JP2005105340A - Thick hot rolled steel plate having excellent workability and ductility, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick steel plate excellent in workability, particularly in stretch-flanging properties, and to provide its production method. <P>SOLUTION: The hot rolled steel plate has a composition comprising 0.02 to 0.2% C, ≤0.5% Si, ≤2.5% Mn, ≤0.02% P and ≤0.005% S, and the balance iron with inevitable impurities, and has a plate thickness of 5 to <15 mm. The surface layer has a bainite single phase structure in 10 to <25% with respect to the plate thickness, and the balance internal layer is composed of ferrite and one or more kinds of hard second phases selected from bainite and pearlite. The hot rolled steel plate can further comprise one or more kinds of metals selected from Ti, Nb, V, Mo, Zr, Cr and B in an amount of 0.01 to 0.3% in total. In the method of producing the hot rolled steel plate, the steel having the above chemical components is continuously cast, is heated at 1,150 to 1,300°C, and is hot-rolled at the finishing temperature of an Ar<SB>3</SB>point or higher, and the central part of the plate thickness is rapidly cooled to the range of 700 to 350°C at ≥100°C/s within 2 s after the completion of the hot rolling and is coiled. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hot-rolled steel sheet having a plate thickness of 5 mm or more and less than 15 mm, having a strength of 400 to 520 MPa and excellent workability, and a method for producing the same. Here, workability is assumed to be hole expandability.

  Conventionally, it is known that a steel sheet having a single phase structure such as ferrite and bainite is excellent in workability. Hot-rolled steel sheets with a strength range of 400 to 520 MPa have been improved by workability design by ensuring the workability and ductility mainly with ferrite and maintaining the strength by controlling the hard second phase. At this strength level, hot-rolled steel sheets mainly composed of ferrite and bainite as the second phase have good workability, and many techniques have been developed. For example, Japanese Patent No. 2255436 discloses a technique for improving workability by using a Ti-added ferrite + bainite steel sheet. Since this bainite has a lower hardness difference than ferrite compared to the other second phases (pearlite and martensite), the propagation of cracks, which is an effective factor for hole expansibility, is low.

In addition, when pearlite and martensite having a high hardness difference from ferrite are used, a technique for improving the workability by reducing the volume fraction of these second phases has been proposed. For example, Japanese Patent Application Laid-Open No. 58-6936 discloses a technique for improving workability by finely dispersing pearlite during rapid cooling on a run-out table. Japanese Patent Application Laid-Open No. 4-88125 provides a technique of high workability high tensile strength of 50 kgf / mm ² or more by setting the volume ratio of martensite to 5%.

  On the other hand, as a prior art related to crack growth, which is an effective factor of hole expansibility, an arrested steel sheet having a non-uniform structure, specifically a fine ferrite structure in the surface layer, has been proposed in the field of steel materials. This serves to reduce the behavior of crack propagation with respect to brittle fracture cracks. For example, in Japanese Patent Application Laid-Open No. 61-235534, the average particle diameter is 5 μm over a distance of 1/8 or more of the plate thickness from both sides of the plate thickness. A thick steel plate having the following ferrite but having excellent arrest characteristics when the area ratio is 50% or more is provided.

  The technology for suppressing the propagation of brittle cracks with respect to the fine ferrite grain size is disclosed in Japanese Patent Application Laid-Open No. 2-301540 and Japanese Patent Application Laid-Open No. 8-299592, which are based on the refinement of the ferrite structure.

  The above-mentioned surface layer ferrite structure refinement technique is applied from brittle fracture to ductile fracture. For example, in Japanese Patent Application Laid-Open No. 2000-328177, the microstructure is composed of ferrite, pearlite, and bainite, and the thickness of both surface portions is 5%. The above is composed of a layer with an area equivalent to a circle equivalent grain size of 7 μm or less and an aspect ratio of 2 to 4 with 50% ferrite and a bainite fraction of 5 to 25% or less. It is reported that it is composed of a layer having a ferrite with an equivalent particle diameter of 4 to 10 μm and an aspect ratio of 2.0 and having a bainite fraction of 10% or less.

