JP5423072B2 - High-strength cold-rolled steel sheet excellent in bending workability and delayed fracture resistance and method for producing the same - Google Patents

Description

The present invention relates to a high-strength cold-rolled steel sheet excellent in bending workability and delayed fracture resistance suitable for use in high-strength vehicle body structural parts such as automobile center pillars and door impact beams, and a method for producing the same. is there.
In the present invention, “excellent in bending workability” means that the radius of curvature at the tip of the minimum bent portion where fracture does not occur when 90 ° bending is performed is 1.5 mm or less. “Excellent delayed fracture resistance” means that no fracture occurs for 100 hours or more in a hydrochloric acid environment at 25 ° C. and pH 1.

In recent years, concern about global warming due to an increase in CO ₂ emissions in Europe is progressing regulation of CO ₂ emissions from motor vehicles is a mobile source of CO _2, and fuel efficiency of the vehicle is strongly required Yes. Although reducing the weight of the vehicle body is effective for improving fuel efficiency, it is also necessary to ensure the safety of passengers. Therefore, it is necessary to reduce the weight of the vehicle body and ensure collision safety more than before. The
In order to ensure both weight reduction and collision safety, the use of high specific strength materials is being considered to reduce the thickness of the steel sheet used. In recent years, high tensile strength of 980 to 1180 MPa class has been studied. The application of steel plates to automobile safety parts represented by center pillars and door impact beams is advancing. However, the demand for reducing the weight of the vehicle body is increasing, and studies are being conducted with a view to further reducing the weight of the vehicle body by applying a steel plate that is stronger than the 1180 MPa class steel plate.

  Automobile safety parts are generally manufactured by press molding. The press formability depends strongly on the ductility of the material, but in the press working of a steel sheet with a tensile strength of over 980 MPa, bending is mainly performed. Excellent workability is also required. Moreover, in the case of a high-strength material having a tensile strength exceeding 980 MPa, there is a concern about the residual stress after press molding and delayed fracture due to hydrogen entering from the environment. Therefore, in order to apply a high-strength cold-rolled steel sheet as an automobile safety part as described above, it is necessary to have high press formability, that is, excellent ductility, bending workability, and delayed fracture resistance.

Various proposals have been made to meet the above demand.
For example, Patent Document 1 discloses a technique relating to an ultrahigh strength thin steel sheet having a tensile strength exceeding 1400 MPa, utilizing the TRIP effect (Transformatoin Induced Plasticity) of the retained austenite phase. However, in all of the examples disclosed in Patent Document 1, Mo is added from the viewpoint of improving hardenability and stable generation of retained austenite, which has a disadvantage of increasing the alloy cost. In addition, since the metallographic structure uses a bainitic ferrite phase that contributes relatively little to strength compared to martensite, the amount of C and Mn required to obtain a tensile strength of 1470 MPa or more increases. For this reason, there remains a manufacturing problem that the rolling load of the steel sheet increases. Furthermore, in order to generate the bainite phase and stabilize the retained austenite phase, it is necessary to keep isothermal during the cooling from the annealing temperature to room temperature. In order to induce fluctuations, extremely strict operations were required, and problems were left in terms of manufacturing stability. Furthermore, Patent Document 1 points out that bending workability is important in press forming of an ultrahigh strength steel sheet, but does not disclose any bending characteristics.

  On the other hand, Patent Document 2 discloses a high-strength steel sheet having a tensile strength of 1470 MPa or more and having a martensite single-phase structure. However, in the example disclosed in Patent Document 2, in addition to the low elongation of 6%, the minimum bending radius at which cracks do not occur in 180 ° bending is as large as 4 mm, and sufficient bending workability is obtained. I can not say.

JP 2007-197819 Japanese Patent No. 3729108

  The present invention has been developed in view of the above situation, and is effective for increasing the strength of V, Mo, etc., but does not include a reverted metal element that significantly increases the alloy cost and may induce casting defects. It is an object of the present invention to provide a cold-rolled steel sheet having a high tensile strength of 1470 MPa or more and an excellent workability together with its advantageous manufacturing method, with a certain Al content as a reduced steel component.

