JPH03264645A - High-strength steel sheet having excellent elongation flanging property or the like - Google Patents

Abstract

PURPOSE:To obtain the high-strength steel sheet having an excellent elongation flanging property, fatigue characteristic and resistance weldability by specifying a compsn. consisting of C, Mn, Si, Cr, Cu, B, and Fe, regulating S, and forming the specific composite structure. CONSTITUTION:The structure of the steel which contains 0.01 to 0.2wt.% C, 0.6 to 2.5% Mn and 0.02 to 1.5% Si, further, contains >=1 kinds of 0.1 to 1.5% Cr, 0.1 to 0.6% Cn, and 0.0005 to 0.1% B, contains >=1 kinds of 0.005 to 0.2% rare earth elements and 0.005 to 0.01 % Ca and/or 0.005 to 0.06% Al at need, consists of the balance Fe and unavoidable impurities and is regulated to <=0.02% S is constituted of the three phases: polygonal ferrite, bainite and martensite. Further, the area rate of the above-mentioned bainite is 4 to 45% and the area rate of the martensite is 1 to 15%. In addition, the latter is set lower than the former and the average diameter of the martensite is confined to >=6mu. This structure is obtd. by adjusting the conditions for rolling, annealing, etc. The high-strength steel sheet having the excellent elongation flanging property, etc., is obtd. in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-strength steel plate with excellent stretch flangeability, fatigue properties, and resistance weldability.

In recent years, various measures have been considered to reduce the fuel consumption of automobiles, and among them, reducing the weight of the car body is the most effective measure. In parallel with this, high-strength steel plate or A7!
Many attempts have been made to change materials such as using alloys. Among these, reducing the weight 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.

As such a high-strength steel sheet, a composite structure steel sheet consisting of ferrite and martensite, that is, Dual Pba
SE steel plate has been proposed. This composite structure steel sheet has a low yield ratio, high elongation compared to strength, and excellent formability and shape fixability, but is inferior in stretch flangeability. Therefore, when applied to wheel discs, problems remain unresolved, such as cracks occurring from the hole enlarged portion during disk molding, and cracks occurring from the hole enlarged portion during fatigue tests or running tests. . Furthermore, when applied to wheel rims, there is a problem in that cracks occur during roll forming performed after flash butt welding.

For example, the incidence of cracking from the heat affected zone during molding is approximately 50%.
It is said that it can reach up to

In order to improve stretch flangeability, the present inventors have conducted detailed research into the relationship between the steel plate structure and these properties, and have already determined that the steel plate structure has a three-phase composite structure of polygonal ferrite, heinite, and martensite. We have found that this is desirable. Therefore, as a result of further research, the present inventors found that even when the area ratio of each structure was adjusted to an appropriate amount in the above three-phase structure, the size of martensite had a large effect on stretch flangeability. It was found that

The present invention was made based on this knowledge, and consists of a three-phase composite structure of polygonal ferrite, bainite, and martensite, and it specifies the area ratio of each structure and also specifies the size of martensite. The object of the present invention is to provide a composite structure high-strength steel plate with excellent stretch flangeability.

The high-strength steel plate with excellent stretch flangeability, fatigue properties, and resistance weldability according to the present invention has (a) C 0
．． (b) Cr 0.1-1.5%, Cu 0.1-0. 6%, and at least one element selected from the group consisting of B 0.0005 to 0.1%,
The balance is iron and unavoidable impurities, the S content is regulated to 0.02% or less, the structure is composed of three phases of polygonal ferrite, bainite and martensite,
Bainite area ratio 4-45%, martensite area ratio 1
~15%, the martensite area ratio is smaller than the bainite area ratio, and the average diameter of martensite 1~ is 6μ or less.

In the present invention, bainite includes not only bainite containing carbide but also bainitic ferrite and so-called acicular ferrite. Furthermore, martensite also includes some retained austenite.

In the high-strength steel plate according to the present invention, the bainite area ratio is in the range of 4 to 45%, particularly from the viewpoint of stretch flangeability.

A three-phase composite steel sheet consisting of polygonal ferrite, bainite and martensite has a yield ratio of 3-phase composite steel sheet consisting of ferrite and martensite.
It has a low yield ratio similar to that of e1iii plate, and has a good strength-elongation balance that is close to that of a composite steel sheet consisting of ferrite and bainite, but it has poor stretch flangeability. As a result of the investigation, it was found that a material having a particularly excellent value even within the range of bainite area ratio of 4 to 45%, which indicates good stretch flangeability, ie, hole expansion rate, was found.

