WO2013094130A1

WO2013094130A1 - High-strength steel sheet and process for producing same

Info

Publication number: WO2013094130A1
Application number: PCT/JP2012/007663
Authority: WO
Inventors: 功一中川; 河村　健二; 横田　毅; 瀬戸　一洋
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2011-12-19
Filing date: 2012-11-29
Publication date: 2013-06-27
Also published as: US20140332123A1; KR101624439B1; JP2013127099A; CN104011240B; EP2796584A1; CN104011240A; EP2796584A4; IN2014KN01170A; EP2796584B1; JP5316634B2; KR20140100994A

Abstract

Provided are a high-strength steel sheet which has a tensile strength (TS) of 600-700 MPa, an elongation (El) of 25% or higher, and a hole expansion ratio (λ) of 80% or higher and has excellent formability and a process for producing the steel sheet. The high-strength steel sheet has a composition which contains, in terms of mass%, 0.10-0.18% C, 0.5-1.5%, excluding 0.5%, Si, 0.5-1.5% Mn, up to 0.05% P, up to 0.005% S, and up to 0.05% Al, with the remainder comprising Fe and unavoidable impurities. The sheet has a microstructure which comprises ferrite and pearlite, the volume proportion of the ferrite being 70-97% and the volume proportion of the pearlite being 3% or higher, and in which cementite is present at the ferrite grain boundaries in an amount of 2% by volume or less, the volume proportion of the phases other than the ferrite, pearlite, and cementite is less than 3% in total, and the ferrite has an average grain diameter of 7 µm or less.

Description

High strength steel plate and manufacturing method thereof

The present invention is a high-strength steel sheet with excellent formability that can be applied to automobile parts and the like, and in particular, tensile strength TS is 600 to 700 MPa, elongation El is 25% or more (plate thickness 1.6 mm, JIS No. 5 specimen), high strength steel plate with a hole expansion ratio λ of 80% or more, which is an index of stretch flangeability, and its manufacturing method (high strength, steel, sheet, and method for for producing same).

In recent years, improving the fuel efficiency of automobiles by reducing the weight of the vehicle body has become an important issue from the viewpoint of environmental conservation. For this reason, the reduction in thickness and weight by increasing the strength of steel plates, which are materials for automobile parts, has been studied. However, generally, since the workability of steel sheets decreases with increasing strength, there is a strong demand for high-strength steel sheets having both high strength and good workability.

Until now, some proposals have been made on high-strength steel sheets having excellent workability.
In Patent Document 1, as chemical components, C: 0.02 to 0.16% by mass, P ≦ 0.010%, S ≦ 0.003%, one or two of Si and Al in a total amount of 0.2 to 4%, One or more of Mn, Ni, Cr, Mo, Cu are included in a total amount of 0.5-4%, C / (Si + Al + P) is 0.1 or less, the remainder Fe and inevitable impurities A steel sheet having a cross-sectional microstructure of one or two of martensite and retained austenite in a total area ratio of less than 3%, ferrite and bainite. The total area ratio of one or two of (bainite) is 80% or more, the balance is pearlite, the maximum length of pearlite, martensite, and retained austenite is 10 microns or less. Strength-hole expansion ratio balance and shape fixability characterized by the inclusion of inclusions of 20 microns or more in the cross section within 0.3 per square mm An excellent high strength steel sheet for processing is disclosed.
Patent Document 2 includes mass%, C: 0.05 to 0.15%, Mn: 0.8 to 1.2%, Si: 0.02 to 2.0%, sol.Al: 0.002% to less than 0.05%, N: 0.001% to 0.005 The balance consists of Fe and impurities, and Ti, Nb and V in the impurities are all less than 0.005%, and the microstructure is ferrite with an average particle size of 1.1-5.0μm as the main phase and pearlite as the second phase. A hot-rolled steel material containing either one or both of cementite and cementite and satisfying Mnθ / Mnα ≦ 1 is disclosed. Here, Mnθ is the amount of Mn in cementite containing cementite in pearlite, and Mnα is the amount of Mn in ferrite as the main phase.
In Patent Document 3, by weight, C: 0.07 to 0.18%, Si: 0.5 to 1.0%, Mn: 0.7 to 1.5%, P: 0.02% or less, S: 0.005% or less, Ca: 0.0005 to 0.0050%, After making a steel slab containing Al: 0.01-0.10% and the balance Fe and inevitable impurities, heated to 1000-1200 ° C, hot-rolled (Ar ₃ transformation point +60) ° C to 950 ° C Finish rolling at temperature, finish cooling at 50 ℃ / sec within 3 seconds after finish rolling, T = 660-450 × [% C] + 40 × [% Si] -60 × By rapidly quenching at a temperature calculated as [% Mn] + 470 x [% P] (T ° C) or lower (T-70) ° C or higher, and then air-cooled and wound at over 350 to 500 ° C. Punched when the tensile strength is 50 kgf / mm ² or more, characterized in that the structure ratio of cementite with an equivalent circle radius of 0.1 μm or more is 0.1% or less and / or the structure ratio of martensite is 5% or less Disclosed is a method for manufacturing hot-rolled steel sheets with stretch flangeability of hole expansion ≧ 1.8 and excellent ductility. It has been.

