JP3790087B2 - High-strength hot-rolled steel sheet with excellent workability - Google Patents

Description

【００３６】
得られた熱延鋼板から試験片を採取し、鋼板断面を光学顕微鏡によりミクロ組織を観察して、表層部を構成するポリゴナルフェライトが９５％以上の領域（表面からの平均深さmm）、および中心部を構成するベイナイト（ベイニティックフェライトを含む。）が９５％以上の領域（断面における面積率％）を測定するとともに、鋼板表面から０．２mm深さ位置および板厚中心位置のビッカース硬度（荷重１００ｇｆ）を測定した。これらの測定結果を表２に示す。
【００３７】
また、得られた鋼板を用いて、ＪＩＳ５号引張試験、形状凍結性評価試験、穴拡げ試験を行った。これらの結果を表２に併せて示す。
【００３８】
形状凍結性評価試験は、図２に示すように、ハット形断面部材（図中の矢印間の数値は寸法mm、アール部の内側半径１０mm）をそのフランジ部１を拘束しながら底壁部３をダイ・ポンチによって絞り曲げ成形し、成形部材を成形型から取り出し、図３に示すように、底壁部３を定盤に水平に載置して、フランジ部１の上面が水平面との成す角度θを測定した。この角度θは、底壁部３と側壁部２との間に生じるスプリングバックと、側壁部２とフランジ部１との間に生じるスプリングバックとが合わさったものであり、θが５度未満であれば例えば平板との接合に不都合がないことから形状凍結性が良好（○）、θが５度以上を不良（×）と評価した。
【００３９】
また、穴拡げ試験は、試験片に直径１０mmの打ち抜き穴を開け、その初期穴（ｄ₀ ＝１０mm）に頂角が６０度の円錐パンチを押し込み、クラックが板厚を貫通するまで押し拡げたときの穴径をｄ₁ としたとき、下記式で算出される穴拡げ率λ（％）を求めるものである。
λ＝（ｄ₁ −ｄ₀ ）／ｄ₀ ×１００
【００４０】
【表２】

【００４１】
表２より、本発明の熱延鋼板は、５９０Ｎ／mm² 級以上の高強度を有し、かつ形状凍結性に優れている。しかも、伸びフランジ性については、ＴＳ（ＭＰa）＊λ（％）の値で７５０００以上が達成されており、伸びフランジ性にも極めて優れていることがわかる。
【００４２】
【発明の効果】
本発明の熱延鋼板によれば、所定厚さの表層部が９５面積％以上のポリゴナルフェライトを有する軟質組織で形成され、６０％以上の中心部が９５面積％以上のベイナイトを有する硬質組織で形成されているので、５９０Ｎ／mm² 以上の高強度を有しながら、加工性、特に形状凍結性および伸びフランジ性に優れる。
【図面の簡単な説明】
【図１】本発明の熱延鋼板の板厚方向断面における硬度分布の一例を示すグラフである。
【図２】形状凍結性評価試験に用いたハット形断面部材の斜視図である。
【図３】形状凍結性の定量的データを示すθの測定要領説明図である。[0001]
[Technical field to which the invention belongs]
The present invention relates to a high-strength hot-rolled steel sheet excellent in workability that can be suitably used as a material steel sheet for an undercarriage member for automobiles, for example.
[0002]
[Prior art]
In recent years, from the viewpoint of improving the safety and fuel consumption of automobiles, high strength and thinning of steel sheets for automobiles has been widely promoted, and application of high strength steel sheets of 590 MPa class and further 780 MPa class has been intensively studied.
[0003]
These steel plates are required to have shape freezing property and bending workability, and are also required to have local ductility, that is, stretch flangeability. These characteristics deteriorate as the strength of the material increases. In particular, shape defects due to springback may occur, and there are cases where the material cannot be used due to cracking or necking.
[0004]
As described in JP-A-10-1748, a hot-rolled steel sheet having good workability under high strength is formed of Ti-containing low-C steel, and has a Vickers hardness (HVs) of a surface layer portion. It is known that the Vickers hardness (HVa) at the center is HVs / HVa ≦ 0.95. This hot-rolled steel sheet has a microstructure of ferrite and bainite, and the Nb-Ti carbide of the surface layer is coarsened to reduce the hardness of the surface layer and improve workability such as bending and burring. It is.
