Steel slabs and rolled steel

Abstract

The present invention provides steel slabs and rolled steel materials that contain at least one of Nb and Ti, and in which the formation of NbTi carbonitrides with an equivalent circular diameter exceeding 50.0 μm is suppressed in the central part of the steel slabs and rolled steel materials. [Solution] Having a predetermined chemical composition, in a cross-section parallel to the main surface, within a range from the center in the thickness direction and from the center to a length of 1 / 100 of the width in the width direction, 10.0 particles / mm of CaZrO3 with an equivalent circular diameter of 1.0 μm or more are provided. 2 The above is true, and of the CaZrO3, at least a portion of the outer circumference is in contact with a carbide, nitride, or carbonitride containing at least one of Nb and Ti, with a value of 2.0 CaZrO3 particles / mm². 2 The above are the steel slabs and rolled steel materials.

Description

[Technical Field]

[0001] This disclosure relates to steel slabs and rolled steel materials. [Background technology]

[0002] Nb and Ti are often used in steel materials to increase their strength and refine their grain size. When Nb and Ti segregate in the center of a steel slab, coarse carbides, nitrides, or carbonitrides (hereinafter, carbides, nitrides, or carbonitrides may be collectively referred to as carbonitrides) containing Nb and Ti may be formed in the central segregation area of ​​the slab. Carbonitrides containing Nb and Ti (hereinafter, sometimes referred to as NbTi carbonitrides) are hard and remain even after the steel slab is rolled, so they can become the fracture initiation point for hydrogen-induced cracking (HIC) in rolled steel materials manufactured from steel slabs. They can also become the fracture initiation point for fatigue fracture in parts manufactured from rolled steel materials. These problems become particularly pronounced when the equivalent circle diameter of the NbTi carbonitride exceeds 50.0 μm. Therefore, the industry is demanding that the formation of NbTi carbonitrides with an equivalent circle diameter exceeding 50.0 μm be prevented.

[0003] Conventionally, measures such as light reduction at the end of solidification during continuous casting and so-called soaking treatment, which involves heating steel slabs at high temperatures for extended periods, have been employed. However, these measures make it difficult to stably suppress coarse carbonitrides containing Nb and Ti in steel slabs containing Nb and Ti, and can lead to excessive increases in manufacturing costs.

[0004] Therefore, further measures are needed to meet the demands of the industrial sector.

[0005] Incidentally, in the technical fields related to cast steel slabs and rolled steel, the properties of steel materials are sometimes improved by utilizing inclusions such as oxides. Patent Document 1 discloses a steel characterized by having two main phases, one containing CaO and Al2O3 and the other containing CaO and ZrO2, with a number ratio of 50% or more of inclusions having a mixture of these two phases, in order to prevent a decrease in the toughness of the steel material. Furthermore, Patent Document 2 addresses the objective of providing a steel slab with a fine solidification structure that can advantageously neutralize segregation and porosity in the steel slab, and describes an oxide in a steel slab with a predetermined chemical composition, in which Ca is essential, and one or two of Zr and Hf are the main components, and the entire or partial surface is in direct contact with the base metal without the intermediary of a third substance, and the oxide has a maximum diameter of 0.1 to 10 μm, and exists as single particles or composite particles with an area of ​​1 mm² in any cross-section. 2 A steel slab with a fine solidification structure is disclosed, characterized by the presence of one or more particles per hit. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2022-175322 [Patent Document 2] Japanese Patent Publication No. 2008-127599 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, the technologies disclosed in Patent Documents 1 and 2 are not suitable for the purpose of suppressing coarse carbonitrides containing Nb and Ti.

[0008] This disclosure has been made in view of the above circumstances, and aims to provide steel slabs and rolled steel materials in which the formation of NbTi carbonitrides with an equivalent circular diameter exceeding 50.0 μm is suppressed in the central part of steel slabs and rolled steel materials containing at least one of Nb and Ti. [Means for solving the problem]

[0009] The above problems are solved by the following means. <1> By mass percentage, C: 0.02 to 0.30%, Si: 0.02 to 2.50%, Mn: 0.40 to 2.50%, Al: 0.001 to 0.080%, Ca: 0.0002 to 0.0050%, Zr: 0.0015 to 0.0650%, P: 0.020% or less, S: 0.020% or less, O: 0.0040% or less, and N: 0.0100% or less, and further contains Ti: 0.005 to 0.050%, and Nb: 0.005 to 0.070%, contains one or two selected from the group consisting of a steel slab having a chemical composition in which the balance is Fe and impurities, a cross-section parallel to the main surface of the steel slab, in the center in the thickness direction and in the range of a length of 1 / 100 of the width of the steel slab from the center of the steel slab in the width direction, the number of CaZrO3 with a circle equivalent diameter of 1.0 μm or more is 10.0 pieces / mm 2 or more, and among the CaZrO3, at least a part of the outer periphery is in contact with any one of carbides, nitrides, or carbonitrides containing at least one of Nb and Ti, and the number of CaZrO3 is 2.0 pieces / mm 2 or more. <2> The chemical composition further contains, by mass percentage, instead of a part of the Fe, Cr: 2.00% or less, Ni: 2.00% or less, Mo: 0.50% or less, V: 0.20% or less, Cu: 0.50% or less, and B: 0.0050% or less The steel slab according to <1>, which contains one or more selected from the group consisting of <3> By mass percentage, C: 0.02 to 0.30%, Si: 0.02~2.50%, Mn: 0.40~2.50%, Al: 0.001~0.080%, Ca: 0.0002~0.0050%, Zr: 0.0015~0.0650%, P: 0.020% or less, S: 0.020% or less, O: 0.0040% or less, and It contains N: 0.0100% or less, and further Ti: 0.005~0.050%, and Nb: 0.005~0.070%, It contains one or two selected from the group consisting of, A rolled steel material having a chemical composition in which the remainder is Fe and impurities, A cross-section parallel to the main surface of the rolled steel material, located at the center in the thickness direction and in the width direction, within a range from the center of the rolled steel material to a length of 1 / 100 of the width of the rolled steel material, 10.0 CaZrO3 particles / mm² with an equivalent circle diameter of 1.0 μm or larger 2 The above is true, and of the CaZrO3, at least a portion of the outer circumference is in contact with a carbide, nitride, or carbonitride containing at least one of Nb and Ti, with a value of 2.0 CaZrO3 particles / mm². 2 The above describes rolled steel materials. <4> The aforementioned chemical composition is further, by mass%, by substituting a portion of the Fe: Cr: 2.00% or less, Ni: 2.00% or less, Mo: 0.50% or less, V: 0.20% or less, Cu: 0.50% or less, B: 0.0050% or less Includes one or more species selected from the group consisting of <3> Rolled steel materials as described above. [Effects of the Invention]

