JP3539546B2 - High-strength hot-dip galvanized steel sheet excellent in workability and method for producing the same - Google Patents

Description

【００５７】
【表２】

【００５８】
【表３】

【００５９】
【表４】

【００６０】
本発明例にめっき原板として用いた鋼板は、いずれもフェライトの平均粒径が3.5 μm 以下で、かつ第２相の平均粒径が3.5 μm 以下で、第２相中のマルテンサイト量が70体積％以上、オーステナイト量が２体積％以上である組織を有し、低降伏比で、ＴＳ×Ｅｌ値が23000MPa・％以上と高く、さらにλ値が80％以上と高い穴拡げ加工性を有し、加工性に優れた高張力熱延鋼板であり、また、伸びの異方性ΔＥｌが絶対値で3.0 以下と異方性が少なくなっている。この熱延鋼板に溶融亜鉛めっき処理を施した溶融亜鉛めっき鋼板はめっき性に優れ、さらに加工性を備えた溶融亜鉛めっき鋼板となっている。
【００６１】
これに対し、スラブ加熱温度が高く、動的再結晶の生起がなく、フェライト平均粒径が大きく、さらに第２相のマルテンサイト量、オーステナイト量が少なく、本発明の範囲を外れる鋼板No. 4 は、ＹＲが大きく、伸び、ＴＳ×Ｅｌ値が低くさらにΔＥｌの絶対値が大となっている。
また、動的再結晶低温域で２パスしか圧下を施さず、フェライト平均粒径が大きく、さらに第２相のマルテンサイト量、オーステナイト量が少なく、本発明の範囲を外れる鋼板No. ５は、ＹＲが大きく、伸び、ＴＳ×Ｅｌ値が低くなっている。
【００６２】
また、焼鈍後の冷却速度が速く、本発明の範囲を外れる鋼板No.6は、第２相のオーステナイト量が少なく、鋼板No. ４、No. ５、No.22 〜No.24 よりは優れた加工性を示すが、ＹＲがやや高く、ＴＳ×Ｅｌ値が若干低くなっている。鋼板No.7は、焼鈍後の冷却速度度が遅く、本発明の好適範囲を外れ、第２相中のマルテンサイト量が少なくなって、鋼板No. ４、No. ５、No.22 〜No.24 より優れた加工性を示すものの、ややＹＲが高く、ＴＳ×Ｅｌ値がやや低くなっている。鋼板No.22 は、Ti含有量が少なく、動的再結晶での圧延ができないため、フェライト平均粒径が大きくなりＹＲが高くなるとともに、伸び、ＴＳ×Ｅｌ値が低くなっている。鋼板No.23 は、Mn量が高く、第２相が52％もあってフェライトが主相とは言い難く、また、第２相もベイナイト主体となるためＹＲが高く、ＴＳ×Ｅｌ値が低い。鋼板No.24 は、Ｃ量が少なく、動的再結晶圧下によってもフェライトの平均粒径を3.5 μm 以下とすることができず、またマルテンサイトの形成も困難であった。
【００６３】
【発明の効果】
本発明によれば、超微細粒を有し、良好な機械的特性を具備し、かつ強度−伸びバランス、強度−穴拡げ性バランスに優れ、めっき性にも優れ、加工性に優れた高張力溶融亜鉛めっき鋼板、高張力合金化溶融亜鉛めっき鋼板を安価に製造でき、産業上格段の効果を奏する。
【図面の簡単な説明】
【図１】ＹＳ、ＴＳにおよぼす焼鈍温度の影響を示すグラフである。
【図２】ＹＲ、ＴＳ×Ｅｌにおよぼす焼鈍温度の影響を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength hot-dip galvanized steel sheet that is advantageous for use in automobiles, home appliances, mechanical structures, construction, and the like, and in particular, has high ductility, toughness, and high-strength hot-dip zinc excellent in strength-elongation balance. It relates to a plated steel sheet. The steel sheet in the present invention includes a steel sheet and a steel strip.
[0002]
[Prior art]
Steel materials used for automobiles, home appliances, mechanical structures, buildings, and the like are required to have excellent mechanical properties such as strength, workability, and toughness. Among them, conventionally, high-strength steel sheets with high strength by various methods have been proposed. For example, with a ferrite single phase structure, a solid solution strengthened steel sheet added with a solid solution strengthening element such as Si, Mn, or P, or a precipitation strengthened steel sheet added with a carbonitride forming element such as Nb or Ti, or a ferrite phase A double phase structure (DP (Dual Phase)) steel sheet strengthened by a second phase such as martensite and bainite, and a steel sheet strengthened by refining crystal grains are known.
[0003]
On the other hand, hot-dip galvanized steel sheets are used in a wide range of applications for automobiles, home appliances, building materials, etc. because of their excellent corrosion resistance. However, there are various difficulties in applying hot-dip galvanizing to a high-strength high-strength steel sheet.
For example, in solid solution strengthened steel sheets, alloy elements such as Si, Mn, and P added for solid solution strengthening cause deterioration of plating adhesion and delay in alloying, and the production of high tensile strength plated steel sheets It was very difficult. Further, the solid solution strengthened steel sheet has a problem in that the added alloy element becomes large in amount, which leads to an increase in cost, a reduction in workability such as ductility, and a limit in the strength obtained.
[0004]
Further, in the dual phase structure type steel sheet, alloy elements such as Mn, Cr, and Mo are added in order to convert the second phase into martensite. However, Mn and Cr deteriorate the plating property, particularly the wettability. In addition, since the alloying after plating is delayed, the characteristics of the formed plating layer deteriorate. In addition, during the alloying process after plating, when the cooling rate after heating is low, the transformation rate to martensite decreases and the properties of the dual phase type steel sheet are lost, so strict cooling control is necessary during the alloying process There was such a problem. Further, the dual-phase structure steel sheet has a good strength-ductility balance, but has some problems such as inferior hole expandability.