  Conventionally, in order to obtain a steel sheet with a tensile strength of 1470 MPa or more, it is effective to use a martensite single-phase structure (hereinafter, unless otherwise specified, the martensite phase includes a tempered martensite phase that has been tempered). It had been. However, in general, a martensite single-phase structure is relatively easy to secure strength, but has a poor ductility and is disadvantageous in terms of workability. For this reason, research and development of steel sheets using the TRIP effect or duplex-structure steel sheets composed of a martensite phase and a ferrite phase excellent in ductility have been promoted as steel sheets having high strength and excellent ductility. Furthermore, in recent years, a steel sheet using a bainite phase as a kind of low-temperature transformation phase has also been proposed.

  However, in order to develop the TRIP effect, it is necessary not only to add a large amount of alloying elements in order to increase the stability of the austenite phase, but also to keep isothermal at temperatures above the Ms transformation point when cooling from the annealing temperature. This is not preferable from the viewpoint of the manufacturing process and manufacturing cost. Furthermore, there is a problem that the sensitivity to material fluctuations such as strength and ductility due to fluctuations in temperature and holding time in the isothermal holding process is higher than martensitic single-phase steel sheets.

  In addition, the two-phase structure of martensite phase and ferrite phase is excellent in ductility, but a large amount of C is required to develop a predetermined strength with a relatively small amount of martensite phase. And it is not preferable from the viewpoint of spot weldability. Furthermore, since the metal structure is composed of a martensite phase and a ferrite phase having different hardnesses, there has been a problem that stretch flangeability and fracture toughness are lower than those of a martensite single phase structure.

  On the other hand, in steel sheets that utilize the bainite phase, it is necessary to cool the austenite phase to the pearlite or ferrite phase during the cooling from the annealing temperature, so it is necessary to cool it to the bainite phase generation temperature range. Mn and Cr are added in a large amount for the purpose of delaying, which is not preferable from the viewpoint of alloy cost. Moreover, in order to produce | generate a bainite phase effectively, it is necessary to cool at high speed, without producing a pearlite and a ferrite phase from an annealing temperature to a bainite phase production temperature. However, unlike steel sheets using the martensite phase, in order to obtain a steel sheet using the bainite phase, the steel sheet must not be cooled below the martensitic transformation point before the bainite phase is generated. In addition to requiring high-speed cooling, it is necessary to stop cooling strictly in the bainite phase generation temperature range, so stable and efficient production in the conventional continuous annealing line has been extremely difficult. .

From the above, in order to obtain an ultra-high-strength steel sheet with a tensile strength exceeding 1470 MPa, although there is a disadvantage that the ductility is relatively inferior, a martensitic single-phase structure can be used to reduce alloy costs and manufacturing stability. From this point of view, it is considered promising.
That is, the martensitic single-phase structure steel sheet does not change the strength characteristics depending on the volume ratio of the constituent phases unlike the double-phase steel sheet, and can stably manufacture a steel sheet having predetermined mechanical characteristics. In addition, since the amount of alloying elements required to develop desired strength is relatively small, there is an advantage that the alloying cost is excellent.

Therefore, the inventors have intensively studied to develop excellent ductility in the martensitic single-phase steel sheet from the above background, and as a result, by adding Si, the work hardening ability of the martensitic phase is increased. The present inventors have found that it is possible to develop higher ductility than conventional martensite single-phase steel sheets. It was also found that the addition of Si suppresses the coarsening of the carbide during tempering, and a structure in which the carbide is finely and uniformly dispersed in the structure can be obtained. Carbide functions as a hydrogen trap site that induces delayed fracture, but in the present invention, by finely and uniformly dispersing carbide in the structure, crack initiation and propagation starting from coarse carbide during bending are suppressed. Therefore, it became clear that bending workability was also improved. Further, from the viewpoint of delayed fracture resistance, local concentration of hydrogen due to coarse carbides can be suppressed by finely and uniformly dispersing carbides in the structure. In addition, because of excellent bending workability, material damage such as microvoids and microcracks when bending with a high strain amount is reduced, the starting point of delayed fracture can be reduced, and delayed fracture resistance is improved. I found out.
On the other hand, in terms of manufacturing process, during annealing and cooling after cold rolling, the annealing temperature and the subsequent cooling process are appropriately controlled, and tempering heat treatment is performed in a temperature range of 100 ° C. or higher and 250 ° C. or lower as necessary. It was also found that this is effective.