[0010] According to this disclosure, steel slabs and rolled steel materials containing at least one of Nb and Ti are provided, in which the formation of NbTi carbonitrides with an equivalent circular diameter exceeding 50.0 μm is suppressed in the central part of the steel slabs and rolled steel materials. [Brief explanation of the drawing]

[0011] [Figure 1A] This diagram schematically shows an example of the central part of a steel slab or rolled steel material with a rectangular cross-section. [Figure 1B] This diagram schematically shows an example of the central part of a steel slab or rolled steel material with a circular cross-section. [Figure 2] This image shows an example of NbTi carbonitride formed using CaZrO3 as a nucleation site, as observed by FE-SEM. [Modes for carrying out the invention]

[0012] Hereinafter, a steel slab and rolled steel material according to one embodiment of the present disclosure will be described in detail. In the following explanation, "%" in chemical composition refers to "mass percent". In this disclosure, a numerical range represented by "~" means a range that includes the numbers before and after "~" as the lower and upper limits, unless otherwise specified. Furthermore, if the numbers before and after "~" are preceded by "greater than" or "less than", the numerical range means a range that does not include those numbers as the lower or upper limit. In the numerical ranges described stepwise in this specification, the upper limit of one stepwise numerical range may be replaced with the upper limit of another stepwise numerical range, or with the values ​​shown in the examples. The same applies to the lower limits of the numerical ranges. The elemental content in a chemical composition is sometimes expressed by adding the "amount" to the element symbol (for example, C amount, Si amount, etc.). When the chemical composition of an element is described as "0~", it means that the element does not need to be included. The term "process" includes not only independent processes, but also any process that cannot be clearly distinguished from other processes, as long as its intended purpose is achieved.

[0013] In order to solve the above problems, the inventors of this disclosure conducted extensive research and studies to suppress the formation of NbTi carbonitrides with an equivalent circular diameter exceeding 50.0 μm, and after carefully analyzing and examining the results, obtained the following findings (a) to (g). (a) Since coarse NbTi carbonitrides tend to form in the central segregation area, attention should be paid to the central part of the steel slab and rolled steel. (b) The size of individual NbTi carbonitrides can be reduced by increasing their number density. (c) As a means of increasing the number density of NbTi carbonitrides, it is advisable to disperse a large number of inclusions in the steel that preferentially nucleate NbTi carbonitrides. Such inclusions are sometimes called "inoculation nuclei." (d) Coarse NbTi carbonitrides are formed when Nb and Ti segregate in the center of the cast slab during the final stage of solidification, and NbTi carbonitrides crystallize in this central segregation area. (e) Therefore, in order to function as an inoculation nucleus, it is necessary to crystallize in the molten steel before the final stage of solidification. In the molten steel, for example, Al2O 3、 While CaO, MnS, and CaS may crystallize, these do not function well as inoculants for NbTi carbonitride. Investigations revealed that CaZrO3, which exhibits good lattice compatibility with NbTi carbonitride based on its crystal structure, is an effective inoculant. An example of these observations is shown in Figure 2. (f) In order for CaZrO3 to function effectively as a nucleation site (inoculation nucleus) for NbTi carbonitride, it is necessary to have a size greater than a certain limit. (g) In CaZrO3 of a certain size or larger, a large number of NbTi carbonitrides are observed to be formed starting from it. A small number of carbonitrides not originating from CaZrO3 are also observed, but the formation of coarse NbTi carbonitrides can be suppressed.

[0014] Based on the findings in (a) to (g) above, the inventors of this disclosure conducted further detailed experiments and research. As a result, they found that the above problems can be solved by adjusting the chemical composition of the steel slab and rolled steel material, the number density and size (equivalent diameter) of CaZrO3 with an equivalent circle diameter of 1.0 μm or more, and the number density of CaZrO3 in which at least a portion of the outer circumference is in contact with NbTi carbonitride, to an appropriate range, leading to this disclosure. Furthermore, the objective of the steel slabs and rolled steel materials relating to this disclosure is that there are no NbTi carbonitrides with an equivalent circular diameter exceeding 50.0 μm in their central part.

[0015] [Steel slabs and rolled steel products] <Chemical composition> The chemical composition of the steel slabs and rolled steel materials relating to this disclosure is described in detail below. The rolled steel materials relating to this disclosure are ultimately processed into various parts. In the following, the amounts of each element may be described from the perspective of the steel slabs, rolled steel materials, or parts, but the chemical composition of the steel slabs and rolled steel materials relating to this disclosure is the same.