[0005]
Furthermore, in the case of high-strength steel by grain refinement, in addition to tensile strength, the yield strength is high, so that the yield ratio is high, and wrinkles during press forming are apt to occur, leaving the problem that press formability is low. I was
In recent years, with regard to high-strength steel sheets, the target is shifting to the development of high-strength steel sheets that can achieve both low cost and high-performance properties. Further, in order to protect an occupant in the event of a collision, a steel sheet for automobiles is required to have not only high strength but also excellent impact resistance.
[0006]
For these reasons, in the case of high-strength plated steel sheets, it is necessary to comprehensively improve the strength and other mechanical properties such as toughness and workability, and further including the plating properties. For the purpose of suppressing deterioration of toughness, durability ratio, and the like, and further suppressing deterioration of plating property, miniaturization of the structure of a high-strength steel sheet as an original plate for plating has become an important issue.
[0007]
Recently, a method in which austenite grains before hot rolling are extremely refined and rolled to utilize dynamic recrystallization and further controlled cooling to refine the structure is disclosed in, for example, JP-A-9-87798 and JP-A-9-87798. This is described in JP-A-9-143570 and JP-A-10-8138.
JP-A-9-87798 discloses that a slab containing Mn: 1.0 to 2.5 wt%, Ti: 0.05 to 0.30 wt%, or Ti: 0.05 to 0.30 wt% and Nb: 0.30 wt% or less is 950 to 1100 ° C. , And rolling at a rolling reduction of 20% or more per pass is performed at least twice, and the finish rolling temperature is Ar_ThreeAfter hot rolling at a temperature above the transformation point, it is cooled at a cooling rate of 20 ° C / s or more, wound up at 350 to 550 ° C, and 75% by volume or more of polygonal ferrite having an average crystal grain size of less than 10 μm. A method for producing a high-tensile hot-rolled steel sheet having an austenite structure of 5 to 20% by volume is disclosed.
[0008]
JP-A-9-143570 discloses that a steel containing one or two of Ti: 0.05 to 0.3 wt% and Nb: 0.10 wt% or less is heated to a temperature of 950 to 1100 ° C. Is performed at least twice or more so that the rolling reduction becomes 20% or more, and the finish rolling temperature is Ar_ThreeHot-rolled to a temperature above the transformation point,_ThreeCool the transformation point to 750 ° C at a cooling rate of 20 ° C / s or more, stay for 5 to 20 seconds in a temperature range of less than 750 ° C to 600 ° C, and then again at a cooling rate of 20 ° C / s or more to 550 ° C or less. And a method for producing a high-tensile hot-rolled steel sheet having an ultrafine structure with a ferrite content of not less than 80% by volume and an average ferrite grain size of less than 10 μm by winding at a temperature of 550 ° C. or less.
[0009]
Japanese Patent Application Laid-Open No. 10-8138 discloses that a slab containing Mn: 1.0 wt% or less, Ti: 0.05 to 0.30 wt%, or Nb in an amount of twice or more of 950 to 1100 ° C. in place of all or part of Ti. , And rolling at a rolling reduction of 20% or more per pass is performed at least twice, and the finish rolling temperature is Ar_ThreeAfter hot rolling to a temperature above the transformation point, cool at a cooling rate of 20 ° C / s or more, wind up at 350 to 550 ° C, and manufacture a high-tensile hot-rolled steel sheet having an ultrafine grain structure consisting of ferrite and retained austenite. A method is disclosed.
[0010]
Japanese Patent Application Laid-Open No. 10-195588 discloses that, in terms of wt%, C: 0.02 to 0.2%, Si: 0.1 to 1.5%, Mn: 0.5 to 3.0%, S: 0.010% or less, P: 0.03 to 0.15% %, Cr: 0.1 to 2.0%, Mo: 0.1 to 1.0%, the balance being Fe and unavoidable impurities, and the ferrite phase having an average grain size of 10 μm or less is 80 to 80%. A hot-rolled high-strength steel sheet having excellent formability and impact resistance comprising a second phase mainly composed of martensite having an average diameter of 0.2 to 1.5 times the average diameter of ferrite and occupying 97% of the balance is disclosed. I have.
[0011]
[Problems to be solved by the invention]
However, JP-A-9-87798, JP-A-9-143570, and JP-A-10-8138 focus on refining the crystal grains, but the particle size is 3.6. Although it can be obtained up to about μm, the strength and ductility of steel sheets manufactured using these technologies are improved, but the anisotropy of mechanical properties is not acceptable, especially from the viewpoint of workability of automotive steel sheets. It is hard to say that it is as small as possible, the yield ratio is high, and wrinkles and the like are likely to occur during press molding.
[0012]
  Also, high-tensile steel sheets manufactured by the technique described in Japanese Patent Application Laid-Open No. H10-195588 have improved formability and impact resistance as compared with conventional steel sheets. From the viewpoint, there remains a problem that the anisotropy of elongation (ΔEl) is large and the yield ratio is high.
The present invention advantageously solves the above-mentioned problems of the prior art, has ultra-fine grains, has a low yield ratio, is excellent in strength-elongation balance, hole expandability, has low elongation anisotropy, and is more favorable. It is an object of the present invention to provide a high-strength hot-dip galvanized steel sheet having a plated layer with excellent characteristics and excellent workability.
[0013]
[Means for Solving the Problems]
The present inventors have conducted intensive studies on the miniaturization of the original plate structure to achieve the above-mentioned object. As a result, during hot rolling, the rolling was repeatedly performed in the austenite dynamic recrystallization temperature range, and the pressure was relatively light. By rapid cooling after the reduction, the ferrite grains, which are the main phase, can be made into ultra-fine grains of 3.5 μm or less, and the second phase is also refined to be equal to or more than the main phase, and is dispersed and formed in an island shape. It was found that it could be done. Further, the hot rolled sheet having ultra-fine grains is Ac₁Above the transformation point (Ac₁Heating to the temperature range of the α-γ2 phase region below the transformation point + 80 ° C.) and then performing annealing to cool the steel can produce a high-strength steel sheet in which the yield ratio is significantly reduced and the strength-elongation balance is significantly improved. I found it. Furthermore, it has been found that the plating layer formed even when hot-dip galvanizing is applied to this high-tensile steel sheet has excellent plating adhesion, has no delay in alloying, and has good characteristics.