[0005]
[Problems to be solved by the invention]
Although the workability was improved by the hot-rolled steel sheet described in the above publication, the surface layer portion has a ferrite-bainite two-phase structure, so it cannot be said that the shape freezing property and the bending workability are sufficient. Since coarse precipitates exist in the part, there is a surface inferior in local ductility, and further improvement in stretch flangeability is desired.
[0006]
The present invention has been made in view of such problems, and an object thereof is to provide a hot-rolled steel sheet having a high strength of 590 N / mm ² or higher and excellent workability, particularly shape freezing property and stretch flangeability. To do.
[0007]
[Means for Solving the Problems]
The high-strength hot-rolled steel sheet of the present invention is coupled to a center part formed by a structure having an area ratio of 60% or more in the steel sheet cross section and having a bainite of 95 area% or more, and both sides of the center part. , And a surface layer portion formed by a structure having polygonal ferrite of 95 area% or more, and the surface layer portion is formed in a region at least 0.2 mm deep from both surfaces of the steel plate. In this hot-rolled steel sheet, as described in claim 2, the hardness ratio HVs / the Vickers hardness (HVs) of the surface layer portion at a depth of 0.2 mm from the surface and the Vickers hardness (HVa) of the central portion at the center of the plate thickness. HVa is preferably 0.9 or less.
[0008]
Moreover, as for the suitable chemical component of the hot-rolled steel plate of this invention, as described in Claim 3, the average composition in a plate | board thickness direction is weight%, C: 0.01-0.1%,
Si: 1% or less,
Mn: 1.0 to 2.5%
P: 0.1% or less,
S: 0.01% or less,
Al: 0.01 to 0.08%,
Ti: 0.1 to 0.4%
And the balance consisting of Fe and unavoidable impurities .
[0009]
Furthermore, as described in claim 4, the steel component may contain one or more components selected from at least one of the following first to fourth groups.
[0010]
First group Nb: 0.1 to 0.4%
Second group Cr: 0.05-0.8%
Mo: 0.05-1.0%,
B: 0.0005 to 0.01%
Third group Ca: 0.005% or less,
Rare earth element: 0.005% or less
DETAILED DESCRIPTION OF THE INVENTION
As a result of studying measures for improving workability, particularly shape freezing property and stretch flangeability, the inventors have a structure mainly composed of bainite in order to ensure the strength of the entire steel sheet while ensuring high strength. It has been found that these properties are very excellent by forming the surface layer portion with a structure mainly composed of soft polygonal ferrite while forming. The present invention has been completed based on such findings.
[0012]
The shape freezing property means the holding property of the mold dimensions given to the raw steel plate after press forming, and the shape freezing property decreases as the springback increases. Spring back refers to deviation from the shape determined by the original mold dimensions when the molded product is taken out from the press mold after press forming, such as bending. The larger the spring back, the other parts in the next process And brings difficulties when joining. This phenomenon is due to the fact that the molding strain amount of the surface layer portion is large and the stress corresponding to the surface strain amount increases as the strength increases, and this phenomenon has been unavoidable as a fate of the high strength material.
[0013]
In the present invention, a bainite (including bainitic ferrite) is formed with a hard structure having 95% by area or more of the center portion of the plate thickness with less distortion, thereby ensuring a predetermined standard strength of 590 MPa or 780 MPa, The surface layer of a hot-rolled steel sheet with large strain is made of a soft structure with 95% by area or more of polygonal ferrite that is easily plastically deformed, which promotes the plastic deformation of the surface layer during press forming and reduces residual stress. By suppressing, the spring back is suppressed and the shape-sintering property is improved.
[0014]
Moreover, since the structure of the surface layer portion is softened by making it a structure having polygonal ferrite of 95 area% or more, it does not generate coarse precipitates that become the starting point of cracks, and the center portion also has Since the structure has a bainite of 95 area% or more, very excellent local ductility, that is, stretch flangeability can be obtained.
[0015]
The central part mainly composed of bainite needs to occupy 60% or more in the cross-sectional area of the steel sheet. If it is less than 60%, the strength will be insufficient even if the central part is formed of a bainite-based structure, and the soft surface layer part becomes thick, so that the sag of the hole edge increases during drilling. On the other hand, the surface layer portion mainly composed of polygonal ferrite needs to occupy a region at least 0.2 mm deep from the surface of the steel sheet in order to ensure good shape freezing property. Furthermore, it is desirable that 5% or more, more preferably 6% or more of the plate thickness is the surface layer portion. In addition, since the surface layer portion inevitably contains bainite in addition to polygonal ferrite as it goes to the thickness center side, 95% of polygonal ferrite is included as a range in which the softening action in the surface layer portion is not substantially impaired. The above region is a surface layer portion. Similarly, since the ferrite portion is included in the central portion toward the surface side, the region where bainite (including bainitic ferrite is included) is 95% or more is set as the central portion.
