JP3879440B2 - Manufacturing method of high strength cold-rolled steel sheet - Google Patents

Description

【００６０】
その後、鋼塊を1250℃で再加熱しラボ熱間圧延をおこなった。圧延後は種々の水冷条件で一旦冷却したのち、続けて巻取り相当温度まで空冷し、その温度の炉で1時間保持したのち炉冷をおこなうことで巻き取り相当の熱処理とした。熱間圧延後は、種々の冷圧率で冷間圧延したのち、種々の加熱条件で再結晶焼鈍をおこない、焼鈍後は空冷した。冷圧後の板厚はいずれも1.2mmとした。表２に実験条件を示す。
【００６１】
【表２】

【００６２】
ここで、鋼板No.4〜7、10〜13、15〜20は発明鋼板である。また、鋼板No.1〜3は焼鈍における加熱速度、鋼板No.8〜9は巻取温度、No.14は冷圧率がそれぞれ本発明範囲外の比較鋼板である。
【００６３】
焼鈍後のサンプルを用いて、引張特性、穴拡げ特性（伸びフランジ特性）、溶接性を調査した。ここで、溶接性は、TIGビードオン溶接で溶接部の硬度分布を調査し、母材硬度に対するHAZ軟化部の硬度比（HAZ軟化部硬度／母材部硬度）で評価した。表３に、これらのサンプルの特性値をまとめて示す。
【００６４】
【表３】

【００６５】
以下、製造条件と特性値の関係を、図を用いて説明する。まず、焼鈍における加熱速度の影響について、図1〜5に示す。ここでは、鋼種Aに関し、熱延条件として最終仕上温度が850℃、冷却開始時間が3ｓ、降下温度が100℃、巻取処理温度が600℃、冷延条件として冷圧率が60％、焼鈍条件として焼鈍温度が800℃、焼鈍時間が1ｓのとき材料特性におよぼす焼鈍時の加熱速度の影響を示す。
【００６６】
図1に降伏および引張強度におよぼす加熱速度の影響を示す。図に示すように、加熱速度が本発明の請求範囲の下限30℃/ｓを下回るとき、降伏強度は490〜500MPa、引張強度は550〜570MPaであるのに対し、加熱速度が本発明の請求範囲である30℃/ｓ以上のとき、降伏強度は400MPa以下と低くなり、引張強度は600MPa以上と高くなった。
【００６７】
図2に降伏比（＝降伏強度／引張強度）におよぼす加熱速度の影響を示す。図に示すように、加熱速度が本発明の請求範囲の下限30℃/ｓを下回るとき、降伏比は86〜88％であるのに対し、加熱速度が本発明の請求範囲である30℃/ｓ以上のときは63％以下と低くなった。
【００６８】
さらに、図3に全伸びにおよぼす加熱速度の影響を示す。図に示すように、加熱速度が本発明の請求範囲外である30℃/ｓを下回るとき、全伸びは29〜31％であるのに対し、加熱速度が本発明の請求範囲である30℃/ｓ以上のときは34％以上と高くなった。
【００６９】
また、図4に穴拡げ率におよぼす加熱速度の影響を示す。図に示すように、加熱速度が本発明の請求範囲の下限30℃/ｓを下回るとき、穴拡げ率は58〜65％であるのに対し、加熱速度が本発明の請求範囲である30℃/ｓ以上のときは78％以上と高くなった。
【００７０】
また、図5にHAZ軟化硬度比におよぼす加熱速度の影響を示す。図に示すように、加熱速度が本発明の請求範囲の下限30℃/ｓを下回るときは、HAZ軟化硬度比は0.79〜0.82であるのに対し、加熱速度が本発明の請求範囲である30℃/ｓ以上のときは0.95以上と高くなった。
【００７１】
以上のように、焼鈍時の加熱速度を30℃/ｓ以上とすることで、低降伏でかつ高強度、さらに延性、穴拡げ性に優れ、溶接時にHAZ軟化のし難い鋼板を製造することができる。
【００７２】
さらに、図6〜10に、鋼種Aに関し、熱延条件として最終仕上温度が850℃、冷却開始時間が3ｓ、降下温度が100℃、冷延条件として冷圧率が60％、焼鈍条件として加熱速度が100℃/ｓ、焼鈍温度が800℃、焼鈍時間が1ｓのとき材料特性におよぼす熱延時の巻取処理温度の影響を示す。
【００７３】
図6に降伏および引張強度におよぼす巻取処理温度の影響を示す。図に示すように、巻取処理温度が本発明の請求範囲外である650℃を上回るとき、降伏強度は470〜480MPa、引張強度は570〜580MPaであるのに対し、巻取処理温度が本発明の請求範囲である650℃以下のとき、降伏強度は380MPa以下と小さくなり、引張強度は605MPa以上と大きくなった。
【００７４】
そして、図7に降伏比（＝降伏強度／引張強度）におよぼす巻取処理温度の影響を示す。図に示すように、巻取処理温度が本発明の請求範囲外である650℃を上回るとき、降伏比は82％であるのに対し、巻取処理温度が本発明の請求範囲である650℃以下のときは63％以下と小さくなった。
【００７５】
さらに、図8に全伸びにおよぼす巻取処理温度の影響を示す。図に示すように、巻取処理温度が本発明の請求範囲外である650℃を上回るとき、全伸びは30〜32％であるのに対し、巻取処理温度が本発明の請求範囲である650℃以下のときは34％以上と大きくなった。
【００７６】
また、図9に穴拡げ率におよぼす巻取処理温度の影響を示す。図に示すように、巻取処理温度が本発明の請求範囲外である650℃を上回るとき、穴拡げ率は66〜67％であるのに対し、巻取処理温度が本発明の請求範囲である650℃以下のときは79％以上と大きくなった。
【００７７】
また、図10にHAZ軟化硬度比におよぼす巻取処理温度の影響を示す。図に示すように、巻取処理温度が本発明の請求範囲外である650℃を上回るときは、HAZ軟化硬度比は0.83〜0.84であるのに対し、巻取処理温度が本発明の請求範囲である650℃以下のときは0.94以上と大きくなった。
【００７８】
以上のように、焼鈍時の巻取処理温度を650℃以下とすることで、低降伏でかつ高強度、さらに延性、穴拡げ性に優れ、溶接時にHAZ軟化のし難い鋼板を製造することができる。
