JP5158271B2 - High-strength steel sheet with excellent stretch flangeability and bending workability and method for producing the molten steel - Google Patents

Description

  The present invention relates to a high-strength steel sheet suitable for use in undercarriage parts of transportation equipment and a method for melting the molten steel, and in particular, a high-strength steel sheet excellent in stretch flangeability and bending workability and the molten steel manufacturing It is about the method.

  There is an increasing demand for high strength and light weight hot-rolled steel sheets for automobiles from the viewpoint of improving the safety of automobiles and improving fuel efficiency leading to environmental conservation. Among the parts for automobiles, the mass of frames and arms, especially called undercarriage systems, occupies a large proportion of the mass of the entire vehicle body, so by reducing the thickness by increasing the strength of the materials used for these parts It becomes possible to realize the weight reduction. Further, the material used for the undercarriage system is frequently used for press forming, and high bending workability is required from the viewpoint of preventing cracking during press forming, and high strength steel sheets are widely used. Among these, hot rolled steel sheets are mainly used because of price advantages. In addition, cold-rolled steel sheets and galvanized steel sheets are used for the purpose of reducing the thickness by using high-strength steel sheets for reinforcing materials and under-floor members, especially small bending members such as seat slide rails. Is mainly used.

  Among them, high strength, good workability, and good formability can be achieved by combining a low yield ratio DP steel plate that combines a ferrite phase and a martensite phase, or a composite of a ferrite phase and a (residual) austenite phase. TRIP steel sheets are known. However, although these steel plates are excellent in high strength, workability and ductility, they cannot be said to have excellent hole expandability, that is, stretch flangeability and bending workability. Stretch flanges such as suspension parts In structural parts that require formability, bainite-based steel sheets are generally used, although the ductility is somewhat inferior.

  One of the reasons why a composite steel sheet such as a ferrite phase and martensite phase composite steel sheet (hereinafter sometimes referred to as “DP steel sheet”) is inferior in stretch flangeability is a soft ferrite phase and hard martensite. It is considered that because it is a composite of the site phase, stress concentrates at the boundary between both phases during hole expansion processing, and it cannot follow deformation and tends to be a starting point of fracture.

  In order to overcome these problems, several steel plates have been proposed that are based on DP steel plates and aiming to achieve both mechanical strength characteristics and bending workability and hole expansibility (workability). For example, as a technique directed to stress relaxation by finely dispersed particles, Patent Document 1 discloses a steel sheet in which fine Cu precipitates or solid solutions are dispersed in a ferrite structure-martensitic phase composite structure steel sheet (DP steel sheet). ing. In the technique shown in Patent Document 1, Cu precipitates having a particle size of 2 nm or less composed of solid solution of Cu or Cu alone are very effective for improving the bending workability, and the workability is not impaired. As a result, the composition ratio of various components is limited.

  In addition, as a technique directed to stress relaxation by reducing the strength difference of the composite phase, for example, in Patent Document 2, the main phase is made bainite structure by reducing C as much as possible, and solid solution strengthening or precipitation strengthening is performed. A technique is disclosed in which a ferrite structure is contained at an appropriate volume ratio, the hardness difference between the ferrite and bainite is reduced, and the formation of coarse carbides is avoided.

  In Patent Document 3, a high-strength steel sheet excellent in bending workability is defined by prescribing the size and number of oxide inclusions because the oxide inclusions cause cracking during bending. Obtaining techniques are disclosed.

  Further, in Patent Documents 4 and 5, stretched MnS-based inclusions that are present in steel and cause deterioration in fatigue characteristics and stretch flangeability (hole expanding workability) become the starting point of crack generation. A technique for obtaining a high-strength steel sheet excellent in stretch flangeability and fatigue characteristics by dispersing in a steel sheet as difficult fine spherical inclusions is disclosed.

Japanese Patent Laid-Open No. 11-199973 Japanese Patent Laid-Open No. 2001-200331 JP 2002-363694 A JP 2008-274336 A JP 2009-299136 A

  By the way, although the steel plate which disperse | distributed the fine Cu precipitation or solid solution in DP steel plate as disclosed in the said patent document 1 certainly shows high fatigue strength, the remarkable improvement in stretch flangeability has been confirmed. Not done. Further, as disclosed in Patent Document 2, a high-strength hot-rolled steel sheet having a steel sheet structure mainly composed of a bainite phase and suppressing the formation of coarse carbides certainly exhibits excellent stretch flangeability, but Cu It cannot be said that the bending workability is necessarily superior to that of the contained DP steel sheet. In addition, the generation of cracks cannot be prevented when severe hole enlargement processing is performed only by suppressing the formation of coarse carbides.

  And the high-strength cold-rolled steel sheet reduced in the amount of coarse oxide inclusions as disclosed in Patent Document 3 shows excellent bending workability, but improved fatigue characteristics, stretch flangeability No significant improvement has been confirmed. In addition, since Mn and S are contained in a predetermined amount, according to the experimental findings of the present inventors, it is considered that coarse MnS inclusions are generated. It is not sufficient to prevent the occurrence of cracks when severe hole enlargement processing is performed only by reducing the amount of production inclusions.

  In addition, the high-strength steel sheet in which MnS inclusions disclosed in Patent Document 4 are dispersed as fine spherical inclusions in the steel sheet exhibits excellent stretch flangeability and fatigue characteristics, but at the stage of melting in steelmaking. Since desulfurization treatment is performed under conditions where relatively high free oxygen exists without substantially using Al, it is difficult to desulfurize even to extremely low sulfur, and substantially Al is used. In addition, in order to perform deoxidation with Ce or La or the like, there is a problem that more addition is necessary and the addition yield of Ce or La or the like is low, so that it is necessary to add excessively.

Furthermore, the high-strength steel sheet in which the MnS inclusions disclosed in Patent Document 5 are dispersed in the steel sheet as fine spherical inclusions is deoxidized with Al at the melting stage in steelmaking, and Ce or La, etc. In order to perform deoxidation, the yield of addition of Ce or La, etc. is good, and it exhibits excellent stretch flangeability and fatigue characteristics even at a relatively high S concentration as well as desulfurization to extremely low sulfur. However, since a large amount of Al ₂ O ₃ —Ce ₂ O ₃ oxide is generated, the ladle nozzle is blocked or the immersion nozzle is blocked in the continuous casting process in the steelmaking stage, resulting in production hindrance and continuous product production. In order to avoid this problem, and when Ca is added to avoid this, a CaO—Al ₂ O ₃ -based low-melting oxide or coarse CaS is produced, so that an MnS-based intervening material is produced. There was a problem of stretching and impairing stretch flangeability in the same manner as the product.

According to the researches of the present inventors, the causes of the problems described in Patent Documents 1, 2, 3, 4 and 5 are the stretched sulfide inclusions mainly composed of MnS in the steel sheet, the low melting point CaO- It was found that Al ₂ O ₃ -based inclusions and coarse extending CaS were present. That is, when subjected to repeated deformation, internal defects are generated around the stretched coarse MnS inclusions existing in the surface layer or in the vicinity thereof, and propagated as cracks, so that fatigue characteristics deteriorate, and hole expansion processing, Since it tends to be the starting point of cracking during bending, it becomes a factor that the stretch flangeability and bending workability deteriorate.

  That is, when the presence of sulfide inclusions mainly composed of MnS described in Patent Documents 1, 2, 3, 4 and 5 is described in detail, Mn contributes effectively to increase the strength of the material together with C and Si. Since it is an element, in a high-strength steel sheet, it is common to set the Mn concentration high in order to ensure the strength. Further, in a normal steelmaking process, S is also included in an amount of about 5 to 50 ppm. Therefore, MnS is usually present in the ingot (slab).

  Therefore, in the invention described in Patent Document 4, MnS inclusions are dispersed as fine spherical inclusions in the steel sheet, thereby improving stretch flangeability (hole expandability) and fatigue characteristics. However, since Al deoxidation is not substantially performed, a high oxygen potential is obtained, and therefore, the desulfurization reaction hardly occurs. For this reason, the extreme value of the inclusion composition and form is obtained with the relatively high S concentration, and the material is improved. Therefore, it cannot respond to desulfurization to extremely low sulfur. That is, it is generally desirable to suppress acid-soluble Al as much as possible because the oxides are likely to be clustered and become coarse and deteriorate stretch flangeability, bending workability, and fatigue characteristics. Therefore, desulfurization treatment is performed at a relatively high oxygen potential such that the acid-soluble Al concentration does not exceed 0.01%, and as a result, it has been impossible to treat even extremely low sulfur. Since the desulfurization reaction is a reduction reaction, it proceeds easily under a low oxygen potential, but under a high oxygen potential, it has a high sulfur potential, and desulfurization to extremely low sulfur is very difficult. Then, although Ce and La are added excessively and the oxygen potential is lowered as much as possible, the oxygen potential is not lowered sufficiently, and the cost is increased. That is, with the idea of detoxifying S in a relatively high S concentration, Ce and La are added excessively to control the inclusion composition and form, thereby improving stretch flangeability and fatigue characteristics.

  However, even if Ce and La are excessively added to render S harmless in a state where the S concentration is relatively high, the inclusion composition and form are controlled, so that the S concentration is relatively high. There is a limit to detoxification of steel, and a high-strength steel sheet having better stretch flangeability (hole expandability) and fatigue properties is desired.

On the other hand, in Patent Document 5, if Al deoxidation is performed to improve the operability by performing Al deoxidation in order to improve the oxygen potential, sulfur potential, and material, Ca addition is required. Accompanying this, a low melting point oxide was formed, which was to reduce the material. Since Ca has a vapor pressure that is liquid or vaporized in molten iron, an oxide having a low melting point is first generated. When such an oxide that is liquid in molten iron is first generated, the liquid inclusions aggregate and coalesce to produce coarse CaO—Al ₂ O ₃ -based low melting point oxide or CaS. Attempts to control the inclusion form by adding Ce or La after that were impossible.

When the ingot is hot-rolled and cold-rolled, the low-melting-point oxide CaO—Al ₂ O ₃ -based oxide, CaS, and MnS-based inclusions that are always generated by adding Mn, Since it is easily deformed, it becomes a stretched CaO—Al ₂ O ₃ -based oxide, coarse CaS or MnS-based inclusion, which causes a decrease in bending workability and stretch flangeability (hole expansion workability). However, until now, from the viewpoint of controlling the precipitation and deformation of such CaO—Al ₂ O ₃ -based oxides, coarse CaS and MnS-based inclusions, high-strength steel sheets excellent in stretch flangeability and bending workability and their molten steels There are no examples of proposed melting methods.

