JP5220343B2 - Ultra-high strength steel plate and automotive strength parts using the same - Google Patents

Description

  The present invention relates to an ultra-high-strength steel sheet and an automotive strength part using the same, and more specifically, an ultra-high-strength steel sheet excellent in formability and delayed fracture resistance suitable for automobile parts and the like, and an automobile using the same. It relates to strength parts.

  In recent years, from the viewpoint of reducing the weight of the vehicle body in order to achieve both car crash safety and environmental issues, it will be applied to parts that require complex press molding, such as front side members, rear side members, lockers, and pillars. Attempts to improve formability in ultra-high strength steel sheets are eagerly desired.

Conventionally, for ultra-high-strength steel sheets, material strength can be secured by various strengthening measures, but formability tends to decrease with increasing strength. In other words, in high-strength steel sheets, due to structural inhomogeneity, local mixing of hard and soft phases, workability decreases greatly with increasing strength, and it is difficult to achieve both higher strength and formability. There was a real situation.
Further, it is known that when the strength becomes 1180 MPa or more, a new harmful effect of delayed fracture due to hydrogen embrittlement occurs.

Against this background, TRIP (Transformation Induced Plasticity) steel sheets have attracted attention as high-strength steel sheets with excellent formability.
A TRIP steel sheet is a steel sheet in which retained austenite is induced and transformed into martensite by processing deformation to obtain a large elongation.
However, it has been reported that delayed fracture is also promoted in TRIP steel sheets due to work-induced transformation of retained austenite (see Non-Patent Document 1, for example).
Yamazaki et al., “Effects of retained austenite and strain on delayed fracture properties of ultra-high strength cold-rolled steel sheets”, Iron and Steel, 1997, Vol 83, No. 11, p66-71

As for delayed fracture characteristics, a high-strength steel sheet having improved delayed fracture resistance by forming precipitates such as niobium (Nb) has been proposed (see, for example, Patent Document 1).
However, no knowledge about formability is described, and there is an urgent need for both delayed fracture resistance and formability in ultra-high strength steel sheets.
JP 2005-68548 A

  On the other hand, the inventors of the present invention have found a method capable of achieving both formability and delayed fracture resistance when sufficiently considering the characteristics of an ultra-high-strength steel sheet as a strength component for automobiles.

  The present invention has been made in view of the problems and new knowledge of the prior art, and the object of the present invention is an ultra-high-strength steel sheet having excellent formability and delayed fracture resistance, and the same. An object of the present invention is to provide automotive strength parts using

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by making the base structure of a predetermined steel plate a lower bainite structure and reducing the prior austenite grain size. The present invention has been completed.

That is, the ultra-high-strength steel sheet of the present invention is, by mass ratio, C: 0.10% to 0.40%, Si: 0.01% to 2.5%, Mn: 0.1% to 1.0% , P: ≦ 0.02%, S : ≦ 0.013%, Cu: 0.05% ~3.0%, Ni: 0.05% ~3.0%, Cr: 0.01% ~3. 5%, containing Mo: 0.10% to 2.0%, the balance being Fe and inevitable impurities, the base structure is the lower bainite, and the average prior austenite particle size is 3 to 10 μm , The tensile strength is 980 MPa or more , and the stress reduction degree (SD), which is the difference between the tensile strength and the rupture stress by the tensile test using a plate-like test piece, is 180 MPa or more.

Moreover, the other suitable form of the ultra-high-strength steel sheet of the present invention further includes aluminum (Al) or niobium (Nb) in the steel, Al: 0.001% to 0.1%, Nb: 0.005% to and Rukoto be contained at 1.0% range, characterized in that it is a hot-rolled steel sheet or cold-rolled steel sheet.

Furthermore, the automotive strength component of the present invention is characterized by using the above ultra-high strength steel plate.

  Because the base structure of the specified steel sheet is the lower bainite structure and the prior austenite grain size is made finer, ultra-high strength steel sheets that have excellent formability and delayed fracture resistance, and automotive strength parts using the same Can provide.

  Hereinafter, the ultra high strength steel sheet of the present invention will be described in more detail. In the present specification and claims, “%” for concentration, content, filling amount and the like represents a mass percentage unless otherwise specified.

As described above, the ultra-high-strength steel sheet according to the present invention contains copper (Cu), nickel (Ni), chromium (Cr), and molybdenum (Mo). The base organization shall be lower bainite. Further, the prior austenite grain size is 3 to 10 μm .

