JP4268535B2 - High-strength cold-rolled steel sheet with excellent balance of strength formability - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet having an excellent balance between strength and formability. The cold-rolled steel sheet of the present invention includes a case where the steel sheet is used as it is, or after being subjected to a surface treatment such as plating or painting.

  High-strength steel sheets are used in various applications such as automobile bodies that require high strength for weight reduction. As these high-strength steel sheets, as a strengthening mechanism that increases the strength of the steel sheet without reducing the press formability, a high-strength structure using solid solution strengthening or a composite structure strengthening in which martensite and bainite are finely dispersed in a ferrite matrix is used. Strength steel sheets have been developed.

  Dual phase type steel (hereinafter also simply referred to as DP steel) and triphase type steel (processing induced transformation type steel: hereinafter also simply referred to as TRIP steel) as high-strength steel sheets using this composite structure strengthening Since it has excellent formability such as stretch flangeability and high strength, it is widely used in automobile parts and the like, and contributes to weight reduction of automobiles.

In this dual phase type steel, the microstructure of the finally obtained steel sheet has a ferrite phase as the main phase and a hard martensite phase with a volume fraction of 3 to 3% after forming with an equivalent strain of the steel sheet. High dynamic deformation resistance when it is a composite structure with other low-temperature generation phase containing 50%. Furthermore, it has excellent dynamic deformation characteristics, high impact energy absorption capability, and suitable for automobile crash safety members It has been known so far (see Patent Documents 1 and 2).
JP 2000-17385 A (pages 1 to 6) Japanese Patent Laid-Open No. 11-80878 (pages 1 to 6)

  However, the quality of the press formability does not depend only on the elongation of the tensile test, but also depends on the quality of the ductility of the shear end face, which is called hole expansibility (λ), in a member having a severe stretch flange process. The elongation and hole expansibility of this tensile test are contradictory issues (a trade-off relationship) and are difficult to make compatible.

  Conventional methods for improving the hole expandability of steel plates include reducing S, which leads to an increase in the number of inclusions expanded like MnS, and tearing sharp voids formed at phase boundaries where the deformation stress is greatly different. It has been proposed to avoid mixing martensite.

  However, extremely reducing S in steel causes not only a significant increase in cost, but also cannot be easily realized industrially, and if inclusions containing S are not found, there is an adverse effect of other inclusions. There is a possibility of actualization, which is not realistic.

  Even if it is intended to increase the strength while avoiding the presence of martensite, if an attempt is made to obtain a high strength of 590 MPa or more, a metal structure mainly composed of bainite or an element such as Ti, Nb, Mo, or V Must be added to enhance precipitation. However, in general, the ratio of the yield strength to the tensile strength, that is, the yield ratio is high in these methods, which is not preferable from the press formability. Further, elements such as Ti, Nb, Mo, and V increase the recrystallization temperature. In cold-rolled steel sheets, it is difficult to fully utilize the precipitation strengthening in a recrystallized state.

  For this reason, in order to achieve both elongation and hole expandability in the tensile test of this dual phase type steel, including main phases such as ferrite phase and martensite phase, and a small amount of retained austenite (also referred to as residual γ), It is necessary to clarify the microstructure of the steel sheet finally obtained.

  However, the residual γ in the dual-phase steel is particularly small and positive from the viewpoint of hindering stretch flangeability, unlike the tri-phase steel that contains a large amount or actively contains the residual γ. Is also regulated so as not to contain residual γ (Patent Document 3). For this reason, the trace residual γ in the dual-phase steel cannot be accurately measured by the X-ray diffraction method or the FE-SEM / EBSP method used for the usual measurement of the residual γ amount. This is due to various reasons such as the influence of the texture on the surface of the measurement sample.

