JP4268559B2 - High strength steel plate with excellent stretch flangeability - Google Patents

Description

  The present invention relates to a high-strength steel sheet excellent in stretch flangeability, which is suitable for building materials, home appliances, automobiles, and the like. The high-strength steel plate in the present invention includes not only a normal cold-rolled steel plate but also a galvanized steel plate or a steel plate subjected to various platings such as an Al-plated steel plate. The galvanized steel sheet includes not only normal hot dip galvanizing but also alloyed hot dip galvanizing. The plating layer includes not only pure zinc but also one containing Fe, Al, Mg, Cr, Mn or the like.

  In recent years, there has been an increasing demand for high-strength steel sheets with good workability for the purpose of improving fuel consumption and durability especially in automobile bodies. In addition, steel plates with a tensile strength of 780 MPa or higher are being used for some parts such as reinforcement because of the need for collision safety and expansion of cabin space.

  When assembling a member using such a high-strength material, ductility, bendability, stretch flangeability, etc. are important. In high-strength steel sheets with a tensile strength of up to about 780 MPa, measures are taken for these. Yes.

  For example, as described in Non-Patent Document 1, with regard to hole expansibility, the main phase is bainite to improve the hole expansibility, and further, with regard to the stretch formability, by generating residual austenite in the second phase, It is disclosed that it shows the same stretchability as the current retained austenitic steel.

  In order to increase the ductility of a high-strength material, it is common to actively utilize a composite structure. However, when martensite or retained austenite is used for the second phase, there is a problem that the hole expandability is significantly lowered (for example, Non-Patent Document 2). Further, this document discloses that the hole expansion ratio is improved by reducing the difference in hardness between the main phase of ferrite and the second phase of martensite.

  In addition, there are some disclosed examples of those subjected to hot dip galvanization. For example, Patent Documents 1 to 4 are representative examples.

Japanese Patent No. 2607906 Japanese Patent No. 2862187 JP 2001-355043 A Japanese Patent No. 3037767 CAMP-ISIJ vol. 13 (2000) p. 395 CAMP-ISIJ vol. 13 (2000) p. 391

  As described above, a large number of steel plates having excellent stretch flangeability represented by hole expandability have been developed. However, a high-strength steel sheet having a tensile strength of 780 MPa or more contains C or a large amount of alloy elements, so that the structure of the product is likely to change depending on manufacturing conditions such as temperature and cooling rate, and good stretch flangeability is not always obtained. Not.

  According to the study by the present inventors, this is caused by the second phase (phase having a smaller area ratio than the main phase) and the main phase (phase having the largest area ratio) generated due to variations in manufacturing conditions such as temperature and cooling rate. It was found that the inclusions existed at the boundary.

  That is, the second phase has a hardness different from that of the main phase, and the boundary between both phases is in an environment where cracks are likely to progress. Therefore, there is a coarse cluster-like inclusion that tends to cause cracks at the boundary between both phases. When present, it was found that cracks occurred starting from coarse cluster-like inclusions during hole expansion, and the cracks propagated through the grain boundaries of both phases and expanded to the surface layer all at once.

  Therefore, if the inclusions in the steel are made as fine as possible and the second phase is generated due to variations in manufacturing conditions, the inclusions at the grain boundaries do not become the starting point for cracking, improving hole expandability. I found out that

  Generally, deoxidation of steel is performed using Al, but alumina inclusions generated by Al deoxidation are easily clustered and remain in the steel as coarse inclusions. This is considered to have reduced the stretch flangeability (hole expansion value) as described above, but there have been examples in which high strength steel plates with excellent stretch flangeability were proposed from the viewpoint of inclusion microsphere control. Absent.

  In view of such a situation, the inventors of the present invention have almost no Al deoxidation in the molten steel component of the high-strength steel plate containing C or a large amount of alloy elements in order not to generate (i) alumina inclusions that are easily coarsened. Without degrading, (ii) the inclusions are not clustered and coarsened, and (iii) the deoxidation method is modified to form spherical inclusions that do not easily start cracking. The present invention was completed by examining the components and the production method.

  The summary is as follows.