[0014]
The experimental results on which the present invention is based will be described.
Hot rolled steel sheet containing 0.12% C, 0.3% Si, 0.8% Mn, 0.005% Pi, and 0.16% Ti and having a ferrite average grain size of 1.5 μm or 4.5 μm as the main phase. (Ac₁(Transformation point: 740 ° C.) and continuous annealing was performed while changing the heating temperature in the range of 650 ° C. to 880 ° C. The holding time at the soaking temperature was constant at 40 sec. The cooling rate after soaking was 30 ° C / s, and the material was rapidly cooled to 300 ° C. After the continuous annealing, a tensile test was performed to determine the yield strength YS, the tensile strength TS, and the elongation El, and the yield ratio YR and the strength-elongation balance TS × El were calculated. In addition, the tensile test was implemented also about the steel plate as hot rolled. The results are shown in FIGS.
[0015]
From FIG. 1 and FIG. 2, the ultra-fine grained steel sheet having a hot-rolled ferrite grain size (initial grain size) of 1.5 μm₁Above the transformation point (Ac₁It can be seen that when heating to the α-γ2 phase region below the transformation point (+ 80 ° C.), TS increases, YS decreases, YR decreases, and TS × El remarkably improves. When the initial particle size is 4.5 μm, such reduction in YR and remarkable improvement in TS × El are not observed.
[0016]
According to further studies by the present inventors, a fine-grained steel sheet having an initial grain size of 3.5 μm or less was obtained from Ac.₁By heating above the transformation point to cause α → γ reverse transformation, the structure after cooling includes a second phase having an average crystal grain size of 3.5 μm or less, and the second phase is mainly a martensite phase. It has an austenite phase. As a result, it has been found that a steel sheet having a low YS, a high TS and an extremely good TS × El balance can be obtained without adding a large amount of alloying elements. Also, Ac₁When heated above the transformation point + 80 ° C, crystal grains grow, the strength decreases, and the material properties deteriorate. On the other hand, in the case of a steel sheet having an initial grain size of more than 3.5 μm, sufficient reverse transformation and concentration of alloying elements in the second phase do not easily occur by short-time annealing, so that martensite and the like hardly occur after cooling. Even if reverse transformation is caused by long-time annealing, the particle size of the second phase exceeds 3.5 μm, and the characteristics are deteriorated.
[0017]
The present inventors have further studied based on the above findings and completed the present invention.
That is, the present invention is a hot-dip galvanized steel sheet obtained by performing hot-dip galvanizing on a hot-rolled steel sheet surface, wherein the hot-rolled steel sheet isqualityIn terms of%, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%,Al: 0.10% or less, the balance being substantially Fe, and the main phaseBy volume 50 %More thanIt has a structure composed of ferrite and a second phase, wherein the average particle size of the ferrite is 3.5 μm or less, the average particle size of the second phase is 3.5 μm or less, and the second phase is an entire second phase. It is a high-strength hot-dip galvanized steel sheet excellent in workability, characterized by being a high-tensile hot-rolled steel sheet having a martensite of 70% or more by volume and an austenite of 2% or more by volume. In the present invention, the hot-rolled steel sheet isqualityIn terms of%, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%,High-strength hot-rolled steel sheet containing Al: 0.10% or less, Nb: 0.3% or less, V: 0.3% or less, and the balance substantially consisting of Fe Also, the hot-rolled steel sheet may bequalityIn terms of%, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%,Further, it contains Al: 0.10% or less, further contains one or more of Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less, with the balance being Even as a high-tensile hot-rolled steel sheet having a composition substantially composed of Fe, the hot-rolled steel sheet isqualityIn terms of%, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%,Al: 0.10% or less, and further, one or two or more of Ca, REM and B are contained in a total of 0.005% or less, and the balance is substantially high iron. As the steel sheet, also, the hot-rolled steel sheet,qualityIn terms of%, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%,Al: 0.10% or less, Nb: 0.3% or less, V: one or two of 0.3% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: a high-tensile hot-rolled steel sheet containing one or more of 1.0% or less, and the balance substantially consisting of Fe;qualityIn terms of%, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%,Al: 0.10% or less, Nb: 0.3% or less, V: 0.3% or less, one or more of Ca, REM, B, 0.005% or more in total. % Or less, and the balance is also a high-strength hot-rolled steel sheet having a composition substantially consisting of Fe,qualityIn terms of%, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%,Further, it contains Al: 0.10% or less, and further contains one or more of Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less, Ca, REM, B A high-tensile hot-rolled steel sheet containing 0.005% or less in total of one or more of the above, and the balance substantially consisting of Fe;qualityIn terms of%, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%,Al: 0.10% or less, Nb: 0.3% or less, V: one or two of 0.3% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: a composition having a composition containing one or more of 1.0% or less and one or two or more of Ca, REM and B in total of 0.005% or less, and the balance substantially consisting of Fe. It may be a tension hot-rolled steel sheet.
[0018]
Further, the present invention provides a method for producing a hot-dip galvanized steel sheet by performing a hot-dip galvanizing treatment on a steel sheet to obtain a hot-dip galvanized steel sheet.In mass%, C: 0.01 ~ 0.3 %, Si : 1.0 %Less than, Mn : 3.0 % Or less, P: 0.5 %Less than, Ti : 0.03 ~ 0.3 % Al : 0.10 %, With the balance being substantially Fe Having a composition consisting ofThe main phaseBy volume 50 %More thanA high-tensile hot-rolled steel sheet having an average ferrite grain size of 3.5 μm or less,MeltingBefore galvanizing, the steel sheet₁Transformation point ~ (Ac₁Heated to the temperature range (transformation point + 80 ° C)rear, Ten ~ 100 ° C / s Cooling stop temperature with cooling rate in the range: 200 ~ 460 Cool down to ℃After the annealing treatment,500A method for producing a high-strength hot-dip galvanized steel sheet, characterized by performing a hot-dip galvanizing process of dipping in a hot-dip galvanizing bath in a temperature range of ° C..