The present invention was completed after further investigation based on the above findings, and the gist of the present invention is as follows.
1. % By mass
C: 0.15-0.20%, Si: 1.0-2.0%,
Mn: 1.5 to 2.5%, P: 0.020% or less,
S: 0.005% or less, Al: 0.01 to 0.05%,
N: 0.005% or less, Ti: 0.1% or less,
Nb: 0.1% or less, B: 5-30 ppm
The balance consists of Fe and inevitable impurities, and the tempered martensite phase is 97% or more by volume and the residual austenite phase is less than 3% by volume (however, the portion within 10μm in depth from the steel sheet surface is excluded) A high-strength cold-rolled steel sheet with excellent bending workability and delayed fracture resistance, characterized by having a metallographic structure, a tensile strength of 1470 MPa or more, and a ratio of 0.2% proof stress to tensile strength of 0.80 or more.

2. The steel plate is more mass%,
Cu: The high-strength cold-rolled steel sheet having excellent bending workability and delayed fracture resistance as described in 1 above, comprising 0.20% or less.

3. % By mass
C: 0.15-0.20%, Si: 1.0-2.0%,
Mn: 1.5 to 2.5%, P: 0.020% or less,
S: 0.005% or less, Al: 0.01 to 0.05%,
N: 0.005% or less, Ti: 0.1% or less,
Nb: 0.1% or less, B: 5-30 ppm
The steel slab consisting of Fe and inevitable impurities is heated to 1200 ° C or higher, and then subjected to hot rolling at a finish rolling exit temperature of 800 ° C or higher, and then cold-rolled. When continuously annealing a rolled steel sheet, hold it for 30 to 1200 s in the temperature range of Ac ₃ transformation point to (Ac ₃ transformation point + 30 ° C.), then primary at an average cooling rate of 1 ° C./s or more to a temperature range of 720 ° C. Tempering after cooling, secondary cooling from the primary cooling end temperature to the average cooling rate: 100-1000 ° C / s to 100 ° C or less, and then holding in the temperature range of 100-250 ° C for 120-1800s It has a metal structure with a martensite phase volume ratio of 97% or more and a residual austenite phase volume ratio of less than 3% (excluding the portion within 10μm depth from the steel sheet surface), and a tensile strength of 1470MPa or more. and bending the ratio of the tensile strength and 0.2% proof stress is 0.80 or more processing and delayed Method for producing a high strength cold rolled steel sheet having excellent fracture properties.
4). The steel slab is further mass%,
Cu: 0.20% or less
4. The method for producing a high-strength cold-rolled steel sheet having excellent bending workability and delayed fracture resistance as described in 3 above.

According to the present invention, the tensile strength is 1470 MPa or more, which has excellent delayed fracture resistance that hardly causes delayed fracture due to hydrogen entering from the environment after press molding, and exhibits excellent workability during molding. The ultra-high strength cold-rolled steel sheet can be obtained with high productivity.
For this reason, as an ultra-high-strength part that does not easily cause delayed fracture, for example, it can be used for automobile safety parts such as a center pillar and an impact beam.

Hereinafter, the present invention will be specifically described.
The present invention was developed after earnest research to obtain a steel sheet having a tempered martensite single phase structure and excellent workability. In other words, the inventors have improved the work hardening ability of the tempered martensite phase by adding Si, and it is possible to finely and uniformly disperse carbides in the structure, and extremely high tensile strength of 1470 MPa or more. The present invention has been completed by finding that a cold-rolled steel sheet having high bending workability and excellent delayed fracture resistance can be obtained.

