipitated together with oxides and oxysulfides (hereinafter referred to as “oxide inclusions”), and the oxide inclusions. The Nd content was 45% by mass or more. Specifically, the oxide inclusions is made 87 mass% of Nd ₂ O ₃ and 13 wt% of Al ₂ O _3, Nd content was 74.6 wt%.
1-2. Second Preliminary Experiment In the second preliminary experiment, high temperature ductility of B-containing steel was examined.

1-2-1. Experimental Method As a sample, a B-containing steel round bar having the composition of steel A and steel B shown in Table 1 was used. The round bar was a forged material having a diameter of 10 mm and a length of 190 mm, and had no as-cast structure. While maintaining the shape of this round bar, a portion having a length of about 30 mm at the center in the length direction was melted. Thereafter, from the molten state to 1200 ° C., it was cooled and solidified at a cooling rate of 10 ° C./s, and continuously cooled at a cooling rate of 0.4 ° C./s in a temperature range of 1200 ° C. or lower.

A tensile test was performed in the cooling process of the sample to break the sample. In that case, the tensile test changed variously the temperature of the center part of a sample in the range of 600-1000 degreeC. The strain rate in the tensile test was 3 × 10 ⁻⁴ s ⁻¹ . This strain rate is approximately the same as the strain rate acting on the slab and the number of digits when the slab is bent and straightened in a vertical bend type or curved type continuous casting machine.

1-2-2. Evaluation method Evaluation was performed on the high temperature ductility of B-containing steel. The high temperature ductility was defined as the reduction rate of the cross-sectional area of the sample before and after breakage by the tensile test defined by the following formula (1).
RA = (A ₀ −A _f ) / A ₀ × 100 (1)
Here, RA is the cross-sectional area reduction rate (%), A _{0 is} the cross-sectional area (m ² ) of the sample before the tensile test, and A _{f is} the cross-sectional area (m ² ) of the fracture portion of the sample.

  Due to the relationship between the amount of tensile strain applied to the surface of the slab and the occurrence of surface cracks in the slab in a vertical bending type or curved type continuous casting machine, the above-mentioned steel with a cross-sectional area reduction rate of 60% or more is cast during continuous casting. It has been found that the surface cracks of the pieces do not occur.

  In addition to the evaluation of the high temperature ductility, the fracture surface observation and the structure observation of the sample after the fracture were performed using a scanning electron microscope. Microscopic observation was performed by cutting the fractured sample in a direction parallel to the tensile direction and corroding the cut surface with a nital solution.

1-2-3. Evaluation of Steel A In Steel A, a decrease in hot ductility was confirmed when the sample temperature was 700 to 950 ° C. when the tensile test was performed. Specifically, the cross-sectional area reduction rate was 20 to 55%, which was a value that could cause surface cracks in the slab during continuous casting.

  When the fracture surface of the sample after fracture was observed, the shape of the fracture surface was a typical austenite grain boundary crack.

  When the structure of the sample after the fracture was subjected to a tensile test at 900 ° C., precipitation of BN having a size of several hundred nm was confirmed along the austenite grain boundary. In addition, voids (voids starting from cracks in steel) starting from BN at the austenite grain boundaries were also found. This void is presumed to be caused by the effect of tensile strain.

1-2-4. Evaluation of Steel B Steel B had a high-temperature ductility higher than that of Steel A, with a sample area of 600 to 1000 ° C. when the tensile test was performed and a cross-sectional area reduction rate of 60% or more. Moreover, the cross-sectional reduction rate was a value at which surface cracks of the slab did not occur during continuous casting.

  When the fracture surface of the sample after fracture was observed, the fracture surface was in the form of intragranular ductility.

  When the structure of the sample after the fracture was subjected to a tensile test at 900 ° C., BN was hardly precipitated on the austenite grain boundaries, but mainly precipitated in the grains. Further, the precipitated BN was deposited integrally with oxide inclusions containing Nd, which is a rare earth element. The oxide inclusions had an Nd content of 45% by mass or more.

