JP3864663B2 - Manufacturing method of high strength steel sheet - Google Patents

Description

【００５７】
【表２】

【００５８】
【発明の効果】
以上、本発明によれば、伸び−フランジ性や破断伸び等の加工性および降伏強度が高く耐衝撃特性に優れた３４０ＭＰａ級以上の高張力鋼薄鋼板が得られ、自動車等の軽量化及び衝突安全性の向上に有効であり、産業上極めて有効である。
【図面の簡単な説明】
【図１】板厚方向断面におけるミクロ組織で、（ａ）は比較法、（ｂ）は本発明法によるものを示す金属組織写真。
【図２】機械的性質に及ぼす一次冷却開始時間の影響を示す図
【図３】引張強さと穴拡げ率の関係における本発明の効果を示す図[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thin steel sheet used for automobiles, home appliances, and industrial machines, and particularly relates to a sheet having excellent workability (stretch flangeability, elongation at break) and impact resistance in press working.
[0002]
[Prior art]
Hot-rolled steel sheets and cold-rolled steel sheets are widely used in automobiles, home appliances, industrial machines and the like. In these applications, it is often used as a member formed by press working, and various workability is required depending on its shape.
[0003]
In recent years, in the field of automobiles, there is a strong need for weight reduction and the usage ratio of high-strength thin steel sheets tends to be high, but high-tensile materials are inferior in workability compared to soft materials of 270 MPa class, so product manufacturing Time yield (cracking during press processing) and quality variation are problems, and improvement of workability, which is a basic characteristic of the material surface, is required.
[0004]
As the workability, for example, in a high-tensile material of 340 MPa class or higher, it is required that hot-rolled steel sheets and cold-rolled steel sheets have high stretch-flange properties during burring. Furthermore, in recent years, in automobile applications, collision safety is required as one of the important performances, and it is required to have excellent impact resistance (high impact absorption energy is one of the evaluation items for collision safety). It has come to be.
[0005]
Japanese Patent No. 2555436 has been proposed as a prior art for improving the workability of high-tensile materials. Using Ti-based precipitation-strengthened steel, the cooling rate after finish rolling is 30 to 150 ° C./s, the coiling temperature is 250 to 540 ° C., and the elongation-flange property of 50-60 K class high tensile strength is achieved by the ferrite + bainite structure. Techniques for improving are disclosed. However, at a cooling rate of 30 to 150 ° C./s after finish rolling, it is difficult to say that the elongation-flangeability is drastically improved, and the elongation at break is low because the winding temperature is low. ing.
[0006]
In Japanese Examined Patent Publication No. 7-56053, 45-50K steel and pearlite steel are used, and the cooling rate after hot rolling is 10 ° C./s or more (in the example, maximum 95 ° C./s). It has been proposed to improve the stretch-flange property of hot dip galvanized steel sheets. However, the cooling rate is at most 95 ° C./s, and no substantial improvement in stretch-flangeability has been achieved.
[0007]
In JP-A-4-88125, using ferrite and pearlite steel, 0.0005 to 0.0050% of Ca is added, and hot rolling is finished at a high temperature of Ar3 +60 to 950 ° C., and then immediately within 3 seconds. Cooling is performed at a cooling rate of 50 ° C./s or more, preferably 150 ° C./s or less, and after cooling is stopped (410 to 620 ° C.) according to the steel components, winding is performed at 350 to 500 ° C. through air cooling. Has proposed improving the stretch-flange property of 50-70K class high tensile steel. However, since a small amount of Ca is added, an RH degassing step is required in the steelmaking stage, which increases the steelmaking cost, and the stretch-flange property is dramatically improved even by the cooling conditions after hot-rolling finish, which is a feature of this technology. In addition, since the coiling temperature is low, the elongation at break is also low.
[0008]
[Problems to be solved by the invention]
As described above, according to any of the prior arts, it is difficult to say that sufficient characteristics are obtained with respect to the stretch-flange property and the elongation at break, and there is no description on improvement of impact resistance.
[0009]
The present invention has been made in view of the above points, and in consumers such as automobiles and industrial machines, there are few defects when molding into a product shape, and it is possible to perform product collection from a coil with a high yield, An object of the present invention is to provide a method for producing a thin steel sheet having a tensile strength of 340 MPa or more, which is excellent in stretch-flange property, workability of elongation at break, and impact resistance.
[0010]
[Means for Solving the Problems]
The present inventors have intensively studied to improve stretch-flange property, elongation at break, and impact resistance properties for high-tensile materials manufactured by reheating a continuous cast slab or by direct hot rolling. went. As a result, the presence of a band structure enriched with C, Mn, etc. at the center of the plate thickness has an effect on elongation-flangeability and elongation at break, and the yield strength of the material is processed to improve impact resistance. It has been found that it is effective to increase it within a range where the properties are not impaired.
[0011]
The present invention has been made based on these findings and further studies. Mass% characterized by comprising the following steps: C: 0.05 to 0.14%, Si: 0.5% or less, Mn: 0.5 to 2.5%, P: 0.05 %, S: 0.01% or less, O: 0.005% or less, Ca: less than 0.0005% , and the manufacturing method of the high strength thin steel plate which consists of remainder Fe and unavoidable impurities .
[0012]
