JP5082649B2 - High-strength cold-rolled steel sheet with excellent manufacturing stability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet excellent in production stability and having a tensile strength of 980 MPa or more, which is suitable for automobile frame members and reinforcing members, and a method for producing the same.

  In recent years, there has been a demand for improving fuel efficiency of automobiles from the viewpoint of global environmental conservation. Further, from the viewpoint of protecting occupants in the event of a vehicle collision, it is also required to improve the safety of the automobile body. For this reason, in order to satisfy both the improvement in fuel efficiency and the improvement in safety, studies are being actively carried out to reduce and strengthen the vehicle body. In order to satisfy the weight reduction and strengthening of automobile bodies at the same time, it is effective to increase the strength and thickness of component materials. Recently, high-strength steel sheets with a tensile strength of 980 MPa or more are used for automobile framework members and automobile seats. It has begun to be used for skeletal members.

Since members such as automobile frame members are formed by press working, the material is required to have high stretch flangeability. In order to make the steel sheet have a high strength with a tensile strength of 980 MPa or more, it is effective to use a martensite phase having a high hardness. However, a DP steel composed of a soft ferrite phase and a hard martensite phase has two phases. Due to the difference in workability, strain concentrates on the two-phase interface and stretch flangeability deteriorates. In order to obtain high stretch flangeability, it is effective to avoid a two-phase structure having a hardness difference and to have a single-phase structure. For example, in Patent Document 1, a martensite single-phase structure is excellent. The hole expandability is achieved.
Japanese Patent No. 3729108

However, the method of Patent Document 1 has a problem in manufacturing stability because the tensile strength of the steel sheet varies greatly depending on the cooling rate after heating in the annealing process (the strength depends greatly on the cooling rate).
Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, have high press formability such as stretch flangeability, and have high strength cold rolling that has low dependency on the cooling rate of strength and has excellent manufacturing stability. The object is to provide a steel plate and a method for producing the same.

  The present inventors diligently studied to solve the above problems, and obtained the following knowledge. Usually, the decrease in strength of martensitic single phase steel is considered to be caused by the formation of ferrite phase or bainite phase until the martensitic transformation during cooling. As means for suppressing the formation, Mn, Cr, Mo It is known that it is effective to add an element that delays the transformation of ferrite and bainite, such as. However, even when the formation of ferrite and the like is not observed, the cooling rate dependence of the strength is recognized, and as a result of detailed structural observation, it has been found that iron carbide is precipitated in the martensite of the steel with reduced strength. It was. Furthermore, the addition of Mn, Cr, Mo, B, etc. is ineffective in suppressing iron carbide precipitation, but by adding 0.1 mass% or more of Si, iron carbide during cooling to room temperature after martensitic transformation. By suppressing the precipitation, it is possible to ensure the strength stably regardless of the cooling rate in the range where the average cooling rate up to 200 ° C after the heating and holding in the continuous annealing is 300 ° C / second or more. I found it. As described above, the reason why the addition of Si has the effect of suppressing the decrease in strength is that Si hardly dissolves in iron carbide, so that diffusion of Si is necessary for the formation and growth of iron carbide. Since Si hardly diffuses at a low temperature below the site transformation point (about 450 ° C.), it is considered that precipitation of iron carbide is suppressed. Furthermore, in a manufacturing method with a relatively high cooling rate in which the average cooling rate to 200 ° C. is 300 ° C./second or more, the time required from the martensite transformation point to 200 ° C. is less than 1 second, and the iron in the meantime It was found that a small amount of Si addition of about 0.1 mass% is sufficient for suppressing carbide precipitation.