  With such a configuration, while exhibiting a tensile strength of 980 MPa or more, the conventional high-strength steel sheet exhibits sufficient formability to satisfy the requirements as automotive parts, and the delayed fracture resistance is improved. By achieving both formability and delayed fracture resistance, excellent effects are sufficiently exhibited, and industrially useful effects are achieved.

  Here, in the ultra-high-strength steel sheet of the present invention, the tensile strength of the steel sheet becomes 980 MPa or more by using the lower bainite structure which is a hard phase as a base. More preferably, the tensile strength of the steel sheet is 1180 MPa or more.

  The prior austenite particle size can be reduced to 1-30 μm. When the prior austenite particle size exceeds 30 μm, improvement in deep drawability, stretchability, and shape freezing property cannot be expected. Further, when the prior austenite particle size is less than 1 μm, the mechanical properties are likely to deteriorate, and the production tends to be difficult.

  Further, the prior austenite particle size is preferably 3 to 10 μm. At this time, since the deep drawability, the stretchability, and the shape freezing property can be further improved, it is possible to satisfy the formability required when molding an automobile part using the ultra-high strength steel sheet.

In addition, the ultra-high strength steel sheet of the present invention, as an additive component,
Carbon (C) and chromium (Cr) are contained in a proportion of C: 0.10% to 0.40%, Cr: 0.01% to 3.5%, and silicon (Si) and manganese (Mn) are contained. , Si: 0.01% to 2.5%, Mn: 0.1% to 1.0%,
Copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), Cu: 0.05% to 3.0%, Ni: 0.05% to 3.0%, Mo: 0.10 % To 2.0%,
Further, phosphorus (P) and sulfur (S) as impurities are contained in a ratio of P: ≦ 0.02%, S: ≦ 0.013 %,
The balance is iron (Fe) and inevitable impurities.

At this time, the steel alloy which is excellent in delayed fracture resistance is ensured, ensuring a moldability by containing a fine alloy carbide.
Each component will be described below.

C: C is the most effective element for increasing the strength. In order to obtain a strength of 980 MPa or more, it is preferable to contain 0.1% or more. However, if it exceeds 0.4%, it tends to cause toughness deterioration, so 0.10 to 0.40% is contained. good.

Cr: Cr is an element effective for improving the hardenability, and is an element effective for increasing the strength of the steel sheet by solid solution in cementite. Therefore, it is preferable to contain at least 0.01%. The content is preferably 1% or more, but if added excessively, the effect is saturated and the toughness is lowered, so the upper limit is preferably made 3.5%.

Mo: Mo is an important element in the present invention. In addition to improving hardenability, Mo is effective for fine graining by forming alloy carbide, and also effective for hydrogen replacement. However, if it is less than 0.10%, formation of alloy carbide tends to be difficult. On the other hand, since Mo is an expensive alloy element, it is preferable to contain 0.1 to 2.0%.

In addition, in order to ensure good delayed fracture resistance while securing formability, Nb that exhibits the same effect as Mo can be added, but the range is 0.005% to 1.0%. That is good.

P: P is an element to be removed as much as possible in order to reduce the grain boundary strength, and the upper limit is preferably made 0.02%.

S: S is an element to be removed as much as possible in order to reduce the grain boundary strength, and the upper limit is preferably made 0.013 %.

Furthermore, the ultra high strength steel sheet of the present invention includes both copper (Cu) and nickel (Ni) as additive components, Cu: 0.05 % to 3.0%, Ni: 0.05 % to 3.0. It is good to contain in the ratio of%.
Each component will be described below.

Cu: Since Cu is effective for strengthening and its fine precipitation contributes to suppression of delayed fracture, it is preferable to contain 0.05 % or more. Moreover, since excessive addition causes deterioration of workability, the upper limit is preferably set to 3.0%.

Ni: Ni is an element effective in improving the corrosion resistance while ensuring the strength of the steel sheet by enhancing the hardenability of the steel sheet. If it is less than 0.05 %, the desired effect cannot be obtained. On the other hand, if it exceeds 3.0%, the workability deteriorates, so 0.05 to 3.0% is preferable.

Furthermore, the ultra-high-strength steel sheet of the present invention contains both silicon (Si) and manganese (Mn) as additive components, Si: 0.01% to 2.5%, Mn: 0.1% to 1. It is good to contain in the ratio of 0%.
Each component will be described below.

