JP2004068051A - High strength hot-dip galvanized steel sheet and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot-dip galvanized steel sheet having high strength and excellent formability represented by uniform elongation and local elongation and also to provide its manufacturing method. <P>SOLUTION: In the hot-dip galvanized steel sheet, the amounts of C, Si, Mn, P, S, Al and N in a base material are regulated and one or more elements among Ti, Nb, V, B, Cr, Mo, Cu, Ni and Ca are further contained if necessary, and the steel sheet has a metallic structure in which ferrite is contained in amounts of ≥30% and the number of ferrite grains containing cementite, martensite or retained austenite in each grain is ≥20% of the total ferrite. The hot-dip galvanized steel sheet can be manufactured by finishing hot rolling at specific temperature, performing coiling under specific conditions, carrying out cold rolling, applying annealing under specific conditions, performing hot-dip galvanizing and then carrying out successive cooling under specific conditions. <P>COPYRIGHT: (C)2004,JPO

Description

【００３１】
【表２】

【００３２】
【表３】

【００３３】
このようにして得られた溶融亜鉛めっき鋼板につき、その母材をナイタル腐食して光学顕微鏡及びＳＥＭ観察、更に電子顕微鏡での観察にて金属組織の観察及び特定を行った。
また、圧延直角方向にＪＩＳ５号試験片と曲げ試験片を採取し、引張試験にて特性を調査した。
更に、粒内にセメンタイト又はマルテンサイト又は残留オ−ステナイトを１種以上含むフェライトの数は、ある任意の母材板厚断面でフェライトが１００個入る領域を観察して、相当する粒数の割合（％）を把握した。
上述のようにして調査した金属組織及び材料特性を、表４に示す。
【００３４】
【表４】