Therefore, the present invention has been devised in view of the above-described problems, and the object of the present invention is to perform complex deoxidation of molten steel at the steel making stage, and to form CaO-Al ₂ O in the ingot. ₃ based oxide, not to produce a coarse CaS, and inclusions in the form of an oxide or oxysulfide was fine composite precipitate MnS, further without being deformed during the rolling, it is difficult fine spherical inclusions as starting points of cracking An object of the present invention is to provide a high-strength steel sheet excellent in stretch flangeability and bending workability, which has improved stretch flangeability and bending workability, and a method for producing the molten steel, by dispersing it as a product in the steel sheet.

  In order to solve the above-mentioned problems, the present inventors have deposited fine MnS inclusions in an ingot (slab), and are not subject to deformation during rolling, and are difficult to cause cracking. Intensive research has been carried out with a focus on elucidating additive elements that do not degrade fatigue properties and methods for improving stretch flangeability and bending workability by dispersing them as inclusions in steel sheets.

As a result, fine and hard Ce oxide, La oxide, Nd oxide, Pr oxide, cerium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide produced by deoxidation by addition of Ce, La, Nd, and Pr , plastics Se gym oxide was formed, by complex and yet further the added Ca, Ce, La, Nd, 1 kind of Pr, 2 kinds, and three or four, and the Ca, and O Inclusion phase consisting of one or two species from S, one species from Ce, La, Nd, Pr, two species, three species, or four species, and one species from Ca and O, S Or a composite inclusion of an inclusion phase containing different components of two types and an inclusion phase consisting of one, two, or three types from Mn, Si, and Al. , Equivalent circle diameter 0.5-5 When one spherical inclusion having a size of μm is formed, deformation of the precipitated MnS hardly occurs during rolling, so that the stretched coarse MnS is remarkably reduced in the steel sheet, and during repeated deformation and holes. It has been found that MnS inclusions are difficult to be the starting point of crack generation and the path of crack propagation during expansion and bending, which leads to improvements in hole expansion.

  In addition to making the precipitates into fine oxides and MnS inclusions, desulfurization treatment is performed until low sulfur, and the remaining sulfur content is securely fixed to fine and hard inclusions. A sequential deoxidation with Ce, La, Nd, Pr) and Ca was also studied. As a result, after deoxidizing with Si, deoxidizing with Al, and then adding one or more of Ce, La, Nd, and Pr to deoxidize the molten steel. When (Ce + La + Nd + Pr) / acid-soluble Al and (Ce + La + Nd + Pr) / S are obtained, and finally Ca is added, the oxygen potential in the molten steel decreases, and under this low oxygen potential, It has been found that desulfurization can proceed to extremely low S relatively easily, and that it can be made into a fine MnS-based inclusion, and the remaining sulfur content can be reliably fixed to a fine and hard inclusion, and In this case, the inventors have found that stretch flangeability and bending workability are dramatically improved, and the present invention has been completed.

  The summary of the high strength steel plate excellent in stretch flangeability and bending workability according to the present invention and the method for producing the molten steel is as follows.

(1) In mass%,
C: 0.03-0.25%,
Si: 0.1 to 2.0%,
Mn: 0.5 to 3.0%
P: 0.05% or less,
T.A. O: 0.0050% or less,
S: 0.0001 to 0.01%,
N: 0.0005 to 0.01%,
Acid soluble Al: more than 0.01%,
Ca: 0.0005 to 0.0050%,
Total of one or more of Ce, La, Nd or Pr: 0.001 to 0.01%, and further, on a mass basis, 70 ≧ 100 × (Ce + La + Nd + Pr) / acid-soluble Al> 0.7, and , (Ce + La + Nd + Pr) / S is 0.2 to 10,
The balance is a steel plate of chemical composition consisting of iron and inevitable impurities,
In the steel plate,
Ce, La, Nd, 1 kind of Pr, 2 kinds, and three or four,
And, and Ca,
And the inclusion phase of the chemical component which consists of 1 type or 2 types from O and S,
1 type, 2 types, 3 types, or 4 types from Ce, La, Nd, and Pr,
And Ca,
In addition, a composite of inclusion phases containing different components, including one or two types of O and S and one or two or three types of chemical components consisting of Mn, Si and Al. The composite inclusions form one composite spherical inclusion having a circle-equivalent diameter of 0.5 to 5 μm, and the number ratio of the spherical inclusions is 0.5 to the equivalent circle diameter. A high-strength steel sheet excellent in stretch flangeability and bending workability, characterized by being 30% or more of the total number of inclusions having a size of 5 μm.

  (2) The spherical inclusions are inclusions having an equivalent circle diameter of 1 μm or more, and the number ratio of elongated inclusions having a major axis / minor axis of 3 or less is 50% or more of the total number of inclusions having an equivalent circle diameter of 1 μm or more. A high-strength steel sheet excellent in stretch flangeability and bending workability as described in (1) above.

  (3) The above-mentioned spherical inclusions contain 0.5 to 95% by mass of one, two, three or four kinds of Ce, La, Nd or Pr in an average composition ( A high-strength steel sheet excellent in stretch flangeability and bending workability as described in 1) or (2).

  (4) The average grain size of crystals in the structure of the steel sheet is 10 μm or less, which is excellent in stretch flangeability and bending workability according to any one of (1) to (3) above Strength steel plate.

(5) The chemical composition of the steel sheet is further mass%,
Nb: 0.01-0.10%,
V: 0.01 to 0.10%,
The high-strength steel sheet excellent in stretch flangeability and bending workability according to any one of the above (1) to (4), characterized in that any one or two of these are contained.

(6) The chemical composition of the steel sheet is further mass%,
Cu: 0.1 to 2%,
Ni: 0.05 to 1%,
Cr: 0.01-1%,
Mo: 0.01 to 0.4%,
B: 0.0003 to 0.005%,
The high strength steel plate excellent in stretch flangeability and bending workability according to any one of the above (1) to (5), characterized by containing any one or more of the above.

(7) The chemical composition of the steel sheet is further mass%,
Zr: 0.001 to 0.01%,
The high-strength steel sheet excellent in stretch flangeability and bending workability according to any one of the above (1) to (6), characterized in that

  (8) In a refining process in steelmaking, C is 0.03 to 0.25% and Si is 0.00% in molten steel that is processed by mass%, P is 0.05% or less, and S is 0.0001% or more. 1 to 2.0%, Mn 0.5 to 3.0%, N is added or adjusted to be 0.0005 to 0.01%, and then Al is 0.01% with acid-soluble Al. Super, T. O is added so as to be 0.0050% or less, and then one or more of Ce, La, Nd, or Pr is added. Furthermore, on a mass basis, 70 ≧ 100 × (Ce + La + Nd + Pr) / Acid soluble Al> 0.7 and (Ce + La + Nd + Pr) / S is 0.2 to 10, and the total of one or more of Ce, La, Nd or Pr is 0.001 to 0.01%. The stretch flangeability and bending workability according to any one of the above (1) to (4), wherein Ca is added or adjusted so that Ca is 0.0005 to 0.0050% later, is excellent A method for producing molten steel for high-strength steel sheets.

  (9) In the refining step, before adding one or more of Ce, La, Nd or Pr, Nb is further 0.01 to 0.10% and V is 0.01% by mass%. Melting of molten steel for high-strength steel sheets with excellent stretch flangeability and bending workability as described in (8) above, which is added so as to be any one or two of ˜0.10% Method.

(10) In the refining step, before adding one or more of Ce, La, Nd or Pr, further, 0.1% to 2% of Cu and 0.05 to 1 of Ni in mass%. %, Cr is 0.01 to 1%, Mo is 0.01 to 0.4%, and B is 0.0003 to 0.005%. The method for producing molten steel for high-strength steel sheets having excellent stretch flangeability and bending workability as described in (8) or (9) above.

  (11) In the refining step, before adding one or more of Ce, La, Nd or Pr, further, Zr is added so as to be 0.001 to 0.01% by mass%. The method for producing molten steel for high-strength steel sheets excellent in stretch flangeability and bending workability according to any one of (8) to (10) above.

  Incidentally, the high-strength steel plate in the present invention includes a case where it is used as it is with a normal hot-rolled / cold-rolled steel plate as it is, or subjected to surface treatment such as plating or painting.

  In the high-strength steel sheet having the above-described structure and excellent in stretch flangeability and bending workability, the adjustment of the composition of the molten steel is stabilized by Al deoxidation, and the formation of coarse alumina inclusions is suppressed. In the steel plate as fine spherical inclusions that are not deformed during rolling and are unlikely to start cracking because they are precipitated as inclusions in the form of fine composite precipitated oxide or oxysulfide in the ingot. In addition, the crystal grain size of the structure can be made fine, and stretch flangeability and bending workability can be improved.

  Further, in the method for producing molten steel of a high strength steel sheet excellent in stretch flangeability and bending workability according to the present invention, the formation of coarse alumina inclusions is achieved while stabilizing the component adjustment of the molten steel by Al deoxidation. It can be suppressed and deposited as a complex inclusion that is a fine composite precipitated oxide or oxysulfide in the ingot, so that it does not undergo deformation during rolling and is difficult to start cracking in the steel plate as a fine spherical inclusion It can be dispersed, the crystal grain size of the structure can be made fine, and a high-strength hot-rolled steel sheet excellent in stretch flangeability and bending workability can be obtained.

In illustration of _Al 2 _{O 3} and MnS is stretched inclusions present in the hot-rolled steel sheet, (a) shows the _Al 2 _O 3, (b) is an explanatory view of MnS. In illustration of the stretched CaOAl ₂ O ₃ inclusions preliminary CaS-based inclusions present in the hot-rolled steel sheet, (a) represents CaOAl ₂ O ₃ inclusions, (b) are explanatory views of CaS inclusions is there. It is explanatory drawing of the composite inclusion in this invention, (a) And (b) is a figure which shows the example of the presence state of a different inclusion.

  Hereinafter, as a form for carrying out the present invention, a high-strength steel sheet excellent in stretch flangeability and bending workability will be described in detail. Hereinafter, the mass% in the composition is simply described as%.

  First, the experiment that led to the completion of the present invention will be described.

  The inventor contains C: 0.06%, Si: 1.0%, Mn: 1.4%, P: 0.01% or less, S: 0.005%, N: 0.003%. The molten steel whose balance is Fe was deoxidized using various elements to produce a steel ingot. The obtained steel ingot was hot-rolled to obtain a hot-rolled steel sheet having a thickness of 3 mm. These manufactured hot-rolled steel sheets were subjected to a tensile test, a hole expansion test, and a bending test, and the number density, form, and average composition of inclusions in the steel sheets were investigated.

