JP2003253388A - High-strength hot-rolled steel sheet and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet with a high-strength composite structure, which has a low yield ratio, and is superior in workability, impact resistance and paintability, and to provide a manufacturing method therefor. <P>SOLUTION: In the hot-rolled steel sheet including C, Si, Mn, P, S, Al, N, and one or more of Cr, Mo, Ti, Nb, V, Cu, Ni, Ca, and B, as needed, this high- strength hot-rolled steel sheet has amounts of Mn, Al, and S moderately balanced, and a metal structure in which an area rate of ferrite is 60% or more, an area ratio of cementite and bainite is 40% or less, an area rate of martensite is 2-20%, an area rate of retained austenite is 5% or less, a ratio of a long side to a short side of a crystal grain is 5 or less, and a difference between an average grain size and a maximum grain size of the crystal grains is 10 μm or less. The manufacturing method is characterized by adjusting conditions of cooling after casting, roughing rolling, finishing rolling, cooling after finishing rolling, and winding, to predetermined ones. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a low yield ratio and workability (elongation, elongation) when it is formed into various shapes by overhanging, drawing, etc. and is used as a structural member for automobiles or industrial equipment. The present invention relates to a high-strength composite hot-rolled steel sheet having excellent stretch flangeability, overhangability, drawability, etc.), impact resistance, and paintability, and a method for producing the same.

[0002]

2. Description of the Related Art So-called "hot-rolled steel sheet" produced by continuous hot rolling is widely used as a relatively inexpensive structural material in various industrial equipment such as automobiles. In particular, in recent years, parts such as automobile body parts that are subject to severe processing such as overhanging and drawing, and have high strength for weight reduction and impact resistance for ensuring collision safety, Furthermore, the application of hot-rolled steel sheets to parts requiring corrosion resistance for long-term rust prevention is increasing more and more. Further, although dimensional accuracy is important when assembling an automobile, a hot-rolled steel sheet having a low yield ratio is often desired for its application.

Conventionally, a method of utilizing retained austenite is known as a method of improving the workability and impact resistance of a hot rolled steel sheet. However, most of these methods involve adding a large amount of Si, and the hot-rolled steel sheet obtained by this method has a problem in paintability. Further, the hot-rolled steel sheet according to this method could not obtain a low yield ratio necessary for obtaining dimensional accuracy.

Therefore, for example, Japanese Patent No. 3066689 discloses a method of adding Al instead of Si, which deteriorates the coating property of a hot rolled steel sheet utilizing the retained austenite. However, the hot-rolled steel sheet obtained by this method has a high yield ratio and is not satisfactory for a molded member that requires strict dimensional accuracy. A technique relating to a hot rolled steel sheet utilizing the retained austenite by adding Al is also disclosed in JP-A-11-106862, but the hot rolled steel sheet disclosed in this publication is subjected to finish rolling in the α + γ2 phase region. The material obtained by this method has problems of insufficient impact resistance and high yield ratio because it is rolled in the α + γ2 phase region.

In addition, as a hot rolled steel sheet to which Al is added in order to utilize the retained austenite, Japanese Patent Laid-Open No. 10-317 is known.
096, JP-A-11-350063, JP-A-11-61327 or JP-286496.
As described in Japanese Patent Publication No. 0, etc., these are not intended for the steel performance capable of achieving sufficient workability, impact resistance, and paintability at a low yield ratio, which has been increasingly demanded.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The object of the present invention is to solve the above-mentioned problems of the prior art, to provide a high strength composite structure hot rolled steel sheet having a low yield ratio and being excellent in workability, impact resistance and paintability, and to provide stable It was to establish the manufacturing method.

[0007]

[Means for Solving the Problems] The inventors of the present invention have conducted extensive studies to achieve the above-mentioned object. As a result, Mn as a constituent component of a hot-rolled steel sheet tends to suppress the formation of ferrite, and the impact resistance It was clarified that the ductility becomes insufficient when the amount of Si is regulated in order to secure the paintability and paintability. Furthermore,
It was also clarified that these problems can be solved by adding an appropriate amount of Al. Therefore, as a result of further research, in order to realize a hot-rolled steel sheet having a low yield ratio and sufficient workability, impact resistance, and paintability, the amounts of Mn, Al, and Si should be appropriately balanced, and We were able to obtain the knowledge that it is important to carry out tissue control.

