JP4288085B2 - Hot-dip galvanized high-strength steel sheet excellent in hole expansibility and method for producing the same - Google Patents

Description

【００７４】
【表２】

鋼種Ａ、Ｈ、Ｉ、Ｊ、Ｋ、Ｍ、Ｎ、Ｒ、Ｔ、Ｕの「発明鋼」を「参考例」
【００７５】
【表３】

鋼種Ａ、Ｈ、Ｉ、Ｊ、Ｋ、Ｍ、Ｎ、Ｒ、Ｔ、Ｕの「発明鋼」を「参考例」
【００７６】
＜参考例＞
表１、表２（表１のつづき）に示した鋼のうち、鋼種Ａ及び鋼種Ｒについて、冷延後の熱処理条件を変化させた。即ち、実施例１における熱処理条件において、最高到達温度及び、冷却過程における（最高到達温度−２０）（℃）から６５０℃までの冷却速度を変化させた。その他の条件は、実施例１における条件と同一とした。（但し、最高到達温度が異なるため、昇温過程において１０℃／秒の加熱速度で昇温する過程の到達温度であるところの（最高到達温度−１１０）（℃）は異なる。）それらの鋼について機械的性質及び穴拡げ性を測定した結果を表４に示す。
【００７７】
一方、本発明の条件から外れる比較鋼は、穴拡げ性が劣勢である。
【００７８】
【表４】

「発明鋼」を「参考例」
【００７９】
＜実施例３＞
表１、表２（表１のつづき）に示した鋼のうち、鋼種Ｓについて鋳造スラブをそのまま実施例１と同じ条件で加熱〜冷延まで行い、冷延後に以下のように処理を行った。即ち、昇温速度５℃／ｓで最高到達温度まで昇温し、９０秒保持し、次に５℃／秒の冷却速度で６８０℃まで冷却し、その後冷却速度を変化させて４６０℃まで冷却した。引き続き４６０℃の溶融亜鉛めっき槽に浸漬し、その後３℃／秒の昇温速度で５００℃まで加熱し、３０秒保持して合金化処理を施した後、冷却した。以上の処理過程において、最高到達温度、及び６８０℃から４６０℃までの冷却速度を変化させた。処理終了後、機械試験及び穴拡げ性の測定を行った。結果を表５に示す。本発明の条件を満たす発明鋼は、穴拡げ性（４５％以上）強度（引張り強度で９００ＭＰａ以上）のバランスに優れていることがわかる。
【００８０】
一方、本発明の条件から外れる比較鋼は、穴拡げ性が劣勢である。
【００８１】
【表５】

【００８２】
【発明の効果】
本発明により、引張り強度が９００ＭＰａ以上であり、穴拡げ性が４５％以上である溶融亜鉛めっき高強度鋼板およびその製造方法を得ることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-dip galvanized and hot-dip alloyed galvanized high-strength steel sheet excellent in hole expansibility and suitable for building materials, home appliances, automobiles and the like, and a method for producing the same. The galvanizing in the present invention includes not only normal hot dip galvanizing but also alloyed hot dip galvanizing. The plating layer may contain Fe, Al, Mg, Cr, etc. in addition to pure zinc.
[0002]
[Prior art]
In recent years, there has been 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 plates with a tensile strength of 780 MPa class or higher are being used for some parts such as the reinforcement due to the need for collision safety and expansion of cabin space.
[0003]
Various properties such as ductility, bendability and hole expansibility are important when assembling members using such high strength materials, but countermeasures against these are taken for high strength steel sheets with a tensile strength of up to about 590 MPa. It has been.
[0004]
For example, with respect to hole expansibility, as disclosed in Non-Patent Document 1, the main phase is bainite to improve hole expansibility, and with respect to the stretch formability, residual austenite is generated in the second phase. It has been proposed to exhibit the same overhangability as the current retained austenitic steel. Furthermore, it is also shown that when retained austenite having a volume ratio of 2 to 3% is generated by austempering at a temperature equal to or lower than the Ms temperature, the tensile strength × the hole expansion ratio is maximized. However, the technique disclosed here is in a steel plate of about 590 MPa, and it is not simply established for a material exceeding 900 MPa.
[0005]
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 rate is improved by setting the main phase as ferrite and the second phase as martensite and reducing the hardness difference between the two, but the hole expansion rate is 70. Less than%, it is not a significant improvement.
[0006]
In addition, there are some disclosed examples of hot-dip galvanized steel having a tensile strength of 900 MPa or more. These are listed below.
