JPH0238525A - Manufacture of hot rolled high strength steel sheet - Google Patents

Abstract

PURPOSE:To manufacture the title steel sheet having excellent press formability in the sheet width edge part by executing specific hot rolling, coiling and continuous annealing to a steel having specific compsn. constituted of C, Si, Mn, P, S, Al and Fe. CONSTITUTION:A steel contg., by weight, 0.10 to 0.25% C, <=2.0% Si, 1.0 to 3.0% Mn, <=0.10% P, <=0.010% S and 0.01 to 0.10% solAl, furthermore contg., at need, 0.015 to 0.10% Nb or Ti and the balance Fe with inevitable impurities is subjected to hot rolling. At this time, the finish temp. in the plate width center part is regulated to >=800 deg.C as well as >=40 deg.C higher than that of the plate width edge part. The steel is furthermore coiled at >=30 deg.C/sec cooling speed at 650 to 400 deg.C in the plate width center part. After pickled, the above hot rolled plate is subjected to continuous annealing in such a manner that it is heated to Ac1 point to 900 deg.C for 1 to 5min and is thereafter cooled to >=400 deg.C at <=50 deg.C/sec cooling speed. In this way, the structure of the steel plate is regulated to a low temp. transformation-forming phase of ferrite and martensite having >=30% volume rate, by which the hot rolled high strength thin steel plate having excellent press formability in the plate width edge part and having >=70kg f/mm<2> tensile strength can be obtd.

Description

[Detailed Description of the Invention] Kokurijo ■Hadaho Public Corporation The present invention provides a hot rolled high tensile strength thin steel plate having a tensile strength of 70 kgf/mm2 or more and a plate thickness of 2 fi or less, which has excellent press formability at the ends in the width direction. Relating to a manufacturing method. Such high tensile strength thin steel sheets are suitably used, for example, in press forming of automobile bumpers and the like.

In recent years, with the aim of improving fuel efficiency by reducing the weight of automobiles,
There is an increasing demand for higher strength of members such as armature bumpers, hangers, center beams, etc., and for example, the use of "81!a plate" with a thickness of 2 cm or less and a tensile strength of 70 kgf/mm2 or more is being considered. As shown in Figure 1, the above-mentioned members are often long, measuring 1,300 to 1,600 mm, and are often drawn at their longitudinal ends, so in addition to high strength, they have poor press formability. It is required to be excellent in.Furthermore,
In order to improve product yield, if the longitudinal direction of the member is taken as the width direction of the coil, generally the edges in the width direction of the coil plate (hereinafter referred to as the edges) where the material deteriorates are drawn. Therefore, improving press formability at the edge of the sheet width has become an important issue.

However, conventionally, it has been extremely difficult to manufacture a wide thin hot-rolled coil with high strength and uniform material. In some cases, edge heaters and the like are used to reduce the deterioration of the material at the ends of the coil width, but this is still not sufficient.

The inventors of the present invention discovered the following facts in the course of various studies related to the production of high-strength hot-rolled thin steel sheets with uniform inner coil material: C0.16%, Si0. Steel consisting of 25%, Mn 1.63%, P 0.014%, S 0.002%, An 0.033%, the balance being iron and unavoidable impurities was heated to 1200°C, and then heated to a finishing temperature of 860°C. Hot rolled, 580℃
Thickness l, 5 mm, width 1000 rolls rolled with heat, rolled T
As shown in FIG. 2, the itWa plate has extremely high strength at the width ends and extremely low ductility. However, after soaking such a hot-rolled thin steel sheet to a temperature of 830°C,
Continuous annealing treatment with water quenching from a temperature of
When over-aging treatment is carried out at 250°C, as shown in Figure 3, the strength is improved overall, and the strength is particularly improved at the center in the width direction of the plate (hereinafter referred to as the center of the plate width). On the other hand, it has been found that the width edges of the plate are softened and the ductility is excellent at the width edges of the plate. However, in FIGS. 2 and 3, the board width end refers to a position 25 m in the width direction from the board width end.

Therefore, in order to take advantage of the above-mentioned changes in material properties due to continuous annealing in the industrial production of hot-rolled thin steel sheets, the present inventors aimed to ensure strength at the center of the sheet width and to maintain softness at the ends of the sheet width. We have conducted extensive research on the conditions for stably achieving this goal. As a result, it was found that the control of the microstructure in the width direction of the as-hot-rolled screen sheet greatly influences the softening of the width end portion of the steel sheet after continuous annealing. In other words, by producing a finer structure or a larger amount of low-temperature transformation products such as bainite and martensite at the edges of the strip width compared to the structure at the center of the strip width, the strip width can be reduced after continuous annealing. The ends can be made softer. The reason for this is not necessarily clear, but it is probably because a finer structure produces finer austenite in the heating process after continuous annealing, and the transformation of austenite accelerates in the subsequent cooling process.