1-3. Examination and consideration In addition to the results of the first and second preliminary experiments described above, based on the results of further examination, a method for suppressing the occurrence of surface cracks in the slab of B-containing steel during continuous casting will be discussed. went.

  As a result, BN precipitated at the austenite grain boundaries is a cause of embrittlement of steel, and it is important to control the precipitation form of BN during continuous casting in order to suppress the occurrence of surface cracks in the slab. Was confirmed.

  In addition, if the steel component composition satisfies the relationship of the above formula (a), BN is combined with oxide inclusions containing rare earth elements uniformly dispersed in the steel to form intragranular particles. It was found that precipitation of BN at the austenite grain boundary can be suppressed.

  The steel B used in the first and second preliminary experiments contained Nd as a rare earth element. However, in addition to Nd, La, Ce, or any other rare earth element can obtain the same effect. I also confirmed that. Here, the rare earth element means one or more metal elements selected from Sc, Y and lanthanoids (La, Ce, etc., 15 elements having atomic numbers 57 to 71).

Moreover, in order to enhance the effect of depositing BN in the grains integrally with oxide inclusions containing rare earth elements dispersed in the steel, the composition of the steel satisfies the relationship of the following formula (b): It was found that the rare earth element content in the oxide inclusions should be 45% by mass or more.
[% Sol. Al] / [% REM] ≦ 7 (b)
Here, [% Sol. [Al] and [% REM] are obtained from Sol. It is the content rate (mass%) of Al and rare earth elements.

2. Component composition of B-containing steel of the present invention and reasons for limitation The B-containing steel of the present invention was made based on the knowledge obtained as a result of the above examination, and the component composition was C: 0.050 0.180%, Si: 0.10-0.40%, Mn: 0.5-2.0%, P: 0.020% or less, S: 0.0030% or less, Ti: 0.005-0 .030%, Sol. Al: 0.005-0.060%, B: 0.0005-0.0050%, N: 0.0015-0.0070%, and rare earth elements: 0.001-0.050%, Furthermore, it contains one or more of Cu, Cr, Ni, Mo, V, Nb and Ca, and at least one of these elements is Cu: 0.1 to 0.5%, Cr: 0.2-2.0%, Ni: 0.3-2.5%, Mo: 0.1-0.8%, V: 0.01-0.10%, Nb: 0.005- 0.050% and Ca: 0.0005 to 0.0060% are satisfied, and the balance is composed of Fe and impurities, and the relationship of the above formula (a) is satisfied.

2-1. Essential element C: 0.050 to 0.180%
Carbon (C) is generally known as an element that greatly affects the strength of steel. When the C content is less than 0.050%, it is difficult to obtain a predetermined strength for applications such as high-strength thick steel plates. On the other hand, when the C content exceeds 0.180%, the hardness of the steel is remarkably increased and causes a new surface flaw, so that a special process is required for heat treatment of the steel. Further, when steel is welded, the welded portion and the heat-affected zone (HAZ) are hardened, so that the weldability required for the thick steel plate is impaired. For these reasons, the C content is set to 0.050 to 0.180%. The C content is preferably 0.060 to 0.180%.

Si: 0.10 to 0.40%
In general, silicon (Si) is one of elements effective for reducing the oxygen (O) content of steel as a deoxidizing element in the steel manufacturing process, and also has an effect of strengthening steel. If the Si content is less than 0.10%, the molten steel cannot be sufficiently deoxidized. If the continuous casting is performed in a state where the molten steel is not sufficiently deoxidized, bubbles are generated in the steel, resulting in product defects, and sometimes a breakout is induced and the operation becomes impossible. On the other hand, when the Si content exceeds 0.4%, striped martensite is generated, and there is a problem that the HAZ toughness is lowered during welding. Therefore, the Si content is set to 0.10 to 0.40%. The Si content is preferably 0.10% or more and less than 0.30%.

