JP2004285366A - Extra-thin ultrahigh tensile strength cold rolled steel sheet, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an extra-thin ultrahigh tensile strength cold rolled steel sheet which has excellent tensile strength, ductility and stretch-flanging properties. <P>SOLUTION: A steel slab containing 0.05 to 0.25% C, ≤0.05% Si, 1.5 to 3.5% Mn, ≤0.01% P, ≤0.003% S, ≤0.10% Al and 0.0050 to 0.0250% N, and in which the total content of Cr and Mo is controlled to ≤0.2%, and the content of Nb to ≤0.005%, and the balance Fe with inevitable impurities is heated to a slab heating temperature of ≥1,000°C, is thereafter subjected to hot rolling in which finish rolling outlet side temperature is controlled to ≥800°C, and coiling temperature is controlled to ≥550°C, is then subjected to cold rolling, is subsequently heated at a temperature in which recrystallization ratio reaches 80% to 900°C, and is cooled at a cooling rate of ≥30°C/s. The extra-thin ultrahigh tensile strength cold rolled steel sheet having a sheet thickness of ≤0.8 mm, a tensile strength of ≥900 MPa, and excellent ductility and stretch flanging properties is obtained. Further, a plating layer may be formed on the surface of the steel sheet. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

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本発明例はいずれも、寸法精度の低下や冷間圧延性の低下もなく、９８０ＭＰａ以上の引張強さと、優れた延性および優れた伸びフランジ性を有する極薄超高張力冷延鋼板あるいは極薄超高張力溶融亜鉛めっき鋼板となっている。一方、本発明の範囲を外れる比較例は、引張強さが９８０ＭＰａ未満であるか、延性、伸びフランジ性が低下するか、あるいは寸法精度の低下や冷間圧延性の低下がみられ、所望の極薄超高張力冷延鋼板あるいは極薄超高張力溶融亜鉛めっき鋼板となっていない。
【００５１】
【発明の効果】
以上のように、本発明によれば、９８０ＭＰａ以上の高い引張強さを有する板厚：０．８ｍｍ 以下の極薄超高張力冷延鋼板、極薄超高張力めっき鋼板が容易にしかも安定して製造でき、産業上格段の効果を奏する。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a cold-rolled steel sheet, in particular, mainly used for automobiles, general home appliances, furniture, etc., specifically, is formed into a hat-shaped cross-sectional shape or a pipe shape by light bending, foam forming, roll forming, or the like. Ultrathin, suitable for applications subject to relatively severe drawing, with a thickness of 0.8 mm or less, a tensile strength of 980 MPa or more, and excellent ductility and stretch flangeability (also referred to as hole expanding properties) It relates to an ultra-high tensile cold-rolled steel sheet.
[0002]
[Prior art]
2. Description of the Related Art In recent years, from the viewpoint of preserving the global environment, there has been a demand for improved fuel efficiency of automobiles. In order to respond to such demands, weight reduction of an automobile body has become an important issue. In order to reduce the weight of automobile bodies, it is effective to increase the strength of component materials and reduce the thickness of the material. It has become.
[0003]
From the viewpoint of further reducing the weight of an automobile body, a thin steel sheet having a higher tensile strength is demanded. Depending on the application, for example, the steel sheet has an ultra-high tensile strength of 980 MPa or more, and has a thickness of: Ultra-thin ultra-high strength steel sheets of 0.8 mm or less are demanded.
As a steel sheet having such an ultra-high tension, for example, Patent Document 1 proposes an ultra-high-tensile cold-rolled steel sheet excellent in ductility having a tensile strength of 980 MPa or more.
[0004]
[Patent Document 1]
JP 2001-81533 A
[0005]
[Problems to be solved by the invention]
However, in the technique described in Patent Document 1, the minimum thickness of an ultra-high tensile cold-rolled steel sheet that contains a large amount of strengthening elements, does not sufficiently define impurities, and can be manufactured stably with good yield is at most 1 However, there is a problem in the production of an ultra-thin ultra-high tensile cold-rolled steel sheet having a thickness of 0.8 mm or less, more preferably 0.75 mm or less. Further, in the technique described in Patent Document 1, the strength of a hot-rolled steel sheet (cold-rolled base sheet) also becomes equal to or higher than that of the hot-rolled steel sheet, so that cold rolling becomes impossible or annealing must be performed in the middle. There was such a problem.
[0006]
A cold-rolled steel sheet is usually manufactured by subjecting a slab adjusted to a predetermined composition to hot rolling and cold rolling to obtain a cold-rolled sheet, and then subjecting the slab to an annealing treatment. However, in producing ultra-high tensile ultra-thin steel sheets with a thickness of 0.8 mm or less, the deformation resistance of the steel increases, and the rolling stability in hot rolling and cold rolling decreases. However, in extreme cases, there is a problem that rolling becomes impossible and it is difficult to control the shape of the steel sheet.