First, in a hot rolled steel sheet manufactured by adding Si to molten steel and then deoxidizing with Al, the melting point of Al ₂ O ₃ inclusions precipitated as inclusions in the steel ingot is 2040 ° C. As shown in FIG. 1 (a), it exists in an angular shape without being stretched during rolling. For this reason, it becomes a starting point of the crack of a steel plate at the time of a hole expansion process, and causes a decrease in bending workability and stretch flangeability (hole expansion processability). Further, the MnS inclusions coarsely precipitated as inclusions in the steel ingot have a melting point as low as 1610 ° C., and as shown in FIG. 1 (b), they are easily drawn during rolling to become drawn MnS inclusions. It becomes the starting point of cracking of the steel sheet during the expansion process.

Further, in a hot rolled steel sheet manufactured by adding Ca after deoxidation with Al, Ca melts and becomes larger due to interfacial energy, and CaOAl ₂ O ₃ inclusions and CaS (Fe, Mn) are contained in the ingot. , Al ₂ O ₃ ) -based inclusions are coarsely precipitated. Since these inclusions have a melting point of about 1390 ° C., as shown in FIGS. 2 (a) and 2 (b), they are easily stretched during rolling and exist as stretched inclusions of about 50 to 100 μm. And the stretch flangeability (hole expansion workability) are reduced.

  Furthermore, Si is added to the molten steel, then deoxidized with Al and stirred for about 2 minutes, and then one, two, three or four of Ce, La, Nd and Pr are added. Then, the stretch flangeability and bending workability of the steel sheet produced by deoxidation were investigated. As a result, the steel sheet that has been sequentially deoxidized in three stages of Si, then Al, and Ce, La, Nd, and Pr, has three types of deoxidation, namely, stretch flangeability and bending workability. It was confirmed that it could be improved. The reason is that fine and hard Ce oxide, La oxide, Nd oxide, Pr oxide, cerium oxysulfide, lanthanum oxysulfide, neodymiumoxy produced by deoxidation by addition of Ce, La, Nd, and Pr. MnS precipitates on sulfide and prasedim oxysulfide, and it becomes possible to suppress deformation of inclusions that are complex precipitated oxide or oxysulfide during rolling. This is because MnS inclusions can be significantly reduced.

The reason why Ce oxide, La oxide, Nd oxide, Pr oxide, cerium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, and prasedium oxysulfide are refined was first generated by Si deoxidation. Al to which SiO 2 inclusions are added later is reduced and decomposed to produce fine Al ₂ O ₃ inclusions, and then Ce, La, Nd and Pr are further reduced and decomposed to form fine Ce oxides, La Forming oxides, Nd oxides, Pr oxides, cerium oxysulfide, lanthanum oxysulfides, neodymium oxysulfides, prasedimoxysulfides, and further generated Ce oxides, La oxides, Nd oxides, Pr oxidations , Cerium oxysulfide, lanthanum oxysulfide, neodymium Shisarufaido is because also suppressed aggregation coalescence after generation for surface energy is low and the plug cell Jim oxysulfide themselves and the molten steel.

  The present inventors subsequently performed deoxidation while changing the composition of Ce, La, Nd, and Pr while performing Al deoxidation, and then added Ca to produce a steel ingot. The obtained steel ingot was hot-rolled to obtain a hot-rolled steel sheet having a thickness of 3 mm. These manufactured hot-rolled steel sheets were subjected to a hole expansion test and a bending test, and the inclusion number density, form and average composition in the steel sheets were investigated.

Through such experiments, after adding Si, deoxidize with Al, then add one, two, three or four of Ce, La, Nd, Pr to deoxidize, then add Ca In the composite deoxidized molten steel, when (Ce + La + Nd + Pr) / acid-soluble Al ratio is 0.7 to 70 and (Ce + La + Nd + Pr) / S ratio is 0.2 to 10 on a mass basis, As a result, the oxygen potential in the molten steel decreased rapidly. In other words, due to the combined deoxidation effect of Al, Si, (Ce, La, Nd, Pr), and Ca, the oxygen potential is the most among the systems that have so far been deoxidized with various deoxidation elements. The effect of decreasing was obtained. Because of the combined deoxidation effect, Al ₂ O ₃ concentration can be very low for the oxide to be produced, so that it is excellent in stretch flangeability and bending workability in the same manner as a steel sheet manufactured with almost no deoxidation with Al. It was found that a steel plate was obtained.

  The reason is considered as follows.

That is, SiO ₂ inclusions are generated upon adding Si, SiO ₂ inclusions are reduced to Si by subsequent addition of Al. Further, Al, together with the reduction of SiO ₂ inclusions, the dissolved oxygen in the molten steel even when deoxidation, generates _Al 2 _{O 3} inclusions, some of _Al 2 _{O 3} inclusions are floated removed, The remaining Al ₂ O ₃ inclusions remain in the molten steel. Thereafter, the added (Ce, La, Nd, Pr) causes the Al ₂ O ₃ inclusions to be reduced and decomposed, resulting in fine and spherical Ce oxide, La oxide, Nd oxide, Pr oxide, cerium oxy REM oxysulfide such as sulfide, lanthanum oxysulfide, neodymium oxysulfide, prasedimoxysulfide is formed. Furthermore, by adding Ca, Al ₂ O ₃ , MnS, CaS, (MnCa) S, etc. are precipitated in these oxides and / or oxysulfides, which is a solid solution inclusion phase. As shown in (a), an Al—O—Ce—La—Nd—Pr—OS—Ca inclusion phase [eg, Al ₂ O ₃ (Ce, La, Nd, Pr) ₂ O ₂ SCa], Ca-Mn-S-Ce- La-Nd-Pr-Al-O -mediated phase [e.g., CaMnS (Ce, La, Nd , Pr) Al 2 O 3] and, Ce-La-Nd-Pr -O- S-Ca inclusion phase [for example, (Ce, La, Nd, Pr) 2 O ₂ SCa] is combined into a single spherical inclusion, or as shown in FIG. Ca-Mn-S-Ce-La-Nd-Pr inclusion phase [e.g., CaM nS (Ce, La, Nd, Pr)] and, Ce-La-Nd-Pr -O-SCa mediated phase [e.g., (Ce, La, Nd, Pr) 2O 2 SCa] and Ce-La- Nd—Pr—O—S—Al—O—Ca inclusion phase [for example, (Ce, La, Nd, Pr) ₂ O ₂ SAl ₂ O ₃ Ca] is combined to form a single inclusion Form things. These composite inclusions are mainly oxysulfide (one or more of Ce, La, Nd, Pr) and are almost spherical, so once added metal such as Ce, La, Nd, Pr, etc. After melting and reacting to form oxysulfide, a large number of very fine nuclei were formed, and then phase separation was performed from among them, or some of the low melting point phases had a high melting point. It is thought that these phases were fused.

  These fine spheroidized composite inclusions have a high melting point of about 2000 ° C., and are not stretched by hot rolling, and show a finely spheroidized form in the hot-rolled steel sheet. Therefore, by forming spherical composite inclusions (REM oxysulfide composite inclusions) in the form of oxides or oxysulfides that have been compositely precipitated in this way, bending workability and stretch flangeability (hole expansion workability) are reduced. Can prevent the cause.

Al ₂ O ₃ remains a little due to the four-step complex deoxidation by the addition of Al, Si, (Ce, La, Nd, Pr), and Ca, but most of them are fine and hard with an equivalent circle diameter of 0.5 to 5 μm. There is an oxide or oxysulfide consisting of one, two, three or four kinds of Ce, La, Nd or Pr of a size of 1, and one, two or three kinds of Si, Al and Ca. Spherical complex inclusions (REM oxysulfide) in the form of oxide or oxysulfide in which one or more of MnS, CaS, or (Mn, Ca) S is complex precipitated Composite inclusions).

  Even if Ca is added before the addition of (Ce, La, Nd, Pr), a fine spherical composite compound cannot be obtained.

Therefore, in the composite deoxidation in the order of addition of Al, Si, (Ce, La, Nd, Pr), and Ca, by performing the deoxidation method appropriately, the above-described hard composite inclusions that have been microspherically formed are described above. (REM oxysulfide composite inclusions) can be deposited, and deformation of the composite precipitate inclusions can be suppressed even during rolling, so that the stretched coarse MnS inclusions are significantly reduced in the steel sheet. In addition to obtaining the effect that the bending workability can be improved by this, in addition to reducing the oxygen potential of the molten steel by combined deoxidation, we have newly found out that the variation in composition can be reduced. Based on the findings obtained, the present inventor examines the chemical composition conditions of the steel sheet and performs the component design of the steel sheet as described below. The present invention has been completed.

  Hereinafter, the reason for limiting the chemical components of the high-strength steel sheet excellent in stretch flangeability and bending workability in the present invention will be described.

(C: 0.03-0.25%)
C is the most basic element for controlling the hardenability and strength of steel, and contributes to the improvement of fatigue strength by increasing the hardness and depth of the hardened hardened layer. That is, this C is an essential element for securing the strength of the steel sheet, and at least 0.03% is required to obtain a high-strength steel sheet. However, if this C is excessively contained and exceeds 0.25%, workability and weldability deteriorate. In order to achieve the required strength and ensure workability and weldability, the C concentration is set to 0.25% or less in the present invention. Preferably it is 0.10 to 0.20%.

(Si: 0.1-2.0%)
Si is one of the main deoxidizing elements, and has the function of increasing the number of austenite nucleation sites during quenching heating, suppressing austenite grain growth, and reducing the grain size of the quenched hardened layer. This Si suppresses the formation of carbides, suppresses the decrease in grain boundary strength due to carbides, and is also effective for the formation of bainite structure, thus improving the strength without greatly impairing the elongation, and the low yield strength ratio. It is an important element for improving hole expandability. In order to reduce the dissolved oxygen concentration in the molten steel, once generate SiO2 inclusions, and to obtain the final dissolved oxygen minimum value by combined deoxidation (Al added with this SiO2 inclusions is reduced In order to reduce alumina inclusions by further reducing Ce, La, Nd, and Pr), it is necessary to add 0.1% or more of Si. In the present invention, the lower limit of Si is set to 0.1%. On the other hand, if the concentration of Si is too high, the toughness becomes extremely poor, and surface decarburization and surface flaws increase, so that the bending workability deteriorates. In addition, excessive addition of Si adversely affects weldability and ductility. Therefore, in the present invention, the upper limit of Si is set to 2.0%. Preferably it is 0.5 to 1.8%.

(Mn: 0.5-3.0%)
Mn is an element useful for deoxidation in the steelmaking stage, and is an element effective for increasing the strength of the steel sheet together with C and Si. In order to obtain such an effect, it is necessary to contain 0.5% or more of this Mn. However, when Mn is contained in an amount exceeding 3.0%, ductility is lowered due to segregation of Mn and increase in solid solution strengthening. Further, since the weldability and the base metal toughness are also deteriorated, the upper limit of Mn is set to 3.0%. Preferably it is 1.0 to 2.6%.

