JP3951282B2 - Hot-dip galvanized steel sheet and manufacturing method thereof - Google Patents

Description

【００３４】
【表２】

【００３５】
【表３】

【００３６】
表４は、本発明成分鋼Ｈについて、とくに連続溶融亜鉛メッキラインでの熱サイクル条件を変化させて、特性の変化を検討した結果を示している。鋼番１と鋼番５では均熱温度が、鋼番６や11では冷却速度が適切でないため、また、鋼番16では400〜600℃での滞留時間が長すぎるため、本発明の規定する組織が得られておらず、所望の耐ＨＡＺ軟化特性が得られていない。これに対し、請求項３に記載する製造条件で製造した本発明鋼においては、請求項１に記載する組織が得られ、いずれも、ΔHv≦20と優れた耐ＨＡＺ軟化特性が得られている。
【００３７】
【表４】

【００３８】
【発明の効果】
以上説明したように、本発明によれば、ＨＡＺ部の硬度変化が小さい高強度溶融亜鉛メッキ鋼板が得られる。すなわち、本発明に係る溶融亜鉛メッキ鋼板をテーラードブランク材に使用した場合には成形性の劣化が防止される。また、本発明に係る溶融亜鉛メッキ鋼板は、溶接で構造体となされたとき、部材の変形強度、破断強度、高速変形強度などの性能の劣化が極めて小さい。よって、本発明に係る溶融亜鉛メッキ鋼板は、テーラードブランク材用、及び溶接された構造体用として用いるのに極めて好適である。
【図面の簡単な説明】
【図１】 Mo、Ｖ含有量とΔHvとの関係を示す図である。
【図２】 Mo、Ｖ、Cr含有量の過不足によるＨＡＺ部での硬度変化を模式的に示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is applied to construction members, machine structural parts, automobile structural parts, and the like, and is particularly suitable for use as a structural member that is assembled as a structural member by welding as a plate or after molding. The present invention relates to a plated steel sheet.
[0002]
[Prior art]
High-strength hot-dip galvanized steel sheets with a tensile strength exceeding 440 MPa have the advantage of excellent rust prevention and high proof stress, and are widely applied to construction members, machine structural parts, automotive structural parts, and the like. For this reason, very many inventions related to high-strength hot-dip galvanized steel sheets have been disclosed. In particular, since the required characteristics for workability are increasing as the application range is expanded, it relates to a high-strength hot-dip galvanized steel sheet excellent in workability, such as JP-A-5-31244 and JP-A-7-54051. A number of inventions have been disclosed.
[0003]
[Problems to be solved by the invention]
In recent years, while the required characteristics for workability of as-manufactured steel sheets have increased, with the expansion of applied technology, it has been processed in a state including a welded part, such as a tailored blank material, or The characteristics of the welded part have been attracting attention as the required characteristics of the product, such as the required characteristics for the high-speed deformation behavior of the included structural members become severe.
[0004]
However, the conventional high-strength hot-dip galvanized steel sheet with excellent workability as described above generally uses a low-temperature transformation phase such as martensite or bainite, whose main strengthening mechanism is obtained by rapid cooling of the austenite phase. There is a big weakness that HAZ is sometimes softened. Such softening of HAZ during welding leads to deterioration of formability in, for example, tailored blank materials, and deteriorates performance as a structural member such as deformation strength, breaking strength, and high-speed deformation strength even when used for other applications. It has the problem of becoming a cause.
[0005]
The present invention has been made in view of such circumstances, and is a novel high-strength melt having the property that the hardness change of the HAZ part is extremely small under a wide range of welding methods such as laser welding, mash seam welding or arc welding. It is an object of the present invention to provide a galvanized steel sheet and a manufacturing method thereof.
[0006]
The first means for solving the above-mentioned problems is, by weight, C: 0.04% to 0.13%, Si: 0.5% or less, Mn: 1 to 2%, P: 0.05% or less, S: 0.01% or less ( Sol.Al: 0.05% or less, N: 0.007% or less (including 0), Mo: 0.05 to 0.5%, Cr: 0.2% or less (including 0), the balance being Fe and inevitable A hot-dip galvanized steel sheet comprising an impurity and having a structure in which ferrite having an average particle diameter of 20 μm or less and martensite having a volume ratio of 5 to 40% exceed 95 % by volume .
[0007]
The second means for solving the above-mentioned problems is that in addition to the components of the first means, V: 0.02 to 0.2%, ferrite having an average particle diameter of 20 μm or less, and martensite having a volume ratio of 5 to 40%. A hot-dip galvanized steel sheet characterized in that a site is composed of a structure exceeding 95 % by volume .
[0008]
The 3rd means for solving the above-mentioned subject is a manufacturing method of the hot dip galvanized steel sheet concerning the 1st means or the 2nd means, and was indicated in the 1st means or the 2nd means, respectively. The steel having the components is made into a hot-rolled steel strip after casting, and after pickling, it is cold-rolled at a reduction rate of 40% or more as necessary, and is soaked at 750 to 850 ° C. in a continuous hot-dip galvanizing line. After that, it is cooled to a temperature range of 600 ° C. or lower at a cooling rate of 1 to 50 ° C./sec, galvanized, further alloyed as necessary, and then a residence time at 400 to 600 ° C. for 200 seconds. The cooling is performed in such a state that it is within the range (Claim 3).