JP 2004-68095 A JP 2004-137564 A Japanese Unexamined Patent Publication No. 4-88125

However, with the high-strength steel sheet described in Patent Document 1 and the hot-rolled steel material described in Patent Document 2, a TS of 600 to 700 MPa cannot be obtained. Further, in the high-strength hot-rolled steel sheet described in Patent Document 3, 25% or more El cannot be obtained with a plate thickness of 1.6 mm.

The present invention provides a high-strength steel sheet having excellent workability with TS of 600 to 700 MPa, El of 25% or more (in the case of a plate thickness of 1.6 mm, JIS No. 5 test piece) and λ of 80% or more, and a method for producing the same. The purpose is to do.

The inventors of the present invention have studied the above-mentioned high-strength steel sheet, and have ferrite and pearlite, the ferrite volume fraction is 70% to 97%, the pearlite volume fraction is 3% or more, and the ferrite It is effective to have a microstructure in which the volume fraction of cementite existing at grain boundaries is 2% or less, the volume fraction of other phases is less than 3% in total, and the average grain size of ferrite is 7 μm or less I found out.

The present invention has been made based on such findings, in mass%, C: 0.10% or more and 0.18% or less, Si: more than 0.5% and 1.5% or less, Mn: 0.5% or more and 1.5% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.05% or less, having a composition composed of the remaining Fe and inevitable impurities, the microstructure has ferrite and pearlite, and the volume fraction of the ferrite is 70% or more 97% or less, the pearlite volume fraction is 3% or more, the volume fraction of cementite present in the ferrite grain boundary is 2% or less, and the volume fraction of phases other than the ferrite, pearlite, and cementite is 3 in total. And providing a high-strength steel sheet having an average grain size of 7 μm or less.

The high-strength steel sheet of the present invention further includes at least one selected from mass%, Cr: 0.01% to 1.0%, Ti: 0.01% to 0.1%, V: 0.01% to 0.1%. It is preferable to do.

The high-strength steel sheet of the present invention preferably has a tensile strength TS of 600 to 700 MPa.
The high-strength steel sheet of the present invention preferably has a hole expansion ratio λ of 80% or more.
The high-strength steel sheet of the present invention preferably has a ferrite volume fraction of 80% to 95%.
The high strength steel sheet of the present invention preferably has a pearlite volume fraction of 3% to 30%. The volume ratio of pearlite is more preferably 5% or more and 28% or less.

The production method of the high strength steel sheet of the present invention is mass%, C: 0.10% or more and 0.18% or less, Si: more than 0.5% and 1.5% or less, Mn: 0.5% or more and 1.5% or less, P: 0.05% or less, S: 0.005% or less, Al: including 0.05% or less, a step of preparing a steel slab having a chemical composition consisting of the balance Fe and inevitable impurities, a step of subjecting the steel slab to hot rolling to form a hot rolled sheet, The hot-rolled sheet is heated to a two-phase temperature range between the Ac ₁ transformation point and the Ac ₃ transformation point, and then the average cooling rate is 5 ° C./s to 30 ° C./s in a temperature range of 450 ° C. to 600 ° C. Cooling and performing annealing for staying in the temperature range for 100 s or more.

The steel slab further contains at least one selected from mass%, Cr: 0.01% to 1.0%, Ti: 0.01% to 0.1%, V: 0.01% to 0.1%. preferable.
The annealing is performed by heating to a two-phase temperature range between the Ac ₁ transformation point and the Ac ₃ transformation point, and then to an average cooling rate of 10 ° C./s to 20 ° C./s to a temperature range of 450 ° C. to 600 ° C. It is preferable that it is cooled at a temperature of 100s to 300s.

The present invention makes it possible to produce a high-strength steel sheet having excellent workability with TS of 600 to 700 MPa, El of 25% or more, and λ of 80% or more.

The reasons for limitation of the high-strength steel sheet and the manufacturing method thereof according to the present invention will be described in detail below.