Si: Si is an element effective for deoxidation and strength increase. Therefore, it is preferable to make the content 0.2% or more including those added as a deoxidizer and remaining in the steel. However, since excessive addition may cause toughness deterioration, the upper limit is preferably 2.5%.

Mn: Mn is an element effective for increasing the strength of the steel sheet. If it is less than 0.1%, it is difficult to obtain a desired effect. On the other hand, when the content is too large, not only co-segregation of P and S is promoted, but also toughness deterioration may be caused.

  Moreover, the ultra-high-strength steel sheet of the present invention preferably contains aluminum (Al) as an additive component at a ratio of Al: 0.001% to 0.1%.

Al: Al is added for deoxidation, but if the addition amount is too large, inclusions increase and workability deteriorates, so 0.001 to 0.1% is preferably contained.

Since the ultrahigh strength steel sheet of the present invention described above has good formability, it can be typically made of a hot rolled steel sheet or a cold rolled steel sheet. The thickness of a typical ultra-high strength steel sheet is 0.5 to 2.3 mm.
Moreover, the surface treatment of galvanization can be performed from a viewpoint of component design.
Similarly, a film laminating process can also be performed.

Next, the automotive strength component of the present invention will be described.
Such an automotive strength component is formed using the above-described high-strength thin steel sheet. As a result, a high-strength part for automobiles having excellent moldability and delayed fracture resistance can be obtained.
Specifically, the high-strength thin steel sheet can be formed by any one of press forming (cold, warm, hot), hydroforming, and blow molding.
Normally, the pierced and trimmed parts have a high residual stress and the risk of delayed fracture is high, but the strength parts for automobiles of the present invention have few delayed fractures even when they have a cut part. It is valid.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.

(Examples 1-5, Comparative Examples 1-6)
Using the steels having the components shown in Table 1, steel plates of each example were produced under the manufacturing conditions shown in Table 2.
For each steel plate, tensile strength, SD (stress reduction after uniform elongation), structure, forming evaluation, and delayed fracture evaluation were performed. These test results are shown in Table 3.
Each characteristic was evaluated in the following manner. As the steels of Comparative Examples 3 to 6 (E, F, G, H), commercially available products were used.

1. Mechanical property value (1) Tensile strength Tensile strength was evaluated by conducting a tensile test in accordance with JIS Z2241 using No. 5 test piece of JIS Z2201.
(2) Degree of stress reduction (SD)
FIG. 1 is a schematic diagram showing a stress-strain diagram by a tensile test using a plate-like test piece (for example, a No. 5 test piece or a No. 13 test piece defined in JIS Z 2201). The difference between the tensile strength (TS) and the breaking stress is defined as the degree of stress reduction (SD).
Those having a stress reduction degree (SD) of 180 MPa or more had good toughness.

2. Tissue (1) Base Tissue The base tissue was evaluated by polishing the cross section, etching with a nital solution, and performing optical microscope 100 to 1000 times and SEM observation 1000 to 5000 times.

(2) Old austenite particle size The prior austenite particle size was evaluated in accordance with JIS G0551 for the base structure having the lower bainite.

3. Formability The formability evaluation method was evaluated by comprehensive evaluation of deep drawability evaluation, stretchability evaluation, and shape freezeability evaluation, with application to automotive parts that require complex press molding in mind. Each formability evaluation method was carried out in the following manner.

(1) Deep drawability evaluation FIG. 2 shows an outline of the deep draw test. Using a test tool composed of a cylindrical punch 4 having a punch shoulder radius of 5 mm and a diameter of 50 mm and a die 1 having a die shoulder radius of 7 mm and a wrinkle presser 2, a pressure of 50 kN is applied to the wrinkle presser 2 and 3 mm / second. The punch 4 was moved at a speed of
At this time, the blank diameter of the test piece 3 was increased, and the blank diameter that could be squeezed without breaking was designated as the maximum blank diameter D. The ratio of punch diameter to maximum blank diameter (D / 50) was defined as LDR. The larger this value, the better the deep drawability.