[0007]
(1) In Patent Document 1 and Patent Document 2, a technique capable of obtaining high strength is disclosed. However, since the composite structure is mainly used, a hardness difference between phases is generated, and a hole expansion ratio is 30. % And lower.
[0008]
(2) In Patent Document 3, the austenite phase is stabilized by adjusting the C concentration to 0.1 to 0.2% and the Mn concentration to 2 to 3%, and after heat treatment in the plating line, 480 ° C to 560 ° C. A method of obtaining strength and workability by leaving an austenite phase by holding at a low temperature at ° C. is disclosed. However, since there is a retained austenite phase, it tends to be a non-uniform composite structure, and the hole expansion rate does not improve.
[0009]
(3) In Patent Document 4, a small amount of Mo, Ti, Cr, Nb, B, V or the like is added to C: 0.05%, Si: 0.55%, Mn: 1.59%, and 93 However, since Mn is low, it becomes a heterogeneous composite structure when the heating temperature during heat treatment is low or when the cooling rate after heat treatment is low. It is difficult to obtain spreadability.
[0010]
(4) Patent Document 5 discloses an example in which a small amount of Nb and B is added to C: 0.16%, Mn: 2.3%, Ti: 0.01%, and the strength is about 1180 MPa. Although it is obtained, it is based on a composite structure, so that the hole expansion rate is only 40% or less due to the hardness difference between the phases.
[0011]
Therefore, there is no hot-dip galvanized steel sheet having excellent hole expansibility and having a tensile strength of 900 MPa or more and a hole expansion ratio of 45% or more.
[0012]
[Non-Patent Document 1]
CAMP-ISIJ vol.13 (2000) p.395
[Non-Patent Document 2]
CAMP-ISIJ, vol.13 (2000), p.391
[Patent Document 1]
Japanese Patent No. 2607906
[Patent Document 2]
Japanese Patent No. 2862187
[Patent Document 3]
JP-A-1-198459
[Patent Document 4]
JP 2001-355043 A
[Patent Document 5]
Japanese Patent No. 3037767
[0013]
[Problems to be solved by the invention]
One of the causes of the problems as described above is the cooling capacity in the continuous galvanizing process. That is, in this process, since a relatively low cooling rate of 0.1 ° C./second to 20 ° C./second is generally employed after the steel sheet reaches the maximum temperature, the structure tends to be non-uniform. As a result, the hole expandability deteriorates. Furthermore, in a steel sheet that has been subjected to alloying hot dip galvanization, the steel sheet is immersed in a hot dip galvanizing tank and then continuously heat treated, alloyed, and then continuously wound. In the heat treatment process for carbonization, super-saturated carbon in iron is precipitated as iron carbide in the structure of the steel sheet that has been built so far, and the iron carbide finely precipitated in the steel sheet is coarsened. Phenomenon occurs, and the organization is altered. As a result, the hole expandability is significantly deteriorated.
[0014]
The present invention provides a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized high-strength steel sheet having excellent hole expansibility with a tensile strength of 900 MPa or more, and a method for producing them. The purpose is to do.
[0015]
[Means for Solving the Problems]
As a result of various studies, the present inventors have found that a hot dip galvanized steel sheet having excellent hole expansibility with a tensile strength of 900 MPa or more can be obtained by limiting chemical components. The high-strength steel sheet according to the present invention has a hole expansion ratio defined by [(inner diameter of hole after hole expansion test / hole diameter before hole expansion test) -1] × 100, and has excellent corrosion resistance. Of course, softening of the heat affected zone is suppressed, and the fatigue durability of the welded portion is excellent.
[0016]
The present invention has been completed based on the above findings, and the gist thereof is as follows.
[0017]
(1) In mass%,
C: 0.03-0.12%,
Si: 0.001 to 0.8%
Mn:2.53 to 3.12%,
P: 0.001 to 0.1%,
S: 0.0001 to 0.01%,
Al: 0.001 to 0.2%
In addition,
Mo: 0.11 to 1.0%,
Ti: 0.005-0.3%
Containing
Mo (mass%) + 10 × Ti (mass%) ≧ 0.3%
Satisfied,A hot-dip galvanized high-strength steel sheet excellent in hole expansibility, characterized in that the balance is Fe and inevitable impurities, the tensile strength is 900 MPa or more, and the hole expansion ratio is 45% or more.
[0018]
(2) Further, the above-mentioned (1) characterized by containing one or two of B: 0.0001 to 0.01% and Nb: 0.003 to 0.3% by mass%.)Hot-dip galvanized high-strength steel sheet with excellent hole expandability.