【００３５】
表４に示される結果から明らかなように、本発明に係る溶融亜鉛めっき鋼板は一様伸びと局部伸びの両方とも高い値を示した。
これに対して、本発明で規定する金属組織が得られなかった材料は、一様伸び或いは局部伸びの一方又は両方が低い値となった。
また、Ｓｉ含有量の高い試験番号３０に係る溶融亜鉛めっき鋼板は、特性に問題は無かったが、めっき合金化処理不足が部分的に生じ、めっき品質に問題が認められた。
【００３６】
【発明の効果】
以上に説明した如く、この発明によれば、優れた一様伸びと局部伸びを有し、ロッカ−インナ−等といった耐食性が必要な自動車の補強部品等に好適な溶融亜鉛めっき鋼板を安定して得ることができるなど、産業上有用な効果がもたらされる。
【図面の簡単な説明】
【図１】鋼板の引張試験を実施したときの公称応力−公称歪み曲線であり、局部伸びと一様伸びについての説明図でもある。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength hot-dip galvanized steel sheet which is excellent in formability and is suitable as a material for a part where corrosion resistance is important, such as a part used for an automobile body for ensuring collision safety and which is important as a material for a portion where corrosion resistance is important.
[0002]
[Prior art]
In recent years, demands for high-strength steel sheets as measures for safety and weight reduction of automobiles have been increasing.
However, as the strength of a steel sheet increases, formability becomes a problem. In particular, it is important to increase local elongation, which greatly affects stretch flange forming and burring forming, and uniform elongation, which greatly affects stretch forming.
FIG. 1 shows a nominal stress-nominal strain curve when a tensile test is performed on a steel sheet, and local elongation and uniform elongation can be grasped.
[0003]
As a method of manufacturing a high-strength cold-rolled steel sheet, for example, Japanese Patent Application Laid-Open No. 7-188767 discloses a method of improving stretch flangeability by forming a bainite-based metal structure.
However, there was a problem that uniform elongation was low only with a bainite-based structure alone.
Furthermore, in the method for producing a high-strength cold-rolled steel sheet described in JP-A-7-188767, a cooling rate of 100 ° C./c or more is required after annealing in order to obtain high strength. There was another problem that it was difficult to secure
[0004]
Japanese Patent Application Laid-Open No. 9-263838 discloses a method of improving the hole expandability by changing the metal structure of a steel sheet to a mixed structure of ferrite and bainite.
However, even with this method, it is insufficient to secure local elongation and uniform elongation. In addition, none of them considers corrosion resistance, which is a difficult production condition for hot-dip galvanizing.
[0005]
[Problems to be solved by the invention]
In view of the above, an object of the present invention is to solve the above-described problems of the prior art, and to provide a high strength having high strength and good moldability expressed by uniform elongation and local elongation. An object of the present invention is to provide a galvanized steel sheet and a method for producing the same.
[0006]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to achieve the above object, and as a result, in order to realize a hot-dip galvanized steel sheet having both high strength and formability expressed by uniform elongation, the metallographic structure of the base steel sheet Is a structure in which ferrite occupies 30% or more by volume, and the number of ferrite grains containing at least one of cementite, martensite, and retained austenite in the grains is 20% or more of the total number of ferrites. Was found to be important.
[0007]
That is, the present inventors first found that the starting point of fracture at local elongation was pearlite containing hard martensite or coarse and brittle carbide present at the ferrite grain boundaries. Furthermore, it has been clarified that martensite, cementite, and residual austenite, which are necessary for obtaining high strength, are unlikely to be crack initiation points if present in ferrite grains.
By examining the elucidated items and the structure of the base steel sheet that can provide high strength, and the hot-dip galvanizing method that can obtain the preferred metal structure of the base steel sheet, the formability expressed by high strength and uniform elongation can be obtained. Thus, the effectiveness of the base steel sheet structure in realizing a compatible hot-dip galvanized steel sheet has been confirmed.
[0008]
The present invention has been completed on the basis of the above findings and the like, and is a high-strength hot-dip galvanized steel sheet excellent in formability suitable as a material for a portion where corrosion resistance is important as shown in the following items (1) to (3). And a method for producing the same.
{Circle around (1)} C: 0.005 to 0.20% (% indicating the component ratio is hereinafter referred to as% by weight), Si: 0.5% or less, Mn: 0.7 to 3.0%, P: 0 0.10% or less, S: 0.010% or less, Al: 0.001 to 0.20%, N: 0.020% or less, the balance being substantially composed of Fe and unavoidable impurities, and a metal structure A structure in which ferrite has a volume fraction of 30% or more, and the number of ferrite grains containing at least one of cementite, martensite, and retained austenite in the grains is 20% or more of the total number of ferrites A high-strength hot-dip galvanized steel sheet comprising a base steel sheet having a hot-dip galvanized layer on a surface thereof.
{Circle around (2)} C: 0.005 to 0.20%, Si: 0.5% or less, Mn: 0.7 to 3.0%, P: 0.10% or less, S: 0.010% or less, Al : 0.001 to 0.20%, N: 0.020% or less, Ti: 0.20% or less, Nb: 0.20% or less, V: 0.10% or less, B: 0.01 % Or less, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Ca: 0.01% or less, and the balance Is substantially composed of Fe and unavoidable impurities, and has a metal structure of ferrite having a volume fraction of 30% or more, and further contains at least one of cementite, martensite, and residual austenite in grains. The surface of a base steel sheet with a structure in which the number of grains is 20% or more of the total number of ferrite Characterized by comprising a lead-plated layer,
High strength galvanized steel sheet.