[0016] (C: 0.02~0.30%) Carbon (C) is an important element for ensuring the strength of rolled steel materials and parts. If the C content is less than 0.02%, this effect is not sufficiently obtained. On the other hand, if the C content exceeds 0.30%, workability may deteriorate. Therefore, the C content should be controlled within the range of 0.02% to 0.30%. The lower limit of the C content is preferably 0.04%, and the upper limit is preferably 0.20%.

[0017] (Si: 0.02~2.50%) Silicon (Si) is an element that acts as a deoxidizing agent and is also effective in improving the strength of rolled steel materials and parts by enhancing hardenability. This effect is not fully obtained when the Si content is less than 0.02%. On the other hand, if the Si content exceeds 2.50%, the workability may deteriorate. Therefore, the Si content should be controlled within the range of 0.02 to 2.50%. The lower limit of the Si content is preferably 0.17%, and the upper limit is preferably 0.50%.

[0018] (Mn: 0.40~2.50%) Manganese (Mn) is an effective element for improving the hardenability and thus increasing the strength of rolled steel materials and parts. This effect is not fully achieved when the Mn content is less than 0.40%. On the other hand, if the Mn content exceeds 2.50%, workability may deteriorate. Therefore, the Mn content should be controlled within the range of 0.40% to 2.50%. The lower limit of the Mn content is preferably 0.90%, and the upper limit is preferably 1.20%.

[0019] (Al: 0.001~0.080%) Aluminum (Al) is an element that acts as a deoxidizing agent. If the Al content is less than 0.001%, this effect cannot be sufficiently obtained. On the other hand, if the Al content exceeds 0.080%, the processability may deteriorate. Therefore, the Al content should be controlled within the range of 0.001 to 0.080%. In this disclosure, Al content refers to the total Al content, which is the sum of acid-soluble Al and acid-insoluble Al content. The lower limit of the Al content is preferably 0.010%, and the upper limit of the Al content is preferably 0.030%.

[0020] (Ca: 0.0002~0.0050) Ca is one of the important elements in the steel slabs and rolled steel materials relating to this disclosure. In order to achieve the equivalent circle diameter and number density of the CaZrO3 inoculum within the scope of this disclosure, the Ca content must be 0.0002% or more. On the other hand, if the Ca content exceeds 0.0050%, the amount of CaO increases and the amount of CaZrO3 decreases, failing to meet the equivalent circle diameter and number density within the scope of this disclosure. Therefore, the Ca content is controlled within the range of 0.0002% to 0.0050%. Preferably, the lower limit of the Ca content is 0.0005%, and preferably the upper limit is 0.0030%.

[0021] (Zr: 0.0015~0.0650%) Zr is one of the important elements in the steel slabs and rolled steel materials relating to this disclosure. In order to achieve the equivalent circle diameter and number density of the CaZrO3 inoculum within the range of this disclosure, the Zr content must be 0.0015% or more. On the other hand, if the Zr content exceeds 0.0650%, the ZrO2 increases and the CaZrO3 decreases, making it difficult to satisfy the equivalent circle diameter and number density within the range of this disclosure. Therefore, the Zr content is controlled within the range of 0.0015 to 0.0650%. Preferably, the lower limit of the Zr content is 0.0030%, and preferably the upper limit is 0.0200%.

[0022] (Contains one or two substances selected from Ti: 0.005-0.050% and Nb: 0.005-0.070%) Titanium (Ti) and niobium (Nb) are elements that form carbonitrides and are effective in preventing grain coarsening and improving workability. This effect is not fully obtained when the Ti content is less than 0.005% and the Nb content is also less than 0.005%. On the other hand, when the Ti content exceeds 0.050%, NbTi carbonitrides with an equivalent circle diameter of more than 50.0 μm are easily formed, even if the other requirements of this disclosure are met. Furthermore, if the Nb content exceeds 0.070%, NbTi carbonitrides with an equivalent circle diameter exceeding 50.0 μm will be formed, even if the other requirements of this disclosure are met. Therefore, the Ti content should be controlled to a range of 0.005-0.050% and Nb content of 0.005-0.070%, or both. The lower limit of the Ti content is preferably 0.008%, and the upper limit of the Ti content is preferably 0.035%. The lower limit of the Nb content is preferably 0.008%, and the upper limit of the Nb content is preferably 0.040%.

[0023] (P:0.020% or less) Phosphorus (P) is an impurity element that tends to segregate at grain boundaries and hinders the workability of rolled steel. Therefore, the P content should be limited to 0.020% or less. However, considering current general refining practices (including secondary refining), the lower limit of the P content may be 0.005% or higher.

[0024] (S:0.020% or less) S (sulfur) is an impurity element that hinders the workability of rolled steel by forming nonmetallic inclusions, mainly MnS. Therefore, the S content should be limited to 0.020% or less, preferably to 0.003% or less. However, considering current general refining processes (including secondary refining), the lower limit of the S content may be 0.0003% or more.

[0025] (O:0.0040% or less) Oxygen (O) is an impurity element that forms oxides (nonmetallic inclusions), and the aggregation and coarsening of these oxides can cause nozzle clogging during casting, and depending on the composition of the oxides, it can stretch during rolling, reducing the workability of rolled steel. Therefore, the O content is limited to 0.0040% or less. Furthermore, considering current general refining (including secondary refining), the lower limit of the O content may be 0.0005% or more. In this disclosure, the O content refers to the total O content, which is the sum of all O content, including O dissolved in the steel and O present in inclusions.