[0019]
(3) Further, it is characterized by containing one or more of Cr: 0.01-2%, Co: 0.01-1%, W: 0.01-0.3% by mass%. Above (1)Or(2The hot-dip galvanized high-strength steel sheet excellent in hole expansibility according to any one of items 1).
[0020]
(4Furthermore, the above (1) to (1), characterized by containing 0.0001 to 0.5% in total of one or more of Ca, Mg and REM in mass%.3The hot-dip galvanized high-strength steel sheet excellent in hole expansibility according to any one of items 1).
[0021]
(5Further, in the mass%, Ni: 0.01-2.0%, Cu: 0.001-2.0% 1 type or 2 types are contained,2)Any one of-(4)Hot-dip galvanized high-strength steel sheet with excellent hole expandability as described in 1.
[0022]
(6) Further, the above-mentioned (%) containing 0.001 to 1% in total of one or more of Zr, Hf, Ta and V in mass% (2) ~ (5The hot-dip galvanized high-strength steel sheet excellent in hole expansibility according to any one of items 1).
[0023]
(7) The cast slab composed of the component described in any one of (1) to (6) above is directly or once cooled to 1000 ° C. or less and then heated again. After washing and cold rolling, the oxygen concentration is 50 ppm or less, the dew point is -20 ° C. or less, and the maximum temperature is_Three (° C.) After heat treatment to 1100 ° C. or less, it is cooled to a temperature range of galvanizing bath temperature −20 ° C. to zinc plating bath temperature + 50 ° C. at a cooling rate of 0.1 to 20 ° C./sec. Hot-dip galvanized high strength excellent in hole expandability with a tensile strength of 900 MPa or more and a hole expansion rate of 45% or more, characterized by holding for 1 second to 1000 seconds including plating immersion in the temperature range A method of manufacturing a steel sheet.
[0024]
(8) In the production method described in (7) above, cooling is performed to a temperature range of galvanizing bath temperature −20 ° C. to galvanizing bath temperature + 50 ° C., followed by plating immersion in the same temperature range for 1 second to 1000 seconds. A method for producing a hot-dip galvanized high-strength steel sheet excellent in hole expansibility, wherein alloying treatment is performed at 430 ° C. to 580 ° C. after holding for 2 seconds.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
[0026]
First, the reason for limiting the suitable range of the steel plate component in the present invention will be described.
[0027]
C: 0.03-0.12%
It is an element added for the purpose of controlling the fraction of the main phase and the second phase to ensure a good strength-hole expansibility balance. It will also affect the fine uniformity of the substrate. In order to ensure the strength and the area ratio of each second phase, 0.03% by mass (hereinafter the same) is required. If it exceeds 0.12%, the hole expandability decreases, so this is the upper limit. Furthermore, the strength of the welded portion tends to deteriorate.
0.04 to 0.10% is a more preferable range.
[0028]
Si: 0.001 to 0.8%
Si is an element added for the purpose of suppressing the formation of relatively coarse carbides that deteriorate the strength and ductility balance, but the upper limit is set to 0.8% because it significantly deteriorates the plateability. In addition, excessive addition adversely affects weldability. From the viewpoint of surface properties, the upper limit is preferably 0.6%. On the other hand, since extremely low Si leads to an increase in manufacturing cost, it is desirable to add 0.001% or more that does not greatly deteriorate the plating property.
[0029]
  Mn:2.53 to 3.12%,
  Mn is important for the present invention. That is, it has been found that limiting Mn to a predetermined range with respect to the relatively slow cooling rate in the above-described hot dip galvanizing line is extremely important from the viewpoint of hole expandability. A uniform structure is obtained by suppressing the ferrite transformation and using bainite or bainitic ferrite as the main phase which is the phase with the largest area ratio. Furthermore, it is 2.3% by mass or more in order to suppress carbide precipitation and pearlite generation, which are one cause of strength reduction and hole expandability deterioration. Preferably, the lower limit is over 2.5%. On the other hand, excessive addition promoted the formation of martensite and caused a significant decrease in ductility and hole expansibility due to segregation and the like, so the upper limit was 3.3% by mass.In addition, the upper limit of the amount of Mn was made into 3.12% or less based on the amount of Mn of the steel type B of Table 1 of the Example of this invention. Moreover, the lower limit of the amount of Mn was 2.53% based on the amount of Mn of steel types S and W in Table 1 of the examples of the present invention.