[0007]
Also, in recent years, scraps have been effectively utilized from the viewpoint of improving the recyclability of steel materials. As a so-called tramp element, elements represented by "Nb, Cu, Ni, Cr, Mo" and the like have been compared with conventional ones. The phenomenon that a large amount is mixed is remarkable. Recently, the amount of Nb, Cu, Ni, Cr, and Mo mixed in more than 0.2% by mass in total has increased in many cases. These elements are strengthening elements for steel, and the strength of the steel increases due to the incorporation of a large amount of these elements.
[0008]
When a large amount of the tramp elements is mixed in this way, the strength of the hot-rolled steel sheet increases, so that the reduction ratio of the subsequent cold rolling is limited, and there is a problem that the production of an ultra-thin cold-rolled steel sheet becomes difficult. In such a case, as a method of manufacturing an ultra-thin cold rolled steel sheet, there is a method of so-called twice cold rolling and twice annealing of hot rolling → pickling → cold rolling → annealing → cold rolling → annealing, but the process is complicated. Thus, there remain industrial problems such as an increase in production cost and an increase in time required for production.
[0009]
The present invention advantageously solves the above-mentioned problems of the prior art, and provides an ultra-thin ultra-high tensile cold-rolled steel sheet having a thickness of 0.8 mm or less and a tensile strength of 980 MPa or more, and a method for producing the same. With the goal. In order to sufficiently reduce the weight of the member by reducing the thickness of the steel sheet, a steel sheet of 780 MPa class is insufficient, and a steel sheet of at least 980 MPa class is required, and excellent ductility and stretch flangeability are also required. Further, from the relationship between the thickness and the strength of the current steel sheet, the sheet thickness is targeted at 0.8 mm or less, more preferably 0.75 mm or less.
[0010]
[Means for Solving the Problems]
The present inventors have diligently studied various factors affecting the cold rolling property in order to achieve the above-described object.
The hot-rolled sheet is usually coiled after being cooled from the austenitic single-phase region to the winding temperature after the end of hot rolling, and then the cooling rate is significantly reduced due to the heat capacity of the coil itself. Undergo a thermomechanical treatment that is finally cooled to room temperature. On the other hand, a cold-rolled annealed sheet to be a product is obtained by cold-rolling a hot-rolled sheet into a cold-rolled sheet, and then rapidly heating the cold-rolled sheet from room temperature to an annealing temperature, and then increasing the production efficiency. Receives a heat history that is rapidly cooled. Due to the difference between the working and the heat history of the hot-rolled sheet and the cold-rolled annealed sheet, the present inventors have found that the working heat treatment applied to the hot-rolled sheet does not increase the strength, but is remarkable after cold rolling and annealing. If a combination of steel composition, heat treatment conditions, etc., is used, the strength of the hot-rolled sheet, which is an intermediate step, becomes low and the load during cold rolling can be reduced, and as a result, the cold rollability improves. Thus, the present inventors have conceived that the production of an ultra-thin ultra-high tensile cold-rolled steel sheet having a thickness of 0.80 mm or less becomes easy.
[0011]
That is, in the production of ultra-thin ultra-high tensile cold-rolled steel sheet, in order to improve the cold rolling property, the hot-rolled sheet is maintained as soft as possible, and thereafter, in the pickling / cold rolling step and the continuous annealing step. It was found that the desired tensile strength should be ensured.
Based on such a concept, the present inventors further manufactured steel plates with various components and manufacturing methods, and performed many material evaluation experiments. As a result, it was found that Mn contributes little to the increase in strength in the heat treatment applied to the hot-rolled sheet, while the heat treatment applied to the cold-rolled sheet makes a significant contribution to the increase in strength. It has been found to be advantageous in the production of thin ultra-high strength steel sheets.
[0012]
In addition, Cr and Mo greatly contribute to the strength under both the heat treatment conditions applied to the hot-rolled sheet and the heat treatment conditions applied to the cold-rolled sheet, and the production of ultra-thin ultra-high-strength steel sheets reduces the content of Cr and Mo severely. I knew that I needed to do it.
Based on such findings, the present inventors further studied and included Mn as much as 1.5% by mass or more, and reduced the total amount of Cr and Mo mixed as impurities to 0.2% by mass or less. By limiting the steel composition, the incorporation of martensite is suppressed, the structure of the hot-rolled sheet becomes ferrite + pearlite or ferrite + bainite structure, and the tensile strength of the hot-rolled sheet is maintained at a low strength of 800 MPa or less. It has been found that the steel sheet strength can be increased to the target strength by increasing the amount of martensite in the subsequent cold rolling and continuous annealing step, and the ductility and stretch flangeability can be sufficiently satisfied. Furthermore, in order to keep the strength of the hot-rolled sheet even lower, it has been found that it is also important to limit the amount of Nb that increases the deformation resistance during hot rolling to 0.005% or less. .
[0013]
The present invention has been completed based on the above findings, with further investigations. That is, in the present invention, C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 1.5 to 3.5%, P: 0.01% or less, S: It contains 0.003% or less, Al: 0.10% or less, and N: 0.0050 to 0.0250%. Cr and Mo as impurities are 0.2% or less in total, and Nb as an impurity is 0.1% or less. 005% or less, having a composition comprising the balance of Fe and unavoidable impurities, having a tensile strength of 980 MPa or more, and being excellent in ductility and stretch flangeability, a sheet thickness of 0.8 mm or less. , And in the present invention, in addition to the above composition, Ca: 0.001 to 0.020% and / or REM: 0.005 to 0. It is preferred to contain 020%.