【００７９】
さらに、発明鋼板の中でも、鋼板No.15に示すように、熱延条件において、仕上最終圧延後の冷却開始時間を0.5ｓ、冷却速度を200℃/ｓ、温度降下量を150℃とすることで、同じ鋼種Aで巻取処理以降は同一条件である鋼板No.5に対し、より高強度、高延性、低降伏比、高穴拡げ率、高HAZ軟化硬度比とすることができた。
【００８０】
また、比較鋼板No.14に示すように、熱間圧延後の冷間圧延において、冷圧率を90％と、本発明の請求範囲を外れる場合は、冷圧率以外は同一条件である本発明鋼板No.5に対し全伸びが著しく低下した。
【００８１】
【発明の効果】
以上のように、本発明では、熱延巻取条件および焼鈍条件を限定することにより、熱間圧延段階では炭窒化物を極力析出させず、それに続く冷間圧延後の再結晶焼鈍過程において急速加熱をおこなうことにより、組織を飛躍的に細粒化させ、析出強化との複合作用で、大幅な強度上昇が得られるとともに、加工性も良好で、かつ、降伏応力を大幅に低下させることができる。その結果、低降伏比で、加工性、溶接性に優れた高強度冷延鋼板の製造方法が提供され、工業上有効な効果がもたらされる。
【図面の簡単な説明】
【図１】降伏、引張強度におよぼす冷圧後焼鈍時の加熱速度の影響を示す図である。
【図２】降伏比におよぼす冷圧後焼鈍時の加熱速度の影響を示す図である。
【図３】全伸びにおよぼす冷圧後焼鈍時の加熱速度の影響を示す図である。
【図４】穴拡げ率におよぼす冷圧後焼鈍時の加熱速度の影響を示す図である。
【図５】 HAZ軟化硬度比におよぼす冷圧後焼鈍時の加熱速度の影響を示す図である。
【図６】降伏、引張強度におよぼす熱間圧延後の巻取処理温度の影響を示す図である。
【図７】降伏比におよぼす熱間圧延後の巻取処理温度の影響を示す図である。
【図８】全伸びにおよぼす熱間圧延後の巻取処理温度の影響を示す図である。
【図９】穴拡げ率におよぼす熱間圧延後の巻取処理温度の影響を示す図である。
【図１０】 HAZ軟化硬度比におよぼす熱間圧延後の巻取処理温度の影響を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-strength cold-rolled steel sheet having a low yield ratio and excellent workability and weldability.
[0002]
[Prior art]
In recent years, high-strength cold-rolled steel sheets with high tensile strength have attracted attention among steel sheets used for automobile parts and home appliances. As shown in p. 41 of the 74th and 75th Nishiyama Memorial Technology Course (Showa 56, Japan Iron and Steel Institute) as a steel strengthening mechanism, (1) solid solution strengthening, (2) precipitation strengthening, Three mechanisms are known: (3) transformation strengthening, (4) grain refinement, and (5) work hardening. Of these, the strengthening by work hardening of (5) significantly deteriorates the workability of steel, and therefore, the strengthening mechanism of (1) to (4) is usually used for the production of high strength thin steel sheets. Yes.
[0003]
Here, the solid solution strengthening (1) is performed by adding a solid solution element such as Si, Mn, and P, and is an effective means for increasing the strength without deteriorating the workability. However, when Si and Mn are added in a large amount, the surface properties of the steel sheet deteriorate. That is, Si generates a red scale, and Mn generates surface defects due to slab cracking, thereby degrading the surface properties of the steel sheet. Moreover, since the solid solution amount of P is limited, it is difficult to produce a high strength material having a tensile strength exceeding 500 MPa only by a solid solution strengthening mechanism.