Furthermore, in order to stably increase the strength at the center of the width of a steel plate after continuous annealing, the present inventors have conducted intensive research on the conditions during the hot rolling process, and an example thereof is shown in Fig. 4. As shown, the strength difference between the edge and center of the steel plate after continuous annealing has a correlation with the difference in hot rolling finishing temperature between the center and edge of the plate, and The difference in bow tensile strength (ΔT S (kgf/mm2)) between the center width and the end of the board width, which is defined as the 50-point position in the width direction from the end of the board width, is determined by the formability in actual press forming. There is a difference in 3
kgf/mm2 or higher, in hot rolling finishing, the finishing temperature at the center of the sheet width should be 40°C higher than that at the ends of the sheet width.
We found that it was necessary to raise the value even higher. in this way,
Setting the finishing temperature lower at the edge of the strip than at the center of the strip makes the austenite before transformation more fine at the edge of the strip, and lowers the winding temperature at the edge of the strip. It seems that lowering δ is related to promotion of bainite transformation due to δ.

The present invention has been made based on the above-mentioned knowledge, and by making the width edges of the plate more flexible, the tensile strength of the width edges of the plate is 70 kgf/mm, which provides excellent press formability.
The object of the present invention is to provide a method for manufacturing a hot-rolled high-strength thin steel sheet having a thickness of 2 inches or less.

The method for manufacturing a hot-rolled high-strength thin steel sheet having a tensile strength of 70 kgf/mm2 or more and having excellent press formability at the width end of the sheet according to the present invention in order to accomplish Section 2 is a method for manufacturing a hot-rolled high-strength thin steel sheet having a tensile strength of 70 kgf/mm2 or more and having excellent press formability at the sheet width end. .25%, Si 2.0% or less, Mn 1.0-3.0%, P 0.10% or less, S 0.010% or less, sol A, Il 0.01-0.10%, balance iron and When hot rolling steel containing unavoidable impurities,
The finishing temperature at the center of the sheet width is 8o'o'c or more, and the temperature at the center of the sheet width is 40 degrees Celsius or more higher than the ends of the sheet width.Furthermore, after finishing rolling, coil winding is carried out. After forming a hot-rolled coil with an average cooling rate of 30°C/sec or more, a coil winding temperature of 650 to 400°C at the center of 41i, 1pM, pickling, and then continuous annealing, Ac + transformation point ~ 1 at a temperature in the range of 900'C
After heating for ~5 minutes, the steel plate structure is transformed to a low-temperature transformation with a volume fraction of 30% or more consisting mainly of noelite and martensite by cooling to a temperature of 400°C or less at an average cooling rate of 50°C/second or more. It is characterized by being a generative phase.

First, the chemical composition of the steel used in the present invention will be explained.

It is necessary to add at least 0.10% of C to ensure a tensile strength of 70 kgf/mm2 or more.

However, when added in excess of 0.25%, spot weldability deteriorates significantly.

Si has a remarkable effect on improving the elongation of high-strength steel sheets, but when the amount added exceeds 2.0%, it not only deteriorates the paintability but also increases the manufacturing cost.

In addition to making the ferrite grains fine after hot rolling, Mn
It is also effective in generating bainite structures.

Thus, the tensile strength after continuous annealing is 70 kgf/mm2.
In order to achieve this, it is necessary to add 1.0% or more, but if it exceeds 3.0%, spot weldability deteriorates.

P is effective in strengthening steel, but addition of an excessive amount not only deteriorates spot weldability but also deteriorates surface and vertical cracking properties, so it is limited to 0.10% or less.

As its content increases, S causes an increase in the amount of nonmetallic inclusions such as MnS, and deteriorates the stretch flange workability, bending workability, etc. of the steel sheet. According to the invention, R.E.M.
The shape of the nonmetallic inclusions can be controlled by adding elements such as XZr and Ca, but even in this case, it is preferable that the amount of S is small, and in the present invention, the amount of S is preferably 0.
0.010% or less.

Since the steel plate according to the present invention is Ap killed steel, 0.01
It is necessary to sufficiently deoxidize the steel by adding % or more. However, even if it is added in an amount exceeding 0.10%, the deoxidizing effect will be saturated and it is also undesirable from the viewpoint of manufacturing costs, so the upper limit of the amount added is set at 0.10%.

In the present invention, in addition to the chemical components described above, at least one element selected from the group consisting of Nb and Ti is further added to the steel in order to refine the microstructure of the steel and improve hardenability. Can be done.

