JP2010255060A - High strength cold-rolled steel sheet excellent in die-galling resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength cold-rolled steel sheet having excellent die-galling resistance. <P>SOLUTION: The high strength cold-rolled steel sheet has a composition containing 0.05 to 0.30 mass% of C, 0.2 to 2.0 mass% of Si, 0.3 to 2.5 mass% of Mn, 0.01 to 0.1 mass% of solution Al, not more than 0.1 mass% of P and the balance comprising iron and inevitable impurities, and shows a tensile strength of not less than 340 MPa. The steel sheet has a maximum depth (Ry) in the surface roughness of 2 to 8 μm, an average interval (Sm) of the surface roughness of 35 to 100 μm, a load length rate (tp40) of the surface roughness of not less than 25%, and a load length rate (tp60) of the surface roughness of not more than 80%. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a high-strength cold-rolled steel sheet excellent in mold galling resistance in press forming.

  Cold-rolled steel sheets are generally processed into a desired shape by press molding or the like, and are widely used for automobiles, home appliances, and the like. In the case of press forming cold-rolled steel sheets, particularly when a large amount of press forming is performed continuously, “galling” between the press-molding mold and the cold-rolled steel sheets may become a problem as the number of presses increases. In particular, in recent years, high-tensile steel sheets have been widely used to reduce the weight of components. In forming high-strength steel sheets, the contact surface pressure between the steel sheet and the mold becomes high, so high-pressure sliding is considered. Measures against mold galling are becoming more important. It is considered that galling is an adhesion reaction between the mold and the steel sheet, and in order to suppress this, conventionally, a method of controlling the material or surface geometry of the steel sheet and mold, and control of the oxide film on the surface of the steel sheet are performed. Methods, optimization of lubricating oil viscosity, work hardening of the steel sheet surface, and the like have been proposed.

  Among these, since the technology for controlling the geometric shape of the steel sheet surface does not impair the original formability of the steel sheet and does not require an additional process in production, various studies have been conducted conventionally. . For example, Patent Document 1 discloses a method of controlling the convex area ratio in consideration of mold galling resistance. Patent Document 2 discloses a method for controlling the surface roughness according to the yield stress in consideration of mold galling resistance. Patent Documents 3 to 10 disclose a method of controlling the surface form in consideration of mold galling resistance.

JP-A-2-163344 JP-A-2-163345 Japanese Patent Laid-Open No. 5-261401 JP-A-6-218403 JP-A-6-87001 Japanese Patent Laid-Open No. 6-87002 Japanese Patent Laid-Open No. 6-87003 JP-A-6-91305 JP-A-6-116745 JP-A-9-29304

However, these conventional techniques are exclusively intended for mild steel, and such techniques cannot effectively suppress die squeezing in press working performed at a high surface pressure such as a high-tensile steel plate.
Therefore, an object of the present invention is to provide a high-strength cold-rolled steel sheet that solves the above-described problems of the prior art and exhibits excellent mold galling resistance.

The gist of the present invention for solving the above problems is as follows.
[1] C: 0.05 to 0.30 mass%, Si: 0.2 to 2.0 mass%, Mn: 0.3 to 2.5 mass%, Sol. A high-strength cold-rolled steel sheet containing Al: 0.01 to 0.1% by mass, P: 0.1% by mass or less, the balance being a component composition composed of iron and inevitable impurities, and a tensile strength of 340 MPa or more Because
The maximum depth (Ry) of the surface unevenness of the steel sheet is 2 to 8 μm, the average interval (Sm) of the surface unevenness is 35 to 100 μm, the load length ratio (tp40) of the surface unevenness is 25% or more, the load length of the surface unevenness A high-strength cold-rolled steel sheet excellent in mold galling resistance, characterized in that the rate (tp60) is 80% or less.

[2] In the high-strength cold-rolled steel sheet of [1], Mo: 0.5% by mass or less, Cr: 1.0% by mass or less, Ti: 0.2% by mass or less, Nb: 0.1% by mass %: V: 0.1% by mass or less, Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, B: 0.002% by mass or less, Ca: 0.005% by mass or less A high-strength cold-rolled steel sheet excellent in mold galling resistance, characterized by containing one or more selected.
[3] In the high-strength cold-rolled steel sheet according to [1] or [2], the mass ratio of the Si content to the Mn content is Si / Mn> 0.4, which is excellent in resistance to mold galling. Strength cold-rolled steel sheet.