In the present invention, “high ductility” means that the elongation at break in JIS No. 5 tensile test is 10% or more, and “excellent delayed fracture resistance” means that fracture occurs for 100 hours or more in hydrochloric acid at 25 ° C. and pH 1. Say nothing.
The “tempered martensite single-phase structure” in the present invention is defined as a structure in which the tempered martensite phase is 97% or more by volume and the residual austenite phase is less than 3% by volume. This martensite single phase structure is obtained by rapid cooling from the annealing temperature, and according to this process, it is possible to stably obtain an ultra-high strength steel sheet without adding a large amount of alloying elements. is there.

Hereinafter, the reason why the metal structure is defined as described above in the present invention will be described.
<Tempered martensite single phase structure>
It is a very important requirement in the present invention that the metal structure is a tempered martensite single phase (except for a portion within a depth of 10 μm from the steel sheet surface). That is, by using a tempered martensite single-phase structure, it is possible to reduce the amount of alloy elements necessary to obtain a tensile strength of 1470 MPa or more. Here, the tempered martensite single phase structure is a structure that does not include a ferrite phase, a pearlite phase, or a bainite phase in the metal structure, and a residual austenite phase that remains without martensite transformation is 3% by volume. It means less than%. However, precipitates such as Fe ₃ C, NbC, and TiN, and inclusions such as unavoidable MnS may be included in the structure.
In addition, a ferrite phase may be generated within a depth of 10 μm from the surface of the steel sheet due to decarburization, etc., but since the ferrite phase in the steel sheet surface layer part does not reduce workability, the surface layer part contains a ferrite phase. It does not matter. For this reason, in the present invention, the structure within a depth of 10 μm from the surface of the steel sheet is not limited.

Next, the reason why the component composition of steel is limited to the above range in the present invention will be described. In addition, unless otherwise indicated, the% display regarding a component shall mean the mass%.
<C: 0.15-0.20%>
C is an element that stabilizes the austenite phase and is an element necessary for obtaining the strength of the steel sheet. If the amount of C added is less than 0.15%, it becomes difficult to obtain a tensile strength of 1470 MPa or more even in a martensite single-phase structure. On the other hand, when added over 0.20%, a predetermined strength can be obtained, but the welded portion generated during welding and the heat-affected zone caused by welding are markedly cured, and the weldability is lowered. Therefore, the C content is in the range of 0.15 to 0.20%. Preferably it is 0.17 to 0.19% of range.

<Si: 1.0-2.0%>
Si is not only a substitutional solid solution strengthening element effective for hardening a steel sheet, but also an element that increases the work hardening ability of the matrix and improves the ductility. In order to express these effects, it is necessary to contain 1.0% or more. However, when the amount of Si increases, scale formation by hot rolling becomes remarkable, and the defect rate in the final product increases, which is not economically preferable. On the other hand, Si has an action of increasing the Ac ₃ point, and if added over 2.0%, the annealing temperature at which an austenite single-phase structure is formed rises remarkably, leading to an increase in production cost. Therefore, the Si content is in the range of 1.0 to 2.0%. More preferably, it is 1.0 to 1.5% of range.

<Mn: 1.5-2.5%>
Mn is an element that stabilizes the austenite phase and reduces the upper critical cooling rate at which a martensite single-phase structure can be obtained, making it easier to obtain a martensite single-phase structure, and is also an effective element for strengthening steel. . However, if the amount of Mn is less than 1.5%, a ferrite phase, a pearlite phase, a bainite phase, and the like are generated during the time required from the annealing temperature to the completion of rapid cooling, and the strength is lowered, and a steel plate having a desired strength is stably produced. It becomes difficult. On the other hand, if it exceeds 2.5%, unfolding becomes remarkable and not only the workability deteriorates, but also the rolling load necessary during hot rolling and cold rolling becomes remarkably large, which is not preferable. Therefore, the Mn content is in the range of 1.5 to 2.5%. Preferably it is 1.5 to 2.0% of range.