A saturation magnetization measurement method has been proposed as a more accurate method for measuring residual γ in steel, replacing the X-ray diffraction method or the FE-SEM / EBSP method (Patent Documents 4 and 5). However, in this saturation magnetization measurement method, too, the measurement object is a steel plate for sub-zero treatment that contains a large amount or actively contains residual γ, and the measurement target is a very small amount of residual γ, such as dual-phase steel. is not. For this reason, it was also unclear whether a small amount of residual γ as in the dual phase type steel could be measured more accurately by the saturation magnetization measurement method than by the X-ray diffraction method.
JP 2000-328186 A (Claim 1, pages 1 to 19) JP 2002-161338 A (Claim 1, pages 1 to 6) Japanese Patent Laying-Open No. 2003-90825 (FIG. 5, pages 1-6)

  Therefore, in order to achieve both the tensile test elongation and hole expansibility of dual-phase steel, it is still unclear how the main phase of ferrite phase, martensite phase, etc. It was enough. As a result, it is difficult for conventional dual-phase steels to achieve both the tensile test elongation and hole expandability, and both the press forming process and the hole expanding process are required. However, there is a limit that is difficult to apply to automotive parts.

  The object of the present invention was made to solve such problems, and is a high-strength cold-rolled steel sheet excellent in a dual-phase strength formability balance that achieves both elongation and hole expandability in a tensile test. Is to provide.

In order to achieve this object, the gist of the high-strength cold-rolled steel sheet having an excellent balance of strength formability according to the present invention is mass%, C: 0.02 to 0.25%, Si: 0.02 to 4. 0%, Mn: 0.15 to 3.5%, B: 0.01% or less (excluding 0%), and Al and P in total 1.0% or less (excluding 0%) Each of which is composed of a balance iron and inevitable impurities, and is a cold-rolled steel sheet having a composite structure of a ferrite phase and a martensite phase, the average hardness of the martensite phase is Hv 400 to 480, and ferrite The volume fraction of the phase is 30 to 60%, and the volume fraction measured by the saturation magnetization measurement method of residual γ in these composite tissues is 1 vol% or less including 0 vol%.

  In the DP steel sheet (cold-rolled steel sheet), the present inventors have elucidated the main phase such as the ferrite phase and martensite phase and the microstructure of the steel sheet including a small amount of residual γ, and the average hardness of the martensite phase and the ferrite It was found that the fraction of residual γ, along with the characteristics of the main phase such as the volume fraction of the phase, greatly affects the balance between elongation and hole expansibility in the tensile test.

  Usually, in the DP steel sheet, the fraction of residual γ in the composite structure is extremely low. However, the present inventors have found that among these small amounts of residual γ, the lower residual γ fraction improves the hole expansibility without decreasing the tensile test elongation.

  As described above, the fraction of residual γ in such DP steel sheet is determined by the conventional X-ray diffraction method or FE-SEM / EBSP method (Electron BackScatter diffraction pattern) for analyzing EBSP that appears when irradiated with electron beams. With this, accurate measurement is not possible. For this reason, the present inventors use the saturation magnetization measurement method that has been conventionally used for the measurement of a large amount of residual γ, and the volume fraction of residual γ is 0 vol% in the conventional X-ray diffraction method. The amount of residual γ of the DP steel sheet measured as (no residual γ) was measured again by the saturation magnetization measurement method. Then, even when the volume fraction of residual γ is measured by the X-ray diffraction method to be 0%, the volume fraction of residual γ is not necessarily 0 vol% according to the saturation magnetization measurement method. It was found to exist with a small volume fraction.

  Moreover, when the residual γ is present in a very small volume fraction according to these saturation magnetization measurement methods, the residual γ volume fraction exceeds 1 vol%, or the residual γ volume fraction is 1 vol% or less (0 vol%). It was also found that the characteristics of elongation and hole expansibility in the tensile test change greatly.

  That is, when the volume fraction of residual γ is 1 vol% or less (including 0 vol%) by the saturation magnetization measurement method, along with the characteristics of the main phase such as the average hardness of the martensite phase and the volume fraction of the ferrite phase Furthermore, it becomes possible to improve the hole expansibility without reducing the elongation of the tensile test in the DP steel sheet.

  In the TRIP steel sheet, when the residual γ is reduced, the elongation in the tensile test is lowered. On the other hand, in the DP steel sheet of the present invention, even if the residual γ is reduced, the elongation in the tensile test does not decrease. The reason for this is considered that the amount of residual γ, which is a problem in the DP steel sheet of the present invention, is an extremely small region as compared with TRIP steel as described above. It is assumed that such a small reduction in residual γ does not affect the elongation of the tensile test.