[0016]
The hardness ratio HVs / HVa between the Vickers hardness (HVs) of the surface layer portion at a depth of 0.2 mm from the surface and the Vickers hardness (HVa) of the central portion at the center of the plate thickness is 0.9 or less, preferably 0.8 The following is recommended. If it exceeds 0.9, the softening structure effect by the surface layer portion becomes small, so the upper limit of the hardness ratio is set to 0.9.
[0017]
FIG. 1 shows the hardness distribution in the thickness direction of a hot-rolled steel sheet (sheet thickness 3.4 mm) according to an example of the present invention. In this example, an area of about 0.5 mm from the steel sheet surface is Polygonal ferrite is a surface layer portion formed of a soft structure of 95% or more, and the hardness at a position 0.2 mm deep from the surface is about 200 Hv. On the other hand, the central portion of bainite having a hard structure of 95% or more occupies about 65% of the cross-sectional area, the hardness of the central portion of the plate thickness is 265 Hv, and the hardness with the surface layer portion at a depth of 0.2 mm. The ratio is 0.75. Further, an intermediate layer mainly composed of bainite and ferrite is integrally and continuously formed between the surface layer portion and the central portion.
[0018]
Here, the suitable chemical composition from which the structure is obtained and the reasons for limiting its components will be described.
C: 0.01 to 0.1%
C has an effect on strengthening of steel. In particular, it is an element necessary for forming bainite. For this purpose, it is necessary to add at least 0.01% or more. However, if excessively added, the ductility is remarkably deteriorated and the weldability is also lowered, so the upper limit is made 0.1%.
[0019]
Si: 1% or less Si is very effective for improving the tensile strength as a solid solution strengthening element. However, excessive addition causes the surface properties and chemical conversion properties to deteriorate, so the upper limit is 1%.
[0020]
Mn: 1.0 to 2.5%
Mn is an element that improves the hardenability and is an element that is necessary for making the structure of the central part bainite. In order to exhibit this effect effectively, it is necessary to add at least 1.0% or more, but adding too much not only lowers the ductility but also harms the weldability, so the upper limit is made 2.5%.
[0021]
P: 0.1% or less P works to improve the strength of steel, but excessive addition deteriorates workability and toughness, so 0.1% was made the upper limit.
[0022]
S: 0.01% or less S is restricted to 0.01% or less in order to improve stretch flangeability.
[0023]
Al: 0.01 to 0.08%
Al needs to be added in an amount of at least 0.01% for deoxidation, but if added over 0.08%, alumina inclusions increase and the workability deteriorates, so the upper limit is set to 0.0. It was set to 08%.
[0024]
Ti: 0.1 to 0.4%
Ti is an important element in achieving both high strength and excellent workability in the steel sheet of the present invention. First, it acts as a hardenable element together with Mn, and has the effect of generating bainite in the center. The other is that the non-recrystallized austenite region can be expanded, and a large strain can be accumulated only in the surface layer portion in combination with a large shear strain generated in the surface layer portion during hot rolling. For this reason, the ferrite transformation of the surface layer part having a large cumulative strain can be promoted in the process after cooling and winding after hot rolling, and polygonal ferrite can be generated and softened. By these two actions, only the surface layer portion can be softened while having high strength, and a steel sheet excellent in shape freezing property and bending workability can be obtained. In order to exert such an effect, addition of 0.1% or more is necessary. However, if excessive addition is performed, the surface layer part also undergoes bainite transformation and a desired structure cannot be obtained. %.
[0025]
A suitable steel plate component includes the above basic components, and consists of the balance Fe and inevitable impurities , and further contains at least one kind of characteristic improving element from at least one of the following first to third groups as necessary. can do.