[0026] (N:0.0100% or less) Nitrogen (N) is an impurity element that forms nitrides (nonmetallic inclusions) and reduces the workability of rolled steel. Therefore, the N content should be limited to 0.0100% or less. Preferably, it should be 0.0050% or less. Also, considering current general refining (including secondary refining), the lower limit of the N content may be 0.0010%.

[0027] The steel slabs and rolled steel materials relating to this disclosure have the above-mentioned basic components controlled, with the remainder being Fe and impurities. However, in addition to these basic components, the steel slabs and rolled steel materials relating to this disclosure may also contain the following optional components as needed, in place of a portion of the remaining Fe. The following optional components can be included primarily to improve strength and workability in terms of material properties, and do not contribute to promoting nucleation of NbTi carbonitrides or thereby reducing their size.

[0028] In other words, the cast slabs and rolled steel materials relating to this disclosure may, in addition to the basic components and impurities described above, further contain one or more selected components from the group consisting of Cr, Ni, Mo, V, Cu, and B. The numerical limit range for the selected components and the reasons for that limit are explained below. The percentages mentioned here are mass percentages.

[0029] (Cr:2.00% or less) The inclusion of chromium (Cr) is optional. Cr is an effective element for improving hardenability and thus increasing the strength of rolled steel materials and parts. To obtain this effect, it is preferable to include 0.03% or more Cr. On the other hand, if the Cr content exceeds 2.00%, the workability may deteriorate. Therefore, the Cr content should be controlled to 2.00% or less. The upper limit of the Cr content is preferably 0.50%.

[0030] (Ni:2.00% or less) The inclusion of nickel (Ni) is optional. Ni is an effective element for improving hardenability and thus increasing the strength of rolled steel materials and parts. To obtain this effect, it is preferable to include 0.03% or more Ni. On the other hand, if the Ni content exceeds 2.00%, the workability may deteriorate. Therefore, the Ni content should be controlled to 2.00% or less. The upper limit of the Ni content is preferably 0.50%.

[0031] (Mo: 0.50% or less) The inclusion of molybdenum (Mo) is optional. Mo is an effective element for improving hardenability and thus increasing the strength of rolled steel materials and parts. To obtain this effect, it is preferable to include 0.03% or more Mo. On the other hand, if the Mo content exceeds 0.50%, the workability may deteriorate. Therefore, the Mo content should be controlled to 0.50% or less. The upper limit of the Mo content is preferably 0.15%.

[0032] (V:0.20% or less) The inclusion of vanadium (V) is optional. Vanadium is an effective element for improving hardenability and thus increasing the strength of rolled steel materials and parts. To obtain this effect, it is preferable to include 0.02% or more vanadium. On the other hand, if the vanadium content exceeds 0.20%, the workability may deteriorate. Therefore, the vanadium content should be controlled to 0.20% or less. The upper limit of the vanadium content is preferably 0.10%.

[0033] (Cu:0.50% or less) The inclusion of copper (Cu) is optional. Cu is an effective element for improving hardenability and thus increasing the strength of rolled steel materials and parts. To obtain this effect, it is preferable to include 0.03% or more Cu. On the other hand, if the Cu content exceeds 0.50%, the workability may deteriorate. Therefore, the Cu content should be controlled to 0.50% or less. The upper limit of the Cu content is preferably 0.20%.

[0034] (B:0.0050% or less) The inclusion of boron (B) is optional. Boron (B) is an effective element for improving hardenability and thus increasing the strength of rolled steel materials and parts. To obtain this effect, it is preferable to include 0.0003% or more of B. On the other hand, if the B content exceeds 0.0050%, the workability may deteriorate. Therefore, the B content should be controlled to 0.0050% or less. The upper limit of the B content is preferably 0.0020%.

[0035] The remainder of the steel slabs and rolled steel materials relating to this disclosure consists of Fe and impurity elements. Here, impurity elements refer to elements that are not intentionally included during the industrial production of steel, but are introduced from raw materials such as ore, scrap, or the manufacturing environment, and are acceptable to the extent that they do not adversely affect the effects or properties of the steel slabs and rolled steel materials relating to this disclosure.

[0036] <cazro3> Next, the size, number density, etc. of CaZrO3 in the steel slabs and rolled steel materials related to this disclosure will be described. Figure 1A schematically shows an example of the central part of a steel slab or rolled steel material with a rectangular cross-section. Figure 1B schematically shows an example of the central part of a steel slab or rolled steel material with a circular cross-section. 10A and 10B are steel slabs or rolled steel materials, and 14A and 14B are cross-sections parallel to the main surfaces 12A and 12B, passing through the center in the thickness direction (at the 1 / 2 position of the thickness). Note that the "main surface" of a steel slab or rolled steel material means, as shown in Figure 1A, the widest surface 12A of the steel slab or rolled steel material 10A, which is perpendicular to the thickness direction, when the cross-sectional shape (cross-section perpendicular to the longitudinal direction) of the steel slab or rolled steel material 10A is rectangular. Also, as shown in Figure 1B, when the cross-sectional shape of the steel slab or rolled steel material 10B is circular, the "main surface" means a hypothetical plane 12B that is in contact with the outer surface. In this disclosure, the "main surface" of a steel slab or rolled steel material may be referred to as the "broad surface."

[0037] In this disclosure, the objective is to reduce the size of NbTi carbonitrides in the central segregation portion of steel slabs and rolled steel materials. Therefore, the central region of the steel slabs and rolled steel materials according to this disclosure is defined as shown in Figure 1A, in a cross-section parallel to the main surface 12A of the steel slab 10A, and is defined as the center in the thickness direction and a length of 1 / 100 of the width from the center of the steel slab in the width direction, in the case where the cross-sectional shape of the steel slab 10 is circular rather than rectangular, then, as shown in Figure 1B, the central region is defined as the plane containing the center in the diameter direction of the steel slab, and is defined as a length of 1 / 100 of the diameter from the center in the diameter direction, in the case where the diameter corresponds to the width. Furthermore, since the ratio of the central region in the cross-section does not change even when the steel slab is rolled, the same range is applied to rolled steel materials.

