JP2005314767A - High strength hot rolled steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet having high strength, further combining satisfactory stretch flange formability and elongation properties, and, if required, capable of exhibiting excellent fatigue properties as well. <P>SOLUTION: The high strength hot rolled steel sheet has a composition comprising 0.015 to 0.08% C, 0.5 to 2.0% Si, 0.8 to 2.0% Mn and 0.08 to 0.20% Ti, and the balance Fe with inevitable impurities, and has a metallic structure composed essentially of a granular bainitic ferrite structure. If required, the maximum height Ry of the surface in the steel sheet is controlled to ≤15 μm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a high-strength hot-rolled steel sheet having excellent workability (stretch flangeability and elongation) and excellent fatigue characteristics, and in particular, automobile parts such as members by taking advantage of the excellent workability and fatigue characteristics. It can be effectively used as a material for parts such as undercarriage such as arms and arms and chassis.

  In recent years, efforts to reduce the weight of automobile bodies have been promoted in order to improve fuel efficiency, and the application of higher-strength steel sheets has been expanded. For these reasons, many techniques for increasing the strength of steel sheets used as raw materials have been developed.

  As a technique for increasing the strength of a high-strength steel sheet, there are a solid solution strengthening method by adding a solid solution element such as Si and Mn, and a composite structure strengthening method including a low temperature transformation product in the structure. As specific examples, a dual-phase steel having a martensite structure introduced in a ferrite structure and a ferritic-bainite steel sheet having a bainite structure are well known. In addition, a method of increasing elongation by introducing a retained austenite phase into such a structure is also employed.

  By the way, steel sheets used as undercarriage parts for automobiles need to have high workability, particularly good stretch flangeability so that they can be processed into complex shapes as well as high strength. There is. In order to enhance such stretch flangeability, it is necessary to avoid inclusions that are the starting points of void generation during processing and two phases (bainite and martensite) having different strengths. However, when high strength is achieved with a ferrite-bainite composite structure as in the prior art, it is necessary to increase the strength of the bainite phase coexisting with the low-strength polygonal ferrite phase, and inclusions exist in this bainite. In some cases, this is the starting point for crack initiation.

  In contrast, the bainiteitic structure has a substructure with a lath-like structure having a high dislocation density in the bainite structure, and the inclusions contained therein are ferrite-bainite having the same strength. It is less likely to be a starting point for voids than inclusions present in the bainite phase of steel, and as a result, stretch flangeability is enhanced. However, the elongation characteristic is about 18% at most for a high strength steel having a tensile strength of 800 MPa class, and the actual situation is that it does not have sufficient elongation characteristics to satisfy the comprehensive formability.

On the other hand, in a hot-rolled steel sheet containing Si, it is known that a scale called Si scale is generated on the surface of the steel material. This Si scale appears as a reddish brown island pattern on the surface of the steel sheet after hot rolling, and the appearance quality of the steel sheet is significantly impaired. Moreover, since the Si scale has unevenness on the surface of the steel plate, an island-like pattern remains after pickling, which may cause fatigue cracking and shorten the life of the part. It is considered that the unevenness generated on the surface of the Si-added steel is caused by 2FeSiO ₂ produced as a compound by the reaction between the Si oxide and the iron oxide.

  For this reason, a method for limiting the Si content to 0.25% or less has been proposed in order to reduce or prevent the Si scale that is the cause of this island pattern (for example, Patent Document 1). Patent Documents 2 and 3 propose methods for reducing Si scale by defining the slab hot rolling temperature and rolling conditions. Furthermore, Patent Documents 4 to 6 propose techniques for reducing the occurrence of island patterns by appropriately defining rolling conditions and scale removal conditions.

