JP3780611B2 - High strength and high yield ratio hot dip galvanized steel sheet and its manufacturing method - Google Patents

Description

【００３７】
次いで、前記で得た鋼板に、実験室で焼鈍、冷却し、めっき浴温度465 ℃で片面当たり60g/m²の溶融亜鉛めっきを施した。その際、めっき前あるいはめっき後に第２焼鈍工程を設け、焼鈍後あるいは第２焼鈍後の冷却は、全て冷却速度が5 〜10℃/secの空冷で行った。
【００３８】
焼鈍条件、第2 焼鈍条件を表２に示す。表２の第２焼鈍パターンの欄の符号は図１の（ａ）〜（ｃ）に示される第２焼鈍工程とめっき工程の順序、あるいは第２焼鈍工程の有無を示している。
１：図１（ａ）のめっき工程の前に第２焼鈍を行うパターン
２：図１（ｂ）のめっき工程の後に第２焼鈍を行うパターン
なし：図１（ｃ）の第２焼鈍がないパターン
【００３９】
前記で得ためっき鋼板の材質（TS、El、YP）を、JIS5号引張試験片によって測定し、また、降伏比YR(=YP/TS)を求めた。得られた結果を表２に示す。なお、表２の皮膜GIは亜鉛めっき皮膜、GAは合金化溶融亜鉛めっき皮膜である。
【００４０】
【表２】

【００４１】
表２の実験No.1、2 、7 〜10、12〜15は本発明例である。いずれも引張強度(TS)が45kg/mm²以上で降伏比（YR）が80％以上になっている。
【００４２】
実験No.3〜6 、11、16、17は比較例である。いずれも降伏比(YR)が80％未満である。
【００４３】
No.3とNo.11 は第２焼鈍を行わなかったため、冷却過程において複合組織となり、降伏比が低くなった。No.4は第２焼鈍温度が高すぎたため、γ→α変態が起こらず、残ったγ相がその後の冷却中にマルテンサイト相になり、降伏比が低くなった。No.5は逆に第２焼鈍温度が低すぎたため、生成したベイナイト相がフェライト相に変態せず、降伏比が低くなった。
【００４４】
No.6も降伏比が低くなった。これは、焼鈍温度が900 ℃と高かったため、焼鈍中にγ相の体積率が多くなりすぎて、第２焼鈍を行っても、γ→α変態が完了しなかったものと推測される。
【００４５】
No.16 は供試材のMn濃度が高かったため、複合組織化し降伏比が低くなった。No.17 は供試材のCr濃度が高かったため、複合組織化し降伏比が低くなった。
【００４６】
【発明の効果】
以上に示したように、本発明によって、引張強度(TS)が45kg/mm²以上で降伏比が80％以上の高強度高降伏比型溶融亜鉛めっき鋼板を得ることができる。本発明の鋼板は、高速変形量が少ないので、衝突安全性の求められる自動車の客室部材等の用途に使用することができる。
【図面の簡単な説明】
【図１】実施例の焼鈍、第２焼鈍およびめっき工程の熱サイクルのパターンを示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength, high-yield ratio hot-dip galvanized steel sheet and a method for producing the same.
[0002]
[Prior art]
Conventionally, automobile bodies have been required to be lighter from the viewpoint of improving fuel consumption and reducing exhaust gas. Moreover, since corrosion resistance is also requested | required in many cases, the high intensity | strength hot-dip galvanized steel plate has been used as a raw material of the member for motor vehicles.
[0003]
On the other hand, in recent years, the impact safety of automobiles has become important. For this reason, the structural parts in the passenger compartment of automobiles are required not only to have high strength but also to have a material with little deformation at the time of collision, that is, a material with a small amount of high-speed deformation. The amount of high-speed deformation of a material is related to the yield point, and it is known that the yield point of a material should be increased to reduce the amount of high-speed deformation (Proceedings of the Spring Conference of the Society of Automotive Engineers of Japan, Showa 48) , p.60).