First group; Nb: 0.01 to 0.4%
Second group: Cr: 0.05-0.8%, Mo: 0.05-1.0%, B: 0.0005-0.01%
Group 3: Ca: 0.005% or less, Rare earth element: 0.005% or less
Nb is an element that exhibits the same effect as Ti, and the lower limit was made 0.01% in order to effectively exhibit this effect. On the other hand, if excessively added, the surface layer part also undergoes bainite transformation and a desired structure cannot be obtained as in the case of Ti, so the upper limit was made 0.4%.
[0027]
Cr, Mo, and B have the effect of improving the hardenability and stably generating bainite, and a lower limit is defined to effectively exhibit this effect. Moreover, when it adds excessively, since a surface layer part will carry out a bainite transformation and it will become impossible to obtain a desired structure | tissue, the upper limit was prescribed | regulated.
[0028]
Ca and rare earth elements have the effect of improving ductility, particularly stretch flangeability, through sulfide morphology control. The lower limit of the addition amount of these elements was determined from the minimum necessary amount that can effectively exhibit the effect, and the upper limit was determined from an economical point of view that the effect was saturated.
[0029]
Next, manufacturing conditions for the hot-rolled steel sheet of the present invention will be described.
First, regarding the heating temperature of the steel slab, these elements are added for the purpose of adding Ti and Nb, that is, to expand the non-recrystallized austenite region during hot rolling, and to promote the bainite transformation by exhibiting hardenability during cooling. Is required to be in a solid solution state as much as possible in the heating stage, and for this purpose, the heating temperature is preferably set to 1200 ° C. or higher.
[0030]
No particular restriction on the rough rolling, but for the finish rolling is required to be a finishing temperature in _three or more points Ar. Below this temperature, the processed structure remains and the workability deteriorates. On the other hand, in order to ferritize the surface layer portion of the steel sheet, at least the cumulative reduction amount is 15% in the non-recrystallized austenite region (particularly, it varies depending on the Ti amount, but the upper limit of the Ti amount of the present invention is about 980 ° C.). It is necessary to secure the above. By rolling in the non-recrystallized austenite region, strain accumulates on the surface portion of the rolled material that comes into contact with the roll, and this acts as a starting force for ferrite transformation to promote ferrite transformation and generate polygonal ferrite in the surface layer portion. By adjusting the cumulative reduction amount, the thickness of the region occupied by the polygonal ferrite structure, that is, the surface layer portion can be adjusted. However, when the finishing temperature exceeds 980 ° C., the non-recrystallized austenite region becomes narrow and it becomes difficult to ensure the accumulated strain in the surface portion. Therefore, the finishing temperature is set to Ar ₃ points or more, 980 ° C. or less, preferably 900 ° C. or less. It is good.
[0031]
The cooling rate after finish rolling needs to be 20 ° C./second or more. If it is less than 20 ° C./second, pearlite is generated, and it becomes difficult to obtain a sufficient amount of bainite of 60% or more, and desired strength and workability cannot be obtained. Although the upper limit of the cooling rate is not particularly limited, it is desirable that the cooling rate is 100 ° C./second or less for temperature control.
[0032]
The coiling temperature is preferably 350 ° C to 600 ° C. Control of the coiling temperature below 350 ° C is difficult to achieve with ordinary coiling equipment. On the other hand, if it exceeds 600 ° C, ferrite is also generated at the center, and the desired strength and workability can be ensured. It becomes difficult.
[0033]
EXAMPLES Hereinafter, although an Example demonstrates this invention further, this invention is not limited by these Examples.
[0034]
【Example】
A 45 mm-thick slab made of steel having the chemical components shown in Table 1 is heated to 1250 ° C. and hot-rolled to a thickness of 3.4 mm within the range of 850 ° C. to 1000 ° C. It cooled to 450 degreeC-650 degreeC of coiling equivalent temperature at -70 degreeC / second, and it furnace-cooled after hold | maintaining at that temperature for 30 minutes. In the invention example, the finishing temperature is set to 850 ° C. to 900 ° C., the cumulative reduction amount at 950 ° C. or less is secured about 15 to 45%, and immediately after the finish rolling is completed, the winding equivalent temperature is increased to 450 ° C. at 70 ° C./s. After cooling and holding at that temperature for 30 minutes, the furnace was cooled.
[0035]
[Table 1]

[0036]
A specimen is taken from the obtained hot-rolled steel sheet, the microstructure of the steel sheet cross section is observed with an optical microscope, and the region of polygonal ferrite constituting the surface layer part is 95% or more (average depth mm from the surface), In addition, a bainite (including bainitic ferrite) constituting the central portion is measured in a region where the area is 95% or more (area ratio% in the cross section), and the Vickers at a depth position of 0.2 mm from the steel plate surface and a central thickness position. Hardness (load 100 gf) was measured. These measurement results are shown in Table 2.