  On the other hand, as a method for uniformizing the properties of the steel sheet surface, by controlling the heating temperature and heating time in the slab heating stage, Si scale is uniformly generated on the slab surface, and this scale is put together in the hot rolling process. Techniques for suppressing the occurrence of island-like patterns by removing them have also been proposed (for example, Patent Documents 7 and 8). Patent Document 9 discloses a technique for uniformly generating Si scales on the entire surface of a steel sheet in a hot rolling stage by containing appropriate amounts of Ni and Cu, and Patent Document 10 discloses heating in a slab heating stage. A technique capable of controlling the temperature and heating time and completely detaching the mixed scale by descaling during hot rolling is disclosed in Patent Document 11 by limiting the amounts of Si and Ni in the hot rolling process. A method for manufacturing a steel sheet with good scaling properties has been proposed.

However, even with the various techniques proposed so far, the Si scale cannot be completely peeled off at the time of descaling, and the actual situation is that an island pattern on the surface of the steel sheet is generated.
JP-A-5-6937, claims, etc. Japanese Patent Laid-Open No. 60-15684, claims, etc. Japanese Patent Laid-Open No. 4-247829, claims, etc. Japanese Patent Publication No. 60-1865, claims, etc. Japanese Patent Application Laid-Open No. 63-68214, claims, etc. JP-A-4-238620, claims, etc. Japanese Patent Laid-Open No. 3-72031, claims, etc. JP-A-3-79718, claims, etc. JP-A-6-346145, claims, etc. JP-A-5-279734, claims, etc. JP-A-6-279923, claims, etc.

  The present invention has been made under such circumstances, and its purpose is to have a high strength, a good stretch flangeability and an elongation characteristic, and to exhibit an excellent fatigue characteristic if necessary. An object of the present invention is to provide a hot-rolled steel sheet.

  The hot-rolled steel sheet of the present invention that has achieved the above-mentioned object is: C: 0.015-0.08% (meaning of mass%, the same applies hereinafter), Si: 0.5-2.0%, Mn: 0 .8 to 2.0%, Ti: 0.08 to 0.20%, respectively, the balance is made of Fe and inevitable impurities, and the metal structure is mainly composed of a granular bainite ferrite structure. It has a gist.

  In the hot-rolled steel sheet of the present invention, if necessary, (a) Nb: 0.10% or less (not including 0%), (b) Cu: 0.30% or less (not including 0%), and Ni: 0.30% or less (not including 0%), (c) B: 0.0030% or less (not including 0%), (d) Ca: 0.0030% or less (not including 0%), and rare earth Element: 0.005% or less (not including 0%) and the like can be contained, and the properties of the hot-rolled steel sheet are further improved depending on the type of the element contained.

  In the hot-rolled steel sheet of the present invention, the surface roughness of the steel sheet surface is preferably 15 μm or less in terms of the maximum height Ry, and the fatigue characteristics of the steel sheet are excellent by adopting such surface properties.

  In the hot-rolled steel sheet of the present invention, a hot-rolled steel sheet with good workability (stretch flangeability and stretch characteristics) is realized by adjusting the chemical composition appropriately and making the structure mainly a granular bainitec ferrite structure. In addition, if necessary, the surface roughness of the steel sheet is adjusted to a maximum height Ry of 15 μm or less, thereby realizing a hot-rolled steel sheet having excellent fatigue characteristics.

  The present inventors studied from various angles in order to realize a hot-rolled steel sheet having excellent workability. As a result, if the granular bainite ferrite structure (hereinafter sometimes abbreviated as “GBF structure”) having a lower dislocation density than the bainitec ferrite structure is mainly used, the moldability can be remarkably improved. I found out.

  The GBF structure has an acicular shape (needle shape) in observation with an optical microscope or a scanning microscope (SEM). To determine a clear difference, identification of a substructure by observation with a transmission electron microscope (TEM) Is required. The GBF structure does not have a lath structure, and has a substructure whose dislocation density is slightly lower than that of the bainiteitic structure. This GBF structure is clearly different from the bainite structure in that it does not have carbide inside, and has a substructure such as polygonal ferrite having a substructure with little or very little dislocation density, or fine subgrains. This is also different from the quasi-polygona ferrite structure (for example, “Stay Bainite Photobook-1”, Japan Iron and Steel Institute Basic Research Group issued in June 1992).