[0014]
The present invention also provides an ultra-thin ultra-high tensile strength sheet having a thickness of 0.8 mm or less and a tensile strength of 980 MPa or more formed by forming a plating layer on the surface layer of each of the ultra-thin ultra-high tensile cold-rolled steel sheets described above. It is a plated steel sheet, and in the present invention, it is preferable that the plated layer is a galvanized layer.
In the present invention, C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 1.5 to 3.5%, P: 0.01% or less, S: 0.003% or less, Al: 0.10% or less, N: 0.0050 to 0.0250%, Cr and Mo as impurities are 0.2% or less in total, and Nb as an impurity is 0.1% or less. After heating a steel slab having a composition consisting of the balance Fe and unavoidable impurities to a slab heating temperature of 1000 ° C. or more, the finish-rolling exit temperature is set to 800 ° C. or more, and a winding temperature is set to 550 ° C. A hot rolling step in which the above hot rolling is performed to form a hot rolled sheet, a cold rolling step in which the hot rolled sheet is subjected to cold rolling to form a cold rolled sheet, and the recrystallization rate of the cold rolled sheet is Heat to an annealing temperature of not less than 80% and not more than 900 ° C. A continuous annealing treatment step of cooling the temperature range up to 0 ° C. at a cooling rate of 30 ° C./s or more into a cold-rolled annealed sheet, wherein the sheet thickness is 0.8 mm or less and the tensile strength is This is a method for producing an ultra-thin ultra-high tensile cold-rolled steel sheet having a tensile strength of 980 MPa or more and excellent in ductility and stretch flangeability. In the present invention, in addition to the above composition, Ca: 0. It is preferable to contain 001 to 0.020% and / or REM: 0.005 to 0.020%.
[0015]
In the present invention, C: 0.05 to 0.25%, Si: 0.05% or less, Mn: 1.5 to 3.5%, P: 0.01% or less, S: 0.003% or less, Al: 0.10% or less, N: 0.0050 to 0.0250%, Cr and Mo as impurities are 0.2% or less in total, and Nb as an impurity is 0.1% or less. After heating a steel slab having a composition consisting of the balance Fe and unavoidable impurities to a slab heating temperature of 1000 ° C. or more, the finish-rolling exit temperature is set to 800 ° C. or more, and a winding temperature is set to 550 ° C. A hot rolling step in which the above hot rolling is performed to form a hot rolled sheet, a cold rolling step in which the hot rolled sheet is subjected to cold rolling to form a cold rolled sheet, and the recrystallization rate of the cold rolled sheet is Heat to an annealing temperature of not less than 80% and not more than 900 ° C. A continuous annealing process of cooling the temperature range up to 0 ° C. at a cooling rate of 30 ° C./s or more into a cold-rolled annealed plate, and then forming a plating layer on the surface of the cold-rolled annealed plate. A method for producing an ultra-thin ultra-high tensile steel sheet having a thickness of 0.8 mm or less, a tensile strength of 980 MPa or more, and excellent ductility and stretch flangeability. In the present invention, it is preferable that Ca: 0.001 to 0.020% and / or REM: 0.005 to 0.020% by mass% in addition to the above composition. In the present invention, the plating layer is preferably a hot-dip galvanized layer.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
The ultra-thin ultra-high tensile cold-rolled steel sheet of the present invention is a cold-rolled steel sheet having a thickness of 0.8 mm or less, a tensile strength of 980 MPa or more, and excellent ductility and stretch flangeability. In the present invention, "excellent in ductility" means that the elongation is 10% or more, and "excellent in stretch flangeability" means that the hole expansion rate evaluated by a hole expansion test is 70% or more. Shall be referred to.
[0017]
First, the reason for limiting the composition of the ultra-thin ultra-high tensile cold-rolled steel sheet of the present invention will be described. Hereinafter, mass% in the composition is simply described as%.
C: 0.05 to 0.25%
C is an element that increases the strength of the steel sheet. In the present invention, the content of 0.05% or more is required to secure the desired strength. The weldability and the like are significantly reduced. For this reason, C was limited to the range of 0.05 to 0.25%. In addition, from the viewpoint of improvement in moldability, the content is preferably 0.20% or less. Further, for applications requiring particularly good ductility, the content is more preferably 0.18% or less. By setting the C content to 0.05% or more, a necessary and sufficient amount of the hard second phase required for increasing the strength can be dispersed.
[0018]
Si: 0.05% or less
Si is an element that increases the deformation resistance in hot rolling and cold rolling. In the present invention, in which it is necessary to reduce the strength of a hot-rolled sheet and to improve the cold-rolling property, it is desirable to reduce as much as possible. If the Si content exceeds 0.05%, the load during cold rolling tends to increase significantly, even if the strength of the hot-rolled sheet is the same. For this reason, Si was limited to 0.05% or less. In the production of a further ultrathin object, the content of Si is preferably set to 0.02% or less.
