JP2020084281A - steel sheet - Google Patents

Abstract

To provide a steel sheet less in the number of inclusions which may become breakage and origin during processing or using, excellent in cleanliness, and suitably usable even in applications requiring high strength.SOLUTION: A steel material for seamless steel pipe contains C, Si, Mn, Al, Ti, Cr, Ca, and REM in prescribed ranges, and the balance iron with impurities, in which P, S, O and N in the impurities are limited to prescribed values or less, 3 zones are differentiated by first threshold [REM_1] representing [REM] with the following (1) formula, and second threshold [REM_2] represented by the (2) formula, in which [M] is a mass ratio of each component element M in the steel, Ca amount [Ca] in the steel in each zone satisfies following (6) formula, (7) formula and (8) formula by using a1 represented by the following (3) formula, b represented by the (4) formula, c represented by the (5) formula, and number density of an inclusion with equivalent circle diameter of 1.0 μm or more in the steel is 100/mmor less.SELECTED DRAWING: None

Description

The present invention relates to a steel sheet mainly used as a material for automobile parts (for example, suspension parts such as stabilizers, safety-related parts such as chassis and side impact beams, etc.), and in particular, there are few inclusions and cleanliness is improved. It relates to an excellent steel plate.

Many steel materials are used for the above-mentioned automobile parts and the like, and further cleanliness is required for the steel plate as the material.
For example, underbody parts are required to have fatigue resistance and impact resistance against vibration and impact constantly applied during traveling. Therefore, it is important to reduce non-metallic inclusions, which are the starting points of fracture, as much as possible. As an example of the steel material for such an application, a steel plate which is a raw material of an electric resistance welded steel pipe processed into a hollow stabilizer can be mentioned.
Further, for the purpose of improving fuel efficiency and safety by reducing the weight of automobiles, the strength of steel materials used is increasing. The use ratio of so-called "HITEN" (tensile strength of 590 MPa or more) and "super-HITEN" (980 MPa or more) is increasing. Originally, automobile parts often have complicated shapes, and the higher the strength, that is, the more difficult the steel sheet is to be deformed, the higher the frequency of occurrence of cracks due to the inclusion becoming a fracture starting point during processing. High cleanliness in high strength steel sheets is a major issue.
As described above, inclusions cause various problems during use of the final product or during parts manufacturing and processing, and therefore, improvement in cleanliness of steel sheets is strongly demanded.

Among the inclusions, MnS, which greatly stretches during rolling and sometimes reaches a length in the rolling direction of several hundreds of μm, is regarded as a particular problem. In order to prevent the formation of coarse MnS in a slab, conventionally, molten steel is treated with Ca to suppress MnS production mainly by fixing S as CaS, or the slab is subjected to unsolidified light reduction in the casting process. Measures such as reducing the center segregation of are taken.
However, in order to increase the strength of the above-mentioned steel materials, in recent years, the adoption of steel types having an increased Mn amount has been increasing, and for example, Mn-B steel “26MnB5” (example of component range, unit is% by mass/C: 0.23 to 0.28%, Si: 0.15 to 0.35%, Mn: 1.10 to 1.40%, B: 0.0015 to 0.0040%) are used as materials for automobile parts. Is increasing. Since the above-mentioned MnS is likely to be generated as the Mn content increases, the reduction of MnS has become an important issue.

Therefore, various techniques have heretofore been proposed in order to improve the cleanliness of the steel sheet and improve the material properties.
For example, in Patent Document 1, the contents of S, Ca, and REM are specified to reduce the amount of S that binds to Mn, thereby reducing the stretched MnS and the equivalent circle diameter of 1.0 μm or more. The number density of inclusions is specified to be 200 pieces/mm ² or more and 2000 pieces/mm ² or less.
Further, in Patent Document 2, in order to set the major axis of inclusions elongated in the rolling direction to 50 μm or less, when Ca is contained in one kind, when REM is contained in one kind, two kinds of Ca and REM are contained in combination. In this case, the contents of Ca and REM are specified respectively.
Further, in Patent Document 3, TiS particles having a particle size of 10 μm or more and MnS particles having a particle size of 10 μm or more have a cleanliness of 0.1% or less (provided that they are obtained by a point calculation method according to JIS G 0555 (however, , Including 0%).

JP 2012-224915 A JP, 2008-081823, A Republished WO 2017/056384

By the way, in recent years, for the purpose of reducing the weight of automobiles and the like, further strengthening has been demanded for steel sheets which are raw materials of automobile parts. With the increase in strength of steel sheets, even the size and amount of inclusions, which have not been a problem in the past, are likely to become the starting point of fracture. Therefore, the importance of reducing inclusions (cleaning countermeasures) including inclusions other than MnS is increasing.
The total oxygen concentration (TO) of steel components is widely used as a control value as a measure of the amount of oxides that occupy most of the inclusions. T. If the O value is high, it can be considered that the amount of oxides in the steel is large. O must be properly managed.
Here, in the above-mentioned patent documents 1-3, T.I. O was not taken into consideration, and the number of problematic inclusions could not be sufficiently reduced.

The present invention has been made in view of the above-mentioned circumstances, and has a small number of inclusions that can be a starting point of breakage during processing or use, is excellent in cleanliness, and is also suitable for applications requiring high strength. The purpose is to provide a steel sheet that can be used for.

In order to solve the above problems, the inventors of the present invention have earnestly studied, and as a result, have obtained the following findings.
Inclusions stretched in the rolling direction are notably harmful because their tips are notched. Even non-stretched inclusions, which are coarse, are highly harmful and it is important to reduce them. Examples of the stretched inclusions include low melting point oxides in addition to MnS described above.

The low-melting point oxide generally refers to an oxide that is in a liquid phase in molten steel (standardly assumed to be 1600° C.). Although it is a solid phase at the rolling temperature, it easily deforms and stretches because of its low melting point. Although the degree of stretching (aspect ratio) is lower than that of MnS, it is becoming a problem for high strength steel sheets. For example, an example in which CaO—Al ₂ O ₃ -based oxide having a low melting point composition is generated during Ca treatment and stretched is observed. As a means for avoiding a low melting point composition, [T. O] is regulated/controlled, or given [T. It is effective to control [Ca] according to O] to control the oxide to have a high melting point composition.

As the non-stretched inclusions, sulfides such as TiS and (carbon)nitrides such as Ti(C)N may be generated depending on the steel type and application, but in the present invention, oxidation that is the main inclusion is Assuming things, we will explain. Conventionally, with respect to non-stretched inclusions, first of all, size has been regarded as a problem, and measures to remove coarse inclusions (oxides) have been emphasized. However, due to the strength of steel sheets and the emphasis on safety, there is a demand for further reduction (cleaning) even in the size and the number (quantity) which have not been regarded as problems in the past. Therefore, in order to float and remove the oxide from the molten steel, it is widely practiced to extend the secondary refining stirring time. The total amount of oxides in steel [T. O] is an index. For example, in the case of bearing steel that is particularly required to have high cleanliness, [T. [O.] is strictly controlled, and [T. O] steadily decreases.