[0019]
In the present invention, the steel sheet,qualityIn terms of%, C: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%And further Al : 0.10 % Or less, with the balance being substantially Fe Consists ofRolling steel material of the composition, or reheat to 1100 ℃ or less, or after it is 1100 ℃ or less,(Minimum temperature of dynamic recrystallization) + 80 ℃ to the lower limit temperature of dynamic recrystallizationDynamic recrystallizationlow temperatureAt least 5 passes in the zone and finish rolling temperature is Ar_ThreeA main phase obtained by performing hot rolling at a transformation point or higher and cooling at a cooling rate of 30 ° C./s or more within 0.5 sec after completion of hot rolling.By volume 50 %More thanIt is preferable to use a high-tensile hot-rolled steel sheet having a structure in which the average grain size of ferrite is 3.5 μm or less..
[0020]
Further, according to the present invention, in the method for producing a galvanized steel sheet to be subjected to a hot dip galvanizing treatment and an alloying treatment on the steel sheet to obtain an alloyed hot-dip galvanized steel sheet,In mass%, C: 0.01 ~ 0.3 %, Si : 1.0 %Less than, Mn : 3.0 % Or less, P: 0.5 %Less than, Ti : 0.03 ~ 0.3 % Al : 0.10 % Or less, with the balance being substantially Fe Having a composition consisting ofThe main phaseBy volume 50 %More thanA high-tensile hot-rolled steel sheet having an average ferrite crystal grain size of 3.5 μm or less, and before the hot-dip galvanizing treatment, the steel sheet is Ac₁Transformation point ~ (Ac₁Heated to the temperature range (transformation point + 80 ° C)rear, Ten ~ 100 ° C / s Cooling stop temperature with cooling rate in the range: 200 ~ 460 Cool down to ℃After the annealing treatment,500A high-strength hot-dip galvanized steel sheet characterized by being subjected to a hot-dip galvanizing treatment immersed in a hot-dip galvanizing bath at a temperature range of 450 ° C, and then to an alloying process of alloying by heating to a temperature range of 450 to 550 ° C. Manufacturing methodYou.In the present invention, the steel sheet,qualityC: 0.01 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%See, further Al : 0.10 % Or less, with the balance being substantially Fe Consists ofRolling steel material of rolling composition is reheated to 1100 ° C or less, or after it reaches 1100 ° C or less, dynamic recrystallizationlow temperatureAt least 5 passes in the zone and finish rolling temperature is Ar_ThreeA main phase obtained by performing hot rolling at a transformation point or higher and cooling at a cooling rate of 30 ° C./s or more within 0.5 sec after completion of hot rolling.By volume 50 %More thanIt is preferable to use a high-tensile hot-rolled steel sheet having a structure in which the average grain size of ferrite is 3.5 μm or less.
[0021]
In the method for producing a high-strength galvanized steel sheet (including a high-strength galvannealed steel sheet) of the present invention, the hot-rolled steel sheet may further include, in addition to the composition,quality% Or less, one or two of Nb: 0.3% or less, V: 0.3% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less The composition may contain one or more selected from each group of one or more, and one or more of Ca, REM and B in total of 0.005% or less.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the reasons for limiting the chemical components of a high-strength hot-rolled steel sheet used as a plating base sheet (steel sheet) for the hot-dip coated steel sheet and the galvannealed steel sheet of the present invention will be described.Hereinafter, mass% is simply described as%.
C: 0.01-0.3%
C is an inexpensive reinforcing component, and contains a necessary amount according to the desired steel sheet strength. If the C content is less than 0.01%, the crystal grains become coarse, and it becomes impossible to achieve the average ferrite grain size of less than 3.5 μm intended in the present invention. On the other hand, if the C content exceeds 0.3%, workability is deteriorated and weldability is also deteriorated. Therefore, C is set in the range of 0.01 to 0.3%. More preferably, it is in the range of 0.05 to 0.2%.
[0023]
Si: 1.0% or less
Si effectively contributes to the increase in strength while improving the strength-elongation balance as a solid solution strengthening component. In addition, it effectively acts to suppress the formation of ferrite and obtain a structure having a desired volume ratio of the second phase, but an excessive content deteriorates ductility and surface properties. Therefore, the content of Si is set to 1.0% or less. Incidentally, the content is preferably 0.01 to 0.7%.
[0024]
Mn: 3.0% or less
Mn is Ar_ThreeIt contributes to the refinement of crystal grains through the action of lowering the transformation point. In addition, it has the effect of enhancing the strength-elongation balance through the effect of promoting the formation of martensite and retained austenite in the second phase. Further, it also has a function of detoxifying harmful dissolved S as MnS. However, a large amount hardens the steel and rather degrades the strength-elongation balance. Therefore, Mn is set to 3.0% or less. The content is preferably at least 0.05%, more preferably 0.3 to 2.0%.
[0025]
P: 0.5% or less
P is useful as a reinforcing component and can be contained according to the desired strength of the steel sheet. However, excessive P segregates at grain boundaries and causes embrittlement. Therefore, P is set to 0.5% or less. The excessive reduction may increase the cost, and is preferably 0.001 to 0.2%, more preferably 0.005 to 0.1%.
[0026]
Ti: 0.03-0.3%
Ti is present as TiC and effectively acts to refine the initial austenite grains during the hot rolling heating step and induce dynamic recrystallization in the subsequent hot rolling process. In order to exert such an effect, the content must be at least 0.03% or more. However, if the content exceeds 0.3%, the effect is saturated and an effect corresponding to the content cannot be expected. Therefore, Ti is set in the range of 0.03 to 0.3%. In addition, Preferably, it is 0.05 to 0.20%.