(P: 0.05% or less)
P is an element inevitably contained, and is effective in that it acts as a substitutional solid solution strengthening element smaller than Fe atoms. However, if this P concentration exceeds 0.05%, it segregates at the austenite grain boundaries and lowers the grain boundary strength, thereby lowering the torsional fatigue strength and causing the workability to deteriorate. The upper limit is 0.05%. Further, if solid solution strengthening is not necessary, it is not necessary to add P, and the lower limit value of P includes 0%.

(T.O: 0.0050% or less)
T.A. O forms an oxide as an impurity. T.A. When O is too high, mainly Al ₂ O ₃ inclusions increase, the oxygen potential of the system cannot be minimized, the toughness becomes extremely poor, and the surface flaws increase, so bending workability is rejected. Deteriorate. For this reason, in the present invention, T.W. The upper limit of O was 0.0050%.

(S: 0.0001% to 0.01%)
S is segregated as an impurity, and S combines with Mn to form a MnS-based coarse stretch inclusion to deteriorate stretch flangeability. Therefore, it is desirable that the concentration be as low as possible. On the other hand, even at a relatively high S concentration of approximately 0.01%, the MnS-based coarse stretched inclusions of the present invention are not subjected to desulfurization load in secondary refining, and without desulfurization costs, by controlling the morphology. More material than the cost can be obtained. Therefore, the range of the S concentration in the present invention is set to a range from 0.0001% to 0.01% from a very low S concentration assuming desulfurization in secondary refining to a relatively high S concentration.

  In the present invention, fine inclusions such as hard and hard Ce oxide, La oxide, Nd oxide, Pr oxide, cerium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, praseodymium oxysulfide and Ca oxide are included. On top of this, the MnS inclusions are precipitated and dissolved, and the shape of the MnS inclusions is controlled, so that deformation does not easily occur during rolling and the inclusions are prevented from being stretched. Therefore, the upper limit of the concentration of S is As will be described later, it is defined in relation to the total amount of one, two, three or four types of Ce, La, Nd and Pr. Furthermore, when it exceeds 0.01%, cerium oxysulfide and lanthanum oxysulfide grow and become a size exceeding 2 μm, and when it becomes coarse, the toughness becomes extremely poor and the surface defects increase. Therefore, the bending workability is worsened. For this reason, in the present invention, the upper limit of S is set to 0.01%.

  That is, in the present invention, as described above, MnS is controlled in form by inclusions such as Ce oxide, La oxide, cerium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, praseodymium oxysulfide, and Ca oxide. Even if it is relatively high within a range of 0.01% or less, the addition of one or two of Ce or La in an amount corresponding thereto can prevent adverse effects on the material. That is, even if the concentration of S is high to some extent, by adjusting the addition amount of Ce or La or the like according to this, a substantial desulfurization effect can be obtained, and the same material as the ultra low sulfur steel can be obtained. In other words, it is possible to increase the degree of freedom for the upper limit by appropriately adjusting the S concentration with the total amount of Ce, La, Nd, and Pr. Therefore, in the present invention, there is no need to perform molten steel desulfurization in secondary refining to obtain ultra-low-sulfurized steel, which can be omitted, simplifying the manufacturing process, and reducing the desulfurization treatment cost associated therewith. It can be realized.

(N: 0.0005 to 0.01%)
N is an element that is inevitably mixed in steel because nitrogen in the air is taken in during the treatment of molten steel. N forms a nitride with Al or the like to promote the refinement of the base material structure. However, when the N content exceeds 0.01%, coarse precipitates such as Al are generated, and the stretch flangeability is deteriorated. For this reason, in the present invention, the upper limit of the concentration of N is set to 0.01%, preferably 0.005%. On the other hand, since it is expensive to make the concentration of N less than 0.0005%, 0.0005% is made the lower limit from the industrially feasible viewpoint.

(Acid-soluble Al: over 0.01%)
In general, acid-soluble Al tends to become coarse due to clustering of its oxides, and it is desirable to suppress it as much as possible in order to degrade stretch flangeability and bending workability. However, in the present invention, a composite and sequential deoxidation effect of Si, Ti, (one or two or more of Ce, La, Nd, and Pr) and an acid can be obtained while performing Al deoxidation. By adjusting the concentration of (Ce, La, Nd, Pr) according to the dissolved Al concentration, as described above, the Al ₂ O ₃ inclusions generated by Al deoxidation while achieving an extremely low oxygen potential are The Al ₂ O ₃ inclusions in the part are levitated and removed, and the remaining Al ₂ O ₃ inclusions in the molten steel are reduced and decomposed by Ce and La added later to break up the clusters, and fine inclusions As a result, a new region was found where the alumina-based oxide was not clustered and coarsened.

  For this reason, in the present invention, there is no need to provide a restriction that Al is not substantially added in order to avoid coarse clusters of alumina-based oxides as in the prior art, and the degree of freedom particularly with respect to the concentration of this acid-soluble Al. Can be increased. By making acid-soluble Al more than 0.01%, it becomes possible to use both Al deoxidation and deoxidation by adding Ce and La, and the amount of Ce and La required for deoxidation as in the past Therefore, the problem of an increase in oxygen potential in the steel due to Ce and La deoxidation can be solved, and the effect that the variation in the composition of each component element can be suppressed can also be enjoyed.

  As will be described later, the upper limit of the concentration of acid-soluble Al is 70 ≧ 100 × (Ce + La + Nd + Pr) / acid on a mass basis that is related to the total amount of one or more of Ce, La, Nd, or Pr. Although it is prescribed | regulated by soluble Al> 0.7, it is preferable to set it as 1% or less from the surface of the addition cost of Al, Ce, La, Nd, and a Pr alloy.

In addition, the acid-soluble Al concentration referred to here is a measurement of the concentration of Al dissolved in an acid, and is an analytical method utilizing the fact that dissolved Al dissolves in an acid and Al ₂ O ₃ does not dissolve in an acid. is there. Here, examples of the acid include a mixed acid mixed at a ratio (mass ratio) of hydrochloric acid 1, nitric acid 1, and water 2. By using such an acid, it can be separated into Al soluble in acid and Al ₂ O ₃ not soluble in acid, and the acid soluble Al concentration can be measured.

(Ca: 0.0005 to 0.0050%)
Ca is an important element in the present invention, and controls the form of desulfurization, such as spheroidizing sulphides, and combines one or more of MnS, CaS, or (Mn, Ca) S. It has the effect of forming a solid inclusion by precipitation and solid solution with the precipitated oxide or oxysulfide, and can also improve the stretch flangeability and bending workability of the steel. In order to obtain these effects, the amount of Ca added is preferably 0.001% or more. However, even if Ca is contained in a large amount, the effect is saturated, and on the contrary, the cleanliness of the steel is impaired and the ductility is deteriorated. Therefore, the upper limit is made 0.0050%. Preferably it is 0.001 to 0.0040%. More preferably, it is 0.0015 to 0.0030%.

(Total of 1 type, 2 types, 3 types, or 4 types of Ce, La, Nd, and Pr: 0.001 to 0.01%)
Ce, La, Nd, and Pr reduce SiO ₂ produced by Si deoxidation, sequentially reduce Al ₂ O ₃ produced by Al deoxidation, and break up Al ₂ O ₃ clusters to be coarsened. Ce oxides (for example, Ce ₂ O ₃ , CeO ₂ ), cerium oxysulfide (for example, Ce ₂ O ₂ S), La oxidation, which are easy to become precipitation sites of system inclusions and are hard, fine and difficult to deform during rolling. Product (eg, La ₂ O ₃ , LaO ₂ ), lanthanum oxysulfide (eg, La ₂ O ₂ S), Nd oxide (eg, Nd ₂ O ₃ ), Pr oxide (eg, Pr ₆ O ₁₁ ), Ce oxidation A main phase (50% or more as a guideline) of a substance-La oxide-Nd oxide-Pr oxide or cerium oxysulfide-lanthanum oxysulfide. It has the effect of forming an object. Of Ce, La, Nd, and Pr, Ce and La are preferably used.

Here, the inclusions may contain a part of MnO, SiO ₂ , or Al ₂ O ₃ depending on deoxidation conditions. However, if the main phase is the oxide, the MnS inclusion precipitation site As well as the effect of making the inclusions finer and harder.

  In order to obtain such inclusions, the total concentration of one, two, three or four of Ce, La, Nd and Pr must be 0.001% or more and 0.01% or less. Were found experimentally.

When the total concentration of one, two, three, or four of Ce, La, Nd, and Pr is less than 0.001%, SiO ₂ and Al ₂ O ₃ inclusions cannot be reduced. One type or two or more types of ruium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, and prasedymium oxysulfide are produced in large amounts, resulting in coarse inclusions, which deteriorate stretch flangeability and bending workability.

  In addition, in the steel sheet of the present invention described above, the presence of inclusions in the form of MnS precipitated on oxides or oxysulfides of one, two, three or four kinds of Ce, La, Nd and Pr As a condition, it is defined by using the concentration of S to capture how MnS is modified with one, two, three or four oxides or oxysulfides of Ce, La, Nd, and Pr. Focusing on the points that can be achieved, the idea was to define and organize the chemical composition (Ce + La + Nd + Pr) / S mass ratio of the steel sheet. Specifically, when this mass ratio is small, there are few oxides or oxysulfides consisting of one, two, three or four kinds of Ce, La, Nd, and Pr, and a large amount of MnS precipitates alone. Become. As this mass ratio increases, oxides or oxysulfides consisting of one, two, three, or four kinds of Ce, La, Nd, and Pr increase in comparison with MnS, and these Ce, Inclusions in the form of MnS deposited on oxides or oxysulfides of one, two, three or four of La, Nd and Pr increase. That is, MnS is modified with an oxide or oxysulfide composed of one, two, three, or four kinds of Ce, La, Nd, and Pr. In this way, in order to improve stretch flangeability and bending workability, MnS is precipitated on oxide, oxysulfide, or one, two, three, or four kinds of Ce, La, Nd, and Pr, and MnS is stretched. It leads to prevention. Therefore, the mass ratio can be organized as a parameter for identifying whether or not these effects are achieved.

  Therefore, in order to clarify the effective chemical composition ratio for suppressing the stretching of MnS inclusions, the (Ce + La + Nd + Pr) / S mass ratio of the steel sheet was changed to evaluate the inclusion morphology, stretch flangeability and bending workability. . As a result, it was found that when the (Ce + La + Nd + Pr) / S mass ratio is 0.2 to 10, both stretch flangeability and bending workability are dramatically improved.