[0019]
Mn: 1.5 to 3.5%
Mn is a useful element that prevents hot cracking due to S, suppresses an increase in strength of a hot-rolled sheet, increases the strength of a cold-rolled steel sheet, and further refines crystal grains. About 5% or more is required. By containing Mn in an amount of 1.5% or more, bainite transformation and pearlite transformation easily occur in a relatively short time even with heat retention at the winding temperature or a heat history of slow cooling from the winding temperature. Since the incorporation of martensite into the structure of the sheet can be suppressed, the strength of the hot-rolled sheet can be maintained relatively low, and the cold-rollability of the hot-rolled sheet is improved.
[0020]
On the other hand, when the content of Mn exceeds 3.5%, the detailed mechanism is unknown, but the hot deformation resistance of the steel sheet tends to increase, and the weldability and the formability of the welded portion tend to deteriorate. In addition, the formation of ferrite is significantly suppressed, and the ductility is significantly reduced. For these reasons, Mn is limited to the range of 1.5 to 3.5%. By setting the Mn content to 2.0% or more, an effect of significantly suppressing a change in mechanical properties of a steel sheet due to a change in hot rolling conditions is expected, and thus Mn is 2.0% or more. It is desirable to contain.
[0021]
P: 0.01% or less
P is an element that increases the strength of the steel, reduces the ductility of the steel sheet, and deteriorates the stretch flangeability of the steel sheet, and P has a strong tendency to segregate in the steel, and the welded portion is caused by the segregation. Causes embrittlement. For this reason, it is preferable to reduce P as much as possible. In the present invention, the upper limit is set to 0.01%. In particular, in applications where emphasis is placed on ductility and stretch flangeability, the content is preferably 0.005% or less.
[0022]
S: 0.003% or less
S is present as an inclusion in the steel, which reduces the ductility of the steel sheet and further deteriorates the corrosion resistance. Therefore, it is preferable to reduce S as much as possible. In the present invention, S is limited to 0.003% or less. In particular, for applications requiring excellent stretch flange workability, the content is desirably 0.002% or less.
[0023]
Al: 0.10% or less
Al is a useful element that acts as a deoxidizing agent, improves the cleanliness of steel, and has a function to refine the structure. In order to obtain such an effect, it is desirable to contain 0.005% or more. On the other hand, when the content exceeds 0.10%, it leads to deterioration of surface properties and remarkable decrease of solid solution N. For this reason, Al was limited to 0.10% or less. In addition, from the viewpoint of material stability, the content is desirably 0.005 to 0.06%.
[0024]
N: 0.0050 to 0.0250%
N is an element having the effect of increasing the strength (yield strength and tensile strength) of a steel sheet by solid solution strengthening and strain age hardening, and such an effect is stably recognized at a content of 0.0050% or more. Will be able to In addition, N also has an effect of lowering the transformation point of the steel, and the N content is effective when it is not desired to perform rolling with a thin material that greatly reduces the transformation point. On the other hand, when the content exceeds 0.0250%, the rate of occurrence of slab cracks and the like during continuous casting is significantly increased, and the rate of occurrence of internal defects in the steel sheet is increased. For this reason, N was limited to the range of 0.0050 to 0.0250%. In addition, as long as N content is within the range of the present invention, there is no adverse effect on weldability such as arc welding, resistance welding, high frequency welding, and electron beam welding. In addition, it is preferable to set it in the range of 0.0070 to 0.0170% from the viewpoint of improving the stability and yield of the material in consideration of the entire manufacturing process.
[0025]
In addition to the limitation of the basic components described above, in the present invention, of the elements mixed as impurities, Cr, Mo, and Nb are limited to the following predetermined values or less.
Cr and Mo as impurities are 0.2% or less in total
Cr and Mo can suppress the formation of soft ferrite and low-strength pearlite during annealing of the cold-rolled sheet, and can increase the strength of the cold-rolled annealed sheet (product). In addition to increasing the deformation resistance of the hot rolled sheet, the bainite and pearlite transformations are suppressed during cooling after hot rolling, and the martensitic transformation is easily caused even at a slow cooling from the winding temperature, and the strength of the hot-rolled sheet is markedly increased. increase. For this reason, in the present invention, it is necessary to reduce Cr and Mo as impurities as much as possible, and in the present invention, Cr and Mo are limited to 0.2% or less in total. More preferably, it is 0.17% or less. When the thickness of the steel sheet is small, the total content is preferably 0.1% or less.
[0026]
In addition, in order to reduce Cr and Mo as impurities, since Mo, Cr, etc. are mixed as impurities into the hot metal used as an iron source and the ferromagnetic iron, an iron source (hot metal, It is important to use ferroalloys and to prevent the mixing of these elements in equipment and processes.
0.005% or less of Nb as an impurity
Nb is an element that significantly increases the deformation resistance during hot rolling. In the present invention, in which the strength of a hot-rolled sheet needs to be low, it must be reduced as much as possible. Nb only increases the strength of the hot-rolled sheet, but has a small effect of increasing the strength of the cold-rolled steel sheet manufactured by continuous annealing, and it is desirable to reduce it from this viewpoint. Further, Nb raises the recrystallization temperature, so that continuous annealing requires annealing at a higher temperature. Annealing at a high temperature is not preferable for an extremely thin material as in the present invention, since the risk of causing operational problems such as breakage of a steel sheet is significantly increased. If Nb is reduced to 0.005% or less, the level becomes almost no problem. For these reasons, Nb is limited to 0.005% or less. Note that the content is more preferably 0.002% or less.