(1) C: 0.03-0.3 mass%, Si: 0.1-2.0 mass%, Mn: 1.0-3.5 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, N: 0.0005 to 0.01 mass%, acid-soluble Al: 0.01 mass% or less, acid-soluble Ti: less than 0.008 mass%, one of Ce and La Total of two types: steel containing 0.0005 to 0.04% by mass, the balance being iron and inevitable impurities, and in the steel, Ce oxide and La oxidation with an average inclusion composition The stretch flangeability is characterized by including inclusions in which the total of one or two of the substances is 10 to 90% by mass, SiO ₂ is 5 to 60% by mass, and Al ₂ O ₃ is 50% by mass or less. Excellent high strength steel plate.

(2) C: 0.03-0.3 mass%, Si: 0.1-2.0 mass%, Mn: 1.0-3.5 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, N: 0.0005 to 0.01 mass%, acid-soluble Al: 0.01 mass% or less, acid-soluble Ti: less than 0.008 mass% (however, 0.005 mass%) Except for the above) , a total of one or two of Ce and La: 0.0005 to 0.04% by mass, and the balance is iron and inevitable impurities, and the steel has intervening A high-strength steel sheet excellent in stretch flangeability, characterized in that 50% or more of the number of objects is composed of spherical and spindle inclusions.

(3) C: 0.03-0.3 mass%, Si: 0.1-2.0 mass%, Mn: 1.0-3.5 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, N: 0.0005 to 0.01 mass%, acid-soluble Al: 0.01 mass% or less, acid-soluble Ti: less than 0.008 mass% (however, 0.005 mass%) Except for the above) , a total of one or two of Ce and La: 0.0005 to 0.04 mass%, the balance being iron and inevitable impurities, and the steel contains 0 A high-strength steel sheet excellent in stretch flangeability, characterized in that inclusions of 5 μm or more and 10 μm or less exist between 1000 pieces / cm ² and 100,000 pieces / cm ² .

  According to the present invention, it is possible to obtain a high-strength steel plate having high strength and being hardly affected by variations in manufacturing conditions, and having excellent stretch flangeability.

  Hereinafter, the present invention will be described in detail.

  First, the reason for limiting the component range of the steel sheet in the present invention will be described.

  C is an essential element for securing the strength of the steel plate, and at least 0.03 mass% is necessary to obtain a high strength steel plate of 780 MPa or more. However, if C exceeds 0.3% by mass, stretch flangeability deteriorates remarkably, so 0.3% by mass is made the upper limit.

  Si is extremely important in the present invention because Si is a major deoxidizing element in molten steel to which Al or Ti is not added as much as possible. In order to reduce the dissolved oxygen concentration in the molten steel and prevent the generation of CO bubbles during casting, it is necessary to add Si by 0.1% by mass or more.

  Further, Si is an element effective for securing the strength without deteriorating stretch flangeability, and at least 0.05% by mass is necessary. However, considering deoxidation conditions together, Si is 0.1%. Mass% is the lower limit. If added in excess, the weldability and ductility are adversely affected, so 2.0 mass% is the upper limit.

  Mn is an element effective for increasing the strength of the steel sheet together with C and Si, and it is necessary to contain 1.0% by mass or more, but if it exceeds 3.5% by mass, the ductility deteriorates. , MnS precipitates a lot and lowers the stretch flangeability, so the upper limit is made 3.5% by mass.

  P is effective as a solid solution strengthening element, but since there is a concern about deterioration of workability due to segregation, it is necessary to make it 0.05% by mass or less. The lower limit value of P includes 0% by mass.

  S forms inclusions such as MnS and deteriorates stretch flangeability. Therefore, it should be suppressed as much as possible, but is acceptable if it is 0.01% by mass or less. The lower limit value of S includes 0% by mass.

  N is effective in increasing mechanical strength and imparting BH properties (seizure curability), but if added too much, coarse precipitates will be formed even in trace amounts of Al and trace amounts of Ti. Since it produces and deteriorates stretch flangeability, 0.01 mass% is made an upper limit. On the other hand, when N is less than 0.0005% by mass, the current process is very heavy and the cost is high, so the lower limit is made 0.0005% by mass.

It is desirable to suppress acid-soluble Al as much as possible because the oxide is likely to cluster and become coarse. However, it is permissible to use up to 0.01% by mass as a preliminary deoxidizer. This is because when the acid-soluble Al concentration exceeds 0.01% by mass, the content of Al ₂ O _{3 in} inclusions exceeds 50% by mass, and inclusions can be clustered even if Ce or La is added. This is because it cannot be prevented, and the hole expandability is reduced.