[0004]
(2) Precipitation strengthening is achieved by adding carbonitride-forming elements such as Ti, Nb, V, etc., and finely depositing carbonitrides. The fine precipitates act as trap sites for dislocations. Can be raised. However, in the case of precipitation strengthening, the yield strength is inevitably increased, and there is a problem that the shape freezing property at the time of press molding is inferior.
[0005]
Furthermore, when manufacturing as a cold-rolled steel sheet that is thinner than hot-rolled steel sheets and is excellent in surface roughness and thickness accuracy, a large amount of carbonitride-forming element is added after annealing. Sometimes the recrystallization temperature increases. For this reason, annealing in the austenite region is unavoidable, and in that case, the workability is significantly deteriorated due to the coarsening of the crystal grains. Therefore, particularly in the case of cold-rolled steel sheets, it is difficult to increase the strength by precipitation strengthening.
[0006]
The transformation strengthening of (3) is performed by rapid cooling from a high temperature region, and by controlling the cooling, a mixed structure of a martensite phase that is a hard transformation phase or a bainite phase and a ferrite phase that is a soft phase is formed with high strength and processing. It can be set as the steel plate which is excellent also in property. However, when manufactured as a hot dip galvanized plate, it cannot be rapidly cooled due to the manufacturing process. For this reason, it is necessary to add a large amount of quenching promoting elements such as Cr and Mo, which causes an increase in manufacturing cost.
[0007]
Furthermore, thin steel plates are often required to have weldability, and there is a problem that the hard phase formed by rapid cooling is tempered and softened by the heat effect during welding. Thus, the strengthening mechanism based on the generation of the low temperature transformation phase is disadvantageous from the viewpoint of weldability.
[0008]
The grain refinement of (4) is performed by carrying out strong hot processing and rapid cooling. Therefore, when manufacturing as a cold-rolled sheet that can be thin and is excellent in surface roughness and sheet thickness accuracy, it is difficult to make fine particles. Furthermore, fine graining can increase the tensile strength by acting as a trap site for dislocations at the grain boundary, but it also inevitably increases the yield strength, so there is a problem that the shape freezing property at the time of press molding is inferior. is there.
[0009]
As described above, in the manufacturing method using the strengthening mechanisms of (1) to (4), a thin steel plate that has low yield strength, excellent shape freezing property during press molding, good workability, and does not soften HAZ during welding. It was difficult to manufacture. Among them, the following technologies have been proposed.
[0010]
With regard to a method for producing a low yield ratio and high strength cold-rolled steel sheet excellent in workability, for example, as disclosed in Japanese Patent Publication No. 60-54373, Ti / C = 2-20 by weight percent of Ti. There is a method in which the steel added to is subjected to hot rolling and cold rolling by a normal method, followed by recrystallization annealing by rapid heating and cooling at an average cooling rate of 20 ° C./s or less.
[0011]
This method aims at fixing C with Ti in particular, and focuses on preventing solid solution C from remaining by slow cooling after recrystallization annealing. Regarding rapid heating, from the viewpoint of productivity, it is more preferable as it is more rapid. In the examples, the expression of heating to 750 ° C. within 1 minute is limited, so that it is substantially about 15-20 ° C./s. The heating rate seems to be intended.
[0012]
Regarding the manufacturing method of a high-tensile cold-rolled steel sheet with excellent stretch flangeability, for example, as disclosed in Japanese Patent No. 2688384, hot rolling and cold rolling of steel with 0.005 to 0.045 wt% of Nb added is performed. Then, there is a method of performing heating and annealing at a heating rate of 5 ° C./s or more. This method is intended to achieve fine graining by adding Nb.
[0013]
Further, a larger heating rate is advantageous for miniaturization, and therefore, it is set to 5 ° C./s or more, preferably 10 ° C./s or more, and examples of up to 20 ° C./s are shown in the examples. . At this time, the average grain size of the ferrite is reduced to 11 μm.
[0014]
Further, as a method for producing a high-strength cold-rolled steel sheet having excellent bake hardenability, for example, as disclosed in JP-A-4-365814, a steel added with at least one of Ti and Nb and Cr and Mo is used. There are methods of performing hot rolling, cold rolling and recrystallization annealing. Here, the heating rate during annealing is said to improve the workability as the (111) surface develops as the speed increases, but it is not important in the case of high-strength cold-rolled steel sheets. As about 5 to 5000 ° C./s.
[0015]
[Problems to be solved by the invention]
However, the above prior art has the following problems. Regarding the technique described in Japanese Patent Publication No. 60-54373, as with solid solution C, solid solution N, which greatly affects the yield strength, is fixed with Al. However, it is clear that Ti preferentially forms nitrides as shown in p.55 of the 74th and 75th Nishiyama Memorial Technology Course (Japan Steel Association). Accordingly, if C and N are to be fixed with Ti, a large amount of Ti needs to be added, resulting in an increase in manufacturing cost.
[0016]
With regard to the technique described in Japanese Patent No. 2688384, the increase in yield strength is inevitable as pointed out in the past in the case of increasing the strength by the method of realizing finer grain formation by adding Nb. Therefore, there exists a problem that the shape freezing property at the time of press molding worsens.
