JP3821073B2 - Hot-rolled material for hydroform forming steel pipe and method for producing the same - Google Patents

Description

【００３２】
【表２】

【００３３】
【表３】

【００３４】
なお、この実施例では、熱延素材が熱延ままの鋼板である場合を示したが、本発明は、これに限るものではなく、熱延素材が、熱延ままの鋼板を焼鈍した鋼板、この焼鈍した鋼板もしくは熱延ままの鋼板を酸洗した鋼板、この酸洗した鋼板に耐錆性を付与する表面処理を施した鋼板のうちのいずれである場合でも、同様の効果が得られることはいうまでもない。
【００３５】
【発明の効果】
本発明によれば、ｒ_L が高く、異方性が小さい、加えてｎ値も優れたハイドロフォーム用鋼管の熱延素材を提供できるようになるという効果を奏する。また、実際のハイドロフォーム加工では強軸押しモードだけではなく管端固定モードの成形加工要素も必要とされるが、本発明の熱延素材は、このような複合成形に適用できる鋼管の安定製造供給を可能にするので、例えば、自動車車体の製造コスト低減および衝突安全性向上に寄与する。
【図面の簡単な説明】
【図１】 LBR に及ぼすｒ_L 、Δｒの影響を示す図である。
【図２】 LBR(0)に及ぼすｎ値、ｒ_L の影響を示す図である。
【図３】Δｒに及ぼす冷却速度の影響を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-rolled material for a steel pipe for hydroforming and a method for producing the same. Here, the hot-rolled material is a hot-rolled steel plate, a steel plate that has been annealed, a steel plate that has been pickled either of the former two, and a surface treatment that imparts rust resistance to the pickled steel plate. It refers to one of the steel plates. The surface treatment refers to hot dip galvanization or electrogalvanization.
[0002]
[Prior art]
There has been a strong demand for collision safety of automobiles, and there has been a tendency for the weight of the vehicle body to increase due to the use of reinforcing members. Therefore, the weight of the vehicle body has been reduced by increasing the strength of the steel material and improving the part structure. Recently, new processing methods such as hydroform molding are being introduced to meet the demand for reducing the number of parts. This hydroform molding is expected to have advantages such as cost reduction and increased design freedom.
[0003]
When a material suitable for this hydroforming is required and hydroforming in the strong axial push mode is performed, the r value in the longitudinal direction of the steel pipe is required to be high. In the tube end fixing mode, a high n value is required. However, in a thick hot-rolled steel sheet used for automobile undercarriage parts, the r value (r _L ) in the rolling direction is the r value (r _D ) in the 45 ° direction from the rolling direction and the 90 ° direction from the rolling direction. Compared to the r value (r _C ) of the film, it is low and compared to the L direction, D direction, and C direction (0 degrees, 45 degrees, 90 degrees from the rolling direction), and exhibits an inverted V-shaped anisotropy.
[0004]
Patent Document 1 proposes a high-strength cold-rolled steel sheet having a specific chemical composition, tensile strength = 320 to 600 MPa, and r _L ≧ 1.3, and a steel pipe made from the same.
[0005]
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-279330 [0006]
[Problems to be solved by the invention]
However, as a material for a steel pipe for hydroforming, there are many required thickness materials of 1.4 mm or more. If a hot-rolled steel sheet can be applied, the cost can be further reduced as compared with a cold-rolled steel sheet. Since the existing hot-rolled steel sheet has a low r _L and a large Δr (= (r _L + r _C ) / 2−r _D ), it is particularly necessary to improve r _L. The r value in the longitudinal direction of the steel pipe can be increased by reducing the r value anisotropy. Further, in actual hydroforming, not only the strong shaft pressing mode but also the forming element in the tube end fixing mode is performed, so an n value is also required.
[0007]
In view of these points, the present invention provides a hot-rolling material for a steel tube for hydroforming, which has a high r _L , a small absolute value of Δr, that is, a small anisotropy, and an excellent n value, and a method for producing the same. The purpose is to provide.
[0008]
[Means for Solving the Problems]
The present invention has been completed as a result of intensive investigations on means for increasing the r _L of the hot-rolled material, decreasing the absolute value of Δr, and increasing the n value in order to obtain a hydroforming steel pipe excellent in formability. Yes, the summary is as follows.