Mn: 0.5 to 2.0%
Manganese (Mn) is generally known as an element that greatly affects the strength of steel. When the Mn content is less than 0.5%, it is difficult to obtain sufficient strength as a high-strength thick steel plate. On the other hand, if the Mn content exceeds 2.0%, the strength of the steel is remarkably increased due to solid solution strengthening, and it becomes difficult to adjust the strength of the product. Further, since Mn is concentrated at the center segregation part, the higher the content ratio, the more the unevenness in strength that occurs in the slab and in the thick steel plate after rolling the slab. Therefore, the Mn content is set to 0.5 to 2.0%. The Mn content is preferably 0.70 to 1.80%.

P: 0.020% or less Phosphorus (P) is one of impurity elements inevitably contained in steel, and the content is preferably low. Since P has a small equilibrium partition coefficient at the solid-liquid interface of steel, it segregates significantly during solidification of the molten steel. For this reason, when P content rate is high, we are anxious about having a bad influence on various product characteristics. Further, since the melting point of the portion where P is concentrated due to segregation is remarkably lowered, the concentrated portion may melt during hot rolling of the slab and cause wrinkling of the product. Therefore, the P content is set to 0.020% or less. The P content is preferably less than 0.01% in order to prevent occurrence of various problems in the portion where P is segregated. Moreover, it is preferable that P content rate shall be 0.005% or more as a range which can be manufactured with a normal industrial refining method.

S: 0.0030% or less Sulfur (S), like P, is one of impurity elements inevitably contained in steel, and the content is preferably as low as possible. S, like P, also has a small equilibrium partition coefficient at the solid-liquid interface of the steel, so that it segregates remarkably during solidification of the molten steel, and the melting point of the portion where S is concentrated by segregation decreases. The portion where S is concentrated becomes a cause of surface flaws, particularly when the slab is rolled. Therefore, the S content is set to 0.0030% or less. The S content is preferably 0.0020% or less. In a strict requirement level, such as when higher strength is required, the S content is preferably 0.0020% or less. As a range that can be produced by a normal industrial refining method, the S content is preferably 0.0002% or more.

Ti: 0.005-0.030%
Titanium (Ti) is an element that generally improves the strength of steel. Moreover, since N in steel is fixed as TiN, the production of BN is also affected by including Ti in the steel. That is, since the amount of BN produced is reduced by containing Ti in the steel, the effect of suppressing the occurrence of surface cracks in the slab at the time of bending and correcting the slab in a continuous casting machine is obtained. It is done. This effect of suppressing the occurrence of surface cracks cannot be obtained when the Ti content is less than 0.005%. On the other hand, if the Ti content exceeds 0.030%, a large number of Ti carbides are generated, and there is a problem that the HAZ toughness is lowered during welding, and this also causes coarse TiN to be generated. Therefore, the Ti content is set to 0.005 to 0.030%. Moreover, when the production amount of TiN is excessive, the surface properties of the slab are lowered. From the viewpoint of stably suppressing both the occurrence of surface cracks in the slab and the deterioration of the surface properties of the slab, the Ti content is preferably 0.010 to 0.020%.

Sol. Al: 0.005-0.060%
Similar to Si, aluminum (Al) is one of the elements effective for reducing the O content of steel as a deoxidizing element. If the Al content is less than 0.005%, the molten steel cannot be sufficiently deoxidized. It also becomes difficult to sufficiently desulfurize in the smelting process. On the other hand, if the Al content is excessive, AlN is likely to be generated, which causes a surface crack of the slab. Therefore, the Al content is set to 0.005 to 0.060%. The Al content is preferably 0.005 to 0.050%. The Al content referred to in this specification is Sol. It means the content of Al (acid-soluble Al).

B: 0.0005 to 0.0050%
Boron (B) has the effect of increasing the hardenability of the grain boundaries, the effect of controlling the structure of the steel material, and the effect of increasing the strength of the steel material. B exhibits a high effect when added in a small amount. However, if the B content is less than 0.0005%, a high strength of 700 to 1200 MPa cannot be obtained in terms of tensile strength. On the other hand, when the B content exceeds 0.0050%, the effect is saturated and the toughness of the steel is lowered. Therefore, the B content is set to 0.0005 to 0.0050%. From the viewpoint of controlling the microstructure of the thick steel plate and clearly expressing the effect of adding B, the B content is preferably 0.0010 to 0.0040%.