The present invention has been made based on the above findings, and the steel sheet according to the present invention is mass%,
C: more than 0.05% and less than 0.48%,
Si: 0% or more and 0.60% or less,
Mn: 0.40% or more and 2.0% or less,
Al: 0.003% or more and 0.080% or less,
Ti: 0% or more and 0.060% or less,
Cr: 0% or more and 0.70% or less,
Ca: 0.0003% or more and 0.0050% or less,
REM: 0.0003% or more and 0.0050% or less,
And the balance consists of iron and impurities, of which P, S, O and N are
P: 0.020% or less,
S: 0.0034% or less,
O: 0.0040% or less,
N: 0.0075% or less,
Limited to
When the mass ratio of each component element M in steel is represented by [M], [REM] is a first threshold value [REM_1] represented by the following equation (1) and a second threshold value represented by the equation (2). It is divided into three regions by [REM_2], and in each region, the amount of Ca [Ca] in the steel is shown in the following equation (3), a in the equation (4), and in the equation (5). Using c, the following formulas (6), (7), and (8) are satisfied, and the number density of inclusions in the steel having a circle equivalent diameter of 1.0 μm or more is 100/mm. It is characterized by being ² or less.
Formula (1): [REM_1]=−0.0161×[Al]−0.2030×[S]+[O]+0.0008
Formula (2): [REM_2]=0.0139×[Al]−0.0791×[S]+1.6×[O]−0.0007
Formula (3): a=2.5765×[Al]+265.56×[S]-266.33×[O]+0.2621
Formula (4): b=0.01469×[Al]−0.0399×[S]+[O]−0.0004
Formula (5): c=0.070×[Al]+0.0951×[S]+0.4×[O]−0.0003
Formula (6): When [REM]<[REM_1], [Ca]≧a×[REM]+b
(7) Formula: [REM] [[REM]<[REM_2] [Ca]≧a×[REM_1]+b
Formula (8): [REM_2]≦[REM]≦0.0050%, [Ca]≧c

According to the steel sheet having such a configuration, as described above, the T.I. Since the relationship between REM and Ca is defined in consideration of O, even if Ca or REM forms an oxide, the amount of Ca or REM that binds to S is secured and MnS-based inclusions are reduced. It becomes possible.
Further, Ca in steel, REM, T. Al, T.A. Since the relationship between the amounts of O and S is defined as described above, the ratio of Al ₂ O ₃ in the inclusions can be reduced and the formation of low melting point oxide can be suppressed. Further, by adding Ca and REM in combination, and making the main composition of the oxide a ternary system containing REM ₂ O ₃ , an Al ₂ O ₃ —CaO—REM ₂ O ₃ system, the crushability during rolling is improved. And the size after rolling becomes finer, so that the harmfulness can be reduced.
Furthermore, since the number density of inclusions having an equivalent circle diameter of 1.0 μm or more is defined to be 100 pieces/mm ² or less, the number of inclusions that are the starting points of fracture is sufficiently reduced.
Therefore, it is possible to provide a high-quality steel plate in which the occurrence of cracks and the like during use and processing is suppressed.

Here, in the steel sheet of the present invention, further in mass %,
Cu: 0% to 0.05%,
Nb: 0% to 0.05%,
V: 0% to 0.05%,
Mo: 0% to 0.05%,
Ni: 0% to 0.05%,
B: 0% or more and 0.0050% or less,
The configuration may include one or two or more selected from the group consisting of
In this case, various properties of the steel sheet can be improved by further containing the above-mentioned elements.

Further, in the steel sheet of the present invention, when the mass ratio of each compound MX in the inclusions present inside is (MX), it is preferable to satisfy the following expression (9) or expression (10). ..
Formula (9): (CaO)/(Al ₂ O ₃ )≧1.35
Formula (10): (REM ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≧0.45
In this case, since the composition of the inclusions is defined as described above, it is possible to suppress lowering of the melting point of the inclusions, and it is possible to further reduce the number of stretched inclusions caused by the low melting point oxide. Become.

As described above, according to the present invention, a steel sheet that has a small number of inclusions that can be the starting point of fracture during processing or use, is excellent in cleanliness, and can be suitably used even in applications requiring high strength. It becomes possible to provide.

It is a graph which shows the generation|occurrence|production state of the stretch inclusion in a lab experiment. [T. It is a graph which shows the result of having stratified by [O]. (A) is [T. O]: 0.0005% or more and less than 0.0012%, (b) is [T. O]: 0.0012% or more and less than 0.0017%, (c) is [T. O]: 0.0017% or more and less than 0.0025% It is a graph which shows the conditions which suppress the production|generation of the stretch inclusion in a lab experiment. 5 is a graph showing the relationship between the number density of drawn inclusions and Charpy absorbed energy. 7 is a graph showing the relationship between the number density of inclusions having a circle equivalent diameter≧1.0 μm and the number density of stretched inclusions.

Hereinafter, a steel plate which is an embodiment of the present invention will be described with reference to the accompanying drawings. The present invention is not limited to the embodiments below.

The steel sheet according to the present embodiment is, in mass %, C: more than 0.05% and less than 0.48%, Si: 0% or more and 0.60% or less, Mn: 0.40% or more and 2.0% or less, Al : 0.003% to 0.080%, Ti: 0% to 0.060%, Cr: 0% to 0.70%, Ca: 0.0003% to 0.0050%, REM: 0 0.003% or more and 0.0050% or less, and the balance is composed of iron and impurities. Further, P, S, O, and N among the impurities are P: 0.020% or less and S : 0.0034% or less, O: 0.0040% or less, N: 0.0075% or less.

Then, when the mass ratio of each component element M in the steel is represented by [M], [REM] is represented by a first threshold value [REM_1] represented by the following equation (1) and a second threshold represented by the equation (2). Two regions are distinguished by three threshold values [REM_2], and in each region, the amount of Ca in steel [Ca] is represented by the following equation (3), a represented by the equation (4), b represented by the equation (5). It is defined so as to satisfy the following expressions (6), (7), and (8) by using c shown in FIG.
Formula (1): [REM_1]=−0.0161×[Al]−0.2030×[S]+[O]+0.0008
Formula (2): [REM_2]=0.0139×[Al]−0.0791×[S]+1.6×[O]−0.0007
Formula (3): a=2.5765×[Al]+265.56×[S]-266.33×[O]+0.2621
Formula (4): b=0.01469×[Al]−0.0399×[S]+[O]−0.0004
Formula (5): c=0.070×[Al]+0.0951×[S]+0.4×[O]−0.0003
Formula (6): When [REM]<[REM_1], [Ca]≧a×[REM]+b
(7) Formula: [REM] [[REM]<[REM_2] [Ca]≧a×[REM_1]+b
Formula (8): [REM_2]≦[REM]≦0.0050%, [Ca]≧c

Further, in the steel sheet according to the present embodiment, the number density of inclusions having a circle equivalent diameter of 1.0 μm or more contained in the steel is 100 pieces/mm ² or less.

In the steel sheet of the present embodiment, if necessary, further, in mass%, Cu: 0% or more and 0.05% or less, Nb: 0% or more and 0.05% or less, V: 0% or more and 0% 0.05% or less, Mo: 0% or more and 0.05% or less, Ni: 0% or more and 0.05% or less, B: 0% or more and 0.0050% or less, or one or more kinds selected from the group consisting of May be included.