In order to effectively obtain the above effects, one or two of the above elements must be used.
It is necessary to add 0.015% of the total amount of seeds. However, addition of an excessive amount exceeding 0.10% saturates the effect and is also unfavorable from an economic point of view.

Next, manufacturing conditions in the present invention will be explained.

The hot-rolled thin steel sheet according to the present invention has a finishing temperature of 800° C. or higher at the center of the sheet width when hot rolling steel having the above-mentioned chemical composition, and the center of the sheet width is lower than the end portions of the sheet width. The temperature is also higher than 40℃, and after finish rolling,
The average cooling rate during coil winding is 30℃/second or more,
The coil winding temperature at the center of the plate width is 650 to 400℃.
After making it into a hot-rolled coil, it is pickled. Next, when continuously annealing this hot-rolled coil, the Ac+transformation point ~9o
After heating for 1-5 minutes at a temperature in the range of o 'c,
By cooling at an average cooling rate of 50°C/sec or more to a temperature of 00°C or lower, the steel sheet structure can be obtained by forming a low-temperature transformation phase with a volume fraction of 30% or more consisting mainly of ferrite and martensite. can.

In the method of the present invention, the hot rolling finishing temperature is 800°C.
That's all. When the temperature is lower than 800° C., the degree of rolling in the ferrite region becomes large, and the Mi weave of the hot rolled steel sheet becomes elongated, which deteriorates the workability of the steel sheet after continuous annealing.

The upper limit of the finishing temperature is not particularly limited, but is usually 920'C in consideration of operability in actual production.

Furthermore, 5 (1++
The difference in tensile strength (
In order to make ΔTS (kgf/1n2) 3 kgf/mm” or higher, which is the difference in formability in actual press forming, the finishing temperature at the center of the sheet width during hot rolling finishing must be adjusted as described above. It is necessary to increase the temperature at least 40°C higher than that at the edge of the sheet width, and according to the present invention, under these conditions, it is possible to stably increase the strength at the center of the width of the steel sheet after continuous annealing. It is preferable that the finishing temperature difference between the central part of the plate width and the ends of the plate width be large, but in consideration of the stability of the material inside the coil, it is desirable that the difference be 100° C. or less.

After such finish rolling, it is necessary to coil at a temperature in the range of 650 to 400°C with an average cooling rate of 30°C/second or more. If the average cooling rate is lower than 30° C./sec, in the present invention, which uses steel with relatively low C and Mni contents, it is not possible to form a fine structure at the width end portion of the sheet after hot rolling. Although the upper limit of the average cooling rate is not particularly limited from a metallurgical point of view, it is usually about 00° C./sec based on equipment capacity. According to high temperature winding where the winding temperature exceeds 650°C and low temperature winding which is lower than 400°C,
In both cases, in practice, the change in the structure in the width direction of the sheet after hot rolling is small, and as a result, according to the present invention, it is not possible to sufficiently control the material quality in the width direction of the sheet after continuous annealing.

After obtaining the hot-rolled coil as described above, it is pickled according to a conventional method, and then, according to the present invention, in continuous annealing, the coil is subjected to continuous annealing for 1 to 5
Heat for a minute. The heating temperature is Ac.

When it is lower than the point, the specified strength cannot be obtained,
On the other hand, when the temperature exceeds 900° C., the strength of the width end portion of the sheet ultimately increases due to the growth of austenite grains, so that softening of the width end portion of the sheet cannot be achieved. When the heating time is shorter than 1 minute, the entire steel plate cannot be heated uniformly in actual operation, and when heating in a low temperature region such as directly above the AC point, it is difficult to uniformly form austenite to achieve high strength. cannot be realized. Heating for more than 5 minutes,
This is mainly disadvantageous from the economic point of view of steel manufacturing.

After the heating, the steel plate is cooled to 400° C. or lower at an average cooling rate of 50° C./second or higher in order to generate martensite in the cooling process. However, from the viewpoint of improving drawability, it is desirable to select the heating temperature and cooling rate to finally generate a certain amount of ferrite in the steel sheet. That is, a steel plate heated in ferrite/austenite I/two-phase regions can be subjected to water quenching treatment at a cooling rate of 1000° C./sec or more. On the other hand, according to the present invention, overaging treatment can also be performed by cooling to a temperature of about 350°C at an average cooling rate of 50 to 150°C/sec. Alternatively, after rapidly cooling to room temperature, overaging treatment may be performed.

In order to obtain a tensile strength of 70 kgf/mm2 or more, as mentioned above, in addition to solid solution strengthening of ferrite with Si, Mn, etc., it is essential to strengthen with a low-temperature transformation generated structure such as martensite according to the present invention. It is necessary to generate a low-temperature transformation phase of 30% or more by volume. The upper limit is not particularly limited, but is 70 to 70 in terms of material strength.
To achieve 120 kgf/mm2, usually 80~
85% is sufficient.