In addition, there is a technique that utilizes a non-uniform structure in the thickness direction of a thin material, and Japanese Patent Application Laid-Open No. 7-258795 proposes application to a can material steel plate. This is because the finishing temperature of hot rolling is set to Ar ₃ or higher, and then rapidly cooled at an average cooling rate of 30 ° C / s or higher, and wound at a winding temperature of 500 to 670 ° C. This is done for sheet thicknesses of up to 100μm, providing steel sheets with excellent surface properties.

Japanese Patent No. 2255436 JP 58-6936 Japanese Unexamined Patent Publication No. 4-88125 JP 61-235534 A JP-A-2-301540 JP-A-8-295982 JP 2000-328177 A Japanese Unexamined Patent Publication No. 7-258795

  Thick steel plates, mainly with a thickness of 5 mm or more, have lower cooling capacity after hot rolling in the prior art than thin steel plates (less than 5 mm). This makes it difficult to refine the ferrite structure and mainly control the bainite in the second phase structure, and pearlite is formed. This pearlite may not be able to ensure strength, and since there is a difference in hardness of ferrite, workability is deteriorated and ductility is poor. For example, when the techniques described in Patent Documents 1 to 3 are applied to a thick steel plate, a coarse ferrite + pearlite structure is formed in the central portion of the plate thickness, the strength is lowered, and the workability is also impaired. In particular, the technique described in Patent Document 3 has a problem that martensite may not be generated at the center of the plate thickness because the runout cooling rate is 50 ° C./s, and the strength and workability deteriorate due to generation of a pearlite structure. is there.

  On the other hand, regarding the method of using a structure that is not uniform in the plate thickness direction, the technique described in Patent Document 4 aims to improve surface properties, and does not mention workability, particularly stretch flangeability. The techniques described in Patent Documents 5 to 7 all improve the arrest characteristics in fracture toughness by refining ferrite grains in the surface layer portion, and are not applied to stretch flangeability.

  In addition, the technique described in Patent Document 8 in which such a structure refinement technique is applied to ductile fracture does not consider stretch flangeability. In addition, the fine ferrite + bainite structure of the surface layer is used, and it is difficult to control the volume ratio. In particular, when this technique is applied to a thick steel plate, the bainite volume fraction in the surface layer structure varies greatly depending on the cooling conditions, and it is difficult to obtain a stable material.

The present invention solves the above problems and provides a steel plate having excellent workability, particularly stretch flangeability, and a method for producing the same in a thick steel plate.

  These problems are solved by the following invention. The invention has a chemical composition of% by mass, C: 0.02 to 0.2%, Si: 0.5% or less, Mn: 2.5% or less, P: 0.02% or less, S: 0.005% or less, the balance being iron and inevitable impurities The steel sheet has a thickness of 5 mm or more and less than 15 mm, the surface layer and the back layer have a bainite single phase structure at 10% or more and less than 25% of the sheet thickness, and the inner layer remainder is one type of ferrite and bainite or pearlite. It is a hot-rolled steel sheet excellent in workability and ductility characterized by comprising the above hard second phase.

  Here, a hot-rolled steel sheet having excellent workability and ductility is characterized in that the chemical composition further includes one or more of Ti, Nb, V, Mo, Zr, Cr, B in a total of 0.01 to 0.3%. You can also.

In addition, the invention of the manufacturing method is that the steel having the above chemical composition is continuously cast into a slab, heated to a temperature range of 1150 ° C to 1300 ° C, and then heated under the condition that the finishing temperature is Ar ₃ point or higher. It is characterized in that hot rolling is performed and rapid cooling is started within 2 seconds after the end of hot rolling, and the central part of the plate thickness is rapidly cooled to a range of 700 to 350 ° C at a cooling rate of 100 ° C / s or more. This is a method for producing a hot-rolled steel sheet having excellent workability and ductility.

  In this invention, the surface layer and the back layer are excellent in workability and ductility characterized by rapid cooling to 550 ° C. or less at a cooling rate of 200 ° C./s or more for 10% or more and less than 25% of the plate thickness, respectively. It can also be set as the manufacturing method of a hot-rolled steel plate.

  In the present invention, the surface layer (both front and back surfaces) of the steel sheet has a bainite single-phase structure, and the inner layer is made of ferrite + hard second phase, thereby improving workability and ductility. Hereinafter, the steel composition, steel structure and production conditions in the present invention will be described.

