JPS5842726A - Manufacture of high strength hot rolled steel plate - Google Patents

Abstract

PURPOSE:To manufacture a steel plate with a favorable stretch flanging property by subjecting a steel contg. specified percentages of C, Mn and Si to hot rolling, slow cooling, rapid cooling and coiling under specified conditions to form a structure contg. a specified percentage by area of an upper bainite structure. CONSTITUTION:A steel contg. 0.02-0.25% C, 0.3-2% Mn and 0.1-1.5% Si or further contg. >=1 kind among 0.01-1% Cr, 0.005-0.06% Nb, 0.02-1.0% V, 0.01-0.08% Ti and 0.02-0.15% Zn is hot rolled at 700-900 deg.C finish temp., slowly cooled for 3-20sec at 4-10 deg.C/sec average cooling rate, rapidly cooled at 50-100 deg.C/sec average cooling rate, and coiled at 370-575 deg.C. Thus, a high strength hot rolled steel plate having a polygonal ferrite + upper bainite structure contg. 3-60% by area of an upper bainite structure is manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention has excellent characteristics for use in severe molding of small wheel rims, discs, bumpers, and other automobile parts, particularly for parts that require good elongation and lunge properties. This relates to a method for manufacturing a high-strength hot-rolled steel sheet that exhibits the following properties.

Recently, ferritic and martensitic steels (D and P grades) have been developed as high-strength steel sheets, but these 1)
We have developed a ferrite + bainite steel (bainite area ratio of 5 to 60%) that does not significantly deteriorate the properties of P @ and has good elongation, lunge properties, flash butt weldability, fatigue properties, etc. (Patent application 1982-110829
, 55-177842, etc.) As a result of further detailed investigation of this ferrite + bainite steel, we found that elongation, 7 runciness, and toughness (impact properties) r. It was discovered that there is a difference in the characteristic values, and that using upper bainite improves the above characteristics, and the manufacturing conditions for this were studied and the present invention was completed.

That is, in the present invention, OO, 02 to 0.25%, M! 10
．． 8 staves 2%, 0t no 0ri e+ 0.1~15% and Cr 0.01 as necessary
- Finishing steel containing 1% or more of Nb in air 11f 7
Average cooling rate after hot rolling at 00-900℃ 4-10
Slowly cool for 8-20 seconds at a temperature of 850-515'c/e, then rapidly cool at an average cooling rate of 50-100'c/e.
By winding the upper bainite with an area ratio of 8-6
This is a method for producing a high-strength hot-rolled steel sheet characterized by having a polygonal ferrite structure containing 0% + upper bainite structure. Furthermore, in the present invention, cooling can be performed at an average cooling rate of 20 to 20 b seconds prior to slow cooling after hot rolling.

The present invention will be explained in more detail below.

FIG. 7 is a diagram showing the relationship between the impact properties (MaTrs, MaEs) and the impact properties (MaTrs, MaEs). As is known from Fig. 1.2, bainite @ (F + B) in ferrite is
Better elongation compared to ferrite-martensite (F+M) m and ferrite-pearlite (F+P) steel 7
Among them, ferrite + upper bainite (p+on) IR has lunge properties and impact properties, but ferrite + upper bainite (p+on) IR is ferrite + lower bainite CF + LB) steel? Compared to this, even better characteristics can be achieved. The upper bainite is the so-called J type, in which carbides (cementite) are precipitated, and the so-called Bl, Bl type, in which carbides are precipitated at the boundaries of the ferrite laths. -What is the lower right bainite? Defined as a type in which carbides are precipitated within ferrite.

Note that these bainite phases were all determined using a thin-film electron microscope, and there are cases where various bainite phases coexist in the same field of view, but in this case, the bainite phase that occupies the largest area ratio should be used as the representative.