[0017]
In the technique described in Japanese Patent Laid-Open No. 4-365814, the obtained material property value also has a high yield ratio (= yield strength / tensile strength). Such a high yield ratio is considered to be the influence of Ti and Nb-based carbonitride precipitation as pointed out conventionally. Therefore, even in this method, there is a problem that the shape freezing property at the time of press molding deteriorates. Is not resolved. About the heating rate at the time of annealing, there is no description in an Example, and it is appropriate to think that rapid heating is not especially directed and is not made also as an Example.
[0018]
The present invention has a high yield ratio, which is a characteristic of precipitation-type, fine-grained high-strength steel sheets, and solves the problem of poor shape freezing properties during press molding, with a low yield ratio, and workability, It aims at providing the manufacturing method of the cold rolled steel plate with the high tensile strength which is excellent in weldability.
[0019]
[Means for Solving the Problems]
The above problems are solved by the following invention. The invention is mass%, C: 0.016-0.2%, Ti: 0.025-1%, Nb: 0.01-1.5%, V: 0.01-1% containing any one or moreAnd the rest Fe And inevitable impuritiesAfter hot rolling steel1s within 100 ℃ / s With the above cooling rate 80 Cooling over a temperature range above ℃,Winding at 650 ° C or less, cold rolling at a reduction rate of 85% or less, from 600 ° CAnnealing temperatureIt is characterized by annealing at a heating rate of 30 ° C / s or more in the temperature range up toPrecipitation type, fine grain typeIt is a manufacturing method of a high-strength cold-rolled steel sheet.
[0020]
The present invention has been made as a result of intensive studies to solve the above-described problems. In the course of research, from the viewpoint of weldability, a strengthening mechanism based on the formation of a low-temperature transformation phase is disadvantageous, and the present invention has been directed to increasing the strength by precipitation. For precipitation, carbonitride is not precipitated as much as possible in the hot rolling stage, and rapid heating is performed in the subsequent recrystallization annealing process after cold rolling, so that the structure is remarkably refined and precipitation strengthened. As a result of the combined action, it was found that a significant increase in strength was obtained, the workability was good, and the yield ratio was greatly reduced.
[0021]
In addition, in the cold-rolled steel plate which this invention makes object, the steel plate which gave surface treatments, such as a hot-dip galvanized material and an electrogalvanized material, is also included.
[0022]
First, the chemical components of the present invention will be described.
[0023]
C: 0.016-0.2%
C is an inexpensive and effective element for increasing the strength of steel. Furthermore, the addition of Ti, Nb, and V precipitates carbides finely, suppresses grain growth, and contributes to strength increase by precipitation strengthening. In order to obtain this effect, the C content is required to be 0.016%. On the other hand, addition of a large amount of C exceeding 0.2% causes an increase in the amount of pearlite, which not only deteriorates ductility and stretch flangeability but also adversely affects weldability. Therefore, the C content is specified in the range of 0.016 to 0.2%.
[0024]
Ti: 0.025 to 1%, Nb: 0.01 to 1.5%, V: 0.01 to 1%
Ti, Nb, and V are all carbonitride-forming elements, and contribute to an increase in strength by finely depositing carbonitride. In order to obtain this effect, it is necessary to contain at least one of Ti ≧ 0.025%, Nb ≧ 0.01%, and V ≧ 0.01%.
[0025]
In the case of a process where the temperature of the slab is once lowered from continuous casting and then reheated in a hot rolling furnace, especially when a large amount of Ti and Nb is added, the carbonitride is re-dissolved in the hot rolling furnace. It cannot be fully crushed and remains coarse, and does not contribute to the increase in strength. Therefore, in this case, it is preferable to add Ti and Nb at about 0.2% or less.
[0026]
In the case where the hot rolling is started directly from the continuous casting without going through the reheating process, there is no upper limit of the amount of Ti and Nb added. However, Ti, Nb, and V having a C equivalent or more do not contribute to an increase in strength but are economically disadvantageous. Therefore, the upper limits of Ti, Nb, and V are defined as 1%, 1.5%, and 1%, respectively.
[0027]
Next, the production conditions of the present invention will be described.
[0028]
Winding temperature: 650 ℃ or less
In the winding after hot rolling, when winding at a high temperature exceeding 650 ° C., Ti, Nb, V carbonitrides precipitate in the cooling process after winding. Therefore, a precipitate-free zone in the vicinity of the grain boundary, which is considered to be formed by precipitation after cold rolling and recrystallization annealing, is not formed, and as a result, yield strength increases.
[0029]
Furthermore, since this carbonitride prevents recrystallization in the recrystallization annealing process after cold rolling, it is necessary to increase the annealing temperature and time, resulting in promoting grain growth and reducing strength. End up. Therefore, it is defined that the film is wound at 650 ° C. or less after hot rolling. The lower limit of the winding temperature is not particularly specified, and the winding may be performed at room temperature from the viewpoint of material characteristics.