[0030]
P: 0.001 to 0.1%
P is a strengthening element. Lowering P improves hole expansibility, but extremely lowering is also economically disadvantageous, so 0.001% by mass was made the lower limit. Moreover, since addition of a large amount has an adverse effect on weldability, manufacturability during casting or hot rolling, and hole expandability, the upper limit is set to 0.1%.
[0031]
S: 0.0001 to 0.01%
As for S, lowering S is effective for improving hole expansibility. On the other hand, since extremely low S is economically disadvantageous, 0.0001% by mass is set as the lower limit, and 0.01% by mass is set as the upper limit. This is because the spreadability is adversely affected. More preferably, the upper limit is 0.003%.
[0032]
Al: 0.001 to 0.2%
Al is effective as a deoxidizing element. For this reason, 0.001 mass% was made into the minimum. On the other hand, excessive addition impairs hole expansibility, weldability and plating wettability, so 0.2% was made the upper limit. Preferably, it is 0.005 to 0.08% of range.
[0033]
Mo: 0.11-1%
Ti: 0.005-0.3%
Mo and Ti are extremely important for the present invention. That is, good hole expandability can be obtained only when Mo and Ti are added in combination and the amount of Mn is 2.3% or more as described above. The effect is acquired when the addition amount of Mo is 0.11% or more. However, if it exceeds 1%, an increase in cost becomes a problem, so the upper limit is made 1.0%. In addition, Mo has an effect of improving hardenability and preventing softening in the heat-affected zone during welding.
[0034]
Ti also sets the lower limit to 0.005 mass% from the above viewpoint. Moreover, excessive addition causes deterioration of ductility and hole expansibility, so the upper limit is made 0.3%. Ti basically suppresses the formation of carbides and pearlite that deteriorate the strength-hole expansibility balance. In addition, it is effective in suppressing ferrite transformation and making the main phase into bainite or bainitic ferrite. It is effective to get a balance. However, even if only Ti is added, the effect is not sufficient for the purpose of the present invention, and as described above, it is essential to add it simultaneously with Mn and Mo. More preferably, Mo: 0.20 or more and Ti: 0.015 or more. In order to further promote the effect of the combined addition of Mo and Ti, it is desirable that Mo (mass%) + 10 × Ti (mass%) ≧ 0.3%.
[0035]
Further, the steel targeted by the present invention can contain Cu and Ni for the purpose of improving the plating property without adversely affecting the strength-hole expansibility balance.
[0036]
Ni has the purpose of improving the hardenability in addition to improving the plating property. If it exceeds 0.01% by mass and exceeds 2% by mass, it contributes to the workability, particularly the hardness increase associated with the formation of martensite, and has an adverse effect. This is the upper limit.
[0037]
Cu has the purpose of improving the strength in addition to improving the plating property, and when added in an amount of 0.001% by mass or more and exceeding 2% by mass, the workability and manufacturability are adversely affected.
[0038]
In particular, when 0.3% or more of Si is added, it is desirable from the viewpoints of plating properties and alloying reactivity that Ni is 0.2% or more and Cu is 0.1% or more.
[0039]
Furthermore, the steel which this invention makes object can add Nb and B for the purpose of the further improvement of a strength-hole expansibility balance.
[0040]
Nb forms fine carbides, nitrides or carbonitrides and is extremely effective for strengthening steel sheets. It also retards ferrite transformation and promotes the formation of bainite and bainitic ferrite. Furthermore, it is also effective for suppressing softening of the weld heat affected zone, so 0.003 mass% or more is added. On the other hand, excessive addition deteriorates ductility and hot workability, so the upper limit was set to 0.3% by mass.
[0041]
B is effective for strengthening grain boundaries and increasing the strength of steel by addition of 0.0001% by mass or more. However, when the addition amount exceeds 0.01% by mass, the effect is not only saturated, but Nb Since the hot workability deteriorates in the same manner as described above, the upper limit was made 0.01 mass%.
[0042]
Furthermore, one or more of Cr, Co, and W can be contained.
[0043]
Cr is an element added for the purpose of strengthening and suppressing the formation of carbides and the formation of bainite and bainitic ferrite. It is 0.01% or more, and if it exceeds 2%, the workability and plating properties are adversely affected. Therefore, this is the upper limit.
[0044]
Co was added in an amount of 0.01% by mass or more for a good balance between strength and hole expansibility by bainite transformation control. On the other hand, although the upper limit of addition is not particularly provided, since it is an expensive element, addition of a large amount impairs economic efficiency, so it is desirable to make it 1% by mass or less.
[0045]
The strengthening effect appears when W is 0.01% by mass or more, and the reason why the upper limit is 0.3% by mass is that if it exceeds this, workability is adversely affected.