First, the requirements of the microstructure in the cold rolled steel sheet of the present invention will be described below.
(Ferrite phase)
In the present invention, the volume fraction of the ferrite phase as the main phase is set to 30 to 60%. When the volume fraction of the ferrite phase is less than the lower limit of 30%, the elongation in the tensile test is lowered, for example, the average El is 12% or less. On the other hand, when the volume fraction of the ferrite phase exceeds the upper limit of 60%, the hole expansibility decreases, and for example, the average λ becomes 50% or less. Therefore, the volume fraction of the ferrite phase is set in the range of 30 to 60% in order to improve the hole expandability without reducing the elongation of the tensile test.

  Here, the average hardness of the ferrite phase dominates the YP of the DP steel sheet, and the average hardness is preferably Hv260 or less in order to reduce the YP. By setting the average hardness of such a ferrite phase, the average YP of a high-strength DP steel sheet of 950 MPa or more can be lowered to 850 MPa or less, the elongation of a tensile test can be secured, and the average EL can be made 13% or more. . When the average hardness of the ferrite phase exceeds Hv260, the average YP exceeds 850 MPa, the tensile test elongation decreases, and the shape freezeability during press molding is likely to decrease.

(Martensite phase)
In the present invention, the average hardness of the martensite phase that is the main phase is set to Hv400 to 480. If the average hardness of the martensite phase is less than the lower limit of Hv400, it is difficult to ensure the strength of the steel sheet. On the other hand, when the average hardness of the martensite phase exceeds the upper limit of Hv480, the martensite phase tends to act as a starting point of fracture during hole expansion processing, and conversely, the hole expansion property decreases, and the average λ becomes It cannot be secured. Therefore, the average hardness of the martensite phase is set to a range of Hv 400 to 480 in order to ensure the strength and hole expandability of the steel plate.

(Residual γ)
As described above, in the DP steel sheet, a small amount of residual γ becomes a controlling factor for achieving both the elongation of the tensile test and the hole expandability. In the present invention, the volume fraction measured by the saturation magnetization measurement method of residual γ in the DP steel sheet is set to 1 vol% or less including 0 vol%. When the volume fraction measured by the saturation magnetization measuring method of the residual γ exceeds the upper limit of 1 vol%, the hole expansibility decreases, for example, the average λ becomes 35% or less.

  The measuring method by saturation magnetization of a small amount of residual γ in the DP steel sheet can be performed as follows by the method described in Patent Document 4 described above.

  The saturation magnetization measurement method is based on the following measurement principle. That is, a structure such as a ferrite phase or a martensite phase in a metal structure exhibits ferromagnetism at room temperature, whereas an austenite phase is paramagnetic. Accordingly, the saturation magnetization amount (Is) per unit volume of the metal structure of the DP steel sheet consisting only of a structure exhibiting ferromagnetism such as a ferrite phase and a martensite phase (substantially no residual γ phase) is obtained in advance. By measuring the saturation magnetization (I) of the sample containing the residual γ phase, the proportion of the γ phase can be obtained from the calculation formula as described later.

  FIG. 1 schematically illustrates a part of a DC magnetometer used for the saturation magnetization measurement. A magnetic field is generated using electrode stones, a 4πI coil 1 for magnetization detection and an H coil 3 for magnetic field detection are mounted between electromagnets, a rod-like sample 2 is inserted into the 4πI coil 1 to form a closed magnetic circuit, It is obtained by measuring the magnetization curve in a state where the influence of the demagnetizing field is eliminated. In addition, 4 is a conducting wire for the coil 1.

That is, the saturation magnetization amount (I) of the measurement target sample and the saturation magnetization amount (Is) when the residual γ amount is substantially the same component as the measurement target sample and the volume fraction is substantially 0 vol%. The amount of residual γ (γR) in the sample to be measured is calculated according to the following formula (1) by actual measurement or calculation.
γR (volume%) = (1−I / Is) × 100 (1)

  For the saturation magnetization measurement, two cold-rolled plates as test materials were stacked and a 4 mmW × 30 mmL test piece was used. Then, the hysteresis loop of the test piece was measured at a gap between the electrode stones of the DC magnetometer of FIG. The saturation magnetization amount was defined as the maximum magnetization average value of both poles. Since this saturation magnetization is easily affected by the measurement temperature condition, the measurement is performed within the range of 23 ° C. ± 3 ° C. when measured at room temperature. The accuracy of the absolute value was obtained by calibrating magnetization using a standard calibration sample (4N pure iron sample).