(3) The steel slab having the component composition described in the above (1) or (2) is heated to 1050 ° C. or higher, and then rough rolling is started. Alternatively, the finish rolling is started after the temperature is held, and after finishing the rolling at the finishing temperature of 740 to 1030 ° C., it is cooled at an average cooling rate of 5 ° C./s or more, wound up at 700 ° C. or less, and further unwound. After cold rolling as it is or after skin pass rolling and pickling, and then annealing at a temperature of 720 to 900 ° C. for 5 seconds or more, 460 at an average cooling rate of 2 to 30 ° C./s.ま で 600 ° C., and kept at the temperature range for 10 to 90 seconds, further cooled, hot-dip galvanized, and further alloyed if necessary. Up to 300 A method for producing a high-strength hot-dip galvanized steel sheet, characterized by cooling within seconds.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, the reason why the metal structure of the base steel sheet, the composition of the steel slab or the base steel sheet, and the manufacturing conditions of the hot-dip galvanized steel sheet in the present invention are limited as described above.
(A) Metal Structure of Base Steel Sheet The metal structure of the base steel sheet is an important element of the present invention. Ferrite occupies at least 30% by volume in the base steel sheet and further contains cementite or martensite or A high-strength melt having excellent uniform elongation and local elongation by forming a metal structure in which the number of ferrite grains having any one or more of retained austenite is 20% or more of the total number of ferrites. A galvanized steel sheet is obtained.
[0010]
Since ferrite itself has excellent ductility, a volume ratio of 30% or more is required to ensure excellent uniform elongation of the steel sheet. When the volume ratio of ferrite is less than 30%, excellent uniform elongation cannot be secured. The volume fraction of ferrite in the steel sheet structure is preferably at least 50%, more preferably at least 70%.
[0011]
However, it is difficult to secure high strength only with ferrite, and thus it is necessary to generate a hard second phase to increase the strength of the steel sheet.
However, hard martensite existing at ferrite grain boundaries, cementite which is a brittle carbide, or retained austenite which changes into hard martensite by molding tends to be a starting point of local elongation cracking. Therefore, these are preferably present in the ferrite grains, and the number of ferrite grains having at least one of cementite, martensite, and retained austenite in the grains is 20% or more of the total number of ferrites. Need to be The number of such ferrite grains is preferably at least 30%, more preferably at least 50%.
[0012]
(B) Chemical composition C of a billet or a base steel sheet: C is an important component for obtaining high tensile strength. If the content of C is less than 0.005%, the necessary high tensile strength cannot be obtained, and if the content of C exceeds 0.20%, the toughness and weldability are reduced and the amount of ferrite generated is insufficient. Therefore, the C content is determined to be 0.005 to 0.20%, but is preferably adjusted to 0.03 to 0.12%.
[0013]
Si: Si is also an effective component for increasing the strength of a steel sheet, but causes non-plating when hot-dip galvanizing is performed and insufficient processing during alloying processing. Therefore, the Si content is set to 0.5% or less, but is preferably adjusted to less than 0.10%.
[0014]
Mn: Mn is an effective component for promoting transformation strengthening and achieving high strength, and therefore, it is necessary to contain 0.7% or more. On the other hand, if the content of Mn exceeds 3.0%, ferrite is not easily formed, and at the same time, the band structure is developed and the local elongation is reduced. Therefore, the Mn content is determined to be 0.7 to 3.0%, but is preferably adjusted to 0.9 to 3.0%, and more preferably 1.3 to 3.0%.
[0015]
P: P is an undesirable element that degrades toughness. Therefore, the allowable amount was confirmed, and the P content was determined to be 0.10% or less.
S: S becomes MnS and deteriorates bendability. Therefore, the allowable amount was confirmed, and the S content was determined to be 0.010% or less, but it is preferably 0.0040% or less, and more preferably 0.0015% or less.
[0016]
Al: Al is an element added for deoxidation, but its effect is insufficient if less than 0.001%, and even if it exceeds 0.20%, the effect is saturated and economical. Disadvantageous. Therefore, the Al content was determined to be 0.001 to 0.20%.
N: N forms a nitride during continuous casting and causes cracking of the slab, so that its content is preferably low. Therefore, the allowable amount was confirmed, and the N content was determined to be 0.020% or less.
[0017]
Ti, Nb, V, B: Since Ti, Nb, V, and B have an effect of delaying recrystallization and refining crystal grains, one or more of them may be contained as necessary. However, the effect is that the Ti content exceeds 0.20%, the Nb content exceeds 0.20%, the V content exceeds 0.10%, and the B content exceeds 0.010%. And it is disadvantageous in terms of cost. Therefore, the Ti content is set to 0.20% or less, the Nb content is set to 0.20% or less, the V content is set to 0.10% or less, and the B content is set to 0.010% or less.
[0018]
Cr, Mo: Both Cr and Mo, like Mn, have the function of stabilizing austenite to promote transformation strengthening, and are effective in increasing the strength of steel sheets. Is contained. However, if the Cr content exceeds 1.0% and the Mo content exceeds 1.0%, there is a problem in the surface treatment properties of the steel sheet. Therefore, the Cr content was set to 1.0% or less, and the Mo content was set to 1.0% or less.
[0019]
Cu, Ni: Cu and Ni have a corrosion inhibitory effect, have a function of concentrating on the surface to suppress the invasion of hydrogen, and suppress delayed destruction. Therefore, one or more of them are contained as necessary. However, when the content exceeds 1.0%, the effect is saturated and the cost is disadvantageous. Therefore, both the Cu content and the Ni content were determined to be 1.0% or less.