[0038] The steel slab according to this disclosure satisfies all of the following conditions (1) to (3) in a cross-section parallel to the broad surface of the steel slab, at the center in the thickness direction and in the width direction, within a range from the center of the steel slab to a length of 1 / 100 of the width. The same applies to the rolled steel material according to this disclosure.

[0039] (1) Equivalent circular diameter of CaZrO3 In order to obtain the effects of the present disclosure, CaZrO3 with an equivalent circle diameter of 1.0 μm or more is required. It is preferably 2.0 μm or more. When CaZrO3 is combined with other inclusions, the equivalent circle diameter is calculated only for CaZrO3. NbTi carbonitride is likely to nucleate starting from CaZrO3 with an equivalent circle diameter of 1.0 μm or more. When there are 10.0 pieces / mm 2 or more present in the central part of the slab as described later, the number of NbTi carbonitrides in the central segregation part of the slab can be sufficiently increased, and the equivalent circle diameter of the NbTi carbonitride can be made 50.0 μm or less. For CaZrO3 with an equivalent circle diameter of less than 1.0 μm, the nucleation effect of NbTi carbonitride cannot be sufficiently obtained. On the other hand, if the equivalent circle diameter of CaZrO3 is 30.0 μm or less, it does not have much adverse effect on the material properties, so the equivalent circle diameter of CaZrO3 is preferably 30.0 μm or less. For the above reasons, in the present disclosure, the equivalent circle diameter of CaZrO3 is made 1.0 μm or more. Preferably it is 2.0 μm or more and 30.0 μm or less.

[0040] (2) Number density of CaZrO3 Within the range defined above, CaZrO3 satisfying the requirement of the equivalent circle diameter is distributed at 10.0 pieces / mm 2 or more. If it is less than 10.0 pieces / mm 2 , the number of nucleations of NbTi carbonitride, and thus the number of NbTi carbonitrides, is not sufficient, so the size of each NbTi carbonitride cannot be sufficiently reduced, and the equivalent circle diameter of the NbTi carbonitride cannot be made 50.0 μm or less. It is preferable that CaZrO3 is distributed at 15.0 pieces / mm 2 or more. Although the upper limit value of the number density of CaZrO3 is not particularly limited, it is preferably 50.0 pieces / mm 2 or less from the viewpoint of manufacturability.

[0041] On the other hand, inclusions that do not satisfy either the composition or the equivalent circle diameter and are therefore not subject to evaluation may be distributed. These have almost no promoting effect on the nucleation of NbTi carbonitride, but do not inhibit the promoting effect of CaZrO3 satisfying the regulations of the present disclosure.

[0042] (3) Number density of CaZrO3 in contact with NbTi carbonitride 10.0 CaZrO3 particles / mm² with an equivalent circle diameter of 1.0 μm or larger 2 Even if the distribution is as described above, at least a portion of the outer circumference of the CaZrO3 is in contact with a carbide, nitride, or carbonitride containing at least one of Nb and Ti, and there are 2.0 CaZrO3 particles / mm² with an equivalent circular diameter of 1.0 μm or more. 2 The following is required: 2.0 pieces / mm 2 Below this level, it is difficult to prevent the formation of NbTi carbonitrides with an equivalent circle diameter exceeding 50.0 μm. Preferably, the number of CaZrO3 particles with an equivalent circle diameter of 1.0 μm or more that are in contact with any of the above-mentioned carbides, nitrides, or carbonitrides is 4.0 particles / mm². 2 That concludes the explanation. While there is no particular upper limit, from a manufacturability standpoint, 40.0 pieces / mm² is recommended. 2 The following are preferable.

[0043] Details of the measurement methods for the equivalent circular diameter, number density, and number density of CaZrO3 in contact with NbTi carbonitride in steel slabs and rolled steel materials will be explained in the examples.

[0044] The steel slabs relating to this disclosure may be slabs or blooms. In particular, slabs are more preferred. The slabs may have a width and thickness in a cross-sectional shape perpendicular to the continuous casting direction, and may also have a length in the continuous casting direction. If the slab is a slab, its width corresponds to the long side (corresponding to the long side of the mold) in the cross-sectional shape perpendicular to the continuous casting direction, and its thickness corresponds to the short side (corresponding to the short side of the mold) in the same cross-sectional shape. Furthermore, when the cast slab is a bloom, its aspect ratio (length of the long side / length of the short side) is generally smaller than that of a slab. When the cast slab is a slab, its width may be, for example, 800 mm or more and 2200 mm or less, and its thickness may be, for example, 100 mm or more and 400 mm or less. When the cast slab is a bloom, its width may be, for example, 300 mm or more and 800 mm or less, and its thickness may be, for example, 200 mm or more and 600 mm or less.

[0045] The rolled steel material relating to this disclosure may be a thick steel plate, a thin steel plate, a steel bar, or a wire rod. In particular, a thick steel plate is more preferable.

[0046] [Method for manufacturing steel slabs and rolled steel products] An example of a method for manufacturing steel slabs and rolled steel products related to this disclosure will be described below. However, the method for manufacturing steel slabs and rolled steel products related to this disclosure is not limited to the method described below.

[0047] Using blast furnace molten iron as the raw material, and after pre-treatment of the molten iron and decarburization in a converter, for example, 300 tons of molten steel are subjected to the following ladle refining process, depending on the Al and Zr content.