(1) By mass%, C: 0.05 to 0.20%, Si: 0.001 to 0.80%, Mn: 0.30 to 1.8%, S: 0.01% or less, P: 0.02% or less, Al: 0.01 to 0.10%, O: 0.005 % Or less, N: 0.01% or less, consisting of remaining Fe and inevitable impurities, tensile strength of 370 to 610 MPa, r _L of 0.85 or more, and | Δr | of 0.10 or less Hot rolled material for forming steel pipe.
[0009]
(2) Further, by mass%, Cu: 0.01 to 0.3%, Ni: 0.01 to 0.3%, Mo: 0.01 to 0.2%, V: 0.01 to 0.05%, Ca: 0.0015 to 0.0040% The hot-rolling material for a steel pipe for hydroforming as set forth in (1), which contains the above.
(3) The hot-rolled material for a steel pipe for hydroforming as described in (1) or (2), wherein the n value is 0.18 or more.
[0010]
(4)% by mass, C: 0.05 to 0.20%, Si: 0.001 to 0.80%, Mn: 0.30 to 1.8%, S: 0.01% or less, P: 0.02% or less, Al: 0.01 to 0.10%, O: 0.005 N: 0.01% or less, or Cu: 0.01 to 0.3%, Ni: 0.01 to 0.3%, Mo: 0.01 to 0.2%, V: 0.01 to 0.05%, Ca: 0.0015 to 0.0040% Finish rolling with a total rolling reduction of 75% or more after finishing rolling, when slabs containing one or more of them, the balance Fe and unavoidable impurities are heated to 1000 ° C or higher and then hot rolled The heat of hydroforming steel pipe is characterized in that the end temperature is A ₃ (° C) + 50 ° C or higher, and after cooling for 1 second or longer after finishing rolling, the steel tube is forcedly cooled and wound at a cooling rate of 35 ° C / s or lower. A manufacturing method for rolled material.
[0011]
A ₃ = 910-310 * C-80 * Mn + 40 * Si-20 * Cu-50 * Ni-80 * Mo + 100 * V
The element symbol on the right side = component content in steel of the same element (mass%), * = product operator (5) The hydroforming temperature is set to 700 ° C or less for hydroforming A method of manufacturing a hot-rolled material for steel pipes.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the chemical composition will be described.
C: 0.05 to 0.20%
In order to obtain a tensile strength of 370 MPa or more, the C content needs to be 0.05% or more. Below this, it is difficult to ensure strength. On the other hand, if the C content exceeds 0.20%, the weldability and the r-value decrease significantly.
[0013]
Si: 0.001 to 0.80%
Si has a function of increasing steel strength by solid solution strengthening, and is added according to a desired tensile strength. It is difficult to add Si so that the amount of Si is less than 0.001% in terms of steelmaking technology. On the other hand, if the Si content exceeds 0.80%, a scale with a low melting point (such as firelite) is likely to be formed, and the surface appearance may be impaired. In addition, Preferably it is 0.50% or less.
[0014]
Mn: 0.30 to 1.8%
Mn is a solid solution strengthening element effective for increasing the strength, and the addition amount is changed according to the desired tensile strength. In order to prevent segregation of S grain boundaries due to MnS precipitation, the Mn content should be 0.30% or more. On the other hand, if the amount of Mn exceeds 1.8%, a strong transformation texture is formed after hot rolling, leading to a decrease in r _L and an increase in | Δr |. Preferably it is 0.50 to 1.0%.
[0015]
S: 0.01% or less If the amount of S exceeds 0.01%, there is a concern of causing grain boundary cracking during hot rolling. Preferably it is 0.006% or less.
P: 0.02% or less P adversely affects ERW weldability, and in particular reduces the fatigue strength of the weld. P is set to 0.02% or less from the viewpoint of preventing defects in the ERW weld.
[0016]
Al: 0.01-0.10%
Al is added as a deoxidizer at the time of melting, but from the viewpoint of ensuring deoxidation, the content of the remaining Al that is consumed in deoxidation and floats as an alumina oxide becomes 0.01% or more. It was supposed to be added. On the other hand, if the Al content exceeds 0.10%, there is a concern that defects in surface properties may occur, so the content was made 0.10% or less.
[0017]
O: 0.005% or less Since an increase in the oxide in steel may cause a burst during hydroforming, the O content is set to 0.005% or less.
N: 0.01% or less Excessive N lowers the moldability, so the N content is set to 0.01% or less. From the viewpoint of further improving the r value, the amount of solid solution N is preferably 30 ppm (meaning mass ppm, the same shall apply hereinafter) or less.
[0018]