[0046]
Furthermore, the steel targeted by the present invention is a total of one or more of Zr, Hf, Ta, and V, which are strong carbide forming elements, for the purpose of further improving the balance between strength and hole expansibility. 001 mass% or more may be added. On the other hand, since it causes deterioration of ductility and hot workability, the upper limit of the total amount of one kind or two or more kinds is set to 1% by mass.
[0047]
Ca, Mg, and REM contribute to inclusion control, particularly fine dispersion, by adding appropriate amounts, so the total of one or two or more is 0.0001% or more, while excessive addition is castability and hot In order to reduce manufacturability such as workability and the ductility of the steel sheet product, the upper limit was set to 0.5 mass%.
[0048]
Inevitable impurities include, for example, N and Sn. Even if these elements are contained in a total amount of 0.2% by mass or less, the effect of the present invention is not impaired.
[0049]
Next, a preferable microstructure of the base steel sheet will be described.
[0050]
In order to achieve both a tensile strength of 900 MPa or more and excellent hole expandability, bainite or bainitic ferrite is suitable as the main phase. In order to obtain excellent hole expansibility, the area ratio is set to 80% or more.
[0051]
Moreover, the bainite said here contains both the upper bainite in which the carbide | carbonized_material has produced | generated in the lath boundary, and the lower bainite in which the fine carbide | carbonized_material has produced | generated in the lath. Bainitic ferrite means bainite having no carbide, and for example, acicular ferrite is one example.
[0052]
In order to improve hole expansibility, it is desirable that lower bainite in which carbide is finely dispersed or bainitic ferrite without carbide is the main phase and the area ratio exceeds 85%.
[0053]
In addition, as a remaining structure, the total of one or more of ferrite, austenite, martensite, and upper bainite structures may be contained in an area ratio of less than 15%. Furthermore, less than 10% is preferable. In order to obtain particularly excellent hole expansibility, austenite and martensite are preferably 5% or less, more preferably 3% or less in terms of area ratio. In addition to the above, as the remaining microstructure of the microstructure, carbides, nitrides, sulfides, oxides, and even those composite compounds containing a total area ratio of 2% or less can be used in the present invention. These were included in the area ratio of the main phase.
[0054]
In addition, each phase of the above microstructure, ferrite (bainitic ferrite), bainite, austenite, martensite, interfacial oxidation phase and remaining structure, identification of existing positions, and measurement of area ratio are performed using Nital reagent and The steel plate rolling direction cross section or the rolling perpendicular direction cross section is corroded with the reagent disclosed in Japanese Patent No. 59-219473, and quantified by observation with an optical microscope of 500 to 1000 times and an electron microscope (scanning type and transmission type) of 1000 to 100,000 times. Is possible. It is possible to obtain an area ratio of each tissue by observing 20 fields of view or more and using a point counting method or image analysis.
[0055]
A high-strength steel sheet having such a structure and excellent in hole expansibility and a method for producing the same will be described below.
[0056]
When the steel sheet of the present invention is manufactured by cold rolling and heat treatment after hot rolling, the slab adjusted to a predetermined component is directly or once cooled to 1000 ° C. or lower and then reheated for hot rolling. It is preferable to heat and heat-roll the as-cast steel slab as it is to reduce the heating unit, and to cool the steel slab to 1000 ° C. or less is preferable from the viewpoint of ductility of the final product.
[0057]
In this case, the reheating temperature is desirably 1100 ° C. or higher and 1300 ° C. or lower. When the reheating temperature is high, coarse grains and thick oxide scales are formed. On the other hand, low temperature heating increases deformation resistance during rolling.
[0058]
The hot rolling completion temperature is determined by the chemical composition of the steel._Three Generally, it is carried out at the transformation temperature or higher, but Ar_Three If it is up to about −100 ° C., the properties of the final steel sheet will not be deteriorated. In addition, the coiling temperature after cooling is higher than the bainite transformation start temperature determined by the chemical composition of the steel, so that the load during cold rolling can be increased more than necessary, but when the total rolling reduction of cold rolling is small Is not limited to this, and even if the steel sheet is wound at a temperature lower than the bainite transformation temperature of the steel, the properties of the final steel sheet are not deteriorated.
[0059]
Moreover, after hot rolling, removing the surface scale by using a high-pressure descaling device or pickling improves the surface cleanliness of the product, which is advantageous when plating. Then, it heat-processes after cold rolling, and it is set as a final product by performing hot dip galvanization. Further, electroplating may be further performed as a post-treatment. Further, the total rolling reduction of cold rolling is set based on the relationship between the final plate thickness and the cold rolling load, but if it is 30% or more, it is sufficient for recrystallization and does not deteriorate the properties of the final steel plate.