N: 0.0015 to 0.0070%
Nitrogen (N) is an element that inevitably penetrates into steel when the steel is melted in an air atmosphere using a converter or the like. N forms a nitride with Ti, B, etc. in steel. Since these nitrides have an effect of refining crystal grains as pinning particles in the process of hot working of a slab, they affect the mechanical properties of steel materials. However, if the N content is less than 0.0015%, the effect of refining crystal grains cannot be obtained. On the other hand, since these nitrides dynamically precipitate at austenite grain boundaries during continuous casting, it causes surface cracks in the slab when the N content is excessive, specifically when it exceeds 0.0070%. . For this reason, N content rate shall be 0.0015 to 0.0070%. From the standpoint of reliably exhibiting the pinning effect of the structure and preventing the deterioration of the toughness of the steel due to the formation of coarse carbides and nitrides in the center of the slab, the N content is 0.002 to 0.00. 004% is preferred.

REM: 0.001 to 0.050%
Rare earth elements (REM) are elements that control the form of oxide inclusions in steel. The oxide inclusions containing the generated REM bind to BN. Therefore, precipitation of BN on the austenite grain boundary is suppressed by precipitating oxide inclusions containing REM in the steel. However, if the REM content is less than 0.001%, the effect of suppressing the precipitation of BN cannot be obtained. On the other hand, REM is an expensive element, and if added excessively, its effect is saturated. Therefore, if it exceeds 0.050%, the effect obtained compared to the cost becomes small. Therefore, the REM content is set to 0.001 to 0.050%. The REM content is preferably 0.001 to 0.035%.

  Here, REM means one or more elements among scandium (Sc), yttrium (Y), and lanthanoids (15 elements from La to Lu (atomic number 57 to 71)) belonging to Group 3 of the periodic table. Means. Among these elements, one or more of Ce, La, Pr and Nd are particularly preferable.

1-2. Selective Essential Elements It is difficult to achieve a high strength of 700 MPa or higher in tensile strength and to develop other characteristics such as weldability and weather resistance only by adjusting the component composition of the above essential elements. In order to obtain these characteristics, it is necessary to contain one or more of Cu, Cr, Ni, Mo, V, Nb, and Ca at the following contents.

Cu: 0.1 to 0.5%
Copper (Cu) is an element that improves the hardenability of steel. However, if the Cu content is less than 0.1%, the effect of improving the hardenability cannot be obtained. On the other hand, when the Cu content exceeds 0.5%, not only the effect of improving the hardenability becomes excessive, but also the hot workability of the steel material decreases. Therefore, when Cu is contained, the content is set to 0.1 to 0.5%. The Cu content is preferably 0.2 to 0.5%.

  Cu is also an element that induces surface cracks of a slab called a star crack during continuous casting of steel. For this reason, when making Cu content rate into 0.2% or more, it is preferable to add Ni mentioned later so that it may become 1/3 or more content rate of Cu content.

Cr: 0.2 to 2.0%
Chromium (Cr) is an element having an effect of increasing the strength and toughness of steel. Moreover, it is a substantially essential element when high strength is required for steel, such as steel of 80 kg class or more. However, if the Cr content is less than 0.2%, the effect of increasing the strength and toughness of the steel cannot be obtained. On the other hand, when the Cr content is 2.0% or more, problems such as weld cracks occur. Therefore, when Cr is contained, the content is made 0.2 to 2.0%. When emphasizing the weldability of steel, the Cr content is preferably 0.2 to 1.5%.

Ni: 0.3-2.5%
Nickel (Ni) is an element having an effect of improving the strength of steel by solid solution strengthening, and also has an effect of improving toughness. If the Ni content is less than 0.3%, these effects cannot be obtained. On the other hand, when the Ni content exceeds 2.5%, the effect of improving the strength and toughness is not only saturated, but also has an adverse effect of deteriorating the weldability. Therefore, when Ni is contained, the content is set to 0.3 to 2.5%. The Ni content is preferably 0.4 to 2.5%.