The present invention has been completed based on the above findings and the like, and provides a high-strength hot-rolled steel sheet excellent in workability, impact resistance, and coatability, and a method for producing the same. C: 0.001 to 0.15% (% representing the component ratio in the present invention is% by weight), Si: less than 1.4%, Mn: 0.5 to 3
%, P: 0.1% or less, S: 0.03% or less, Al: more than 0.1% ~
Less than 1.5%, N: 0.0005 to 0.02%, the balance consisting essentially of Fe and unavoidable impurities, and all metallic structures from the central part of the plate thickness to the surface layer are ferrite area ratio: 60% or more , Area ratio of cementite and bainite: 40% or less, area ratio of martensite: 2 to 20
%, And the area ratio of retained austenite: 5% or less, and the ratio of long pieces / short pieces of crystal grains is 5 or less, and the difference between the average grain size of crystal grains and the maximum grain size is within 10 μm. High strength hot rolled steel sheet. C: 0.001 to 0.15%, Si: less than 1.4%, Mn: 0.5 to
3%, P: 0.1% or less, S: 0.03% or less, Al: more than 0.1% ~
Less than 1.5%, including N: 0.0005 to 0.02%, Cr: 0.00
1 to 1%, Mo: 0.001 to 1%, Ti: 0.001 to 0.1%, N
b: 0.001 to 0.1%, V: 0.001 to 0.1%, Cu: 0.002 to
1%, Ni: 0.002-1%, Ca: 0.0001-0.01% and B:
Including at least one selected from the group consisting of 0.0001 to 0.01%, the balance consists essentially of Fe and unavoidable impurities, and all metallographic structures from the center of the plate thickness to the surface layer are ferrite area ratios. : 60% or more, area ratio of cementite and bainite: 40% or less, area ratio of martensite:
2-20%, and area ratio of retained austenite: 5%
A high-strength hot-rolled steel sheet having a ratio of long pieces / short pieces of crystal grains of 5 or less and a difference between an average grain size of crystal grains and a maximum grain size of 10 μm or less. After casting, a steel slab having the component composition described in the above item or the item is cooled at an average cooling rate of 3 to 70 ° C./min in a section of 1080 to 1420 ° C., and then heated to 1050 ° C. or more and then rough rolling. After finishing the rough rolling, heating of the rough bar or temperature retention is carried out to finish the finishing side temperature of 900-
Finish rolling started at 1200 ° C, finish outlet temperature 780
A method for manufacturing a high-strength hot-rolled steel sheet, which comprises cooling at an average cooling rate of 5 ° C./s or more and winding at 600 ° C. or lower after the rolling is completed at -1030 ° C.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the reasons why the composition of the billet or hot rolled steel sheet, the metallographic structure of the hot rolled steel sheet, and the manufacturing conditions are limited as described above in the present invention will be explained. (A) Ingredient composition C of steel billet or hot rolled steel sheet: When the C content exceeds 0.15%, stretch flangeability and weldability deteriorate. On the other hand, if the C content is less than 0.001%, the martensite necessary for obtaining the properties of low yield ratio cannot be sufficiently obtained. Therefore, the C content is 0.0
It was set at 01 to 0.15%.

Si: Si is a component that should be added in a large amount from the viewpoint of mechanical properties because it is effective in strengthening the steel sheet and has little adverse effect on ductility. However, if added in excess, it will cause red scale formation. In addition to the deterioration of the appearance, the paintability and impact resistance intended by the present invention are significantly impaired. Therefore, although the Si content is set to less than 1.4%, it is preferably adjusted to 1.0% or less.

Mn: Mn is an effective strengthening component although it does not reach C or Si. Moreover, Mn has the function of suppressing the excessive generation of pearlite and refining the crystal grains. In order to exert these effects, it is necessary to contain Mn in an amount of 0.5% or more, but even if Mn is contained in an amount of more than 3%, the effect of improving the strengthening ability is saturated. P: P is a component to be added in a large amount because it is effective in strengthening the steel sheet, but if the P content exceeds 0.1%, it becomes brittle and the impact resistance intended by the present invention deteriorates. Therefore, the P content is set to 0.1% or less.

S: When S is contained in excess of 0.03%
Since it is an impurity element that forms Mn and A type inclusions and deteriorates workability (stretch flangeability), it is desirable to reduce it as much as possible. Therefore, as a result of examining the allowable range, the S content was determined to be 0.03% or less.

Al: Al is an important component in the hot-rolled steel sheet according to the present invention, and contributes to the improvement of ductility of the hot-rolled steel sheet by accelerating the formation of ferrite like Si. Moreover,
Like Si, it is a beneficial component that does not cause deterioration in paintability and impact resistance. And in order to exert its effect, 0.1
% Al content is required, but even if more than 1.5% is contained, the effect is saturated, so Al content is more than 0.1% to 1.5%.
Defined as less than%. More preferable range of Al content is 0.2%
It is over ~ 1%.