[0048] (Pattern a: When the amount of Al in the steel is less than or equal to twice the amount of Zr) In ladle smelting, the Zr addition is divided into two stages: after Al is added, the first Zr addition, the second Zr addition, and then Ca addition are performed in that order. The amount of Zr in each stage is approximately half of the total amount. The time between the first and second Zr additions should be between 2 and 10 minutes. If the time is less than 2 minutes, the concentration of the Zr added in the first addition will not become uniform throughout the molten steel. If the time exceeds 10 minutes, the ZrO2-containing inclusions formed by the first Zr addition will become coarser and their number will decrease. The time between the second Zr addition and the Ca addition should be between 2 minutes and 30 minutes. The upper limit reflects the fact that Ca addition is generally performed in a separate facility from the ladle refining, and therefore requires time including transfer time before Ca addition. If the time is less than 2 minutes, the Zr concentration will not become uniform throughout the molten steel. If it exceeds 30 minutes, the ZrO2-containing inclusions will become coarser and their number will decrease significantly. It is preferable to keep the time within 20 minutes.

[0049] (Pattern b: When the amount of Al in the steel component is more than twice but four times or less than the amount of Zr) In ladle smelting, the addition of Al is divided into two stages: the first Al addition is followed by the addition of Zr, the second Al addition, and then the addition of Ca. After the first addition of Al, the amount of Al in the steel component should be less than twice the amount of Zr. The time between the first addition of Zr and the second addition of Al should be between 2 and 10 minutes. If the time is less than 2 minutes, the Zr concentration will not become uniform throughout the molten steel. If the time exceeds 10 minutes, the ZrO2-containing inclusions will become coarser and their number will decrease. The time between the second addition of Al and the addition of Ca should be between 2 minutes and 30 minutes. If the time is less than 2 minutes, the concentration of the Al added in the second addition will not be uniform throughout the molten steel. If the time exceeds 30 minutes, the ZrO2-containing inclusions will become coarser and their number will decrease significantly. It is preferable to keep the time to within 20 minutes.

[0050] (Pattern c: When the amount of Al in the steel component exceeds four times the amount of Zr) In ladle smelting, the addition of Al is divided into two stages: the first Al addition is followed by Zr addition, Ca addition, and then the second Al addition. The amount of Al in the steel composition after the first Al addition should be no more than twice the amount of Zr. The time between Zr addition and Ca addition should be between 2 minutes and 30 minutes. If the time is less than 2 minutes, the Zr concentration will not become uniform throughout the molten steel. If the time exceeds 30 minutes, the ZrO2-containing inclusions will become coarser and their number will decrease significantly. It is preferable to keep the time within 20 minutes. The time between the first addition of Ca and the second addition of Al should be between 2 minutes and 30 minutes. If the time is less than 2 minutes, the Ca concentration will not become uniform throughout the molten steel. If the time exceeds 30 minutes, the CaZrO3-containing inclusions will become coarser and their number will decrease significantly. It is preferable to keep the time to within 20 minutes.

[0051] After ladle refining according to one of the above patterns a, b, or c, depending on the amount of Al and Zr, the steel slab according to the disclosure can be manufactured by casting under the conditions that the degree of superheating of the molten steel in the tundish (the difference between the actual molten steel temperature and the liquidus temperature determined from the chemical composition of the steel) is within the range of +10 to +50°C, and the fluctuation range of the casting speed in the steady state after the casting speed reaches the target value is within ±0.3 m / min. Furthermore, the rolled steel material according to the disclosure can be manufactured by rolling the manufactured steel slab at a heating temperature of 1250°C or less. [Examples]

[0052] The following describes examples of the steel slabs and rolled steel materials relating to this disclosure. However, the steel slabs and rolled steel materials relating to this disclosure are not limited to the examples described below.

[0053] Using blast furnace molten iron or electric furnace molten steel as raw materials, 300 tons of molten steel were produced by ladle refining. When blast furnace molten iron was used as the raw material, after pre-treatment of the molten iron and decarburization in the converter, ladle refining was performed, and components other than calcium were adjusted using an RH vacuum degassing unit. When using electric furnace molten steel as the raw material, the components other than Ca were adjusted using an LF (Lamb Refining) device as part of ladle refining. Next, the mixture was moved to the Ca-adding facility where Ca was added. In some cases, Al was added after the Ca was added. As a result, steel with the chemical composition shown in Table 1 below was melted down.

[0054] [Table 1]

[0055] The order and intervals of Al, Zr, and Ca addition in the ladle smelting process described above are shown in Table 2. Based on the ratio of Al to Zr content in the chemical composition of the steel (indicated as [Al] / [Zr] in Table 1), the order and intervals of Al, Zr, and Ca addition are described separately for the three methods of adding Zr or Al in stages (patterns a, b, and c) and the normal method without staged addition (pattern d). (Pattern a) When the amount of Al in the chemical composition of the steel is less than or equal to twice the amount of Zr: Add Al → Add Zr for the first time → Add Zr for the second time → Add Ca. The interval between the first and second Zr additions is shown as ta1, and the interval between the second Zr addition and the Ca addition is shown as ta2. (Pattern b) When the amount of Al in the chemical composition of the steel is more than twice but four times or less than the amount of Zr: First Al addition → Zr addition → Second Al addition → Ca addition. The interval between the Zr addition and the second Al addition is shown as tb1, and the interval between the second Al addition and the Ca addition is shown as tb2. (Pattern c) When the amount of Al in the chemical composition of the steel exceeds four times the amount of Zr: First Al addition → Zr addition → Ca addition → Second Al addition. The interval between Zr addition and Ca addition is shown as tc1, and the interval between Ca addition and the second Al addition is shown as tc2. (Pattern d) Normal addition method: Al addition → Zr addition → Ca addition, etc. The interval between Zr addition and Ca addition is indicated by td. Note that in comparative example No. 104, which does not include Zr addition, td indicates the interval between Al addition and Ca addition. Also, in comparative example No. 101, which does not include Ca addition, the interval between Al addition and Zr addition was set to 5 minutes. In Table 2, under "Al, Zr, Ca Addition Order," (1) and (2) refer to the first and second additions, respectively.