Further, in the steel sheet according to the present embodiment, when the mass ratio of each compound MX in the inclusions present inside is (MX), the following formula (9) or formula (10) is satisfied: May be
Formula (9): (CaO)/(Al ₂ O ₃ )≧1.35
Formula (10): (REM ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≧0.45

Hereinafter, the reasons for defining each component as described above in the steel sheet according to the present embodiment will be described.

(C: more than 0.05% and less than 0.48%)
C (carbon) is an important element for ensuring the strength (hardness) of the steel sheet. By setting the C content to exceed 0.05%, the strength of the steel sheet suitable for the application and the processing method is secured. On the other hand, if the C content is 0.48% or more, the strength becomes too high, and the heat treatment for ensuring the workability requires a long time, so the workability of the steel sheet may deteriorate unless the heat treatment is extended. .. Therefore, the C content is controlled within the range of more than 0.05% and less than 0.48%.

(Si: 0% or more and 0.60% or less)
Si (silicon) acts as a deoxidizing agent and is an element effective for enhancing hardenability and improving the strength (hardness) of the steel sheet, while the steel sheet is caused by scale flaws during hot rolling. May deteriorate the surface quality of For example, in a component such as a power transmission shaft to which a very large torsional stress is applied, minute unevenness on the surface may be a starting point of crack generation, which directly affects safety and may not be intentionally added. In other normal use, 0.10% or more is often added to obtain the above effect. However, even in normal use, if it exceeds 0.60%, the deterioration of the surface properties of the steel sheet due to scale flaws during hot rolling becomes remarkable. Therefore, the Si content is controlled within the range of 0% to 0.60%.
When intentionally added, the lower limit of the Si content is preferably 0.10% or more, and the upper limit of the Si content is preferably 0.55% or less.

(Mn: 0.40% or more and 2.0% or less)
Mn (manganese) is an element that acts as a deoxidizing agent, and is an element that is effective in enhancing the hardenability and improving the strength (hardness) of the steel sheet. If the Mn content is less than 0.40%, the effect cannot be sufficiently obtained. On the other hand, if the Mn content exceeds 2.0%, the workability of the steel sheet may deteriorate. Therefore, the Mn content is controlled within the range of 0.40% or more and 2.0% or less.
The lower limit of the Mn content is preferably 0.90% or more, and the upper limit of the Mn content is preferably 1.65% or less.

(Al: 0.003% or more and 0.080% or less)
Al (aluminum) is an element that acts as a deoxidizer, and is an element that is effective for improving the workability of the steel sheet by fixing N. If the Al content is less than 0.003%, the above effect cannot be sufficiently obtained, so 0.003% or more must be contained. On the other hand, if the Al content exceeds 0.080%, the above-described content effect is saturated, and coarse inclusions increase. The coarse inclusions may deteriorate workability or may easily cause surface defects. Therefore, the Al content is controlled within the range of 0.003% to 0.080%.
The lower limit of the Al content is preferably 0.020% or more, and the upper limit of the Al content is preferably 0.050% or less.

(Ti: 0% or more and 0.060% or less)
Since Ti (titanium) has the effect of increasing the strength by forming carbonitride, it may be contained in the range of 0.060% or less, if necessary. On the other hand, if the Ti content exceeds 0.060%, coarse angular carbonitrides are likely to be formed, resulting in the deterioration of workability. Therefore, the Ti content is limited to 0.060% or less. The lower limit of the Ti content may be 0%. Further, considering the current general refining (including secondary refining), the lower limit of the Ti content may be 0.0005% or more.

(Cr: 0% or more and 0.70% or less)
Cr (chromium) is an element effective for enhancing the hardenability and improving the strength (hardness) of the steel sheet. Therefore, if necessary, Cr may be contained within the range of 0.70% or less. Further, when the lower limit of the Cr content is 0.10%, the above effect can be preferably obtained. When the Cr content exceeds 0.70%, the cost is increased, but the effect of inclusion is saturated. Therefore, the Cr content is controlled to 0.70% or less.

(Ca: 0.0003% or more and 0.0050% or less)
Ca (calcium) is an element effective for reducing MnS and controlling the form of inclusions, thereby improving the workability of the steel sheet. If the Ca content is less than 0.0003%, the above effect cannot be sufficiently obtained. On the other hand, when the Ca content exceeds 0.0050%, the oxides and/or CaS-based inclusions become coarse, which may deteriorate the workability of the steel sheet. Further, if the Ca content exceeds 0.0050%, the nozzle refractory material is likely to be melted and damaged, and the continuous casting operation may become unstable. Therefore, the Ca content is controlled within the range of 0.0003% or more and 0.0050% or less.
The lower limit of the Ca content is preferably 0.0010% or more, and the upper limit of the Ca content is preferably 0.0035% or less.

(REM: 0.0003% or more and 0.0050% or less)
REM (Rare Earth Metal) means a rare earth element, and is 17 elements of scandium Sc (atomic number 21), yttrium Y (atomic number 39) and lanthanoid (15 elements from lanthanum with atomic number 57 to lutecium with atomic number 71). Is a general term for. The steel sheet according to the present embodiment contains at least one element selected from these. In general, REM is often selected from Ce (cerium), La (lanthanum), Nd (neodymium), Pr (praseodymium), etc. because of its easy availability. As a method of addition, for example, addition as a misch metal which is a mixture of these elements into steel is widely performed. The main components of misch metal are Ce, La, Nd, and Pr. In the steel sheet according to the present embodiment, the total amount of these rare earth elements contained in the steel sheet is the REM content. In this embodiment, since the average atomic weight of misch metal is about 140, the atomic weight of REM is 140.

REM is an element effective in reducing MnS, controlling the form of inclusions, and improving the workability of a steel sheet. If the REM content is less than 0.0003%, the above effect cannot be sufficiently obtained. On the other hand, if the REM content exceeds 0.0050%, nozzle clogging during continuous casting is likely to occur. Further, when the REM content exceeds 0.0050%, the number density of the REM-based inclusions (oxides and oxysulfides) generated becomes relatively high, so the lower surface side of the slab that curves during continuous casting of the slab. These REM-based inclusions are deposited. This may cause internal defects in the product obtained by rolling the slab and further deteriorate the workability of the steel sheet. Therefore, the REM content is controlled within the range of 0.0003% or more and 0.0050% or less.
The lower limit of the REM content is preferably 0.0010% or more, and the upper limit of the REM content is preferably 0.0030% or less.

Further, the contents of Ca and REM are set to S, T. O, T. It is necessary to control according to the content of Al. Specifically, it is necessary to control the content of each element in the chemical components expressed in mass% to a range represented by the above formulas (1) to (8). The reasons for the expressions (1) to (8) will be described later.