[0027]
Ca: 0.001 to 0.020% and / or REM: 0.005 to 0.020%
Both Ca and REM have an effect of improving stretch flangeability without controlling the morphology of inclusions and without causing surface defects, etc., and can be selected as necessary and contained alone or in combination. . In addition, even if these are contained in combination, the effects are not offset. Such effects are observed when the content of Ca is 0.001% or more and the content of REM is 0.005% or more. However, when the content of Ca exceeds 0.020% and the content of REM exceeds 0.020%, the amount of inclusions is reduced. The cleanliness decreases with an increase. For this reason, Ca is preferably limited to 0.001 to 0.020%, and REM is preferably limited to 0.005 to 0.020%.
[0028]
The balance other than the components described above is Fe and inevitable impurities. As the inevitable impurities, for example, Ti: 0.010% or less can be allowed.
Next, a method for producing the steel sheet of the present invention will be described.
After smelting the molten steel having the above composition by a known smelting method such as a converter, it is preferable to form a steel slab by a known casting method. As the casting method, any known method can be applied, but it is preferable to use a continuous casting method in order to prevent macro segregation of components. There is no problem even if the ingot making method or the thin slab casting method is used.
[0029]
After the steel slab is heated to a predetermined temperature or higher, the steel slab is subjected to a hot rolling process in which the steel slab is subjected to hot rolling to form a hot rolled sheet.
The steel slab is once cooled to room temperature and then heated again to a predetermined temperature or higher and hot rolled. Alternatively, if the temperature of the steel slab is high, do not cool the steel slab to room temperature, insert it into a heating furnace as it is, and heat it to a specified temperature or higher, or insert it in a heating furnace and keep a small amount of heat. After the temperature is increased to a predetermined temperature or more, an energy saving process such as direct rolling or direct rolling in which hot rolling is performed immediately can be applied. In particular, direct rolling is one of useful techniques for effectively securing solid solution N.
[0030]
Slab heating temperature (SRT): 1000 ° C or more
The steel slab is heated to a predetermined slab heating temperature of 1000 ° C. or higher and hot-rolled. If the slab heating temperature is lower than 1000 ° C., a uniform structure cannot be obtained as an initial state. By setting the slab heating temperature to 1000 ° C. or higher, the structure at the start of hot rolling is sufficiently uniform. The upper limit of the slab heating temperature does not need to be particularly limited, but is preferably 1280 ° C. or lower from the viewpoint of a decrease in yield due to an increase in oxidized weight.
[0031]
In the hot rolling, it is preferable that a steel slab heated to a temperature equal to or higher than the slab heating temperature is preferably subjected to rough rolling and finish rolling to obtain a hot rolled sheet.
Rough rolling is only required to obtain a sheet having a predetermined thickness, and the conditions are not particularly limited.
The finish rolling is a rolling in which the finish rolling exit temperature FT is 800 ° C. or higher.
[0032]
Finishing roll exit temperature (FT): 800 ° C or more
If the finish-rolling exit temperature is less than 800 ° C., the structure of the hot-rolled sheet becomes uneven. Since the non-uniformity of the structure of the hot-rolled sheet remains without disappearing even after cold rolling and annealing, there is an increased risk of causing various problems during press forming. When the finish-rolling exit temperature is lower than 800 ° C., if a high winding temperature is employed to avoid the retention of the processed structure, coarse grains are generated, which increases the risk of causing various problems during press forming. I do. If a high winding temperature is employed, the amount of solid solution N is remarkably reduced, and the desired strength cannot be secured. For this reason, it is preferable that the finish-rolling exit side temperature be 800 ° C. or higher. By setting the finish-rolling exit temperature to 800 ° C. or higher, a uniform and fine hot-rolled structure can be obtained. It is more preferable that the temperature be 820 ° C. or higher from the viewpoint of improving the mechanical properties. The upper limit of the finish-rolling discharge side temperature is not particularly limited, but is preferably about 1000 ° C. in consideration of prevention of scale flaws.
[0033]
After finishing rolling, the hot rolled sheet is cooled and wound into a coil. The cooling rate after hot rolling is not particularly limited, but in order to reduce the strength of the hot rolled sheet, water cooling is performed after the completion of hot rolling, and the average cooling rate during cooling is set at 10 ° C to 150 ° C / s and winding is performed. Cooling to temperature is preferred. If the cooling rate is within this range, the control is relatively easy, and it is possible to avoid the occurrence of non-uniformity of the material and shape due to the non-uniformity of the cooling rate.