The present invention has been made on the basis of such findings and has the following gist.
[1] C: 0.03-0.12 mass%, Si: 0.1-0.6 mass%, Mn: 1.5-3.0 mass%, P: 0.10 mass% or less, S: 0.01 mass% Hereinafter, Al: 0.01 to 0.1 mass%, N: 0.005 mass% or less, the balance is made of Fe and unavoidable impurities, steel sheet structure (however, the structure of the region from the steel sheet surface to a depth of 20 μm) Is a tempered martensite single-phase structure, and has a tensile strength of 980 MPa or more, a high-strength cold-rolled steel sheet excellent in manufacturing stability.
[2] A high-strength cold-rolled steel sheet excellent in manufacturing stability, wherein the high-strength cold-rolled steel sheet according to [1] further contains B: 0.0005 to 0.005 mass% .

[3] A method for producing a cold-rolled steel sheet in which hot rolling, cold rolling, and continuous annealing are sequentially performed on a slab having the component composition described in [1] or [2] , wherein in the continuous annealing, Is heated to a temperature range of Ae ₃ transformation point to 900 ° C., then rapidly cooled to 200 ° C. or less at an average cooling rate of 300 ° C./second or more, and then tempered at 200 ° C. or less. For producing high-strength cold-rolled steel sheets with excellent resistance.
In the present invention, “excellent in production stability” means that the average cooling rate up to 200 ° C. after heating and holding in the temperature range of Ae ₃ transformation point to 900 ° C. in the continuous annealing step is 300 ° C. / It means that the difference in strength between the case of manufacturing in seconds and the case of manufacturing at 1000 ° C./second is 50 MPa or less.

  The high-strength cold-rolled steel sheet of the present invention has a tensile strength of 980 MPa or more, is excellent in press formability such as stretch flangeability, and the tensile strength is not greatly changed by the cooling rate in the annealing process, and is manufactured. Excellent stability. For this reason, it is particularly suitable as a material for automobile framework members and reinforcing members that require good press formability, and as other materials for automobile parts, home appliances, building members, etc. that require high strength and high stretch flangeability. Is also suitable. Moreover, according to the manufacturing method of this invention, the high intensity | strength cold-rolled steel plate which has the above performances can be manufactured stably.

  The cold-rolled steel sheet of the present invention has C: 0.03-0.12 mass%, Si: 0.1-0.6 mass%, Mn: 1.5-3.0 mass%, P: 0.10 mass% or less, S : 0.01 mass% or less, Al: 0.01 to 0.1 mass%, N: 0.005 mass% or less, and if necessary, B: 0.0005 to 0.005 mass%, or further , Ti: 0.005 to 0.05 mass% and Nb: 0.005 to 0.05 mass%, one or two of them, the balance being made of Fe and unavoidable impurities, steel sheet structure (however, from the surface of the steel sheet (Excluding the structure in the region up to a depth of 20 μm) is a tempered martensite single-phase structure and has a tensile strength of 980 MPa or more.

First, the reasons for limiting the component composition of the steel sheet are as follows.
C: 0.03-0.12 mass%
C is one of the important elements for ensuring the strength. In the present invention, it is necessary to contain 0.03 mass% or more in order to set the tensile strength to 980 MPa or more. On the other hand, inclusion exceeding 0.12 mass% significantly deteriorates weldability. For this reason, C is 0.03-0.12 mass%, preferably 0.04-0.10 mass%.
Si: 0.1 to 0.6 mass%
Si is the most important element in the present invention, and has the effect of suppressing precipitation of iron carbide during cooling to room temperature after martensite transformation in continuous annealing and reducing the strength cooling rate dependency. Such an effect can be obtained by addition of 0.1 mass% or more. When the average cooling rate up to 200 ° C. is 300 ° C./second or more, it is produced at 300 ° C./second and at 1000 ° C./second. In this case, the strength difference can be 50 MPa or less. On the other hand, the addition of Si exceeding 0.6 mass% not only deteriorates the chemical conversion property of the steel sheet, but also promotes ferrite transformation during cooling, making it difficult to obtain a tempered martensite single phase structure. It is necessary to add a large amount of an element that delays the ferrite transformation such as Cr and Mo. For this reason, Si is 0.1 to 0.6 mass%, preferably 0.2 to 0.5 mass%.