N: N acts as various nitrides and has a function of preventing coarsening during heating during hot rolling. However, if the amount of nitride is too large, it becomes a starting point of cracking.
Therefore, the N content is set to 0.0005 to 0.02%.

Cr, Mo, Ti, Nb, V, Cu, Ni and B: All of these are effective components for increasing the tensile strength of the hot-rolled steel sheet, and by adding a required amount, cooling in actual production. Since a hard phase is easily generated without taking a complicated cooling pattern such as winding in a relatively low temperature range where it is difficult to control the speed, one or more kinds are added if necessary. However, the effect is insufficient if the content of each is less than the lower limit, and if the content of each exceeds the upper limit, the coatability is deteriorated and the yield ratio is increased.

Ca: Ca is a component effective in combining with S to make the sulfide spherical and improve the stretch-flange formability of the hot-rolled steel sheet, so Ca is contained if necessary. However, if the content is less than 0.0001%, the above-mentioned effect is insufficient, and even if the content exceeds 0.01%, the effect is saturated and it is disadvantageous in terms of cost, so the Ca content is 0.0001 to 0.01. Defined as%.

Components other than the above are Fe and unavoidable impurities. Examples of unavoidable impurities include O and RE
M, Sn, Sb, and W are included, but the content is 0.02% or less. Further, the steel having the above composition is melted by, for example, a converter, an electric furnace, an open furnace, or the like. The steel type may be any of rimmed steel, capped steel, semi-killed steel or killed steel. Furthermore, casting of billets is performed by "ingot-making".
Either slab rolling or continuous casting may be used.

(B) Metallographic Structure In the hot-rolled steel sheet according to the present invention, all the metallic structures from the central portion of the plate thickness to the surface layer have a ferrite area ratio of 60% or more,
The area ratio of cementite and bainite: 40% or less, the area ratio of martensite: 2 to 20%, and the area ratio of retained austenite: 5% or less, and the ratio of long pieces / short pieces of crystal grains is 5. In the following, the difference between the average grain size of the crystal grain size and the maximum grain size is within 10 μm. Ferrite contributes to ductility and martensite contributes to strength, but a low yield ratio is achieved due to the strength difference between ferrite and martensite.

Here, in order to obtain the desired ductility in the hot rolled steel sheet, the area ratio of ferrite is 60% or more, preferably 85%.
It is necessary to be ˜95%. Further, the ferrite is preferably polygonal ferrite. When the area ratio of martensite exceeds 20%, the hot-rolled steel plate becomes hard and the workability deteriorates, so the area ratio is set to 2 to 10%. The remaining structure is unavoidable cementite and
Any of light, retained austenite, and bainite may be used, but bainite and cementite have an area ratio of 40% or less, preferably 5 to 15%, because they prevent the yield ratio from decreasing.
Should be. Here, bainite includes acicular ferrite in addition to upper bainite and lower bainite. Area ratio of residual austenite is 5% for the same reason.
It should be below, and preferably less than 3%.

In the hot-rolled steel sheet, if the crystal grains are partially expanded or mixed grains in the thickness direction portion, workability and impact resistance are deteriorated.
The structure is such that the ratio of the short pieces is 5 or less and the difference between the maximum grain size and the average grain size of the crystal grains is within 10 μm. Then, the central part of the plate thickness and the metal structure of the surface layer both need to have the above structure.

Generally, in a steel sheet requiring workability such as the hot rolled steel sheet of the present invention, in order to positively generate a ferrite structure having good workability, just above the Ar ₃ transformation point which is the lower limit temperature of the austenite temperature range. Hot rolling finish rolling is performed. However, when pursuing the rolling immediately above the Ar ₃ transformation point, the transformation point falls below the transformation point partially or at the plate thickness surface layer where it is easy to cool due to temperature fluctuations during rolling, and mixed grain structure and expanded grain structure occur, resulting in workability and impact resistance. Therefore, in order to obtain the target performance of the present invention, it is necessary to have the above-described structure over the entire plate thickness. The central part of the plate thickness is ¹ / _4th to ^3rd from the surface layer of the plate thickness.
/ ₄ thickness range. Further, the surface layer means a plate thickness surface layer 0 to 30.
The range is μm.