From the viewpoint of preventing clustering, the acid-soluble Al concentration should be low, and the lower limit includes 0% by mass. The acid-soluble Al concentration is a value obtained by measuring the amount of Al dissolved in an acid. The value analyzed by an analysis method using the fact that dissolved Al is dissolved in an acid and Al ₂ O ₃ is not dissolved in an acid. It is.

  The acid-soluble Ti is also less than 0.008% by mass because its oxides are likely to be clustered and become coarse, and it is easy to form coarse TiN inclusions in combination with N in the steel. Contains 0% by weight. The acid-soluble Ti concentration is a value obtained by measuring the amount of Ti dissolved in an acid, and is a value analyzed by an analysis method utilizing that dissolved Ti dissolves in an acid and Ti oxide does not dissolve in an acid. .

Ce and La are Al ₂ O ₃ —SiO ₂ inclusions produced by preliminary Al deoxidation and Si deoxidation, and fine spherical Ce oxides (eg, Ce ₂ O ₃ , CeO ₂ ) —Al ₂ O ₃ — SiO ₂ composite inclusion, La oxide (for example, La ₂ O ₃ , LaO ₂ ) -Al ₂ O ₃ —SiO ₂ composite inclusion, or Ce oxide-La oxide-Al ₂ O ₃ —SiO ₂ composite It is the most important component that improves the inclusion and improves the stretch flangeability.

  In order to obtain such an inclusion modification effect, the total concentration of Ce and La needs to be 0.0005 mass% or more and 0.04 mass% or less.

When the total concentration of Ce and La is less than 0.0005 mass%, the Al ₂ O ₃ —SiO ₂ inclusion cannot be modified into a fine sphere, and when it exceeds 0.04 mass%, the SiO ₂ content in the inclusion is reduced. Then, almost Ce oxide-Al ₂ O ₃ inclusions and La oxide-Al ₂ O ₃ inclusions are formed, the inclusions are clustered, and the stretch flangeability is lowered.

  Next, conditions for inclusions in the steel sheet will be described.

  In order to obtain a steel sheet excellent in stretch flangeability, it is important that inclusions in the steel sheet are spherically and finely dispersed so that cracks are unlikely to become the starting point of cracking. The average composition, morphology, and particle size distribution of inclusions in the steel sheet of the present invention were investigated. The average composition of inclusions can be obtained by analyzing the composition of a plurality of randomly selected inclusions (for example, about 20) and calculating the average concentration.

As a result, one or a total 10 to 90 wt% or more of Ce oxide and La oxide with an average composition of inclusions, SiO ₂ is 5 to 60 wt%, Al ₂ O ₃ is less than 50 wt% It was found that the stretch flangeability is improved in the steel sheet whose composition is controlled so as to be in the above range.

  When the total of one or two of Ce oxide and La oxide is less than 10% by mass with an average composition, the inclusion modification effect due to addition of Ce or La is small. If the total of 1 type or 2 types exceeds 90% by mass, over-reformation occurs, and in any case, inclusions do not spheroidize, so that one or two types of Ce oxide and La oxide are used in average composition. The lower limit of the total was 10% by mass, and the upper limit was 90% by mass.

On the other hand, if the average composition of SiO ₂ exceeds 60% by mass, very long stretched inclusions are formed during rolling, and many of these inclusions are present in the steel sheet, resulting in reduced stretch flangeability. Conversely, SiO ₂ is less than 5% by mass. Then, since inclusions do not disperse into fine spheres, the upper limit of SiO ₂ is 60% by mass and the lower limit is 5% by mass in the average composition.

Furthermore, it is preferable not to contain Al ₂ O ₃ in inclusions from the viewpoint of fine spheroidization, but due to the influence of Al preliminary deoxidation and refractory melting, the content of Al ₂ O _{3 in} inclusions is reduced. May be high. In this case, Al ₂ O ₃ may be mixed in the inclusions only to 50% by mass or less in the average composition, and the lower limit includes 0% by mass.