[0027]
Al: 0.10% or less
AlActs as a deoxidizing agent and also has a function of refining crystal grains as AlN. However, when the content exceeds 0.10%, oxide diameter inclusions increase and the cleanliness decreases. For this reason, Al is limited to 0.10% or less.You.The content is preferably 0.005 to 0.07%.
[0028]
Nb: one or two of 0.3% or less, V: 0.3% or less
Both Nb and V form carbonitrides and have an action of refining the initial austenite grains in the hot rolling and heating stage, and if necessary, by overlapping and containing Ti, Effectively affects dynamic recrystallization. However, even if it is contained in a large amount exceeding 0.3%, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, it is desirable that both Nb and V be 0.3% or less.
[0029]
One or more of Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less
Any of Cu, Mo, Ni, and Cr can be contained as a reinforcing component, if necessary. However, a large amount of the component rather deteriorates the strength-ductility balance. For this reason, it is desirable that Cu, Mo, Ni, and Cr all be 1.0% or less. In order to sufficiently exhibit the above-mentioned effects, it is preferable to contain at least 0.01% or more.
[0030]
0.005% or less in total of one or more of Ca, REM and B
Ca, REM, and B all have the effect of improving workability by controlling the shape of sulfides and increasing the grain boundary strength, and can be contained as necessary. However, excessive content may adversely affect cleanliness and recrystallization, so that the total content is desirably 0.005% or less.
[0031]
The hot-rolled steel sheet having a hot-dip galvanized surface formed thereon according to the present invention is substantially composed of Fe, except for the composition described above.
The hot-rolled steel sheet having a hot-dip galvanized surface formed thereon according to the present invention has a main phase of ferrite having an average grain size of 3.5 μm or less, and has a structure composed of a main phase and a second phase. The main phase is preferably at least 50% by volume, preferably at least 70% by volume. If it is less than 50%, workability is low and strength is too high. The second phase has an average particle size of 3.5 μm or less, and has a martensite of 70% or more and an austenite of 2% or more by volume relative to the entire second phase.
[0032]
If the ferrite grains are refined, the desired strength can be secured with a smaller amount of alloying elements compared to conventional high-tensile steel, and there is little deterioration in properties other than strength, and the subsequent plating properties will be good. . However, when the average grain size of ferrite exceeds 3.5 μm, the increase in strength due to the refinement of crystal grains is small, the amount of alloy addition is increased, and the subsequent plating property is adversely affected. For this reason, the average grain size of ferrite was limited to 3.5 μm or less. Further, when the average particle size of the second phase exceeds 3.5 μm, the improvement in toughness and ductility decreases, so the average particle size of the second phase is limited to 3.5 μm or less.
[0033]
The volume ratio of the second phase is desirably 3 to 30%. The volume fraction of the second phase is 3%If it is less than 1, the effect of increasing the strength due to the refinement of ferrite grains is obtained, but the ductility is reduced and the balance between strength and ductility is low. If it exceeds 30%, the hole expandability decreases.
The second phase shall have a martensite of 70% or more by volume relative to the entire second phase and an austenite of 2% or more by volume.
[0034]
If the volume fraction of martensite in the second phase is less than 70%, a low yield ratio cannot be obtained, resulting in a high yield ratio which is a defect of a steel sheet having fine grains, and a low strength-elongation balance. If the volume fraction of austenite in the second phase is less than 2%, only a low strength-elongation balance can be obtained. Only when the volume ratio of austenite is 2% or more, TS × El becomes 23000 MPa ·% or more.
[0035]
In the present invention, the ferrite and the secondPhaseThe average particle size is defined as an average particle size in a cross section in the rolling direction according to a conventional method.
Next, a method for producing a hot-rolled steel sheet according to the present invention will be described. The steps before the annealing (welding to hot rolling / coil winding) are a suitable example for obtaining a high-strength hot-rolled steel sheet having an average crystal grain size of ferrite as a main phase of 3.5 μm or less. It is not limited to this manufacturing method.
[0036]
The molten steel adjusted to the above component composition range is made into a rolled material by continuous casting or ingot-bulking rolling, and the rolled material is subjected to hot rolling to obtain a hot-rolled steel sheet.
The hot rolling may be reheating rolling, in which the rolled material is once cooled and then reheated, or may be direct rolling or hot charge rolling. Further, a continuously cast slab such as a thin slab continuous casting method may be directly hot-rolled. In the case of reheating, it is desirable to heat to 1100 ° C. or less in order to make the initial austenite grains fine. Also, in the case of direct rolling, it is preferable to start rolling after cooling to 1100 ° C. or lower in order to promote dynamic recrystallization. In order to set the finish rolling temperature in the austenite range, it is preferable that the reheating temperature or the direct rolling start temperature is 950 ° C. or higher.
[0037]
When hot rolling the rolling material at the above temperature, in the present invention is a low temperature range of the dynamic recrystallization temperature,(Minimum temperature of dynamic recrystallization) + 80 The temperature range from ℃ to the lower limit temperature of the dynamic recrystallization is defined as the dynamic recrystallization low temperature range,It is preferable to apply at least 5 or more repetitive rolling reductions in the dynamic recrystallization low temperature range. The austenite grains are refined by repeatedly reducing the temperature in the dynamic recrystallization low temperature range. Since the finer austenite grains progress as the number of times of the dynamic recrystallization increases, it is preferable to reduce the pressure in at least 5 passes and more than 5 consecutive passes. With less than 5 passes, the degree of austenite grain refinement is small and it is difficult to achieve fine grains with an average ferrite grain size of 3.5 μm or less.
[0038]
The rolling reduction in the low temperature range of the dynamic recrystallization is not particularly limited as long as the dynamic recrystallization occurs, but it is preferably 4 to 20% per pass, preferably less than 20%. desirable. When the rolling reduction per pass is less than 4%, dynamic recrystallization does not occur.
On the other hand, when the rolling reduction per pass exceeds 20%, the anisotropy of the mechanical properties increases.
