JP2007291421A - High strength steel plate with excellent stretch flangeability - Google Patents

Abstract

A high-strength steel sheet excellent in stretch flangeability is provided.
SOLUTION: 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 0.005% by mass, one of Nd and Pr Or the total of the two types: 0.0002 to 0.02% by mass, and the balance is steel made of iron and inevitable impurities, and the steel contains Nd oxide or Pr oxide with an average inclusion composition The stretch flangeability is characterized by including inclusions in which the total of one or two of the materials is 2 to 70% by mass, SiO ₂ is 2 to 60% by mass, and Al ₂ O ₃ is 70% by mass or less. Excellent high strength steel plate.
[Selection figure] None

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 those subjected to various platings such as a galvanized steel plate and 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 is an increasing demand for high-strength steel sheets with good workability for the purpose of improving fuel efficiency and durability particularly in automobile bodies. In addition, steel sheets 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 hole expansibility, and the stretch formability is also improved by generating residual austenite in the second phase. It is disclosed that it shows the same stretchability as that of retained austenitic steel.

  Moreover, 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 several disclosed examples in which ductility, bendability, and hole expansion rate are improved by the above-described structure control as those that have been subjected to hot dip galvanization. For example, Patent Documents 1 to 5 are typical examples.
Japanese Patent No. 2607906 Japanese Patent No. 2862187 JP-A-1-198459 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 flange formability represented by hole expansibility have been developed. However, since a high strength steel sheet having a tensile strength of 780 MPa or more contains C or a large amount of alloying elements, 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 necessarily 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 is different in hardness from the main phase, and the boundary between the two 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. It was found that cracks originated from coarse cluster-like inclusions during hole expansion and 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 so that the inclusions at the grain boundaries do not become the starting point of cracking, the hole expandability is improved. I found out.

  In general, 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. Although this is considered to reduce the stretch flangeability (hole expansion value) as described above, no example of suggesting a high-strength steel sheet excellent in stretch flangeability from the viewpoint of inclusion fine spheroidization control has been found.

  In view of such a situation, the present inventors perform almost Al deoxidation in the molten steel component of the high-strength steel plate containing C or a large amount of alloy elements in order to 1) not generate alumina inclusions that are likely to be coarsened. 2) Intensive research on deoxidation methods to improve inclusions into spherical inclusions that do not become coarse due to clustering of inclusions, and 3) are difficult to start cracking. The present invention has been completed through examination.

The summary is as follows. That is,
(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% 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 0.005% by mass, one of Nd and Pr Total of two types: steel containing 0.0002 to 0.02 mass% with the balance being iron and inevitable impurities, and the steel contains Nd oxide or Pr oxide with an average inclusion composition Excellent stretch flangeability, characterized by including inclusions in the range of 1 to 2 mass%, ₂ to 70 mass% of SiO ₂ , ₂ to 60 mass% of SiO ₂ , and 70 mass% or less of Al ₂ O ₃ 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% 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 0.005% by mass, one of Nd and Pr Total of two types: Steel containing 0.0002 to 0.02% by mass with the balance being iron and inevitable impurities, in which 50% or more of the inclusions are spherical and spindle-shaped A high-strength steel sheet excellent in stretch flangeability, characterized by comprising any one or both 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% by mass or less, N: 0.0005 to 0.01% by mass, acid-soluble Al: 0.01% by mass or less, acid-soluble Ti: 0.005% by mass or less, one of Nd and Pr Total of two types: Steel containing 0.0002 to 0.02% by mass, the balance being iron and inevitable impurities, and 500 inclusions / cm of inclusions of 0.5 μm to 10 μm in the steel ² or more, a high strength steel sheet excellent in stretch flange formability, characterized in that there 100,000 / cm ² or less.

  According to the present invention, a high-strength steel plate that is stable and excellent in stretch flangeability can be obtained.

  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 this is the upper limit.