The hot-rolled material of the present invention further includes Cu: 0.01 to 0.3%, Ni: 0.01 to 0.3%, Mo: 0.01 to 0.2%, V: 0.01 to 0.05%, Ca: 0.0015 to 0.0040% as necessary. It may be included. Cu, Ni, Mo, and V are effective elements for increasing the strength, and can be used without reducing r _L within the above range.
Further, Ca can be added in the above-described range, which has the effect of preventing the formation of oxide inclusions having a tendency toward coarse extensibility by forming Ca sulfide and improving the toughness and strength of the weld. If the Ca effect is less than 0.0015%, it is insufficient. On the other hand, if it exceeds 0.0040%, the effect is not only saturated but also the workability may be lowered.
[0019]
Next, the reasons for limiting the mechanical properties will be described.
In the hot-rolled material of the present invention, when the processing mode of the hydroforming steel pipe is the strong shaft pressing mode, the r value (corresponding to r _L of the hot-rolled material) is particularly high in the tube axis direction (matching the rolling direction) Therefore, r _L is restricted to 0.85 or more, and | Δr | is restricted to 0.10 or less in order to eliminate anisotropy. Thereby, hydroform moldability improves.
[0020]
In addition, actual parts are often molded in the tube end fixing mode in addition to the processing in the simple strong shaft pressing mode. However, in the present invention, when the n value is restricted to 0.18 or more, the hydroforming in the tube end fixing mode is performed. It is preferable because moldability is also improved.
In the hot rolled material of the present invention, it is more preferable that the average crystal grain size of ferrite exceeds 8 μm because the r value and the n value are further increased.
[0021]
In the present invention, an experiment in which r _L , Δr, or n value is determined as described above, and the results thereof will be described below.
Of the hot-rolled steel sheets having the chemical composition within the scope of the present invention as described above, steel pipes having different r _L , Δr and n values are used as raw materials, and a steel pipe having a wall thickness of 1.6 mm × outer diameter of 63.5 mm is changed in the direction of the pipe axis by the ERW method. A free bulge test was carried out in accordance with the rolling direction of the steel sheet. The free bulge test is a test in which both ends are constrained with a mold and the center is unconstrained, and internal pressure is applied from one end or both ends of the tube, and the center is expanded in the strong shaft push mode or tube end fixing mode. Say.
[0022]
FIG. 1 shows the effects of r _L and Δr on the limiting bulging rate LBR (Limiting Buldging Ratio) in the free bulge test. Here, LBR is defined by d / d0-1 (× 100%) (where d is the maximum pipe diameter that can be expanded without a burst, and d0 is the pipe diameter). Than 1, in order to increase the LBR is simply insufficient only increases r _L, r _L, specific range i.e. r _L ≧ 0.85 and a [Delta] r | [Delta] r | be a ≦ 0.10 It can be seen that it is important .
[0023]
FIG. 2 shows the influence of the n value and r _L on the limit tube expansion ratio LBR (0) without pipe end movement, which shows the formability of the pipe end fixed mode. Obviously, increasing both the n value and r _L increases LBR (0), which improves the moldability. In particular, it is good in the range of r _L ≧ 0.85 and n value ≧ 0.18. Thus, a steel pipe corresponding to both forming modes of the tube end fixing mode and the strong shaft pushing mode is obtained.
[0024]
With respect to the tensile strength (TS), if it is less than 370 MPa, a sufficient lightening effect cannot be obtained. On the other hand, if it exceeds 610 MPa, the ductility is lowered and hydroform molding becomes difficult, so 370 to 610 MPa.
Next, a manufacturing method will be described.
If the heating temperature of the slab is too low, the deformation resistance at the time of hot rolling will increase, or the initial structure will be inherited by hot rolling, so it will be 1000 ° C. or higher. The heating temperature is preferably set to 1350 ° C. or lower because the amount of surface scale generation increases if it is too high.
[0025]