[0039]
In addition, dynamic reconnectionCrystalThe final rolling pass in the low temperature range is desirably set to a rolling reduction of 13 to 30% in order to reduce the size of the second phase.
Note that Ac₁Although annealing is performed at a temperature equal to or higher than the transformation point, it is not preferable that the second phase exists in an agglomerated state before annealing, and an island shape (here, the island shape means another island at an interval equal to or less than the particle size of the second phase). The ratio in which the second phase is present is a dispersion state of 20% or less). Under the above hot rolling conditions, an island-like second phase distribution can be obtained.
[0040]
The dynamic recrystallization temperature range referred to in the present invention is a strain-stress obtained by simulating rolling conditions by a measuring device capable of independently controlling temperature and strain (for example, “Processing for Master” manufactured by Fuji Denki Koki Co., Ltd.). In this case, a value measured in advance is used. Dynamic recrystallization temperatureAreaAlthough it varies with the steel composition, heating temperature, reduction ratio, reduction distribution, etc., it is said that it exists in the temperature range of 850 to 1100 ° C, usually in the range of 250 to 100 ° C. The temperature width of the dynamic recrystallization temperature range increases as the rolling reduction per pass is higher or as the Ti content is higher.
[0041]
Further, from the viewpoint of microstructural refinement, it is advantageous to perform rolling in a temperature range as low as possible in the dynamic recrystallization temperature range, because the number of transformation sites of γ → α transformation increases. Therefore, in order to promote the refining of austenite grains, the dynamic recrystallization is performed from (lower limit temperature of dynamic recrystallization) + 80 ° C, preferably (lower limit temperature of dynamic recrystallization) + 60 ° C.CrystalThe temperature range up to the lower limit temperature is defined as a dynamic recrystallization low temperature range, and it is preferable to apply the above-mentioned 5 or more passes in this temperature range.
[0042]
In order to minimize the temperature drop of the material to be rolled during rolling in the low temperature range of dynamic recrystallization within a predetermined range, it is preferable to install a heating means between the rolling stands and heat the material to be rolled or the roll. In particular, it is effective to install the heating means at a position where the temperature drops significantly. As the heating means, the steel plate may be heated by a high-frequency heating device, the roll may be heated by using an electric heater, or the heating may be performed by direct electric heating.
[0043]
In addition, at the time of hot rolling, it goes without saying that the reduction may be performed while applying lubrication.
In the present invention, rolling conditions other than rolling in the dynamic recrystallization temperature range are not particularly limited, but the rolling finishing temperature is Ar_ThreeAbove the transformation point. The rolling finish temperature is Ar_ThreeIf the temperature is lower than the transformation point, ductility and toughness of the steel sheet deteriorate.
[0044]
In the hot-rolled steel sheet that has been hot-rolled under the above-described conditions, the austenite grains at this point are almost equiaxed crystal grains. There are many transformation nuclei of the α transformation, the grain growth of ferrite grains is suppressed, and the structure is refined. For this reason, it is preferable to start cooling within 0.5 sec, preferably within 0.3 sec after the end of rolling. If the start of cooling exceeds 0.5 sec after the end of rolling, grain growth becomes remarkable.
[0045]
The cooling rate is 30 ° C./sec or more. If the cooling rate is less than 30 ° C./sec, ferrite grains grow, making it difficult to achieve finer grains and making it difficult to distribute the second phase finely and in islands.
The hot-rolled steel sheet cooled at a cooling rate of 30 ° C./sec or more, preferably to a temperature range of 350 to 650 ° C., is immediately wound around a coil. The winding temperature and the cooling rate after winding are not particularly limited. What is necessary is just to determine suitably according to the steel plate to manufacture. However, when the winding temperature is high, the second phase has a structure mainly composed of pearlite, and the ferrite grains tend to grow. On the other hand, if the winding temperature is too low, winding becomes difficult. For this reason, it is desirable that the winding temperature be in the range of 350 to 650 ° C.
[0046]
Next, the hot-rolled steel sheet is annealed before the hot-dip galvanizing treatment. Although there is no particular limitation on the annealing method, continuous annealing is preferable in terms of production efficiency. The soaking temperature is Ac₁Transformation point ~ (Ac₁(Transformation point + 80 ° C). By heating to this temperature range, a part is transformed into the γ phase. The soaking time is preferably from 1 to 300 sec, more preferably from 20 to 100 sec.
[0047]
Next, it is immersed in a hot-dip galvanizing bath.460After cooling to a temperature of ° C., it is preferable to immerse it in a hot-dip galvanizing bath. Preferably, the cooling rate is 10 to 100 ° C./s. If the cooling rate is less than 10 ° C./s, diffusion of C occurs, and it is difficult to form a second phase mainly composed of martensite and having a structure containing austenite. On the other hand, even if the cooling rate is higher than 100 ° C./s, the structural fraction of the second phase does not change, and it is very difficult to increase the cooling rate to more than 100 ° C. It is desirable to make s the upper limit. The quenching stop temperature after soaking is 200 to460It is desirable to set the temperature range to ° C. If the quenching stop temperature is less than 200 ° C, shape defects are likely to occur.460If the temperature exceeds ℃, it is difficult to obtain a structure containing a predetermined amount of martensite and austenite.
[0048]
Hot-dip galvanizing process is 450 ~500Preferably, the steel sheet is immersed in a hot-dip galvanizing bath maintained in a temperature range of ° C. After the hot-dip galvanizing treatment, an alloying treatment for heating to a temperature range of 450 to 550 ° C. for alloying may be performed.
The hot-dip galvanizing treatment is preferably performed by a continuous galvanizing equipment, but is not limited thereto.
[0049]
After the hot-dip galvanizing treatment, an alloying treatment for heating to a temperature range of 450 to 550 ° C. for alloying may be performed. The alloying treatment temperature is a temperature for obtaining an appropriate alloy layer structure.When the temperature is lower than 450 ° C, sufficient alloying does not proceed, whereas when the temperature exceeds 550 ° C, alloying proceeds excessively, and the plating layer is formed during forming. The so-called powdering property of peeling is deteriorated.