  Since Si is a major deoxidizing element in molten steel to which Al and Ti are not added as much as in the present invention, it is extremely important in the present invention. 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. Si is an element effective for securing strength without deteriorating stretch flangeability, and at least 0.05% by mass is necessary. However, considering deoxidation conditions as well, Si is 0.1% by mass. % Is the lower limit. If added excessively, the weldability and ductility are adversely affected, so 2.0 mass% is made 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 and MnS Therefore, the upper limit is set to 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 hardenability), but if added too much, coarse precipitates are generated even with trace amounts of Al and Ti. Since the stretch flangeability is deteriorated, the upper limit is 0.01% by mass. On the other hand, if N is less than 0.0005 mass%, the current process is very heavy and the cost is high, so the lower limit is set to 0.0005 mass%.

It is desirable to suppress acid-soluble Al as much as possible because the oxide is likely to cluster and become coarse. However, Al is allowed to be used as a preliminary deoxidizer up to 0.01% by mass at an acid-soluble Al concentration. As described later, when the acid-soluble Al concentration exceeds 0.01% by mass, the content of Al ₂ O _{3 in the} inclusion exceeds 70% by mass, and even if Nd or Pr is added, Since the clustering cannot be prevented, the hole expandability is reduced. From the viewpoint of preventing clustering, the acid-soluble Al concentration should be low, and the lower limit value includes 0% by mass. The acid-soluble Al concentration is an analytical method that measures the amount of Al dissolved in an acid, and utilizes the fact that dissolved Al dissolves in an acid and Al ₂ O ₃ does not dissolve in an acid. Here, the acid is, for example, a mixed acid mixed at a ratio of hydrochloric acid 1, nitric acid 1, and water 2.

  The acid-soluble Ti also tends to be coarse due to its oxides clustering, and easily forms coarse TiN inclusions in combination with N in the steel, so the acid-soluble Ti should be less than 0.005% by mass. The lower limit value includes 0% by mass. The acid-soluble Ti concentration is an analytical method that measures the amount of Ti dissolved in an acid, and uses that dissolved Ti dissolves in an acid and Ti oxide does not dissolve in an acid. Here, the acid is, for example, a mixed acid mixed at a ratio of hydrochloric acid 1, nitric acid 1, and water 2.

Nd and Pr are fine spherical Nd oxides (for example, Nd ₂ O ₃ , NdO ₂ ) -Al ₂ O ₃ —SiO _2, which are Al ₂ O ₃ —SiO ₂ inclusions formed by preliminary Al deoxidation and Si deoxidation. Composite inclusions, Pr oxides (eg, Pr ₂ O ₃ , PrO ₂ ) -Al ₂ O ₃ —SiO ₂ composite inclusions, or Nd oxide—Pr oxide—Al ₂ O ₃ —SiO ₂ composite inclusions It is the most important component to improve and improve stretch flangeability.

In order to obtain such an inclusion modification effect, the total concentration of one or two of Nd and Pr needs to be 0.0002 mass% or more and 0.02 mass% or less. If the total concentration of one or two of Nd and Pr is less than 0.0002% by mass, the Al ₂ O ₃ —SiO ₂ inclusion cannot be modified into a fine sphere, and if it exceeds 0.02% by mass, the inclusion in the inclusion The SiO ₂ content is reduced and almost becomes Nd oxide-Al ₂ O ₃ inclusions or Pr oxide-Al ₂ O ₃ inclusions, and the inclusions cluster, and the stretch flangeability decreases. There are other lanthanoid elements such as Ce, La, and Sm, but these elements have a weaker effect of making the inclusions into a fine sphere than Nd and Pr. For this reason, even when a large amount of Al ₂ O ₃ produced by preliminary Al deoxidation remains due to operational variations, the use of Nd or Pr having a high reforming ability prevents stable inclusion clustering and increases the elongation. Flangeability can be improved.
The inevitable impurities other than iron contained in the steel of the present invention mean that Sn, Ni, Cu, etc. contained in the scrap are inevitably mixed in the steel.

  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, 20 or more) and calculating the average concentration.