[0020]
Ca: Ca has an effect of improving local ductility by binding to S and spheroidizing sulfides, so Ca is added as necessary. However, if the content exceeds 0.01%, the effect is saturated and the cost becomes disadvantageous. Therefore, the Ca content is set to 0.01% or less.
[0021]
The composition other than the above is Fe and inevitable impurities.
The steel having the above composition is melted by, for example, a converter, an electric furnace, a flat furnace, or the like. The steel type may be any of rimed steel, capped steel, semi-killed steel and killed steel. Further, the casting of the billet may be performed by any means of "ingot-bulking rolling" or "continuous casting".
The type of galvanized steel sheet according to the present invention is not particularly limited as long as it is a zinc-based galvanized steel sheet. And so on. Further, the plating layer may be provided on both sides of the steel sheet, or may be provided on one side.
[0022]
(C) Manufacturing Conditions In order to manufacture the “high-strength hot-dip galvanized steel sheet excellent in formability suitable as a material for a portion where corrosion resistance is important” according to the present invention, first, a steel slab having a component composition specified by the present invention is subjected to 1050. After heating to a temperature of at least ℃, start rough rolling. After finishing the rough rolling, finish the rolling as it is or after heating or maintaining the temperature of the rough bar as necessary, and finish the rolling at a finishing temperature of 740 to 1030 ° C. Thereafter, hot rolling is performed by cooling at an average cooling rate of 5 ° C./s or more and winding at 700 ° C. or less.
[0023]
Heating the billet to 1050 ° C. or higher is necessary to secure the finishing temperature. A finishing temperature of 740 ° C. or higher is a condition necessary to suppress ferrite from being coarsened with rolling at a temperature lower than the transformation point, and a finishing temperature of 1030 ° C. or lower makes the structure of the hot-rolled sheet finer. This is a condition necessary for forming sufficient ferrite after annealing of the cold rolled sheet.
It is effective to heat or maintain the temperature of the rough bar before finishing rolling to secure the finishing temperature. There is no problem even if the rough bar is joined and subjected to continuous rolling.
In hot rolling, the slab to be inserted into the heating furnace may be “a slab kept at a high temperature after casting” or “a slab left at room temperature”.
[0024]
After finishing the rolling, the material is cooled at an average cooling rate of 5 ° C./s or more to reduce the band-like structure, and is wound at 700 ° C. or less. If the winding temperature is higher than 700 ° C., a band-like structure that lowers the bendability of the product develops, which is not preferable.
[0025]
After hot rolling, if necessary, skin pass rolling for flatness correction and pickling for scale removal are performed, and preferably cold rolling at a rolling reduction of 30% or more is performed to perform annealing (continuous annealing) and Hot-dip galvanized. In the annealing, after performing a process of holding for 5 seconds or more in a temperature range of 720 to 900 ° C., cooling to 460 to 600 ° C. at a cooling rate of 2 to 30 ° C./s, and holding for 10 to 90 seconds in the temperature range. After further cooling, hot-dip galvanizing is performed, and alloying treatment is performed if necessary, and then rapidly cooled from 200 ° C. to 100 ° C. or less within 300 seconds.
[0026]
In this case, at an annealing temperature of less than 720 ° C. and a heating holding time of less than 5 seconds, austenitization is insufficient, and the amount of cementite or martensite or residual austenite in ferrite grains effective for increasing the strength is small. . On the other hand, at an annealing temperature exceeding 900 ° C., ferrite with a volume ratio of 30% or more cannot be obtained.
After the annealing, cooling is performed at an average cooling rate of 2 ° C./s or more, in order to suppress precipitation of the hard phase at the ferrite grain boundaries. In addition, the upper limit of the average cooling rate was set to 60 ° C./s so as not to cause flat defects.
In the cooling process after the annealing, it is necessary to hold the ferrite and the second phase in a temperature range of 460 to 600 ° C. for 10 to 90 seconds in order to separate the ferrite and the second phase.
[0027]
Then, following the cooling, hot-dip galvanizing is performed, and alloying is performed if necessary. However, these processing conditions are not particularly limited.
After plating, it is important to cool from 200 ° C to 100 ° C or less within 300 seconds. Thereby, precipitation of coarse cementite at the ferrite grain boundary can be suppressed. The cooling from 200 ° C. to 100 ° C. is preferably performed within 60 seconds, more preferably within 30 seconds.
[0028]
It should be noted that there is no problem if a skin pass with an elongation of 4% or less is applied for flatness correction after cooling following the plating process. Further, a film having a lubricating effect may be provided on the plating layer.
[0029]
【Example】
After smelting steel having the chemical composition shown in Table 1 in a converter, a slab was formed by continuous casting. Then, the slab was hot-rolled under the conditions shown in Table 2 to produce a hot-rolled steel sheet having a thickness of 2.6 mm.
Next, the obtained hot-rolled steel sheet was pickled, cold-rolled to a thickness of 1.2 mm, and then subjected to continuous annealing and hot-dip galvanizing under the conditions shown in Table 3.
After the hot-dip galvanizing, some materials were subjected to alloying treatment by heating to reduce the Fe content in the plating to about 10% by weight.
[0030]
[Table 1]