  At this time, when the saturation magnetization (Is) of the DP steel plate consisting only of the ferrite phase and the martensite phase is obtained by actual measurement, a long-time austempering temperature of 400 ° C. × 15 h is applied to the steel having substantially the same composition as the measurement target sample. A sample that was treated and substantially free of residual γ phase was used.

(Component composition of steel sheet)
In the following, the present invention has the above-described microstructure, and is necessary or preferable to have the characteristics such as high strength, high formability, and high hole expansibility required for applications such as automobile bodies. The chemical component composition of the cold-rolled steel sheet and the reasons for limitation of each element will be described.

The basic chemical composition of the cold-rolled steel sheet according to the present invention is, by mass%, C: 0.02 to 0.25%, Si: 0.02 to 4.0%, Mn, in order to have the necessary characteristics. : 0.15 to 3.5%, B: 0.01% or less (excluding 0%), Al and P in total containing 1.0% or less (excluding 0%), respectively And the balance iron and inevitable impurities. With such a chemical component composition, it is preferable to ensure the characteristics of the DP steel sheet having TS of 950 MPa or more, YP of 850 MPa or less, EL of 13% or more, and λ of 50% or more, in addition to the definition of the above microstructure. Is done.

  Other elements such as Al, P, Cr, Ni, Cu, Mo, Nb, Ti, V, Zn, S, and N are basically impurities, and it is preferable that the content is small. However, since some elements have useful effects, B is 0.01% by mass or less, Al and P are 1.0% by mass or less in total, and one or more of Cr, Ni, Cu, and Mo are added in total. And 1.0% by mass or less, and one or more of Nb, Ti, V and Zn are allowed to be contained in total of 1.0% by mass or less.

(C: 0.02-0.25%)
C is an element that ensures high strength and strongly influences the structure of the steel sheet. When the content is reduced to less than 0.02%, a martensite phase having the above desired amount and hardness (strength) is obtained. It becomes difficult. On the other hand, if the content exceeds 0.25%, unnecessary carbide precipitation occurs, and the elongation and hole expansibility of the tensile test are lowered, and the moldability is deteriorated. Also, the weldability is deteriorated. Therefore, the C content is in the range of 0.02 to 0.25%.

(Si: 0.02 to 4.0%)
Si is an element useful for generating martensite, and has the effect of improving the hole expansibility and securing the martensite by promoting the formation of ferrite and suppressing the formation of carbides. In addition, it has a solid solution strengthening action and a deoxidizing action in which the strength is increased without greatly reducing the elongation of the steel sheet. Thus, Si has a low yield ratio and is important for realizing both elongation and hole expansibility in a tensile test. If the Si content is less than 0.02%, these effects cannot be exhibited, and the Si content needs to be 0.02% or more. On the other hand, excessive content exceeding 4.0% lowers the ductility of the DP steel sheet and lowers the formability, so the upper limit is made 4.0%. Therefore, the Si content is in the range of 0.02 to 4.0%.

(Mn: 0.15 to 3.5%)
Mn stabilizes austenite to ensure martensite and is also a strengthening element. If it is less than 0.15%, these effects cannot be exhibited, and the Mn content needs to be 0.15% or more. On the other hand, an excessive content exceeding 3.5% saturates the above effect, and adversely affects ferrite transformation and the like. Therefore, the Mn content is in the range of 0.15 to 3.5%.

(Al, P)
Al and P have the effect of generating martensite, have the effect of ensuring the martensite by promoting the formation of ferrite and suppressing the formation of carbides, and also have a solid solution strengthening effect. Al also has a deoxidizing action. Accordingly, the total content is 1.0% by mass or less .