The steel sheet according to the present embodiment contains impurities in addition to the above basic components. Here, the impurities are P, S, O, N, Cd, Zn, Sb, W, Mg, Zr, As, Co, Sn, Pb, etc., which are mixed in from the raw materials such as scraps and the manufacturing process. Means the element. Since the content of these elements is not essential, the lower limit of the content of these elements is 0%. Among these, P, S, O, and N are limited as follows in order to exert the above effects favorably. Further, it is preferable to limit the above-mentioned impurities other than P, S, O, and N to 0.01% or less. However, even if these impurities are contained in an amount of 0.01% or less, the above effects are not lost. Here, the stated% is mass %.

(P: 0.020% or less)
P (phosphorus) has a function of solid solution strengthening. However, the inclusion of an excessive amount of P hinders the workability of the steel sheet. Therefore, the P content is limited to 0.020% or less. The lower limit of the P content may be 0%. Further, considering the current general refining (including secondary refining), the lower limit of the P content may be 0.005% or more.

(S: 0.0034% or less)
S (sulfur) is an impurity element that impairs the workability of the steel sheet by forming non-metallic inclusions mainly containing MnS. Therefore, the S content is limited to 0.0034% or less, preferably 0.0020% or less. The lower limit of the S content may be 0%. Further, considering the current general refining (including secondary refining), the lower limit of the S content may be 0.0003% or more.

(O: 0.0040% or less)
O (oxygen) forms an oxide (non-metallic inclusion), and when the oxide aggregates and coarsens, and depending on the composition of the oxide, it is stretched during rolling to improve the workability of the steel sheet. It is an impurity element that lowers. Therefore, the O content is limited to 0.0040% or less. The lower limit of the O content may be 0%. Further, considering the current general refining (including secondary refining), the lower limit of the O content may be 0.0005% or more. The O content of the steel sheet according to the present embodiment is the total O content (TO content) that is the sum of all O contents such as O dissolved in steel and O existing in inclusions. Means

T. Since the O content greatly affects the composition of the oxide and the total amount of the oxide, it is very important to control the content. Therefore, T. O is contained together with S and Al in the above formulas (1) to (8) that define the contents of Ca and REM. Qualitatively, in order to reduce the drawn oxide (for the same combination of REM and Ca content) and to reduce the total oxide content, T.S. It is preferable to reduce O. T. In order to reduce O, for example, the stirring time of the secondary refining is extended to promote the floating removal of inclusions.

(N: 0.0075% or less)
N (nitrogen) is an impurity element that forms a nitride (non-metallic inclusion) and reduces the workability of the steel sheet. Therefore, the N content is limited to 0.0075% or less. The lower limit of the N content may be 0%. Further, considering the current general refining (including secondary refining), the lower limit of the N content may be 0.0010%.

In the steel sheet according to the present embodiment, the above basic components are controlled, and the balance is iron and the above impurities. However, in the steel sheet according to the present embodiment, in addition to this basic component, the following selective components may be contained in the steel, if necessary, instead of part of the balance Fe.

That is, the hot-rolled steel sheet according to the present embodiment may further contain at least one of Cu, Nb, V, Mo, Ni, and B as a selective component in addition to the basic components and impurities described above. Good. The numerical range of selection components and the reason for the limitation are described below. Here, the stated% is mass %.

(Cu: 0% to 0.05%)
Cu (copper) is a selective element having the effect of improving the strength (hardness) of the steel sheet. Therefore, Cu may be contained within the range of 0.05% or less, if necessary. Further, when the lower limit of the Cu content is 0.01% or more, the above effect can be preferably obtained. On the other hand, if the Cu content exceeds 0.05%, hot work cracking may occur during hot rolling due to molten metal embrittlement (Cu cracking). In addition, the preferable range of Cu content is 0.02% or more and 0.04% or less.

(Nb: 0% to 0.05%)
Nb (niobium) is a selective element that forms carbonitrides and is effective in preventing the coarsening of crystal grains and improving the workability of steel sheets. Therefore, Nb may be contained in the range of 0.05% or less, if necessary. Further, when the lower limit of the Nb content is 0.01% or more, the above effect can be preferably obtained. On the other hand, if the Nb content exceeds 0.05%, coarse Nb carbonitrides may precipitate and the workability of the steel sheet may deteriorate. In addition, the preferable range of Nb content is 0.02% or more and 0.04% or less.

(V: 0% or more and 0.05% or less)
V (vanadium) forms a carbonitride similar to Nb, and is a selective element effective for preventing coarsening of crystal grains and improving workability. Therefore, V may be contained within the range of 0.05% or less, if necessary. Further, when the lower limit of the V content is 0.01% or more, the above effect can be preferably obtained. On the other hand, if the V content exceeds 0.05%, coarse V carbonitrides may be generated, and the workability of the steel sheet may be deteriorated. In addition, the preferable range of V content is 0.02% or more and 0.04% or less.

(Mo: 0% to 0.05%)
Mo (molybdenum) is a selective element that has the effect of improving the strength (hardness) of the steel sheet by improving the hardenability and the temper softening resistance. Therefore, Mo may be contained within the range of 0.05% or less, if necessary. Further, when the lower limit of the Mo content is 0.01% or more, the above effect can be preferably obtained. On the other hand, when the Mo content exceeds 0.05%, the cost increases and the effect of inclusion is saturated. Furthermore, if the Mo content exceeds 0.05%, the workability of the steel sheet, especially the cold workability, deteriorates, which makes it difficult to process the steel sheet into a complicated shape (for example, a gear shape). .. For the above reasons, the upper limit of the Mo content is set to 0.05%. In addition, the preferable range of Mo content is 0.01% or more and 0.05% or less.

(Ni: 0% to 0.05%)
Ni (nickel) is a selective element effective for improving the strength (hardness) of the steel sheet by improving the hardenability and for improving the workability. Further, it is a selective element which also has an effect of preventing molten metal embrittlement (Cu cracking) when Cu is contained. Therefore, Ni may be contained in the range of 0.05% or less, if necessary. Further, when the lower limit of the Ni content is 0.01% or more, the above effect can be preferably obtained. On the other hand, if the Ni content exceeds 0.05%, the cost increases, but the effect of inclusion is saturated, so the upper limit of the Ni content is made 0.05%. In addition, the preferable range of Ni content is 0.02% or more and 0.05% or less.

(B: 0% or more and 0.0050% or less)
B (boron) is a selective element that has the effect of enhancing the hardenability and improving the strength (hardness) of the steel sheet. Therefore, B may be contained in the range of 0.0050% or less, if necessary. Further, when the lower limit of the B content is 0.0008% or more, the above effect can be preferably obtained. On the other hand, if the B content exceeds 0.0050%, a B-based compound is formed and the workability of the steel sheet is reduced, so the upper limit is made 0.0050%. In addition, the preferable range of B content is 0.0015% or more and 0.0040% or less.

Next, the results of a laboratory experiment in which the knowledge for defining the number of inclusions and the expressions (1) to (10) are obtained as described above will be described.