The area ratio of this upper bainite phase must be 8 to 60%; if it is less than 8%, pearlite or cementite will substantially be mixed in, resulting in deterioration of mechanical properties. Furthermore, if the area ratio is 60% or more, the elongation deteriorates significantly, making it difficult to apply it to parts that require particularly severe processing.

Next, the chemical components and hot rolling conditions for forming a polygonal aerite +5 to bite structure will be described.

First, regarding the chemical components, C is the cheapest and most effective element in controlling the second phase to upper bainite, and it needs to be contained at 0.02% or more in order to exhibit its effect. . However, if it exceeds 0.25%, lower bainite is likely to occur, and it may cause problems with the characteristics of welds such as flash butt welding, spot welding, or arc welding, so the upper limit has been set at 0.25%.
It is necessary to specify.

1i4n is an essential element for compensating for the decrease in strength due to lower carbon content and for obtaining a bainitic structure. Contained value is 0
If it is less than 68%, the required strong electricity and organization cannot be obtained,
On the other hand, if it exceeds 2%, the 7-elite transformation will be delayed, and not only will it be impossible to obtain the desired 7-elite fraction, but it will also cause problems in terms of production costs.

Therefore, it is added in a range of OJ to 2%.

8i promotes the production of polygonalbuerite.

It is an effective element for obtaining a proper structure, and is also a suitable element for imparting high strength and high ductility. 411 L
, if added in excess, the weld will become brittle (1-increase in transition humidity)
Alternatively, since it causes deterioration of surface properties, it is necessary to specify the content within the range of 0.1 to 1.5%.

In addition, in the present invention, in addition to the component t of J double series, if necessary, C
It can contain one or more of r, Nb, V, Ti, and Zr.

Cr is an effective element for improving hardenability and obtaining a desired structure, and is preferably 0.01% or more, but an excess of 1%
If this is added, the formation of ferrite will be delayed, so the upper limit is kept at 1%.

Nb, V, Ti, and Zr have precipitation-strengthening and grain-refining effects, so they are less effective as auxiliary elements for increasing strength, but if they are added in excess and the amount of precipitation strengthening is increased, they may cause a decrease in ductility. It must be invited and avoided. Furthermore, these elements are effective in preventing decomposition of the bainite structure and decrease in hardness in the heat affected zone of welding such as flash butt welding, and preventing the decrease in fatigue strength and ductility caused by these. It also contributes to Considering these conditions, the amount of addition is NbO,005-0,06'4,V
O,01~1.0%, Ti O,01~G-08%
, ZrO, 01 to 0.15%. In addition to these common effects, Nb affects the transformation behavior of the structure after hot rolling,
It is an effective element for obtaining a bainite structure.

Incidentally, Aj is contained in an amount of 0.06% or less after being deoxidized during melting. In addition, to control the shape of sulfides, rare earth elements, Cs
The hot rolling conditions will be described next. The hot rolling conditions are determined in relation to the chemical components, but are required to satisfy two conditions: obtaining a desired ferrite fraction and making the second phase upper bainite.

FIG. 3 schematically shows the hot rolling process in the present invention. 1st
In the figure, the hot rolling finishing temperature (FT) needs to be 900 to 700°C.

Normally, finishing is done in the γ region between 825 and 900°C, but so-called two-phase rolling at a lower temperature of 700 to 825'c was also adopted due to a mismatch with the subsequent cooling and winding conditions.
There is no effect on material properties.

Next, describe the cooling rate v: (CR)In after hot rolling.

The cooling pattern after the hot needle can be broadly classified as shown in Figure 8. (υ is to obtain the desired ferrite fraction.
a L, < , (p), ■ is a convenient pattern for obtaining the desired result n. Hold (waha O, Mn, Wb
It is used for the 11I type steel, which contains a large amount of ferrite and has a slow ferrite transformation, and (:) is a relatively low-component steel type and is used for a fast ferrite transformation.