  Inclusions in which MnS is precipitated on oxides or oxysulfides of one, two, three or four types of Ce, La, Nd and Pr when the (Ce + La + Nd + Pr) / S mass ratio is less than 0.2 Since the number ratio is too small, the number ratio of MnS-based stretched inclusions, which are likely to be the starting point of cracking, is excessively increased and stretch flangeability and bending workability are lowered.

  On the other hand, when the (Ce + La + Nd + Pr) / S mass ratio exceeds 10, the effect of precipitating MnS on cerium oxysulfide and lanthanum oxysulfide and improving stretch flangeability and bending workability is saturated, resulting in a cost reduction. It will not be commensurate. From the above results, the (Ce + La + Nd + Pr) / S mass ratio is limited to 0.2-10. By the way, if the (Ce + La + Nd + Pr) / S mass ratio becomes excessive and exceeds 70, for example, one or more of cerium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, and prasedimoxysulfide are produced in large quantities. On the contrary, since it becomes a coarse inclusion, the upper limit of the (Ce + La + Nd + Pr) / S mass ratio is 10 because the stretch flangeability and bending workability are deteriorated.

  Hereinafter, the reason why the chemical component is limited for the selected element in the present invention will be described. Since these elements are selective elements, the presence or absence of addition is arbitrary, and only one kind may be added, or two or more kinds may be added.

Regarding Nb and V, Nb and V form carbides, nitrides, and carbonitrides with C or N to promote the fine graining of the base material structure and contribute to the improvement of toughness.

(Nb: 0.01-0.10%)
In order to obtain the above-described composite carbide, composite nitride, etc., the Nb concentration is preferably set to 0.01% or more. However, even if the Nb concentration exceeds 0.10%, the effect of refining the base material structure is saturated and the manufacturing cost increases. For this reason, the upper limit of the Nb concentration is 0.10%.

(V: 0.01-0.10%)
In order to obtain the above-described composite carbide, composite nitride, etc., the V concentration is preferably 0.01% or more. However, even if the V concentration exceeds 0.10%, the effect is saturated and the production cost is increased. For this reason, the upper limit of the V concentration is 0.10%.

About Cu, Ni, Cr, Mo, and B Cu, Ni, Cr, Mo, and B improve strength and improve the hardenability of steel.

(Cu: 0.1 to 2%)
Cu contributes to the precipitation strengthening of ferrite and the improvement of fatigue strength, and can be contained as needed to secure the strength of the steel sheet. To obtain this effect, 0.1% or more should be added. Is preferred. However, this large amount of Cu deteriorates the balance between strength and ductility. Therefore, the upper limit is 2%.

(Ni: 0.05 to 1%)
Since Ni can strengthen the solid solution of ferrite, it can be contained as necessary to further secure the strength of the steel sheet. To obtain this effect, 0.05% or more may be added. preferable. However, this large amount of Ni deteriorates the balance between strength and ductility. Therefore, the upper limit is 1%.

(Cr: 0.01 to 1%)
In order to further secure the strength of the steel sheet, Cr can be contained as necessary. To obtain this effect, it is preferable to add 0.01% or more. However, this large amount of Cr deteriorates the balance between strength and ductility. Therefore, the upper limit is 1%.

(Mo: 0.01-0.4%)
Mo can be added as necessary to further secure the strength of the steel sheet. To obtain these effects, it is preferable to add 0.01% or more. However, this large amount of Mo deteriorates the balance between strength and ductility. Therefore, 0.4% is made the upper limit.

(B: 0.0003 to 0.005%)
B can be contained as necessary in order to further strengthen the grain boundaries and improve workability. In order to obtain these effects, B is preferably added in an amount of 0.0003% or more. However, even if this B is contained in a large amount exceeding 0.005%, the effect is saturated, and on the contrary, the cleanliness of the steel is impaired and the ductility is deteriorated. Therefore, the upper limit is made 0.005%.

About Zr Zr can be contained as needed in order to strengthen the grain boundary and improve the workability by controlling the form of sulfide.

(Zr: 0.001 to 0.01%)
Zr is preferably added in an amount of 0.001% or more in order to obtain the effect of improving the toughness of the base material by spheroidizing the aforementioned sulfide. However, this large amount of Zr deteriorates the cleanliness of the steel and deteriorates the ductility. Therefore, the upper limit is 0.01%.

  Next, the presence conditions of inclusions in the steel sheet of the present invention will be described. The steel plate here means a rolled plate obtained through hot rolling or further cold rolling. Moreover, the presence conditions of the inclusion in the steel plate of this invention are prescribed | regulated from various viewpoints.

  In order to obtain a steel sheet excellent in stretch flangeability and bending workability, it is important to reduce as much as possible the stretched and coarse MnS-based inclusions in the steel sheet, which are likely to become the starting point of crack generation and the path of crack propagation.

Therefore, as described above, the present inventor added Si, deoxidized with Al, and then added one, two, three, or four of Ce, La, Nd, and Pr. When the above-mentioned (Ce + La + Nd + Pr) / acid-soluble Al ratio and (Ce + La + Nd + Pr) / S ratio are obtained on a mass basis in a steel sheet that has been deoxidized by adding Ca, it is abrupt in the molten steel due to complex deoxidation. Similar to steel sheet manufactured with almost no deoxidation with Al to reduce Al ₂ O ₃ produced by Al deoxidation and to break up Al ₂ O ₃ clusters to be coarsened as the oxygen potential decreases. In addition, it was found that it has excellent stretch flangeability and bending workability.

In addition, fine and hard Ce oxides, La oxides, and Nd oxides, which occupy a large portion of those containing a little Al ₂ O ₃ by addition of Ce, La, Nd, and Pr followed by addition of Ca. Pr oxide, cerium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, praseodymium oxysulfide and Ca oxide or Ca oxysulfide are solid solution, MnS is precipitated and solid solution, inclusion phase of different components It was also found that the coarse inclusions that were stretched in the steel sheet were remarkably reduced because the composite inclusions were formed and deformation of the composite inclusions hardly occurred during rolling.

  Therefore, when the (Ce + La + Nd + Pr) / acid-soluble Al ratio and the (Ce + La + Nd + Pr) / S ratio are obtained on a mass basis, the number density of fine inclusions having a circle-equivalent diameter of 2 μm or less increases rapidly. It was found that fine inclusions were dispersed in the steel.

  Since the fine inclusions are difficult to aggregate, the shape is almost spherical or spindle-shaped. In addition, it is 3 or less, preferably 2 or less when expressed in terms of major axis / minor axis (hereinafter sometimes referred to as “stretch ratio”). In the present invention, these inclusions are called spherical inclusions.

  Experimentally, identification was easy by observation with a scanning electron microscope (SEM) or the like, and attention was paid to the number density of inclusions having an equivalent circle diameter of 5 μm or less. Incidentally, the lower limit value of the equivalent circle diameter is not particularly specified, but it is preferable to target inclusions of about 0.5 μm or more as the size that can be counted with numbers. Here, the equivalent circle diameter is defined as (major axis × minor axis) 0.5 obtained from the major axis and minor axis of the inclusion observed in the cross section.

  These fine inclusions of 5 μm or less are dispersed because of the decrease in the oxygen potential of molten steel due to Al deoxidation and the oxidation of one, two, three or four of Ce, La, Nd or Pr. Or oxides containing one, two or three kinds of Si, Al, and Ca are precipitated and solid-dissolved therein, and further one kind of MnS, CaS, or (Mn, Ca) S. Alternatively, it is considered to be due to a synergistic effect with the refinement of oxide and / or oxysulfide composite inclusions in which two or more kinds are precipitated and dissolved.

Composite inclusions to be generated is, Ce, La, Nd, one from Pr, two, and three kinds, or four, and consists of a Ca, and, O, with one or the S-mediated Physical phase (hereinafter sometimes referred to as [Ce, La, Nd, Pr] -Ca- [O, S] first group), Ce, La, Nd, and Pr, 1 type, 2 types, 3 types An inclusion phase comprising one species, four species, Ca, one or two species from O and S, and one, two, or three species from Mn, Si, and Al ( [Ce, La, Nd, Pr] -Ca- [O, S]-[Mn, Si, Al] may be described as a second group in the following). This composite inclusion forms a large number of one composite spherical inclusion having a circle equivalent diameter of 0.5 to 5 μm, Hardly becomes a path for crack propagation, rather contribute to the relaxation of the fine is the stress concentration, stretch flangeability, is considered to have led to the improvement of such resistance to bending workability.

  On the other hand, the present inventor investigated whether stretched and coarse MnS-based inclusions that tend to become crack initiation points and crack propagation paths can be reduced in the steel sheet.

  The present inventor has found through experiments that if the equivalent circle diameter is less than 1 μm, even stretched MnS is harmless as a starting point of cracking and does not deteriorate stretch flangeability and bending workability. Inclusions with an equivalent circle diameter of 1 μm or more can be easily observed with a scanning electron microscope (SEM), etc. Therefore, the shape and composition of the inclusions with an equivalent circle diameter of 1 μm or more in a steel sheet are investigated and stretched. The distribution state of MnS was evaluated.

  In addition, although the upper limit of the circle equivalent diameter of MnS is not specified in particular, MnS of about 1 mm may be observed in practice.

  The ratio of the number of stretched inclusions was determined by analyzing the composition of a plurality of inclusions (for example, 50) having a circle-equivalent diameter of 1 μm or more selected at random using SEM, and measuring the major axis and minor axis of the inclusions from the SEM image. To do. Here, the elongated inclusions are defined as inclusions having a major axis / minor axis (ratio of stretching) exceeding 3, and the number of the detected elongated inclusions is the total number of investigated inclusions (50 in the above example). The number ratio of the above-described stretched inclusions can be determined by dividing by the number.

The reason why the stretching ratio was set to 3 or less was that inclusions exceeding the stretching ratio of 3 in the comparative steel sheet to which Ce, La, Nd, and Pr were not added were mostly Ce when MnS, Ce, La, Nd, and Pr were added. , La, Nd, Pr oxide and oxysulfide as the core, MnS precipitates around it, low melting point CaO—Al ₂ O ₃ type inclusions and coarse stretched CaS This is because. In addition, although the upper limit of the extending | stretching ratio of MnS is not prescribed | regulated in particular, MnS of about 50 extending | stretching ratio may be observed actually.

As a result, it was found that the stretch flangeability and bending workability are improved in the steel sheet whose form is controlled so that the number ratio of the stretched inclusions having a stretching ratio of 3 or less is 50% or more. That is, when the number ratio of the stretched inclusions with a stretching ratio of 3 or less is 50% or more, the oxides of Ce and La and oxysulfide when MnS, Ce, and La, which are likely to start cracking, are used as the core. The number ratio of inclusions in the case where MnS precipitates around, low-melting point CaO—Al ₂ O ₃ -based inclusions and coarsely extending CaS is excessively increased, and stretch flangeability and bending workability are deteriorated. Therefore, in the present invention, the number ratio of stretched inclusions having a stretching ratio of 3 or less is set to 50% or more.