  The high-strength cold-rolled steel sheet of the present invention has excellent mold galling resistance by optimizing the chemical composition of steel and the surface shape of the steel sheet.

Drawing for explaining the concept of the maximum depth (Ry) of surface irregularities Drawing for explaining the concept of average interval (Sm) of surface irregularities Drawing for explaining concepts of load length ratio (tp40) and load length ratio (tp60) of surface irregularities

First, the reasons for limiting the component composition of the high-strength cold-rolled steel sheet of the present invention will be described.
C: 0.05-0.30 mass%
C is an important element for increasing the strength of the steel sheet. If it is less than 0.05% by mass, its effect is insufficient and the desired strength cannot be obtained stably. On the other hand, if it exceeds 0.30% by mass, a welded portion such as spot welding becomes brittle. For this reason, the amount of C is made 0.05 to 0.30 mass%.

-Si: 0.2-2.0 mass%
Si forms a Si oxide on the surface of the steel sheet, suppresses adhesion between the mold and the steel sheet, and improves mold galling resistance. If the amount is less than 0.2% by mass, the effect is not sufficient. On the other hand, if the amount exceeds 2.0% by mass, the effect is not only saturated, but also deformation resistance in rolling increases, and the production yield decreases. For this reason, Si amount shall be 0.2-2.0 mass%.
Mn: 0.3 to 2.5% by mass
Mn is an element effective in increasing the strength of the steel sheet, and if it is less than 0.3% by mass, its effect is not sufficient. On the other hand, if it exceeds 2.5% by mass, the workability is remarkably impaired. For this reason, the amount of Mn shall be 0.3-2.5 mass%.

-Sol. Al: 0.01 to 0.1% by mass
Al is added as a deoxidizer. If it is less than 0.01% by mass, the effect is not sufficient. On the other hand, if it exceeds 0.1% by mass, the effect is saturated and uneconomical. For this reason, Sol. The amount of Al is 0.01 to 0.1% by mass.
-P: 0.1 mass% or less P is contained as an impurity, but is added as necessary to further increase the strength. However, if it exceeds 0.1% by mass, the strength of the welded portion such as spot welding is significantly deteriorated, so the P content is 0.1% by mass or less.

  The cold-rolled steel sheet of the present invention can contain one or more selected from Mo, Cr, Ti, Nb, V, Cu, Ni, B, and Ca, if necessary. Although these elements have an effect of increasing the strength, if the content is too large, the ductility and surface properties are deteriorated, so Mo is 0.5% by mass or less, Cr is 1.0% by mass or less, and Ti is 0.2%. Mass% or less, Nb is 0.1 mass% or less, V is 0.1 mass% or less, Cu is 1.0 mass% or less, Ni is 1.0 mass% or less, B is 0.002 mass% or less, Ca Is preferably contained in each range of 0.005 mass% or less.

・ Si / Mn> 0.4
The mass ratio between the Si amount and the Mn amount is preferably Si / Mn> 0.4, which can further improve mold galling resistance. The reason for this is that, by setting Si / Mn> 0.4, an oxide of Si alone is more easily generated as an oxide film formed on the surface of the steel sheet in the annealing process than a complex oxide of Si and Mn. This is probably because of this.
The balance of the component composition of the high-strength cold-rolled steel sheet of the present invention is iron and inevitable impurity elements. In addition, a small amount of other elements may be contained as long as the above-described effects of the component elements are not impaired.

Next, the geometric shape of the surface of the high-strength cold-rolled steel sheet of the present invention will be described.
The high-strength cold-rolled steel sheet of the present invention has a maximum depth (Ry) of surface unevenness of the steel sheet of 2 to 8 μm, an average interval (Sm) of surface unevenness of 35 to 100 μm, and a load length ratio (tp40) of surface unevenness. The load length ratio (tp60) of the surface irregularities is 25% or more and 80% or less. By adopting such a geometric shape of the steel sheet surface, excellent mold galling resistance can be obtained.