[0031]
[Table 2]

[0032]
[Table 3]

[0033]
With respect to the hot-dip galvanized steel sheet obtained in this manner, the base material was subjected to nital corrosion, and the metal structure was observed and specified by an optical microscope, an SEM observation, and an electron microscope.
Further, JIS No. 5 test pieces and bending test pieces were sampled in the direction perpendicular to the rolling direction, and the characteristics were examined by a tensile test.
Further, the number of ferrites containing at least one kind of cementite or martensite or retained austenite in the grains is determined by observing a region containing 100 ferrites in a given cross section of the base material plate thickness. (%).
Table 4 shows the metallographic structure and material properties investigated as described above.
[0034]
[Table 4]

[0035]
As is clear from the results shown in Table 4, the hot-dip galvanized steel sheet according to the present invention exhibited high values in both uniform elongation and local elongation.
On the other hand, in the material in which the metal structure specified in the present invention was not obtained, one or both of the uniform elongation and the local elongation were low.
The hot-dip galvanized steel sheet according to Test No. 30 having a high Si content had no problem with the properties, but the plating alloying treatment was partially insufficient, and a problem was observed with the plating quality.
[0036]
【The invention's effect】
As described above, according to the present invention, a hot-dip galvanized steel sheet having excellent uniform elongation and local elongation and suitable for reinforcing parts of automobiles requiring corrosion resistance such as rocker-inner and the like is stably provided. An industrially useful effect such as being obtained can be obtained.
[Brief description of the drawings]
FIG. 1 is a nominal stress-nominal strain curve when a tensile test is performed on a steel sheet, and is also an explanatory diagram of local elongation and uniform elongation.

Claims (3)

By weight, C: 0.005% to 0.20%, Si: 0.5% or less, Mn: 0.7% to 3.0%, P: 0.10% or less, S: 0.010% or less , Al: 0.001% to 0.20%, N: 0.020% or less, and the balance substantially consists of Fe and inevitable impurities, and has a ferrite of 30% or more by volume as a metal structure. Hot dip galvanizing on the surface of a base steel sheet having a structure in which the number of ferrite grains containing at least one of cementite, martensite, and retained austenite in the grains is 20% or more of the total number of ferrites; A high-strength hot-dip galvanized steel sheet comprising a layer. By weight, C: 0.005% to 0.20%, Si: 0.5% or less, Mn: 0.7% to 3.0%, P: 0.10% or less, S: 0.010% or less , Al: 0.001% to 0.20%, N: 0.020% or less, further Ti: 0.20% or less, Nb: 0.20% or less, V: 0.10% or less, B: 0 0.01% or less, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Ca: 0.01% or less In addition, the balance substantially consists of Fe and inevitable impurities, and has a metal structure of ferrite of 30% or more by volume, and further contains one or more of cementite or martensite or residual austenite in grains. Has a structure in which the number of ferrite grains contained is 20% or more of the total number of ferrites On the surface of the wood steel sheet, characterized by comprising a galvanized layer, a high strength galvanized steel sheet. After the steel slab having the component composition according to claim 1 or 2 is heated to 1050 ° C. or more, rough rolling is started, and after the rough rolling is completed, the raw bar is heated or the temperature of the rough bar is maintained. Finish rolling is started, and after finishing the rolling at a finishing temperature of 740 to 1030 ° C., it is cooled at an average cooling rate of 5 ° C./s or more, wound up at 700 ° C. or less, further unwound and then unrolled or skin pass rolling, pickling After performing cold rolling, and then performing annealing at a temperature range of 720 to 900 ° C. for 5 seconds or more, cooling to 460 to 600 ° C. at an average cooling rate of 2 to 30 ° C./s. Hold in the temperature range for 10 to 90 seconds, further cool, then apply hot-dip galvanizing, further apply alloying treatment as needed, and then cool from 200 ° C to 100 ° C or less in 300 seconds in the subsequent cooling process Do Characterized the door, the method of producing a high strength galvanized steel sheet.

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