[0037]
Moreover, the JIS No. 5 tensile test, the shape freezing evaluation test, and the hole expansion test were performed using the obtained steel plate. These results are also shown in Table 2.
[0038]
As shown in FIG. 2, in the shape freezing evaluation test, the bottom wall portion 3 while constraining the flange portion 1 of the hat-shaped cross-section member (the numerical value between the arrows in the figure is dimension mm and the inner radius of the radius portion is 10 mm). Is formed by drawing and bending with a die punch, and the molded member is taken out of the mold, and as shown in FIG. 3, the bottom wall portion 3 is horizontally placed on a surface plate, and the upper surface of the flange portion 1 forms a horizontal plane. The angle θ was measured. This angle θ is a combination of a spring back generated between the bottom wall portion 3 and the side wall portion 2 and a spring back generated between the side wall portion 2 and the flange portion 1, and θ is less than 5 degrees. If there is, for example, there is no inconvenience in joining with a flat plate, the shape freezing property is good (◯), and θ of 5 degrees or more is evaluated as defective (×).
[0039]
In the hole expansion test, a punched hole having a diameter of 10 mm was formed in the test piece, a conical punch having an apex angle of 60 degrees was pushed into the initial hole (d ₀ = 10 mm), and the sample was expanded until the crack penetrated the plate thickness. When the hole diameter is d ₁ , the hole expansion ratio λ (%) calculated by the following equation is obtained.
λ = (d ₁ −d ₀ ) / d ₀ × 100
[0040]
[Table 2]

[0041]
From Table 2, the hot-rolled steel sheet of the present invention has a high strength of 590 N / mm ² or higher and is excellent in shape freezing property. Moreover, as for stretch flangeability, the value of TS (MPa) * λ (%) is 75000 or more, and it can be seen that the stretch flangeability is extremely excellent.
[0042]
【The invention's effect】
According to the hot-rolled steel sheet of the present invention, a hard layer having a surface layer portion having a predetermined thickness formed of a soft structure having 95% by area or more of polygonal ferrite and a center part of 60% or more having a bainite having a surface area of 95% by area or more. 590N / mm ² While having the above-mentioned high strength, it is excellent in workability, particularly shape freezing property and stretch flangeability.
[Brief description of the drawings]
FIG. 1 is a graph showing an example of hardness distribution in a cross section in the thickness direction of a hot-rolled steel sheet of the present invention.
FIG. 2 is a perspective view of a hat-shaped cross-sectional member used in a shape freezing evaluation test.
FIG. 3 is an explanatory diagram of a measuring procedure of θ showing quantitative data of shape freezing property.

Claims (4)

A central portion formed by a structure having an area ratio of 60% or more in the steel plate cross section and having a bainite of 95 area% or more, and polygonal ferrite of 95 area% or more coupled to both sides of the center portion. A high-strength hot-rolled steel sheet having excellent workability, wherein the surface layer part is formed at least in a region from both surfaces of the steel sheet to a depth of 0.2 mm.   2. The processing according to claim 1, wherein the hardness ratio HVs / HVa between the Vickers hardness (HVs) of the surface layer portion at a depth of 0.2 mm from the surface and the Vickers hardness (HVa) of the center portion at the thickness center is 0.9 or less. High strength hot-rolled steel sheet with excellent properties. C: 0.01 to 0.1% in terms of weight percent average composition in the thickness direction,
Si: 1% or less,
Mn: 1.0 to 2.5%
P: 0.1% or less,
S: 0.01% or less,
Al: 0.01 to 0.08%,
Ti: 0.1 to 0.4%
The high-strength hot-rolled steel sheet having excellent workability according to claim 1 or 2 , comprising the balance Fe and unavoidable impurities . The high-strength hot-rolled steel sheet having excellent workability according to claim 3, further comprising at least one component selected from at least one of the following first to fourth groups.
First group Nb: 0.1 to 0.4%
Second group Cr: 0.05-0.8%
Mo: 0.05-1.0%,
B: 0.0005 to 0.01%
Third group Ca: 0.005% or less,
Rare earth elements: 0.005% or less

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