[0056] [Table 2]

[0057] Molten steel was poured into a tundish, and the superheat of the molten steel in the tundish (the difference between the actual molten steel temperature and the liquidus temperature determined by the chemical composition of the steel) was kept within the range of +20 to +40°C. Subsequently, continuous casting at the target casting speed of 1.1 m / min was performed to produce slabs with a thickness of 250 mm and a width of 1500 mm, or blooms with a thickness of 340 mm and a width of 460 mm. The fluctuation range of the casting speed in the steady-state section after reaching the target casting speed was within ±0.2 m / min. Furthermore, the slabs were manufactured as 19mm thick steel plates or 3.8mm thick thin steel plates by hot rolling. For hot rolling, the heating temperature was set to 1200°C, and the rolling temperature was set to general conditions. Furthermore, for the bloom, the steel was subjected to bract rolling at a heating temperature of 1250°C to form 150mm square steel billets. Then, the heating temperature was reduced to 1200°C, and the rolling temperature was adjusted to typical conditions to produce 25mm diameter steel bars. Table 1 lists the types of cast slabs and rolled materials. Slabs are denoted as SL, and blooms as BL.

[0058] As shown in Figures 1A and 1B, samples were taken from the above-mentioned steel slabs (or blooms) and rolled steel materials (thick steel plates, thin steel materials, or steel bars) such that the cross-section was parallel to the broad surface of the steel slabs and rolled steel materials, and the observation surface was located at the center in the thickness direction and within a range of 1 / 100th of the width from the center of the steel slab or rolled steel material in the width direction. The observation surface was then mirror-polished. In this observation area, 5 fields of view were randomly selected, with 1 mm per field of view. 2 Observations and measurements were taken regarding this matter. In each field of view, surface (mapping) analysis was performed at an acceleration voltage of 15 kV using an EDS (energy-dispersive X-ray analyzer) attached to a FE-SEM (field emission scanning electron microscope) to identify inclusions containing Ca, Zr, and O. When the central part (intersection of the major and minor axes) of the identified inclusions was measured using the EDS attached to the FE-SEM at an acceleration voltage of 15 kV and a magnification of 5000 to 30000x, those that simultaneously satisfied the following equations (1) and (2) were defined as CaZrO3. Ca(mass%)+Zr(mass%)+O(mass%)≧80 ···(1) However, in order to exclude the influence of Fe, which is present in large quantities in the matrix phase, Fe was excluded from equation (1) even if it was detected. Zr(mass%) / Ca(mass%)=1.77~2.77 ···(2)

[0059] For the above CaZrO3, image data acquired by FE-SEM and EDS was measured for area using ImageJ version 1.54i, an image analysis software developed and distributed by the National Institutes of Health (NIH), and the equivalent circle diameter was calculated based on this. Of the calculated equivalent circle diameters, the number of particles with an equivalent circle diameter of 1.0 μm or more was determined, and the number of particles in the five fields of view obtained above was summed up. From this sum, 1 mm 2 The number of items was calculated.

[0060] Even in the case of rolled steel, the same method can be used for determination as for cast steel slabs, but since the cross-section of rolled steel is smaller than that of cast steel slabs, the area per field of view is 1 mm². 2 If this is not possible, the observation and measurement range per field of view should be 0.1 mm. 2 The field of view was set to 10. If 10 fields of view could not be obtained from one sample, another sample was prepared and used for measurement.

[0061] Furthermore, the number density of CaZrO3 particles in contact with the NbTi carbonitride was calculated using the following method. If there are precipitates in contact with CaZrO3 particles with an equivalent circle diameter of 1.0 μm or more, as identified by the above method, the center of each precipitate (the intersection of the major and minor axes) was measured using the EDS attached to the FE-SEM at an acceleration voltage of 15 kV and a magnification of 5000x. If the following equation (3) was satisfied, it was determined that the NbTi carbonitride was in contact with CaZrO3. Ti(mass%)+Nb(mass%)+C(mass%)+N(mass%)≧75···(3) However, in order to exclude the influence of Fe, which is present in large quantities in the matrix phase, Fe was excluded from equation (3) even if it was detected.

[0062] Furthermore, coarse NbTi carbonitrides in cross-sections parallel to the main surface of steel slabs (or blooms) and rolled steel (thick steel plates, thin steel bars, or steel bars) were also investigated using the same samples and observation surfaces as described above. In each field of view, surface (mapping) analysis was performed at an acceleration voltage of 15 kV using an EDS (energy-dispersive X-ray analyzer) attached to a FE-SEM (field emission scanning electron microscope) to identify inclusions containing Ti and / or Nb. The largest equivalent circular diameter among the inclusions that satisfied equation (3) above was determined when the center of the identified inclusions (intersection of the major and minor axes) was measured using the EDS attached to the FE-SEM at an acceleration voltage of 15 kV and a magnification of 5000x. The results of the above survey are shown in Table 3. In the evaluation in Table 3, ○ means pass, and × means fail.