  The reason why the workability is improved by using such a GBF structure has not been fully clarified, but the GBF structure probably improves stretch flangeability and lowers the dislocation density to increase elongation. Is considered to have improved. The “GBF structure main body” means that the ratio of the GBF structure to the whole is 90 area% or more, and a structure such as bainite or bainite ferrite may be included in less than 10 area%. .

  In order to obtain the GBF structure as described above, the time from rolling to the start of primary cooling after hot rolling is performed such that the final pass end temperature is 900 ° C. or higher and the final pass rolling rate is 11% or higher. May be performed at a cooling rate of 50 ° C./second or more (primary cooling) within 2 seconds. When the final rolling pass end temperature is less than 900 ° C., coarse grains are likely to be generated on the surface layer, and when the final pass rolling ratio is less than 11%, the crystal grains are likely to become coarse. Further, when the time to primary cooling exceeds 2 seconds, the crystal grain size tends to be coarsened, and when the cooling rate is less than 50 ° C., the displacement density decreases, and in any case, the target GBF structure is obtained in the present invention. I can't. In addition, it is preferable that the coiling temperature after hot rolling shall be about 350-550 degreeC from a viewpoint of reducing the quantity of a precipitate as much as possible and maintaining stretch flangeability favorable.

  On the other hand, in the hot rolled steel sheet having the above-described structure, it has been found that if the surface roughness is 15 μm or less at the maximum height Ry (JIS B0601), the fatigue characteristics can be remarkably improved. The reason why the fatigue characteristics can be improved by setting the surface roughness of the steel sheet to 15 μm or less at the maximum height Ry is probably because the degree of stress concentration when a repeated load is applied is reduced.

  In order to adjust the surface roughness of the hot-rolled steel sheet as described above, a roll whose surface roughness is adjusted after hot rolling is used, and skin pass rolling (SKP rolling) with a rolling rate of about 0.5 to 3% is performed. However, from the viewpoint of making the surface roughness of the steel sheet 15 μm or less, the load during SKP rolling is desirably 250 tons or more. Further, excessive SKP rolling raises the yield point to impart unnecessary strain to the steel sheet, and lowers the elongation, so the upper limit is 600 tons, preferably 500 tons.

  In the hot-rolled steel sheet of the present invention, in order to provide the basic mechanical properties (yield point YP, tensile strength TS, elongation EL, etc.), it is necessary to appropriately adjust the chemical composition, The reasons for limiting the range of the chemical composition defined in the present invention are as follows.

C: 0.015-0.08%
C is a basic component as a strength improving element and also contributes to the generation of a GBF structure. In order to exhibit such an effect, it is necessary to contain 0.015% or more. However, when the C content is excessive, a martensite structure is easily generated, and stretch flangeability is deteriorated. Therefore, the C content needs to be 0.08% or less. In addition, the preferable minimum of C content is 0.03%, and a preferable upper limit is 0.05%.

Si: 0.5 to 2.0%
Si effectively works to increase the strength of the GBF structure and to improve the stretch flangeability. In order to exert such an effect, it is necessary to contain 0.5% or more. However, if it is contained excessively, polygonal ferrite is likely to be generated, and the surface property is greatly affected (for example, an island-like scale pattern). Generation), it is necessary to make it 2.0% or less. In addition, the preferable minimum of Si content is 0.8%, and a preferable upper limit is 1.5%.