<P: 0.020% or less>
P is an element that promotes grain boundary fracture due to grain boundary deflection, and its content is preferably as low as possible, so its upper limit is made 0.020%. Preferably it is 0.010% or less. In particular, from the viewpoint of improving weldability, the content is preferably 0.008% or less.

<S: 0.005% or less>
Since S becomes an inclusion such as MnS and induces deterioration of impact resistance and delayed fracture resistance, its content is preferably reduced as much as possible, and its upper limit is made 0.005%. Preferably it is 0.001% or less.

<Al: 0.01-0.05%>
Al is an element effective for deoxidation, so 0.01% or more should be contained. However, if added in a large amount, inclusions in the steel sheet increase and ductility is lowered, so the upper limit is made 0.05%.

<N: 0.005% or less>
N is an unavoidable impurity and forms a nitride. In particular, when the content exceeds 0.005%, ductility at high and low temperatures decreases due to the formation of nitrides. Therefore, the N content is 0.005% or less.

<Ti: 0.1% or less, Nb: 0.1% or less>
Both Ti and Nb are effective elements for increasing the yield strength by forming precipitates such as carbides and nitrides and increasing the strength of the steel and by making the crystal grains finer. In order to obtain these effects, addition of 0.01% or more is preferable, but when it exceeds 0.1%, the effects are saturated. Therefore, the Ti and Nb contents are each 0.1% or less.

<B: 5 to 30 ppm>
B is an effective element for improving the hardenability of steel and obtaining a martensite single phase structure more easily. However, if the addition amount is less than 5 ppm, the effect of addition is poor. On the other hand, if it exceeds 30 ppm, the effect of improving the hardenability is saturated, and there is a concern that the ductility is lowered. Therefore, the B content is in the range of 5 to 30 ppm. Preferably it is the range of 5-20 ppm.

The basic components have been described above, but in the present invention, other components described below can be appropriately contained as necessary.
<Cu: 0.20% or less>
Cu not only stabilizes the austenite phase and makes it easy to obtain a martensite single-phase structure, but also suppresses the penetration of hydrogen into the steel by forming a concentrated layer on the surface of the steel sheet in a corrosive environment. It has the effect of improving delayed fracture characteristics. However, when the added amount exceeds 0.20%, these effects are saturated, so Cu is contained at 0.20% or less.

Next, a method for producing a high-strength cold-rolled steel sheet according to the present invention will be described.
The high-strength cold-rolled steel sheet according to the present invention is a steel slab adjusted to the above component composition, heated to 1200 ° C or higher, and then hot-rolled at a finish rolling exit temperature of 800 ° C or higher, and then cold-rolled. Then, during the continuous annealing, after holding for 30 to 1200 s in the temperature range of Ac ₃ transformation point to (Ac ₃ transformation point + 30 ° C.), primary cooling is performed at an average cooling rate of 1 ° C./s or more to a temperature range of 720 ° C. or more. Subsequently, after secondary cooling from the primary cooling end temperature to 100 ° C. or lower at an average cooling rate of 100 to 1000 ° C./s, the temperature can be obtained by holding for 120 to 1800 s in a temperature range of 100 to 250 ° C.
Hereinafter, the reason why each processing condition is limited to the above range in the manufacturing method of the present invention will be described.

<Slab heating temperature: 1200 ℃ or higher>
The slab heating temperature is preferably high in order to reduce the deformation resistance during hot rolling and stabilize the productivity by dissolving undissolved precipitates and inclusions. When the heating temperature is less than 1200 ° C., the rolling load increases and the risk of trouble occurring during hot rolling increases. Therefore, the slab heating temperature is set to 1200 ° C. or higher. However, if the heating temperature is too high, the slab heating temperature is desirably 1300 ° C. or lower because it leads to an increase in scale loss accompanying an increase in the oxidized weight.

