JP3911972B2 - Manufacturing method of high strength hot-dip galvanized steel sheet - Google Patents

Description

【００４０】
【表２】

【００４１】
続いて、実験室の熱処理シミュレーターにて、連続溶融亜鉛メッキライン相当の熱処理条件で焼鈍した。表３に、この熱処理条件を連続溶融亜鉛メッキ条件として、主な圧延条件（表２に同じ）と共に示す。これらの鋼板の組織と、圧延方向と直角に試験片どりしたJIS5号試験片を用いた引張強度(TS)と延性(El)を、表３に併せて記載した。
【００４２】
【表３】

【００４３】
図１は、鋼AとBの延性に及ぼす、熱延終了後の一次冷却速度の影響を示す。この図より、本発明が開示する一次冷却速度の範囲に制御することで、延性が向上していることがわかる。とくに急冷開始時間を0.5秒を超えて2.5秒以内に制御することで、上記の効果は顕著となる。また、前述したとおり、フェライト＋マルテンサイト鋼である鋼Aでは、フェライト＋パーライト（＋セメンタイト）組織をベースに析出強化により強化した鋼Bに対して、延性向上しろが1％程度大きいことがわかる。
【００４４】
【発明の効果】
本発明は、化学成分の制御に加え、熱延ラインでは仕上げ圧延後の冷却パターンの制御により、熱延板段階においていわゆるバンド状組織の形成を抑制して、組織を均一微細化している。その結果、本発明によれば、鋼成分や組織の制約によらず、延性に優れる高強度溶融亜鉛メッキ鋼板が得られるので、工業的価値は極めて高い。
【図面の簡単な説明】
【図１】延性に及ぼす熱延終了後の一次冷却速度の影響を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-strength hot-dip galvanized (plated) steel sheet capable of imparting excellent ductility, which is applied to structural parts of automobiles.
[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 yield strength, and are widely applied especially to structural parts of automobiles. For this reason, many inventions related to high strength hot dip galvanized steel sheets for automobiles have been disclosed.
[0003]
High-strength hot-dip galvanized steel sheets are manufactured in various component systems and structures, such as precipitation strengthening, solid solution strengthening, and transformation structure strengthening using low-temperature transformation phases such as martensite and bainite, depending on the application and required characteristics. Yes.
In general, in order to improve ductility, a technique such as adding Si to the steel component or controlling to ferrite + martensite in the structure is used.
[0004]
For example, Japanese Patent Application Laid-Open No. 7-54051 proposes a method for obtaining a high-strength hot-dip Zn-plated steel sheet having excellent ductility and stretch flangeability. This technology uses 1.0 to 2.0% Mn-Nb-added steel (and Ti addition) slab as a raw material, and after hot rolling at a finish rolling temperature of 1000 to 850 ° C, an average of over 40 ° C / sec up to 600 ° C Cooling is performed at a cooling rate, cooling is performed at 600 ° C. or less at an average cooling rate of 30 ° C./sec or less, winding is performed in a temperature range of 550 to 400 ° C., and then hot-dip Zn plating is performed.
[0005]
[Problems to be solved by the invention]
However, in the hot dip galvanized steel sheet, there is a great restriction on the amount of Si added effective in improving ductility from the viewpoint of plating adhesion, and there is a limit in improving ductility. In applications where a high yield ratio is required, ferrite + martensitic steel, which is advantageous in terms of ductility, cannot be applied, and ferrite + pearlite steel, which is disadvantageous in terms of ductility, is used. That is, as for the hot-dip galvanized steel sheet, no means has been found to improve ductility without receiving these restrictions.
[0006]
An object of this invention is to provide the manufacturing method of the high intensity | strength hot-dip galvanized steel plate which can solve said problem and can provide the outstanding ductility.
[0007]
[Means for Solving the Problems]
The above problem is solved by the following invention. The invention is mass%, C: 0.01 to 0.3%, Si: 0.7% or less, Mn: 1 to 3%, P: 0.08% or less, S: 0.01% or less, sol.Al: 0.08% or less, N: Finishing hot rolling of steel containing 0.007% or less and the balance Fe and inevitable impurities at Ar ₃ points or more, and starting rapid cooling with an average cooling rate of 100 ° C / sec or more within 2.5 seconds, 700 ° C or less Winding after cooling to a temperature exceeding 500 ° C, then pickling or further cold rolling to perform continuous hot dip galvanizing and annealing at 720 ° C or higher manufacture of high strength hot dip galvanized steel sheet Is the method.
[0008]
In view of the above-mentioned problems, the present invention was made in consideration of how to control under manufacturing conditions other than steel components in order to improve ductility after hot dip galvanization. In particular, in order to solve the above-mentioned problems, intensive studies were conducted on the effects of hot rolling conditions and thermal cycles during hot dip galvanizing on the ductility of high strength hot dip galvanized steel sheets. As a result, the formation of a so-called band-like structure in which pearlite is distributed in layers in the hot-rolled sheet stage is suppressed, the structure is made uniform and fine, and the layered structure of ferrite and cementite in the pearlite grains is made fine (perlite). It has been clarified that the refinement of the lamellar spacing in the above is effective in improving the ductility of the high-strength hot-dip galvanized steel sheet.
[0009]
The above technique requires control of the cooling pattern after finish rolling in the hot rolling line in addition to control of chemical components, and the reasons for limitation will be described below.
First, the reason for component limitation will be described.
[0010]
C: 0.01% to 0.3%