[0060]
Temperature Ac determined by the chemical composition of steel when heating after cold rolling_Three When the temperature is lower than the temperature (for example, “Steel Material Science”: W. C. Leslie, translated by Koyasu Naruyasu, Maruzen P273), the amount of austenite obtained at the time of heating is small. Nitic ferrite cannot be generated. In addition, the higher the maximum temperature reached, the higher the crystal grain coarsening and surface oxidation, and the higher the maximum temperature reached 1100 ° C. in order to increase the manufacturing cost. Ac_Three+100 (° C.) is a more preferable upper limit. The heat treatment time in this temperature range requires 1 second or more to make the temperature of the steel plate uniform and to secure austenite. However, if it exceeds 10 minutes, formation of a grain boundary oxidation phase is promoted, and the cost is increased. Here, the atmosphere during heating was an oxygen concentration of 50 ppm or less and a dew point of −20 ° C. or less. If the oxygen concentration exceeds 50 ppm or the dew point exceeds −20 ° C., the plateability of the steel sheet, in particular the wettability, deteriorates and causes non-plating.
[0061]
Subsequent cooling is important for producing bainite or bainitic ferrite while suppressing the transformation from the austenite phase to the ferrite phase to some extent. Setting this cooling rate to less than 0.1 ° C./second may promote the formation of ferrite and pearlite and cause a decrease in strength, so the lower limit of the cooling rate was set to 0.1 ° C./second. On the other hand, it is advantageous that the upper limit of the cooling rate is high, but since the object of the present invention is to ensure high hole expandability even by slow cooling during continuous hot-dip galvanizing line passing, the upper limit is intentionally set to 20 ° C / Seconds. The steel plate of the present invention has a cooling rate in the region of 10 ° C./second or less, that is, the target of 900 MPa or more even under the cooling rate condition in which it is difficult to stably ensure the strength and the hole expandability in the conventional steel plate. The strength and the hole expandability of 45% or more can be secured sufficiently stably. Furthermore, in this cooling process, not only cooling to a target temperature at a single cooling rate in the range of 0.1 ° C./second to 20 ° C./second, but also combining a plurality of cooling rates within the above range. You may cool to target temperature by changing a cooling rate in steps.
[0062]
When cooling is performed to a plating bath temperature of less than -20 ° C, there are operational problems such as a large heat removal at the time of entering the plating bath. Further, if the cooling stop temperature exceeds the plating bath + 50 ° C., in addition to operational problems, carbides are generated during the subsequent holding, leading to a decrease in strength, so this was made the upper limit. If the retention time in this temperature range is long, it is not preferable in terms of productivity, and carbides are generated, so it is desirable that the retention time be within 1000 seconds. Further, the bainite transformation is allowed to proceed, and the plating wettability is maintained for 1 second or longer, preferably 15 seconds to 10 minutes.
[0063]
Moreover, when performing an alloying process, it was set as 430 degreeC or more and 580 degrees C or less. When the alloying treatment temperature is lower than 430 ° C., the progress of alloying is slow and the productivity is poor. Moreover, when it exceeds 580 degreeC, a carbide | carbonized_material precipitation will accompany and material quality will deteriorate.
[0064]
The tensile strength of the steel sheet obtained by the present invention is 900 MPa or more. Preferably, it is 980 MPa or more. Although an upper limit is not specifically limited, since it is difficult to set it as 1600 MPa or more, this is made an upper limit. The hole expandability is 45% or more, preferably 60% or more. Since it is difficult to make it 200% or more, this is a practical upper limit.
[0065]
The steel of the present invention is also excellent in weldability. The welding method is adapted to a welding method that is usually performed, such as arc, spot, TIG, MIG, mash, and laser.
[0066]
【Example】
The steel sheet according to the present invention was subjected to microstructural observation, hole expansion test specified by the Federation of Steels, and tensile test based on JIS.
[0067]
Hereinafter, the present invention will be described in more detail with reference to examples.
<Example 1>
After cooling the casting slab having the composition as shown in Table 1 and Table 2 (continued in Table 1) to room temperature, it is heated to 1200 ° C., and Ar_Three Hot-rolling is completed at 880 ° C. to 910 ° C., which is higher than the transformation temperature, and after cooling, a 2.5 mm thick steel strip wound up at 550 ° C., which is higher than the bainite transformation start temperature determined by the chemical composition of each steel, is pickled. Thereafter, it was cold-rolled to obtain a steel plate having a thickness of 1.2 mm.