<Finishing rolling delivery temperature: 800 ° C or higher>
By setting the finish rolling exit temperature to 800 ° C. or higher, a uniform hot rolled matrix phase structure can be obtained, and can be used without any problem in use. In this respect, when the finish rolling exit temperature is lower than 800 ° C., the structure of the steel sheet becomes non-uniform, not only the ductility is lowered, but also the possibility of various problems occurring during forming increases. Further, when the rolling exit temperature is less than 800 ° C., the same problem associated with the generation of coarse grains occurs even if a high coiling temperature is employed to avoid the remaining of the processed structure. Therefore, the finish rolling exit temperature is set to 800 ° C. or higher. In addition, although there is no restriction | limiting in particular about the upper limit of finish rolling exit side temperature, When it rolls at an excessively high temperature, since it becomes a cause of a scale flaw etc., it is preferable to set it as about 1000 degrees C or less.

  A winding process is performed after said hot rolling. In the present invention, the coiling temperature is not particularly limited, but if the coiling temperature is too high, coarse grains are generated as described above, and the steel sheet structure becomes non-uniform, resulting in a decrease in ductility. On the other hand, when the coiling temperature is too low, the processed structure generated by hot rolling remains, and the rolling load in cold rolling, which is the next step, is increased. Therefore, the winding temperature is preferably about 400 to 700 ° C. Preferably, it is around 650 ° C.

  Subsequently, cold rolling is performed, but there is no particular limitation on the cold rolling conditions, and any conventionally known method may be followed.

Next, continuous annealing is performed. Hereinafter, this continuous annealing condition will be described.
<Annealing treatment: Ac ₃ transformation point to (Ac ₃ transformation point + 30 ° C) for 30 to 1200 s>
In order to obtain a martensite single phase structure, it is necessary to have an austenite single phase structure during annealing. Therefore, the annealing temperature needs to be higher than the Ac ₃ transformation point. However, even at a temperature at which an austenite single-phase structure is formed, if the annealing temperature is excessively high, the crystal grains become extremely coarse, and the yield strength and toughness of the steel sheet decrease. Therefore, the upper limit of the annealing temperature is (Ac ₃ transformation point + 30 ° C.).
In addition, when the holding time at the above-described annealing temperature is less than 30 s, not only does the structure become an austenite single-phase structure and a martensite single-phase structure cannot be obtained, but a work structure formed by cold rolling is not obtained. It remains and the structure after annealing becomes non-uniform, resulting in a decrease in ductility. On the other hand, when the holding time exceeds 1200 s, the crystal grains are remarkably coarsened, and the yield strength and the inertia of the steel sheet are also lowered. Therefore, the holding time at the time of annealing shall be 30-1200s. Preferably it is the range of 300-900 s.

<Primary cooling at an average cooling rate of 1 ° C./s or higher to a temperature range of 720 ° C. or higher, and further average cooling rate: secondary cooling to 100 ° C. or lower at 100 to 1000 ° C./s>
In order to obtain a martensite single phase structure, it is necessary to cool to 100 ° C. or less without forming a ferrite phase, a pearlite phase, and a bainite phase after forming an austenite single phase structure by annealing. However, when primary cooling to a temperature range of 720 ° C or higher is performed at an average cooling rate of less than 1 ° C / s, a ferrite phase is formed during cooling, and a martensite single phase structure having a tensile strength of 1470 MPa or more is obtained. Absent.
The reason why the cooling end temperature in the primary cooling is set to 720 ° C. or more is that the generation of the ferrite phase and the pearlite phase occurs most significantly in the temperature region below 720 ° C. The upper limit of the primary cooling end temperature is not particularly limited, and rapid cooling may be performed directly from the above-described annealing temperature under the secondary cooling conditions described below.
In addition, when the subsequent secondary cooling is performed at an average cooling rate of less than 100 ° C / s, ferrite, pearlite, and bainite phases are generated while reaching a temperature range of 100 ° C or less, and tensile strength of 1470 MPa or more is also achieved. It is not possible to obtain a martensite single-phase structure having On the other hand, if the average cooling rate in the secondary cooling is higher than 1000 ° C./s, there is a risk of shrinkage cracking of the steel sheet due to cooling, so the upper limit is set to 1000 ° C./s. In addition, as secondary cooling, it is preferable to perform water quenching.