[0004]
However, conventionally developed high-strength hot-dip galvanized steel sheets, especially high-strength hot-dip galvanized steel sheets with a tensile strength of 60 kg / mm ² or more, have a composite structure such as ferrite phase + martensite phase or ferrite phase + bainite phase. there were. For example, JP-B-59-5649, JP-A-4-36741, JP-B-57-61819, JP-B-59-43975, JP-A-3-94018, JP-A-5-125485, The steel sheets disclosed in JP-A-5-47586, JP-A-6-93340, etc. are so-called high-strength, low-yield ratio hot-dip galvanized steel sheets with a low yield ratio. There are few examples of yield ratio hot dip galvanized steel sheets developed.
[0005]
Conventionally developed high-strength hot-dip galvanized steel sheet has a low yield ratio. The reason is that high strength can be obtained with a relatively small amount of additive elements, and the lower the yield point, the easier the processing during press molding. The point which has the advantage of a steel plate manufacture surface and a use surface etc. is mentioned.
[0006]
[Problems to be solved by the invention]
In order to produce a high yield ratio type high strength hot dip galvanized steel sheet, the steel sheet structure is changed to a non-composite structure (hereinafter simply referred to as a non-composite structure) composed of a ferrite phase and a pearlite phase without a martensite phase or a bainite phase. do it. However, in order to strengthen the steel sheet to the required strength, it is necessary to add a large amount of elements for the purpose of solid solution strengthening or precipitation strengthening. However, the addition of a large amount of elements improves the hardenability of the steel sheet, and martens There is a problem that a site phase or a bainite phase is generated and it is difficult to stably obtain a non-composite structure.
[0007]
If it is cooled at a sufficiently low cooling rate, a non-composite structure can be obtained regardless of the composition of the steel sheet. However, in an actual continuous line, the cooling rate can be extremely reduced due to equipment limitations and productivity problems. Have difficulty.
[0008]
In consideration of such circumstances, the present invention provides a yield strength with a tensile strength of 45 kg / mm ² or more that can stably obtain a non-composite structure without extremely reducing the cooling rate in an actual continuous line. An object is to provide a high-strength, high-yield ratio hot-dip galvanized steel sheet having a ratio of 80% or more and a method for producing the same.
[0009]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors have studied from both aspects of the steel plate component and the steel plate production method, and by means of a realistic steel plate component and production method, high strength and high yield ratio type melting of the non-composite structure It has been found that a galvanized steel sheet can be obtained stably.
[0010]
A configuration for solving the above-described problem is as follows.
(1) In the first invention, the composition is% by weight, C: 0.07 to 0.25%, Mn: 1.5 to 2.5%, Nb: 0.10% or less, Ti: 0.3% or less, Si: 0.1% or less, Cr: 0.1% P: 0.05% or less, Sol.Al: 0.010 to 0.100%, S: 0.01% or less, N: 0.01% or less, the balance being Fe and inevitable impurities , steel sheet structure is ferrite phase and pearlite phase A high-strength and high-strength material having a tensile strength of 45 kg / mm ² or more and a yield ratio of 80% or more formed by forming a hot-dip galvanized film or an alloyed hot-dip galvanized film on a cold-rolled steel sheet made of It is a yield ratio hot-dip galvanized steel sheet.
[0011]
(2) The second invention is a hot dip galvanized steel sheet obtained by hot rolling, pickling, and cold rolling the steel having the composition of (1), followed by annealing in a continuous hot dip galvanizing line, followed by hot dip galvanizing. In the manufacturing process, after annealing at a temperature range of 750 ° C. to 880 ° C. for 30 seconds to 90 seconds, either before or after hot dip galvanization or after hot dip galvanization, either 515 ° C. to 600 ° C. There line a second annealing for holding at the temperature range 15sec or more, a high strength and high yield ratio-type production method of galvanized steel sheet, wherein a steel sheet structure and texture comprising ferrite phase and a pearlite phase.
[0012]
(Function)
The present invention is described in detail below. First, the reasons for limiting the steel plate components of the present invention will be described.