In a vacuum melting furnace, in mass %, C: 0.25% or more and 0.28% or less, Si: 0.22% or more and 0.27% or less, Mn: 1.20% or more and 1.31% or less, P: 0.006% or more and 0.008% or less, S: 0.0017% or more and 0.0023% or less, T.I. Al: 0.022% to 0.028%, Cr: 0.10% to 0.17%, Ti: 0.022% to 0.026%, B: 0.0022% to 0.0033% Hereinafter, T. O: A plurality of types of molten steel containing 0.0005% or more and 0.0025% or less were melted, and REM and Ca were added in different amounts to produce a 50 kg ingot. For REM, a misch metal containing Ce, La and Nd was added. These ingots were hot-rolled to a thickness of 5 mm at a finish rolling temperature of 920° C. and air-cooled to obtain hot-rolled steel sheets. In addition, each component value described in the present invention described above and thereafter is not an addition amount but an analysis value (content) of the steel sheet.

In the present embodiment, the content in steel and the composition of inclusions are described, but all units are expressed in mass %. The content in steel is expressed as [M] with the element symbol M sandwiched between rectangular brackets. The content of the compound MX in the inclusion is represented by (MX) by sandwiching the compound MX between parentheses. For example, the content of Ca in steel is expressed as [Ca]. Further, the content of CaO in the inclusions is expressed as (CaO).

The cross section (L section) parallel to the rolling direction and the plate thickness direction of the hot rolled steel sheet was used as an observation surface, and the inclusions in the hot rolled steel sheet were magnified 400 times with an optical microscope (however, the shape of the inclusions was measured in detail. At that time, the magnification was 1000 times, and the observation was performed in a total of 60 visual fields. In each observation visual field, inclusions having a particle size (in the case of spherical inclusions) or a long diameter (in the case of deformed inclusions) of 1 μm or more are observed, and these inclusions are expressed as (long diameter)/( The aspect ratio calculated by (minor axis) was classified into those having an aspect ratio of 3.0 or less and those having an aspect ratio of more than 3.0, and the number densities thereof were measured. Inclusions number density of aspect ratio> 3.0, "inferior" samples exceeded 6 / mm ^2, and the six / mm ² or less was evaluated as "good".

Further, an SEM (scanning electron microscope, Scanning Electron Microscope) using an SEM (scanning electron microscope) equipped with EPMA (electron probe micro analysis, Electron Probe Micro Analysis) or EDX (energy dispersive X-ray analysis, Energy Dispersive X-Ray Analysis) is used. The inclusions in the steel sheet were analyzed.

Then, the Charpy impact value at room temperature (20° C.) was measured as an index of the toughness of the hot rolled steel sheet obtained above. The Charpy test piece is a subsize sampled from the C direction of the steel sheet. That is, the test piece length 55 mm was taken as the width direction of the steel sheet, the test piece height was taken as the steel sheet longitudinal (rolling) direction 10 mm, and the test piece width was taken as the steel sheet thickness direction 2.5 mm. A 2 mm V notch was machined on a surface of 55 mm length×2.5 mm width. When a test piece is sampled in this direction, the L cross section of the steel plate becomes a fracture surface, so the influence of the stretched inclusions can be easily evaluated. The Charpy impact value decreases as the number of inclusions at the starting point of fracture in the steel sheet increases, and the size of the inclusions (stretching length in the rolling direction) increases, that is, the cleanliness of the steel sheet deteriorates. To do.

Composition of inclusions observed in the steel _{sheet, MnS,} CaS, an _{Al 2 O 3 -CaO-REM 2} O 3 oxide. In addition to the case where each of the above-mentioned three types is present alone, for example, when MnS or CaS is attached to a part of the periphery of the Al ₂ O ₃ —CaO—REM ₂ O ₃ -based oxide, a plurality of types are present. The phases may be mixed.

First, the results of investigation of the cleanliness of hot rolled steel sheets will be described. FIG. 1 shows the slab content of Ca and REM and the generation state of stretched inclusions. In FIG. 1, “◯” indicates a good material having an aspect ratio of> 3.0 and having 6 or less inclusions/mm ² , and “x” in FIG. 1 indicates a poor material having more than 6/mm ^2. ..
Here, in FIG. 1 in which all the samples are plotted, the good material and the poor material are mixed, and the boundary condition for preventing the formation of harmful stretch inclusions is unclear.

As a result of analyzing the inclusion composition stretched with the aspect ratio exceeding 3.0 by EDS attached to the SEM, (i) the case where only MnS was stretched, and (ii) Al ₂ O ₃ -CaO-REM ₂ O. ₃ type oxide with low melting point composition (MnS and CaS may adhere to a part of the surroundings. From the EDS analysis results, the MnS and CaS adhering to the surroundings are separated from the oxide to determine the composition. It can be calculated.)

The latter (ii) the composition result of detailed analysis of the ratio of the inclusions in (CaO) and _{(Al 2 O 3) (CaO} ) / (Al 2 O 3) < 1.35, and, (REM ₂ It was found that the oxide having O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}<0.45 stretched to the aspect ratio>3.0.
On the other hand, (CaO)/(Al ₂ O ₃ )≧1.35 or (REM ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≧0 If it was 0.45, the aspect ratio of the oxide was 3.0. It is considered that as a result of the melting point of the oxide becoming higher, it becomes difficult to stretch during rolling.

The composition of inclusions (CaO), (Al ₂ O ₃ ), and (REM ₂ O ₃ ) can be calculated in consideration of the mass balance of the elements based on the inclusion analysis result by the EDS attached to the SEM. .. The EDS analysis result is output in mass% for each element. In the following description, the content of the element M detected from inclusions will be referred to as (M).
From each of (Ca), (Al), and (REM), the amount of oxide may be calculated using the atomic weight, and the formulas (9) and (10) may be calculated. For example, (Al ₂ O ₃ ) can be calculated by (27×2+16×3)/(27×2)×(Al) using the atomic weight 27 of Al and the atomic weight 16 of oxygen. The inclusions were observed by SEM and it was confirmed that AlN was not generated in the inclusions (since it has a characteristic square shape, it can be easily identified by SEM observation. Here, it affects the composition of inclusions. The problem is the formation of micron-order or submicron-order size AlN. The formation of smaller AlN precipitates does not affect the inclusion composition.).
Basically, by the same method, (CaO) can be obtained from (Ca) and (REM ₂ O ₃ ) can be obtained from (REM). Here, (REM) is the total amount of detected rare earth elements, specifically, the total amount of Ce, La, and Nd detected in this experiment is (REM).

Since Ca and REM form sulfides in addition to oxides, in consideration of the mass balance of S, Ca and REM that combine with S to form sulfides are subtracted to form oxides. Care should be taken in determining the amounts of Ca and REM.
Specifically, first, the amount of S forming MnS is calculated from 32/55×(Mn) using the atomic weights of Mn and S, and is excluded from the total amount of S. This is because the present experiment is an Al deoxidized steel, and therefore Mn detected from inclusions can be regarded as detected from MnS. This remaining amount of S is the amount of S forming CaS, REM ₂ O ₂ S (oxysulfide), and REMS (a sulfide in which REM atoms and S atoms are combined at a molar ratio of 1:1). Thermodynamically, CaS is most likely to be produced, REM ₂ O ₂ S is produced next, and finally REMS is produced. In this order, the amount of sulfide or oxysulfide produced may be calculated in consideration of the mass balance of S.
The residual amount obtained by subtracting Ca and REM forming sulfides and oxysulfides thus obtained from their respective total amounts is the amount of Ca and REM forming oxides. (CaO) and (REM ₂ O ₃ ) may be calculated using the atomic weights of Ca, REM, and O. In many cases, S was present as MnS and CaS and was not bound to REM on the mass balance. That is, Ca was often present as CaS and CaO, and REM was often present as an oxide. In particular, in the case of a large amount of S, there was an example in which REM ₂ O ₂ S or REMS was generated.