For the next winding 2, the temperature needs to be 350 to 575°C, which is important for making the @2 phase into an upper bainite structure.
If it is wound at a temperature of 575°C or higher, there is a risk of pearlite appearing, and if it is less than FJ50t, mano 1 tenside may appear.

Examples of the present invention will be shown below along with comparative examples.

Table 2 shows the chemical composition of the test materials, Table 2 shows the hot rolling conditions and the structure of the produced hot rolled steel sheets, and Table 8 shows the mechanical properties.

Table 1 Chemical composition of sample material (wt%) Table 8 Mechanical properties 1) JII No. 118 B 2) Initial hole: 80-φ Punching allowance, punch: 6oφ,
800 conical 5) J1a+ 1/4 size C direction The relationship between tensile strength, hole expansion ratio (stretch flangeability) and impact value for each structure of the steel plate in Table 2.8 is shown in Figure 1.2, but as mentioned above. As shown, F+υB manufactured by the present invention
#I has good elongation 7 lunge properties and impact properties.

Next, for lines B and C in Table 1, B- in Table 2 respectively.
Hot-rolled steel sheets were manufactured under almost the same hot-rolling conditions as No. 3 and C-6, and were actually formed into wheel rims and disks to examine formability. The results are shown in Table 4.

Table 4 Results of wheel rim and disk forming As can be seen from Table 4, the hot rolled steel sheet according to the present invention has a fairly low defect rate during forming Nile rims and disks, and has no practical problems. The biggest reason for this is that no softening phenomenon occurred in the heat-affected zone during rim forming and butt welding, and cracking around the hub holes and bolt holes occurred in disc forming. Because it didn't happen.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between tensile strength and hole expansion ratio for a with various structures, Figure 2 is a diagram showing the relationship between tensile strength and impact properties, and Figure 48 is a diagram showing the cooling pattern after hot rolling. FIG. Patent applicant: Kobe Steel, Ltd.

Claims (1)

[Claims] (υOO, 0290, 25%, MJI 0.8-d2%
, 8i 0l-1ji%, finishing temperature 700 ~
Average cooling speed Ff4~10'' after hot rolling at 9001X
c/f! slow cooling at 8 to 2 p, then at an average cooling rate of [50
A method for manufacturing a high-strength hot-rolled steel sheet characterized by forming a polygonal ferrite + upper bainite structure containing an upper bainite structure in an area ratio of 8 to 6004 by ~b 0/ZrO,02→one or more % alcohol Finishing steel containing temperature 7
Average cooling rate after hot rolling at 00~900″C 4~
10 for 7 seconds, slow cooling for 8 to 20 seconds, then average cooling rate of 5
It is characterized by a polygonal anilite + upper bainite structure containing an upper bainite structure of 8 to 60% in iw ratio by rapidly cooling it at 0 to 100 in 7 seconds and winding it with 850-575. A method for producing high-strength hot-rolled steel sheets. (3) 00.02-025%, MnO, 8-2%, 8
Finishing temperature of steel containing 10.1-1.5% 700-900
t) After hot rolling at C, cool for 1 to 10 seconds in 7 seconds at an average cooling rate of 20 to 50, then average cooling rate of 4 to]ot/
e) for 8 to 20 seconds, and further at an average cooling rate of 50 to 10
High-strength hot rolling characterized by being rapidly cooled at 0° C. and 400° C. and rolled to 850 to 575″Ct, resulting in a polygonal anilite + upper bainite structure containing an area ratio of 8 to 60% upper bainite structure. Method of manufacturing steel plates. After hot rolling at ~900t, the average cooling rate is 20~5
Cool at 0°C for 1 to 10 seconds at 4°C, then at an average cooling rate of 4°C.
~10℃ 1 hour for 8~20 seconds, then an average cooling time of 50~100℃, 1~20 seconds rapid cooling, 850~575 4 rolls to obtain an upper bainite structure with an area ratio of 8~60
% of polygonal ferrite + upper bainite structure.