Mn: 1.5 to 3.0 mass%
Mn is an element that stabilizes austenite and delays ferrite transformation, and by adding an appropriate amount of Mn, it suppresses the formation of ferrite during cooling after continuous annealing and stably obtains a tempered martensite single phase structure. Can do. In order to obtain such an effect, addition of 1.5 mass% or more is necessary. On the other hand, the addition of Mn exceeding 3.0 mass% deteriorates workability. For this reason, Mn is 1.5 to 3.0 mass%, preferably 1.6 to 2.5 mass%.
P: 0.10 mass% or less P has an effect of strengthening steel, and may be added according to the strength level of the steel sheet. However, if it exceeds 0.10 mass%, weldability deteriorates. Therefore, P is 0.10 mass% or less. Moreover, when more excellent weldability is required, it is preferably 0.05 mass% or less.

S: 0.01 mass% or less S is present as an inclusion in the steel sheet and deteriorates stretch flangeability. Therefore, it is preferable to reduce S as much as possible, and in order to eliminate the adverse effect on stretch flangeability, it is necessary to be 0.01 mass% or less. Moreover, when the more excellent stretch flangeability is requested | required, it is preferable to set it as 0.005 mass% or less.
Al: 0.01 to 0.1 mass%
Al is added as a deoxidizing element for steel, is an element useful for improving the cleanliness of steel, and is also an element that is desirable for addition to refine the structure of steel. Further, the aluminum killed steel added with an appropriate range of Al is superior in mechanical properties to the conventional rimmed steel not containing Al. For this reason, the lower limit of Al is set to 0.01 mass%. On the other hand, when the Al content increases, the surface properties deteriorate, so the upper limit is made 0.1 mass%.

N: 0.005 mass% or less N is treated as an impurity in the present invention. If N exceeds 0.005 mass%, strength variation will be caused, so 0.005 mass% or less.
The steel sheet of the present invention can achieve the desired characteristics with the above component composition, but can contain the following elements according to the desired characteristics.
B: 0.0005 to 0.005 mass%
B is an element that delays the ferrite transformation, and by adding an appropriate amount of B, it is possible to suppress the formation of ferrite at the time of cooling during continuous annealing and stably obtain a tempered martensite single phase structure. In order to obtain such an effect, addition of 0.0005 mass% or more is necessary. On the other hand, the addition of B exceeding 0.005 mass% not only saturates the above-described effect, but also increases the deformation resistance of hot rolling, making it difficult to manufacture. For this reason, B is set to 0.0005 to 0.005 mass%.

When adding 1 type or 2 types B of Ti: 0.005-0.05 mass% and Nb: 0.005-0.05 mass%, it is preferable to add Ti and Nb in an appropriate amount. In order for B to exhibit the above effect, it needs to be in a solid solution state. When B is combined with N to become BN, the effect decreases. At that time, by adding Ti and Nb, N is preferentially bonded to Ti and Nb, and the formation of BN can be suppressed. Such an effect can be obtained by adding 0.005 mass% or more of Ti and Nb, respectively, but addition exceeding 0.05 mass% brings about deterioration of workability. Therefore, Ti and Nb are 0.005 to 0.05 mass%, respectively.
The balance other than the above is Fe and inevitable impurities. Inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. The allowable ranges of these contents are Sb: 0.01 mass% or less, Sn: 0.1 mass% or less, Zn: 0. It is the range of 01 mass% or less and Co: 0.1 mass% or less. Moreover, in this invention, even if it contains Cr, Mo, V, Ni, Cu, Mg, Ca, Zr, and REM within the range of a normal steel composition, the effect is not lost.