(Cr, Ni, Cu, Mo)
Cr, Ni, Cu, and Mo are austenite stabilizing elements like Mn, and have the effect of enhancing the hardenability of steel and facilitating the formation of martensite. Therefore, the total content up to 1.0% by mass is allowed.

(Nb, Ti, V, Zn)
These elements form carbides, nitrides, and carbonitrides, and are effective in increasing the strength of the steel sheet. Therefore, the total content up to 1.0% by mass is allowed. However, the excessive content exceeding 1.0 mass% in total increases the fraction of residual γ in the DP steel sheet without sufficiently decomposing the C-enriched region such as carbide even by annealing. . As a result, the elongation and hole expansibility of the tensile test are lowered, and the moldability is deteriorated. Therefore, the total content up to 1.0% by mass is allowed.

(B ≦ 0.01%)
B increases the hardness of the ferrite phase and is effective in ensuring or controlling the average hardness. When B is contained, B is fixed as BN at a low temperature, but decomposes and becomes a solid solution B when the steel plate is held at a high temperature. When quenching from the state where this solid solution B exists, martensite increases. As a result, it is presumed that the constraint of the ferrite increases and the hardness of the ferrite increases. B is contained at an impurity level of about 5 ppm in a normal steel plate. Even if the content is about this level, the above effect is obtained, and the average hardness of the ferrite phase can be ensured within a range of Hv 260 or less. Therefore, in the present invention, the B content at a normal impurity level is allowed and the upper limit is 0.01% . If the B content exceeds 0.01%, the average hardness of the ferrite phase may exceed Hv260, the average YP exceeds 850 MPa, the tensile test elongation decreases, and the shape during press molding There is a high possibility that the freezing property will decrease.

(Production method)
Next, preferable production conditions for the cold-rolled steel sheet of the present invention will be described below.
The cold-rolled steel sheet according to the present invention is cold-rolled and subjected to annealing after undergoing the respective steps after hot rolling and winding.

  At this time, the coiling temperature for hot rolling is preferably higher than 600 ° C. If the coiling temperature is low, the average hardness of the ferrite phase easily exceeds Hv260 and the average YP is 850 MPa even by annealing of the cold-rolled steel sheet. It becomes high exceeding this, and the elongation of a tensile test may fall.

  The cold-rolled steel sheet is annealed in an annealing process such as batch annealing or continuous annealing to obtain a final product DP steel sheet (cold-rolled steel sheet). However, during this annealing, annealing heating (soaking) is performed in two stages so that the fraction of residual γ in the DP steel sheet is measured by a saturation magnetization measurement method and is 1 vol% or less including 0 vol%. It is preferable.

  A normal DP steel sheet is heated by one stage of heating (soaking) in a temperature range of Ac1 to Ac3. In this normal annealing, the pre-anneal structure before annealing is decomposed in this heating (soaking) process, and it has been said so far that the influence of the pre-structure does not affect the residual γ forming ability. That is, in the DP steel sheet structure after the normal annealing, it has been said that the residual γ does not substantially exist from the measurement result by the X-ray diffraction method.

  However, according to the knowledge of the present inventors, there is of course the influence of the chemical composition of the DP steel sheet, but the C-enriched region of the previous structure before annealing is completely C in the normal heating (soaking) process. Is not diffused, and the austenite around the C-enriched region is stabilized and is likely to remain as residual γ.

  This is because when the residual γ content of DP steel, which is measured by the conventional X-ray diffraction method, is 0, the volume fraction of the residual γ content is not necessarily measured. This is also because the rate does not become 0 vol% and the residual γ exists in a small volume fraction inside and outside 1 vol%.

  Therefore, when the steel sheet is subjected to a thermal history that sufficiently decomposes the C-enriched region of the previous structure such as carbide during annealing, the fraction of residual γ in the DP steel sheet is measured by the saturation magnetization measurement method. And 1 vol% or less including 0 vol%. For this thermal history, it is preferable to perform annealing heating (soaking) in two stages with respect to normal annealing. Alternatively, a heating process may be performed before annealing, and the heat history may be performed in two stages.

