JP3951537B2 - Hot-rolled galvanized high-tensile steel sheet with excellent workability and method for producing the same - Google Patents

Description

【００３６】
【表２】

【００３７】
[実施例２]
表３に示すＣｒとＶを複合添加した本発明の化学成分を有する鋼種Ｇ（表示しない残部はＦｅ及び不可避不純物とする。）を転炉にて溶製し、連続鋳造でスラブとした後、仕上温度をＡｒ3点以上の８６０℃、巻取温度（ＣＴ）を４００〜７５０℃として熱間圧延を行い、板厚２．０ｍｍの鋼帯とした。酸洗後、ＣＧＬにより８００℃で２ｍｉｎ加熱保持後、両面４５ｇ／ｍ2の目付け量で亜鉛めっきし、その後、５５０℃×１０ｓｅｃの合金化処理を行なった。
【００３８】
この際、１コイル毎にコイルＨｅａｄからＥｎｄにかけてライン速度を上げた。ＣＧＬ通板後のコイルより、ライン速度３０，８０，１６０ｍｐｍのいずれかに相当する部分よりサンプル採取し、ＪＩＳ５号引張試験およびミクロ組織の観察を行った。表４に結果を示す。尚、各部分の合金化温度（５５０℃）からＭｓ点までの冷却速度はライン速度に応じて決まり，表中に冷却速度として示す。
【００３９】
本発明例１〜５は、巻取温度が７００℃以下であるため、いずれのライン速度でもフェライトおよびマルテンサイトの二相組織が得られて適正な強度と良好な延性を有する。比較例６〜８は、巻取温度が７５０℃と高く、本発明範囲外となっている。巻取温度が７５０℃と高温の場合、炭化物は熱間圧延巻取り以降に粗大炭化物として析出し、ＣＧＬのめっき前加熱によっても十分に溶解しない。比較例７，８では、フェライトおよびマルテンサイト以外に、一部主としてセメンタイトからなる炭化物が含まれるため、強度が適正でも強度―延性バランスが不十分である。比較例６は、ライン速度が３０ｍｐｍと小さいので、炭化物の溶け込みは十分であるが、生産効率が低く、好ましくない。
【００４０】
【表３】