[0013]
C: 0.07 to 0.25%
C is an indispensable component for securing the strength of steel, and 0.07% is made the lower limit in order to obtain high strength. However, excessive addition of C deteriorates weldability and delayed fracture resistance and improves the hardenability of the steel sheet, making it difficult to stably obtain a non-composite structure. And
[0014]
Mn: 1.5-2.5%
Mn is a component necessary for improving the strength and toughness of the steel sheet by solid solution strengthening and grain refinement strengthening. However, it also has the effect of stabilizing the austenite phase and generating a transformation phase such as a bainite phase and a martensite phase to make the steel sheet structure a composite structure. In the present invention, the lower limit is set to 1.5% for obtaining the required strength and toughness, and the upper limit is set to 2.5% for obtaining a non-composite structure stably.
[0015]
Nb: 0.10% or less
Nb is added according to the intended strength in order to form carbides and refine the structure to improve the material. It also has the effect of promoting transformation from the austenite phase to the ferrite phase during the cooling process. Even if an amount exceeding 0.10% is added, the effect is saturated, so the content is made 0.10% or less (including no addition).
[0016]
Ti: 0.3% or less
Ti, like Nb, has the effect of forming carbides to refine the structure and improve the strength, so it is added according to the intended strength. It also has the effect of promoting transformation from the austenite phase to the ferrite phase during the cooling process. Even if an amount exceeding 0.3% is added, the above effect is saturated, so the content should be 0.3% or less (including no addition).
[0017]
Si: 0.1% or less
Si has been conventionally used as a solid solution strengthening element and is effective for strengthening steel sheets, but has the disadvantages that it degrades the plating properties and significantly delays the alloying of the plating film. In the present invention, in order to prevent the deterioration of the plating property and the delay of alloying of the plating film, the upper limit is made 0.1% (including no addition).
[0018]
Cr: 0.1% or less
Cr, like Mn, stabilizes the austenite phase and makes it difficult to form a non-composite structure. It is also detrimental to the wettability of the plating. Therefore, in the present invention, the amount is reduced as much as possible, and the upper limit is set to 0.1% (including no addition).
[0019]
P: 0.05% or less
P is preferably as low as possible in terms of workability, plating adhesion and alloying of the plating film, and the upper limit is made 0.05%.
[0020]
Sol.Al: 0.010 to 0.100%
Al is added for the purpose of deoxidizing steel. In order to obtain a desired effect, the lower limit is set to 0.010%, and even if an amount exceeding 0.100% is added, this effect is saturated, so the upper limit is set to 0.100%.
[0021]
S: 0.01% or less
S is desirably lower in terms of workability, and the upper limit is made 0.01%.
[0022]
N: 0.01% or less
N is also preferably low in terms of workability, and the upper limit is set to 0.01%.
[0023]
Next, the reasons for limiting the manufacturing conditions of the steel sheet of the present invention will be described.
Annealing temperature: 750 ° C to 880 ° C (preferable range: 800 ° C to 880 ° C)
Annealing is performed in order to completely eliminate the cold rolled structure and the band structure, to obtain a homogeneous structure and improve the workability of the steel sheet, and to improve the shape of the steel sheet.
[0024]
In the present invention, in order to obtain a tensile strength of 45 kg / mm ² or more and a yield ratio of 80% or more, the steel sheet structure needs to be a fine ferrite phase + pearlite phase. For this purpose, it is necessary to have a two-phase structure of ferrite and austenite (hereinafter referred to as a two-phase structure of α + γ) during annealing. This is because the austenite crystal (hereinafter referred to as γ crystal) is formed at the interface of the ferrite crystal (hereinafter referred to as α crystal) during annealing, and this γ crystal is transformed into α crystal during the subsequent cooling process. This is because the structure becomes a fine α crystal. If the temperature is lower than 750 ° C., the formation of γ crystals becomes insufficient, and the effect of strengthening the steel sheet due to the refinement of the α crystal grains is reduced, so the lower limit is set to 750 ° C. At 800 ° C. or higher, the above action is more stable, so the lower limit is more preferably 800 ° C. On the other hand, if the temperature exceeds 880 ° C, the crystal becomes coarse and the strength decreases, so the upper limit is set to 880 ° C.