As described above, the steel sheet according to the present invention has a uniform material in the longitudinal direction of the coil, has high strength in the center of the sheet width, and has excellent press formability at the ends of the sheet width. , it can be used as it is to manufacture a member that is press-formed up to the end of the coil width. However, in order to bring the surface quality closer to that of a cold-rolled steel sheet, a skin bath may be applied, if necessary, prior to continuous annealing, or the continuously annealed steel sheet may be electroplated.

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

A slab having the chemical components shown in Table 1 is melted and hot-rolled under the hot rolling conditions shown in Table 1.
The hot-rolled coil had a width of 1.6 mm and a plate width of 950 to 1,450 mm. Next, this was pickled and subjected to continuous annealing using a water quenching method under the conditions shown in Table 1 to obtain a steel plate. Its mechanical properties are shown in Table 1. The tensile properties were examined using a JIS No. 5 test piece, and the hole expansion rate was examined using a test piece punched with a hole of 10 diameter. Further, width trimming was not performed after hot rolling, and trimming after continuous annealing was not performed.

As is clear from the results shown in Table 1, the steel plate according to the present invention has softened width edges and has excellent press formability. In Comparisons 1ii12 and 4 to 6, although the ΔTS is 3 kgf/mm2 or more, the difference in finishing temperature between the center part of the board width and the ends of the board width is smaller than 40°C, so the board width is The ductility of the edges is lower than that of the center of the plate width. Comparative Steel 11 has a tensile strength of less than 70 kgf/mm because the C content and Mn content are smaller than the amounts prescribed in the present invention. Comparative Steel 12 has a tensile strength of Since the difference in finishing temperature between the two is smaller than 40° C., the softening of the width end portion of the plate is insufficient and ΔTS is small.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing an example of a bumper center beam, Fig. 2 is a graph showing an example of mechanical properties of a hot rolled coil in the sheet width direction, and Fig. 3 is a perspective view showing an example of a bumper center beam. A graph showing the mechanical properties in the width direction of the sheet after continuous annealing of the rolled coil.
FIG. 4 is a graph showing the relationship between the difference (ΔFDT) between the strip width center and the strip width end at the hot rolling finishing temperature and the strength difference in the strip width direction (ΔTS) of the steel sheet after continuous annealing. Patent applicant Kobe Steel, Ltd. Representative Patent attorney Itsube Makino

Claims (2)

[Claims] (1) C0.10-0.25% by weight, Si2.0% or less, Mn1.0-3.0%, P0.10% or less, S0.010% or less, solAl0.01-0.10 %, when hot rolling steel consisting of the remainder iron and unavoidable impurities, the finishing temperature at the center of the plate width is 800°C or higher, and the center of the plate width is 40°C or more higher than the ends of the plate width. After final rolling, the average cooling rate during coil winding was set at 30°C/sec or more, and the coil winding temperature at the center of the sheet width was set at 650 to 400°C to form a hot rolled coil, and then pickled. Then, during continuous annealing, Ac
_1 After heating at a temperature in the range of transformation point to 900℃ for 1 to 5 minutes, the average cooling rate to a temperature of 400℃ or less is 50℃.
By cooling at a rate of ℃/second or more, the steel sheet structure is reduced to a volume ratio of 30% consisting mainly of ferrite and martensite.
A tensile strength of 70 kgf/mm^ with excellent press formability at the edge of the plate width, characterized by the above-mentioned low-temperature transformation formation phase.
2 or more methods for producing hot rolled high tensile strength thin steel sheets. (2) In weight% (a) C0.10-0.25%, Si2.0% or less, Mn1.0-3.0%, P0.10% or less, S0.010% or less, and solAl0.01 ~0.10%, and further contains (a) at least one element selected from the group consisting of Nb and Ti in a total amount of 0.015 to 0.10%, the balance being iron and unavoidable impurities. When hot rolling steel made of After forming a hot-rolled coil with an average cooling rate of 30 ° C / sec or more during coil winding and a coil winding temperature of 650 to 400 ° C at the center of the plate width, pickling, and then continuous annealing. , Ac
_1 After heating at a temperature in the range of transformation point to 900℃ for 1 to 5 minutes, the average cooling rate to a temperature of 400℃ or less is 50℃.
By cooling at a rate of ℃/second or more, the steel sheet structure is reduced to a volume ratio of 30% consisting mainly of ferrite and martensite.
A tensile strength of 70 kgf/mm^ with excellent press formability at the edge of the plate width, characterized by the above-mentioned low-temperature transformation formation phase.
2 or more methods for producing hot rolled high tensile strength thin steel sheets.