[0030]
Cold rolling rate: 85% or less
In cold rolling, excessive cold rolling with a cold rolling rate exceeding 85% promotes the development of a recrystallized texture that is disadvantageous for workability. Moreover, since the load of a cold rolling mill will also become high, a cold rolling rate is prescribed | regulated as 85% or less.
[0031]
Heating rate in annealing: 30 ℃ / s or more
The heating process in recrystallization annealing after cold rolling is the most important part of the present invention. When this heating rate is slow, strain recovery proceeds in the middle of heating, and when the annealing target temperature is reached, the driving force for generating recrystallization nuclei becomes small, and as a result, fine grains can be obtained. It becomes impossible.
[0032]
By making the heating rate in annealing sufficiently high and setting it to 30 ° C / s or more, the recovery of strain during heating can be suppressed, and recrystallization nuclei can be generated at a stretch at the annealing target temperature. can get. At the same time, the precipitation of carbonitride from the grain boundary after recrystallization is promoted, and a precipitate-free zone is formed in the vicinity of the grain boundary, whereby a low yield ratio steel can be obtained.
[0033]
Furthermore, at temperatures above the two-phase region, as the recrystallization of the fine ferrite grains progresses, the austenite grains become finer and the C concentration is promoted, so that it is easy to be tempered during cooling after annealing. The fact that a martensite phase is easily generated) also contributes to a lower yield ratio. Therefore, the heating rate in recrystallization annealing after cold rolling is defined as 30 ° C./s or more.
[0034]
In addition, although a heating method is not specifically limited, you may heat by induction heating, direct electricity supply, etc. Here, from the viewpoint of productivity, it is preferable to heat from room temperature to the annealing temperature at a heating rate of 30 ° C / s or more, but in the range from room temperature to 600 ° C, the strain recovery amount even at low speed heating Since it is small, it may be less than 30 ° C./s. Therefore, the heating rate of 30 ° C./s or more as referred to in the present invention is a heating rate of 30 ° C./s or more in the region from at least 600 ° C. to the annealing temperature, more specifically from 600 ° C. to the recrystallization completion temperature. It means to do.
[0035]
In the above invention, the method for producing a high-strength cold-rolled steel sheet further comprising cooling within a temperature range of 80 ° C. or more at a cooling rate of 100 ° C./s or less within 1 s after the final rolling of the hot rolling. You can also
[0036]
The present invention more reliably suppresses the precipitation of carbonitride by defining the cooling conditions after finish rolling. Cooling is performed at 80 ° C. or more at a cooling rate of 100 ° C./s or more within 1 s immediately after the end of hot final finishing rolling on the run-out table after hot rolling until winding. Thus, the cooling is started before the precipitation of carbonitride, and the temperature region where the precipitation is remarkable is rapidly cooled, thereby suppressing the precipitation of carbonitride.
[0037]
DETAILED DESCRIPTION OF THE INVENTION
In carrying out the invention, first, the steel having the above chemical components is melted. As a melting method, a normal converter method, an electric furnace method, or the like can be appropriately applied. In the invention, other elements are not particularly defined, but the following components are preferable in the production of the high-strength cold-rolled steel sheet of the invention.
[0038]
Si: preferably 2.0% or less
Since Si has the effect of strengthening the solid solution of ferrite without degrading workability and increasing the balance between strength and workability, it is preferably added according to the required strength level. However, if a large amount of Si is added, the toughness and weldability are deteriorated, so the upper limit is preferably about 2.0%.
[0039]
Furthermore, a large amount of Si addition promotes the generation of firelite on the surface of the slab during hot rolling heating, promotes the generation of a surface pattern called a so-called red scale, and when used as a hot dip galvanized steel sheet, Since non-plating defects due to Si are also induced, in the case of a steel sheet or hot dip galvanized steel sheet that requires surface properties, the upper limit is preferably about 0.5%, more preferably about 0.2%.
[0040]
Mn: preferably 2.5% or less
Mn is an element effective for increasing the strength as a solid solution strengthening, and is preferably added according to the required strength level. However, adding a large amount of Mn causes deterioration of weldability, so it is preferable that the upper limit is about 2.5%.
[0041]
P: preferably 0.1% or less
P is an element effective for increasing the strength as a solid solution strengthening. Further, in the case of Si-added steel, it is preferable to add it as necessary because it suppresses the generation of red scale. However, the addition of a large amount of P promotes segregation to the grain boundary and lowers the ductility and toughness, so the upper limit is preferably about 0.1%.
[0042]
S: preferably 0.01% or less
S significantly lowers the hot ductility, thereby inducing hot cracking and significantly deteriorating the surface properties. Furthermore, it not only contributes to strength, but also forms coarse MnS as an impurity element, and in the case of Ti-added steel, a large amount of coarse Ti-based sulfides are produced, thereby improving ductility and stretch flangeability. It is desirable to reduce as much as possible in order to reduce. Accordingly, the upper limit of S is preferably 0.01%.