  When the heat history is performed in two stages, the first stage heating (soaking) temperature is increased so that the C-enriched region of the previous tissue is sufficiently decomposed by the first stage heating (soaking). This temperature is selected from the temperature range of Ac1 to Ac3, including the holding time, according to the chemical composition of the DP steel sheet, the precipitation state of carbides, and the like. Further, the second stage heating (soaking) is performed in the sense of securing the quenching start temperature.

  If the heating (soaking) temperature is less than Ac1, no austenite is formed, and then martensite cannot be obtained, and if it exceeds Ac3, a coarse austenite single phase structure is formed. The volume fraction of martensite cannot be obtained.

  About the cooling after the said annealing, it is preferable that an average cooling rate shall be 5 degrees C / sec or more. When the average cooling rate is less than 5 ° C./second, a desired ferrite fraction / martensite fraction cannot be obtained. The upper limit is not particularly provided, but 300 ° C./second is preferable from the viewpoint of temperature controllability during cooling.

  By adopting the steel sheet composition and manufacturing method as described above, the microstructure of the cold-rolled steel sheet of the present invention is composed of a composite structure of a ferrite phase and a martensite phase, and the average hardness of the martensite phase is Hv 400 to 480. In addition, the volume fraction of the ferrite phase is 30 to 60%, and the volume fraction measured by the saturation magnetization measurement method of residual γ in these composite structures can be 1 vol% or less including 0 vol%. And it can be set as the DP steel plate excellent in the strength formability balance in which the elongation of the tensile test and the hole expandability were compatible.

  In addition, the cold-rolled steel sheet according to the present invention can be further subjected to annealing, temper rolling, electric / hot-plating, chemical conversion treatment, coating, and the like as desired products.

Examples of the present invention will be described below.
As Example 1, a steel slab 1 within the composition range of the present invention shown in Table 1 was melted and a slab was produced by continuous casting, and then the slab was heated to 1150 ° C. and hot-rolled to a thickness of 2.5 mm. Then, it was cooled and wound up at the temperature shown in Table 2. The hot rolled steel was pickled and then cold rolled to a thickness of 1.4 mm to produce a cold rolled steel sheet.

  In addition, in the constituent elements other than those described in Table 1, in each example, P, Cr, Ni, Cu, Mo, Nb, Ti, V, and Zn were 1.0% by mass or less in total. It was.

  Each cold-rolled steel sheet was annealed under the conditions shown in Table 2 for Examples 1 to 6. For soaking, after soaking 1 at each temperature and time (° C. × S), cool to room temperature 2 or cool to room temperature 2 without cooling to room temperature or low temperature. Then, soaking 2 was performed as it was.

  After this soaking, water quenching was performed from each quenching start (immediately before) temperature shown in Table 2, and then tempering was performed at each temperature shown in Table 2 to obtain DP steel sheets.

The microstructure of each DP steel sheet was investigated and the characteristics were evaluated. These results are also shown in Table 2. The microstructure includes the average hardness Hvα ′ (Hv) of the martensite phase, the volume fraction Vfα (%) and the average hardness Hvα (Hv) of the ferrite phase, and the volume fraction Vfγ _R (% ) And volume fraction Vfγ _R (%) of residual γ by X-ray diffraction.

  The volume fraction Vfα of the ferrite phase was obtained by analyzing the image of five fields of view of a sample polished with a mirror-polished and further etched with a mixed solution of nitric acid and ethanol with a SEM (scanning electron microscope) at a magnification of 1000 times. The average value was obtained.

  The average hardness Hvα ′ of the martensite phase and the average hardness Hvα of the ferrite phase were obtained by conducting a micro Vickers test of each phase portion with a load of 3 gf and calculating an average value of five points.

The volume fraction Vfγ _R of residual γ by saturation magnetization measurement was performed under the preferable conditions of the above-described measurement method. For comparison, the volume fraction Vfγ _R of residual γ was also measured by X-ray diffraction.

  The steel sheet was measured for tensile strength TS (MPa), yield point YP (MPa), elongation EL (%), and hole expansibility λ (%). At this time, the tensile strength TS is 980 MPa or more, the yield point YP is 850 MPa or less, the elongation is 11% or more, and the hole expansibility λ is 60% or more. It was set as a standard for cold-rolled steel sheets.