This is because when the Al ₂ O ₃ content in the inclusions exceeds 50% by mass, the reforming effect by Ce or La is impaired, and the inclusions are clustered. In the present invention, inevitable impurity oxides mixed from slag, refractory, etc. are allowed in addition to the oxide having the above composition.

  Next, the form of inclusions in the steel sheet will be described. Regarding the form of inclusions, a plurality of inclusions selected at random can be observed with an optical microscope. For example, 100 inclusions selected at random are observed at 100 times and 1000 times of an optical microscope, and are classified into spherical, spindle, cluster, and others, and the number ratio of spherical and spindle-like inclusions is determined. It is recommended to seek.

  As a result, it has been found that the stretch flangeability is improved in a steel plate in which the number ratio of spherical and spindle inclusions is 50% or more. When the number ratio of spherical and spindle-shaped inclusions is less than 50%, cluster-like inclusions are relatively increased and stretch flangeability is lowered, so the lower limit was made 50%. The upper limit is 100%.

  Furthermore, the particle size distribution of inclusions in the steel sheet will be described. The particle size distribution of the inclusions can be measured by observing the inclusions with an optical microscope (for example, 100 times and 1000 times). Here, the particle diameter means an equivalent circle diameter.

  In addition, regarding the particle size of inclusions, when the number of large inclusions exceeding 10 μm, which is harmful to stretch flangeability, decreases, the number of inclusions between 0.5 μm and 10 μm increases. In order to correspond well with this, attention was focused on 0.5 μm or more and 10 μm or less.

As a result, it has been found that the stretch flangeability is improved in the steel sheet in which the inclusions of 0.5 μm or more and 10 μm or less exist at 1000 pieces / cm ² or more and 100,000 pieces / cm ² or less.

If the number of inclusions of 0.5 μm or more and 10 μm or less is less than 1000 / cm ² , large inclusions that are harmful to stretch flangeability exceeding 10 μm are observed in the steel sheet and become the starting point of cracking. The lower limit was 1000 / cm ² .

Further, when there are more than 100,000 inclusions / cm ² in the range of 0.5 μm or more and 10 μm or less, the number of inclusions is too large and the stretch flangeability deteriorates, so the upper limit is 100,000 / cm ² . did.

  The present invention can naturally be applied to steel of a strength class having a TS (tensile strength) of less than 780 MPa. However, since the problem of stretch flangeability is almost solved only by the structure control, the preferable applied strength is 780 MPa or more. .

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

  In the present invention, stretch flangeability is improved by inclusion control, and the microstructure of the steel sheet is not particularly limited, but in order to obtain excellent stretch flangeability, a single-phase structure should be used as much as possible. The area ratio of the main phase is preferably 80% or more.

  Finally, conditions for manufacturing the high-strength steel sheet of the present invention will be described.

  The molten steel adjusted to the component composition set in the present invention is continuously cast to produce a slab, which is hot-rolled to obtain a hot-rolled steel sheet. The hot rolling method is not particularly limited, but it is preferable to wind at 650 ° C. or lower after performing normal hot rolling.

  If the coiling temperature exceeds 650 ° C., compounds such as coarse carbides are likely to appear, and stretch flangeability deteriorates. More preferably, it is 600 degrees C or less. The lower limit of the coiling temperature is not particularly defined, but it is difficult to set it to room temperature or lower, so this is preferably set as the lower limit.

  The hot-rolled steel sheet thus manufactured may be pickled and skin-passed as necessary. The reduction rate of the skin pass is not particularly limited, but is preferably about 40% for shape correction, improvement of normal temperature aging resistance, strength adjustment, and the like. If it is less than 0.1%, the effect is small and control is difficult, so it is preferable to make 0.1% the lower limit.

  After cold-rolling the hot-rolled steel sheet, it is heat-treated at a maximum temperature of 600 to 1100 ° C., and then continuously cooled to room temperature, or further maintained at a temperature of 100 to 550 ° C. for 30 seconds or more. Also good. If the maximum temperature is less than 600 ° C., α-γ transformation is unlikely to occur, recrystallization may not occur, and workability is likely to deteriorate, so 600 ° C. is preferably set as the lower limit.

  On the other hand, in order to make the maximum temperature higher than 1100 ° C., the cost increases remarkably, and operation troubles such as plate breakage are induced. More preferably, it is the range of 700-950 degreeC.