(1) The process of manufacturing a slab by continuous casting which performs a segregation reduction process.
[0013]
(2) A hot rolling process in which the finish rolling finish temperature is Ar3 or higher.
[0014]
(3) A step of starting primary cooling at a cooling rate of 100 to 2000 ° C./s within 2 seconds after hot rolling and cooling to a temperature range of 600 to 750 ° C.
[0015]
(4) A step of performing secondary cooling after primary cooling at a cooling rate of less than 50 ° C./s and winding at 450 to 650 ° C.
[0016]
2. Mass% characterized by comprising the following steps: C: 0.05 to 0.14%, Si: 0.5% or less, Mn: 0.5 to 2.5%, P: 0.05 %, S: 0.01% or less, O: 0.005% or less, Ca: less than 0.0005% , and the manufacturing method of the high strength thin steel plate which consists of remainder Fe and unavoidable impurities .
[0017]
(1) The process of manufacturing a slab by continuous casting which performs a segregation reduction process.
[0018]
(2) A step of reheating the slab before hot rolling.
[0019]
(3) A hot rolling step in which the finish rolling finish temperature is Ar3 or higher.
[0020]
(4) A step of starting primary cooling at a cooling rate of 100 to 2000 ° C./s within 2 seconds after the hot rolling and cooling to a temperature range of 600 to 750 ° C.
[0021]
(5) A step of performing secondary cooling after primary cooling at a cooling rate of less than 50 ° C./s and winding at 450 to 650 ° C.
[0022]
3. Furthermore, the manufacturing method of the high intensity | strength thin steel plate of 1 or 2 characterized by providing either of the following processes.
[0023]
(1) A step of annealing after pickling.
[0024]
(2) A step of cold rolling and annealing after pickling.
[0025]
4). The high-strength thin steel sheet according to any one of 1 to 3, to which 0.01 to 0.3% of Ti, Nb, V, Mo, Zr, or Cr is added as a steel component in a total amount of 0.01 to 0.3%. Manufacturing method.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
In this invention, in order to acquire the effect, a component composition and manufacturing conditions are prescribed | regulated. The reasons for limitation will be described in detail below.
[0027]
1. Ingredient composition C
C is added to ensure the strength of the steel sheet. If it is less than 0.05%, the strength of 340 MPa or more targeted in the present invention cannot be obtained, and if it exceeds 0.14%, the workability deteriorates significantly, so 0.05% or more and 0.14% or less. And
[0028]
Si
Si is a solid solution strengthening element, and is added to strengthen the steel sheet. However, if it exceeds 0.5%, the surface properties deteriorate, so the content is made 0.5% or less.
[0029]
Mn
Mn is added in an amount of 0.5% or more in order to improve the toughness of the steel sheet and improve the strength by solid solution strengthening. If it exceeds 2.5%, the deterioration of workability becomes remarkable, so the content is made 0.5% or more and 2.5% or less.
[0030]
P
P has an effect of solid solution strengthening of the steel sheet, but if the content exceeds 0.05%, workability deterioration due to segregation occurs, so 0.05% or less.
[0031]
S
S generates sulfides, and when it exceeds 0.01%, the amount increases and the workability deteriorates, so the content is made 0.01% or less.
[0032]
O
O suppresses the generation of cracks on the surface of the slab during continuous casting or below the surface layer of the slab, so the content is restricted to 0.005% or less.
[0033]
Ca
Ca uses alumina oxide, which is a deoxidation product when Al is used for deoxidation at the time of melting, as a low melting point Al—Ca—O-based oxide. Since the Al-Ca-O-based oxide spreads during hot rolling and degrades workability (elongation-flangeability), in the present invention, Ca is treated as an inevitable impurity, and is not added, and its content is not added. The level is regulated to less than 0.0005%.
[0034]
In the present invention, the above is the basic component composition, but it is possible to add one or more of Ti, Nb, V, Mo, Zr, and Cr in order to further improve the characteristics.
[0035]
Ti, Nb, V, Mo, Zr, Cr
In the present invention, in order to improve the strength, one or more of Ti, Nb, V, Mo, Zr, and Cr can be added in a total of 0.01 to 0.3%.
[0036]
In the present invention, it is allowed to contain elements other than those described above within a range not impairing the function and effect. For example, it is allowed to contain 2% or less of Cu and 0.04% or less of Sn.
[0037]
2. Manufacturing conditions (1) Process for manufacturing slab by continuous casting for performing segregation reduction treatment In the present invention, continuous casting is performed in order to reduce the production cost and to manufacture the slab with a high yield.
[0038]
Suppresses segregation of C, Mn, etc. during continuous casting, prevents the formation of a band structure at the center of the plate thickness, etc., combined with control of the primary cooling rate after finish rolling described later, excellent workability ( In order to obtain (elongation-flangeability), segregation reduction treatment is performed during casting. Examples of the segregation reduction treatment include electromagnetic stirring, light pressure casting, and an increase in the cooling rate of a slab such as a slab. These can be applied alone or in combination.
[0039]
(2) A step of reheating the slab before hot rolling.
[0040]
In order to improve the temperature uniformity of the slab, make the mechanical properties in the coil width direction uniform, and improve the workability, it is desirable to reheat without continuous cooling and start rough rolling after continuous casting. The reheating temperature is preferably 1250 ° C. or lower.
[0041]
(3) A hot rolling step in which the finish rolling finish temperature is Ar3 or higher.