[0050]
After the alloying treatment, the steel sheet is cooled, and temper rolling is performed as necessary.
[0051]
【Example】
Molten steel having the composition shown in Table 1 was used as a slab (rolled material) by a continuous casting method. These slabs were heated, hot-rolled, and rolled and cooled under various conditions shown in Table 2 to obtain a hot-rolled steel sheet (sheet thickness 1.8 to 2.9 mm). The finish rolling temperature of hot rolling is the Ar_ThreeIt was above the transformation point. In addition, lubrication rolling was performed under the manufacturing condition No. 2. Subsequently, these hot-rolled steel sheets were subjected to continuous annealing for heating and cooling under the conditions shown in Table 2, and then subjected to a hot-dip galvanizing treatment under the conditions shown in Table 2 to obtain hot-dip galvanized steel sheets. Immediately after the hot-dip galvanizing treatment, some of the steel sheets were subjected to a heat alloying treatment under the conditions shown in Table 2 to obtain an alloyed hot-dip galvanized steel sheet.
[0052]
The obtained steel sheets were examined for structure, tensile properties, hole expandability, and plating properties.
The microstructure of the section of the steel sheet in the rolling direction was determined by a cutting method using an optical microscope or an electron microscope in accordance with the provisions of JIS G 0551. The diameter was measured. The particle size of each of the ferrite and the second phase was measured in five or more visual fields (structure photograph at a magnification of 1000 times with an optical microscope or an electron microscope), and the equivalent circle diameter was determined as the average particle diameter.
[0053]
The tensile properties were measured by using JIS No. 5 test pieces taken from the rolling direction of the steel sheet, a direction perpendicular to the rolling direction, and a direction of 45 ° in the rolling direction (yield point YS, tensile strength TS, elongation El). . The tensile test was performed at room temperature at a tensile speed of 10 mm / min. From the measured elongation, ΔEl = 1/2 · (El₀+ El₉₀) -El₄₅The anisotropy ΔEl of elongation of each steel sheet defined by the equation was calculated. Where El₀Is the elongation value in the rolling direction, El₉₀Is the elongation at right angles to the rolling direction, El₄₅Represents an elongation value in the direction of 45 ° in the rolling direction.
[0054]
The hole expandability is 10 mmφ (D₀) Is punched and then expanded by a conical punch with a vertex angle of 60 °, the hole diameter D immediately after the crack penetrates the plate thickness is obtained, and λ = ｛(D−D₀) / D₀The evaluation was made based on the λ value obtained from｝ × 100%.
The plating property was evaluated based on the iron content in the plating layer, the powdering property, and the plating property (degree of non-plating). The iron content in the plating layer was measured by X-ray. The degree of non-plating was visually evaluated on a five-point scale where "1" had no non-plating defect and "5" had the most non-plating defect.
[0055]
The powdering resistance was evaluated by measuring the zinc powder adhered to the cellophane tape after the 90 ° bending back by fluorescent X-ray. The fluorescent X-ray intensity of the zinc from the zinc powder was measured for 2 minutes by a counter tube, and if it was 2500 CPS or less, it was determined that it could withstand press molding of an automobile or the like as “1”.
Table 3 shows the results.
[0056]
[Table 1]

[0057]
[Table 2]

[0058]
[Table 3]

[0059]
[Table 4]

[0060]
In each of the steel sheets used as the plating base sheet in the present invention, the average grain size of ferrite was 3.5 μm or less, the average grain size of the second phase was 3.5 μm or less, and the amount of martensite in the second phase was 70 vol. %, The austenite amount is 2% by volume or more, and has a low yield ratio, a TS × El value as high as 23000 MPa ·% or more, and a λ value as high as 80% or more. It is a high-tensile hot-rolled steel sheet excellent in workability, and has anisotropy of elongation ΔEl less than or equal to 3.0 or less in absolute value. A hot-dip galvanized steel sheet obtained by subjecting this hot-rolled steel sheet to a hot-dip galvanizing treatment is a hot-dip galvanized steel sheet having excellent plating properties and further having workability.
[0061]
On the other hand, steel sheet No. 4 having a high slab heating temperature, no occurrence of dynamic recrystallization, a large average ferrite grain size, a small amount of martensite and austenite in the second phase, and out of the range of the present invention was obtained. Has a large YR, elongation, a low TS × El value, and a large absolute value of ΔEl.
Further, the steel sheet No. 5 which undergoes rolling reduction only in two passes in the low temperature range of dynamic recrystallization, has a large ferrite average grain size, further has a small amount of martensite and austenite in the second phase, and is out of the range of the present invention, The YR is large, elongated, and the TS × El value is low.
[0062]
Further, the steel sheet No. 6 having a high cooling rate after annealing and deviating from the range of the present invention has a small amount of austenite of the second phase, and is superior to the steel sheets No. 4, No. 5, No. 22 to No. 24. However, the YR is slightly higher and the TS × El value is slightly lower. In steel sheet No. 7, the cooling rate after annealing was low, outside the preferred range of the present invention, the amount of martensite in the second phase was reduced, and steel sheets No. 4, No. 5, No. 22 to No. .24 Higher workability but slightly higher YR and slightly lower TS × El value. Steel sheet No. 22 has a low Ti content and cannot be rolled by dynamic recrystallization, so that the average ferrite grain size increases, the YR increases, the elongation, and the TS × El value decreases. In steel sheet No. 23, the Mn content is high, the second phase is as much as 52%, and it is difficult to say that ferrite is the main phase, and since the second phase is also mainly bainite, the YR is high and the TS × El value is low. . In steel sheet No. 24, the C content was small, the average grain size of ferrite could not be reduced to 3.5 μm or less even under dynamic recrystallization pressure, and the formation of martensite was difficult.