(1) Steel structure Steel plate surface layer: 10% or more and less than 25% of the plate thickness from the surface layer and the back layer, respectively. A bainite single-phase steel plate with a thickness of 5 mm or more and less than 15 mm, with a strength of 400 MPa or more and a hole expansion rate of 100%. In order to achieve the above, the surface layer and the back layer bainite single phase structure needs to be 10% or more in the thickness direction. On the other hand, when the surface layer and back layer bainite single-phase structures are 25% or more in the thickness direction, ductility is lowered, so the content was made less than 25%. A more desirable strength-ductility balance can be obtained by setting the single-phase structure of the surface layer and the back layer bainite to 20 to 10% of the plate thickness.

Steel plate inner layer: Ferrite and hard second phase To obtain good ductility with a steel plate with a thickness of 5 mm or more and less than 15 mm, the balance is made of ferrite, pearlite, or bainite with one or more types of hard second phase. It is assumed that it has an organization. As a result, about the inside of a steel plate, the same ductility as a normal composite structure steel plate can be ensured.

  By the structure shown above, ductility can be ensured by the structure inside the plate thickness, and the strength and workability can be improved by the single-phase structure of the surface layer.

(2) Steel composition
C: 0.02-0.2%
C is added to ensure the strength of the steel sheet. If it is less than 0.02%, strength of 400 MPa or more cannot be obtained, and if it exceeds 0.2%, workability and ductility deteriorate, so 0.02 to 0.2% is added.

Si: 0.5% or less
Si is a solid solution strengthening element and is added to ensure strength. If it exceeds 0.5%, the surface properties deteriorate, so 0.5% or less.

Mn: 2.5% or less
Mn is a solid solution strengthening element and is added to ensure strength. However, if Mn exceeds 2.5%, workability and ductility deteriorate, so 2.5% or less. In addition, since the effect of ensuring strength is small at 0.5% or less, Mn is set to 0.5% or more, preferably 0.5 to 2.5%.

P: 0.02% or less
P is a harmful element that promotes cracking on the slab surface or under the slab surface during continuous casting. If P exceeds 0.02%, cracking becomes remarkable on the slab surface or under the slab surface during continuous casting, so 0.02% or less.

S: 0.005% or less
S is 0.005% or less in order to form MnS and cause deterioration of workability and ductility.

The selective elements are as follows.
Ti, Nb, V, Mo, Zr, Cr, B: 0.01 to 0.3% of one or more types in total
These elements form carbonitrides and improve strength. If the total of one or more types is less than 0.01%, the effect cannot be obtained, and if it exceeds 0.3%, the workability is deteriorated, so 0.01 to 0.3%.

  In the present invention, other elements are not particularly defined, and can contain, for example, Cu: 2% or less, Sn: 0.01% or less, and the like.

(3) Manufacturing conditions Slab heating temperature: 1150-1300 ° C
The continuously cast slab is heated to a temperature range of 1150 ° C. to 1300 ° C. at the plate thickness central temperature. When the heating temperature is less than 1150 ° C., it is difficult to ensure the finishing temperature of hot rolling. On the other hand, when the heating temperature exceeds 1300 ° C., surface defects are likely to occur.

Finishing temperature: Ar ₃ or more When the finishing temperature is lower than the Ar ₃ temperature, ferrite is generated in the surface layer, and bainite single-phase generation is impossible. Therefore, the hot rolling finishing temperature is set to Ar ₃ or higher.

Cooling after hot rolling: Rapid cooling starts within 2 seconds after hot rolling is completed. After hot rolling, rapid cooling is performed to obtain good ductility and strength by promoting the refinement of ferrite. If the rapid cooling start time exceeds 2 seconds, phase transformation may start during rapid cooling, and the surface layer portion may not have a bainite single phase structure. Therefore, rapid cooling is started within 2 seconds after the end of hot rolling.

Rapid cooling rate: 100 ° C / s or more at the center of the plate thickness If the cooling rate at the center of the plate thickness is less than 100 ° C / s, strength of 400 MPa or more and hole expandability of 100% or more cannot be obtained. 100 ℃ / s or more.