[0034]
Winding temperature CT: 550 ° C or more
As the winding temperature increases, the strength of the hot-rolled sheet tends to decrease. In the present invention, the winding temperature is preferably set to 550 ° C. or higher. By adjusting the steel sheet composition to a high Mn-low Cr, Mo-low Nb system as described above, and adjusting the winding temperature to 550 ° C. or higher, the incorporation of martensite can be prevented, and the upper bainite or bainitic ferrite-based A hot rolled sheet having a structure can be obtained, and the tensile strength of the hot rolled sheet can be effectively reduced to 800 MPa or less. Such a hot-rolled sheet cannot be softened in a hot-rolled sheet having a Cr or Mo content of more than 0.2% or a Nb content of more than 0.005%. Although the upper limit of the winding temperature is not particularly limited, it is more preferably approximately 750 ° C. or less from the viewpoint of uniformity in the width direction of the hot rolled sheet (coil). When a softer hot rolled sheet is required, the temperature is more preferably set in a range of 580 to 720 ° C. By the above-described hot rolling step, the tensile strength of the hot-rolled sheet can be made 800 MPa or less. By setting the tensile strength of the hot-rolled sheet to 800 MPa or less, the cold rollability of a hot-rolled sheet having a normal product width of about 920 to 1600 mm can be significantly improved.
[0035]
In the above-mentioned hot rolling process, applying the continuous rolling technology that is currently partially used is intended to improve the shape and dimensional accuracy of the steel sheet and to make the material uniform in the longitudinal and width directions of the steel sheet. It is extremely effective for. In addition to this continuous rolling technique, it is extremely effective to use a sheet bar edge heater that makes the rolling temperature uniform in the longitudinal direction and the width direction of the coil in order to make the material uniform. Further, a technique of masking the cooling water in the width direction in order to prevent overcooling of the edge portion in cooling after rolling is also effective from the viewpoint of material uniformity.
[0036]
The hot-rolled sheet wound in a coil shape is then preferably subjected to pickling, and then subjected to a cold-rolling process in which the sheet is cold-rolled to form a cold-rolled sheet. The pickling is preferably performed according to a conventional method, but if it is in a very thin scale state, direct cold rolling can be performed without pickling.
The cold rolling is not particularly limited as long as a cold-rolled sheet of a predetermined size can be obtained, and the conditions such as the rolling reduction and the rolling method are not particularly limited. From the viewpoint of uniform and fine structure, the cold rolling reduction is 30% or more. It is preferable that Further, from the viewpoint of productivity, it is preferable that the cold rolling reduction is higher.
[0037]
Next, the cold-rolled sheet is heated to an annealing temperature of not less than 900 ° C. and a temperature at which the recrystallization ratio becomes 80%, and a temperature range from the annealing temperature to 600 ° C. is cooled at a cooling rate of 30 ° C./s or more. A continuous annealing process is performed to form a spread annealing plate. In addition, it is preferable to perform the pickling process on the cold rolled sheet before the continuous annealing process.
Annealing temperature: not less than the temperature at which the recrystallization ratio becomes 80% and not more than 900 ° C
The annealing temperature is preferably not lower than the temperature at which the recrystallization ratio is 80% and not higher than 900 ° C. The annealing temperature does not necessarily need to be in a completely recrystallized state, and may be any temperature as long as a temperature at which a recrystallization rate of about 80% or more can be ensured. Here, when the α → γ transformation occurs at a lower temperature than that at which complete recrystallization occurs, the γ phase is considered as a recrystallized structure. If the recrystallization ratio is less than 80%, a cold-rolled annealed sheet having sufficient formability cannot be obtained. On the other hand, when the annealing temperature is higher than 900 ° C., the formability is remarkably improved, but the strength of the steel sheet is reduced, and the risk of sheet breakage during continuous annealing is increased. Elemental concentration becomes remarkable, and paintability decreases and generation of non-plating becomes remarkable. Therefore, it is preferable that the annealing temperature be approximately 900 ° C. or less. The temperature at which the recrystallization ratio becomes 80% is determined by annealing the cold-rolled steel sheet at various temperatures and observing the microstructure. The value is used as the crystal ratio.
[0038]
Cooling rate in the temperature range from annealing temperature to 600 ° C: 30 ° C / s or more
In the present invention, so-called transformation structure strengthening is used to make the strength of a final product (a cold-rolled annealed sheet) a desired strength. Therefore, the cooling in the continuous annealing process is made relatively rapid, and the formation of a so-called diffusion-type transformed structure is suppressed, and the microstructure of the final product (cold rolled annealed plate) is made of ferrite containing mainly martensite and partially containing bainite. And a mixed structure of martensite or bainite, or a mixed structure of bainite and martensite, thereby achieving a tensile strength of 980 MPa or more even with the above-described component composition. In order to obtain such a microstructure, it is preferable to cool the temperature range from the annealing temperature to at least 600 ° C. at a cooling rate of 30 ° C./s or more, more preferably 50 ° C./s or more. However, when rapid cooling is performed at a rate exceeding 300 ° C./s, since the cooling rate is non-uniform due to the extremely thin steel sheet, the occurrence of poor shape and non-uniform mechanical properties in the width direction becomes remarkable. It is preferred that the upper limit be ° C / s. The cooling in the temperature range of 600 ° C. or less is not particularly limited. For example, rapid cooling may be performed in the temperature range up to 600 ° C., or air cooling may be performed.
