JPH04276015A - Manufacture of hot rolled steel sheet excellent in bore expandability - Google Patents

Abstract

PURPOSE:To offer a hot rolled steel sheet constituted of ferrite and pearlite, having >55kgf/mm<2> strength and excellent in surface properties and bore expandability. CONSTITUTION:A slab having a steel compsn. constituted of, by weight, 0.05 to 0.12% C, 0.8 to 1.8% Mn, 0.3 to 1.5% Si, 0.10 to 1.2% Cr, <=0.02% S, 0.0005 to 0.0100% Ca and/or 0.005 to 0.050% rare earth metal and the balance Fe with inevitable impurities is heated to the temp. range of >=1280 deg.C and is descaled to remove the formed fayalite. After that, this slab is subjected to hot rolling, and the above hot rolling is finished at >=880 deg.C finishing temp. Then, this rolled stock is cooled at <=20 to 30 deg.C/s cooling rate and is coiled at the temp. range of 450 to 600 deg.C.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing hot rolled steel sheets with excellent hole expandability. More specifically, the present invention is made of ferrite and pearlite, and has a weight of 55 kgf/mm.
The present invention relates to a method for producing a hot-rolled steel sheet having a strength (tensile strength, hereinafter the same) exceeding 2.0 and excellent surface texture and hole expandability.

0002

BACKGROUND OF THE INVENTION In recent years, in the automobile industry, lightweight and high-strength hot-rolled steel sheets have been used to reduce the weight of vehicles in order to improve their fuel efficiency. As such high-strength hot-rolled steel sheets, hot-rolled steel sheets having a mixed structure consisting of a ferrite-martensitic structure have conventionally been widely used. Some of the proposals regarding such conventional high-strength hot-rolled steel sheets are listed below.

[0003] Japanese Patent Publication No. 57-42127 After cooling a steel plate having a specific composition from the austenite single phase region at a cooling rate within a specific value range to generate ferrite grains with a volume fraction of 40% or more, A method of manufacturing a low-yield point, high-strength steel plate with excellent workability and a mixed structure mainly composed of ferrite and martensite by coiling and cooling this steel plate at a specific temperature range.

[0004] ■ Japanese Patent Publication No. 58-27329 A steel billet having a specific composition is hot-rolled at a temperature below (Ar point 3 + 60°C) or at a temperature in the ferrite-austenite two-phase coexistence region, and then heated to 450°C or below. A method of manufacturing high-strength, low-yield-ratio hot-rolled steel sheets with excellent ductility.

[0005] ■ Japanese Patent Publication No. 58-24489 A steel billet having a specific composition is heated at (Ar3 point -30°C)
r3 point + 60℃) Finish hot rolling in the following temperature range,
By winding at a temperature of 250 to 480 °C, Si
- A method for producing a Cr-based high-ductility, low-yield-ratio hot-rolled high-strength steel sheet.

[0006] Japanese Patent Publication No. 61-10009 A steel billet having a specific composition is heated to 1050 to 1180°C and hot rolled, the hot rolling is completed in a temperature range of 800 to 900°C, and then A method for producing a dual-phase high-strength steel sheet with excellent workability and having ferrite-martensite as the main structural phase by cooling it to 350 to 500°C and winding it up.

[0007] ■ Japanese Patent Publication No. 11291/1983 A yield ratio of 65% or less is achieved by performing stepwise cooling control in which a hot rolled steel sheet having a specific composition is rapidly cooled under specific conditions after finishing rolling, and then rapidly cooled again. , the value of the strength-elongation balance parameter M is 60 or more, and there is little material variation;
A method for producing a low yield ratio high tensile strength steel sheet having a mixed structure that is a dispersed mixture of a ferrite phase and a martensitic phase and having excellent cold workability.

[0008] JP-A-57-143433 A steel billet having a specific composition is hot rolled at 750 to 900°C, and after finish rolling is cooled at an average cooling rate of 4 to 50°C/sec. By winding at ~650°C and making the structure after hot rolling into a composite structure consisting of ferrite and low-temperature transformation products (martensite), a low yield ratio hot-rolled high-strength steel sheet with excellent stretch flangeability is manufactured. Method. ■Unexamined Japanese Patent Publication No. 62-93003 A high-temperature slab is obtained by continuous casting of steel having a specific composition so that the cooling rate during solidification becomes a specific value, and before the temperature of this slab reaches 550 °C. , the temperature of the slab is 1
If the temperature of the slab is less than 050°C, heating is performed, and if the temperature of the slab is 1050°C or higher, heating is not performed,
By starting hot rolling, finishing the hot rolling at a temperature above Ar3 point, and then performing controlled cooling at a predetermined temperature, the martensite grains produced are finely and uniformly dispersed, resulting in extremely high A method of producing ductile high strength hot rolled steel sheet.