Quenching stop temperature: 700 to 350 ° C. at the center of the plate thickness When the quenching stop temperature exceeds 700 ° C., ferrite recovery or austenite is generated, and the surface layer portion does not have a bainite single phase structure. After that, when the coiling temperature exceeds 700 ° C., the strength cannot be ensured to 400 MPa due to the recovery of the ferrite structure. On the other hand, when the quenching stop temperature is less than 350 ° C., the central portion of the plate thickness also has a bainite or martensite structure, and the steel plate material is hard and has low ductility. Furthermore, since there is a problem that the load on the winding device becomes large, the temperature is set to 350 ° C. or higher.

Cooling condition of the surface layer part of the plate thickness: Rapid cooling to 200 ° C./s or more and 550 ° C. or less Further, by defining the cooling condition of the surface layer part of the sheet thickness, a predetermined portion in the sheet thickness direction is made a bainite single phase. When the cooling rate of the plate thickness surface layer portion is less than 200 ° C./s, at least 10% or more of the plate thickness surface layer portion cannot be made into a bainite single phase. On the other hand, if the cooling stop temperature of the plate thickness surface layer portion is higher than 550 ° C., ferrite is generated, and the plate thickness surface layer portion cannot be made into a bainite single phase. Therefore, it is necessary to cool to a transformation point of 550 ° C. preferably at a cooling rate of 200 ° C. or higher as a cooling condition for the plate thickness surface layer portion.

  Here, after the bainite is formed on the surface layer, the strength of the surface layer portion is high due to the low temperature, and therefore the winding is performed after the surface layer temperature matches the center temperature of the plate thickness, that is, after the surface layer temperature has sufficiently recovered. It is desirable to reduce the load on the apparatus.

In the present invention, the workability and ductility are improved by making the plate thickness surface layer portion of the steel plate have a bainite single-phase structure and the inner layer (plate thickness center portion) is made of ferrite + hard second phase. Even in a thick hot-rolled steel sheet, in the present invention, it is possible to manufacture a steel sheet having such a structure by defining the cooling rate and quenching stop temperature of the surface layer portion of the sheet thickness.

  Hereinafter, the best mode for carrying out the invention will be described with reference to examples.

  The results of the steel sheet hole expansion test will be described. After melting the steel having the chemical composition shown in Table 1, slabs were produced by continuous casting. In addition, hot-rolled steel sheets were produced from these slabs under the rolling conditions shown in Table 2, and samples used for the tests were collected. Here, Nos. 1 to 8 are invention steels within the scope of the present invention, and No. 9 is a comparative steel in which Mn, P and S are outside the scope of the present invention.

Table 2 shows manufacturing conditions and mechanical property values. The steel slab shown in Table 1 above was heated to 1200 ° C. and then rolled at a finishing temperature of normal operation of 850 ° C. (Ar ₃ points or more) to a thickness of 8 mm. Thereafter, for Nos. 1 to 3 of the inventive examples, the center of the plate thickness is set to a cooling rate of 150 to 250 ° C./s and a stop temperature of 650 to 470 ° C., and the surface layers are 220 to 340 ° C./s and 450 to 520, respectively. C. On the other hand, as for the comparative examples, No. 5 to 8 have a cooling rate at the center of the plate thickness lower than the invention range, and Nos. 4 to 8 have a cooling rate of the surface layer lower than the invention range. Of the comparative examples, only No. 9 has these cooling conditions within the scope of the invention.

  Here, the tensile test was implemented using the JIS5 test piece. The materials satisfying the chemical composition and production conditions of the present invention are Nos. 1 to 3, and the comparative examples that deviate from the production conditions of the present invention are materials Nos. 4 to 8. The comparative example will be described. No. 4 is an example in which the cooling start time exceeded 2 seconds (both in the center of the plate thickness and the surface layer). No. 5 has a cooling start time of 2 seconds or less, but the cooling rate is outside the scope of the invention (normal cooling to 40 ° C / s), and No. 6 and 7 have a cooling start time of more than 2 seconds. Is also outside the scope of the invention (normal cooling). In No. 8, the cooling start time exceeds 2 seconds, and only the surface layer has a cooling rate of 100 ° C./s, but it is also outside the scope of the invention (<200 ° C./s).