【００４１】
【表４】

【００４２】
【発明の効果】
以上説明したように、本発明によれば、５９０ＭＰａ以上の引張強さと良好な加工性を有する二相組織型の熱延下地溶融亜鉛めっき高張力鋼板を生産性良く製造可能で、自動車の軽量化など産業上極めて有用な効果が得られる。
【図面の簡単な説明】
【図１】Ｃｒ＋Ｖ量がマルテンサイト体積分率に及ぼす影響を示す図。[0001]
BACKGROUND OF THE INVENTION
This invention is suitable for applications that require high ductility and corrosion resistance, such as automotive structural members, underbody members, and machine structural parts, which can withstand the severe press molding required to form parts with high strength. In addition, the present invention relates to a dual-phase hot-dip galvanized high-tensile steel sheet excellent in workability on a hot-rolled steel sheet and a method for producing the same.
[0002]
[Prior art]
In order to improve automobile fuel efficiency and collision safety, high-strength hot-rolled steel sheets are required for vehicle body structural members and undercarriage members, and higher strength has been required for some time. In addition, in recent years, hot-rolled steel sheets used for vehicle body structural members and underbody members are subjected to severe forming mainly by stretch forming, and therefore are required to have excellent press formability, particularly good ductility. Two-phase hot rolled steel sheets based on the microstructure of ferrite + martensite have been developed.
[0003]
Further, a steel sheet obtained by hot dip galvanizing on a dual phase hot rolled steel sheet is required as a steel sheet having both good ductility and corrosion resistance, and Japanese Patent Application Laid-Open No. 56-142821 is disclosed. In this patent, a steel plate having a composition comprising, by mass, C: 0.15% or less, Mn + Cr: 1.0-2.5% as a basic component, and the balance Fe and unavoidable impurities is pre-plating heating temperature. It is characterized by having a two-phase structure by a continuous hot dip galvanizing line (hereinafter, CGL) in which the cooling rate up to the plating bath, alloying temperature, and cooling rate after alloying are defined in detail.
[0004]
That is, after making into a two phase of a ferrite phase and an austenite phase in the heating process before plating, the austenite phase is made into a martensite phase by quenching in CGL to form a steel sheet having a two-phase structure.
[0005]
[Problems to be solved by the invention]
However, in order to ensure hardenability in CGL, alloy elements must be added as steel composition or the CGL line speed must be increased. The former increases the cost of steel and the latter controls the amount of zinc deposited in the majority of CGLs. When the line speed is determined by the stability of the alloy and the reaction rate of alloying, the hardenability cannot be ensured. The present invention provides a method for producing a hot dip galvanized steel sheet that does not use an expensive alloy element and is not subject to the limitations of CGL facilities and has excellent workability, and the steel sheet.
[0006]
[Means for Solving the Problems]
The inventors of the present invention have studied a component composition that is excellent in hardenability and has a two-phase structure mainly composed of ferrite and martensite even when the CGL line speed is relatively low. As a result, it has been found that the restriction of the line speed is relieved greatly by containing a proper amount of C, Si, Mn and the like and by the combined addition of Cr and V. The present invention has been made based on the above findings and further studies. The gist of the present invention is as follows.
[0007]
1. In mass%, C: 0.04 to 0.12%, Si: 0.1% or less, Mn: 1.0 to 2.0%, P: 0.05% or less, S: 0.005% or less, Cr: 0.05-1.0%, V: 0.005-0.2%, sol. Al: 0.033 to 0.10% , N: 0.01% or less , having a component composition composed of the balance Fe and inevitable impurities, and composed of ferrite and martensite, or composed of ferrite, martensite and bainite A hot-rolled base hot-dip galvanized high-tensile steel sheet with excellent workability, characterized by having a structure .
[0008]
2. In mass%, C: 0.04 to 0.12%, Si: 0.1% or less, Mn: 1.0 to 2.0%, P: 0.05% or less, S: 0.005% or less, Cr: 0.05-1.0%, V: 0.005-0.2%, sol. After rough rolling a steel having a component composition consisting of Al: 0.033 to 0.10% , N: 0.01% or less , the balance Fe and inevitable impurities , finish rolling at Ar ₃ points or more, at 700 ° C. or less A method for producing a hot-rolled base hot-dip galvanized high-tensile steel sheet excellent in workability, characterized by hot-dip galvanizing with a pre-plating heating temperature of Ac ₁ to Ac ₃ after winding.
[0009]