Mo: 0.1 to 0.8%
Molybdenum (Mo) is an element that improves the hardenability of the steel sheet and contributes to the improvement of strength. Mo, like Cr, is a substantially essential element when high strength is required for steel, such as steel of 80 kg class or higher. However, when the Mo content is 0.1%, these effects cannot be obtained. On the other hand, Mo is an expensive element, and increasing the amount of addition not only leads to an increase in manufacturing cost, but if it is contained at 0.8% or more, a hardened phase such as bainite and martensite phase is generated, Deteriorates workability and weldability. Therefore, when Mo is contained, the content is set to 0.1 to 0.8%. The Mo content is preferably 0.1 to 0.7%.

V: 0.01-0.10%
Vanadium (V) is an element effective for improving the strength of steel because it dissolves in the ferrite phase of steel and forms carbonitrides in the steel. However, if the V content is less than 0.01%, the effect of improving the strength of the steel cannot be obtained. On the other hand, if the V content exceeds 0.10%, the state of carbonitride precipitation in the HAZ changes during welding, which adversely affects toughness. Further, when the V content is excessive, specifically, when it exceeds 0.10%, it precipitates as VN inside the slab and causes surface cracks of the slab. Therefore, when V is contained, the content is set to 0.01 to 0.10%. The V content is preferably 0.03 to 0.10%.

Nb: 0.005 to 0.050%
Niobium (Nb) is an element that forms carbonitrides in steel and is effective in improving the strength and toughness of steel. Nb is used to control the microstructure of the steel sheet by controlling solid solution and precipitation in the steel in TMCP (Thermo-Mechanical Control Process). However, if the Nb content is less than 0.005%, the effect of improving the strength and toughness of the steel and the effect of controlling the structure cannot be obtained. On the other hand, if the Nb content is 0.050% or more, even if the slab is heated, Nb cannot be dissolved, and the structure cannot be controlled. In addition, when the Nb content is excessive, specifically, when it is 0.050% or more, NbC precipitates inside the slab and causes surface cracks in the slab. Therefore, when Nb is contained, the content is made 0.005 to 0.050%. The Nb content is preferably 0.005 to 0.040%.

Ca: 0.0005 to 0.0060%
Unlike other component elements, calcium (Ca) is an element that does not significantly affect the material properties of steel. On the other hand, it is an element having an effect of suppressing clogging of a nozzle that injects molten steel from a tundish into a mold during continuous casting. Therefore, it may be added to molten steel for the purpose of suppressing nozzle blockage. Moreover, when Ca is added to steel, the S content in steel is reduced and the effect which suppresses the production | generation of MnS is also acquired. Therefore, it may be added to molten steel for the purpose of controlling the form of sulfide in steel. However, if the Ca content is less than 0.0005%, these effects cannot be obtained. On the other hand, if the Ca content exceeds 0.0060%, the effect is saturated and not only increases the manufacturing cost but also promotes blocking of the nozzle. Therefore, when Ca is contained, the content is set to 0.0005 to 0.0060%. The Ca content is preferably 0.0010 to 0.0050%.

  The balance other than the above essential elements and selective essential elements is Fe and impurities. Here, the “impurities” are those mixed from ore and scrap as raw materials in the industrial production of steel materials, components eluted from production equipment, and the like, and in a range that does not adversely affect the performance of the steel.

2-3. Relationship between content ratios of Ti, REM, B and N in steel In the B-containing steel of the present invention, the content ratios of Ti, REM, B and N in the steel satisfy the relationship of the following formula (a). .
([% Ti] + [% REM]) / ([% B] + [% N]) ≧ 2.5 (a)
Here, [% Ti], [% REM], [% B], and [% N] are the contents (mass%) of Ti, REM, B, and N in the steel, respectively.