(2) Overhang property evaluation FIG. 3 shows an outline of the deep drawing test. Using a test tool comprised of a ball head punch 4 with a radius of 50 mm, a die 1 with a bead with a die shoulder radius of 5 mm, and a wrinkle retainer 2, a high pressure is applied to the wrinkle retainer 2, and no material flows in from the surroundings. The punch 4 was moved at a speed of / min. The dimension of the test piece 3 was 200 mm × 200 mm, and the moving distance from when the punch 4 contacted the test piece 3 to immediately before breaking was defined as the maximum molding height (LDH). The larger this value, the better the overhanging property.

(3) Shape Freezing Evaluation FIG. 4 shows an outline of a hat bending test for evaluating a shape freezing index. Using a test tool composed of a punch 4 having a width of 75 mm and a punch shoulder radius of 5 mm, and a die 1 and a wrinkle presser 2 having a die shoulder radius of 5 mm, a pressurizing force of 200 kN is applied to the wrinkle presser 2 at a speed of 10 mm / min. 4 was moved 80 mm. The dimension of the test piece 3 was 300 mm × 50 mm. The test piece 3 after the hat bending molding was taken out from the testing machine, and the curvature of the test piece 3 was measured by the method shown in FIG. The smaller this value, the better the shape freezing property.

4). Delayed Fracture Delayed Fracture Test Evaluation Method is a test method in which a 100 mm × 50 mm strip test piece is bent with a hat bending tester, bent back, and then subjected to piercing on the wall portion to give a high residual stress. Evaluation was based on the presence or absence of cracks when immersed in a 1 mol / m ³ hydrochloric acid aqueous solution for 100 hours.

As shown in Table 3, the ultra-high-strength steel plates of Examples 1 to 5 within the scope of the present invention have a tensile strength of 980 MPa or more, and are sufficient to satisfy the requirements as automotive parts. Since moldability is shown and no crack was generated in the delayed fracture test, it has both moldability and delayed fracture resistance.
On the other hand, although the steel plates of Comparative Examples 1 and 2 exhibited a tensile strength of 980 MPa or more, they were unable to achieve both formability and delayed fracture property. Further, the steel plates of Comparative Examples 3 to 6 have components that deviate from the scope of the present invention, and some of the tensile strength is less than 980 MPa, and there is no problem with delayed fracture, but the formability is inferior to the steel of the present invention. ing.

It is a schematic diagram which shows the stress-strain diagram by the tension test using a plate-shaped test piece (For example, No. 5 test piece prescribed | regulated to JISZ2201, and No. 13 test piece). It is the schematic which shows the calculation method of LDR which is a deep drawing test outline | summary and a deep drawing index. It is the schematic which shows LDH which is an overhang | projection test outline | summary and an overhang | projection parameter | index. It is the schematic which shows a hat bending test outline | summary. It is the schematic which shows the amount of wall warpage (curvature) which is a shape freezing index.

Explanation of symbols

1 Die 2 Wrinkle presser 3 Test piece (steel plate)
4 punches

Claims (9)

By mass ratio, C: 0.10% to 0.40%, Si: 0.01% to 2.5%, Mn: 0.1% to 1.0%, P: ≦ 0.02%, S: ≦ 0.013%, Cu: 0.05% to 3.0%, Ni: 0.05% to 3.0%, Cr: 0.01% to 3.5%, Mo: 0.10% to 2 0.0% content, the balance being Fe and inevitable impurities, the base structure being lower bainite, the average prior austenite grain size being 3 to 10 μm , the tensile strength being 980 MPa or more , An ultra-high strength steel sheet having a stress reduction degree (SD) which is a difference between a tensile strength and a breaking stress by a tensile test used is 180 MPa or more . Furthermore, Al: 0.001%-0.1% is contained in steel, The ultra high strength steel plate of Claim 1 characterized by the above-mentioned. Furthermore, Nb: 0.005%-1.0% is contained in steel, The ultra high strength steel plate of Claim 1 or 2 characterized by the above-mentioned. It is a hot-rolled steel plate or a cold-rolled steel plate, The ultra high strength steel plate as described in any one of Claims 1-3 characterized by the above-mentioned. The ultra-high strength steel sheet according to any one of claims 1 to 4 , wherein a surface treatment of galvanization is performed. The ultra high strength steel sheet according to any one of claims 1 to 5 , wherein a film laminating process is performed. An automotive strength component comprising the ultra-high strength steel sheet according to any one of claims 1 to 6 . The strength part for automobiles according to claim 7 , which is formed by any one of press molding, hydro molding, and blow molding. Automotive intensity component according to claim 7 or 8, wherein a cutting portion.

Images