(1) Composition Hereinafter, “%” as a unit of content of component elements means “mass%”.

C: 0.10% or more and 0.18% or less C forms a second phase such as pearlite, martensite, and cementite, and contributes to an increase in the strength of the steel sheet. In order to obtain a TS of 600 MPa or more, a C amount of 0.10% or more is necessary. However, if it exceeds 0.18%, the second phase increases too much, so TS exceeds 700MPa and El and λ decrease. From the above, the C content is 0.10% to 0.18%. Preferably it is 0.12% or more and 0.16% or less.

Si: more than 0.5% and less than 1.5% Si is an element contributing to solid solution strengthening. In order to obtain a TS of 600 MPa or more, a Si amount exceeding 0.5% is required. However, if it exceeds 1.5%, the surface properties of the steel sheet deteriorate due to the generation of scale. From the above, the Si content is 0.5% to 1.5%. Preferably they are 0.7% or more and 1.2% or less.

Mn: 0.5% or more and 1.5% or less Mn is an element contributing to solid solution strengthening. In order to obtain a TS of 600 MPa or more, an Mn amount of 0.5% or more is necessary. However, if it exceeds 1.5%, TS exceeds 700 MPa, or λ decreases due to segregation. From the above, the Mn content is 0.5% or more and 1.5% or less. Preferably they are 1.1% or more and 1.5% or less.

P: 0.05% or less P is an element contributing to solid solution strengthening. However, if it exceeds 0.05%, El decreases due to segregation. Based on the above, the P content is 0.05% or less. Preferably it is 0.03% or less.

S: 0.005% or less When the S content exceeds 0.005%, S segregation occurs in the prior austenite grain boundaries, or MnS precipitates in the steel sheet, leading to a decrease in λ. From the above, the amount of S is set to 0.005% or less, but the smaller the amount, the better.

Al: 0.05% or less Al is added as a deoxidizer for steel and is an effective element for improving the cleanliness of steel. However, if it exceeds 0.05%, a large amount of inclusions are generated, which causes surface defects of the steel sheet. From the above, the Al content is 0.05% or less. Preferably it is 0.03% or less.

The balance is Fe and inevitable impurities, and further contains at least one selected from Cr: 0.01% to 1.0%, Ti: 0.01% to 0.1%, V: 0.01% to 0.1%. be able to. This is because Cr, Ti, and V suppress the recrystallization and recovery of austenite in the hot rolling temperature range, promote ferrite refinement, form carbides, or form a solid solution of ferrite. This is because it works to strengthen Note that Nb is an element that has the same effect. However, the addition of these elements does not lower the ductility (El) as much as when the same amount of Nb is added. Preferably, Cr: 0.02% to 0.5%, Ti: 0.02% to 0.05%, and V: 0.02% to 0.05%.

As unavoidable impurities, for example, O is 0.003% or less, and Cu, Ni, Sn, and Sb are each 0.05% or less.

(2) Microstructure In order to increase the strength and improve the workability of the steel sheet, a microstructure with ferrite and pearlite is adopted.

Ferrite volume fraction: 70% or more and 97% or less If the volume fraction of the entire ferrite structure is less than 70%, TS exceeds 700 MPa or λ of 80% or more cannot be obtained. On the other hand, when the volume ratio exceeds 97%, the amount of pearlite decreases, so that a TS of 600 MPa or more cannot be obtained. From the above, the volume fraction of ferrite is 70% or more and 97% or less. It is preferably 95% or less, and more preferably 80% or more and 90% or less.

Perlite volume ratio: 3% or more λ improves when the pearlite volume ratio is 3% or more. Preferably it is 5% or more. This is because pearlite is soft compared to cementite, martensite, and retained austenite, so compared to the number of voids generated at the interface between ferrite and martensite and the interface between ferrite and retained austenite after processing, This is probably because the number of voids generated at the interface is small.

The volume fraction of cementite present at the ferrite grain boundaries: 2% or less The steel sheet of the present invention may contain cementite, martensite, etc. in addition to ferrite and pearlite. If the volume fraction of cementite in the entire grain structure of cementite exceeds 2%, the number of voids generated at the interface between ferrite and cementite during hole expansion increases, leading to a decrease in λ. Therefore, the volume fraction of cementite existing at the ferrite grain boundary is set to 2% or less. It may be 0%.

