JPS58167750A - High strength steel plate excellent in elongation flange property - Google Patents

Abstract

PURPOSE:To provide a high strength steel plate excellent in elongation flange property and fatigue characteristics, constituted such that C, Mn and Si are respectively contained in a predetermined ratio while S content is limitted and each area ratio in the three-phase structure comprising polygonal ferrite, bainite and martensite is adjusted. CONSTITUTION:The aforementioned high strength steel plate contains 0.01-0.2% C, 0.6-2.5% Mn, 0.02-1.5% Si and S content thereof is limitted to 0.02% or less. In this case, the structure of this steel plate comprises a three-phase of polygonal ferrite (RF), bainite (B) and martensite (M) and the area ratio of B is 4-45% while that of M is 1-5% as well as the average diameter of M is limitted to 6mum or less. The adjustment of this structure is obtained, in consideration of the aforementioned each components, by adjusting hot rolling and a cooling condition after hot rolling or a succeeding annealing condition with respect to a hot rolled or a cold rolled plate.

Description

[Detailed Description of the Invention] The present invention provides a high Ii film with excellent stretch flangeability, fatigue properties, and aging resistance. Regarding l111 copper plate.

Recently, various measures are being considered to reduce the fuel consumption of automobiles, and reducing the weight of the vehicle body is the most effective measure.
This vehicle was made smaller to make it lighter.

In parallel with this, material changes such as the adoption of high-strength copper plates or U)1 alloys were introduced. Many attempts have been made. Among these, massization of wheels is an extremely effective means for reducing fuel consumption, and the application of high-strength steel plates to wheel rims and discs is being actively studied. This high-strength counterfeit steel plate is a ferrite + martensite machine structure steel plate (Dual phas
e steel plate) has been proposed.

This steel plate has a low yield ratio, high elongation compared to 1 strength, and excellent formability and shape fixability. However, this −
The plate has poor stretch flangeability when applied to wheel discs.

(1) Occurrence of cracks from the hole expansion part during disk molding (2)
) It has not been possible to solve the problem of cracks occurring from the hole enlargement during fatigue tests or chemotaxis tests. When applied to wheel rims, the above-mentioned dual phase steel plate is used.

There is a problem in that cracks occur during roll forming after flash pad bathing. (It is said that the incidence of cracking from the heat-affected zone during forming reaches approximately 50 cm.) The present inventors investigated the relationship between the steel sheet structure and these properties in order to improve the elongation and lunge properties mentioned above. We have conducted detailed research and have found that it is desirable to have a three-phase interlocking structure of polygonal ferrite + bainite + martensite.

However, as a result of further research, it has become clear that even when each structure is adjusted to an appropriate cap (area ratio) Kv4 in the above-mentioned three-phase structure, the size of martensite greatly influences stretch flangeability.

The present invention was completed based on this knowledge, and its gist is as follows: OO,01~OJ%, MnO
, 6-2°6%, 8i 0. Og-1,5cIb
Contains OrO, l ~ 1.6%, as required by history,
Ou O, 1-0.6%, Nl (1, 1-1%,
BO,0005-0,1*, NbO,01~0
．． 1%, VO,01l-0,2%, TlO,01
~0.1%, ZrO, (12~IIJ% lak or more than 2 woodcutter).

Rare earth element 90.005-0.2%, Oa 0.000
5-0.01% (including D 11m or 2 woodcutter, 80.
02% or less, and its structure is polygonal ferrite (PF).

It consists of 8 phases of bainitic (baini) (B) and martensite) (M), has a bainite area ratio of 4 to 45 inches, a martensite area ratio of 1 to 15 inches, and has an average diameter of martensite of 6 μ or less. Features: High Ii with excellent elongation 7 lunge properties, resistance bath weldability, and fatigue properties! It is an 11 degree steel plate.

Note that it includes not only bainite defined in the present invention and bainite containing a softened Fi, but also bainitic ferrite and so-called ashcu-2-ferrite. tOW rudensite also includes some residual austenite.

The PF+B+M 8-pair mechanical steel plate has a low yield ratio similar to that of the F+M (dual phase) steel plate, and the strength-elongation balance is similar to that of the F1B steel plate, giving it a good yield ratio. However, when we further investigated the elongation and lunge properties, we found that
A bainitic steel sheet exhibiting good elongation properties (two-hole expansion ratio) was found to exhibit particularly excellent values within the range of 4 to 45 inches. ,.