  Although the heat treatment time in this temperature range is not particularly defined, it is preferably 1 second or more in order to make the temperature of the steel sheet uniform. However, if it exceeds 10 minutes, the generation of grain boundary oxidized phase is promoted and the cost is increased. Various plating may be performed after the heat treatment. A skin pass can also be implemented.

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

Steel slabs having chemical components shown in Table 1 were produced by a continuous casting method. This slab was heated to 1200 ° C., and the hot rolling was completed at 880 ° C. to 910 ° C., which is higher than the Ar ₃ transformation temperature, and wound up at 580 ° C. The rolled steel strip having a thickness of 2.3 mm was pickled and then cold rolled to a plate thickness of 1.2 mm.

  Subsequently, the heat treatment was carried out by holding at a maximum attained temperature of 870 ° C. for 90 seconds and cooling to 740 ° C. at 5 ° C./second. Thereafter, the temperature was lowered to 350 ° C. at a cooling rate of 80 ° C./second, and an additional heat treatment was performed at that temperature for about 300 seconds. The skin pass was 0.5%.

  The steel sheet thus obtained was examined for strength, ductility, stretch flangeability, inclusion particle size distribution, morphology, and average composition. The results are shown in Table 2.

  Strength and ductility were determined by a tensile test of a JIS No. 5 test piece taken in a direction perpendicular to the rolling direction. Stretch flangeability is measured by measuring the hole diameter D (mm) when a through-thickness crack is generated by punching out 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.

  Inclusions were observed 100 times and 1000 times with an optical microscope, and the particle size and morphology of 100 randomly selected inclusions were measured. Furthermore, using a quantitative analysis function of a scanning electron microscope, composition analysis was performed on 20 inclusions randomly selected.

  As is apparent from Table 2, the steel of the present invention that satisfies the requirements of the present invention has a larger hole expansion value than the comparative steel of the same strength, and is excellent in the balance between stretch flangeability and strength. I understand.

  As described above, since the present invention can provide a high-strength steel plate having excellent stretch flangeability, it is highly usable in the steel industry.

Claims (3)

C: 0.03-0.3 mass%, Si: 0.1-2.0 mass%, Mn: 1.0-3.5 mass%, P: 0.05 mass% or less, S: 0.01 % By mass or less, N: 0.0005 to 0.01% by mass, acid-soluble Al: 0.01% by mass or less, acid-soluble Ti: less than 0.008% by mass, one or two of Ce and La Total: 0.0005 to 0.04% by mass, the balance being steel consisting of iron and inevitable impurities, and the steel contains 1 of Ce oxide and La oxide with an average inclusion composition species or two total 10 to 90 wt%, SiO ₂ is 5 to 60 wt%, high _Al 2 _{O 3} is superior in stretch flange formability, which comprises the inclusions in the range of 50 wt% or less Strength steel plate. C: 0.03-0.3 mass%, Si: 0.1-2.0 mass%, Mn: 1.0-3.5 mass%, P: 0.05 mass% or less, S: 0.01 % By mass or less, N: 0.0005 to 0.01% by mass, acid-soluble Al: 0.01% by mass or less, acid-soluble Ti: less than 0.008% by mass (excluding 0.005% by mass or more) ) , A sum of one or two of Ce and La: 0.0005 to 0.04% by mass, the balance being iron and inevitable impurities, and the number of inclusions in the steel A high-strength steel sheet excellent in stretch flangeability, characterized in that 50% or more is composed of spherical and spindle-shaped inclusions. C: 0.03-0.3 mass%, Si: 0.1-2.0 mass%, Mn: 1.0-3.5 mass%, P: 0.05 mass% or less, S: 0.01 % By mass or less, N: 0.0005 to 0.01% by mass, acid-soluble Al: 0.01% by mass or less, acid-soluble Ti: less than 0.008% by mass (excluding 0.005% by mass or more) ) , A sum of one or two of Ce and La: 0.0005 to 0.04% by mass, the balance being iron and inevitable impurities, and the steel contains 0.5 μm or more A high-strength steel sheet excellent in stretch flangeability, wherein inclusions of 10 μm or less are present in an amount of 1000 pieces / cm ² or more and 100,000 pieces / cm ² or less.