3. In mass%, C: 0.04 to 0.12%, Si: 0.1% or less, Mn: 1.0 to 2.0%, P: 0.05% or less, S: 0.005% or less, Cr: 0.05-1.0%, V: 0.005-0.2%, sol. After rough rolling a steel having a component composition consisting of Al: 0.033 to 0.10% , N: 0.01% or less , the balance Fe and inevitable impurities , finish rolling at Ar ₃ points or more, at 700 ° C. or less A method for producing a hot-rolled base hot-dip galvanized high-tensile steel sheet having excellent workability, characterized by hot-dip galvanizing with a pre-plating heating temperature of Ac ₁ to Ac ₃ after winding.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the reason for limiting the components, the reason for limiting the microstructure, the hot rolling conditions, and the hot dip galvanizing conditions of the present invention will be described.
[0011]
1. Component composition C: 0.04% or more, 0.12% or less C is essential for generating martensite and ensuring the target strength, and requires 0.04% or more. On the other hand, if it exceeds 0.12%, the workability deteriorates, so the content is made 0.04% or more and 0.12% or less.
[0012]
Si: 0.1% or less Plating in hot dip galvanizing is difficult when the content of Si increases, and if it exceeds 0.1%, the adhesion of the plating deteriorates, so the content is made 0.1% or less.
[0013]
Mn: 1.0% or more and 2.0% or less Mn is added to increase the strength by solid solution strengthening because it has an advantageous effect on the formation of the structure. In order to secure the required strength, 1.0% or more is added, but if it exceeds 2.0%, workability such as press formability deteriorates, so 1.0% or more and 2.0% or less.
[0014]
P: 0.05% or less P is an impurity element that deteriorates weldability and press formability, and is limited to 0.05% or less. However, it is desirable to reduce as much as possible within the economically allowable range.
[0015]
S: 0.005% or less S is an impurity element that makes M-based inclusions with Mn and lowers press formability, and is limited to 0.005% or less. However, it is desirable to reduce as much as possible within the economically allowable range.
[0016]
Cr: 0.05% or more, 1.0% or less, V: 0.005% or more, 0.2% or less In the present invention, the hardenability of steel is improved by the combined addition of Cr and V. In CGL, Cr: 0.05% or more and V: 0.005% or more are added in combination in order to greatly relax the restriction of the line speed that enables quenching to the dual phase structure type steel sheet.
[0017]
On the other hand, even if these elements are added in a large amount, the effect is saturated and the manufacturing cost increases, so Cr: 1.0% or less and V: 0.2% or less. When only one of Cr and V is added alone, sufficient hardenability cannot be ensured.
[0018]
sol. Al: 0.033 to 0.10%
sol. Al is essential as a deoxidizing element, but in the range exceeding 0.10%, the effect is saturated, Al inclusions increase, and press formability deteriorates, so the content is made 0.10% or less.
[0019]
N: 0.01% or less N is contained in an amount of 0.01% or less because a large amount of N deteriorates ductility.
[0020]
2. Microstructure In the present invention, the microstructure of steel has a structure mainly composed of ferrite and martensite in order to ensure necessary strength and good ductility. In the present structure, bainite can be further included as long as the effects thereof are not impaired.
[0021]
3. Next, hot rolling conditions will be described. In the present invention, the two phases of ferrite and austenite are separated and subjected to a quenching treatment in the hot dip galvanizing step after hot rolling. In the hot rolling process, a finishing temperature and a winding temperature in finish rolling are defined so that a desired structure is obtained in the hot dip galvanizing process.
[0022]
Finishing temperature: When the finishing temperature is higher than the Ar3 transformation point and lower than the Ar3 transformation point, it becomes α + γ two-phase rolling, resulting in a mixed grain structure. Do not exceed the point.
[0023]
Winding temperature: 700 ° C. or less When the winding temperature exceeds 700 ° C., carbides precipitated in the cooling process become coarse, and it takes a long time to dissolve the carbides required before plating. Therefore, the line speed in CGL must be reduced, which is disadvantageous for the steel sheet quenching process and reduces the production efficiency. Accordingly, the winding temperature is set to 700 ° C. or lower. This tendency becomes stronger when the CGL is charged without cold rolling.
[0024]
The hot rolling may be a normal ingot forming method, a method using a slab produced by continuous casting, or a method by direct hot rolling without passing through a heating furnace, and is not particularly limited. The heating temperature of the slab is not particularly limited as long as the weight loss due to scale generation is appropriate, rough rolling and finish rolling are possible, and the finish rolling temperature can be secured at the Ar3 transformation point or higher. Moreover, you may heat the semi-finished product after rough rolling before finishing rolling by an atmospheric furnace, high frequency heating, etc.