<Holds for 120 to 1800s in the temperature range of 100 to 250 ° C>
This tempering process is performed in order to soften the martensite phase and improve workability. That is, after the secondary cooling, in order to temper the martensite phase, the temperature is maintained at a temperature of 100 to 250 ° C. for 120 to 1800 s. When the tempering temperature is less than 100 ° C., the softening of the martensite phase is insufficient, and the effect of improving the workability cannot be expected. On the other hand, when the tempering temperature exceeds 250 ° C., not only the cost for reheating is increased, but also a significant decrease in strength is caused and a desired effect cannot be obtained.
Further, if the holding time is less than 120 s, the martensite is not sufficiently modified at the holding temperature, so that the workability improvement effect cannot be expected. On the other hand, if the holding time exceeds 1800 s, the softening of martensite proceeds excessively, resulting in a significant decrease in strength, and an increase in reheating time leads to an increase in manufacturing cost. Note that the cooling method and speed after being held at the temperature are not limited.

A steel slab having the composition shown in Table 1 was hot-rolled under the conditions shown in Table 2, and then cold-rolled according to a conventional method to obtain a cold-rolled steel sheet having a thickness of 1.4 mm. Next, continuous annealing and tempering were performed under the conditions shown in Table 2.
Table 3 shows the results of investigation on the tensile properties, bending properties, metal structure and delayed fracture resistance of the cold-rolled steel sheet thus obtained.

The tensile properties, bending properties, metal structure and delayed fracture resistance were each evaluated by the following methods.
<Tensile test>
JIS No. 5 tensile test specimens were collected in the direction perpendicular to the rolling direction of the obtained cold-rolled steel sheet, and 0.2% proof stress (YS), tensile strength (TS), and elongation at break (EL) were obtained in accordance with the provisions of JIS Z 2241. It was.

<Bending test>
The bending test was performed by shearing a 110 mm × 35 mm plate with the rolling direction as the longitudinal direction from the obtained cold-rolled steel plate and grinding each end face to obtain a test piece having a size of 100 mm × 30 mm. These specimens are subjected to 90 ° bending with a punch with various curvature radii using a three-point bending tester, and the critical bending radius (hereinafter referred to as the critical bending radius) that causes fracture at the bending apex is obtained. I asked for each.

<Metallic structure evaluation>
Specimens were collected from the obtained cold-rolled steel sheet, the cross-section parallel to the rolling direction was mirror-polished and etched with nital, and the microstructure was observed and photographed using an optical microscope or scanning electron microscope, and tempered martensite. The types of constituent phases such as phases were identified. About the test piece by which the ferrite phase was recognized, the volume ratio of the tempered martensite phase and the ferrite phase was calculated | required by binarizing a structure | tissue photograph using an image analyzer, respectively. Further, the retained austenite fraction in the structure was measured by an X-ray diffraction method using a Mo tube.

<Delayed fracture property evaluation test>
From the obtained cold-rolled steel sheet, use a test piece whose end face is ground after cutting into 30 mm x 100 mm with the rolling direction as the long side, and this test piece is bent 180 ° with a predetermined bending radius that does not cause breakage by 180 ° bending. Processed. The spring back generated on the bent test piece is tightened by a specified amount with bolts, and after stress is applied to the test piece, it is immersed in an aqueous HCl solution at 25 ° C and pH 1, and the time until failure occurs is 100 hours at maximum. Until measured.

  As is apparent from Table 3, all of the inventive examples (Nos. 1 to 8) obtained by producing the steel sheets adjusted to the proper composition according to the present invention under the production conditions according to the present invention have a tensile strength of 1470 MPa or more. In addition, the ratio of YS (0.2% proof stress) and TS (tensile strength) is 0.80 or higher, and it has excellent workability with a breaking elongation of 10% or more and a 90 ° limit bending radius of 1.5 mm or less. A cold-rolled steel sheet having a tempered martensite single-phase structure is obtained. Moreover, it can be seen that all of these invention examples have good delayed fracture resistance that does not cause destruction for 100 hours or more in hydrochloric acid at pH 1. In particular, No. 8 to which an appropriate amount of Cu was added had a much improved diffusible hydrogen content in steel after being immersed in hydrochloric acid for 24 hours compared to the other invention examples, and diffusible hydrogen induced delayed fracture. It can be seen that intrusion into the steel is suppressed.