[0063] [Table 3]

[0064] In Table 3, Nos. 1-48 are examples of the present invention that satisfy all the requirements of this disclosure, and Nos. 101-111 are comparative examples. In the comparative examples, items and values ​​that do not meet the requirements of this disclosure are underlined. Examples No. 1 to 48 of this invention satisfy all the requirements of this disclosure, and therefore the central part of the cast slab contains 10.0 CaZrO3 particles / mm² with an equivalent circular diameter of 1.0 μm or more. 2 The above distribution was observed. Furthermore, 2.0 particles / mm³ were in direct contact with the NbTi carbonitride. 2 The above results were obtained. As a result, the equivalent circular diameter of the NbTi carbonitride in the central segregation region was kept below 50.0 μm.

[0065] Comparative Example No. 101 was an example where Ca was not added, and No. 104 was an example where Zr was not added. In both cases, CaZrO3 was not formed, so the NbTi carbonitride in the central segregation region was coarse, and the equivalent circle diameter exceeded 50.0 μm. Comparative Example No. 102 is an example where the Ca content was below the lower limit, and No. 105 is an example where the Zr content was below the lower limit. Comparative Example No. 103 is an example where the Ca content was above the upper limit, No. 106 is an example where the Zr content was above the upper limit, and No. 107 is an example where the Al content was above the upper limit. Nos. 110 and 111 are examples where ladle refining was performed without the separate addition of Al and Zr. As a result, the number density of CaZrO3 particles with an equivalent circle diameter of 1.0 μm or more distributed in the center of the cast slab was 10.0 particles / mm². 2 The number density of CaZrO3 in contact with NbTi carbonitride is also below 2.0 particles / mm³. 2 Because the temperature fell below a certain level, the NbTi carbonitrides coarsened, and the equivalent circle diameter exceeded 50.0 μm. Comparative Example No. 108 is an example where the Ti content exceeded the upper limit, and the number density of CaZrO3 particles with an equivalent circular diameter of 1.0 μm or more distributed in the center of the cast slab was 10.0 particles / mm². 2 Because the temperature fell below a certain level, the NbTi carbonitride material coarsened, and the equivalent circle diameter exceeded 50.0 μm. Comparative Example No. 109 is an example where the Nb content exceeded the upper limit, and the number density of CaZrO3 in contact with the NbTi carbonitride was 2.0 particles / mm³. 2 Because the temperature fell below a certain level, the NbTi carbonitride material coarsened, and the equivalent circle diameter exceeded 50.0 μm.

Explanation of Symbols

[0066] 10A, 10B Steel Cast Slabs, Rolled Steel Products 12A, 12B Main Surfaces 14A, 14B Cross Sections Parallel to the Main Surfaces and Passing through the Center in the Thickness Direction (Position at 1 / 2 of the Thickness) 20A Center in the Thickness Direction and Range of a Length of 1 / 100 of the Width from the Center of the Steel Cast Slab in the Width Direction 20B Range of a Length of 1 / 100 of the Diameter from the Center in the Diameter Direction

Claims

1. In mass percent, C: 0.02-0.30%, Si: 0.02-2.50%, Mn: 0.40-2.50%, Al: 0.001-0.080%, Ca: 0.0002-0.0050%, Zr: 0.0015-0.0650%, P: 0.020% or less, S: 0.020% or less, O: 0.0040% or less, N: Contains 0.0100% or less, and further, Ti: 0.005 to 0.050%, and Nb: 0.005-0.070%, It contains one or two selected from the group consisting of, A steel slab having a chemical composition in which the remainder is Fe and impurities, A cross-section parallel to the main surface of the steel casting, located at the center in the thickness direction and in the width direction, within a range from the center of the steel casting to a length of 1 / 100 of the width of the steel casting, CaZrO with an equivalent circular diameter of 1.0 μm or more ３ 10.0 pieces / mm ２ The above is true, and the CaZrO ３ Among them, CaZrO is one in which at least a portion of the outer circumference is in contact with a carbide, nitride, or carbonitride containing at least one of Nb and Ti. ３ 2.0 pieces / mm ２ The above is a steel casting.

2. The aforementioned chemical composition is further, in mass%, by substituting a portion of the Fe: Cr: 2.00% or less, Ni: 2.00% or less, Mo: 0.50% or less V: 0.20% or less, Cu: 0.50% or less, B: 0.0050% or less The steel slab according to claim 1, comprising one or more selected from the group consisting of the following.

3. In mass percent, C: 0.02-0.30%, Si: 0.02-2.50%, Mn: 0.40-2.50%, Al: 0.001-0.080%, Ca: 0.0002-0.0050%, Zr: 0.0015-0.0650%, P: 0.020% or less, S: 0.020% or less, O: 0.0040% or less, N: Contains 0.0100% or less, and further Ti: 0.005 to 0.050%, and Nb: 0.005-0.070%, It contains one or two selected from the group consisting of, A rolled steel material having a chemical composition in which the remainder is Fe and impurities, A cross-section parallel to the main surface of the rolled steel material, within the center in the thickness direction and within a range of 1 / 100 of the width of the rolled steel material from the center in the width direction, CaZrO with an equivalent circle diameter of 1.0 μm or more ３ is 10.0 pieces / mm ２ or more, and among the above-mentioned CaZrO ３ at least a part of the outer periphery of which is in contact with any one of carbides, nitrides, or carbonitrides containing at least one of Nb and Ti ３ is 2.0 pieces / mm ２ or more, and a rolled steel material

4. The aforementioned chemical composition is further, in mass%, by substituting a portion of the Fe: Cr: 2.00% or less, Ni: 2.00% or less, Mo: 0.50% or less V: 0.20% or less, Cu: 0.50% or less, B: 0.0050% or less The rolled steel material according to claim 3, comprising one or more selected from the group consisting of the following.

Images