[0009]
In the specification and drawings,% indicating the components of the steel, the meaning taste weight unless otherwise specified.
[0010]
(Background to the invention and reasons for limitation of Mo, V, Cr and structure)
In order to solve the above-mentioned problems, the present inventors have intensively studied the influence of the steel composition and the structure on the strength change of the welded portion. As a result, the present invention has an appropriate amount for a steel having a certain basic component such as C, Si, Mn. The hardness reduction of the HAZ part is extremely small by making the structure mainly composed of ferrite having an average particle diameter of 20 μm or less and martensite whose volume ratio is limited to 5 to 40%. It has been found that a high-strength hot-dip galvanized steel sheet can be produced. It has also been found that this effect is further enhanced by containing an appropriate amount of V.
[0011]
In general, the low-temperature transformation phase obtained by rapid cooling of austenitic phases such as martensite and bainite, when kept at a high temperature such as 400 to 800 ° C., is easily tempered even in a short time, and the carbides become coarse, resulting in a rapid increase. It is known that the strength decreases. As a result of detailed examination of the effects of the steel composition and the microstructure on the hardness change during welding of such a low-temperature transformation strengthened high-strength steel, the present inventors have confirmed that the following control prevents a decrease in strength. Found to be effective.
[0012]
{Circle around (1)} By using martensite with a high dislocation density as a hard phase and utilizing secondary precipitation strengthening, the strength reduction of the hard phase can be reduced at a short temperature rise. For this purpose, it is effective to contain Mo or V. However, if these contents increase, the HAZ part partially increases in hardness compared to the base material, which is not preferable. In addition, Cr, which is known as a secondary precipitation strengthening element, like Mo and V, is precipitating quickly at a high temperature, so the hardness change in the HAZ part becomes large, so the Cr content is increased. Is not preferred.
[0013]
(2) The volume ratio of the martensite phase, which has a large hardness change during welding, is limited to 40% or less, and the balance is ferrite, so that the overall hardness change can be reduced. However, if the volume fraction of martensite becomes too small, the effect of secondary precipitation strengthening of the martensite phase cannot be effectively utilized for the HAZ softening resistance, so the lower limit is defined as 5%.
[0014]
{Circle around (3)} Further, control of the ferrite grain size is also important. By increasing the grain boundary area by setting the average grain size to 20 μm or less, precipitation of austenite at the grain boundary is promoted when the temperature is raised for a short time. As a result, an increase in Ac3 transformation point at which the most significant decrease in the hardness of the martensite phase is avoided, and a decrease in the hardness of the martensite phase is suppressed.
[0015]
Hereinafter, the reasons for limitation of Mo, V, and Cr will be described.
Mo: 0.05-0.5%
Mo is an essential element for obtaining the effects of the present invention. As described above, this is because the temper softening of the martensite phase due to the temperature rise in the HAZ part during welding is suppressed by precipitation of Mo carbides. Therefore, 0.05% at which the effect is manifested is contained as the lower limit. However, if it is excessively contained, the hardness increase is greatly increased in the HAZ part, and the hardness change of the HAZ part is increased. For this reason, the upper limit is defined as 0.5%.
[0016]
Cr: 0.2% or less (including 0)
In carrying out the present invention, other elements that are considered effective for temper softening resistance of the martensite phase based on the Mo content, specifically, V and Cr were also examined. As a result, the effect of the type of element is different when the temperature is raised for a short time like the HAZ part during welding, and even if Cr is contained in a trace amount, the hardness increase in the HAZ part is large, and if it is contained exceeding 0.2% It became clear that the hardness change of the HAZ part became large. For this reason, in the present invention, the Cr content is limited to 0.2% or less (including 0).
[0017]
V: Preferably 0.02 to 0.2%