(C) Manufacturing Conditions The hot-rolled steel sheet according to the present invention has a thickness of 1080-1 after casting of billets.
After cooling the section of 420 ℃ at an average cooling rate of 3 to 70 ℃ / min, and then heating it to 1050 ℃ or more, rough rolling is started, and after the rough rolling is finished, heating of the rough bar or temperature retention is performed. After finishing rolling at a finishing inlet temperature of 900 to 1200 ° C. and finishing at a finishing outlet temperature of 780 to 1030 ° C., the coil is cooled to an average cooling rate of 5 ° C./s or more until winding up to 600 ° C. It can be stably manufactured by winding the following.

Here, after casting, the section of 1080 to 1420 ° C. is cooled at an average cooling rate of 3 to 70 ° C./min to obtain 105
The reason for heating or holding at 0 ° C. or higher is that it is necessary to sufficiently dissolve the additive elements such as Si, Ti, Nb and the like to form a solid solution, and to utilize them effectively for grain refinement and high strength. After the rough rolling is finished, the rough bar is heated or retained for the purpose of controlling the finishing inlet side temperature and the finishing outlet side temperature with high accuracy to obtain the above-described structure over the entire plate thickness. As a heating or retaining method, a method of winding a coarse bar into a coil,
Alternatively, there are a crude bar heater for heating the coarse bar by an induction heating method, a gas burner heating method, and an electric heating method for supplying an electric current directly to the coarse bar. Particularly, a coarse heater is suitable. Further, a method of joining a rough bar and continuously rolling before finish rolling is also effective because the finish rolling can be completed at a high speed in a short time without slowing down the speed.

The temperature of the finishing entry side is set to 900 to 1200 ° C. When the temperature is less than 900 ° C., ferrite transformation occurs during finish rolling, and a band-like structure or a mixed grain structure is formed to reduce ductility and impact resistance. On the other hand, when the temperature exceeds 1200 ° C., the scale is actively produced and surface defects occur. Preferably, the finishing inlet temperature is 950-11
It is better to set it to 00 ° C.

The temperature on the finishing delivery side is set to 780 to 1030 ° C. When the temperature is less than 780 ° C., the amount of working increases due to the rolling applied to the transformed ferrite before the end of hot rolling, and a band structure or a mixed grain structure occurs. However, the ductility and impact resistance are deteriorated, while at a temperature higher than 1030 ° C., scale generation becomes active, resulting in defective surface defects. The finish outlet temperature is preferably 820 to 900 ° C.

Cooling at a cooling rate of 5 ° C./s or more after hot rolling is necessary to form martensite. Here, if the cooling stop temperature is high, pearlite is generated, a low yield ratio cannot be obtained, and impact resistance is also reduced. The winding temperature is 600 ° C. or lower in order to suppress the precipitation of cementite.

The hot-rolled steel sheet produced is usually subjected to shape correction by skin pass rolling and scale removal by pickling, and its surface is coated with rust preventive oil.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE After casting a steel having the composition shown in Table 1, 10
Average cooling rate of 3 to 70 ° C / min in the section of 80 to 1420 ° C
After that, the rough rolling is started after heating to 1050 ° C. or higher, and after the rough rolling is finished, the rough bar is heated or temperature retention is performed to start the finish rolling.
After the rolling was completed at 1030 ° C., it was cooled at an average cooling rate of 5 ° C./s or more and wound at 600 ° C. or less to produce a hot-rolled steel sheet having a plate thickness of 2.6 mm. Table 2 shows the hot rolling conditions at this time.

[0029]

[Table 1]

[0030]

[Table 2]

Next, the obtained hot rolled steel sheet was subjected to flattening with a skin pass and scale removal by pickling, and then,
The metallographic structure of the plate thickness center part of the rolling direction cross section of steel and the surface layer,
The areas of ferrite, martensite, cementite, and bainite were calculated by area percentage (JIS G 0552) by observing them with an optical microscope (× 500 times) and an SEM (× 1000 times). The ratio of long pieces / short pieces of crystal grains was determined from the above-mentioned structure observation. The difference between the maximum grain size and the average grain size of the crystal grains was determined by the cutting method. The amount of retained austenite was measured by X-ray,
The types of some tissues were confirmed by SEM and transmission electron microscopy.

Also, a tensile test (JIS Z2241) on a JIS No. 5 test piece taken in the direction perpendicular to the rolling, a hole expansion test (JIS Z 2242, U notch, t2.5 mm) with a 60 ° conical punch, and a paintability test I also went. In addition,
The paintability was evaluated by a chemical conversion treatment test. The chemical conversion treatment test conditions at this time were as follows. Chemical conversion treatment agent… SD500 made by Nippon Paint, treatment time… 120 seconds, evaluation method… confirm crystal size with SEM × 100 times,
The amount of adhesion is calculated from the weight difference after dissolving the film with chromic acid. The results of these measurements are shown in Tables 3 and 4.