  Tensile strength TS (MPa), yield point YP (MPa), and elongation EL (%) were obtained by taking a JIS No. 5 test piece and conducting a tensile test at room temperature.

The hole expansion property λ was evaluated by a hole expansion test. Specifically, a cold-rolled steel sheet is cut out into a rectangle of 150 mm × 150 mm, a hole with a diameter d ₀ = 10 mm is punched at the center with a clearance of about 12%, and the burr is placed on the testing machine so as to be on the die side. Set and constrain the material so that it does not flow into the expanded portion with a 5 ton wrinkle pressing force, then expand the hole with a 60 ° conical punch and measure the hole diameter d when the crack penetrates the edge of the hole edge. Then, the increase rate of the hole diameter, that is, λ = (d−d ₀ ) / d ₀ was evaluated as the hole expansion rate.

As is clear from Tables 1 and 2, Invention Examples 1 and 4 are made of steel type 1 having a chemical composition within the scope of the present invention, and annealing heating (soaking) is performed in two stages. As a result, the average hardness of the martensite phase is in the range of Hv 400 to 480, and the volume fraction of the ferrite phase is in the range of 30 to 60%. The volume fraction Vfγ _R measured by the saturation magnetization measurement method of residual γ in these composite tissues is 0 vol%, which is 1 vol% or less. Therefore, Invention Examples 1 and 4 have a high tensile strength of 1000 MPa or more, a low yield point of 850 MPa or less, an elongation of 14%, and a hole expansibility λ of 70% or more.

On the other hand, Invention Example 3 has a relatively short time for first-stage annealing (soaking), which is slightly insufficient. As a result, the volume fraction Vfγ _R measured by the saturation magnetization measurement method of residual γ in the composite tissue is relatively high at 0.5 vol%. For this reason, compared with the said invention example 1 and 4, although elongation is as high as 15%, hole expansibility (lambda) is comparatively low as 60%.

In Invention Example 5, the hot rolling coiling temperature is low, and the average hardness of the ferrite phase is Hv270, which exceeds the preferable upper limit Hv260. For this reason, the average YP is 880 MPa, which is higher than the preferable upper limit of 850 MPa, the average hardness of the martensite phase, the volume fraction of the ferrite phase, and the volume fraction measured by the saturation magnetization measurement method of residual γ. Vfganma _R is 0 vol% although the hole expandability lambda 80% and higher, the elongation of the tensile test and 11%, lower than the above invention examples. Although the first stage annealing (soaking) temperature of Invention Example 5 is low, since the heating time is long, the annealing conditions are sufficient for reducing the residual γ.

Moreover, although the comparative example 2 consists of the steel type 1 within the scope of the present invention, the annealing heating (soaking) is performed in only one ordinary stage. As a result, the volume fraction Vfγ _R measured by the saturation magnetization measurement method of residual γ in the composite tissue is more than 1 vol%, which is 1.5 vol%. For this reason, Comparative Example 2 has an elongation as high as 15% at a high tensile strength of 1000 MPa or more, but the hole expansibility λ is extremely low as 20%, and both cannot be used together.

  Although the comparative example 6 consists of the steel type 1 within the range of this invention, the tempering temperature after annealing is as high as 500 degreeC. As a result, the average hardness of the martensite phase is Hv360, which is lower than the lower limit Hv400, and has a low strength of about 800 MPa.

  Although the comparative example 7 consists of the steel type 1 within the scope of the present invention, there is no tempering after annealing, and the average hardness of the martensite phase is Hv500, which is lower than the upper limit Hv480. As a result, the hole expansibility λ is 70%, but the elongation in the tensile test is 9%, which is significantly lower than that of the above-described invention example.

As is apparent from these results, in Table 2, even when the volume fraction Vfγ _R of residual γ by X-ray diffraction is 0%, the volume fraction Vfγ _R of residual γ by saturation magnetization measurement is 0 to 1. It was confirmed that there was a clear significant difference in the range of 5 vol%. In addition, it was confirmed that the volume fraction Vfγ _R of residual γ measured by saturation magnetization greatly changes the characteristics of elongation and hole expansibility in the tensile test with ₁ vol% as a boundary. Moreover, the significance of the organization requirements of the present invention and the significance of preferable production conditions are supported.