  As the B content and N content in the steel are higher, BN is more likely to be generated in the slab during continuous casting and the amount of generation is increased, so that the resulting slab is more likely to be embrittled. However, TiN is generated prior to precipitation of BN from a high temperature range of 1300 to 1400 ° C. Therefore, if the Ti content in the steel is high, the amount of BN generated can be reduced.

  Moreover, REM produces | generates an oxide and an oxysulfide (oxide inclusion) in steel, and disperse | distributes in steel. Since BN produced as the temperature of the steel decreases, it precipitates together with the REM oxide inclusions dispersed in the steel, so that precipitation of BN on the austenite grain boundaries can be suppressed.

  When the content of Ti, REM, B and N in the steel satisfies the relationship of the above formula (a), the amount of BN produced can be reduced, and precipitation of BN on the austenite grain boundaries can be suppressed. The embrittlement of the contained steel can be sufficiently suppressed. Therefore, even when the B-containing steel of the present invention is produced using a vertical bending type or a curved type continuous casting machine, occurrence of surface cracks is suppressed.

2-4. Sol. Relationship between Al and REM content ratios and definition of REM oxide-based inclusions in the slab In the B-containing steel of the present invention, Sol. It is preferable that the content of Al and REM satisfy the relationship of the following formula (b), and the REM content in the oxide inclusions present in the steel is 45% by mass or more.
[% Sol. Al] / [% REM] ≦ 7 (b)
Here, [% Sol. [Al] and [% REM] are obtained from Sol. It is the content rate (mass%) of Al and rare earth elements.

Sol. When the content of Al and REM does not satisfy the relationship of the above formula (b), the oxide inclusions present in the steel are mainly composed of Al ₂ O ₃ because the amount of REM oxide produced is small. It is easy to become. Further, such oxide-based inclusions mainly composed of Al ₂ O ₃ tend to form coarse clusters. Furthermore, when the REM content in the oxide inclusions is less than 45% by mass, the formation of coarse clusters due to the inclusions becomes prominent and unevenly distributed in the steel. It is difficult to obtain a uniformly dispersed state in steel. Further, when the REM content in the oxide inclusions is low, it becomes difficult to deposit BN integrally with the oxide inclusions.

  On the other hand, Sol. When the content of Al and REM satisfies the relationship of the following formula (b) and the REM content in the oxide inclusions present in the steel is 45% by mass or more, the oxide inclusions are It is easy to obtain a uniformly dispersed state in steel, and it is easy to effectively precipitate BN in crystal grains by integrating BN with this oxide inclusion. Therefore, precipitation of BN on the austenite grain boundary can be further suppressed, high hot ductility can be obtained, and occurrence of surface cracks in the slab can be further suppressed.

  In order to confirm the effect of the B-containing steel of the present invention, the following two continuous casting tests were conducted and the results were evaluated.

1. First test 1-1. Test Method The molten steel was melted by 2.5 t using a high-frequency induction furnace having a capacity of 2.5 t and subjected to preliminary combined deoxidation with Si and Mn. The molten steel was transferred from the high-frequency induction furnace to the ladle from above. Metal la and Nd were charged in the ladle in advance, and molten steel was poured to dissolve Al and Nd in the molten steel, so that the component composition of the molten steel was finally adjusted to the target composition to be cast.

  As the continuous casting machine, a vertical bending type continuous casting machine having a vertical length of 1.3 m was used. Molten steel with adjusted component composition was poured from a ladle through a tundish into a continuous casting machine to continuously cast a slab having a thickness of 100 mm and a width of 500 mm. The casting speed was 0.70 to 1.20 m / min, and the specific water amount for secondary cooling was 0.7 to 1.6 L / kg-steel.

The component composition of the cast steel is shown in Table 2. In the same table, the value of the left side (([% Ti] + [% REM]) / ([% B] + [% N])) of the following formula (a) is also shown as “TR / BN”.
([% Ti] + [% REM]) / ([% B] + [% N]) ≧ 2.5 (a)
Here, [% Ti], [% REM], [% B] and [% N] are the contents (mass%) of Ti, rare earth elements, B and N in the steel, respectively.