Therefore, in order to find the cause of this, we focused on the difference in the size of martensite in steel sheets in the above range.

The relationship between the average diameter of martensite and the hole expansion rate was investigated. It has become clear that martensite having an average diameter of 6 μm, preferably 5 μm or less, exhibits particularly excellent hole expansion rate, that is, stretch flangeability.

Next, the area ratio of martensite should be 1 to 15%; if it exceeds 15-, the yield ratio will increase, while if it is less than 1%, the effect of introducing martensite will be small. In addition, the martensite area ratio is 1 to 1 (l is desirable,
The area ratio of bainite is preferably 11 to 86%, and the area ratio of martensite is preferably smaller than the area ratio of paynite.

The structure can be adjusted by adjusting the hot rolling and subsequent cooling conditions in consideration of the chemical components shown in 9 below, or by adjusting the subsequent annealing conditions (continuous annealing,
patch annealing).

Next, the reasons for limiting the chemical components in the present invention will be described.

0 is an essential element for maintaining the necessary strength and forming low-temperature products such as bainite and martensite, but if it exceeds 01 g, ductility tends to deteriorate and weldability is impaired, so it is limited. be done. In addition, when moldability is particularly required, it is desirable to set it to 0.09 or less.

The lower limit is set to o and o1% in order to exhibit the effects of strengthening and improving hardenability.

Mn #i is an element necessary to increase hardenability and obtain a desired structure. 1f(1,
6 or more is required, and if it exceeds 2.5 inches, it will be difficult to contact, and at the same time, the abrasiveness and bath contactability will be "H", and the price of the steel plate will be expensive, so the upper limit is set at 2.5. −.

Sl is a 7c element necessary for deoxidizing bath steel, and is also an effective element to have as a substitutional solid solution element.

High Ii! It is an indispensable element for producing steel sheets with high ductility and high ductility. Furthermore, it has the function of favoring the formation of clean polygonal ferrite. In order to exhibit these characteristics, the lower limit is set at 0.02% to prevent embrittlement of the joint (1 degree increase in transition ff), and the upper limit is set at 1.5% to prevent deterioration of the surface oxide scale state. .

S is o, o to improve formability, especially stretch flangeability.
It is preferably 20'16 or less, and particularly 11.009 inches or less when elongation 7 lunge properties are required.

In the present invention, in addition to the above elements, the following elements may be contained as necessary.

Or, Ou, Ni, B, Nb, V, T
Both i and Zr are elements that contribute to the strengthening of steel, and of these, 0'' is particularly important.

Ou, Ni, and B are all elements necessary to improve hardenability and obtain a desired composition. The lower limit is when the effect can be exerted, and the upper limit is when the effect reaches saturation.
Determined from the amount that becomes uneconomical.

Or 0.1~t, s%, Ou o, t~o, s%
, Ni 011-1.0-, Bo, ooos~0.
It is one song. On the other hand + Nb, V, Ti, Zr
is a precipitation-strengthening element and is not only necessary for increasing 9 strength, but also! Coexisting with iln, etc., it affects the transformed structure after hot rolling, and has the function of making it easier to form a bainite structure. Furthermore, it refines the structure, improves elongation, and prevents a decrease in the hardness of the heat-affected zone of the bath, helping to improve the fatigue strength of not only the base plate but the entire disk. It has the function 4 of maximizing the hardenability improvement effect of Maku B. Ninth to produce these effects, Nb O, 01~0.1%
, VO, 02~O, 2%, 1' insect 0. O1~o,
It is necessary to contain t s + Zr (1,0111 to 0.2%).

The rare earth element (REM), Os, contributes to improving ductility, especially elongation and lungability, through the control of sulfide morphology. .

The lower limit is the minimum amount necessary to exhibit the effect.

The upper limit is determined by the amount at which the effect reaches saturation and becomes uneconomical, and by the amount that worsens the reverse KmpP degree.REVo, 005
~0.2chi, Oa 0.005~0. Contains 1.

In addition, M may be included as a deoxidizing agent in the range of o, oos to 0.06%, and P is in a range that does not cause grain boundary embrittlement (0,1
1! You can add shares using (below). P has a strong reinforcing effect and, like 8i, has the function of purifying ferrite, and is useful for improving elongation and the like.

Next, examples of the present invention will be shown together with comparative examples.