C is an essential element for securing a desired strength, and needs to be 0.01% or more in order to obtain a strength of 440 MPa or more. On the other hand, when a large amount exceeding 0.03% is added, the pearlite fraction in the hot-rolled sheet increases, and even if the large cooling after finishing hot rolling is used, the reduction of the layered structure becomes insufficient.
[0011]
Si: 0.7% or less
Si is an effective element for improving the ductility of steel. However, since the adhesiveness and surface appearance of galvanizing are significantly deteriorated when the addition amount is increased, the upper limit is defined as 0.7%.
[0012]
Mn: 1-3%
Like C, Mn is an essential element for ensuring a desired strength. In order to obtain a desired strength, 1% is necessary as the lower limit. However, when excessively added, as in the case of excessive addition of C, even if large cooling after finishing hot rolling is used, the reduction of the layered structure becomes insufficient. For this reason, the upper limit is defined as 3%.
[0013]
P: 0.08% or less
P is an element having a large solid solution strengthening ability and an essential element for the production of high-strength steel sheets. However, as the addition amount increases, the plating quality deteriorates like Si, so the upper limit is specified as 0.08%.
[0014]
S: 0.01% or less
S is an impurity, and if the content is high, inclusions in the steel increase and workability deteriorates. For this reason, the upper limit is defined as 0.01%.
[0015]
sol.Al: 0.08% or less
The amount of sol.Al may be 0.08% or less without impairing the effects of the present invention as long as it is contained in ordinary steel.
[0016]
N: 0.007% or less
N may be 0.007% or less without impairing the effects of the present invention as long as it is contained in ordinary steel.
[0017]
In addition, in the present invention, one or more selected from Nb, Ti: 0.005 to 0.5%, B: 0.0002 to 0.005%, or one more selected from V, Cr, Mo: 0.01 to 1% Alternatively, steel containing two or more types can be used.
[0018]
Nb, Ti: 1 type or more 0.005-0.5% each
Nb and Ti are added for the purpose of increasing strength by refining the structure and strengthening precipitation. Each lower limit is the minimum amount for obtaining a desired effect, and the upper limit is defined not only because the effect is saturated but also the ductility deteriorates even if it is added more than this.
[0019]
B: 0.0002-0.005%
B is added for the purpose of increasing the volume ratio of the low-temperature transformation phase by suppressing the precipitation of ferrite and increasing the strength. The lower limit is the minimum amount at which a desired effect can be obtained, and the upper limit is specified because not only the effect is saturated but also the ductility deteriorates even if added more.
[0020]
V, Cr, Mo: 1 ~ 1% each 0.01 ~ 1%
V, Cr, and Mo are added for the purpose of improving the hardenability of the steel to increase the strength, but the lower limit of each is the minimum amount that can achieve the desired effect, and the upper limit is added more than this. Even though the effect is saturated, it was specified.
One or more of Nb and Ti and one or more of V, Cr and Mo may be added together.
[0021]
Next, the reason for limiting the manufacturing conditions in the present invention will be described.
In the production method of the present invention, the cooling rate after finish hot rolling is set to a cooling rate exceeding 100 ° C./sec. This is because, as a result of the examination, it became clear that there is a clear ductility improving effect in this cooling rate region as will be described later. The lower limit of the cooling rate, 100 ° C./sec, is specified because the desired effect cannot be obtained below this value.
[0022]
This large cooling (rapid cooling) needs to be stopped in a temperature range of 500 ° C. to 700 ° C. This is because when rapidly cooled to an extremely low temperature of 500 ° C. or lower, low-temperature transformation structures such as bainite and martensite are formed, and when this is annealed in a continuous hot dip galvanizing line, acicular ferrite is formed and ductility is reduced. This is to prevent deterioration.
[0023]
Also, when the rapid cooling stop temperature exceeds 700 ° C. (higher than 700 ° C.), the diffusion rate of C is sufficiently high, and thus it tends to form a band, and the band structure is not sufficiently suppressed. Or, since pearlite is generated at a high temperature, the layered structure of pearlite is not sufficiently refined, and the desired effect cannot be obtained. Therefore, it is specified that large cooling exceeding 100 ° C / sec is stopped in a temperature range exceeding 500 ° C and 700 ° C or less.
[0024]
In addition, if the time from the end of hot rolling to the start of quenching becomes extremely long, not only the structure becomes coarse due to grain growth, but also precipitation of coarse pearlite starts. It is easy to perform rapid cooling exceeding 100 ° C / sec within 2.5 seconds after the end of hot rolling.
[0025]