[0033]

[Table 3]

[0034]

[Table 4]

From the results shown in Tables 3 and 4, hot-rolled steel sheets satisfying the specified conditions of the present invention can be obtained according to the manufacturing conditions specified by the present invention, and have a high strength and a low yield ratio. It can be seen that it satisfies the characteristics of excellent properties, impact resistance, and paintability.

On the other hand, in Test Nos. 7 to 11, any of the manufacturing conditions deviates from the specified conditions of the present invention, so that the obtained hot-rolled steel sheet has poor performance. Also,
The hot-rolled steel sheets obtained in Test Nos. 23 to 26, 28, and 29 are inferior in performance because the steel sheet composition does not satisfy the specified conditions of the present invention.

[0037]

As described above, according to the present invention, a high-strength hot-rolled steel sheet having a low yield ratio and excellent workability, impact resistance, and paintability (particularly 490 to 7 in terms of tensile strength in the direction perpendicular to the rolling direction).
80MPa class high strength steel plate), which is formed into various shapes by overhanging, drawing, etc. and is an undercarriage part of automobiles such as lower arm and wheel, or structural member of industrial equipment. It becomes possible to meet the recent required characteristics in "applications requiring workability, impact resistance, and paintability", and industrially useful effects are brought about.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/58 C22C 38/58 F term (reference) 4E002 AA07 AD02 AD04 BC07 BD07 CB01 4K037 EA01 EA02 EA04 EA05 EA06 EA11 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA23 EA25 EA27 EA28 EA31 EA32 EB02 EB05 EB07 EB08 EB09 EB11 EB12 FA02 FA03 FC03 FC04 FC05 FD02 FD03 FD04 FE01 FE02 FE06

Claims (3)

[Claims] 1. A weight ratio of C: 0.001 to 0.15%, S
i: less than 1.4%, Mn: 0.5 to 3%, P: 0.1% or less,
S: 0.03% or less, Al: over 0.1% to less than 1.5%, N: 0.00
05-0.02% and the balance consisting essentially of Fe and unavoidable impurities, and the entire metal structure from the central part of the plate thickness to the surface layer has an area ratio of ferrite of 60% or more, an area of cementite and bainite Ratio: 40% or less, martensite area ratio: 2 to 20%, and retained austenite area ratio: 5% or less, and long pieces of crystal grains /
A high-strength hot-rolled steel sheet, wherein the ratio of the short pieces is 5 or less, and the difference between the average grain size of the crystal grain size and the maximum grain size is within 10 μm. 2. A weight ratio of C: 0.001 to 0.15%, S
i: less than 1.4%, Mn: 0.5 to 3%, P: 0.1% or less,
S: 0.03% or less, Al: over 0.1% to less than 1.5%, N: 0.00
Including 05 to 0.02%, Cr: 0.001 to 1%, Mo: 0.001
~ 1%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%,
V: 0.001 to 0.1%, Cu: 0.002 to 1%, Ni: 0.002 to
1%, Ca: 0.0001 to 0.01%, and B: 0.0001 to 0.01%, and at least one selected from the group consisting of the balance is substantially contained.
Area ratio of ferrite is 6 which is composed of Fe and unavoidable impurities, and whose metallographic structure from the central part of plate thickness to the surface layer
0% or more, area ratio of cementite and bainite: 40
%, The area ratio of martensite: 2 to 20%, and the area ratio of retained austenite: 5% or less, and the ratio of long pieces / short pieces of crystal grains is 5 or less, and the average crystal grain size. A high-strength hot-rolled steel sheet, wherein the difference between the grain size and the maximum grain size is within 10 μm. 3. A steel slab having the composition according to claim 1 or 2 is cast, and then cooled in an area of 1080 to 1420 ° C. at an average cooling rate of 3 to 70 ° C./min, and then 1050 ° C.
After the above heating, the rough rolling is started, and after the rough rolling is finished, the rough bar is heated or the temperature is held to finish the finishing side temperature 9
After finishing rolling at 0 to 1200 ° C and finishing rolling at finishing temperature 780 to 1030 ° C, cool up to winding at an average cooling rate of 5 ° C / s or more, and wind at 600 ° C or less. A method for producing a high-strength hot-rolled steel sheet, characterized by: