JPS6196057A - Hot-rolled steel plate having maximum strength - Google Patents

Abstract

PURPOSE:To obtain a maximum-strength hot-rolled steel plate having superior formability, resistance weldability, and fatigue characteristic, by providing specific amounts of C, Si, and Mn and also providing a composite structure consisting of polygonal ferrite and bainite, with specifying the area ratio of bainite. CONSTITUTION:The hot-rolled steel plate consists of, by weight, 0.01-0.12% C, 0.1-1.6% Si, 0.7-2.5% Mn, and the balance Fe. More over, the steel plate has a composite structure consisting of polygonal ferrite and bainite, with specifying the area ratio of the bainite to 5-60%. And further, >=1 kind among 0.02-1.5% Nb, 0.02-1.5% V, 0.01-0.08% Ti, and 0.02-0.18% Zr is combined with said components, as required.

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention has excellent formability, resistance weldability, and fatigue properties, and is suitable for wheel rims, discs, bumpers, and other automobile parts. Regarding high strength hot rolled steel sheets.

(Prior Art) As a measure to reduce vehicle weight in order to save fuel consumption, attempts have been made to reduce the size of vehicle bodies and to change materials by using high-strength steel materials. Among these, reducing the weight of wheels is considered to be extremely effective in reducing fuel consumption, and the application of high-strength hot-rolled steel sheets to wheel rims, discs, etc. is being actively studied. For example, in the United States, ferrite-martensitic composite structure hot-rolled steel sheets (dual-phase steel sheets) are considered to be the most suitable material for these members, and prototype tests are underway. However, problems with material properties have become apparent, and it has not yet been put into practical use.

That is, the above dual phase steel plate
When applied to a 1-ru disk, there are problems such as cracks occurring at the hole enlarged portion during molding, and the moldability, especially stretch flangeability, is poor, and the fatigue properties are also not sufficient. Furthermore, when applied to wheel rims, flash butt welding has the problem of softening of the weld heat affected zone, which causes cracks to occur in this area during subsequent molding.

(Object of the Invention) The present invention was made to solve the problems of the above-mentioned dual phase steel as a high-strength hot-rolled steel sheet applied to wheel rims, discs, etc. The purpose of the present invention is to provide a high-strength hot-rolled steel sheet with excellent stretch flangeability, resistance weldability, and fatigue properties.

(Structure of the Invention) The high-strength hot-rolled steel sheet with excellent formability, resistance weldability, and fatigue properties according to the present invention has a carbon content of 0.01 to 0.12% by weight.
, Si 0.1 to 1.6%, and Mn 0.7 to 2.5%, and has a composite structure consisting of polygonal ferrite and bainite, and the area ratio of bainite is 5 to 6.
It is characterized by being 0%.

The present invention will be explained in detail below.

First, the reason for limiting the chemical components in the steel of the present invention will be explained.

C has the effect of strengthening steel and improving hardenability.

In order to effectively obtain such an effect, it is necessary to add 0.01% or more. However, if too much is added, the hardness will decrease due to decarburization of the joint surface during flash butt welding, and the difference in hardness between the weld line and its vicinity will increase.
The upper limit of the amount added is 0.12%, preferably 0.09%.

Mn is an essential element for compensating for the decrease in strength of the steel due to low carbon content and for obtaining a bainitic structure.

In order to effectively obtain such effects, in the steel of the present invention,
It is necessary to add 0.7% or more. That is, the amount added is 0.
．． If it is less than 7%, the required strength and! IJl texture cannot be obtained. However, if too much is added, melting becomes difficult and steel ductility deteriorates, so the upper limit of the amount added is set at 2.5%.

Si is an element that promotes the formation of polygonal ferrite and is effective in obtaining an appropriate structure, and is also an element suitable for obtaining high strength and high ductility.

In order to effectively obtain such effects, in the steel of the present invention,
It is necessary to add 0.1% or more. However, when adding too much, it causes embrittlement of the welded part, that is, an increase in the transition temperature, so the upper limit of the amount added is set at 1.6%.

In the steel of the present invention, in addition to the above components, if necessary, 0.01 to 0.08% of Nb, 0.02 to 1.5% of V, 0.01 to 0.08% of Ti, and 0.00% of Zr. At least one element selected from the group consisting of 0.02 to 0.18% can be added.

Nb, (2), Ti, and Zr are all effective elements for preventing decomposition of the bainite structure and reduction in hardness in the heat-affected zone during flash butt welding. Furthermore, since these elements have a precipitation strengthening effect, they are also effective as auxiliary elements for increasing strength. However, if excessive amounts are added to increase the amount of precipitation strengthening, not only will the ductility of the steel decrease, but the precipitates will re-dissolve in the weld heat affected zone, causing softening. It is desirable that the amount added be within the above range.

Furthermore, in the steel of the present invention, Po, 02 to 0.15%, and/or Al, 0.005 to 0.06%, may be added together with the above elements or independently of the above elements.

P is an element effective in increasing strength without impairing ductility, and in order to effectively obtain this effect, 0.02
It is added in a range of 0.15%. Further, Al is added as a deoxidizing agent in a range of 0.005 to 0.06%.

Furthermore, the steel of the present invention contains REM 0.005 to 0.1%, Ca 0.0005 to 0.01%, and Mg 0 together with or independently of the above elements.
．． 0005~0. At least one element selected from the group consisting of O1% can be added.

REV (rare earth element) 6, Ca, and Mg each have the effect of rendering inclusions harmless and improving formability through the effect of controlling the shape of sulfide. In order to effectively obtain this effect, REM must be 0.005-0.1%, Ca 0.0005-0.01%, Mg 0.
．． It is added in a range of OOO5 to 0.01%.

Next, the steel of the present invention has the chemical composition described above, is composed of borigonal ferrite and bainite as a copper structure, and has an area ratio of bainite in a range of 5 to 60%.

As described above, in accordance with the present invention, by making the steel structure ferrite-haynite having bainite at a predetermined area ratio, the steel structure is made to be ferrite-haynite having bainite in a predetermined area ratio, which makes it possible to improve the performance of the embodiments described below, compared to the conventional ferritic-martensitic (dual phase) steel described above. As is clear from FIG. 4, resistance weldability is excellent, and in particular, there is no softening of the heat affected zone. However, in ferrite-bainite steel with a bainite area ratio exceeding 60%, softening occurs in the weld heat-affected zone, similar to ferrite-martensitic steel.

Furthermore, as is clear from FIG. 5, the fatigue properties are also excellent.

Regarding formability, as shown in Figure 2, the ferrite-bainitic steel according to the present invention has a lower stretch flangeability than both the ferritic-martensitic steel and the ferritic-pearlitic steel. - Looking at the elongation balance, as shown in Figures 1 and 6, even with the same ferrite-bainite steel, the steel of the present invention has a bainite area ratio in the range of 5 to 60%. Superior to ferritic and bainite steels.

That is, according to the present invention, by setting the area ratio of bainite in the ferrite-bainite steel to a range of 5 to 60%, it is possible to particularly improve the strength-elongation balance and prevent softening in the weld heat affected zone. can.

In addition, the bainite referred to in the present invention refers to lower bainite, upper bainite (BII,
In addition to BII[), it also includes a structure that is not accompanied by a carbide reaction and is called Bl, Widmanstätten, or acicular ferrite.

Next, a method for producing the steel of the present invention will be explained.

In manufacturing the steel of the present invention, a steel slab having the above chemical composition is hot rolled according to a conventional method. After completion of this hot rolling, first, in order to generate ferrite with a desired area ratio, cooling is performed to a temperature of 700 to 500°C, preferably 600°C, at an average cooling rate of 5 to b (primary cooling). . It is practically technically difficult to reduce the cooling rate to less than 5°C/sec. On the other hand, if the cooling rate is faster than 35°C/sec, sufficient ferrite will not be produced, so the cooling rate should be within the above range. limited to. Further, the temperature at which the cooling rate is changed is also determined from the same purpose. That is, 700
Ferrite is not sufficiently generated at high temperatures above 500°C.
This is because if the temperature is lower than °C, productivity will decrease.