[0043]
sol.Al: preferably 0.1% or less
sol.Al has the effect of reducing inclusions in steel as a deoxidizing element, but when added in a large amount, alumina inclusions increase and ductility decreases, so about 0.1% is the upper limit. Is preferable.
[0044]
N: preferably 0.01% or less
If N is added in a large amount, it may cause slab cracking during hot rolling and surface flaws may occur, so the upper limit is preferably about 0.01%.
[0045]
Cu, Ni, Cr, Mo, B: Add as necessary
Furthermore, additional elements such as Cu, Ni, Cr, Mo, and B may be added according to the required strength level. However, addition of Cu exceeding 1% tends to cause surface defects due to hot cracking. In addition, addition of Ni, Cr, and Mo exceeding 1% increases the alloy cost. For B, the effect is saturated even if added over 0.01%. Therefore, when Cu, Ni, Cr, and Mo are added, each is 1% or less, and when B is added, the content is 0.01% or less.
[0046]
The molten steel is cast into a slab, heated as it is or cooled, and hot-rolled. The hot-rolled steel sheet after finish rolling is wound at the above-described winding temperature and subjected to normal cold rolling.
[0047]
About annealing, rapid heating is performed on the above-mentioned heating conditions. Although the annealing temperature (recrystallization temperature) is not particularly defined, it is preferable to anneal each chemical component to a temperature necessary for recrystallization annealing, and within that range, it is preferable to make the temperature as low as possible. Although the annealing time is not particularly specified, the time for staying in the temperature range above the recrystallization temperature is not particularly required.
[0048]
Cooling after the annealing may be performed by cooling or rapid cooling. In particular, when it is manufactured as a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet, the thermal history in the process may be used.
[0049]
Thus, according to the present invention, it is possible to manufacture a cold-rolled steel sheet having a low yield ratio and a high tensile strength that is excellent in workability and weldability. This reason is not intended to limit the scope of the present invention, but is considered as follows.
[0050]
That is, in the recrystallization annealing process after cold rolling, when slow heating is performed, strain recovery progresses during the heating, and much of the strain accumulated by cold rolling disappears. Requires long-term holding at high temperatures. The recrystallization nuclei are also preferentially generated from the grain boundaries in the hot-rolled steel sheet stage, and the recrystallization proceeds as the nuclei grow.
[0051]
On the other hand, when rapid heating is performed, the recrystallization temperature range can be reached without recovery of strain during heating. And since the strain is immediately transferred to a high temperature region without recovery of strain, the driving force for the recrystallization nucleation becomes enormous, and not only from the grain boundary in the hot rolling stage but also from the inside of the grain instantly. Nucleation is considered to progress. It is presumed that an ultrafine structure is formed by the increase in the nucleation sites.
[0052]
Since the recrystallization process by rapid heating proceeds instantaneously, the carbonitride-forming element that is in a solid solution state in the hot rolling stage cannot be precipitated in the heating stage, but is deposited after completion of the recrystallization. As a result, the precipitation of carbonitride occurs preferentially at the recrystallized grain boundary, but the rapid heating material is an ultrafine grain and the grain boundary area is large, so that most of the precipitation occurs at the grain boundary. become.
[0053]
As a result, carbonitrides precipitated at the grain boundaries suppress grain growth, and a precipitate-free zone (PFZ) is formed near the grain boundaries. In such a composite reinforced steel with an ultrafine grain structure and precipitation, high strength is achieved, and when processing strain is applied as in press molding, strain is not generated in the precipitate-free zone near the grain boundary. It is thought that the yield strength decreases due to the concentration of slag.
[0054]
Furthermore, in the rapid heating of the two-phase region or more, as the recrystallization of the fine ferrite grains progresses, the austenite grains become finer and the C concentration is promoted extremely. Therefore, it is presumed that, when cooling after annealing, it becomes easy to be tempered (a martensite phase is easily generated), and the remaining strain in the steel sheet also contributes to a lower yield ratio. As a result, according to the present invention, a low yield ratio can be realized while achieving high strength.
[0055]
Thus, in the structure of the cold-rolled steel sheet of the present invention, a fine martensite phase is generated at the grain boundaries of the fine ferrite grains. Therefore, although it can be said to be a dual-phase steel in a strict sense, the volume ratio of the second phase is very small (several percent) and is not so large that strength can be secured, so the structure of the cold-rolled steel sheet of the present invention is substantially Can be said to be a fine ferrite structure.
[0056]
Although the fine martensite at the ferrite grain boundary affects the yield stress, it is thought that the fine martensite is not so large as to be the starting point of cracks during plastic deformation because it is fine. As a result, it is possible to prevent a decrease in stretch flangeability, which is unavoidable with a dual phase steel.
[0057]
【Example】
Examples of the present invention will be described. In addition, this invention is not limited only to these Examples.
[0058]
Steels having the components shown in Table 1 were melted in a laboratory vacuum melting furnace and once cooled to room temperature.