[0042]
The finishing temperature at the finish rolling mill is set to Ar3 or higher, the ferrite crystal grain size and pearlite after transformation are refined, and the elongation-flange property and impact resistance are improved.
[0043]
(4) A step of starting primary cooling at a cooling rate of 100 to 2000 ° C./s within 2 seconds after the hot rolling and cooling to a temperature range of 600 to 750 ° C.
[0044]
Cooling at the runner-out after hot rolling (primary cooling) is to refine the ferrite crystal grain size and pearlite after transformation, and to improve impact resistance characteristics due to excellent workability and high yield strength. Start within 2 seconds, more preferably within 1 second. FIG. 2 shows the influence of the primary cooling start time on the mechanical properties. When finishing cooling within 2 seconds after finishing rolling, excellent workability and high strength can be obtained.
[0045]
The cooling rate of the primary cooling is specified in order to improve the elongation-flange property by reducing the ferrite crystal grain size after transformation and pearlite refinement and suppressing the band structure at the center of the plate thickness. The band structure corresponds to the enriched part of C and Mn in the solidification stage, and at the normal cooling rate of 100 ° C./s or lower, the transformation temperature from austenite to ferrite is low and the transformation is the slowest compared with other parts. Therefore, a lot of pearlite is generated and the stretch-flange property is deteriorated.
[0046]
When the cooling rate is 100 ° C./s or more, the ferrite transformation is facilitated even in the concentrated portion of C and Mn, and as a result, the elements are homogenized and the band structure is suppressed. The faster the cooling rate, the better. However, from the viewpoint of industrial feasibility, the upper limit is set to 2000 ° C./s. FIG. 1 shows the influence of the cooling rate on the microstructure. In the comparative method in which the cooling rate is outside the range of the present invention, a band structure is observed, and the crystal grain size is also large compared to the microstructure by the method of the present invention.
[0047]
The cooling rate is more preferably 200 ° C./s or more from the viewpoint of ferrite crystal grain size and pearlite refinement, and 400 ° C./s or more is preferable for further improving workability.
[0048]
When the end temperature of the primary cooling exceeds 750 ° C., it becomes difficult to refine the ferrite. When the temperature is less than 600 ° C., the second phase becomes a hard low-temperature transformation phase.
[0049]
(5) A step of performing secondary cooling after primary cooling at a cooling rate of less than 50 ° C./s and winding at 450 to 650 ° C.
[0050]
Secondary cooling is performed following primary cooling. The secondary cooling may be started immediately after stopping the primary cooling or may be started after standing to cool for a while, and is not particularly defined. The cooling rate of the secondary cooling is set to 50 ° C./s or less in order to appropriately pearlite transform the austenite structure to obtain excellent workability.
[0051]
When the coiling temperature exceeds 650 ° C., coarse pearlite harmful to ductility is generated. When the coiling temperature is less than 450 ° C., the structure mainly consists of a low-temperature transformation phase and the workability deteriorates. The following. When more uniform mechanical properties are desired, the temperature difference in the coil is preferably within 50 ° C. by using a cooling facility with excellent cooling controllability.
[0052]
According to the present invention, even if pickling-annealing or pickling-cold rolling-annealing after manufacturing a hot-rolled steel sheet, the effect is not impaired. Furthermore, the effect of the present invention is not impaired as a hot-dip and cold-rolled base hot-dip galvanized material.
[0053]
In the present invention, more uniform mechanical properties can be obtained by heating the edge portion in the width direction with an induction heating device or the like after rough rolling, before finish rolling, or between the stands of finish rolling. It becomes. Moreover, the effect of this invention is not impaired also in the continuous hot rolling which welds a rough rolling bar and performs finish rolling continuously after rough rolling.
[0054]
【Example】
After melting the steel having the chemical composition shown in Table 1 that satisfies the chemical composition of the present invention, a hot-rolled sheet having a thickness of 2.0 mm was prepared by the production method shown in Table 2. Material No. Nos. 1-2 and 5-9 show the mechanical properties as hot rolled, with the material No. No. 3 is hot-rolled, pickled, cold-rolled and hot-dip galvanized. No. 4 was hot-rolled, pickled, hot-dip galvanized, and examined for mechanical properties. For the evaluation of stretch-flange property, the hole expansion rate (λ) was measured. Table 2 shows these evaluation results together.
[0055]
Material No. of the present invention example satisfying the chemical components and production conditions of the present invention. Nos. 1 to 4 are materials Nos. Which are comparative examples when any one of the provisions of the manufacturing conditions is outside the provisions of the present invention. Compared with 5-9, it is clear that the processability (strength-hole expansion balance) is excellent, the yield strength is high, and the impact resistance is excellent. FIG. 3 shows the tensile strength and the hole expansion ratio of the inventive example and the comparative example together. Obviously, the present invention provides excellent properties.
[0056]
[Table 1]