In the present invention, the total rolling reduction after the finish rolling, the finish rolling end temperature, the cooling start timing and the cooling rate are important. Only when these conditions are combined and controlled, r _L ≧ 0.85, | Δr | ≦ 0.10, n ≧ 0.18 hot-rolled steel sheet can be manufactured stably.
Total rolling reduction after finishing rolling: The total rolling reduction (= cumulative rolling reduction) at the latter stage of the tandem rolling mill used for finishing rolling is 75% or more. Here, when the number of all passes of finish rolling is N, the latter stage refers to the (N + 1) / 2 pass to the final N pass when N is an odd number, and the N / 2 pass when N is an even number. To the final Nth pass. If the total reduction ratio in the subsequent stage is less than 75%, the austenite structure is not sufficiently homogenized, r _L is lowered, and | Δr | is increased.
[0026]
Finishing rolling finish temperature (FDT) and cooling start timing (Cst): FDT must be A ₃ (° C) + 50 ° C or more below, and Cst must be 1 second or more after the finish final pass (cooling during this time) is there.
A ₃ = 910-310 * C-80 * Mn + 40 * Si-20 * Cu-50 * Ni-80 * Mo + 100 * V
Element symbol on the right side = content of the same element in steel (mass%), * = product operator This is enough to recrystallize by increasing the FDT above the ferrite transformation start temperature and delaying the start of cooling. Is a means for suppressing the formation of a transformation texture by transforming ferrite from the austenite structure in which advancing, and this realizes a reduction in | Δr | and an increase in r _L. Furthermore, since ferrite transformation is performed from an austenite structure in which recrystallization has sufficiently progressed, the ferrite grains after the transformation become relatively coarse sized grains having an average crystal grain size exceeding 8 μm, and thus the n value increases.
[0027]
Cooling rate: As the cooling rate of forced cooling after hot rolling (hot ranking) increases, the average r value increases, but r _L decreases. As a result, for example, as shown in FIG. 3, Δr is much less than −0.10. Therefore, the cooling rate was controlled to 35 ° C./s or less. When the cooling rate exceeds 35 ° C./s, it is considered undesirable because compositional supercooling causes selectivity in the formation of ferrite transformation nuclei. In addition, if the cooling rate is too low, non-uniform coarsening occurs due to abnormal grain growth of ferrite grains, and there is a concern that the material may vary. Therefore, the cooling rate is preferably 10 ° C./s or more.
[0028]
Winding temperature: If the winding temperature exceeds 700 ° C, there is a risk of uneven coarsening due to abnormal grain growth of ferrite grains, resulting in material variations. Therefore, the winding temperature should be 700 ° C or less. preferable.
[0029]
【Example】
Steel having the chemical composition shown in Table 1 was melted and rolled under the hot rolling conditions shown in Table 2. Table 3 shows the mechanical properties of the obtained hot-rolled steel sheet (as-rolled steel sheet) together with the amount of solute N and the average ferrite crystal grain size. The mechanical properties of the hot-rolled steel sheet were evaluated by preparing JIS No. 5 tensile test pieces, and the r value was measured by 10% or 15% tension (if the uniform elongation is less than 15%, 10% (Tensed).
[0030]
As is apparent from Table 3, the inventive examples within the scope of the present invention show good r _L , Δr, and n values, and the comparative examples outside the scope of the present invention do not satisfy the characteristics.
In addition, an electric resistance welded steel pipe having a thickness of 1.6 mm × outer diameter of 63.5 mmφ (the rolling direction of the material steel sheet coincides with the pipe axis direction) was produced from these hot-rolled steel sheets, and a free bulge test was performed. In the free bulge test, the critical expansion ratio LBR in the strong shaft push mode and the LBR (0) in the tube end fixed mode were determined and are shown together in Table 3. The inventive examples showed good hydroform moldability in both modes.
[0031]
[Table 1]