[0025]
Annealing time: 30 ~ 90sec
As described above, the steel sheet structure needs to have a two-phase structure of α + γ during annealing. However, if the annealing time is less than 30 seconds, the formation of γ crystals becomes insufficient, and the fineness of α crystal grains is reduced. The effect | action of strengthening a steel plate falls by forming. Further, even if annealing is performed for more than 90 seconds, the γ transformation has reached a steady state, and therefore the γ transformation does not proceed any further. Therefore, the lower limit is 30 sec and the upper limit is 90 sec.
[0026]
Second annealing temperature: 515 ° C-600 ° C
In order to make the steel sheet structure stable and non-composite structure, the cooling rate after annealing should be as small as possible, and as a result of our study, the cooling rate should be 2 ° C / sec or less if possible. preferable. However, it is difficult to actually obtain this cooling rate considering the restrictions on the equipment of the line and productivity. However, for the steel sheet having the composition described in claim 1, the present inventors perform a second annealing which is held for a certain time or more in a temperature range of 515 ° C. to 600 ° C. in the cooling process after annealing, thereby austenite. It was found that even when the cooling rate is high by proceeding the transformation of the phase to the ferrite phase (hereinafter referred to as γ → α transformation), the formation of martensite phase and bainite phase can be prevented.
[0027]
That is, the austenite phase (hereinafter referred to as γ phase) is completely transformed into the ferrite phase (hereinafter referred to as α phase) during the second annealing, and the subsequent phases such as bainite phase and martensite phase are not generated in the subsequent cooling. Even if these second phases are generated in the cooling process after annealing, the second phase can be transformed into an α phase during the second annealing. When this holding temperature is less than 515 ° C., a bainite phase is formed. Further, if the temperature exceeds 600 ° C., the γ → α transformation does not occur sufficiently, so that a martensite phase or a bainite phase may be formed in the subsequent cooling process. Therefore, the lower limit of the holding temperature was 515 ° C and the upper limit was 600 ° C.
[0028]
Second annealing time: 15 seconds or more. If the second annealing time is less than 15 seconds, the desired effect cannot be obtained. Therefore, the lower limit of the holding time was set to 15 seconds.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
The steel sheet of the present invention is a steel sheet obtained by hot rolling, pickling and cold rolling the steel melted in the component range according to claim 1, and annealing and hot dip zinc using a continuous hot dip galvanizing line. Manufacture by plating. Steel melting, hot rolling, pickling, cold rolling methods, hot dip galvanizing conditions and alloying conditions other than the annealing conditions and second annealing conditions defined in claim 2 are not particularly limited, and are usually The method used is acceptable.
[0030]
In terms of the material, the second annealing may be performed before the plating step or after the plating step.
[0031]
When it is not desired to alloy the plating film, it is necessary to perform the second annealing before the plating step. In this case, if it is possible in terms of equipment, cooling equipment associated with the annealing furnace may be used, but a dedicated equipment may be provided between the annealing furnace and the plating bath.
[0032]
Moreover, when forming an alloying hot-dip galvanization film by alloying a plating film, if a 2nd annealing is performed after a plating process, it can serve as the alloying process of a plating film. Of course, also when alloying hot dip galvanization is performed, plating may be performed after the second annealing, and then alloying treatment may be performed.
[0033]
Further, the present invention also includes a method in which the coil is once wound on the hot dip galvanizing line and then subjected to the second annealing with another facility.
[0034]
FIG. 1A shows an example of a thermal cycle pattern when the second annealing is performed before the plating step, and FIG. 1B shows an example of a thermal cycle pattern when the second annealing is performed after the plating step. However, the thermal cycle pattern of the present invention is not limited to these patterns.
[0035]
【Example】
Examples of the present invention are shown below. Steels shown in Table 1 (invention examples: a to f, comparative examples: g to h) were melted by vacuum melting in a laboratory, and the ingot obtained by casting was hot-rolled to a thickness of 2.6 mm. In the hot rolling, the finishing temperature was set to 900 ° C, and after the final rolling, it was held in a furnace at 650 ° C for 1 hour. Then, after cooling, pickling and cold rolling were performed to obtain a steel plate having a thickness of 1.0 mm.