  Next, as Example 2, each steel type 2-5 shown in Table 1 was manufactured into a cold-rolled steel sheet under the same conditions as Steel Type 1 of Example 1 as shown in Table 3, and annealed (quenched, Each DP steel sheet was tempered.

  The microstructure of each DP steel sheet and the characteristics of the steel sheet were investigated in the same manner as in Example 1 to evaluate the characteristics. These results are shown in Table 3.

As is apparent from Table 3, Invention Example 8 is composed of steel type 2 having a chemical composition within the scope of the present invention, as well as Invention Example 1 (same as Invention Example 1 of Example 1), and is annealed (soaking). Is performed in two stages. As a result, the average hardness of the martensite phase is within the scope of the present invention, and the volume fraction of the ferrite phase is within the scope of the present invention. The volume fraction Vfγ _R measured by the saturation magnetization measurement method of residual γ in these composite tissues is 0 vol%, which is 1 vol% or less. Therefore, Invention Example 8 has a high tensile strength of 1010 MPa, a low yield point of 848 MPa or less, an elongation of 14%, and a hole expandability of 75% or more.

  On the other hand, Comparative Examples 9, 10, and 11 are each made of steel types 3, 4, and 5 of Table 1, C is 0.01% below the lower limit of 0.02%, and Si is below the lower limit of 0.02%. 0.01% and Mn are 0.13% lower than the lower limit of 0.15%, and the essential components are off.

As a result, in Comparative Example 9, although the volume fraction of residual γ Vfγ _R measured by saturation magnetization is within the range, since C is small, the volume fraction of the ferrite phase is too high and the hardness of the martensite phase is low. The strength is low. In Comparative Example 10 as well, although the volume fraction of residual γ Vfγ _R measured by saturation magnetization is within the range, the volume fraction of the ferrite phase is too low and the elongation is low due to the small amount of Si. Comparative Example 11 also, although the volume fraction Vfganma _R of residual γ by saturation magnetization measurement is in the range, for Mn is small, low hardness of the martensite phase, low strength, hole expandability is also low.

These results support the significance of the chemical component composition of the present invention.

  As described above, according to the present invention, it is possible to provide a high-strength cold-rolled steel sheet excellent in a dual-phase strength formability balance in which both elongation in tensile testing and hole expandability are achieved. For this reason, both the press molding process and the hole expanding process are performed, and both characteristics are required, and it is optimal for applications such as automobile members that require high strength for weight reduction.

It is a perspective view which shows a part of DC magnetic measuring apparatus used for saturation magnetization measurement.

Explanation of symbols

1: Coil, 2: Sample, 3: H coil for magnetic field detection, 4: Conductor

Claims (4)

In mass%, C: 0.02 to 0.25%, Si: 0.02 to 4.0%, Mn: 0.15 to 3.5%, B: 0.01% or less (including 0%) A total of 1.0% or less (excluding 0%) of Al and P, each of which is composed of the balance iron and unavoidable impurities, and is composed of a composite structure of a ferrite phase and a martensite phase. It is a rolled steel sheet, the average hardness of the martensite phase is Hv 400 to 480, and the volume fraction of the ferrite phase is 30 to 60%, measured by the saturation magnetization measurement method of residual γ in these composite structures A high-strength cold-rolled steel sheet having an excellent balance of strength formability, wherein the volume fraction is 1 vol% or less including 0 vol%. The strength formability balance according to claim 1, further comprising a total of one or more selected from Cr, Ni, Cu, and Mo in an amount of 1.0% by mass or less (excluding 0%). Excellent high-strength cold-rolled steel sheet. The strength formability according to claim 1 or 2, further comprising one or more selected from Nb, Ti, V, and Zn in a total amount of 1.0 mass% or less (excluding 0%). High-strength cold-rolled steel sheet with excellent balance. The high-strength cold-rolled steel sheet excellent in strength formability balance according to any one of claims 1 to 3, wherein an average hardness of the ferrite phase is Hv260 or less.

Images