[0025]
4). Hot-dip galvanizing conditions As described above, in the present invention, the hot-dip galvanizing step is adjusted to a two-phase structure having necessary strength and workability. Therefore, pre-plating heating conditions are specified.
[0026]
Pre-plating heating conditions: Heating temperature is Ac1 point or higher, Ac3 point or lower, holding time 5 seconds to 10 minutes Pre-plating heating, heating to Ac1 point or higher and Ac3 point or lower, separating into two phases, after plating or plating In the case of post-alloying treatment, quenching is performed in the cooling after the alloying temperature to obtain a structure mainly composed of ferrite and martensite. In order to sufficiently perform the two-phase separation, the holding time should be at least 5 seconds, and if it is longer than this, there is no problem in terms of tissue control.
[0027]
In CGL, it is difficult to strictly control the thermal cycle, and it is usually difficult to control the microstructure so as to obtain desired characteristics. However, in the present invention, it is not necessary to specifically limit the production conditions of CGL except for the provision of the pre-plating heating temperature by the combined addition of Cr and V. Even when the cooling rate is as low as 3.5 to 9.3 ° C./S, it is possible to obtain a structure mainly composed of ferrite and martensite.
[0028]
In addition, when making the quality of hot dip galvanization further stable, it is preferable to pickle before hot dip galvanization after hot rolling. It is also possible to perform an alloying treatment after hot dip galvanization.
[0029]
【Example】
[Example 1]
Steel having the component composition shown in Table 1 was melted in a converter and slab was formed by continuous casting. The balance not shown in Table 1 is Fe and inevitable impurities. Steel types A and B are steels to which Cr and V are added in combination and have a composition within the scope of the present invention. In steel type C, both Cr and V are not added, and in steel types D to F, only one of Cr or V is added, and the composition is outside the scope of the present invention.
[0030]
Next, after finishing rolling to a plate thickness of 2.0 mm at 860 ° C. above the Ar 3 point, winding at 500 ° C., pickling, heating and holding at 800 ° C. for 2 min in CGL, and hot-dip zinc with a basis weight of 45 g / m 2 on both sides After plating, an alloying treatment at 550 ° C. × 10 sec was performed. At this time, the line speed was increased from coil Head to End for each coil.
[0031]
Samples were taken from the portion corresponding to the line speed of 30, 80, and 165 mpm from the coil after passing through the CGL, and yield strength (YS), tensile strength (TS), yield ratio (YR) using a JIS No. 5 tensile test piece. ) And elongation (El) were obtained, and the microstructure was observed. Table 2 shows the results. The cooling rate from the alloying temperature (550 ° C.) to the Ms point is determined according to the line speed, and is shown in the table as the cooling rate.
[0032]
Invention Examples A1 to B3 are examples of steel type A to which Cr and V are added in a composite manner, and a two-phase structure mainly composed of ferrite and martensite is obtained regardless of the CGL line speed, and necessary strength is ensured. In addition, it has good ductility. On the other hand, Comparative Examples C1 to F3 are examples of steel grades outside the scope of the present invention in which Cr and V are not added in combination. In the case of steel grades C, D, and E, the CGL line speed is 165 mpm. Except for D3 and E3 as examples, the hardenability is insufficient, and a two-phase structure mainly composed of ferrite and martensite cannot be obtained, and the strength and ductility are insufficient.
[0033]
Steel type F has a structure conforming to a two-phase structure at any line speed, and a strength of 590 MPa or more is ensured. However, since a large amount of Cr is added in a Cr-added system, the manufacturing cost is high. Incidentally, the line speed of 165 mpm is close to the operational limit, and the defective rate of alloying is high, which is not preferable.
[0034]
FIG. 1 shows the effect of the amount of Cr + V in the steel on the martensite volume fraction of the steel sheet manufactured under the conditions shown in Table 2. In the case of a composite addition system of Cr and V, it is stable regardless of the line speed. A martensite volume fraction of 7% or more is obtained, whereas in the case of a Cr or V single addition system, a martensite volume fraction of 3% or more is obtained only at a line speed of 165 mpm. It is clear that the combined addition of is effective.
[0035]
[Table 1]