[0036]
[Table 1]

[0037]
Next, the steel sheet obtained above was annealed and cooled in a laboratory, and was subjected to hot dip galvanizing at a plating bath temperature of 465 ° C. at a rate of 60 g / m ² per side. At that time, a second annealing step was provided before or after plating, and cooling after annealing or after the second annealing was all performed by air cooling at a cooling rate of 5 to 10 ° C./sec.
[0038]
Table 2 shows the annealing conditions and the second annealing conditions. The symbols in the column of the second annealing pattern in Table 2 indicate the order of the second annealing step and the plating step shown in FIGS. 1A to 1C, or the presence or absence of the second annealing step.
1: Pattern 2 for performing the second annealing before the plating step of FIG. 1 (a): No pattern for performing the second annealing after the plating step of FIG. 1 (b): No second annealing of FIG. 1 (c) Pattern [0039]
The material (TS, El, YP) of the plated steel sheet obtained above was measured with a JIS No. 5 tensile test piece, and the yield ratio YR (= YP / TS) was determined. The obtained results are shown in Table 2. In Table 2, the film GI is a galvanized film, and GA is an alloyed hot dip galvanized film.
[0040]
[Table 2]

[0041]
Experiments Nos. 1, 2, 7 to 10, and 12 to 15 in Table 2 are examples of the present invention. In both cases, the tensile strength (TS) is 45 kg / mm ² or more and the yield ratio (YR) is 80% or more.
[0042]
Experiments Nos. 3-6, 11, 16, and 17 are comparative examples. In both cases, the yield ratio (YR) is less than 80%.
[0043]
Since No. 3 and No. 11 did not perform the second annealing, they became a composite structure in the cooling process, and the yield ratio was low. In No. 4, since the second annealing temperature was too high, the γ → α transformation did not occur, and the remaining γ phase became a martensite phase during the subsequent cooling, resulting in a low yield ratio. In contrast, in No. 5, the second annealing temperature was too low, so the produced bainite phase did not transform into a ferrite phase, and the yield ratio was low.
[0044]
No. 6 also had a low yield ratio. This is presumably because the annealing temperature was as high as 900 ° C., so that the volume fraction of the γ phase increased during annealing, and the γ → α transformation was not completed even when the second annealing was performed.
[0045]
No. 16 had a high Mn concentration in the specimen, resulting in a composite structure and a low yield ratio. No. 17 had a high Cr concentration in the specimen, resulting in a composite structure and a low yield ratio.
[0046]
【The invention's effect】
As described above, according to the present invention, a high-strength, high-yield ratio hot-dip galvanized steel sheet having a tensile strength (TS) of 45 kg / mm ² or more and a yield ratio of 80% or more can be obtained. Since the steel plate of the present invention has a small amount of high-speed deformation, it can be used for applications such as automobile cabin members that require crash safety.
[Brief description of the drawings]
FIG. 1 is a diagram showing a thermal cycle pattern of annealing, second annealing, and plating steps in an example.

Claims (2)

Composition:% by weight, C: 0.07 to 0.25%, Mn: 1.5 to 2.5%, Nb: 0.10% or less, Ti: 0.3% or less, Si: 0.1% or less, Cr: 0.1% or less, P: 0.05% or less, Sol.Al: 0.010 to 0.100%, S: 0.01% or less, N: 0.01% or less, the balance is composed of Fe and inevitable impurities, and the steel sheet structure is composed of a ferrite phase and a pearlite phase , A high-strength, high-yield ratio hot-dip galvanized steel sheet having a tensile strength of 45 kg / mm ² or more and a yield ratio of 80% or more formed by forming a galvanized film or an alloyed galvanized film. The steel having the composition according to claim 1 is hot rolled, pickled, cold rolled, annealed in a continuous hot dip galvanizing line, and then hot dip galvanized to produce a hot dip galvanized steel sheet. After annealing at a temperature range of 880 ° C or lower for 30 seconds or more and 90 seconds or less, hold at least 15 seconds in the temperature range of 515 ° C to 600 ° C either before or after hot-dip galvanization or after hot-dip galvanization. There line a second annealing, a high strength and high yield ratio-type production method of galvanized steel sheet, wherein a steel sheet structure and texture comprising ferrite phase and a pearlite phase.

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