The metal structure of the steel sheet of the present invention (excluding the structure from the steel sheet surface to a depth of 20 μm, which will be described later) is a tempered martensite single phase structure. Thus, the extensibility flangeability which was excellent by using the tempered martensite single phase structure which does not contain soft phases, such as a ferrite, is acquired.
Here, tempered martensite is a structure obtained by tempering martensite at a low temperature of 200 ° C. or less, and is composed of lath-like ferrite and fine plate-like iron carbide precipitated in the lath and at the lath boundary. Similarly, the lower bainite is known as a structure composed of lath-like ferrite and fine plate-like iron carbide, but the iron carbide generated in the lower bainite is characterized by the fact that the length is aligned in one direction within the same lath and is random. It can be distinguished from tempered martensite by observation with a transmission electron microscope. The tempered martensite single phase means that the structure is quantitatively measured by a scanning electron microscope, a transmission electron microscope, and an X-ray diffraction method, and means that ferrite, bainite, and retained austenite are not contained in total of 1% or more.

In addition, ferrite may be generated on the outermost layer within a depth of 20 μm from the steel sheet surface due to decarburization or the like, but such ferrite does not affect the stretch flangeability but rather improves the bendability. Ferrite may be contained within a depth of 20 μm from the surface. For this reason, in this invention, a steel plate structure | tissue is not limited about the area | region from a steel plate surface to a depth of 20 micrometers.
Since the steel sheet of the present invention is intended for application to automobile frame members, reinforcing members, and automobile seat frame members that require high tensile strength, the tensile strength is set to 980 MPa in consideration of such applications. That's it.

Next, the manufacturing method of the cold rolled steel sheet of this invention is demonstrated.
In this manufacturing method, a slab is manufactured from molten steel adjusted to the above-described component composition, and hot rolling, cold rolling, and continuous annealing are sequentially performed on the slab to manufacture a cold rolled steel sheet.
The slab to be used is preferably manufactured by a continuous casting method in terms of a small amount of macro-segregation of components, but may be manufactured by an ingot-bundling rolling method or a thin slab casting method.
As a method of hot rolling, the cast slab is once cooled to room temperature, then reheated and rolled in a heating furnace, and the cast slab is kept as a warm piece without cooling to room temperature. Any method may be employed, such as a method of rolling after reheating in a heating furnace, a method of rolling immediately after keeping the cast slab heated, and a direct feed rolling / direct rolling method of rolling the cast slab as it is.
When the slab cooled to room temperature after casting is reheated, the slab heating temperature is preferably 1000 ° C. or higher. The upper limit is not particularly limited, but if it exceeds 1300 ° C., scale loss accompanying an increase in oxidized weight increases, and therefore, it is preferably 1300 ° C. or lower. Moreover, also when it reheats with a heating furnace with a hot piece, without cooling to normal temperature, it is preferable that slab heating temperature shall be 1000 degreeC or more.

In hot rolling, after rough rolling is performed as necessary, finish rolling is preferably performed at a finish rolling temperature of 800 ° C. or higher. When the finish rolling temperature is lower than 800 ° C., the structure of the steel sheet becomes non-uniform and workability deteriorates. The upper limit of the finish rolling temperature is not particularly limited, but if it is rolled at an excessively high temperature, scale wrinkles and the like are caused. Then, it is preferable to cool to 700 degrees C or less at an average cooling rate: 30 degrees C / sec or more, and to wind up at 650 degrees C or less. If the average cooling rate is less than 30 ° C./second, the ferrite grain size becomes coarse, so the austenite grain size becomes coarse during annealing after cold rolling, which may adversely affect workability. On the other hand, when the winding temperature exceeds 650 ° C., the scale loss after winding increases.
In this hot rolling, part or all of the finish rolling may be lubricated rolling in order to reduce the rolling load. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.10-0.25. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.