  Moreover, since the stretch flangeability and bending workability are so good that there are few stretched MnS type inclusions etc., the lower limit of the number ratio of the stretch inclusion exceeding the stretch ratio 3 includes 0%. Here, the inclusion having an equivalent circle diameter of 1 μm or more and the lower limit of the number ratio of the drawn inclusions exceeding the draw ratio of 3 means 0%, but is an inclusion having an equivalent circle diameter of 1 μm or more. This is the case where there is no material with a stretch ratio exceeding 3, or even when the stretch inclusion exceeds the stretch ratio 3, the equivalent circle diameter is less than 1 μm.

  Further, it was confirmed that the maximum equivalent circle diameter of the stretched inclusions was smaller than the average grain size of the structure crystals, which is considered to be a factor that the stretch flangeability and the bending workability could be greatly improved.

  Further, in a steel sheet whose shape is controlled so that the number ratio of stretched inclusions having a (Ce + La + Nd + Pr) / S mass ratio of 0.2 to 10 and a stretching ratio of 3 or less is 50% or more, Ce, Inclusion phase containing 1 type, 2 types, 3 types, or 4 types from La, Nd, Pr, Ca, and 1 type or 2 types from O, S ([Ce, La , Nd, Pr] -Ca- [O, S]) and an inclusion phase ([Ce, La, Nd) containing one, two, or three of Mn, Si, and Al. , Pr] —Ca— [O, S] — [Mn, Si, Al])), which is a complex inclusion having an inclusion phase containing a different component. In many cases, a large number of composite spherical inclusions having a size of 5 to 5 μm are formed.

  In addition, since the composite spherical inclusion having a circle equivalent diameter of 0.5 to 5 μm is a hard inclusion having a high melting point, it does not easily deform during rolling, and is not stretched even in a steel plate. That is, it is a spherical or spindle-shaped inclusion (sometimes generically referred to as a spherical shape in the present invention).

  Here, the spherical inclusions judged not to be stretched are not particularly defined, but are inclusions having a stretching ratio of 3 or less in the steel sheet, preferably inclusions of 2 or less. This is because, in the ingot stage before rolling, the inclusions are [Ce, La, Nd, Pr] —Ca— [O, S] inclusion phase of the first group and [Ce of the second group. , La, Nd, Pr] —Ca— [O, S] — [Mn, Si, Al], which are composed of composite inclusions containing different inclusion phases, and have an equivalent circle diameter of 0.5 to 5 μm. This is because one combined spherical inclusion was formed and the stretching ratio was 3 or less. In addition, since the spherical inclusion that is determined not to be stretched is completely spherical, the stretch ratio is 1, so the lower limit of the stretch ratio is 1.

  This investigation of the number ratio of inclusions was carried out in the same manner as the number ratio investigation of stretched inclusions. As a result, the first group contains one, two, three, or four from Ce, La, Nd, and Pr, contains Ca, and contains one or two from O and S. Inclusion phase ([Ce, La, Nd, Pr] -Ca- [O, S]) and a second group of inclusions containing one, two, or three of Mn, Si, and Al It consists of complex inclusions of inclusion phases containing different components from the physical phase ([Ce, La, Nd, Pr] -Ca- [O, S]-[Mn, Si, Al]), A composite spherical inclusion with a circle-equivalent diameter of 0.5 to 5 μm is formed, and the number ratio is 30% or more of the total number of inclusions with a circle-equivalent diameter of 0.5 to 5 μm. It was found that the stretched flangeability and bending workability were improved in the finished steel sheet.

  If the number ratio is less than 30%, the number ratio of the stretched inclusions of MnS increases correspondingly, and the stretch flangeability and bending workability deteriorate, which is not preferable.

  For this reason, the number ratio of one compounded spherical inclusion having a circle equivalent diameter of 0.5 to 5 μm is set to 30% or more. Here, the number ratio is obtained by measuring the major axis and minor axis of 50 stretch inclusions randomly selected using SEM from the SEM image. And the number ratio of the above-mentioned extending | stretching inclusion can be calculated | required by remove | dividing the number of the extending | stretching inclusions whose major axis / short diameter (stretching ratio) is 3 or less by the total number of inclusions investigated (50).

  The stretch flangeability and bending workability are better when a large number of composite spherical inclusions having a circle-equivalent diameter of 0.5 to 5 μm are deposited. Therefore, the upper limit of the number ratio is 100. %including.

  In addition, since one composite of spherical inclusions having a circle equivalent diameter of 0.5 to 5 μm is unlikely to be deformed even during rolling, the circle equivalent diameter is not particularly limited and may be 1 μm or more. However, if it is too large, there is a concern that cracking will start, so the upper limit is preferably about 5 μm.

  On the other hand, this composite inclusion is not easily deformed during rolling, and when the equivalent circle diameter is less than 0.5 μm, it does not become a starting point for cracking. Therefore, the lower limit of the equivalent circle diameter is not particularly specified. .

  Next, the existence condition of the composite inclusions in the steel sheet of the present invention described above is defined by the number density of inclusions per unit volume.

  The particle size distribution of the inclusion was carried out by SEM evaluation of the electrolytic surface by the speed method. The SEM evaluation of the electrolytic surface by the speed method is to evaluate the size and number density of inclusions by polishing the surface of the sample piece, performing electrolysis by the speed method, and directly observing the sample surface by SEM. The speed method is a method in which the sample surface is electrolyzed using 10% acetylacetone-1% tetramethylammonium chloride-methanol to extract inclusions. As the amount of electrolysis, the amount of electrolysis per cm 2 area of the sample surface is used. Was electrolyzed until 1C was reached. The SEM image of the surface electrolyzed in this manner was subjected to image processing, and the frequency (number) distribution with respect to the equivalent circle diameter was obtained. The average equivalent circle diameter was calculated from the frequency distribution of the particle diameters, and the number density of inclusions per volume was also calculated by dividing the frequency by the area of the observed visual field and the depth determined from the amount of electrolysis.

  On the other hand, in the steel sheet of the present invention described above, the inclusion group of [Ce, La, Nd, Pr] —Ca— [O, S] of the first group and the [Ce, La, Nd, Pr] -Ca- [O, S]-[Mn, Si, Al] is composed of composite inclusions containing different inclusion phase components, and a composite having an equivalent circle diameter of 0.5 to 5 μm. The existence condition of one spherical inclusion was defined by the content of the average composition of Ce, La, Nd or Pr in the inclusion.

  Specifically, as described above, in improving stretch flangeability and bending workability, the composite inclusion exists as a composite spherical inclusion having a circle-equivalent diameter of 0.5 to 5 μm, It is important to prevent stretching of MnS inclusions and the like.

  As a form of the composite inclusion, a composite spherical inclusion having a circle equivalent diameter of 0.5 to 5 μm or a spindle-like inclusion is formed.

  The spindle-shaped inclusion is not particularly specified, but is an inclusion having a drawing ratio of 3 or less in the steel plate, preferably an inclusion having 2 or less. Here, if it is perfectly spherical, the stretching ratio is 1, so the lower limit of the stretching ratio is 1.

  Therefore, in order to clarify the composition effective for stretch restriction, stretch flangeability and bending workability, a composition analysis of composite inclusions was conducted.

  However, if the inclusion has a circle equivalent diameter of 1 μm or more, the observation is easy, and therefore, for convenience, a circle equivalent diameter of 1 μm or more was targeted. However, if the observation is possible, inclusions having an equivalent circle diameter of less than 1 μm may be included.

  Moreover, since the form of the said composite inclusion was not extended | stretched, it was confirmed that all the extending | stretching ratios are the inclusion of 3 or less. Therefore, a composition analysis was performed on inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 3 or less.

  As a result, the inclusion having an equivalent circle diameter of 1 μm or more and an elongation ratio of 3 or less contains one, two, three, or four kinds of Ce, La, Nd, and Pr as shown in FIG. And the inclusion phase of the 1st group of the ingredient which contains Ca and contains 1 type or 2 types from O and S, and also 1 type, 2 types, or 3 types from Mn, Si, and Al It turned out that it consists of a component composition of the composite inclusion of the form which contains two or more inclusion phases containing the different component of the inclusion group of the 2nd group of the component to contain. And when the composite inclusion contains 0.5 to 95% of the total of one, two, three or four kinds of Ce, La, Nd and Pr with an average composition, stretch flangeability and bending workability Was found to improve.

  The total average content of 1 type, 2 types, 3 types or 4 types of Ce, La, Nd, and Pr in inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 3 or less is 0.5 mass%. If the ratio is less than 1, the number ratio of inclusions in the above form is greatly reduced, and accordingly, the number ratio of MnS-based inclusions that tend to be the starting point of cracking is excessively increased. Sex is reduced.

  On the other hand, when the total average content of one, two, three, or four of Ce, La, Nd, and Pr in inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 3 or less exceeds 95%. Since cerium oxysulfide and lanthanum oxysulfide are produced in large amounts and become coarse inclusions having an equivalent circle diameter of about 50 μm or more, stretch flangeability and bending workability are deteriorated.

  Next, the structure of the steel plate will be described.

  In the present invention, fine MnS inclusions are precipitated in the ingot, and further, they are not deformed during rolling and are dispersed in the steel plate as fine spherical inclusions that are unlikely to start cracking. Workability is improved, and the microstructure of the steel sheet is not particularly limited.

  The microstructure of the steel sheet is not particularly limited, but a steel sheet having a structure with bainitic ferrite as the main phase, a steel sheet with a composite structure having the ferrite phase as the main phase, the martensite phase, and the bainite phase as the second phase. And any structure of a steel sheet having a composite structure composed of ferrite, retained austenite, and low-temperature transformation phase (martensite or bainite).

  Therefore, any structure is preferable because the crystal grain size can be reduced to 10 μm or less, and the hole expandability and bending workability can be improved. When the average particle size exceeds 10 μm, the improvement in ductility and bending workability becomes small. In order to improve hole expansibility and bending workability, the thickness is more preferably 8 μm or less. However, in general, when it is necessary to obtain excellent stretch flangeability such as undercarriage parts, it is desirable that the ferrite or bainite phase is the largest phase by area ratio although it is slightly inferior in ductility. preferable.

  Next, manufacturing conditions will be described.

  In the present invention, deoxidization and component adjustment are performed by adding and stirring an alloy such as C, Si, Mn, etc. in molten steel blown in a converter and decarburized, or further decarburized using a vacuum degasser. I do.