It was V that attracted attention in this study, and the hardness change in the HAZ part became extremely small due to the combined inclusion of Mo and V. This is because precipitation strengthening due to V carbide at the time of temperature rise in the martensite phase is not so large, and is different from the temperature at which Mo carbide is precipitated, and therefore, uniform temper softening resistance in a wide thermal history region of the HAZ part. It was thought that it was obtained. The lower limit of the V amount for obtaining such an effect is 0.02%. If excessively contained, the increase in hardness at the HAZ part will also increase, so the upper limit is defined as 0.2%. The reason for limiting the lower limit of V to 0.02% in the second means (Claim 2) is as described above. Therefore, the first means (Claim 1) has a V content of 0.02%. It does not exclude what is less than%.
[0018]
FIG. 2 schematically shows a change in hardness at the HAZ portion due to the excess or deficiency of the Mo, V, and Cr contents described above. (a) is a case where the contents of Mo and V are less than appropriate amounts, and the difference ΔHv between the softest part of the HAZ part and the hardness of the base material is large. (b) is a case where the contents of Mo, V, and Cr exceed appropriate amounts, and the softening amount in the HAZ portion is small, but the base material is also cured, so ΔHv eventually increases. (c) is the content of Mo, V, Cr within the range of the present invention, and ΔHv becomes small.
[0019]
(Reason for limitation of other ingredients)
C: 0.04-0.13%
C is an essential element for securing a desired strength. However, when the content increases, the martensite volume fraction increases excessively, and the hardness of the HAZ part decreases greatly. That is, the lower limit is defined as the minimum amount for securing the strength, and the upper limit is defined as described above so that the martensite volume ratio at which the hardness reduction of the HAZ part is large does not exceed 40%.
[0020]
Si: 0.5% or less,
Si is an indispensable element for obtaining a ferrite + martensite two-phase structure stably. However, as the content increases, the adhesion and surface appearance of galvanizing deteriorate significantly, so the upper limit is specified to 0.5%. To do.
Mn: 1-2%
Mn, like C, is an essential element for securing a desired strength. In order to obtain a desired strength, 1% is necessary as the lower limit. However, if excessively contained, the martensite volume ratio increases and the hardness of the HAZ part decreases greatly, so the upper limit is defined as 2%.
[0021]
P: 0.05% or less P is an essential element in order to stably obtain a ferrite + martensite two-phase structure as in Si. However, as the content increases, the toughness of the welded portion deteriorates. Set to 0.05%.
S: 0.01% or less S is an impurity, and if the content is high, the toughness of the welded portion deteriorates as in the case of P. Therefore, the upper limit is specified as 0.01%.
[0022]
sol.Al: 0.05% or less
If the amount of sol.Al is an amount contained in ordinary steel, the effect of the present invention is not impaired, and if it is 0.05% or less, there is no problem, so the upper limit is specified as 0.05%.
N: 0.007% or less (including 0)
If N is an amount contained in ordinary steel, the effect of the present invention is not impaired, and if it is 0.007% or less, there is no problem, so the upper limit is defined as 0.007%.
[0023]
In addition, elements not mentioned are not particularly impaired unless they are contained in an extremely large amount. For example, when Nb or Ti is added for the purpose of increasing the strength or refining of steel, there is no problem as long as it is within 0.05%.
[0024]
(Production method)
Next, the manufacturing method of the hot dip galvanized steel sheet of the present invention will be described.
In order to obtain the steel sheet of the present invention, it is necessary to limit each component element as described above and to control the structure mainly composed of ferrite having an average particle size of 20 μm or less and martensite having a volume ratio of 5 to 40%.
[0025]
First, after casting a steel having a predetermined component, a hot-rolled steel strip is formed, and after pickling, cold rolling is further performed at a reduction rate of 40% or more as necessary to prepare a plating base. Hot rolling conditions are not specified. There is a particular problem unless the rolling method is such that the hot-rolled sheet particle size becomes significantly large, such as when the finish hot rolling is below the Ar3 transformation point or the cooling rate after hot rolling is 10 ° C / sec or less Does not occur. On the contrary, the action of reducing the hot-rolled plate particle size, such as utilizing large cooling such as 100 to 300 ° C./sec within 1 second after the end of hot-rolling, and further combining this with hot rolling under finish, is the present invention. Does not hinder the effects of The reason why the rolling reduction during cold rolling is specified to be 40% or more is that if the rolling reduction is less than this, the grain size tends to increase due to annealing.