Next, the hot-rolled steel sheet obtained as described above is cold-rolled to obtain a cold-rolled steel sheet. The cold rolling condition is not particularly limited as long as it can be a cold-rolled steel sheet having a desired size and shape, but the rolling reduction is preferably 20% or more from the viewpoint of surface flatness and structure uniformity. . In addition, it is sufficient to perform pickling according to a conventional method before cold rolling, but when the scale on the surface of the hot-rolled steel sheet is extremely thin, direct cold rolling may be performed.
Next, the obtained cold-rolled steel sheet is subjected to continuous annealing. In this continuous annealing step, the steel sheet is heated and held in a temperature range of ₃ Ae or more and 900 ° C. or less. When the heating holding temperature is less than ₃ points of Ae, an austenite single phase structure is not obtained, and a tempered martensite single phase structure cannot be obtained after cooling and tempering. On the other hand, when the heating and holding temperature exceeds 900 ° C., the austenite grains are coarsened, so that the bendability and toughness of the steel sheet are deteriorated. In addition, it is preferable that the holding time is 60 seconds or more when the Ae is ₃ points or more from the viewpoint of the uniformity of the steel sheet. More preferably, it is 120 seconds or more.

Next, it is rapidly cooled to 200 ° C. or less at an average cooling rate of 300 ° C./second or more. In the present invention, when the average cooling rate is 300 ° C./second or more, by adding 0.1 mass% or more of Si, when the average cooling rate up to 200 ° C. is manufactured at 300 ° C./second and 1000 ° C./second The difference in strength when manufactured with can be 50 MPa or less. At a cooling rate of less than 300 ° C / second, in order to suppress precipitation of iron carbide during cooling to room temperature after martensitic transformation, a large amount of Si exceeding 0.6 mass% is required, which adversely affects chemical conversion treatment properties. Effect. Therefore, in the present invention, the average cooling rate is set to 300 ° C./second or more, preferably 400 ° C./second or more. In addition, when the steel plate temperature is cooled to less than 200 ° C., carbide precipitation in a short time within a few seconds is negligible compared to carbide precipitation at the time of subsequent tempering at 200 ° C. or less. The cooling rate to room temperature is not particularly defined, but is preferably 50 ° C./second or more from the viewpoint of productivity. In addition, it may be rapidly cooled immediately after the heating and holding, or may be rapidly cooled after being gradually cooled to a certain temperature in the manufacturing process. However, when gradually cooling, it is necessary to set the rapid cooling start temperature to ₃ or more points of Ar. is there.

Subsequently, in order to improve toughness, tempering is performed at 200 ° C. or lower. When the tempering temperature exceeds 200 ° C., coarse carbides are precipitated and the strength is remarkably lowered, so that the strength corresponding to the additive element cannot be obtained, which is uneconomical. The lower limit of the tempering temperature is not particularly defined, but is preferably 100 ° C. or higher. Moreover, it is preferable that the holding time of tempering is 3 to 30 minutes.
In addition, you may give the temper rolling of elongation rate 5% or less after continuous annealing for adjustment of shape correction, surface roughness, etc.
The high-strength cold-rolled steel sheet of the present invention is subjected to a chemical treatment, weldability, corrosion resistance, anti-resistance, etc. after annealing, by subjecting it to pickling treatment or a treatment for adhering about 5 to 500 mg / m ² of components such as Ni. Improvements such as galling may be made.

  A steel slab having the chemical composition shown in Table 1 is manufactured by continuous casting, and after reheating to 1250 ° C., finish rolling temperature: about 850 ° C., winding temperature: about 600 ° C., and hot rolling to a plate thickness of 3.0 mm. It was. After pickling, cold rolling was performed to obtain a cold-rolled steel sheet having a thickness of 1.2 mm, and then annealing and tempering were performed in the continuous annealing line under the conditions shown in Table 2. The obtained cold-rolled steel sheet was subjected to temper rolling with an elongation of 0.5%, and then a test piece was collected and subjected to structure observation and material investigation. Details of each test method are as follows.

・ Structure observation Specimens are collected from the obtained cold-rolled steel sheet, and the microstructure parallel to the rolling direction is observed using an optical microscope, scanning electron microscope, and transmission electron microscope to identify the type of structure. The volume ratio of the tempered martensite phase was determined. Here, since the region from the steel plate surface to the depth of 20 μm has little influence on the steel plate characteristics (strength, hole expansion rate), the structure is identified for the portion excluding the region from the steel plate surface to the depth of 20 μm. (Note that, in the region from the steel sheet surface to a depth of 20 μm, the volume ratio of the tempered martensite phase was 20 to 100% in all steels, and the balance was a structure composed of a ferrite phase). Table 2 shows the types of constituent phases and the tempered martensite volume fraction.