[0032]
[Table 2]

[0033]
[Table 3]

[0034]
In addition, in this example, the case where the hot-rolled material is a hot-rolled steel plate is shown, but the present invention is not limited thereto, and the hot-rolled material is a steel plate obtained by annealing a hot-rolled steel plate, The same effect can be obtained in any of the annealed steel sheet or the steel sheet obtained by pickling a hot-rolled steel sheet and the steel sheet subjected to surface treatment for imparting rust resistance to the pickled steel sheet. Needless to say.
[0035]
【The invention's effect】
According to the present invention, there is an effect that it is possible to provide a hot-rolled raw material for a steel pipe for hydroforming that has a high r _L , a small anisotropy, and an excellent n value. In addition, the actual hydroforming requires not only the strong shaft push mode but also the tube end fixed mode forming element. However, the hot rolled material of the present invention is a stable production of steel pipe that can be applied to such composite forming. Since supply is enabled, it contributes to, for example, reducing the manufacturing cost of automobile bodies and improving collision safety.
[Brief description of the drawings]
FIG. 1 is a diagram showing the influence of r _L and Δr on LBR.
FIG. 2 is a diagram showing the influence of n value and r _L on LBR (0).
FIG. 3 is a diagram showing the influence of the cooling rate on Δr.

Claims (5)

% By mass, C: 0.05 to 0.20%, Si: 0.001 to 0.80%, Mn: 0.30 to 1.8%, S: 0.01% or less, P: 0.02% or less, Al: 0.01 to 0.10%, O: 0.005% or less, N: 0.01% or less, comprising the remainder Fe and inevitable impurities, tensile strength of 370 to 610 MPa, r _L of 0.85 or more, and | Δr | Hot rolled material. Furthermore, by mass%, Cu: 0.01 to 0.3%, Ni: 0.01 to 0.3%, Mo: 0.01 to 0.2%, V: 0.01 to 0.05%, Ca: 0.0015 to 0.0040%, or one or more of them The hot-rolling material for a steel pipe for forming a hydroform according to claim 1. The hot rolled material for a steel pipe for hydroforming according to claim 1 or 2, wherein the n value is 0.18 or more. % By mass, C: 0.05 to 0.20%, Si: 0.001 to 0.80%, Mn: 0.30 to 1.8%, S: 0.01% or less, P: 0.02% or less, Al: 0.01 to 0.10%, O: 0.005% or less, N: 0.01% or less, or Cu: 0.01-0.3%, Ni: 0.01-0.3%, Mo: 0.01-0.2%, V: 0.01-0.05%, Ca: 0.0015-0.0040% Alternatively, when the slab containing two or more types, the balance Fe and inevitable impurities is heated to 1000 ° C. or higher and then hot rolled, the total rolling reduction after the finish rolling is 75% or more, and the finish rolling finish temperature is A hot rolled material of a steel pipe for hydroforming, characterized by the following A ₃ (° C.) + 50 ° C. or higher, cooling for 1 second or more after finishing rolling and then forcibly cooling and winding at a cooling rate of 35 ° C./s or less. Production method.
A ₃ = 910-310 * C-80 * Mn + 40 * Si-20 * Cu-50 * Ni-80 * Mo + 100 * V
Element symbol on right side = component content in steel of same element (mass%), * = product operator The method for producing a hot-rolled raw material for a steel pipe for hydroforming according to claim 4, wherein the winding temperature is 700 ° C or lower.

Images