[0059]
[Table 1]

[0060]
Thereafter, the steel ingot was reheated at 1250 ° C. and subjected to laboratory hot rolling. After rolling, it was once cooled under various water-cooling conditions, then air-cooled to a temperature equivalent to winding, and kept in a furnace at that temperature for 1 hour, and then cooled in the furnace to obtain a heat treatment equivalent to winding. After hot rolling, after cold rolling at various cold pressure ratios, recrystallization annealing was performed under various heating conditions, and after annealing, air cooling was performed. The plate thickness after cold pressing was 1.2 mm. Table 2 shows the experimental conditions.
[0061]
[Table 2]

[0062]
Here, steel plates Nos. 4 to 7, 10 to 13, and 15 to 20 are invention steel plates. Steel plates No. 1 to No. 3 are comparative steel plates having heating rates in annealing, steel plate Nos. 8 to 9 are coiling temperatures, and No. 14 is a comparative steel plate having a cold pressure ratio outside the range of the present invention.
[0063]
Using the samples after annealing, the tensile characteristics, hole expansion characteristics (stretch flange characteristics), and weldability were investigated. Here, the weldability was evaluated by examining the hardness distribution of the welded portion by TIG bead-on welding and evaluating the hardness ratio of the HAZ softened portion to the base metal hardness (HAZ softened portion hardness / base metal portion hardness). Table 3 summarizes the characteristic values of these samples.
[0064]
[Table 3]

[0065]
Hereinafter, the relationship between manufacturing conditions and characteristic values will be described with reference to the drawings. First, the influence of the heating rate in annealing is shown in FIGS. Here, regarding steel type A, the final finishing temperature is 850 ° C. as the hot rolling condition, the cooling start time is 3 seconds, the temperature drop is 100 ° C., the winding temperature is 600 ° C., the cold rolling rate is 60%, and annealing is performed. As the conditions, when the annealing temperature is 800 ° C. and the annealing time is 1 s, the influence of the heating rate during annealing on the material properties is shown.
[0066]
Figure 1 shows the effect of heating rate on yield and tensile strength. As shown in the figure, when the heating rate falls below the lower limit of 30 ° C./s of the claimed range of the present invention, the yield strength is 490 to 500 MPa, and the tensile strength is 550 to 570 MPa, whereas the heating rate is the claimed value of the present invention. When the range was 30 ° C / s or higher, the yield strength was as low as 400 MPa or less, and the tensile strength was as high as 600 MPa or more.
[0067]
Figure 2 shows the effect of heating rate on the yield ratio (= yield strength / tensile strength). As shown in the figure, when the heating rate falls below the lower limit of 30 ° C./s of the claimed range of the present invention, the yield ratio is 86 to 88%, whereas the heating rate is 30 ° C./s of the claimed range of the present invention. When it was over s, it was as low as 63% or less.
[0068]
Further, FIG. 3 shows the effect of the heating rate on the total elongation. As shown in the figure, when the heating rate falls below 30 ° C./s, which is outside the claimed range of the present invention, the total elongation is 29-31%, whereas the heating rate is 30 ° C., which is the claimed range of the present invention. When it was more than / s, it increased to 34% or more.
[0069]
FIG. 4 shows the effect of the heating rate on the hole expansion rate. As shown in the figure, when the heating rate is below the lower limit of 30 ° C./s of the claimed range of the present invention, the hole expansion rate is 58 to 65%, whereas the heating rate is 30 ° C. which is the claimed range of the present invention. When it was more than / s, it was higher than 78%.
[0070]
FIG. 5 shows the influence of the heating rate on the HAZ softening hardness ratio. As shown in the figure, when the heating rate is below the lower limit of 30 ° C./s of the claimed range of the present invention, the HAZ softening hardness ratio is 0.79 to 0.82, whereas the heating rate is the claimed range of the present invention. When it was higher than ℃ / s, it became higher than 0.95.
[0071]
As described above, by setting the heating rate during annealing to 30 ° C / s or more, it is possible to produce a steel plate that has low yield, high strength, excellent ductility and hole expandability, and is difficult to soften HAZ during welding. it can.
[0072]
6-10, regarding steel type A, the final finishing temperature is 850 ° C as the hot rolling condition, the cooling start time is 3 seconds, the temperature drop is 100 ° C, the cold pressure rate is 60% as the cold rolling condition, and the heating is performed as the annealing condition. When the speed is 100 ° C./s, the annealing temperature is 800 ° C., and the annealing time is 1 s, the influence of the coiling temperature during hot rolling on the material properties is shown.
[0073]
Figure 6 shows the effect of coiling temperature on yield and tensile strength. As shown in the figure, when the coiling temperature exceeds 650 ° C., which is outside the scope of the present invention, the yield strength is 470 to 480 MPa and the tensile strength is 570 to 580 MPa, whereas the coiling temperature is When the temperature was 650 ° C. or less, which is the claim of the invention, the yield strength was reduced to 380 MPa or less, and the tensile strength was increased to 605 MPa or more.
[0074]
FIG. 7 shows the influence of the winding treatment temperature on the yield ratio (= yield strength / tensile strength). As shown in the figure, when the coiling temperature exceeds 650 ° C., which is outside the scope of the present invention, the yield ratio is 82%, whereas the coiling temperature is 650 ° C., which is the scope of the present invention. In the following cases, it decreased to 63% or less.