  Moreover, about S, since it is not necessary to desulfurize by a refining process as above-mentioned, a desulfurization process can be abbreviate | omitted. However, when molten steel desulfurization is necessary in secondary refining in order to melt extremely low-sulfur steel with S ≦ 20 ppm, component adjustment may be performed by desulfurization.

About 3 minutes after adding Si, it is preferable to secure a flying time of about 3 minutes in order to add Al and perform Al deoxidation to separate Al ₂ O ₃ by floating.

Thereafter, one, two, three or four of Ce, La, Nd and Pr are added, and 70 ≧ 100 × (Ce + La + Nd + Pr) / acid-soluble Al ≧ 2 and (Ce + L) on a mass basis.
Component adjustment is performed so that a + Nd + Pr) / S is 0.2 to 10.

  By the way, when adding a selective element, do it before adding one, two, three or four of Ce, La, Nd, and Pr, stir well, and adjust the components of the selected element as necessary Is added, 1 type, 2 types, 3 types, or 4 types of Ce, La, Nd, and Pr are added. Thereafter, the mixture is sufficiently stirred and Ca is added. The molten steel thus melted is continuously cast to produce an ingot.

  For continuous casting, not only is it applied to normal slab continuous casting of about 250 mm thickness, but it is also used for thin slab continuous casting where the mold thickness of blooms and billets and slab continuous casting machines is thinner than usual, for example 150 mm or less. And is fully applicable.

  The hot rolling conditions for producing a high strength hot rolled steel sheet will be described.

  The heating temperature of the slab before hot rolling requires that the carbonitrides in the steel are once dissolved, and for that purpose, it is important to set the temperature above 1200 ° C.

  When these carbonitrides are dissolved, a ferrite phase preferable for improving ductility can be obtained in the cooling process after rolling. On the other hand, when the heating temperature of the slab before hot rolling exceeds 1250 ° C., oxidation of the slab surface becomes remarkable, and in particular, wedge-shaped surface defects due to selective oxidation of grain boundaries remain after descaling, Since the surface quality after rolling is impaired, the upper limit is preferably set to 1250 ° C.

  After being heated to the above temperature range, normal hot rolling is performed. In the process, the finish rolling completion temperature is important when the structure of the steel sheet is controlled. If the finish rolling completion temperature is less than Ar3 point + 30 ° C., the crystal grain size of the surface layer portion tends to be coarse, which is not preferable in terms of bending workability. On the other hand, if the Ar3 point exceeds + 200 ° C., the austenite grain size after the end of rolling becomes coarse, making it difficult to control the composition and fraction of the phase generated during cooling, so the upper limit is preferably set to the Ar3 point + 200 ° C.

  In addition, when the average cooling rate of the steel sheet after finish rolling is 10 to 100 ° C./second and the coiling temperature is in the range of 450 to 650 ° C., the air cooling is maintained at about 5 ° C./second until 680 ° C. after finish rolling. Then, cooling is performed at a cooling rate of 30 ° C./second or more, and the coiling temperature is set to 400 ° C. or less, and the selection is made according to the target tissue configuration. By controlling the cooling rate and coiling temperature after rolling, the former rolling conditions had one or more microstructures and fractions from polygonal ferrite, bainitic ferrite, and bainite phase. With the latter rolling conditions, a DP steel sheet having a large amount of a polygonal ferrite phase and a martensite phase composite structure excellent in ductility can be obtained.

  If the above average cooling rate is less than 10 ° C./second, pearlite which is unfavorable for stretch flangeability tends to be generated, which is not preferable. On the other hand, there is no need to set an upper limit on the cooling rate in terms of structure control, but too high a cooling rate may cause uneven cooling of the steel sheet, and it is expensive to manufacture equipment that enables such cooling. It is thought that this will lead to an increase in the price of the steel sheet. From such a viewpoint, the upper limit of the cooling rate is preferably set to 100 ° C./second.

  The high-strength cold-rolled steel sheet according to the present invention is manufactured by cold rolling and annealing a steel sheet that has undergone processes such as pickling and skin pass after hot rolling and winding. The final cold-rolled steel sheet is obtained by annealing in an annealing process such as batch annealing or continuous annealing.

  Needless to say, the high-strength steel sheet according to the present invention may be applied as a steel sheet for electroplating. Even if electroplating is applied, there is no change in the mechanical properties of the high-strength steel sheet of the present invention.

  Examples of the present invention will be described below together with comparative examples.

Molten steel having chemical components shown in Table 1 was melted via a converter and an RH process. In that case, when not passing through the molten steel desulfurization process in secondary refining, S was made into 0.003-0.011 mass%.
Moreover, when performing molten steel desulfurization, it was set as S <= 20ppm.

After adding Si and adjusting the components as shown in Table 1, after about 3 to 5 minutes, Al is added and Al deoxidation is performed to float and separate Al ₂ O ₃ . An ascent time of about 3 to 6 minutes was secured.

  Then, depending on the charge of the experiment, 1 type, 2 types, 3 types, or 4 types of Ce, La, Nd, and Pr were added, and 70 ≧ 100 × (Ce + La + Nd + Pr) / acid-soluble Al ≧ 2, on a mass basis, And component adjustment was performed so that (Ce + La + Nd + Pr) / S might be 0.2-10.

  Depending on the charge of the experiment in which the selective element is added, it is performed before adding one, two, three or four of Ce, La, Nd, and Pr. After the adjustment, one, two, three, or four kinds of Ce, La, Nd, and Pr were added. Then, it fully stirred and Ca addition was performed. The molten steel thus produced was continuously cast to produce an ingot.

  For continuous casting, a normal slab continuous casting machine having a thickness of about 250 mm was used.

  The continuously cast ingot was heated in the range of more than 1200 ° C to 1250 ° C under the hot rolling conditions shown in Table 2.

  Thereafter, rough rolling was performed and finish rolling was performed. The completion temperature of the finish rolling was Ar3 point + 30 ° C. or higher and Ar3 point + 200 ° C. or lower. Here, the calculation derived from the normal component was used to calculate the Ar3 point.

  The average cooling rate of the steel sheet after finish rolling was 10 to 100 ° C./second. Further, depending on the charge of the experiment, when the coiling temperature is set in the range of 450 to 650 ° C., it is air-cooled at about 5 ° C./second until 680 ° C. after finish rolling, and then at a cooling rate of 30 ° C./second or more Cooled down.

  By this cooling, a steel sheet having one or more structures could be obtained from polygonal ferrite, bainitic ferrite, and bainite phase.

  On the other hand, depending on the charge of the experiment, a DP steel sheet having a composite structure of a polygonal ferrite phase and a martensite phase could be obtained by winding at 400 ° C. or lower.

  When obtaining a high-strength cold-rolled steel sheet, the hot-rolled steel sheet was cold-rolled and subjected to continuous annealing after hot rolling, winding, pickling, skin pass, and the like to obtain a cold-rolled steel sheet. Furthermore, when obtaining the steel plate for plating, it was set as the steel plate for plating by the electroplating or the hot dip galvanizing line.

  The chemical composition of the slab is shown in Table 1.

  Table 2 shows the hot rolling conditions. As a result, a hot-rolled sheet having a thickness of 3.2 mm was obtained.

  In Table 1, steel numbers (hereinafter referred to as steel numbers) 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, About 35 and 37, it comprises with the composition within the range of the high-strength steel plate which concerns on this invention, and steel numbers 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, and 38, (Ce + La + Nd + Pr) / acid-soluble Al ratio, (Ce + La + Nd + Pr) / S ratio, S, T. The O, Ca, Ce + La + Nd + Pr concentration is configured as a slab that deviates from the range of the high-strength steel sheet according to the present invention.

  By the way, in this table 1, steel number 1 and steel number 2, steel plate 3 and steel number 4, steel number 5 and steel number 6, steel number 7 and steel number 8, steel number 9 and steel number 10, steel number 11 Steel No. 12, Steel No. 13 and Steel No. 14, Steel No. 15 and Steel No. 16, Steel No. 17 and Steel No. 18, Steel No. 19 and Steel No. 20, Steel No. 21 and Steel No. 22, Steel No. 23 and Steel No. 24, Steel No. 25 and Steel No. 26, Steel No. 27 and Steel No. 28, Steel No. 29 and Steel No. 30, Steel No. 31 and Steel No. 32, Steel No. 33 and Steel No. 34, Steel No. 35 and Steel No. 36, In order to be able to compare between the steel number 37 and the steel number 38, Ce + La and the like are made different from each other after they are configured with substantially the same composition.

  In Table 2, as Condition A, the heating temperature is 1250 ° C., the finish rolling completion temperature is 845 ° C., the cooling rate after finish rolling is 75 ° C./second, and the winding temperature is 450 ° C. As condition B, the heating temperature is 1250 ° C., the finish rolling completion temperature is 860 ° C., and after the finish rolling is maintained at about 5 ° C./second until 680 ° C. After that, the cooling rate is 30 ° C./second or more, and the winding temperature is 400 ° C. It is said. As condition C, the heating temperature is 1250 ° C., the finish rolling completion temperature is 825 ° C., the cooling rate after finish rolling is 45 ° C./second, and the winding temperature is 450 ° C.

  For Steel No. 1 and Steel No. 2, Condition B, for Steel No. 3 and Steel No. 4, Condition B, for Steel No. 5 and Steel No. 6, Condition A, Furthermore, for Steel No. 7 and Steel No. 8, Condition A, for Steel No. 9 and Steel No. 10, Condition A, and for Steel No. 11 and Steel No. 12, Condition C. By applying the condition B to the steel numbers 13 and 14, the influence of the chemical composition can be compared under the same manufacturing conditions.

  The critical bending radius (mm) was investigated as the strength (MPa), ductility (%), stretch flangeability (λ%), and bending workability of the basic properties of the steel sheet thus obtained.

  In addition, as for the presence of the stretched inclusions in the steel sheet, the inclusion has an area number density of inclusions of 2 μm or less and a stretching ratio of 3 or less for all inclusions of about 1 μm or more by observation with an optical microscope or SEM. The product was examined for number ratio, volume number density, and average equivalent-circle diameter (herein, the average is an arithmetic average, and the same applies hereinafter).

  Furthermore, the inclusion state of the non-stretched inclusions in the steel sheet contains 1 type, 2 types, 3 types, or 4 types of Ce, La, Nd, and Pr, all targeting inclusions of about 1 μm or more. And the inclusion group of the 1st group which contains Ca and contains 1 type or 2 types from O and S, and also contains 1 type, 2 types, or 3 types from Mn, Si, and Al The number ratio and volume number density of composite inclusions in the form of an inclusion phase comprising two or more inclusion phases containing different components of the second group of inclusion phases, and Ce and La in inclusions with a drawing ratio of 3 or less The average value of the total content of 1, 2, 3 or 4 of Nd, Pr was examined.