As a result, in the average composition of inclusions, the total of one or two of Nd oxide or Pr oxide is 2 to 70% by mass, SiO ₂ is 2 to 60% by mass, and Al ₂ O ₃ is 70% by mass or less. 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 average composition of one or two of Nd oxide or Pr oxide is less than 2% by mass, the effect of inclusion modification by addition of Nd or Pr is small, and conversely, 1 of Nd oxide or Pr oxide. If the total of seeds or two types exceeds 70% by mass, over-reformation occurs, and in any case, inclusions do not become fine spheroidized, so the average composition of one or two of Nd oxide or Pr oxide The lower limit was 2% by mass, and the upper limit was 70% by mass.

The average SiO ₂ is greater than the 60% by mass in the composition, become very long stretched inclusions during rolling, which reduces the stretch flangeability due to the presence in many steel sheet, interposed in SiO ₂ is less than 2 wt% in the opposite Since the product does not disperse into fine spheres, the upper limit value of SiO ₂ is 60% by mass and the lower limit value is 2% by mass in the average composition.

Furthermore, it is preferable not to contain Al ₂ O ₃ in the inclusions from the viewpoint of fine spheroidization, but the content of Al ₂ O _{3 in the} inclusions increases due to the influence of Al preliminary deoxidation and refractory melting. Sometimes. In this case, Al ₂ O ₃ may be mixed in the inclusions with an average composition of 70% by mass or less, and the lower limit includes 0% by mass. This is because when the Al ₂ O ₃ content in the inclusions exceeds 70% by mass, the modification effect by Nd and Pr is impaired, and the inclusions are clustered. In the present invention, inevitable impurity oxides mixed from slag, refractory, etc. other than oxides having the above composition are allowed.

Next, the form of inclusions in the steel sheet will be described. As for the form of the inclusion, a plurality of (for example, 50 or more) inclusions selected at random can be observed with an optical microscope. For example, 100 randomly selected inclusions 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 inclusions is obtained. Is recommended. In addition, the spherical shape means that the major axis and minor axis of the inclusions are observed as being substantially equal and circular, and on the spindle means that the major axis / minor axis of the inclusion is 3 or less and is observed as an ellipse, Is defined as one in which two or more inclusion particles are densely packed, and the other as, for example, an angular single piece.
As a result, it has been found that the stretch flangeability is improved in the steel plate in which the number ratio of one or both of the spherical shape and the spindle shape is 50% or more. If the number ratio of one or both of the spherical and spindle-shaped inclusions is less than 50%, the cluster-like inclusions are relatively increased and the stretch flangeability is lowered, so the lower limit value was set to 50%. The upper limit is 100%.

Here, “the ratio of the number of inclusions in either or both spherical and spindle shapes is 50% or more”
“When the number of spherical inclusions alone is 50% or more,
When the number ratio of spindle-shaped inclusions is 50% or more,
It indicates that “both spherical inclusions and spindle inclusions are mixed, and the total of the number ratios thereof is 50% or more”.

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 size means the equivalent circle diameter, and was obtained by (inclusion major axis × inclusion minor axis) ^0.5 .

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 500 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 500 / 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 value was 500 pieces / cm ² . Moreover, 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 so large that the stretch flangeability is lowered, so the upper limit is 100,000 / cm ² . did.

  The present invention is naturally applicable to steel of a strength class having a tensile strength of less than 780 MPa, but the problem of stretch flangeability is almost solved only by the structure control, so the inclusion refinement control of the present invention is only the structure control. Therefore, it is preferable to apply to steel of 780 MPa or more where stretch flangeability is difficult to solve.

  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. Although it is suitable, even when it has the second phase, the area ratio of the main phase is preferably 80% or more.

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

In the present invention, decarburization is performed by blowing in a converter, or further decarburization using a vacuum degassing apparatus, or further C is added to the molten steel in which the C concentration is 0.03 to 0.3% by mass. In addition, Si, Mn, P and other alloys are added to perform deoxidation and component adjustment, and when Al or Ti is not added or oxygen adjustment is required, acid-soluble Al or acid-soluble A small amount of Al or Ti is added such that Ti remains slightly (acid-soluble Al is 0.01% by mass or less, and acid-soluble Ti is less than 0.005% by mass), and then one or two of Nd and Pr Add seeds or more to adjust ingredients. In the present invention, it is preferable that Al ₂ O ₃ and Ti oxide do not remain as much as possible from the viewpoint of inclusion spheroidization. Therefore, when adding Al or Ti, a stirring time of 3 minutes or more is provided from the addition. Thereafter, it is desirable to add one or more of Nd and Pr.