[0068]
Then, according to the following formula from the component (mass%) of each steel, Ac_Three The transformation temperature was determined by calculation.
Ac_Three = 910-203 × (C%)^1/2-15.2 x Ni%
+ 44.7 × Si% + 104 × V% + 31.5 × Mo%
−30 × Mn% −11 × Cr% + 400 × Al%,
Ac_Three The maximum temperature reached was determined from the transformation temperature. 870 ° C. was selected as the temperature satisfying the conditions for each steel, and the maximum temperature reached for each steel was constant at 870 ° C. The temperature raising and cooling process was performed as follows. That is, the temperature is increased to 10 ° C / s (maximum reached temperature −110) (° C.), that is, here, the temperature is increased to 760 ° C., and then the temperature is increased to the maximum temperature of 870 ° C. After being heated, the temperature is (maximum temperature -20) (° C) at a cooling rate of 0.2 ° C / second, that is, here, cooled to 850 ° C, and further cooled to 650 ° C at 2 ° C / second. Then, the cooling rate was 10 ° C./second and the cooling was performed to 500 ° C., and then the cooling rate was 2 ° C./second and the cooling rate was reduced to 460 ° C. Subsequently, it was immersed in a hot dip galvanizing bath at 460 ° C., then heated to 500 ° C. at a temperature rising rate of 3 ° C./second, held for 30 seconds, subjected to alloying treatment, and then cooled.
[0069]
JIS No. 5 tensile test specimens were collected from these steel plates and measured for mechanical properties. In addition, a hole expansion test was performed in accordance with the Steel Federation standard, and the hole expansion rate was obtained. Table 3 shows the mechanical properties and hole expandability of each steel. It can be seen that the inventive steel that satisfies the outline of the present invention has an excellent balance of hole expansibility (45% or more) strength (tensile strength of 900 MPa or more). In the steel of the present invention, there is no particular problem in the adhesion of the plating and the plating property is good. However, in the steel types AB and AC to which Ni and Cu are added, the finish of the plating appearance may be even better. I understood. That is, it can be seen that the addition of these further improves the adhesion of the plating and the like, and the plating property is improved.
[0070]
Moreover, in steel type Q which is a comparative example, Mo and Ti are not added, and it is understood that other elemental compositions are within the scope of the present invention and inferior in hole expandability. That is, it is an example showing the effect of Mo and Ti in the present invention. Steel types T and U are steels of the present invention, and both strength and hole expandability are good, but the hole expandability is slightly inferior to other invention steels.
[0071]
  In the steel type T and the steel type U, Mo (mass%) + 10 × Ti (mass%) is 0.23 (%) and 0.21 (%), respectively. In the steels according to the present invention other than these two steel types, all the amounts are 0.3 (%) or more, and Mo (mass%) + 10 × Ti (mass%) ≧ 0. It can be seen that 3 (%) is desirable. Steel types J and K areReference exampleAlthough the hole expansion ratio is 76% and 71%, respectively, it shows sufficient hole expansion performance.,BookSlightly inferior to the inventive steel. This is because in steel types J and K, Nb and B are not added, so that the structure is likely to be uneven in the cooling process. this is, NIt shows that it is preferable to add b and B within the composition range of the present invention.
[0072]
Moreover, all the comparative steels that deviate from the conditions of the present invention are inferior in hole expansibility.
[0073]
[Table 1]

[0074]
[Table 2]

“Invention steel” of steel types A, H, I, J, K, M, N, R, T, U is used as “reference example”
[0075]
[Table 3]

“Invention steel” of steel types A, H, I, J, K, M, N, R, T, U is used as “reference example”
[0076]
<Reference example>
  Among the steels shown in Tables 1 and 2 (continued in Table 1), the heat treatment conditions after cold rolling were changed for steel types A and R. That is, in the heat treatment conditions in Example 1, the maximum temperature reached and the cooling rate from (maximum temperature -20) (° C.) to 650 ° C. in the cooling process were changed. The other conditions were the same as those in Example 1. (However, since the maximum temperature reached is different, the temperature reached in the process of increasing the temperature at a heating rate of 10 ° C./sec in the temperature increasing process (the maximum temperature −110) (° C.) is different.) Table 4 shows the results of measuring the mechanical properties and hole expandability of.
[0077]
On the other hand, comparative steels that deviate from the conditions of the present invention are inferior in hole expansibility.