The oxide composition is [T. O]. Therefore, T. By arranging the data for each O, it was found that it is possible to clarify the condition (hereinafter referred to as the generation prevention condition) that can prevent the number of stretched inclusions having an aspect ratio>3.0 to 6 or less/mm ² (FIG. 2). The symbols in the figure are the same as in FIG. 1, ◯: the number of stretched inclusions having an aspect ratio>3.0≦6/mm ² (generation prevention range), Δ: the number of stretched inclusions having an aspect ratio>3>6/mm ² is shown.
The generation prevention condition (boundary line) is divided into three regions, region I, region II, and region III, according to [REM] (FIG. 3).

(Region I)
The region I is a region where [REM]<[REM_1] and the [REM] is low. This region I is a region in which the lower limit [Ca] needs to be increased as [REM] increases.
First, consider the case where [Ca] is increased at a certain content with a low [REM]. Initially, both [REM] and [Ca] are low, so MnS is generated and stretched during rolling. As [Ca] increases, S bonds with Ca, MnS decreases, and generation can be prevented in due course. On the other hand, an Al ₂ O ₃ —CaO—REM ₂ O ₃ type oxide is generated, but when [REM] is low, it is a low-melting point oxide mainly composed of Al ₂ O ₃ —CaO type, and therefore is stretched during rolling. .. In this way, the apparent exchange between MnS and the low melting point oxide occurs. When [Ca] is further increased, the melting point of the oxide is increased, so that the degree of stretching of the oxide is reduced. Next, consider the case where [REM] is increased from this state (state described in the preceding sentence, in which the lower limit [Ca] is contained and the oxide melting point is increased). When [REM] increases, in the region I, low-melting-point composition inclusions that stretch again during rolling are generated. The reason for this is that the amount of effective Ca that binds to S increases with an increase in [REM], which is a strong deoxidizing element, so that (CaO) in the Al ₂ O ₃ —CaO—REM ₂ O ₃ -based oxide increases. This is because the melting point of the oxide decreases as a result of the decrease. Therefore, it is necessary to increase [Ca] in order to increase the oxide melting point and prevent stretching. Thus, in the region I, when [REM] increases, the [Ca] in steel required for preventing low melting point oxide, that is, the lower limit [Ca] increases.

In this way, in the region I, the composition of the oxide in the generated inclusions containing the lower limit [Ca] or more defined by the formula (6) is represented by the formula (9):(CaO)/( Since Al ₂ O ₃ )≧1.35 is satisfied, the composition of the low melting point oxide can be avoided, and the stretching during rolling can be prevented.
When the formula (6) is not satisfied, an Al ₂ O ₃ —CaO—REM ₂ O ₃ -based low-melting point oxide that does not satisfy the formula (9) is generated, so that this is easily stretched during rolling and the aspect ratio>3. A large number of inclusions having a value of 0.0 are generated, resulting in inferior cleanliness of the steel sheet.

(Area II)
The area II is in the range of [REM_1]≦[REM]<[REM_2], and the lower limit [Ca] defined by the equation (7) is a high area. The range of [Ca] in the steel where low melting point oxides are formed is the widest. Within this region, there is almost no [REM] effect on the lower limit [Ca] for preventing the low melting point oxide, so the lower limit [Ca] is constant.
In the region II, the composition of the oxide in the inclusions generated by containing the lower limit [Ca] or more defined by the formula (7) is expressed by the formula (9): (CaO)/(Al ₂ O ₃ )≧ Satisfies 1.35.
If the formula (7) is not satisfied, an Al ₂ O ₃ —CaO—REM ₂ O ₃ -based low-melting point oxide that does not satisfy the formula (9) is generated, so that it easily stretches during rolling, and the aspect ratio>3. A large number of inclusions having a value of 0.0 are generated, resulting in inferior cleanliness of the steel sheet.

(Region III)
The region III is a region where [REM_2]≦[REM]≦0.0050%, and the lower limit [Ca] defined by the equation (8) is relatively low and constant. In [REM_2]≦[REM], (REM ₂ O ₃ ) in the Al ₂ O ₃ —CaO—REM ₂ O ₃ system oxide increases and becomes 45% or more, so (Al ₂ O ₃ ) and ( The melting point of the oxide is high irrespective of CaO) and it is difficult to stretch during rolling. That is, it is a region where the low melting point oxide is not generated. Therefore, [Ca] required to prevent MnS is the lower limit [Ca] of this region. In this region, since it is high [REM], in steel [T. O] mainly binds to REM (and Al), and the effective [Ca] that binds to S is sufficient, so the lower limit [Ca] is constant regardless of [REM].
In the region III, the composition of the oxide in the inclusions generated by containing the lower limit [Ca] or more is expressed by the formula (10): (REM ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+. Since it satisfies (REM ₂ O ₃ )}≧0.45, it has a high melting point and is difficult to stretch during rolling.
If the formula (8) is not satisfied in the region III, MnS cannot be sufficiently prevented, so that MnS stretched to have an aspect ratio>3.0 during rolling remains in the steel sheet, resulting in poor cleanability of the steel sheet.

From FIGS. 2A to 2C, [REM_1] and [REM_2] that divide the regions I to III, boundaries (lower limits) [Ca] of the respective regions, [Al], [S], and [T. The regression equation of [O] was obtained and the above equations (1) to (8) were defined.

Next, FIG. 4 shows the relationship between the number density of stretched inclusions having an aspect ratio>3.0 and the Charpy impact value. When the number of stretched inclusions exceeds 6/mm ² , the impact value sharply decreases and falls below the standard value of 100 (J/cm ² ).
Then, as shown in FIG. 5, the number of stretched inclusions correlates with the number of inclusions having a circle equivalent diameter ≧1.0 μm. This is because when all the number of inclusions (the number of inclusions having an equivalent circle diameter as an index of ≧1.0 μm) is small, the size distribution of inclusions is shifted to the smaller size side, and the aspect ratio is reduced during rolling. It is considered that the low-melting-point oxides stretched over >3.0 decrease. (Observation of inclusions stretched after rolling shows that the equivalent circle diameter of the inclusions, and thus the volume of the inclusions, tends to have a smaller aspect ratio than the one of large volume. However, it is considered that the thickness does not become infinitely thin but converges to a certain thickness, for example, about 0.1 μm.Since the thickness after rolling is distributed in a certain range, if the volume is conserved, the volume is large. It can be understood that the more the composition has a lower melting point and the coarser the size, the more the inclusions having the aspect ratio>3.0 during rolling increase.
As described above, by controlling the oxide composition and reducing the total number of inclusions, it is possible to reduce the harmful inclusions having an aspect ratio of >3.0, which are fracture initiation points. For this reason, in the present embodiment, the number density of inclusions having an equivalent circle diameter of 1.0 μm or more is defined as 100/mm ² or less.