Volume fraction of phases other than cementite present in ferrite, pearlite, and ferrite grain boundaries: less than 3% in total Other phases other than cementite present in ferrite, pearlite, and ferrite grain boundaries include martensite and retained austenite However, if the amount of these phases is less than 3% of the total volume ratio of the entire structure, it does not have a significant effect on the required steel sheet properties, so it exists in ferrite, pearlite, and ferrite grain boundaries. The volume fraction of phases other than cementite should be less than 3% in total. Preferably it is 2.5% or less, and may be 0%.

Average ferrite particle diameter: 7 μm or less Since the average particle diameter of ferrite exceeds 7 μm, the strength is reduced, so a TS of 600 MPa or more cannot be obtained. From the above, the average grain size of ferrite is 7 μm or less. Preferably, it is 5 μm or less.

Here, the volume fraction of the entire structure of ferrite, pearlite, cementite, martensite, and retained austenite is corroded with a nital solution after polishing the plate thickness section parallel to the rolling direction of the steel plate, and the magnification is 1000 times with an optical microscope. 3 fields of view were taken, and the type of tissue was selected by image processing. At the same time, the average particle diameter of the ferrite was calculated by a cutting method. Here, in order to obtain the average grain size of ferrite, an image taken with an optical microscope at a magnification of 1000 times (equivalent to 84 μm in the rolling direction and 65 μm in the plate thickness direction) is divided into 20 lines vertically and 20 lines horizontally. The segment was plotted, and the value obtained by dividing the total length of ferrite grains cut by one line segment by the number of ferrite to be cut was taken as the cut length, and the average intercept length L at each line segment was calculated. And the average particle diameter d was calculated | required by following Formula.
d = 1.13 × L
The volume ratio of the cementite existing in the ferrite grain boundary in the entire structure was obtained by taking three fields of view with a scanning electron microscope at a magnification of 3000 and extracting the cementite existing in the ferrite grain boundary by image processing.

(3) Manufacturing method Steel slab: The steel slab to be used is preferably manufactured by a continuous casting method in order to prevent macrosegregation of the component molten steel melted in the above component composition by a known method such as a converter. However, it can also be produced by an ingot-making method.

Hot rolling: The steel slab thus produced is hot-rolled after being cooled to room temperature or reheated in a heating furnace without being cooled to room temperature, or kept at a high temperature without passing through the heating furnace. The hot rolling conditions need not be particularly limited, but after heating the steel slab to the range of 1100 ° C to 1300 ° C, the hot rolling (finish rolling) is finished at 850 ° C to 950 ° C, and the coil is wound at 720 ° C or lower. It is preferable to take. This is due to the following reason. That is, if the heating temperature is less than 1100 ° C., the deformation resistance of the steel is high, so that hot rolling may be difficult, and if it exceeds 1300 ° C., the crystal grain size becomes coarse and TS may decrease. Also, if the finish rolling finish temperature is less than 850 ° C, ferrite is formed during rolling, so that extended ferrite is formed, which may lead to a decrease in λ, and if it exceeds 950 ° C, the crystal grain size becomes coarse , TS may decrease. Furthermore, when the winding temperature exceeds 720 ° C., the formation of the internal oxide layer becomes remarkable, and the chemical conversion property and the corrosion resistance after coating may be deteriorated.

The hot-rolled sheet after hot rolling is pickled to remove scale generated on the surface of the steel sheet.

Annealing: The hot-rolled sheet after pickling treatment is heated to a two-phase temperature range between the Ac ₁ transformation point and Ac ₃ transformation point, and then an average cooling rate of 5 ° C / s in the temperature range of 450 ° C to 600 ° C. It is cooled at 30 ° C./s or less and annealed to stay in the temperature range for 100 s or more. The reason for heating to the two-phase temperature range between the Ac ₁ transformation point and the Ac ₃ transformation point is to form a microstructure having ferrite and pearlite. In addition, after heating, cooling to a temperature range of 450 ° C to 600 ° C at an average cooling rate of 5 ° C / s to 30 ° C / s is because the volume fraction of cementite present at the ferrite grain boundaries exceeds 600 ° C. Since it exceeds 2%, the target λ cannot be obtained, and if it is less than 450 ° C, the amount of martensite increases, TS exceeds 700 MPa, λ decreases, and if the average cooling rate is less than 5 ° C / s This is because ferrite grains are coarsened, and a TS of 600 MPa or more cannot be obtained, and if it exceeds 30 ° C / s, the volume fraction of cementite that exists at the ferrite grain boundary exceeds 2% and λ of 80% or more cannot be obtained. . The average cooling rate is preferably 10 ° C./s or more and 20 ° C./s or less. The reason for staying in the temperature range of 450 ° C. or higher and 600 ° C. or lower for 100 seconds or longer is that the amount of pearlite decreases and λ decreases below 100 seconds. It is preferable that the staying time be 150 seconds or longer. In addition, since the effect is only saturated when staying for an excessively long time, it is preferably set to 300 s or less from the viewpoint of production efficiency. Moreover, annealing can be performed by a continuous annealing facility or the like.