[0078]
[Table 4]

“Invention Steel” as “Reference Example”
[0079]
<Example 3>
Of the steels shown in Tables 1 and 2 (continued in Table 1), the steel slab S was subjected to the casting slab as it was under the same conditions as in Example 1 until heating to cold rolling, and after cold rolling, the following treatment was performed. . That is, the temperature is raised to the maximum temperature at a heating rate of 5 ° C./s, held for 90 seconds, then cooled to 680 ° C. at a cooling rate of 5 ° C./s, and then cooled to 460 ° C. by changing the cooling rate. did. Subsequently, it was immersed in a hot dip galvanizing bath at 460 ° C., then heated to 500 ° C. at a temperature rising rate of 3 ° C./second, held for 30 seconds, subjected to alloying treatment, and then cooled. In the above process, the maximum temperature reached and the cooling rate from 680 ° C. to 460 ° C. were changed. After the treatment, a mechanical test and hole expansibility measurement were performed. The results are shown in Table 5. It can be seen that the inventive steel that satisfies the conditions of the present invention has an excellent balance of hole expansibility (45% or more) strength (tensile strength of 900 MPa or more).
[0080]
On the other hand, comparative steels that deviate from the conditions of the present invention are inferior in hole expansibility.
[0081]
[Table 5]

[0082]
【The invention's effect】
According to the present invention, it is possible to obtain a hot-dip galvanized high-strength steel sheet having a tensile strength of 900 MPa or more and a hole expansibility of 45% or more and a method for producing the same.

Claims (8)

% By mass
C: 0.03-0.12%,
Si: 0.001 to 0.8%,
Mn: 2.53 to 3.12%
P: 0.001 to 0.1%,
S: 0.0001 to 0.01%,
Al: 0.001 to 0.2%
In addition,
Mo: 0.11 to 1.0%,
Ti: 0.005-0.3%
Containing
Mo (mass%) + 10 × Ti (mass%) ≧ 0.3%
A hot-dip galvanized high-strength steel sheet excellent in hole expansibility, characterized in that the balance is Fe and inevitable impurities, the tensile strength is 900 MPa or more, and the hole expansion ratio is 45% or more. Furthermore, in mass%,
B: 0.0001 to 0.01%
Nb: 0.003-0.3%
The hot-dip galvanized high-strength steel sheet excellent in hole expansibility according to claim 1, comprising one or two of the following. Furthermore, in mass%,
Cr: 0.01 to 2.0%
Co: 0.01 to 1%
W: 0.01 to 0.3%
One or claim 1 or hole expandability excellent hot dip galvanized high strength steel sheet according to any one of claims 2, characterized by containing two or more. Furthermore, by mass%, Ca, Mg, according to any one of claims 1 to 3, characterized in that it contains 0.0001 to 0.5% in total of one or more of REM Hot-dip galvanized high-strength steel sheet with excellent hole expandability. Furthermore, in mass%,
Ni: 0.01 to 2.0%,
Cu: 0.001 to 2.0%,
The hot-dip galvanized high-strength steel sheet excellent in hole expansibility according to any one of claims 2 to 4, wherein one or two of the above are contained. Furthermore, 0.001 to 1% of 1 type (s) or 2 or more types of Zr, Hf, Ta, and V is contained in the mass% in total, The any one of Claims 2-5 characterized by the above-mentioned. Hot-dip galvanized high-strength steel sheet with excellent hole expandability. The cast slab comprising the component according to any one of claims 1 to 6 is directly or once cooled to 1000 ° C or less and then heated again, and the hot-rolled steel sheet wound after hot rolling is pickled and then cold-rolled. Then, after performing heat treatment with an oxygen concentration of 50 ppm or less, a dew point of −20 ° C. or less, and a maximum temperature of Ac ₃ (° C.) to 1100 ° C., a cooling rate of 0.1 to 20 ° C./second The tensile strength is 900 MPa, characterized in that it is cooled to a temperature range of galvanizing bath temperature −20 ° C. to zinc plating bath temperature + 50 ° C. and subsequently held for 1 second to 1000 seconds including plating immersion in the same temperature range. A method for producing a hot-dip galvanized high-strength steel sheet that is excellent in hole expandability and has a hole expansion ratio of 45% or more.   In the production method according to claim 7, after cooling to a temperature range of galvanizing bath temperature -20 ° C to galvanizing bath temperature + 50 ° C, and subsequently holding the plating immersion in the same temperature range for 1 second to 1000 seconds, A method for producing a hot-dip galvanized high-strength steel sheet excellent in hole expansibility, wherein the alloying treatment is performed at 430 ° C to 580 ° C.