  Inventive Examples 1 to 6 satisfied the provisions of the present invention. Comparative Examples 1 to 6 did not satisfy the relationship of the above formula (a) in the definition of the present invention.

  The obtained slab was pickled by removing the scale from the surface. Thereafter, for both the top side of the slab (the surface that was the inner peripheral surface side in the curved portion of the continuous casting machine) and the ground side of the slab (the surface that was the outer peripheral surface side in the curved portion of the continuous casting machine) The presence or absence of cracks in the slab and the occurrence of flaws were visually observed and confirmed by a dyeing penetrant flaw detection test (so-called color check method) defined in JIS Z2343.

1-2. Test result The evaluation item was the degree of surface cracking of the slab. The degree of surface cracking was evaluated by indexing into four grades of 0, 1, 2, and 3. Table 2 shows the soot index together with the steel component composition. The meaning of each index is as follows.

“0”: No flaws were observed on the surface of the slab, and the surface state of the slab was very sound.
“1”: The number of defects per unit length of the confirmed slab is as small as 10 pieces / m or less, and the identified defects are easy to lightly caret by turning the slab surface up to 3 mm with a grinder. However, it was at a level where there was no problem in practical use.
“2”: The number of ridges per unit length of the slab confirmed is 30 to 40 pieces / m, and the stagnation is found on the entire surface of the slab, and the confirmed defects are on the surface of the slab. It was a level that could not be removed completely by light care of turning 3 mm with a grinder.
“3”: A crack having a depth of 3 mm or more is remarkably observed in the corner portion of the slab, and in order to use the slab in the next process, both ends in the width direction of the slab must be cut by 30 mm or more. It was a level of drought accompanied by a significant drop in yield.

  As shown in Table 2, all of Examples 1 to 6 of the present invention had a haze index of 0 or 1, and the surface quality of the slab was at a level with no practical problem. In particular, Examples 3 and 6 of the present invention had a wrinkle index of 0, no flaws were observed on the surface of the slab, and the surface state of the slab was very sound.

  On the other hand, all of Comparative Examples 1 to 4 had a crack index of 2 or 3, and the surface quality of the cast slab was at an unacceptable level in practice. This is considered to be because the amount of BN produced in the steel was large and BN precipitated on the austenite grain boundaries because the relationship of the above formula (a) was not satisfied among the provisions of the present invention. Although the comparative example 4 did not satisfy | fill the relationship of the said (a) formula, although steel contained Nd as REM, it is thought that the surface crack generate | occur | produced.

  In Comparative Examples 1 and 3, the hull index is 3, and not only is the state in which wrinkles are scattered all over the slab, but also cracks at the corners of the slab are prominent, and both ends in the width direction of the slab are cut. Otherwise, it could not be used.

  FIG. 2 is a diagram showing the relationship between ([% Ti] + [% REM]), ([% B] + [% N]) and the heel index of the slab. As can be seen from the figure, the power index was 0 or 1 when the relationship of the above equation (a) was satisfied, and the power index was 2 or 3 when the relationship was not satisfied.

2. Second test 2-1. Test method The conditions were the same as those in the first test except that the molten steel used had the composition shown in Table 3. The table also shows the value on the left side of the following equation (b).
[% Sol. Al] / [% REM] ≦ 7 (b)
Here, [% Sol. [Al] and [% REM] are obtained from Sol. It is the content (mass%) of Al and REM (here Nd).

  Invention Examples 2a to 2e shown in Table 3 are all Sol. Since the content of components other than Al and Nd is the same as that of Invention Example 2 shown in Table 2 above, Sol. Only the contents of Al and Nd were shown. Inventive Examples 2a to 2e are inventive examples satisfying the relationship between the component composition defined in the present invention and the above formula (a).

  Of these, Examples 2a, 2b and 2c of the present invention are Sol. The contents of Al and REM satisfied the relationship represented by the following formula (b), and Invention Examples 2d and 2e were not satisfied.

  The obtained slab was visually observed for the presence or absence of cracks or flaws in the slab by the color check method as in the first test, and the degree thereof was confirmed.