  Fig. 1 shows the workability (TS-λ (hole expansion ratio) balance). From FIG. 1, it is clear that the invention example has an improved strength-hole expansion ratio balance as compared with the comparative example.

  FIG. 2 shows the structure of the plate thickness surface and the plate thickness center of these steel plates. Inventive Example (No. 1) is composed of a bainite single phase on the plate thickness surface as shown in FIG. 2, and other structures consisting of ferrite and second phase. On the other hand, in the comparative example (No. 6), the plate thickness surface layer portion does not have a bainite single phase structure, but has the same ferrite + pearlite structure as the plate thickness central portion, and the particle size is larger than that of the invention example.

Comparative Example No. 9 is an example in which P and S deviated from the composition of the present invention, and the structure was the same as in the present invention, the plate thickness surface layer portion being a bainite single phase structure, and the plate thickness center portion being ferrite + second phase. However, the ductility and hole expansibility are extremely low.

  The results of a study on the crack propagation characteristic test of the punched hole end face, which is an effective factor for stretch flangeability, are described. The crack propagation characteristics in the hole expansion test in the present invention were performed using the test piece shown in FIG. The test piece was sampled from the width direction (C direction) of the hot-rolled steel strip, 8 t × 100 L × 30 w, machined (polished) on one side of the end face of the test piece 50L, and notched by punching on one side. The stress state in the vicinity of the hole end face of this test is uniaxial tension, which simulates a hole expansion test. The elongation of this test corresponds to the hole expansion rate λ, and the effect of surface bainite on λ was examined by observing the crack growth due to the hole expansion rate.

  The sample is No.1 (λ = 140%) with bainite in the surface layer of Table 2 where the strength is 470 MPa and there is a difference in λ, and ferrite + pearlite uniform structure No.6 (λ = 70%). A tensile test was carried out, and the crack propagation at the notch portion of the punched hole with respect to the elongation was observed to measure the crack length that propagates in the thickness direction. Elongation was performed with a distance between the scores of 10 mm.

The results are shown in FIG. In the figure, B represents bainite, and F + P represents ferrite + pearlite. According to Fig. 4, both samples have almost the same strength, and cracks are generated at the hole end faces, but the elongation is almost the same, and the crack length of the comparative material and the invention material progresses to the surface layer at almost the same speed. However, when the crack reaches the surface layer part, the progress of the inventive material decreases with respect to the elongation, and it is clear that the inventive material suppresses crack propagation compared to the comparative material. From the above, it is clear that the bainite on the surface layer improves the hole expandability.

The figure which shows the effect of this invention in intensity | strength and ductility. The figure which shows the microstructure in a plate | board thickness direction cross section. Notch specimen for crack propagation property test. The figure which shows a crack propagation characteristic.

Claims (4)

C: 0.02-0.2%, Si: 0.5% or less, Mn: 2.5% or less, P: 0.02% or less, S: 0.005% or less, with the balance being iron and inevitable impurities. The plate thickness is 5 mm or more and less than 15 mm, the surface layer and the back layer have a bainite single-phase structure at 10% or more and less than 25% of the plate thickness, and the inner layer balance is one of one or more hard ferrite or bainite or pearlite. A hot-rolled steel sheet excellent in workability and ductility characterized by comprising two phases.
The hot-rolled steel sheet having excellent workability and ductility according to claim 1, wherein the chemical composition further contains one or more of Ti, Nb, V, Mo, Zr, Cr, and B in a total amount of 0.01 to 0.3%. A hot-rolled steel sheet with excellent workability and ductility.
A steel having the chemical composition according to claim 1 or 2 is continuously cast to form a slab, heated to a temperature range of 1150 ° C to 1300 ° C, and then hot-rolled under a condition where the finishing temperature is set to Ar ₃ or higher. Workability is characterized by starting rapid cooling within 2 seconds after the end of hot rolling, rapidly cooling the center of the plate thickness to a range of 700 to 350 ° C at a cooling rate of 100 ° C / s or more And the manufacturing method of the hot-rolled steel plate excellent in ductility.
The heat excellent in workability and ductility according to claim 3, characterized in that 10% or more and less than 25% of the plate thickness from the surface layer and the back layer is rapidly cooled to 550 ° C or less at a cooling rate of 200 ° C / s or more. A method for producing rolled steel sheets.

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