  Here, the maximum depth (Ry) of the surface irregularities is the distance between the highest peak (Rt) and the deepest valley bottom (Rb) of the surface roughness curve as shown in FIG. Further, as shown in FIG. 2, the average interval (Sm) of the surface irregularities is the interval from the change point to the next change point (S1, The average value of S2... Sn). Further, the load length ratio (tp) of the surface unevenness is the cut portion length (l1, l2... Ln) when the surface roughness curve is cut at a certain cutting line level (p) as shown in FIG. ) Is expressed as a percentage of the measured length (L), and the cutting line level (p) is the highest peak (Rt) is 0 (zero) (tp0), the deepest valley (Rb) Is represented by 100 (tp100). And what expressed the said cutting part length (l1 + l2 + l3 + ... ln) in percentage with respect to measurement length (L) when cutting line level (p) is respectively "40" and "60", The load length ratio (tp40) and the load length ratio (tp60).

・ Maximum depth of surface irregularities (Ry): 2 to 8 μm
If Ry is less than 2 μm, sufficient lubricating oil cannot be retained on the surface of the steel sheet during press forming, so that the steel sheet and the mold are likely to adhere to each other, and the resistance to mold galling under high surface pressure deteriorates. On the other hand, when Ry exceeds 8 μm, the appearance quality of the steel sheet surface deteriorates and becomes unsuitable for automobile parts, so Ry is set to 2 to 8 μm.
-Average spacing of surface irregularities (Sm): 35-100 μm
If the Sm is less than 35 μm, the unevenness interval is too short and the unevenness becomes steep, and the strength of the convex part is deformed due to insufficient strength when sliding at a high surface pressure in press working, so that it becomes impossible to hold the lubricating oil, This causes adhesion between the steel plate and the mold. On the other hand, if it exceeds 100 μm, the area of the convex part becomes too large, and the supply of lubricating oil from the concave part becomes insufficient during press deformation, so that the mold galling resistance deteriorates.

・ Load length ratio of surface irregularities (tp40): 25% or more tp40 ≧ 25% means that there is a relatively large area (area) of protrusions protruding on the surface, Even during sliding under pressure, the surface is deformed and is difficult to flatten, and adhesion with the mold can be suppressed.
-Load length ratio of surface irregularities (tp60): 80% or less tp60 ≦ 80% means that there is a relatively large area (area) of a concave portion recessed on the surface, thereby By improving the holding property, adhesion to the mold can be suppressed.

  The means for obtaining the geometrical shape of the steel sheet surface as described above is not particularly defined. For example, when surface-adjusting a skin-pass rolling roll with a discharge dull, the processing current is increased and the surface roughness of the roll is Ra ≧ 5 μm. To do. Using such a skin pass rolling roll, it has been confirmed that the geometric shape can be achieved by performing skin pass rolling a plurality of times at a low elongation rate of 0.3% or less. Moreover, you may provide the above geometric shapes to the steel plate surface in a cold rolling process.

  The manufacturing method of the high-strength cold-rolled steel sheet of the present invention is not particularly defined, but after normal continuous casting or ingot forming, hot rolling, cold rolling, annealing process, and further each skin pass rolling process performed as necessary. Can be manufactured. When high strength is desired, it is effective to use a continuous annealing facility in the annealing process, and in particular to perform jet water quenching with a high cooling rate. You may perform the pickling for surface adjustment after annealing.

  The cold-rolled steel sheet of the present invention is intended for a high-strength material of 340 MPa class or higher, in particular, which has a problem of mold galling resistance at a high surface pressure. It is particularly useful for strength materials and is particularly suitable for automotive structural parts. Examples of automotive structural parts include front and rear side members and crash parts such as crash boxes, pillars such as center pillar reinforcements, body structure parts such as roof rail reinforcements, side sills, floor members, and kick parts. Can be mentioned.