Following the above primary cooling, in order to transform untransformed austenite into bainite, cooling is performed to 575 to 250°C at an average cooling rate of 25 to 25°C (25°C to 250°C) at an average cooling rate of 25 to
(next cooling). Then, it is wound at that temperature.

If this cooling rate is slower than 25°C/sec, there is a risk that pearlite will appear, while on the other hand, a cooling rate faster than 80°C/sec is difficult to employ in practice.

(Effects of the Invention) As described above, the steel of the present invention has a predetermined chemical composition, a composite structure of polygonal ferrite and bainite, and an area ratio of bainite in the range of 5 to 60%. Therefore, the formability, especially 1. Not only does it have excellent stretch flangeability, it also has excellent welding resistance, with no softening in the weld heat-affected zone, and it also has excellent fatigue properties. In addition, in composite structure steels in which the area ratio of haynite is outside the above range, softening in the weld heat affected zone is unavoidable, and
Whereas the elongation-strength balance is poor, the steel of the present invention does not soften in the weld heat-affected zone and has an excellent elongation-strength balance.

(Example) The present invention will be explained below by giving examples together with comparative examples, but the present invention is not limited by these examples in any way.

Steel having the chemical composition shown in Table 1 is melted, hot rolled (finishing temperature 800-855°C) to a thickness of 3.21 mm, and then hot rolled according to the cooling and winding conditions shown in Table 2. Manufactured steel plates.

Table 3 shows the results of microscopic measurements and mechanical property measurements of the hot rolled steel sheets thus obtained.

Figure 1 also shows the strength-elongation balance of these hot-rolled steel sheets.
Figure 2 shows the stretch flangeability, Figure 3 shows the toughness, Figure 4 shows the hardness distribution of the flash butt weld, and Figure 5 shows the Sienck fatigue test results. Furthermore, regarding steel R shown in Table 1, after hot rolling, by variously changing the cooling rate and coiling temperature, it is a ferritic bainite steel,
Hot-rolled steel sheets with different area ratios of these structures were manufactured and their mechanical properties were investigated. The results are shown in Figure 6.
+p) indicates ferrite/pearlite steel.

The flash butt welding conditions in FIG. 4 are as follows.

Flashing: 3R Flash time = 3 seconds Upset: 311 Upset time: 2/60 seconds Upset speed: 150 x 1/second Base plate shape: Width 30 mm, length 75 mm, thickness 3.2
m In the following, F represents pearlite, B represents bainite, and M represents martensite. As is clear from Table 3 and FIG.
F+P) has an excellent strength-elongation balance compared to the comparison steel (F+75-90%B), and is equal to or greater than the comparison m (F+M). In addition, as shown in Figure 6, the bainite area ratio in ferrite-bainite steel has a large effect on the strength-elongation balance, and by setting the bainite area ratio in the range of 5 to 60%, the strength-elongation balance can be improved. It can be improved significantly.

On the other hand, when the bainite area ratio exceeds 60%, the strength-elongation balance is poor.

Furthermore, as is clear from FIG. 2, F+B according to the present invention
The steel has significantly improved stretch flangeability compared to F+M steel. Regarding toughness, as shown in Figure 3,
F+Bi4 has excellent toughness.

Next, regarding flash hat weldability, which is a problem when applied to wheel rims, etc., in particular, the hardness distribution of the welded part,
As shown in FIG. 4, the softening (drop in hardness) in the heat affected zone, which is a problem with F4-Ml, is not observed in the F+B steel according to the present invention. However, the bainite area ratio is 75
%, the softening in the weld heat affected zone is obvious.