Mn: 0.8 to 2.0%
Mn is an element useful as a solid solution strengthening element, and has an action of promoting transformation and promoting the formation of a GBF structure. In order to exert such effects, the Mn content needs to be 0.8% or more. However, if it is excessive, the hardenability becomes too high and a large amount of transformation product is formed, and the stretch flangeability deteriorates. Therefore, it should be 2.0% or less. In addition, the minimum with preferable Mn content is 1.0%, and a preferable upper limit is 1.8%.

Ti: 0.08 to 0.20%
Ti suppresses the formation of polygonal ferrite during rapid cooling after the hot end, promotes the formation of the GBF structure, and effectively acts to improve the strength of the steel sheet. In order to exert such effects, the Ti content needs to be 0.08% or more. However, if the Ti content is excessive, the hot-worked structure is likely to remain, and excessive precipitates are generated to deteriorate the stretch flangeability. Therefore, the content must be 0.20% or less. In addition, the minimum with preferable Ti content is 0.10%, and a preferable upper limit is 0.18%.

  In the hot-rolled steel sheet of the present invention, in addition to the above components, it is composed of Fe and inevitable impurities (for example, P and S), but if necessary, Nb, Cu, Ni, B, Ca, rare earth elements, etc. are contained. It is also effective. The reason for defining the range when these elements are contained is as follows.

Nb: 0.10% or less (excluding 0%)
Nb effectively acts to suppress the formation of polygonal ferrite during rapid cooling after the hot end and to obtain a fine structure. Such an effect increases as the content increases, but even if it is contained excessively, the effect is saturated and the cost is increased.

Cu: 0.30% or less (not including 0%) and Ni: 0.30% or less (not including 0%)
Cu acts effectively as a solid solution strengthening element and also effectively improves fatigue characteristics. Such an effect increases as the content thereof increases. However, when the content is excessive, it becomes easy to generate haze. Therefore, the content is preferably 0.30% or less. In addition, the more preferable minimum of Cu content is 0.1%, and a more preferable upper limit is 0.2%.

  On the other hand, Ni effectively acts to prevent the occurrence of surface defects (hegging) when Cu is contained. Such an effect increases as the content increases, but if it is excessive, the cost increases, so 0.30% or less is preferable. In addition, the more preferable minimum of Ni content is 0.1%, and a more preferable upper limit is 0.2%.

B: 0.0030% or less (excluding 0%)
B effectively acts to improve the hardenability of the steel sheet. Such an effect increases as the content increases. However, if the content is excessive, the elongation characteristics deteriorate, so 0.0030% or less is preferable. In addition, the minimum with preferable B content is 0.0005%, and a more preferable upper limit is 0.0020%.

Ca: 0.0030 or less (not including 0%) and rare earth elements: 0.005% or less (not including 0%)
These elements effectively act to spheroidize sulfides in the steel sheet and improve stretch flangeability. These effects increase as the content increases, but even if they are contained excessively, the effects are saturated. Therefore, it is preferable that the content is 0.0030% or less for Ca and 0.05% or less for rare earth elements. More preferable lower limits of these elements are 0.0010% for Ca and 0.0020% for rare earth elements, and more preferable upper limits are 0.0020% for Ca and 0.0040% for rare earth elements. The rare earth elements used in the present invention include Sc and Y in addition to lanthanoid series rare earth elements such as La and Ce, and one or more of these may be appropriately selected and contained.

  Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are not intended to limit the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are technical aspects of the present invention. It is included in the range.

Example 1
Various steel slabs having the chemical composition shown in Table 1 below were hot-rolled under the hot rolling conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 3.2 mm.

  For each steel plate obtained, a JIS No. 5 test piece was created to investigate the mechanical properties (yield point YP, tensile strength TS, elongation EL, etc.), and the expansion ratio was measured by the following method. Evaluation was performed using λ. The microstructure of each steel plate was observed with a scanning electron microscope (SEM) at t / 4 part of the plate thickness t.