However, if rapid cooling is started in an extremely short time after the end of rolling, the rolled structure is not completely recrystallized, and the structure tends to be non-uniform. As a result, since the variation in the material in the coil longitudinal direction and the width direction tends to increase, it is preferable that the start of rapid cooling exceeds 0.5 seconds after the end of hot rolling.
[0026]
The hot rolling finishing temperature is set at ₃ or more points so that the processed structure remains and ductility does not deteriorate.
[0027]
After controlling the hot rolling conditions as described above, after pickling, or further cold rolling, annealing and plating are performed in a continuous hot dip galvanizing line. It is necessary to promote the re-solution of the large pearlite of the colonies formed in step 1 or the pearlite pulverized in the cold rolling process. By controlling the annealing temperature in this way, when strengthening with pearlite or cementite, or when strengthening with precipitation strengthening, ductility is improved by the disappearance of large collite of colonies or pulverized pearlite.
[0028]
Also, when strengthening in low-temperature transformation phase such as bainite and martensite, the re-solid solution of pearlite is promoted, so that pearlite does not remain undissolved during annealing, and reverse transformed austenite can be obtained stably. Will improve.
[0029]
As shown in the examples, in the case of strengthening with the low-temperature transformation phase such as the latter, such a ductility improving effect is somewhat large. This is presumably because the ductility improvement effect by eliminating the dissolution of pearlite in the latter is larger than the ductility improvement effect by disappearance of pearlite having large colonies or disappearance of pulverized pearlite in the former.
[0030]
After annealing, it is galvanized, but even if further alloyed thereafter, the effect of the present invention is not impaired.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
In carrying out the invention, a chemical composition is adjusted within the above range by a normal steelmaking process, and hot-rolled steel sheets are manufactured by controlling hot-rolling conditions, particularly cooling conditions after finishing rolling. Thereafter, the hot dip galvanized steel sheet of the present invention can be manufactured by hot dip galvanizing. Although not specifically mentioned, slab manufacturing method by ingot casting or continuous casting, continuous hot rolling by hot-rolling rough hot-rolling bar connection, and heating within 200 ° C using an induction heater in the hot-rolling process. Temperature and the like do not affect the effect of the present invention.
[0032]
In the production method of the present invention, the cooling rate after finish hot rolling is most important. In the patents disclosed so far, for example, as in Japanese Patent Application Laid-Open No. 7-54051, the cooling rate up to 100 ° C./sec is examined, and the influence of the cooling rate on the ductility has not been clarified. As a result of using the experimental equipment capable of large cooling at a maximum of 1000 ° C./sec used for the examination of the invention, it was found that there is a clear ductility improving effect in a cooling rate region exceeding 100 ° C./sec.
[0033]
The reason why this effect of improving ductility is obtained is considered as follows. As described above, by applying a large cooling rate exceeding 100 ° C./sec to the steel strip after hot rolling, the formation of a so-called band-like structure in which pearlite is distributed in layers in the hot-rolled sheet stage is suppressed, The tissue can be uniformly refined. In addition, since the layered structure of ferrite and cementite in the pearlite grains is refined, during the subsequent soaking in continuous hot dip galvanizing, colonies formed in the hot rolling stage have large pearlite or cold rolling processes. Re-solution of pulverized perlite is promoted. As a result, since the starting point of the crack when subjected to plastic deformation is reduced, it is considered that the ductility is improved.
[0034]
Note that after the rapid cooling, the cooling may be stopped as it is and the winding may be performed, or further, the winding may be performed while being controlled to a predetermined temperature at a normal cooling rate of 30 ° C./sec or less.
[0035]
The chemical components are preferably as follows from the viewpoint of materials.
[0036]
First, C is preferably 0.05% or more for ease of securing the strength and 0.2% or less from the viewpoint of ductility. Mn may be 2% or less when the low temperature transformation phase is not used. Nb and Ti are preferably 0.1% or less from the viewpoint of ductility.
[0037]
As for the cooling rate, the lower limit is preferably set to 110 ° C./sec in order to ensure the effect of improving ductility.
[0038]
【Example】
The effect by this invention is shown concretely below.
First, the present invention component steels A to E whose components are shown in Table 1 were melted, and then rolled. These were made into hot-rolled steel strips under the hot-rolling conditions shown in Table 2, and pickled or partially cold-rolled at a cold rolling rate of 62% to prepare an annealing base.
[0039]
[Table 1]