  The molten steel thus produced is continuously cast to produce a slab, which is hot rolled to obtain a hot rolled steel sheet. The form and composition of the oxide in steel aimed by the present invention is not affected by the hot rolling process, so the hot rolling method is not particularly limited, and after normal hot rolling, it is wound at 650 ° C. or lower. It is preferable. If it exceeds 650 ° C., compounds such as coarse carbides tend to appear, and stretch flangeability deteriorates. More preferably, it is 600 degrees C or less. Although the lower limit is not particularly defined, it is preferable to set this as the lower limit because it is difficult to make it at room temperature or lower.

  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 may be up to about 40% for shape correction, improvement of room temperature aging resistance, strength adjustment, and the like. If it is less than 0.1%, the effect is small and control is difficult, so this is preferably set to the lower limit.

  After pickling and cold rolling the hot-rolled steel sheet, after heat treatment to make the maximum temperature 600 to 1100 ° C, it is 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 and recrystallization may not occur. On the other hand, in order to set the maximum temperature to be 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 for uniform temperature of the steel sheet. 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 performed.

  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., hot rolling was completed at 880 ° C. to 910 ° C., which is higher than the Ar ₃ transformation temperature, and a 2.3 mm thick steel strip wound up at 580 ° C. was pickled, The plate thickness was set to 1.2 mm by rolling. Subsequently, the heat treatment was carried out under the conditions of the highest temperature reached in Table 2. The temperature was maintained at the maximum temperature for 90 seconds and cooled to (maximum temperature-130) ° C. at 5 ° C./second. Then, it cooled at the cold speed shown in Table 2, and also performed additional heat treatment for about 300 seconds at the overaging temperature of Table 2. 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) at the time when a through-thickness crack is generated by expanding a punched hole with a diameter of 10 mm with a 60 ° conical punch in the center of a 150 mm × 150 mm steel plate. The hole expansion value λ = (D−10) / 10. Furthermore, the inclusions were observed 100 times and 1000 times with an optical microscope, and the particle size and morphology of 100 inclusions selected at random were measured. Furthermore, using a quantitative analysis function of a scanning electron microscope, composition analysis was performed on 20 inclusions randomly selected. As can be seen from Table 2, the inventive steel that satisfies the requirements of the present invention has a larger hole expansion value than the comparative steel of the same strength, and shows excellent stretch flangeability.

Claims (3)

C: 0.03-0.3 mass%,
Si: 0.1 to 2.0% by mass,
Mn: 1.0 to 3.5% by 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 0.005% by mass,
A total of one or two of Nd and Pr: 0.0002 to 0.02% by mass, the balance being steel made of iron and inevitable impurities,
One or two total 2 to 70% by weight of the inclusions of the average in the steel Nd oxides composition or a Pr oxide, SiO ₂ is 2 to 60 wt%, Al ₂ O ₃ is 70 wt% A high-strength steel sheet excellent in stretch flangeability, characterized by including inclusions in the following range. C: 0.03-0.3 mass%,
Si: 0.1 to 2.0% by mass,
Mn: 1.0 to 3.5% by 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 0.005% by mass,
A total of one or two of Nd and Pr: 0.0002 to 0.02% by mass, the balance being steel made of iron and inevitable impurities,
A high-strength steel sheet with excellent stretch flangeability, characterized in that in the steel, 50% or more of inclusions are composed of either spherical or spindle inclusions or both. C: 0.03-0.3 mass%,
Si: 0.1 to 2.0% by mass,
Mn: 1.0 to 3.5% by 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: 0.005% by mass or less,
A total of one or two of Nd and Pr: 0.0002 to 0.02% by mass, the balance being steel made of iron and inevitable impurities,
A high-strength steel sheet excellent in stretch flangeability, characterized in that inclusions of 0.5 μm or more and 10 μm or less are present in the steel of 500 pieces / cm ² or more and 100,000 pieces / cm ² or less.