[0063]
【The invention's effect】
According to the present invention, it has ultrafine grains, has good mechanical properties, and is excellent in strength-elongation balance, strength-hole expandability balance, excellent in plating properties, and excellent in workability and high tension. Hot-dip galvanized steel sheet and high-strength alloyed hot-dip galvanized steel sheet can be manufactured at a low cost, and it has a remarkable industrial effect.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of annealing temperature on YS and TS.
FIG. 2 is a graph showing the effect of annealing temperature on YR and TS × El.

Claims (10)

A galvanized steel sheet subjected to hot-dip galvanized hot-rolled steel sheet, the hot-rolled steel sheet, in mass%, C: 0.01~0.3%, Si : 1.0% or less, Mn: 3.0% or less, P : 0.5% or less, Ti: 0.03 to 0.3% , Al : 0.10 % or less , the balance being substantially composed of Fe, and the main phase of ferrite having a volume fraction of 50 % or more ; The ferrite has a structure composed of two phases, wherein the ferrite has an average particle size of 3.5 μm or less, the second phase has an average particle size of 3.5 μm or less, and the second phase has a volume ratio to the entire second phase. A high-strength hot-dip galvanized steel sheet having excellent workability, characterized by being a high-tensile hot-rolled steel sheet having 70% or more of martensite and austenite of 2% or more by volume. The hot-rolled steel sheet, in addition to the composition, further, in mass%, Nb: 0.3% or less, V: is a high tensile hot-rolled steel sheet having a composition containing one or two of 0.3% or less high-tensile galvanized steel sheet according to claim 1, characterized in that. The hot-rolled steel sheet, in addition to the composition, further, in mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1 kind or two kinds of the 1.0% The high-strength hot-dip galvanized steel sheet having excellent workability according to claim 1 or 2 , which is a high-tensile hot-rolled steel sheet having the above composition. The hot-rolled steel sheet, in addition to the composition, further, in mass%, Ca, REM, with one or high-tensile hot-rolled steel sheet having a composition containing 0.005% or less of two or more in total of B The high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 3 , wherein In the method for producing a hot-dip galvanized steel sheet by subjecting the steel sheet to a hot-dip galvanizing treatment, the steel sheet is expressed in terms of % by mass as C: 0.01 to 0.3 %, Si : 1.0 % or less, and Mn : 3.0 % or less. , P: 0.5 % or less, Ti : 0.03 to 0.3 %, Al : 0.10 % or less, the balance substantially consisting of Fe , and at least 50 % by volume as the main phase the average crystal grain size of the ferrite is a high-tensile hot-rolled steel sheet is 3.5 [mu] m or less, before the hot-dip galvanizing treatment, after heating to a temperature range of Ac ₁ transformation point - the steel plate (Ac ₁ transformation point + 80 ° C.) , 10 ~ 100 ℃ / S in the range of the cooling rate in a quench stop temperature: 200 after subjected to annealing to cool to ~ 460 ℃, 450 ~ immersed in the molten galvanizing bath temperature range of 500 ° C. galvanizing For producing high-strength hot-dip galvanized steel sheet, characterized by performing a heat treatment . In a method for producing a hot-dip galvanized steel sheet by subjecting the steel sheet to a hot-dip galvanizing treatment and an alloying treatment to obtain an alloyed hot-dip galvanized steel sheet, the steel sheet is expressed in terms of % by mass, C: 0.01 to 0.3 %, and Si : 1.0. %, Mn : 3.0 % or less, P: 0.5 % or less, Ti : 0.03 to 0.3 %, Al : 0.10 % or less, the balance being substantially Fe , and a high-tensile hot-rolled steel sheet having an average crystal grain size of more than 50% of ferrite at a volume ratio which is the main phase is a 3.5 [mu] m or less, the prior galvanizing treatment, ~ Ac ₁ transformation point to said steel plate (Ac ₁ transformation After heating to a temperature range of + 80 ° C) , quench at a cooling rate of 10 to 100 ° C / s. Stop temperature: After performing an annealing treatment to cool to 200 to 460 ° C , melting in a temperature range of 450 to 500 ° C. Perform hot-dip galvanizing treatment by immersion in a galvanizing bath, and then apply a temperature range of 450 to 550 ° C. Heating method for manufacturing a high-tensile galvanized steel sheet characterized by performing alloying treatment for alloying to. The steel sheet, in mass%, C: 0.01~0.3%, Si : 1.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3% only contains further Al: 0.10% or less Rolling steel material with a composition consisting essentially of Fe , with the balance being substantially Fe , is reheated to 1100 ° C or lower, or after the temperature reaches 1100 ° C or lower , from (lower limit temperature for dynamic recrystallization) + 80 ° C. At least 5 passes are reduced in the low temperature range of the dynamic recrystallization to the lower limit temperature of the dynamic recrystallization, hot rolling is performed so that the finishing rolling temperature is equal to or higher than the Ar ₃ transformation point, and 0.5 sec after the completion of the hot rolling. A high-tensile hot-rolled steel sheet having a structure in which the average grain size of ferrite with a volume fraction of 50 % or more as a main phase is 3.5 μm or less, obtained by cooling at a cooling rate of 30 ° C./s or more within The method for producing a high-strength hot-dip galvanized steel sheet according to claim 5 or 6 , wherein: 8. The composition according to claim 5, further comprising , in addition to the above composition , one or more of Nb : 0.3 % or less and V: 0.3 % or less by mass% . The method for producing a high-strength galvanized steel sheet according to item 1 . In addition to the above-mentioned composition, the composition further has a composition containing one or more of Cu : 1.0 % or less, Ni : 1.0 % or less, Cr : 1.0 % or less, and Mo : 1.0 % or less by mass%. The method for producing a high-strength hot-dip galvanized steel sheet according to any one of claims 5 to 8, characterized in that: 10. The composition according to claim 5, further comprising, in addition to the composition, a composition containing , by mass%, one or more of Ca , REM , and B in total of 0.005 % or less. A method for producing a high-tensile hot-dip galvanized steel sheet according to any of the claims .

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