[Method for measuring hole expansion rate λ]
A punched hole with a diameter of 10 mm (d ₀ ) is expanded from the punched side with a 60 ° conical punch, and the hole diameter d (mm) is measured when the crack penetrates in the plate thickness direction. It was measured.
λ = [(d−d ₀ ) / 10] × 100 (%)

  These results are shown in Table 3 below, and can be considered as follows from this result. First, test no. Nos. 1 to 11 satisfy all of the requirements defined in the present invention, and both a mechanical property and a hole expansion rate are good, and a hot-rolled steel plate having high strength and good workability can be realized. I understand that.

  In contrast, test no. Those of 12 to 20 lack any of the requirements defined in the present invention, and at least any of the properties of strength, elongation and stretch flangeability is deteriorated.

  First, test no. In No. 12, the C content exceeds the range specified in the present invention, the microstructure is a two-phase structure of (ferrite + martensite), and the hole expansion ratio λ is low. In addition, Test No. In No. 13, the Ti content is less than the range defined in the present invention, the microstructure is a two-phase structure of (ferrite + martensite), the strength is low, and the elongation is low. .

  Test No. In No. 14, the Si content is less than the range defined in the present invention, and the strength is low. Test No. In No. 15, the Mn content is less than the range defined in the present invention, and the strength is low.

  Test No. In the samples 16 and 17, the coiling temperature is high and the amount of precipitates is so large that the hole expansion ratio λ is low. Test No. In No. 18, the microstructure is a two-phase structure of (ferrite + martensite), and the elongation and the hole expansion ratio λ are low.

  Test No. In Nos. 19 and 20, the microstructure is a two-phase structure of (ferrite + martensite), and there are many precipitates, indicating a low elongation value.

Example 2
Test No. shown in Table 2 above. About each steel plate obtained by 1, 2, and 4, SKP rolling (roll diameter: 585 mm, load: 200 to 400 tons) is performed on a part of the coil, and the surface of the steel plate is roughened with and without SKP rolling. In addition to measuring the height (maximum height Ry according to JIS B0601), a fatigue test was performed by the following method. For SKP rolling, no. For the coil No. 1, the SKP load was changed to 200, 250 and 300 tons, and the No. 1 coil was passed. With respect to the coil No. 4, the load was changed to 400, 500, and 600 tons, and a sample for investigation was collected from a site where each coil was stable at each load.

[Fatigue test]
The shape of the test piece used for the fatigue test is shown in FIG. A test piece of this shape was taken from a sample cut out from the coil, processed, and subjected to a full swing plane bending fatigue test.

  The results are shown in Table 4 below. In the case where the SKP rolling was performed, the surface roughness was smaller than that in the case where the SKP rolling was not performed, and the fatigue characteristics (maximum stress) were all improved (50 MPa or more). You can see that

It is a schematic explanatory drawing which shows the shape of the test piece used by the fatigue test.

Claims (6)

  C: 0.015-0.08% (meaning of mass%, the same applies hereinafter), Si: 0.5-2.0%, Mn: 0.8-2.0%, Ti: 0.08-0. High-strength hot-rolled steel sheet excellent in workability and fatigue characteristics, characterized by containing 20% each, the balance being Fe and inevitable impurities, and the metal structure being mainly composed of a granular bainitec ferrite structure .   The high strength hot rolled steel sheet according to claim 1, further comprising Nb: 0.10% or less (not including 0%).   The high-strength hot-rolled steel sheet according to claim 1 or 2, further comprising Cu: 0.30% or less (not including 0%) and Ni: 0.30% or less (not including 0%). .   The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, further comprising B: 0.0030% or less (not including 0%).   Furthermore, one or more selected from the group consisting of Ca: 0.0030% or less (not including 0%) and rare earth elements: 0.005% or less (not including 0%) is contained. A high-strength hot-rolled steel sheet according to any one of the above. The high-strength hot-rolled steel sheet according to any one of claims 1 to 5, wherein the steel sheet surface has a maximum height Ry of 15 µm or less.

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