[0009] JP-A No. 62-93006 A slab is obtained by continuous casting, and before the slab reaches 550°C, after heating if the temperature of the slab is less than 1050°C, If the temperature of the slab is 1050°C or higher, it is heated directly to 1050 to 1150°C without heating.
By starting the hot rolling at a temperature of 100 mL, finishing the hot rolling at a temperature of Ar3 or higher, and then performing predetermined controlled cooling, a transformation-strengthened high-temperature steel having a specific composition and containing bainite is produced. A method of producing high-strength hot-rolled steel sheets.

0010

[Problem to be solved by the invention] However, when these hot-rolled steel sheets having a ferrite-martensitic structure are subjected to hole expansion processing, microvoids are generated around the martensite from the start of deformation, causing cracks. Since this occurs easily, there is a problem that the hole expandability is poor. Insufficient hole expandability is an extremely important problem when used as steel sheets for automobiles, and a solution to this problem is desired.

[0011] In addition to these steel types having a mixed structure of ferrite and martensite, for example, even in steel types to which a large amount of Si is added, island-like scales occur on the surface, impairing the aesthetic appearance, and requiring severe forming. When subjected to processing, there was also the problem that cracks were likely to occur at the interface between the island scale and the base iron. Here, the purpose of the present invention is to solve the problems of the above-mentioned conventional technology,
High-strength hot-rolled steel sheet with excellent surface texture and pore expandability, specifically, made of ferrite and pearlite and has a tensile strength of over 55 kgf/mm2 and has excellent surface texture and pore expandability. The object of the present invention is to provide a method for manufacturing hot-rolled steel sheets.

0012

[Means for Solving the Problems] As a result of extensive research in order to solve the above problems, the present inventors have determined that the composition of the slab is limited to a specific range, and the heating temperature of the slab is limited to a specific temperature range. By performing descaling, the fieryite generated on the slab surface is completely removed, and the finishing temperature of the subsequent hot rolling, the cooling rate after the hot rolling, and the coiling temperature are limited to specific ranges. By this,
The present invention was completed by discovering that it is possible to produce a hot-rolled steel sheet that has a high strength of over 55 kgf/mm2 and has excellent surface texture and hole expandability.

The gist of the present invention is that C: 0.05-0.12%, Mn: 0.8-0.8% by weight.
1.8%, Si: 0.3-1.5%, Cr: 0.1
0 to 1.2%, S: 0.02% or less, as necessary, Ca: 0.0005 to 0.0100% and/or REM: 0.005 to 0.050%, balance: F
A slab with a steel composition from e and unavoidable impurities,
After heating to a temperature range of 1280°C or higher, descaling is performed to remove the generated fireite, hot rolling is performed, and the hot rolling is finished at a finishing temperature of 880°C or higher, and then 20 to 30°C/ Cool at a cooling rate of 450~
This is a method for producing a hot-rolled steel sheet made of ferrite and pearlite that is characterized by being rolled in a temperature range of 600° C., has a strength of over 55 kgf/mm2, and has excellent surface texture and hole expandability.

[0014]

[Function] The present invention will be explained in detail below along with its functions and effects. In addition, in this specification, unless otherwise specified, "
%” shall mean “% by weight”. First, the reason for limiting the composition of the slab used in the present invention will be explained.

C: 0.05 to 0.12% C is an element effective in increasing strength, but on the other hand, it is an element that has a significant detrimental effect on moldability. Therefore, the C content is
From the viewpoint of improving moldability, it is desirable that the content be as low as possible, but if it is less than 0.05%, the target strength cannot be obtained, while if it exceeds 0.12%, moldability will deteriorate. Therefore, the C content is limited to 0.05% or more and 0.12% or less.

Mn: 0.8 to 1.8% Mn increases the strength of steel through solid solution strengthening, but if the content is less than 0.8%, this effect is small, while if it exceeds 1.8%, there is no good effect. Elongation is not ensured. Therefore, the Mn content is 0.8
% or more and 1.8% or less.