  The reason why inclusions of about 1 μm or more are targeted is that observations are easy, and inclusions of less than about 1 μm do not affect the deterioration of stretch flangeability and bending workability.

  The results are shown in Table 3 for each combination of steel and rolling conditions.

  Strength and ductility were determined by a tensile test of a JIS No. 5 test piece taken from the steel plate in parallel with the rolling direction. Stretch flangeability is measured by measuring the hole diameter D (mm) when a through-thickness crack is generated by punching and expanding a punched hole with a diameter of 10 mm in the center of a 150 mm x 150 mm steel plate with a 60 ° conical punch. The hole expansion value λ = (D−10) / 10. The critical bending radius (mm) used as an index representing bending workability was obtained by a V-bending test using a die having a die and a punch taken from a bending test piece. A die having a V-shaped recess and an opening angle of 60 ° was used. A punch having a convex portion that fits into the concave portion of the die was used. Prepare a punch with the bend radius of the tip of the punch changed in 0.5mm increments, conduct a bending test, and make the tip of the tip of the punch with the smallest limit that causes cracks in the bent part of the specimen to be tested The curvature radius was obtained and evaluated as the critical bending radius.

  Note that the test piece is a No. 1 test piece defined in the same standard, with a parallel part of 25 mm, a radius of curvature R of 100 mm, and a thickness of 3.0 mm obtained by equally grinding both surfaces of the original plate (hot rolled plate). .

  Further, the inclusions were observed by SEM, and the major axis and the minor axis were measured for 50 inclusions having a circle-equivalent diameter of 1 μm or more selected at random. Further, using the quantitative analysis function of SEM, composition analysis was performed on 50 inclusions with a diameter of 1 μm or more selected at random. Using these results, the number ratio of inclusions with a stretching ratio of 3 or less, the average equivalent circle diameter of inclusions with a stretching ratio of 3 or less, the number ratio of composite inclusions, and the Ce in the inclusion with a stretching ratio of 3 or less, The average value of the total of one type, two types, three types, or four types of La, Nd, and Pr was determined. Further, the volume number density of inclusions by shape was calculated by SEM evaluation of the electrolytic surface by the speed method.

  As is clear from Table 3, in the case of odd steel numbers such as steel numbers 1, 3, 5, 7, 9, 11, and 13 to which the method of the present invention is applied, the composite inclusions defined in the present invention are generated. The stretched MnS inclusions could be reduced in the steel sheet. That is, there are fine spherical composite inclusions having a circle-equivalent diameter of 0.5 to 5 μm present in the steel sheet, and the component composition of the composite inclusions is the first group of [Ce, La, Nd, Pr] -Ca- [O, S] inclusion phase and the second group of [Ce, La, Nd, Pr] -Ca- [O, S]-[Mn, Si, Al] Among the inclusion phases, the inclusion phase comprised two or more inclusion phases containing different components. In addition, the ratio of the number of the composite spherical inclusions having the equivalent circle diameter of 0.5 to 5 μm is 30% or more of the total number of inclusions having the equivalent circle diameter of 0.5 to 5 μm. And the ratio of the number of elongated inclusions having a major axis / minor axis of 3 or less is 50% or more of the total number of inclusions having an equivalent circle diameter of 1 μm or more, and Ce in the inclusions. , La, Nd, and Pr, the total average content of one type, two types, three types, or four types could be 0.5% to 95%. In any steel sheet structure, the average crystal grain size was 1 to 8 μm, and the present invention and the comparative example had almost the same average crystal grain size.

  As a result, in comparison with the comparative steel, the steel numbers 1, 3, 5, 7, 9, 11, 13 and so on as the steel of the present invention can be used to obtain a steel plate having excellent stretch flangeability and bending workability. I was able to. However, in the comparative steel (even steel numbers such as steel numbers 2, 4, 6, 8, 10, 12, 14 etc.), the average crystal grain size is over 10 μm and almost contains Ce, La, Nd, and Pr. There is no longer major axis / minor minor axis of 3 or more stretched inclusions, that is, stretched MnS-based inclusions. It became the starting point of crack generation, and stretch flangeability and bending workability were reduced.

  Tables 4 and 5 show the composition of inclusions and the hole expansion ratio when the addition order of Ca, Ce, La, Nd, and Pr is changed to 1, 2, 3 or 4 according to the present invention. The result of having compared with Comparative Example 20 is shown. In the present invention 20, Ca is added after the addition of Ce among Ce, La, Nd, and Pr. On the contrary, in Comparative Example 20, when Ce is added after adding Ca, inclusions are added. Is an inclusion in which an oxide or oxysulfide of Ce and MnS are precipitated in CaS, and an inclusion comprising an inclusion phase including two or more inclusion phases containing different components as defined in the present invention. In contrast, the rate of extension of inclusions was large, and the hole expansion rate was lower than that of the inventive example of the present application.

  In Tables 6 and 7, the inclusion composition and the hole expansion ratio of Comparative Example 21 in the case where Ca was not added after the addition of Ce and La, the present invention 21 (two types of addition of Ce and La) The result compared with Ca added later is shown. If Ca was not added after the addition of Ce and La, the immersion nozzle was blocked during casting in the continuous casting facility, and all the molten steel in the ladle could not be completely cast. After that, the pan could not be cast, causing production problems. Among them, a slab that could be cast halfway was processed after hot rolling to obtain a product. Inclusions in the product were Ce and La oxides or oxysulfide containing MnS. Unlike the inclusion composed of inclusion phases containing two or more inclusion phases containing different components of the present application, the inclusions have a large extension ratio and the hole expansion ratio is the present invention 21. Compared to

Claims (11)

% By mass
C: 0.03-0.25%,
Si: 0.1 to 2.0%,
Mn: 0.5 to 3.0%
P: 0.05% or less,
T.A. O: 0.0050% or less,
S: 0.0001 to 0.01%,
N: 0.0005 to 0.01%,
Acid soluble Al: more than 0.01%,
Ca: 0.0005 to 0.0050%,
Total of one or more of Ce, La, Nd or Pr: 0.001 to 0.01%, and further, on a mass basis, 70 ≧ 100 × (Ce + La + Nd + Pr) / acid-soluble Al> 0.7, and , (Ce + La + Nd + Pr) / S is 0.2 to 10,
The balance is a steel plate of chemical composition consisting of iron and inevitable impurities,
In the steel plate,
Ce, La, Nd, 1 kind of Pr, 2 kinds, and three or four,
And, and Ca,
And the inclusion phase of the chemical component which consists of 1 type or 2 types from O and S,
1 type, 2 types, 3 types, or 4 types from Ce, La, Nd, and Pr,
And Ca,
In addition, a composite of inclusion phases containing different components, including one or two types of O and S and one or two or three types of chemical components consisting of Mn, Si and Al. The composite inclusions form one composite spherical inclusion having a circle-equivalent diameter of 0.5 to 5 μm, and the number ratio of the spherical inclusions is 0.5 to the equivalent circle diameter. A high-strength steel sheet excellent in stretch flangeability and bending workability, characterized by being 30% or more of the total number of inclusions having a size of 5 μm.   The spherical inclusion is an inclusion having an equivalent circle diameter of 1 μm or more, and the ratio of the number of elongated inclusions having a major axis / minor axis of 3 or less is 50% or more of the total number of inclusions having an equivalent circle diameter of 1 μm or more. The high-strength steel sheet excellent in stretch flangeability and bending workability according to claim 1.   3. The spherical inclusion contains 0.5 to 95% by mass of one, two, three or four kinds of Ce, La, Nd or Pr in an average composition. High-strength steel sheet with excellent stretch flangeability and bending workability as described in 1.   The high-strength steel sheet excellent in stretch flangeability and bending workability according to any one of claims 1 to 3, wherein an average grain size of crystals in the structure of the steel sheet is 10 µm or less. The chemical composition of the steel sheet is further mass%,
Nb: 0.01-0.10%,
V: 0.01 to 0.10%,
Any one or two of these are contained, The high strength steel plate excellent in stretch flangeability and bending workability of any one of Claims 1-4 characterized by the above-mentioned. The chemical composition of the steel sheet is further mass%,
Cu: 0.1 to 2%,
Ni: 0.05 to 1%,
Cr: 0.01-1%,
Mo: 0.01 to 0.4%,
B: 0.0003 to 0.005%,
The high strength steel plate excellent in stretch flangeability and bending workability of any one of Claims 1-5 characterized by containing any 1 type, or 2 or more types of these. The chemical composition of the steel sheet is further mass%,
Zr: 0.001 to 0.01%,
The high-strength steel sheet excellent in stretch flangeability and bending workability according to any one of claims 1 to 6.   In the refining process in steelmaking, C is 0.03 to 0.25% and Si is 0.1 to 2% by mass%, P is 0.05% or less, and S is 0.0001% or more. 0.0%, Mn 0.5-3.0%, N is added or adjusted to be 0.0005-0.01%, and then Al is acid-soluble Al and exceeds 0.01%. . O is added so as to be 0.0050% or less, and then one or more of Ce, La, Nd, or Pr is added. Furthermore, on a mass basis, 70 ≧ 100 × (Ce + La + Nd + Pr) / Acid soluble Al> 0.7 and (Ce + La + Nd + Pr) / S is 0.2 to 10, and the total of one or more of Ce, La, Nd or Pr is 0.001 to 0.01%. The high strength excellent in stretch flangeability and bending workability according to any one of claims 1 to 4, wherein Ca is added or adjusted so that Ca is 0.0005 to 0.0050% later. A method for producing molten steel for steel plates.   In the refining step, before adding one or more of Ce, La, Nd, or Pr, Nb is further 0.01 to 0.10% and V is 0.01 to 0.00 in terms of mass%. The method for melting molten steel for high-strength steel sheets excellent in stretch flangeability and bending workability according to claim 8, which is added so as to be any one or two of 10%.   In the refining step, before adding one or more of Ce, La, Nd or Pr, further, 0.1% to 2% Cu, 0.05% to 1% Ni, Cr, Cr 0.01 to 1% of Mo, 0.01 to 0.4% of Mo, and 0.0003 to 0.005% of B are added so as to be any one kind or two kinds or more. Item 10. A method for producing molten steel for high-strength steel sheets excellent in stretch flangeability and bending workability according to Item 8 or 9.   In the refining step, before adding one or more of Ce, La, Nd or Pr, Zr is further added so as to be 0.001 to 0.01% by mass%. The method for melting molten steel for high-strength steel sheets excellent in stretch flangeability and bending workability according to any one of claims 8 to 10.

Images