In the present embodiment, particularly [T. It is important to control the components in steel such as [Ca] and [REM], and hence the composition of inclusions, according to O].
Therefore, [T. The lower limit [Ca] condition changes according to [O], and strictly speaking, the target values of [Ca] and [REM] change. In actual operation, in order to respond to this, based on the conventional operation result, [T. The target range may be set in consideration of the fluctuation of [O]. As far as possible, [T. It is actually possible to predict the [O] range including the fluctuation range. This [T. [O] The common range of [Ca] and [REM] set corresponding to the predicted range (change) may be set as the target range of the actual operation.

In the steel sheet according to this embodiment, only inclusions having a grain size (in the case of substantially spherical inclusions) or a long diameter (in the case of deformed inclusions) of 1.0 μm or more are considered. Inclusions having a grain size or major axis of less than 1.0 μm have a small effect on the workability of steel even if they are contained in the steel, so such inclusions are not considered in the present embodiment. Further, the above-mentioned major axis is defined as the line segment having the maximum length among the line segments connecting the vertices that are not adjacent to each other in the cross-sectional contour of the inclusion on the observation surface. Similarly, the above-mentioned minor axis is defined as the line segment having the minimum length among the line segments connecting the non-adjacent vertices in the cross-sectional contour of the inclusion on the observation surface. Hereinafter, the description of “particle diameter (in the case of inclusions having a substantially spherical shape) or long diameter (in the case of deformed inclusions)” may be abbreviated as “particle diameter or long diameter”.

Next, an example of the method for manufacturing a steel sheet according to this embodiment will be described.

The steel sheet according to the present embodiment, like a general steel sheet, for example, using blast furnace molten pig iron as a raw material, molten steel produced by performing converter refining and secondary refining, after forming a slab by continuous casting, the slab Then, hot rolling, and if necessary, cold rolling and/or annealing are performed to obtain a steel sheet. At that time, after the decarburization process in the converter, the secondary refining in the ladle is performed to control the composition of the steel and to control inclusions by adding Ca and REM. In addition to blast furnace hot metal, molten steel obtained by melting iron scrap as a raw material in an electric furnace may be used as a raw material.
Using the steel sheet thus manufactured as a raw material, an automobile parts manufacturer appropriately heat-treats and processes it to manufacture a product.

Ca and REM are added after adjusting the components of other contained elements and further raising Al ₂ O ₃ generated by Al deoxidation from the molten steel. If a large amount of Al ₂ O ₃ remains in the molten steel, Ca and REM are consumed by the reduction of Al ₂ O ₃ . Therefore, the contents of Ca and REM used for fixing S are reduced, and the formation of MnS cannot be sufficiently prevented.

Since Ca has a high vapor pressure, it is preferable to add Ca as a Ca-Si alloy, a Fe-Ca-Si alloy, a Ca-Ni alloy, or the like in order to improve the yield. For the addition of these alloys, alloy wires composed of these alloys may be used. REM may be added in the form of Fe-Si-REM alloy, misch metal, or the like. The misch metal is a mixture of rare earth elements, and specifically, often contains about 40 to 50% Ce and about 20 to 40% La. For example, misch metal composed of Ce 45%, La 35%, Nd 9%, Pr 6% and other impurities can be obtained.

The order of adding Ca and REM is not particularly limited. However, when Ca is added after REM addition, the size of inclusions tends to decrease. Therefore, it is preferable to add Ca after adding REM.

Al ₂ O ₃ is generated after Al deoxidation, and a part of this Al ₂ O ₃ is clustered. However, if the REM addition is performed before the Ca addition, a part of the cluster is reduced/decomposed, and the size of the cluster can be reduced. On the other hand, when Ca addition is performed before REM addition, Al ₂ O ₃ changes to CaO—Al ₂ O ₃ type inclusions having a low melting point, and the Al ₂ O ₃ clusters aggregate to form a single coarse particle. there is a possibility that a _CaO-Al 2 _{O 3} inclusions. Therefore, it is preferable to add Ca after adding REM.

As described above, the steel sheet according to this embodiment is manufactured.

According to the steel plate of the present embodiment configured as described above, the T.I. Since the relationship between REM and Ca is defined in consideration of O, even if Ca or REM forms an oxide, the amount of Ca or REM that binds to S is secured and MnS-based inclusions are reduced. It becomes possible.
Further, Ca in steel, REM, T. Al, T.A. Since the relationship between the amounts of O and S is defined as described above, the ratio of Al ₂ O ₃ in the inclusions can be reduced and the formation of low melting point oxides can be suppressed. Furthermore, by adding Ca and REM in combination, and making the main composition of the oxide a ternary system containing REM ₂ O ₃ , an Al ₂ O ₃ —CaO—REM ₂ O ₃ system, the crushability during rolling is improved. And the size becomes finer after rolling, and the harmfulness can be reduced.
Furthermore, since the number density of inclusions having an equivalent circle diameter of 1.0 μm or more is defined to be 100 pieces/mm ² or less, the number of inclusions that are the starting points of fracture is sufficiently reduced.
Therefore, it is possible to provide a high-quality steel plate in which the occurrence of cracks during use or processing is suppressed.

In the steel sheet according to the present embodiment, Cu: 0% or more and 0.05% or less, Nb: 0% or more and 0.05% or less, V: 0% or more and 0.05% or less, Mo: 0% or more and 0. In the case of including one or more selected from the group consisting of 05% or less, Ni: 0% or more and 0.05% or less, B: 0% or more and 0.0050% or less, depending on the required characteristics, It is possible to improve various properties of the steel sheet.

Further, in the steel sheet according to the present embodiment, when the mass ratio of each compound MX in the inclusions present inside is (MX) and the above formula (9) or (10) is satisfied, Can prevent the melting point of the inclusions from decreasing, and can further reduce the number of stretched inclusions.

Although the steel plate which is the embodiment of the present invention has been specifically described above, the present invention is not limited to this and can be appropriately changed without departing from the technical idea of the invention.
For example, in the embodiment, an example of the steel plate manufacturing method for manufacturing the steel plate of the present invention has been shown, but the present invention is not limited to this, and other manufacturing methods may be applied.

Below, the result of an experiment conducted to confirm the effect of the present invention will be described.

Using blast furnace hot metal as a raw material, after performing hot metal pretreatment and decarburizing treatment in a converter, the components were adjusted by ladle refining to melt 300 tons of molten steel. In ladle refining, first, Al was added to deoxidize, and then the components of other elements such as Ti were adjusted, and then held for 5 minutes or more to float Al ₂ O ₃ generated by Al deoxidation. Later, REM was added and held for 3 minutes for uniform mixing before adding Ca. MIS used misch metal. The REM element contained in this misch metal was Ce 50%, La 25%, and Nd 10% in mass ratio, and the balance was impurities. Therefore, the ratio of each REM element contained in the obtained steel sheet was almost the same as the ratio of each REM element described above. Since Ca has a high vapor pressure, a Ca-Si alloy was added to increase the yield.