[0017] Si: 0.3 to 1.5% Si improves the strength of steel through solid solution strengthening due to its increased content, and is also effective in producing and purifying ferrite, which contributes to improving formability. It is an element that improves the strength of steel and has the effect of not deteriorating formability. This effect is not sufficiently exhibited at a content of less than 0.3%, while at a content of more than 1.5%, the effect is saturated and may actually deteriorate weldability. Therefore, the Si content is
Limited to 0.3% or more and 1.5% or less.

Cr: 0.10-1.2% Cr increases the strength of steel by solid solution strengthening as its content increases. Furthermore, although the reason is not clear, it is thought that in the present invention, when Cr is added, the pearlite structure, which is the second phase, is uniformly dispersed, which particularly contributes to improving the pore expandability. This effect is not sufficiently exhibited at a content of less than 0.10%, and the effect is saturated at a content of more than 1.2%, which may actually deteriorate weldability. Therefore, C
The r content is limited to 0.10% or more and 1.2% or less.

S: 0.02% or less S is an element harmful to pore expandability, and the content is 0.02% or less.
If it exceeds 2%, the pore expandability will be significantly degraded. Therefore,
The S content is limited to 0.02% or less. In addition, 0.00
It is desirable to reduce it to 1% or less.

Ca: 0.0005 to 0.0100% and/or REM: 0.005 to 0.050% Ca or REM is added as necessary in the present invention. In order to improve the pore expandability, it is effective to reduce the S content to reduce sulfide inclusions and to make the sulfide inclusions spheroidal. In order to make the sulfide inclusions spheroidal in this way, Ca is 0.0
It is effective to contain 0.005% or more and REM 0.005% or more, while more than 0.0100% and 0.0100% or more, respectively.
If the content exceeds 0.050%, the effect of spheroidization is saturated and inclusions increase, resulting in the opposite effect. Therefore,
In the present invention, Ca: 0.0005% or more 0.0
It is desirable to contain 100% or less and/or REM: 0.005% or more and 0.050% or less. Compositions other than the above are Fe and inevitable impurities. Next, the manufacturing conditions of the present invention will be explained.

Heating temperature: 1280°C or higher First, a slab having the above composition is heated to a temperature of 1280°C or higher. By heating to a temperature of 1280°C or higher and subsequent descaling, the generated firelite (
Fe2SiO4) can be completely removed. Firelite is formed at temperatures above 1170°C, but since the slab was conventionally heated to a temperature range of about 1200 to 1230°C, firelite grows as if its roots are growing on the surface of the steel base beneath the scale layer. It was forming as strongly as before. For this reason, when descaling is performed, the scale layer can be removed, but the firelite generated under the scale layer cannot be removed, and remains as island scale on the surface of the hot rolled steel sheet. The surface quality deteriorated.

On the other hand, in the present invention, in order to prevent fireite from remaining as island-like scales after descaling, the firelight is removed so that the strongly formed parts that look like roots are removed. Generate light uniformly on the surface of the substructure. By uniformly generating firelite on the surface of the base metal in this way, it becomes possible to remove firelite by descaling. That is, in the present invention, the slab is heated to a higher temperature (1200 to 12
(30℃→1280℃ or higher) to uniformly generate firelite on the surface of the base metal, which enables subsequent removal by descaling. The heating temperature is desirably 1300°C or higher from the viewpoint of firelight removal effect, and 1330°C or higher from the viewpoint of energy consumption.
Desirably below ℃.

[0023] Hot rolling finishing temperature: 880°C or higher The slab from which fireite has been removed is hot rolled. The rolling reduction during hot rolling is not particularly limited. In the present invention, the hot rolling finishing temperature is limited to 880°C or higher. If the hot rolling finishing temperature is less than 880 °C, multiple band structures are formed in the microstructure parallel to the rolling direction, and cracks occur from the periphery of the band structures when holes are expanded. This is because the hole expandability deteriorates.

[0024] When the hot rolling finishing temperature is 880°C or higher,
The reason why the band structure is not formed is not clear, but it is thought to be as follows. That is, what appears in the band structure are ferrite grains crushed in the rolling direction, and it is thought that these ferrite grains are formed from austenite grains crushed in the rolling direction. Therefore, by finishing hot rolling at 880°C or higher, rolling in the unrecrystallized austenite region can be avoided to a large extent, and austenite grains crushed in the rolling direction are not generated, so that the band structure is not formed. It is thought that there is no.