Therefore, in order to clarify the cause of this, we focused on the difference in the size of martensite for steel plates in the above range and investigated the relationship between the average diameter of martensite and the hole expansion rate. As a result, the average diameter of martensite was 6μm or less, especially 5μm
It has become clear that when the diameter is less than m, the hole has a low expansion rate, that is, it has stretch flangeability. Therefore, in the three-phase composite steel sheet according to the present invention, the average diameter of martensite is limited to 6 μm or less, preferably 5 μm or less.

Next, the martensite area ratio is in the range of 1 to 15%. When the martensite area ratio exceeds 15%, the yield ratio increases, while when it is less than 1%, the effect of introducing martensite is small. In particular, the martensite area ratio is preferably in the range of 1 to 10%, and the bainite area ratio is preferably in the range of 11 to 35%. Furthermore, it is desirable that the martensite area ratio is smaller than the bainite area ratio.

Adjustment of the structure as described above can be achieved by adjusting the hot rolling and subsequent cooling conditions in consideration of the chemical components described below, or by adjusting the subsequent annealing conditions (
It can be obtained by adjusting continuous annealing, batch annealing).

Next, the chemical components of the high-strength steel plate according to the present invention will be explained.

C is an essential element for maintaining the necessary strength and forming a low-temperature transformation product consisting of bainite and martensite, but when it exceeds 0.2%, the ductility deteriorates significantly and weldability is impaired. .

0 If moldability is particularly required, the amount of C should be 0°09
% or less. The lower limit amount of C is 0.01 in order to effectively exhibit the effect of strengthening and improving hardenability.
%.

Mn is an essential element for increasing hardenability and obtaining a desired structure. In order to effectively obtain these effects, 0
．． A content of 6% or more is required, but if it exceeds 2.5%, it causes difficulties in welding, impairs ductility and weldability, and further increases the price of the steel plate.

Si is an element necessary for deoxidizing molten steel, and is the most effective element as a substitutional solid solution element, and is an essential element in order to obtain a steel plate with high strength and high ductility.

Furthermore, Si also has the effect of favoring the formation of clean polygonal ferrite. In order to effectively obtain this effect, the lower limit is set at 0.02% to prevent embrittlement of the weld (increase in transition temperature), and on the other hand, the upper limit is set to prevent deterioration of the surface scale condition. It shall be 1.5%.

S is for improving formability, especially stretch flangeability,
0.020% or less, and when strict stretch flangeability is required, 0.020% or less. OO9% or less is desirable.

Cr −, Cu, and B are all elements that contribute to strengthening steel, and are particularly useful elements for improving hardenability and obtaining a desired structure. The lower limit of the amount of these elements to be added is from the amount that can effectively exhibit their effects,
Also, the upper limit is 2. It is determined from an economic point of view that the effect is saturated. Therefore, the addition amount is Cr 0.1-1.5
%, Cu 0.1-0.6%, Bo, 0005-0.1
% range.

In the present invention, in addition to the above elements, the steel plate may contain the following elements.

Rare earth elements (REM) and Ca have the effect of improving ductility, particularly stretch flangeability, through sulfide morphology control. The lower limit of the amount of these elements to be added is the minimum necessary amount to effectively exhibit their effects, and the upper limit is from an economic standpoint that saturates the effect, and on the other hand, from an amount that worsens cleanliness. It is determined. Therefore, the amount added is REM
0. It is desirable that the content is in the range of o01 2 5 to 0.2% and Ca 0.005 to 0.1%.

A6 may be contained as a deoxidizing agent in a range of 0.005 to 0.06%. Furthermore, P can be added within a range that does not cause grain boundary embrittlement, that is, within a range of 0.1% or less. P has a strong reinforcing effect and, like Si, has an effect of purifying ferrite, and is useful for improving elongation and the like.

The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way.

EXAMPLES Test materials having the chemical components shown in Table 1 were melted in a vacuum melting furnace and roughly rolled into 3Q*m thick slabs, which were made into 4n thick hot-rolled plates by 3 passes. In addition, some of the hot rolled sheets were cold rolled (75%).

For these hot rolled steel sheets and cold rolled steel sheets, 780 to 950
It was heated for 5 to 10 minutes at each temperature of °C, and then cooled at various cooling rates to produce test materials with different Mi weaves. Heat treatment conditions 3 for these test materials are shown in Table 2. Furthermore, Table 3 shows the mechanical properties and microscopic structure observation results of the obtained steel plate.

As is clear from Table 3, all of the test materials according to the present invention have an appropriate balance of polygonal ferrite, bainite, and martensite, and the average diameter of martensite is 6 μm or less, It has high strength, low yield ratio, and excellent strength-elongation balance and stretch flangeability.