After melting steel with the composition shown in Table 1 into a slab, it was heated to 1200 ° C, hot-rolled at a rolling end temperature of 890 ° C, wound at 600 ° C, and a hot-rolled sheet with a thickness of 1.6 mm did. Next, the hot-rolled sheet was pickled and then annealed under the annealing conditions shown in Table 2 using a continuous annealing facility. The Ac ₁ transformation point and Ac ₃ transformation point of the steel shown in Table 1 were calculated from the following formulas, respectively.
Ac ₁ transformation point (℃) = 723 + 29.1 (% Si) -10.7 (% Mn) +16.9 (% Cr)
Ac ₃ transformation point (° C) = 910-203 (% C) ^1/2 +44.7 (% Si) -30 (% Mn) +700 (% P) +400 (% Al) -11 (% Cr)
+104 (% V) +400 (% Ti)
However, (% M) represents mass% of the element M.

The steel sheet thus obtained was examined for the microstructure by the above method, and a tensile test was performed according to JIS Z 2241 using a JIS No. 5 test piece to measure TS and El. Further, using a 100 mm square test piece, a hole expansion test was performed in accordance with JFST 1001-1996, and λ was measured.

The results are shown in Table 3.

It can be seen that the steel plates of the examples of the present invention are high-strength steel plates having excellent workability with TS of 600 to 700 MPa, El of 25% or more, and λ of 80% or more. On the other hand, the target TS or λ was not obtained in the steel plate of the comparative example.

Claims

In mass%, C: 0.10% to 0.18%, Si: 0.5% to 1.5%, Mn: 0.5% to 1.5%, P: 0.05% or less, S: 0.005% or less, Al: 0.05% or less In addition, the composition comprising the balance Fe and inevitable impurities, the microstructure has ferrite and pearlite, the ferrite volume fraction is 70% to 97%, the pearlite volume fraction is 3% or more, The volume fraction of cementite present in the ferrite grain boundaries is 2% or less, the volume fraction of phases other than the ferrite, pearlite, and cementite is less than 3% in total, and the average grain size of the ferrite is 7 μm or less. High strength steel plate.
Further, in mass%, Cr: 0.01% or more and 1.0% or less, Ti: 0.01% or more and 0.1% or less, V: 0.01% or more and 0.1% or less, containing at least one selected from claim 1, High strength steel plate.
The high-strength steel sheet according to claim 1, further having a tensile strength TS of 600 to 700 MPa.
The high-strength steel sheet according to claim 1, further having a hole expansion ratio λ of 80% or more.
2. The high-strength steel sheet according to claim 1, wherein a volume ratio of the ferrite is 80% or more and 95% or less.
2. The high-strength steel sheet according to claim 1, wherein a volume ratio of the pearlite is 3% or more and 30% or less.
2. The high-strength steel sheet according to claim 1, wherein the volume ratio of the pearlite is 5% or more and 28% or less.
In mass%, C: 0.10% to 0.18%, Si: 0.5% to 1.5%, Mn: 0.5% to 1.5%, P: 0.05% or less, S: 0.005% or less, Al: 0.05% or less Preparing a steel slab having a chemical composition comprising the balance Fe and inevitable impurities;
The steel slab is hot-rolled by subjecting it to hot rolling,
The hot-rolled sheet is heated to a two-phase temperature range between the Ac 1 transformation point and the Ac 3 transformation point, and then the average cooling rate is 5 ° C./s to 30 ° C./s in a temperature range of 450 ° C. to 600 ° C. Cooling and applying annealing to stay in the temperature range for 100 s or more;
A method for producing a high-strength steel sheet having
The steel slab further contains at least one selected from mass%, Cr: 0.01% to 1.0%, Ti: 0.01% to 0.1%, V: 0.01% to 0.1%, Item 9. A method for producing a high-strength steel sheet according to Item 8.
The annealing step is performed after heating in a two-phase temperature range between the Ac 1 transformation point and the Ac 3 transformation point, and an average cooling rate of 10 ° C./s to 20 ° C./s in a temperature range of 450 ° C. to 600 ° C. The method for producing a high-strength steel sheet according to claim 8, comprising cooling at a temperature of 100 s to 300 s in the temperature range.