[0057]
[Table 2]

[0058]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain a high-strength steel sheet of 340 MPa class or higher that has high workability such as elongation-flange property and elongation at break and high yield strength and excellent impact resistance characteristics, and it is possible to reduce the weight and collision of automobiles and the like. It is effective for improving safety and extremely effective in industry.
[Brief description of the drawings]
FIG. 1 is a microstructure in a cross section in the plate thickness direction, (a) is a metallographic photograph showing a comparative method, and (b) is a metallographic structure showing the method according to the present invention.
FIG. 2 is a diagram showing the influence of primary cooling start time on mechanical properties. FIG. 3 is a diagram showing the effect of the present invention on the relationship between tensile strength and hole expansion rate.

Claims (4)

Mass% characterized by comprising the following steps: C: 0.05 to 0.14%, Si: 0.5% or less, Mn: 0.5 to 2.5%, P: 0.05 %, S: 0.01% or less, O: 0.005% or less, Ca: less than 0.0005% , and the manufacturing method of the high strength thin steel plate which consists of remainder Fe and an unavoidable impurity .
(1) The process of manufacturing a slab by continuous casting which performs a segregation reduction process.
(2) A hot rolling process in which the finish rolling finish temperature is Ar3 or higher.
(3) A step of starting primary cooling at a cooling rate of 100 to 2000 ° C./s within 2 seconds after hot rolling and cooling to a temperature range of 600 to 750 ° C.
(4) A step of cooling to the temperature range, followed by secondary cooling at a cooling rate of less than 50 ° C./s and winding at 450 to 650 ° C. Mass% characterized by comprising the following steps: C: 0.05 to 0.14%, Si: 0.5% or less, Mn: 0.5 to 2.5%, P: 0.05 %, S: 0.01% or less, O: 0.005% or less, Ca: less than 0.0005% , and the manufacturing method of the high strength thin steel plate which consists of remainder Fe and an unavoidable impurity .
(1) The process of manufacturing a slab by continuous casting which performs a segregation reduction process.
(2) A step of reheating the slab before hot rolling.
(3) A hot rolling step in which the finish rolling finish temperature is Ar3 or higher.
(3) A step of starting primary cooling at a cooling rate of 100 to 2000 ° C./s within 2 seconds after hot rolling and cooling to a temperature range of 600 to 750 ° C.
(4) A step of cooling to the temperature range, followed by secondary cooling at a cooling rate of less than 50 ° C./s and winding at 450 to 650 ° C. Furthermore, one of the following processes was given, The manufacturing method of the high strength thin steel plate of Claim 1 or 2 characterized by the above-mentioned.
(1) A step of annealing after pickling.
(2) A step of cold rolling and annealing after pickling.   The high-strength thin steel sheet according to any one of claims 1 to 3, wherein one or more of Ti, Nb, V, Mo, Zr, and Cr is added as a steel component in a total amount of 0.01 to 0.3%. Manufacturing method.

Images