  On the other hand, Nos. 9 to 15 whose steel components are outside the scope of the present invention have a predetermined tensile strength, a 0.2% proof stress / tensile strength ratio, a 90 ° limit bending radius of 1.5 mm or less, and a tempered marten containing no ferrite. It can be seen that not all of the site single-phase structure can be satisfied and is not compatible with the present invention. Further, even if the steel component is within the scope of the present invention, if the production conditions are not appropriate, as shown in No. 16-24, ferrite is generated and a tempered martensite single phase structure cannot be obtained, Neither the prescribed strength can be obtained by excessive tempering, or good bending workability cannot be obtained due to insufficient tempering, and there is no sufficient delayed fracture resistance. A steel sheet that satisfies all the various properties targeted by the present invention has not been obtained.

  The present invention is a thin steel plate for quenching and tempering suitable for the use of ultra-high-strength car body safety parts such as automobile door impact beams and center pillars. Automotive parts using such steel plates are In manufacturing, by properly controlling the steel composition, rolling conditions, and annealing conditions, it is possible to develop excellent strength-ductility balance, bending workability and delayed fracture resistance while having a tempered martensite single phase structure. it can.

Claims (4)

% By mass
C: 0.15-0.20%, Si: 1.0-2.0%,
Mn: 1.5 to 2.5%, P: 0.020% or less,
S: 0.005% or less, Al: 0.01 to 0.05%,
N: 0.005% or less, Ti: 0.1% or less,
Nb: 0.1% or less, B: 5-30 ppm
The balance consists of Fe and inevitable impurities, and the tempered martensite phase is 97% or more by volume and the residual austenite phase is less than 3% by volume (however, the portion within 10μm in depth from the steel sheet surface is excluded) A high-strength cold-rolled steel sheet with excellent bending workability and delayed fracture resistance, characterized by having a metallographic structure, a tensile strength of 1470 MPa or more, and a ratio of 0.2% proof stress to tensile strength of 0.80 or more. The steel plate is more mass%,
The high-strength cold-rolled steel sheet having excellent bending workability and delayed fracture resistance according to claim 1, characterized by containing Cu: 0.20% or less. % By mass
C: 0.15-0.20%, Si: 1.0-2.0%,
Mn: 1.5 to 2.5%, P: 0.020% or less,
S: 0.005% or less, Al: 0.01 to 0.05%,
N: 0.005% or less, Ti: 0.1% or less,
Nb: 0.1% or less, B: 5-30 ppm
The steel slab consisting of Fe and inevitable impurities is heated to 1200 ° C or higher, and then subjected to hot rolling at a finish rolling exit temperature of 800 ° C or higher, and then cold-rolled. When continuously annealing a rolled steel sheet, hold it for 30 to 1200 s in the temperature range of Ac ₃ transformation point to (Ac ₃ transformation point + 30 ° C.), then primary at an average cooling rate of 1 ° C./s or higher to a temperature range of 720 ° C. or higher. Tempering after cooling, secondary cooling from the primary cooling end temperature to the average cooling rate: 100-1000 ° C / s to 100 ° C or less, and then holding in the temperature range of 100-250 ° C for 120-1800s It has a metal structure with a martensite phase volume ratio of 97% or more and a residual austenite phase volume ratio of less than 3% (excluding the portion within 10μm depth from the steel sheet surface), and a tensile strength of 1470MPa or more. and bending the ratio of the tensile strength and 0.2% proof stress is 0.80 or more processing and delayed Method for producing a high strength cold rolled steel sheet having excellent fracture properties. The steel slab is further mass%,
Cu: 0.20% or less
The method for producing a high-strength cold-rolled steel sheet having excellent bending workability and delayed fracture resistance according to claim 3.