[0036]
[Table 2]

[0037]
[Example 2]
After steel grade G having the chemical composition of the present invention combined with Cr and V shown in Table 3 (the remainder not shown is Fe and inevitable impurities) is melted in a converter and made into a slab by continuous casting, Hot rolling was performed at a finishing temperature of 860 ° C. above the Ar 3 point and a coiling temperature (CT) of 400 to 750 ° C. to form a steel strip having a thickness of 2.0 mm. After pickling, after heating and holding at 800 ° C. for 2 minutes with CGL, galvanization was performed with a weight per unit area of 45 g / m 2 on both sides, and then an alloying treatment at 550 ° C. × 10 sec was performed.
[0038]
At this time, the line speed was increased from coil Head to End for each coil. A sample was taken from a portion corresponding to any of line speeds 30, 80, and 160 mpm from the coil after passing through the CGL, and a JIS No. 5 tensile test and observation of the microstructure were performed. Table 4 shows the results. The cooling rate from the alloying temperature (550 ° C.) of each part to the Ms point is determined according to the line speed, and is shown as a cooling rate in the table.
[0039]
In Invention Examples 1 to 5, since the coiling temperature is 700 ° C. or less, a two-phase structure of ferrite and martensite is obtained at any line speed, and has appropriate strength and good ductility. In Comparative Examples 6 to 8, the coiling temperature is as high as 750 ° C., which is outside the scope of the present invention. When the coiling temperature is as high as 750 ° C., the carbide precipitates as coarse carbide after the hot rolling coiling, and is not sufficiently dissolved even by heating before plating of CGL. In Comparative Examples 7 and 8, since carbides mainly composed of cementite are included in addition to ferrite and martensite, the strength-ductility balance is insufficient even if the strength is appropriate. In Comparative Example 6, since the line speed is as low as 30 mpm, the penetration of carbide is sufficient, but the production efficiency is low, which is not preferable.
[0040]
[Table 3]

[0041]
[Table 4]

[0042]
【The invention's effect】
As described above, according to the present invention, it is possible to manufacture a hot-rolled base hot-dip galvanized high-tensile steel sheet having a tensile strength of 590 MPa or more and good workability with high productivity and to reduce the weight of an automobile. It is possible to obtain extremely useful effects in the industry.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of Cr + V content on martensite volume fraction.

Claims (3)

In mass%, C: 0.04 to 0.12%, Si: 0.1% or less, Mn: 1.0 to 2.0%, P: 0.05% or less, S: 0.005% or less, Cr: 0.05-1.0%, V: 0.005-0.2%, sol. Al: 0.033 to 0.10% , N: 0.01% or less , having a component composition composed of the balance Fe and inevitable impurities, and composed of ferrite and martensite, or composed of ferrite, martensite and bainite A hot-rolled base hot-dip galvanized high-tensile steel sheet with excellent workability, characterized by having a structure . In mass%, C: 0.04 to 0.12%, Si: 0.1% or less, Mn: 1.0 to 2.0%, P: 0.05% or less, S: 0.005% or less, Cr: 0.05-1.0%, V: 0.005-0.2%, sol. After rough rolling a steel having a component composition consisting of Al: 0.033 to 0.10% , N: 0.01% or less , the balance Fe and inevitable impurities , finish rolling at Ar ₃ points or more, at 700 ° C. or less A method for producing a hot-rolled base hot-dip galvanized high-tensile steel sheet excellent in workability, characterized by hot-dip galvanizing with a pre-plating heating temperature of Ac ₁ to Ac ₃ after winding. In mass%, C: 0.04 to 0.12%, Si: 0.1% or less, Mn: 1.0 to 2.0%, P: 0.05% or less, S: 0.005% or less, Cr: 0.05-1.0%, V: 0.005-0.2%, sol. After rough rolling a steel having a component composition consisting of Al: 0.033 to 0.10% , N: 0.01% or less , the balance Fe and inevitable impurities , finish rolling at Ar ₃ points or more, at 700 ° C. or less after winding, the pre-plating heating temperature Ac ₁ and to Ac ₃ and the galvanized, method of manufacturing a hot-dip galvanized high-strength steel sheet of the hot-rolled base having excellent workability, characterized by the alloying process.

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