[0026]
In the continuous hot dip galvanizing line, after soaking at 750 to 850 ° C, it is cooled to a temperature range of 600 ° C or less at a cooling rate of 1 to 50 ° C / sec, and the residence time at 400 to 600 ° C is within 200 seconds. Zinc plating is performed, and further alloying treatment is performed as necessary. The soaking temperature needs to be 750 ° C. or higher in order to stably obtain the austenite phase, but if it exceeds 850 ° C., the particle size becomes large and desired characteristics cannot be obtained, so this is the upper limit. After that, it is cooled to a temperature range of 600 ° C. or less at a cooling rate of 1 to 50 ° C./sec. This does not cause pearlite and precipitates the desired volume fraction of fine ferrite. Is defined below because pearlite is produced and the ferrite grain size is increased. The upper limit of the cooling rate is specified because not only ferrite is sufficiently precipitated but also the martensite volume fraction exceeds 40%.
[0027]
The pickled plate or cold-rolled plate is cooled to a temperature range of 600 ° C. or lower, galvanized, and further subjected to alloying treatment as necessary, and finally cooled to room temperature. According to the study by the present inventors, it has been clarified that the residence time at 400 to 600 ° C. greatly affects the formation of the structure in the cooling process to room temperature. That is, when the residence time becomes longer, precipitation of cementite from austenite becomes remarkable, the volume fraction of the martensite phase decreases and the strength decreases, and the HAZ softening resistance effect due to precipitation of Mo and V cannot be obtained. Based on the results of the study by the present inventors, the upper limit of this residence time is defined as 200 seconds.
[0028]
Here, in the present invention, in addition to ferrite and martensite having a volume ratio of 5 to 40%, the structure may include cementite, bainite, or retained austenite having a volume ratio of 5% or less. The effect of the invention is not impaired.
[0029]
In addition, although not specifically mentioned, within 200 ° C using an induction heater in the slab manufacturing method such as ingot forming or continuous casting, continuous hot rolling by hot-rolling rough hot-rolling bar connection, hot-rolling process The temperature rise or the like does not affect the effects of the present invention.
[0030]
【Example】
Examples of the present invention and comparative examples will be described below.
Invention component steels A to X having components as shown in Table 1 and comparative component steels a to m having components in a range deviating from the components of the present invention were produced in a converter and made into slabs by continuous casting. These slabs were made into hot-rolled steel strips under the heating temperatures and hot-rolling conditions shown in Table 2, pickled, and a part of the slabs were cold-rolled at a cold rolling rate of 65% to prepare a plating base. Then, the hot dip galvanized steel plate or the alloyed hot dip galvanized steel plate was manufactured on the conditions shown in Table 3 in the continuous hot dip galvanizing line. In addition, the heat cycle in the continuous hot dip galvanizing line was made into the preferable range shown in Claim 3.
[0031]
Table 3 shows the results of evaluating the microstructure, tensile strength, and hardness change amount ΔHv (defined in FIG. 2) of the HAZ part by laser welding. The steel numbers in Table 2 correspond to the steel numbers in Table 3. The laser welding conditions were an output of 5 kW and a speed of 2 m / min. The welding speed was particularly slow so that HAZ softening was likely to occur.
[0032]
Figure 1 shows the three-stage evaluation of ΔHv of steel shown in Table 3 as ○ (ΔHv ≦ 10), Δ (10 <ΔHv ≦ 20), and × (ΔHv> 20). FIG. As can be seen from FIG. 1, by setting the contents of Mo and other elements within the range prescribed in the present invention, ΔHv ≦ 20 and excellent HAZ softening resistance can be obtained. By setting it as the range of description, the thing of (DELTA) Hv <= 10 can be obtained now. (In FIG. 1, C is outside the range of this invention like the steel numbers 26 and 27 of Table 3.) And those in which Cr is outside the scope of the present invention, such as steel numbers 36 to 38, are excluded).
[0033]
[Table 1]