Cooling rate from finishing temperature to coiling temperature: 2
0~30℃/s After finishing hot rolling at the above finishing temperature, 20~30℃/s
Cool at a cooling rate of s and wind up. The cooling rate is 20℃
If the temperature is less than /s, the microstructure becomes a coarse structure consisting of ferrite and pearlite, and when the hole is expanded, cracks are likely to occur around the pearlite portion, deteriorating the hole expandability. On the other hand, if the cooling rate exceeds 30° C./s, bainite structure will be mixed in and cracks will easily occur around the bainite when the hole is expanded, which will also deteriorate the hole expandability.

Coiling temperature: 450°C or more and 600°C or less After cooling at the above cooling rate, 450°C or more and 600°C
Wind at the following winding temperature. If the coiling temperature exceeds 600°C, the target strength cannot be obtained, whereas if the coiling temperature is lower than 450°C, high strength can be obtained, but the pore expandability deteriorates. Therefore, in order to ensure high strength and excellent hole expandability of the hot rolled steel sheet obtained by the present invention, the coiling temperature is limited to 450°C or more and 600°C or less.

In this way, according to the present invention, it is possible to produce a hot-rolled steel sheet made of ferrite and pearlite and having a strength of over 55 kgf/mm 2 and excellent surface texture and hole expandability. Further, the present invention will be described in detail with reference to Examples, but these are merely illustrative of the present invention and are not intended to limit the present invention.

[0028]

[Example] Slabs A to Slab H having the compositions shown in Table 1 were heated and then descaled to remove fireite generated on the slab surface, followed by hot rolling, cooling, and winding. Sample No., which is a hot-rolled steel plate,
．． 1 to sample no. I got 14.

[0029]

[Table 1]

[0030] The heating temperature during the heating, the finishing hot rolling temperature during the hot rolling, the cooling rate during the cooling, and the winding temperature during the winding are shown in Table 2. It was made a condition.

Sample No. thus obtained. 1 to sample no. 14, ■ Prepare a JIS No. 5 test piece and conduct a tensile test to measure the yield point, tensile strength, and elongation. ■ Use a conical punch to perform a hole expansion test on a hole with an initial diameter of 12 mm. , hole expansion rate = 100
0x(D-12)/12 (where D is the diameter at the time of crack initiation) was measured, and the surface texture was visually inspected. No island scale was rated as ○, and island scale was rated as x. The results are summarized in Table 2.

[0032]

[Table 2]

[0033] In Table 2, when the tensile strength is 55 to 60 kgf/mm2 and the pore expansion rate is 120% or more, the tensile strength is 60 to 70 kgf/mm2.
/mm2, the pore expansion rate was 110% or more, and when the tensile strength was over 70 kgf/mm2, the pore expansion rate was 100% or more, respectively, as passing. As is clear from Table 2, the specimen produced by manufacturing the slab having the steel composition within the range specified by the present invention under the conditions within the range specified by the present invention has a tensile strength of 55 kgf/
mm2 and has excellent surface properties and pore expandability.

[0034]

[Effects of the Invention] As detailed above, the present invention provides a high-strength hot-rolled steel sheet with excellent hole expandability, specifically made of ferrite and pearlite, and with a diameter of 55 kgf/mm.
It has become possible to provide a hot-rolled steel sheet that has a strength exceeding 2.0 and has excellent surface properties and hole expandability. The significance of the present invention having such effects is extremely significant.

Claims (2)

[Claims] Claim 1: C: 0.05 to 0.12 in weight%
%, Mn: 0.8-1.8%, Si: 0.3-1.5
%, Cr: 0.10-1.2%, S: 0.02%
Hereinafter, the remainder: A slab having a steel composition consisting of Fe and unavoidable impurities is heated to a temperature range of 1280 °C or higher, descaled to remove the generated fireite, and then hot rolled to a temperature of 880 °C or higher. The hot rolling is finished at a finishing temperature of
/mm2 A method for producing a hot-rolled steel sheet with excellent surface texture and hole expandability with super strength. 2. The steel composition further comprises Ca in weight percent.
:0.0005~0.0100% and/or RE
2. The method for producing a hot-rolled steel sheet with excellent surface texture and hole expandability according to claim 1, characterized in that M: 0.005 to 0.050% is contained.