JP2013127099A - High-strength steel sheet excellent in workability and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet excellent in workability, in which TS is 600-700 MPa, El is ≥25% (sheet thickness 1.6 mm, measured with a JIS No.5 test piece) and λ is ≥80%, and to provide a method for manufacturing the same.SOLUTION: The high-strength steel sheet excellent in workability has a composition comprising, by mass, 0.10-0.18% C, >0.5% but ≤1.5% Si, 0.5-1.5% Mn, ≤0.05% P, ≤0.005% S, ≤0.05% Al and the balance being Fe and unavoidable impurities and has a microstructure comprising ferrite and pearlite, wherein the volume fractions of ferrite and pearlite are 70-97% and ≥3%, respectively, the volume fraction of cementite present at a ferrite grain boundary is ≤2%, the total volume fraction of phases other than ferrite, pearlite and cementite is <3%, and the average grain size of ferrite is ≤7 μm.

Description

  The present invention is a high-strength steel sheet excellent in workability applicable to automobile parts and the like, in particular, tensile strength TS is 600 to 700 MPa, elongation El is 25% or more (plate thickness 1.6 mm, JIS No. 5 test piece) The present invention also relates to a high-strength steel sheet having a hole expansion ratio λ of 80% or more, which is an index of stretch flangeability, and a method for producing the same.

  In recent years, from the viewpoint of environmental protection, improving the fuel efficiency of automobiles by reducing the weight of the vehicle body has become an important issue. 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. For example, in Patent Document 1, as a chemical component, C: 0.02 to 0.16% in 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 to 4%, C / (Si + Al + P) is 0.1 or less, the remainder Fe and inevitable A steel plate made of mechanical impurities, and as a microstructure of the cross section of the steel plate, one or two of martensite and retained austenite is less than 3% in total area ratio, one or two of ferrite and bainite The total area ratio is 80% or more, the balance is pearlite, the maximum length of pearlite, martensite, and retained austenite is 10 microns or less, and more than 20 microns of inclusions per square mm in the steel plate cross section A high-strength steel sheet for processing excellent in strength-hole expansion rate balance and shape freezing property characterized by being 0.3 or less is disclosed. Further, in Patent Document 2, in mass%, C: 0.05 or more and less than 0.15%, Mn: 0.8 to 1.2%, Si: 0.02 to 2.0%, sol.Al: 0.002% or more and less than 0.05%, N: 0.001 %% More than 0.005% is included, the balance consists of Fe and impurities, Ti, Nb and V in the impurities are all less than 0.005%, and the structure is ferrite with an average particle size of 1.1 to 5.0 μm as the main phase, the second phase As such, a hot rolled steel material containing either or both of pearlite 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. Patent Document 3 includes, 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 the steel consisting of Al: 0.01-0.10% and the balance Fe and inevitable impurities into a slab, heated to 1000-1200 ° C, hot-rolled (Ar ₃ transformation point +60) ° C to 950 ° C Finish rolling at the temperature, cool at 50 ℃ / second or more within 3 seconds after finishing rolling, T = 660-450 × [% C] + 40 × [% Si] -60 × [% Mn] A circle obtained by ending quenching at a temperature calculated at + 470 × [% P] (T ° C) or less (T-70) ° C or higher, and then winding at 350 ° C to 500 ° C after air cooling. Punching hole expansion ≧ 1.8 when tensile strength is 50 kgf / mm ² or more, characterized in that the structure ratio of cementite with an equivalent radius of 0.1 μm or more is 0.1% or less and / or the structure ratio of martensite is 5% or less A method for producing a hot-rolled steel sheet having a stretch flangeability and excellent ductility is disclosed

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 excellent in 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 boundaries is 2% or less, and the volume fraction of phases other than the ferrite, pearlite, and cementite is 3 in total. The present invention provides a high-strength steel sheet excellent in workability, characterized in that the average particle diameter of the ferrite is less than 7% and the average particle diameter of the ferrite is 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 includes a step of subjecting a steel slab having the above composition to hot rolling to form a hot-rolled sheet, and the hot-rolled sheet having two phases between the Ac ₁ transformation point and the Ac ₃ transformation point. By heating to a temperature range, cooling to a temperature range of 450 ° C. to 600 ° C. at an average cooling rate of 5 ° C./s to 30 ° C./s, and subjecting to annealing for allowing the temperature range to stay for 100 s or more. It can be manufactured.

  According to the present invention, it is 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 the limitation of the high-strength steel sheet of the present invention and the manufacturing method thereof will be described in detail below.

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

C: 0.10% to 0.18%
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 that contributes 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% to 1.5%
Mn is an element that contributes 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
If the S content exceeds 0.005%, S segregation occurs in the prior austenite grain boundaries, or MnS precipitates in the steel sheet, causing 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%, and 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 has the function of strengthening. 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 is 0.02% to 0.5%, Ti is 0.02% to 0.05%, and V is 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 manufactured 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 the steel slab is heated to a range of 1100 ° C to 1300 ° C, hot rolling (finish rolling) is finished at 850 ° C to 950 ° C and 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 temperature of finish rolling is less than 850 ° C, ferrite is generated 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 using JIS No. 5 test pieces in accordance with JIS Z 2241 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 (3)

  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 boundary 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 sheet with excellent workability characterized by   Further, in mass%, Cr: 0.01% or more and 1.0% or less, Ti: 0.01% or more and 0.1% or less, V: containing at least one selected from 0.01% or more and 0.1% or less A high-strength steel sheet having excellent workability as described in 1. The steel slab having the chemical composition according to claim 1 or 2 is subjected to hot rolling to form a hot rolled sheet, and the hot rolled sheet has a two-phase temperature between the Ac ₁ transformation point and the Ac ₃ transformation point. And heating at a temperature range of 450 ° C. to 600 ° C. and cooling at an average cooling rate of 5 ° C./s to 30 ° C./s and performing annealing to stay in the temperature range for 100s or more. A method for producing a high-strength steel sheet having excellent workability.