[0040]
[Table 2]

[0041]
Then, it annealed on the heat processing conditions equivalent to a continuous hot dip galvanizing line in the heat processing simulator of a laboratory. Table 3 shows this heat treatment condition as a continuous hot dip galvanizing condition together with main rolling conditions (same as in Table 2). Table 3 shows the structure of these steel sheets and the tensile strength (TS) and ductility (El) using JIS No. 5 test pieces that were cut at right angles to the rolling direction.
[0042]
[Table 3]

[0043]
FIG. 1 shows the influence of the primary cooling rate after the end of hot rolling on the ductility of steels A and B. From this figure, it can be seen that the ductility is improved by controlling to the range of the primary cooling rate disclosed in the present invention. In particular, by controlling the rapid cooling start time from 0.5 seconds to 2.5 seconds, the above effect becomes remarkable. Further, as described above, it is understood that the steel A, which is a ferrite + martensitic steel, has a ductility improvement margin of about 1% larger than that of the steel B strengthened by precipitation strengthening based on the ferrite + pearlite (+ cementite) structure. .
[0044]
【The invention's effect】
In the present invention, in addition to the control of chemical components, the control of the cooling pattern after finish rolling in the hot rolling line suppresses the formation of a so-called band-like structure in the hot-rolled sheet stage, thereby making the structure uniform and fine. As a result, according to the present invention, a high-strength hot-dip galvanized steel sheet having excellent ductility can be obtained regardless of the steel component and structure restrictions, and thus the industrial value is extremely high.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of a primary cooling rate after hot rolling on ductility.

Claims (5)

At mass%, C: 0.01 to 0.3%, Si: 0.7% or less, Mn: 1 to 3%, P: 0.08% or less, S: 0.01% or less, sol.Al: 0.08% or less, N: 0.007% or less Contain steel, which consists of the remainder Fe and inevitable impurities , finish hot-rolled at ₃ or more points of Ar, and start rapid cooling with an average cooling rate of 100 ° C / sec or more within 2.5 seconds. A method for producing a high-strength hot-dip galvanized steel sheet, which is wound after being cooled to temperature and then pickled or further cold-rolled to perform continuous hot-dip galvanizing and annealing at 720 ° C or higher. In the method for producing a high-strength hot-dip galvanized steel sheet according to claim 1, instead of describing the steel as mass%, C: 0.01 to 0.3%, Si: 0.7% or less, Mn: 1 to 3%, P: 0.08% or less, S: 0.01% or less, sol.Al: 0.08% or less, N: 0.007% or less, Nb, Ti: 0.005-0.5%, B: One or more of 0.0002-0.005% And a method for producing a high-strength hot-dip galvanized steel sheet, characterized in that the steel is composed of the remaining Fe and inevitable impurities . In the method for producing a high-strength hot-dip galvanized steel sheet according to claim 1, instead of describing the steel as mass%, C: 0.01 to 0.3%, Si: 0.7% or less, Mn: 1 to 3%, P: 0.08% or less, S: 0.01% or less, sol.Al: 0.08% or less, N: 0.007% or less, and V, Cr, Mo: contain one or more of 0.01 to 1% , the balance Fe and A method for producing a high-strength hot-dip galvanized steel sheet, characterized in that the steel is made of inevitable impurities . In the method for producing a high-strength hot-dip galvanized steel sheet according to claim 1, instead of describing the steel as mass%, C: 0.01 to 0.3%, Si: 0.7% or less, Mn: 1 to 3%, P: 0.08% or less, S: 0.01% or less, sol.Al: 0.08% or less, N: 0.007% or less, Nb, Ti: 0.005-0.5%, B: One or more of 0.0002-0.005% And a method for producing a high-strength hot-dip galvanized steel sheet, wherein the steel contains one or more of V, Cr, and Mo: 0.01 to 1%, and the balance is Fe and inevitable impurities . 5. The method for producing a high-strength hot-dip galvanized steel sheet according to claim 1, wherein the rapid cooling is started within 2.5 seconds after the hot rolling is finished.

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