[0039]
The thus obtained cold-rolled annealed sheet may be further subjected to a plating step of forming a plating layer on the surface of the steel sheet. When the galvanizing process is a hot-dip galvanizing process, it is preferable that the galvanizing process be performed continuously during the cooling in the continuous process. Alternatively, the plating treatment may be performed separately and independently after cooling to room temperature without performing the continuous treatment.
[0040]
The type of the formed plating layer is not particularly limited, but is preferably a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an aluminum plated layer, and a zinc-aluminum alloy plated layer of various compositions. The method for forming the plating layer is not particularly limited, and the plating layer can be formed by immersion in a molten metal bath according to a conventional method. It is preferable to perform pickling on the entrance side of the processing line before performing the plating process. By performing pickling, the plating adhesion of the final product can be improved.
[0041]
In addition, after the continuous annealing process or the plating process, temper rolling (skin pass rolling) and / or leveler processing may be performed for the purpose of shape correction, roughness adjustment, and stable improvement of strain aging hardening characteristics. Good. It is preferable that the elongation rate of the temper rolling or the elongation rate of the leveler processing is 0.3 to 15% alone or in total. The elongation percentage of the passivation (skin pass) rolling or leveling is sufficient if it is approximately 0.3% or more, but if it exceeds 15%, the ductility is reduced. In addition, it has been confirmed that there is no significant difference in the effect between the temper rolling (skin pass rolling) and the leveler processing.
[0042]
【Example】
Molten steel containing the components shown in Table 1 and the balance substantially consisting of Fe was smelted in a converter and made into a steel slab (sheet thickness: 260 mm) by a continuous casting method. These steel slabs are subjected to hot rolling under the conditions shown in Table 2 to form a hot-rolled sheet, and further subjected to pickling, and then cold-rolled under the conditions shown in Table 2. The product (cold rolled steel sheet) was successively subjected to a cold rolling step of giving a cold rolled sheet and a continuous annealing step of subjecting the cold rolled sheet to a continuous annealing treatment to form a cold rolled annealed sheet. The temperature at which the recrystallization rate is 80% is set at an annealing temperature of 600 ° C. or higher at an interval of 10 ° C. before annealing the cold-rolled unannealed sheet, and then observing the microstructure of the annealed sheet at each annealing temperature. The area ratio at which a structure other than the recrystallized structure (rolled structure) is observed is determined, and this is defined as the recrystallized ratio. Was. In the cold rolling, the cold rolling property was measured based on the rolling load and the presence or absence of cracks. When the load was large or cracks occurred, it was evaluated that cold rolling was difficult. Further, the thickness and shape of the cold-rolled steel sheet were measured, and when the thickness accuracy had a variation exceeding 70 μm in the longitudinal direction, it was determined that the dimensional accuracy was poor, and the flatness described in JIS G3135, ie, the wave height measured on the surface plate, was measured. When it was 10 mm or more, the shape was determined to be defective.
[0043]
Also, in some steel sheets, hot-rolled steel sheet is pickled, then cold-rolled, and then formed into a cold-rolled annealed sheet using a continuous hot-dip galvanizing line, and then a hot-dip galvanized layer is formed on the surface thereof. Product (galvanized steel sheet).
A test piece was collected from the obtained product and subjected to a tensile test and a hole expanding test. The test method was as follows.
(1) Tensile test
A JIS No. 5 tensile test piece is sampled from each product (cold rolled steel sheet, hot-dip galvanized steel sheet) and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to obtain tensile properties (yield stress YS, tensile strength TS, elongation). El) was determined. The longitudinal direction of the tensile test piece was made to coincide with the direction perpendicular to the rolling of the steel sheet. In addition, a JIS No. 5 tensile test piece was similarly sampled from the hot-rolled sheet before the cold rolling step, and a tensile test was similarly performed to determine the tensile strength TS of the hot-rolled sheet.
(2) Hole expansion test
A test piece was collected from each product (cold-rolled steel sheet, hot-dip galvanized steel sheet) and subjected to a hole expansion test in accordance with the rules of the Japan Iron and Steel Federation Standard JFS-T1001-1996. A test piece (size: 100 x 100 mm) with a hole diameter d₀(= 10mm) initial hole was punched with a clearance of 12.5%, and then the burr of the initial hole was used as the die side (ie, the side opposite to the conical punch), and a conical punch (vertical angle 60 °) was inserted into the initial hole to crack. The hole was expanded until it penetrated the steel plate, and the hole diameter d when the crack penetrated the steel plate was determined. d₀, D value, and the following equation
λ = 100 × (dd₀) / D₀
(Where d₀: Initial hole diameter (= 10 mm), d: hole diameter at the time when the crack penetrated the steel plate)
, The hole expansion ratio λ (%) was calculated, and the stretch flangeability was evaluated.
[0044]
Table 3 shows the obtained results.