-Material investigation From the obtained cold-rolled steel sheet, a JIS No. 5 tensile test piece was taken with the direction orthogonal to the rolling direction as the tensile direction, and a tensile test was performed in accordance with the provisions of JIS-Z-2241. Moreover, the hole expansion rate was measured based on the steel federation standard (JFST1001-1996). When yield strength (YS / MPa), tensile strength (TS / MPa), elongation (El /%) and average cooling rate up to 200 ° C. obtained by a tensile test were 300 ° C./second and 1000 ° C. / Table 2 shows the strength difference (ΔTS / MPa), hole expansion rate (λ /%) and the like when manufactured in seconds.
The hole expansibility was good at λ ≧ 70%, and the production stability was good (◯) when ΔTS: 50 MPa or less, and poor (×) when ΔTS: more than 50 MPa.

According to Table 2, the inventive examples (No. 3-5, 7-9, 13, 15-17, 19 ) all have a tempered martensite single phase structure, and the tensile strength is 980 MPa or more, Moreover, since ΔTS is 50 MPa or less, it can be seen that the production stability is excellent. On the other hand, in the comparative example of No. 1 in which the addition amount of Si and the addition amount of Mn is less than the lower limit of the present invention, ferrite is generated during cooling after annealing, and λ is low, and the value of ΔTS is also It is large and the dependence of the strength on the cooling rate is large. Further, in the comparative example of No. 2 in which the Si addition amount is less than the lower limit of the present invention as in No. 1, the Mn addition amount is increased, and B is added, the formation of ferrite is suppressed and the tempered martensite single phase structure. However, ΔTS is still large and it can be seen that the production stability is poor. Further, in the comparative example of No. 6, since the annealing temperature is less than ₃ points of Ae, a tempered martensite single phase structure cannot be obtained, and λ is a low value. In each comparative example of No. 10 and No. 11, the steel components satisfy the conditions of the present invention, but the average cooling rate during continuous annealing is less than 300 ° C./sec. It can be seen that the difference in tensile strength of each is as great as 54 MPa and 72 MPa, respectively, and that the production stability is poor when the average cooling rate is less than 300 ° C./sec. The comparative example of No. 12 has a high tempering temperature, and the tensile strength does not reach 980 MPa. In the comparative example of No. 14, the C amount is less than the lower limit of the present invention, so the tensile strength does not reach 980 MPa. Further, in the comparative example of No. 18, since the amount of Mn added is less than the lower limit of the present invention, ferrite is generated during cooling after annealing, and λ is a low value.

Claims (3)

  C: 0.03-0.12 mass%, Si: 0.1-0.6 mass%, Mn: 1.5-3.0 mass%, P: 0.10 mass% or less, S: 0.01 mass% or less, Al : 0.01-0.1mass%, N: 0.005mass% or less, the balance consists of Fe and inevitable impurities, steel plate structure (however, excluding the structure of the region from the steel plate surface to a depth of 20μm) Is a tempered martensite single-phase structure and has a tensile strength of 980 MPa or more, a high-strength cold-rolled steel sheet excellent in manufacturing stability.   B: 0.0005-0.005 mass% is further contained, The high-strength cold-rolled steel plate excellent in manufacturing stability of Claim 1 characterized by the above-mentioned. A method for producing a cold-rolled steel sheet, in which hot rolling, cold rolling, and continuous annealing are sequentially performed on a slab having the component composition according to claim 1 or 2 ,
In the continuous annealing, the steel sheet is heated and held in a temperature range of Ae ₃ transformation point or higher and 900 ° C. or lower, then rapidly cooled to 200 ° C. or lower at an average cooling rate of 300 ° C./second or higher, and then tempered at 200 ° C. or lower. A method for producing a high-strength cold-rolled steel sheet having excellent production stability.