  Steel having the component composition shown in Table 1 (where S and N contents are equivalent to impurities) was melted and cast into slabs. This slab was hot-rolled to a thickness of 2.8 mm, pickled, and then cold-rolled to a thickness of 1.4 mm. The steel plate was subjected to annealing, pickling, and skin pass rolling in order to obtain a product steel plate (cold rolled steel plate). Table 2 shows hot rolling conditions, annealing conditions, pickling methods, and skin pass conditions in this manufacturing process. In Table 2, the skin pass stretching rate is the total value for the skin pass rolling performed a plurality of times (skin pass stretching rate / each time is equal). In some examples, pickling and / or skin pass rolling was not performed.

The cooling method after annealing in continuous annealing was selected from jet water quenching, roll cooling, and gas cooling. In the case of jet water quenching, it was once cooled to the water temperature, reheated to a predetermined temperature, and held at the holding temperatures shown in Table 2. In the case of other cooling methods, it was cooled to the holding temperature shown in Table 2 and held as it was.
Moreover, as shown in Table 3, the surface geometric shape was imparted to the steel plate by skin pass rolling or cold rolling.
Table 2 shows the measurement results (metal phase area fraction) of the mechanical properties and cross-sectional metal structure of the obtained cold-rolled steel sheet, and Table 3 shows the measurement / evaluation results of the surface shape and anti-galling resistance.

The cross-sectional metal structure was obtained by corroding the longitudinal section of the test steel plate with nital, and then observing the structure at a magnification of 1000 using a scanning electron microscope to obtain the structure identification and area ratio. The volume fraction of retained austenite was determined by X-ray diffraction (XRD).
In addition, the surface of the obtained cold-rolled steel sheet was observed with a 50 × objective lens with a laser microscope (model number “VK-8550” manufactured by Keyence Corporation). By scanning an area of .3 mm × 0.2 mm, the average spacing (Sm), maximum depth (Ry), load length ratio (tp40) and (tp60) of surface irregularities were determined.
The evaluation of mold galling resistance was performed using a SKD11 mold having the same shape as the flat plate sliding device disclosed in JP 2005-240148 A, applying a load with a surface pressure of 50 kgf / mm ² and sliding at a sliding distance of 100 mm. The dynamic test was repeated, and the number of sliding times until galling occurred was measured. In this test, it was judged that the number of sliding times until the occurrence of galling was 20 times or more was good.

  According to Table 3, the examples of the present invention show good mold galling resistance. In contrast, No. 2 comparative example in which the steel components do not satisfy the present invention conditions, Ry in No. 5 and 6 comparative examples in which the present invention conditions are not satisfied, and Sm in which No. does not satisfy the present invention conditions. 9, No. 10 comparative example, tp40 or tp60 No. 16 and No. 17 comparative examples where the conditions of the present invention are not satisfied, both galling occurs with a small number of sliding times, and the mold galling resistance is poor. Yes.

Claims (3)

C: 0.05-0.30 mass%, Si: 0.2-2.0 mass%, Mn: 0.3-2.5 mass%, Sol. A high-strength cold-rolled steel sheet containing Al: 0.01 to 0.1% by mass, P: 0.1% by mass or less, the balance being a component composition composed of iron and inevitable impurities, and a tensile strength of 340 MPa or more Because
The maximum depth (Ry) of the surface unevenness of the steel sheet is 2 to 8 μm, the average interval (Sm) of the surface unevenness is 35 to 100 μm, the load length ratio (tp40) of the surface unevenness is 25% or more, the load length of the surface unevenness A high-strength cold-rolled steel sheet excellent in mold galling resistance, characterized in that the rate (tp60) is 80% or less.   Furthermore, Mo: 0.5 mass% or less, Cr: 1.0 mass% or less, Ti: 0.2 mass% or less, Nb: 0.1 mass% or less, V: 0.1 mass% or less, Cu: 1 0.0% by mass or less, Ni: 1.0% by mass or less, B: 0.002% by mass or less, Ca: 0.005% by mass or less selected from one or more types The high-strength cold-rolled steel sheet excellent in mold galling resistance according to claim 1.   The high-strength cold-rolled steel sheet having excellent mold galling resistance according to claim 1 or 2, wherein the mass ratio of the Si content and the Mn content is Si / Mn> 0.4.

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