[0045]
[Table 1]

[0046]
[Table 2]

[0047]
[Table 3]

[0048]
[Table 4]

[0049]
[Table 5]

[0050]
In each of the examples of the present invention, there is no decrease in dimensional accuracy or cold rolling property, and an ultra-thin ultra-high tensile cold-rolled steel sheet or an ultra-thin steel sheet having a tensile strength of 980 MPa or more, excellent ductility and excellent stretch flangeability. It is an ultra-high tensile galvanized steel sheet. On the other hand, the comparative examples out of the range of the present invention have a tensile strength of less than 980 MPa, a decrease in ductility and stretch flangeability, or a decrease in dimensional accuracy and a decrease in cold rolling property. Not ultra-thin ultra-high tensile cold rolled steel sheet or ultra-thin ultra-high tensile hot-dip galvanized steel sheet.
[0051]
【The invention's effect】
As described above, according to the present invention, an ultra-thin ultra-high tensile cold-rolled steel sheet and an ultra-thin ultra-high tensile plated steel sheet having a high tensile strength of 980 MPa or more and a thickness of 0.8 mm or less can be easily and stably formed. It can be manufactured with great effect in industry.

Claims (9)

In mass%,
C: 0.05 to 0.25%, Si: 0.05% or less,
Mn: 1.5 to 3.5%, P: 0.01% or less,
S: 0.003% or less, Al: 0.10% or less,
N: 0.0050 to 0.0250%
And Cr and Mo as impurities are adjusted to a total of 0.2% or less, and Nb as an impurity is adjusted to 0.005% or less. The composition has a balance of Fe and unavoidable impurities, and has a tensile strength. An ultra-thin ultra-high tensile cold-rolled steel sheet having a thickness of 0.8 mm or less, which is 980 MPa or more and is excellent in ductility and stretch flangeability. The ultrathin film according to claim 1, further comprising, by mass%, 0.001 to 0.020% of Ca and / or 0.005 to 0.020% of REM in addition to the composition. Ultra high tensile cold rolled steel sheet. The ultra-thin ultra-high tensile cold-rolled steel sheet according to claim 1 or 2, wherein a plating layer is formed on a surface layer of the ultra-high tensile strength cold-rolled steel sheet. Ultra-thin ultra-high tensile plated steel sheet. The ultra-thin ultra-high tensile strength coated steel sheet according to claim 3, wherein the plating layer is a hot-dip galvanized layer. In mass%,
C: 0.05 to 0.25%, Si: 0.05% or less,
Mn: 1.5 to 3.5%, P: 0.01% or less,
S: 0.003% or less, Al: 0.10% or less,
N: 0.0050 to 0.0250%
In addition, Cr and Mo as impurities are adjusted to a total of 0.2% or less, Nb as an impurity is adjusted to 0.005% or less, and a steel slab having a composition of balance Fe and unavoidable impurities is heated to 1000 ° C. or more. Hot rolling at a finish-rolling outlet temperature of 800 ° C. or higher and a winding temperature of 550 ° C. or higher after hot-rolling to form a hot-rolled sheet; A cold rolling step of performing cold rolling to form a cold-rolled sheet, and heating the cold-rolled sheet to an annealing temperature of not less than a temperature at which a recrystallization ratio is 80% and not more than 900 ° C., and from the annealing temperature to 600 ° C. A continuous annealing treatment step of cooling a temperature range of 30 ° C./s or more at a cooling rate of 30 ° C./s or more to form a cold-rolled annealed sheet, characterized in that the sheet thickness is 0.8 mm or less and the tensile strength is 980 MPa or more. Of ultra-thin ultra-high tensile cold-rolled steel sheets with cracks . The ultrathin film according to claim 5, further comprising, by mass%, 0.001 to 0.020% of Ca and / or 0.005 to 0.020% of REM in addition to the composition. Manufacturing method of ultra-high tensile cold rolled steel sheet. In mass%,
C: 0.05 to 0.25%, Si: 0.05% or less,
Mn: 1.5 to 3.5%, P: 0.01% or less,
S: 0.003% or less, Al: 0.10% or less,
N: 0.0050 to 0.0250%
In addition, Cr and Mo as impurities are adjusted to a total of 0.2% or less, Nb as an impurity is adjusted to 0.005% or less, and a steel slab having a composition of balance Fe and unavoidable impurities is heated to 1000 ° C. or more. Hot rolling at a finish-rolling outlet temperature of 800 ° C. or higher and a winding temperature of 550 ° C. or higher after hot-rolling to form a hot-rolled sheet; A cold rolling step of performing cold rolling to form a cold-rolled sheet, and heating the cold-rolled sheet to an annealing temperature of not less than a temperature at which a recrystallization ratio is 80% and not more than 900 ° C., and from the annealing temperature to 600 ° C. A continuous annealing step of cooling the temperature range of 30 ° C./s or more at a cooling rate of 30 ° C./s or more to form a cold-rolled annealed sheet, and then performing a plating step of forming a plating layer on the surface of the cold-rolled annealed sheet. When the plate thickness is 0.8mm or less, tensile strength There method for producing ultra-thin ultra high-strength plated steel sheet having the above 980 MPa. The ultrathin film according to claim 7, further comprising, by mass%, 0.001 to 0.020% of Ca and / or 0.005 to 0.020% of REM in addition to the composition. Manufacturing method of ultra-high tensile plated steel sheet. The method according to claim 7, wherein the plating layer is a hot-dip galvanized layer.