The molten steel after refining was cast into a slab having a thickness of 250 mm by continuous casting. After that, this slab was heated to 1200° C. and then held for 1 hour, and then hot-rolled to a finishing temperature of 920° C. to a plate thickness of 5 mm, and then a winding temperature of 520° C. I wound it up.

With respect to the obtained hot-rolled steel sheet, the composition of inclusions and the deformation behavior (ratio of major axis/minor axis after rolling; aspect ratio) were investigated. Using an optical microscope, a cross section parallel to the rolling direction and the plate thickness direction was used as an observation surface, and an optical microscope was used to observe 60 fields of view at a magnification of 400 times (however, a magnification of 1000 times when measuring the shape of inclusions in detail). .. In each observation field, inclusions having a particle size (in the case of spherical inclusions) or a long diameter (in the case of deformed inclusions) of 1.0 μm or more are observed, and the inclusions have an aspect ratio of 3. It was classified based on 0. Inclusions number density of aspect ratio> 3.0, "inferior" samples exceeded 6 / mm ^2, and the six / mm ² or less was evaluated as "good".

In addition, 10 representative inclusions having a particle size (in the case of spherical inclusions) or a long diameter (in the case of deformed inclusions) of 1.0 μm or more are randomly selected, and each inclusion is selected. The average composition of the entire cross section was analyzed using the EDX device attached to the SEM, and the average composition of 10 pieces was obtained.
The composition of the inclusions, for example inclusions center instead of being represented by the composition of, the measured average composition of the entire cross-section, for example, than the composition is uniform inclusions as single MnS, Al ₂ O ₃ This is because a plurality of phases are often mixed, as in the case where MnS or CaS adheres to a part of the periphery of the —CaO—REM ₂ O ₃ based oxide. The average composition of such inclusions can be determined by analyzing the entire cross section of inclusions in the EDX analysis range. Alternatively, the average composition may be calculated by performing point analysis for each inclusion phase and multiplying the area ratio of each phase obtained from the SEM image.

Then, the Charpy impact value at room temperature (20° C.) was measured as an index of the toughness of the hot rolled steel sheet obtained above. The subsize test piece was taken from the C direction of the steel plate. That is, a test piece length of 55 mm was sampled from the width direction of the steel sheet, a test piece height was 10 mm in the steel sheet longitudinal (rolling) direction, and a test piece width was 2.5 mm in the steel sheet thickness direction. A 2 mm V notch was machined on a surface of 55 mm length×2.5 mm width. When a test piece is sampled in this direction, the L cross section of the steel plate becomes a fracture surface, so the influence of the stretched inclusions can be easily evaluated. Charpy absorbed energy (impact value) (J/cm ² ) is improved as the number of inclusions that become crack initiation points, especially inclusions stretched in the rolling direction, decreases in the steel sheet.

No. 51 to 60 are comparative examples. In any case, since the component conditions defined by the formulas (6) to (8) are not satisfied, the number of stretched inclusions exceeds 6/mm ^2, and the number of inclusions having a circle equivalent diameter of 1.0 μm or more is 100/ It exceeds mm ² .
When the composition of inclusions was investigated, No. Nos. 52, 53, 55 to 58, and 60 did not satisfy the formulas (9) and (10), and thus a stretched low melting point oxide was observed. Among these, No. 1 having a relatively large difference between the content [Ca] and the lower limit [Ca]. In 53, 57, and 58, since S bound to Mn was relatively large, it was observed that MnS of a larger size was attached to the stretched low-melting oxide and that MnS was also stretched. ..
No. 51, 54, and 59 did not satisfy the formula (9), but satisfied the formula (10) and the oxide had a high melting point, so that the stretching of the oxide was suppressed, and the mainly observed stretching intercalation was observed. The product was MnS.
In this way, No. Since the cleanliness of Nos. 51 to 60 was inferior, the Charpy impact value did not reach the standard value of 100 (J/cm ² ).

Inventive Example No. 1 satisfying the component conditions defined by the formulas (6) to (8). In Nos. 1 to 22, the number of stretched inclusions was 6/mm ² or less, and the number of inclusions having a circle equivalent diameter of 1.0 μm or more was 100/mm ² or less. The Charpy absorbed energy was 100 or more (J/cm ² ) as the reference value, which was good. The average composition of the inclusions satisfied at least one of formulas (9) and (10).

From the above, according to the present invention, regardless of MnS or low melting point oxide, the formation of stretched inclusions can be prevented, and the number density of inclusions having a circle equivalent diameter of 1.0 μm or more is 100 ( It was possible to manufacture a steel sheet having excellent cleanliness, which is less than the number of pieces/mm ² ).

Claims (3)

In mass %,
C: more than 0.05% and less than 0.48%,
Si: 0% or more and 0.60% or less,
Mn: 0.40% or more and 2.0% or less,
Al: 0.003% or more and 0.080% or less,
Ti: 0% or more and 0.060% or less,
Cr: 0% or more and 0.70% or less,
Ca: 0.0003% or more and 0.0050% or less,
REM: 0.0003% or more and 0.0050% or less,
And the balance consists of iron and impurities, of which P, S, O and N are
P: 0.020% or less,
S: 0.0034% or less,
O: 0.0040% or less,
N: 0.0075% or less,
Limited to
When the mass ratio of each component element M in steel is represented by [M], [REM] is a first threshold value [REM_1] represented by the following equation (1) and a second threshold value represented by the equation (2). It is divided into three regions by [REM_2], and in each region, the amount of Ca [Ca] in the steel is shown in the following equation (3), a in the equation (4), and in the equation (5). Using c, the following expressions (6), (7), and (8) are satisfied, and
A steel sheet having a number density of inclusions having an equivalent circle diameter of 1.0 μm or more contained in the steel of 100/mm ² or less.
Formula (1): [REM_1]=−0.0161×[Al]−0.2030×[S]+[O]+0.0008
Formula (2): [REM_2]=0.0139×[Al]−0.0791×[S]+1.6×[O]−0.0007
Formula (3): a=2.5765×[Al]+265.56×[S]-266.33×[O]+0.2621
Formula (4): b=0.01469×[Al]−0.0399×[S]+[O]−0.0004
Formula (5): c=0.070×[Al]+0.0951×[S]+0.4×[O]−0.0003
Formula (6): When [REM]<[REM_1], [Ca]≧a×[REM]+b
(7) Formula: [REM] [[REM]<[REM_2] [Ca]≧a×[REM_1]+b
Formula (8): [REM_2]≦[REM]≦0.0050%, [Ca]≧c Furthermore, in mass %,
Cu: 0% to 0.05%,
Nb: 0% to 0.05%,
V: 0% to 0.05%,
Mo: 0% to 0.05%,
Ni: 0% to 0.05%,
B: 0% or more and 0.0050% or less,
The steel sheet according to claim 1, comprising one or more selected from the group consisting of: When the mass ratio of each compound MX in the inclusions present inside is (MX), the following formula (9) or (10) is satisfied: Steel plate described in.
Formula (9): (CaO)/(Al ₂ O ₃ )≧1.35
Formula (10): (REM ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≧0.45

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