  In the second test, the composition of each slab was also evaluated for the oxide inclusions in the steel. A sample was cut out from the surface of the center part in the width direction of the obtained slab in the thickness direction at a quarter of the thickness. The cut sample was observed with a scanning electron microscope, and 20 oxide inclusions (oxide or oxysulfide) were randomly extracted. The extracted oxide inclusions were analyzed by EDS (energy dispersive X-ray spectroscopy) for the component composition. The content of REM (here, Nd) of 20 oxide inclusions was defined as the REM content in the oxide inclusions in the slab.

2-2. Evaluation results The evaluation items were the degree of surface cracking of the slab and the Nd content in the oxide inclusions in the steel. Similar to the first test, the degree of surface cracking was evaluated by indexing into four grades of 0, 1, 2, and 3. Table 3 shows the N index content and the Nd content in the oxide inclusions in the steel, together with the component composition of the steel.

  As shown in Table 3, Examples 2d and 2e of the present invention satisfied the provisions of the present invention, the haze index was 1, and the surface quality of the slab was at a level that did not impede practical use. However, although it was mild, moths that require care were identified. This does not satisfy the relationship of the above formula (b), and the REM content in the oxide inclusions in the steel was less than 45% by mass, so that the precipitation of BN on the austenite grain boundaries was suppressed. However, it is thought that it was slightly precipitated.

  On the other hand, Examples 2a to 2c of the present invention all had surface properties at a level where no flaws were confirmed on the surface of the slab and no practical surface maintenance was required. This satisfies the relationship of the above formula (b) and the Nd content in the oxide inclusions in the steel is 45% by mass or more, so that the form of the oxide inclusions in the slab is more appropriate. This is considered to be because BN was precipitated integrally with the inclusions, and the precipitation of BN on the austenite grain boundaries was further suppressed.

  When the continuous cast slab of B-containing steel of the present invention is produced using a vertical bending type or curved type continuous casting machine, the occurrence of surface cracks is suppressed. Furthermore, it has high hot ductility and has excellent strength and surface properties as a material for thick steel plates. In addition, since the occurrence of surface cracks is suppressed, the surface cracks can be removed by unnecessary or light care, and therefore, it is possible to produce with high production efficiency. The steel plate obtained by using the continuous cast slab of the B-containing steel of the present invention as a raw material and hot rolling it has excellent strength and surface properties.

Claims (2)

In mass%, C: 0.050 to 0.180%, Si: 0.10 to 0.40%, Mn: 0.5 to 2.0%, P: 0.020% or less, S: 0.0030 % Or less, Ti: 0.005 to 0.030%, Sol. Al: 0.005-0.060%, B: 0.0005-0.0050%, N: 0.0015-0.0070%, and rare earth elements: 0.001-0.050%,
Further, at least one of Cu, Cr, Ni, Mo, V, Nb and Ca is Cu: 0.1 to 0.5%, Cr: 0.2 to 2.0%, Ni: 0.3 to 2 0.5%, Mo: 0.1-0.8%, V: 0.01-0.10%, Nb: 0.005-0.050% and Ca: 0.0005-0.0060% ,
The balance consists of Fe and impurities,
A continuous cast slab of B-containing steel, wherein the contents of Ti, rare earth elements, B and N in the steel satisfy the relationship of the following formula (a).
([% Ti] + [% REM]) / ([% B] + [% N]) ≧ 2.5 (a)
Here, [% Ti], [% REM], [% B], and [% N] mean the contents (mass%) of Ti, rare earth elements, B, and N in the steel, respectively.
Sol. The content of Al and rare earth elements satisfies the relationship of the following formula (b):
2. The continuous cast slab of B-containing steel according to claim 1, wherein the content of rare earth elements in the oxide inclusions present in the steel is 45% by mass or more.
[% Sol. Al] / [% REM] ≦ 7 (b)
Here, [% Sol. [Al] and [% REM] are obtained from Sol. It means the content (mass%) of Al and rare earth elements.

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