Furthermore, regarding the fatigue characteristics that become a problem when applied to wheel discs, etc., as shown in FIG.
+Bljl is superior to F+M steel.

[Brief explanation of drawings]

Fig. 1 is a graph showing the strength-elongation balance of the invention steel and comparative steel, Fig. 2 is a graph showing the stretch flangeability (hole expansion ratio), and Fig. 3 is a graph showing the toughness. Figure 4 is a graph showing the hardness distribution of a welded part in flash-hut welding, and Figure 50 is a graph showing the results of a Sienck fatigue test, where (a) is a blank plate, (b) is a 9% tensile deformation and a 5 inch diameter. Showing the punched Omura. FIG. 6 is a graph showing the relationship between bainite area ratio and mechanical properties in ferrite-bainite steel. In1-C of the drawing: "": -: '-Two deaths)
Fig. 1 g14 Roasted fern j (q/mfrl) Fig. 4 6 Rakko S Momme ρ・4's distance (control) Fig. 5 (a) (b) Hashiri, Shihagi (NJ n Zaikunoshi 116 = □ (・-1/'f) Figure 6 (F-?P,) Key to bainite face Correction procedure amendment form (method) % formula % 2. Name of invention High-strength hot rolled steel plate 3. Person making the amendment Relationship Patent applicant address 1-3-18 Wakihama-cho, Chuo-ku, Kobe City Name
Kobe Steel, Ltd. 4, Agent Address: Shinmachi Shichifuku Building 5, 1-8-3 Shinmachi, Nishi-ku, Osaka City, Date of Amendment Order: October 9, 1985 (Shipping Date: October 29, 1985) 6 , number of inventions increased due to amendment This is an engraving of the originally attached drawing, and there is no change in the content)

Claims (4)

[Claims] (1) Contains C 0.01 to 0.12%, Si 0.1 to 1.6%, and Mn 0.7 to 2.5% by weight, and has a composite structure consisting of polygonal ferrite and bainite. In addition, the area ratio of bainite is 5 to 6.
A high-strength hot-rolled steel sheet with excellent formability, resistance weldability, and fatigue properties. (2) Contains (a) 0.01 to 0.12% of C, 0.1 to 1.6% of Si, and 0.7 to 2.5% of Mn in weight%, and further (b) 0.0% of Nb. 01 to 0.08%, V 0.02 to 1.5%, Ti 0.01 to 0.08%, and Zr 0.02 to 0.18%. ,
Excellent formability, resistance weldability, and fatigue properties according to claim 1, which has a composite structure consisting of polygonal ferrite and bainite, and has an area ratio of 5 to 60% of bainite. High strength hot rolled steel plate. (3) Contains (a) 0.01 to 0.12% of C, 0.1 to 1.6% of Si, and 0.7 to 2.5% of Mn in weight percent, and further (b) REM 0. 0.005 to 0.1%, Ca 0.0005 to 0.01%, and Mg 0.0005 to 0.01%,
Excellent formability, resistance weldability, and fatigue properties according to claim 1, which has a composite structure consisting of polygonal ferrite and bainite, and has an area ratio of 5 to 60% of bainite. High strength hot rolled steel plate. (4) Contains (a) 0.01 to 0.12% of C, 0.1 to 1.6% of Si, and 0.7 to 2.5% of Mn in weight%, and further (b) 0.0% of Nb. At least one element selected from the group consisting of: 01 to 0.08%, V 0.02 to 1.5%, Ti 0.01 to 0.08%, and Zr 0.02 to 0.18%; (c) contains at least one element selected from the group consisting of REM 0.005-0.1%, Ca 0.0005-0.01%, and Mg 0.0005-0.01%, and is polygonal. High strength with excellent formability, resistance weldability, and fatigue properties as claimed in claim 1, which has a composite structure consisting of ferrite and bainite, and has an area ratio of bainite of 5 to 60%. Hot rolled steel plate.