The sample material having the chemical composition shown in Table 1 is vacuum melted using fPK.
A 3θ maroelectric slab made by ## and roughly rolled was rolled in 8 passes for 41
It was made into a hot rolled sheet of t. In addition, some hot-rolled sheets are cold-rolled (75
%)did. 780 for these hot-rolled steel sheets and cold-rolled steel sheets
~95 (PiC) was heated for 5 to 10 minutes at each temperature, and then cooled at various cooling rates to prepare test materials with different structures.The heat treatment conditions for these test materials are shown in Table 2. Furthermore, Table 8 shows the mechanical properties and microstructural observation results of the obtained steel sheets.

Table 1 Chemical composition (wt appetizer) Table 3 Heat treatment conditions Table 3 Sample material 4 according to the present invention as shown in Table IIs
1 to 9 and A I5.16 #i, both have an appropriate balance of polygonal aerite, bainite, and martensite, and the average diameter of martensite is 6 μ or less, and have high strength and low retention ratio. There it is.

O has excellent strength-elongation balance, elongation 7 and hardness.Next, C@0. A hot-rolled steel sheet was manufactured using the test material to which Og % was added under the hot-rolling and cold-rolling conditions shown in No. 4.

Table 5 shows the mechanical properties and microscopic IIA microstructure observation results of this hot rolled steel sheet. In addition, the results of 72 Tushput welding of this thermal IIt steel plate, [labor characteristics and wheel disc forming test]
(Satomi) The results are shown in the 6th barley.

4th barley hot rolling, cooling 1 roll conditions table! II 6 Omote Todoroki 1) Fi contact conditions Flash cost 3■, 7 kushi time 3 seconds, upset cost 3 yen, upsect time 2/60
seconds, upset speed 150 as 7 seconds, blank shape L
1 m' x 80J
As known from JE, sample material M2-2 which is the present invention
~! - Pound is like the above-mentioned properties.

7 Rush pad has excellent contact properties and fatigue resistance, and has the characteristics of extremely low defective rate even when molded into a wheel disc.〇Special applicant: U Co., Ltd.
Made in Kobe.

Claims (4)

[Claims] (1) O0001~0.2 fb, 10.6~B-
5% + 810-OR ~ 1.5-, including 80.01
Copper regulated to 1 or less. The tenso fibers consist of 8 phases of polygonal aerite, bainite, and martensite, and the bainite area ratio is 4 to 4.
5%, -r rutinsite area ratio of 1 to 15-, and the average diameter of martensite is 6 μ or less. 7. Lunge resistance - Excellent fatigue properties and resistance weldability. 9.I! 1 strength copper plate. (2) OO,01-0,119G, hAn O06~
i1.5%, 8i0.08-1.61G. and Or 0.1 = 1.5%, Ou O, 1~0
．． 6%, Ni 0.1 = 11G. B O, 0005~0.1%, Nb O, 01~0.
19b, VO, 02~OJ%. Ti 0.01-0.1%, Zr 0.011-0.
Steel containing one or more strains of 111, regulated to less than SO, OG-, whose structure is not composed of three phases of borigonal ferrite, bainite and martensite,
Bainite loading truck 4-4596. The martensite area ratio is 1-15 mm, and the average diameter of martensite is 6
A high-strength copper plate with excellent elongation, fatigue properties, and resistance weldability, characterized by an elongation of less than 7μ. (3) 00.01-0.2%, MnO,6-2
, 5%, 8io, og~t, ss. and rare earth elements 0.005~o, gs, Oa 0.
005-0.01% of one or two types, go
It is a 9-pot with an area ratio of 4 to 459 cm, and a martensite area ratio of 1 to 15 cm. A high-strength steel plate with excellent lunge properties, fatigue properties, and resistance weldability. (4) O0001-0.2%, Mn 0-6-2-
5%, 8i 0. Og −1-5% + and Or 0
．． 1-L, S %, σu O, 1-0,6%,
NiO-1-I SsB 0.0005-0.1%
, NbO,01-0,1%, VO,0i9-0.
9%. Ti 0.01-0.1 To, Zr 0.02-
oJ%, (F) 1st class Xttgau, * rare earth principal 0.005-0.2%, Oa 0.005-0.01
Copper containing 1fiI or Fiz species of -, regulated to less than 80.02%, and whose structure consists of eight phases: polygonal aerite, bainite, and martensite. Bainite pile rate 4-45 cm, martensite pile rate 1
-15% and the average diameter of martensite is 6μ
A high-strength copper plate with excellent elongation properties, fatigue properties, and resistance weldability, characterized by the following properties: 0