[0075]
Further, FIG. 8 shows the influence of the winding treatment temperature on the total elongation. As shown in the figure, when the coiling temperature exceeds 650 ° C., which is outside the scope of the present invention, the total elongation is 30 to 32%, whereas the coiling temperature is within the scope of the present invention. When the temperature was 650 ° C or lower, it increased to 34% or higher.
[0076]
FIG. 9 shows the influence of the winding temperature on the hole expansion rate. As shown in the figure, when the coiling temperature exceeds 650 ° C., which is outside the scope of the present invention, the hole expansion ratio is 66 to 67%, whereas the coiling temperature is within the scope of the present invention. When the temperature was below 650 ° C, it increased to 79% or more.
[0077]
FIG. 10 shows the influence of the winding treatment temperature on the HAZ softening hardness ratio. As shown in the figure, when the coiling temperature exceeds 650 ° C., which is outside the scope of the present invention, the HAZ softening hardness ratio is 0.83 to 0.84, whereas the coiling temperature is within the scope of the present invention. When the temperature was 650 ° C. or lower, it became 0.94 or higher.
[0078]
As described above, by setting the coiling temperature during annealing to 650 ° C or less, it is possible to produce a steel sheet that has low yield, high strength, excellent ductility and hole expandability, and is difficult to soften HAZ during welding. it can.
[0079]
Furthermore, among the invented steel plates, as shown in steel plate No. 15, under the hot rolling conditions, the cooling start time after finishing final rolling is 0.5 s, the cooling rate is 200 ° C./s, and the temperature drop is 150 ° C. Thus, with the same steel type A, it was possible to achieve higher strength, higher ductility, lower yield ratio, higher hole expansion ratio, and higher HAZ softening hardness ratio than steel plate No. 5, which had the same conditions after the winding treatment.
[0080]
Further, as shown in comparative steel plate No. 14, in the cold rolling after hot rolling, the cold pressure ratio is 90%, and the present conditions are the same except for the cold pressure ratio when it is outside the scope of the present invention. The total elongation was significantly lower than that of the inventive steel plate No. 5.
[0081]
【The invention's effect】
As described above, in the present invention, by limiting the hot rolling winding condition and the annealing condition, carbonitride is not precipitated as much as possible in the hot rolling stage, and is rapidly changed in the subsequent recrystallization annealing process after cold rolling. By heating, the structure can be drastically refined and combined with precipitation strengthening can provide a significant increase in strength, good workability, and a significant reduction in yield stress. it can. As a result, a method for producing a high-strength cold-rolled steel sheet having a low yield ratio and excellent workability and weldability is provided, and an industrially effective effect is brought about.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of the heating rate during annealing after cold pressure on the yield and tensile strength.
FIG. 2 is a diagram showing the influence of the heating rate during post-cold annealing on the yield ratio.
FIG. 3 is a diagram showing the influence of the heating rate during post-cooling annealing on the total elongation.
FIG. 4 is a diagram showing the influence of the heating rate during post-cold annealing on the hole expansion rate.
FIG. 5 is a graph showing the influence of the heating rate during post-cold annealing on the HAZ softening hardness ratio.
FIG. 6 is a diagram showing the influence of the coiling temperature after hot rolling on the yield and tensile strength.
FIG. 7 is a graph showing the influence of the coiling temperature after hot rolling on the yield ratio.
FIG. 8 is a diagram showing the influence of the coiling temperature after hot rolling on the total elongation.
FIG. 9 is a diagram showing the influence of the coiling temperature after hot rolling on the hole expansion rate.
FIG. 10 is a graph showing the influence of the coiling temperature after hot rolling on the HAZ softening hardness ratio.

Claims (3)

  Contains at least one of mass%, C: 0.016-0.2%, Ti: 0.025-1%, Nb: 0.01-1.5%, V: 0.01-1%, the balance being Fe and inevitable impurities The steel consisting of is cooled over a temperature range of 80 ° C. or more at a cooling rate of 100 ° C./s or more within 1 s after hot rolling, wound up at 650 ° C. or less, and after cold rolling at a reduction rate of 85% or less, A method for producing a precipitation-type, fine-grained high-strength cold-rolled steel sheet, characterized in that annealing is performed at a heating rate of 30 ° C / s or more in a temperature range from 600 ° C to the annealing temperature.   Furthermore, use steel containing mass%, Si: 2.0% or less, Mn: 2.5% or less, P: 0.1% or less, S: 0.01% or less, sol.Al: 0.1% or less, N: 0.01% or less The manufacturing method of the precipitation type | mold and fine grain type | mold high-strength cold-rolled steel plate of Claim 1 characterized by these.   Furthermore, it is characterized by using steel containing at least one of mass%, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 1% or less, B: 0.01% or less. The manufacturing method of the precipitation type | mold and fine grain type | mold high-strength cold-rolled steel plate of Claim 1 or Claim 2.

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