Mat, -1 For some hot rolled steel sheets, Flash Yatsu H
8 contact results, fatigue properties and wheel disc forming test (
r+-25) The results are shown in Table 3.

The conditions for flash butt welding are as follows.

Flashing 3ta Flash time 3 seconds Upset 3■ Upset time 2/60 seconds Upset speed 150 cranes/second Plain plate shape 3.2 Dragon thickness x 3 Q mm width x 75 Dragon length 5 Processing conditions Table 2 ( (continued) Next, a hot rolled steel plate was manufactured using a test material in which 0.02% of Ce was added to the corresponding components of test material 1 under the hot rolling, cooling and winding conditions shown in Table 4.

Table 5 shows the mechanical properties and microscopic structure observation results of this hot rolled steel sheet. Furthermore, the results of flash hat welding of this hot rolled steel sheet, fatigue properties and wheel disc forming test (n
-25) The results are shown in Table 5. Note that the conditions for flash hat welding are the same as described above.

As is clear from Table 5, sample material 2-2 according to the present invention
In addition to the above-mentioned properties, No. 2-4 has excellent flash hat weldability and fatigue strength, and has an extremely low defective rate even when molded into a wheel disk.

Claims (4)

[Claims] (1) Contains (a) 0.01 to 0.2% of C, 0.6 to 2.5% of Mn, and 0.02 to 1.5% of Si in weight%, and further contains (b) 0.1 to 1.5% of Cr. 5%, Cu0.1-0.6%, and B0.0005-0.1%,
A steel whose balance consists of iron and unavoidable impurities, and whose S content is regulated to 0.02% or less, whose structure consists of three phases of polygonal ferrite, bainite and martensite,
Bainite area ratio 4-45%, martensite area ratio 1
~15%, the martensite area ratio is smaller than the bainite area ratio, and the average diameter of martensite is 6μ or less.It has excellent stretch flangeability, fatigue properties, and resistance weldability. Strength steel plate. (2) Contains (a) 0.01 to 0.2% of C, 0.6 to 2.5% of Mn, and 0.02 to 1.5% of Si, and further contains (b) 0.1 to 1.5% of Cr by weight. 5%, Cu0.1-0.6%, and B0.0005-0.1%; (c) rare earth element 0.005-0.2%; and Ca0. ．．
At least one element selected from the group consisting of 0.005 to 0.01%, the balance consisting of iron and unavoidable impurities, and 0.02% of S.
A steel regulated as below, whose structure consists of three phases of polygonal ferrite, bainite, and martensite, with a bainite area ratio of 4 to 45%, a martensite area ratio of 1 to 15%, and a martensite area ratio of 4 to 45%. A high-strength steel plate with excellent stretch flangeability, fatigue properties, and resistance weldability, characterized in that the area ratio of martensite is smaller than the area ratio of bainite, and the average diameter of martensite is 6μ or less. (3) Contains (a) C0.01-0.2%, Mn0.6-2.5%, Si0.02-1.5%, Al0.005-0.06% in weight%, and further, (b) Contains at least one element selected from the group consisting of Cr0.1-1.5%, Cu0.1-0.6%, and B0.0005-0.1%,
A steel whose balance consists of iron and unavoidable impurities, and whose S content is regulated to 0.02% or less, whose structure consists of three phases of polygonal ferrite, bainite and martensite,
Bainite area ratio 4-45%, martensite area ratio 1
~15%, the martensite area ratio is smaller than the bainite area ratio, and the average diameter of martensite is 6μ or less.It has excellent stretch flangeability, fatigue properties, and resistance weldability. Strength steel plate. (4) Contains (a) C0.01-0.2%, Mn0.6-2.5%, Si0.02-1.5%, Al0.005-0.06% in weight%, and further, (b) at least one element selected from the group consisting of 0.1% to 1.5% Cr, 0.1% to 0.6% Cu, and 0.0005% to 0.1% B; and (c) 0.0% rare earth element. 005-0.2%, and Ca0.
At least one element selected from the group consisting of 0.005 to 0.01%, the balance consisting of iron and unavoidable impurities, and 0.02% of S.
A steel regulated as below, whose structure consists of three phases of polygonal ferrite, bainite, and martensite, with a bainite area ratio of 4 to 45%, a martensite area ratio of 1 to 15%, and a martensite area ratio of 4 to 45%. A high-strength steel plate with excellent stretch flangeability, fatigue properties, and resistance weldability, characterized in that the area ratio of martensite is smaller than the area ratio of bainite, and the average diameter of martensite is 6μ or less.