[0034]
[Table 2]

[0035]
[Table 3]

[0036]
Table 4 shows the results of examining changes in properties of the steel H of the present invention, particularly by changing the heat cycle conditions in the continuous hot dip galvanizing line. Since the soaking temperature is not appropriate for steel numbers 1 and 5 and the cooling rate is not appropriate for steel numbers 6 and 11, and the residence time at 400 to 600 ° C. is too long for steel numbers 16 and 11, the present invention stipulates. The structure is not obtained, and the desired HAZ softening resistance is not obtained. On the other hand, in the steel of the present invention manufactured under the manufacturing conditions described in claim 3, the structure described in claim 1 is obtained, and in all cases, excellent HAZ softening resistance characteristics such as ΔHv ≦ 20 are obtained. .
[0037]
[Table 4]

[0038]
【The invention's effect】
As described above, according to the present invention, a high-strength hot-dip galvanized steel sheet having a small hardness change in the HAZ part can be obtained. That is, when the hot-dip galvanized steel sheet according to the present invention is used for a tailored blank material, deterioration of formability is prevented. Moreover, when the hot dip galvanized steel sheet according to the present invention is formed into a structure by welding, the deterioration of performance such as deformation strength, breaking strength, and high-speed deformation strength of the member is extremely small. Therefore, the hot dip galvanized steel sheet according to the present invention is extremely suitable for use as a tailored blank material and a welded structure.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between Mo and V content and ΔHv.
FIG. 2 is a diagram schematically showing a change in hardness in the HAZ part due to excess or deficiency of Mo, V, and Cr contents.

Claims (3)

% By weight, C: 0.04% to 0.13%, Si: 0.5% or less, Mn: 1-2%, P: 0.05% or less, S: 0.01% or less (including 0), sol.Al: 0.05% or less, N: 0.007% or less (including 0), Mo: 0.05 to 0.5%, Cr: 0.2% or less (including 0), the balance consisting of Fe and inevitable impurities, and an average grain size of 20 μm or less A hot-dip galvanized steel sheet characterized in that martensite having a volume ratio of 5 to 40% is composed of a structure having a volume ratio exceeding 95 % . In addition to the components according to claim 1, V: 0.02 to 0.2% is further contained, ferrite having an average particle diameter of 20 μm or less and martensite having a volume ratio of 5 to 40% are composed of a structure exceeding 95 % by volume ratio. Hot-dip galvanized steel sheet.   A steel having the components described in claim 1 or claim 2 is cast into a hot-rolled steel strip, pickled, and then cold-rolled at a reduction rate of 40% or more as necessary, followed by a continuous galvanizing line , After soaking at 750 to 850 ° C., cooling to a temperature range of 600 ° C. or lower at a cooling rate of 1 to 50 ° C./sec. The method for producing a galvanized steel sheet according to claim 1 or 2, wherein cooling is performed in a state where the residence time at ~ 600 ° C is within 200 seconds.

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