WO2008093815A1

WO2008093815A1 - High tensile cold-rolled steel sheet and process for production thereor

Info

Publication number: WO2008093815A1
Application number: PCT/JP2008/051598
Authority: WO
Inventors: Kazuhiro Hanazawa; Nobuko Nakagawa; Koichiro Fujita; Eiko Yasuhara
Original assignee: Jfe Steel Corporation
Priority date: 2007-01-29
Filing date: 2008-01-25
Publication date: 2008-08-07
Also published as: KR20090098900A; JP5151504B2; KR20120055743A; JP2009167513A; CN101595235A; CN101595235B

Abstract

A high tensile cold-rolled steel sheet which has a composition containing by mass C: 0.001 to 0.05%, Si: 0.4% or below, Mn: 0.5 to 2.0%, P: 0.08% or below, S: 0.005% or below, Al: 0.05% or below, and N: 0.0080 to 0.0250% with at least 0.0050% of solid-soluted N at an N/Al ratio of 0.30 or above and an N/C ratio of 0.40 or above with the balance being Fe and unavoidable impurities and a composite structure composed of 95.0 to 99.5% (in volume fraction) of ferrite phase and 0.5 to 5.0% (in volume fraction) of a phase formed at low temperature and which exhibits a tensile strength of 340MPa or above. The steel sheet does not contain a large amount of expensive alloying elements and makes it possible to produce 340 to 440MPa grade high tensile cold-rolled steel sheets which have yield ratios of 55% or below and are excellent in press formability or hot-dip zinc-coated steel sheets easily at low costs even when the cooling rate after annealing varies.

Description

Technical field

The present invention relates to a 340 MPa class to 440 MPa class high-tensile cold-rolled steel sheet having a tensile strength of 340 MPa or more and approximately 500 MPa or less, which is suitable mainly for parts in the electrical machinery, building materials, and automobile fields, and in particular, improved formability. About. The “steel sheet” herein includes a steel band.

In general, in the electrical and automotive fields, soft copper (railed steel) with a tensile strength of 270 MPa is widely used. However, in recent years, the use of high-strength steel sheets as raw materials has also been increasing in this field. For example, in the automotive field, there is a strong demand for improvement in the fuel consumption of automobiles due to the environmental protection of the earth and review, ^; force. The application of high-strength copper plates is increasing because it is effective to reduce the thickness by applying steel plates. Furthermore, from the viewpoint of occupant protection in the event of a vehicle collision, there is a demand for improving the safety of automobile bodies, and in order to increase the strength of members, the application of high-tensile steel sheets as materials used is increasing. Recently, in the consumer electronics industry, as the competition for sales has intensified, there has been an increasing demand for lower material costs and a further reduction in freight cost. As a material, there is a strong tendency to apply high-strength steel sheets. In addition, in the can field such as battery cans and drum cans, an increase in battery capacity, an increase in pressure strength and a reduction in weight are expected, and it is considered to use high-tensile steel sheets as materials. Since many parts made of steel sheets are formed by press working, the high-tensile steel sheets used are required to have excellent press formability. Various types of multi-phase steel have already been developed as steel sheets with excellent formability. A representative example of the steel sheet having a composite structure is a steel sheet having a composite structure of soft ferrite and hard martensite. This steel sheet is said to be a steel sheet having a low yield ratio, an excellent balance of strength and ductility, and an excellent bake-hardenability. At the same time, the yield point elongation is also low, so that the occurrence of non-uniform pattern surface defects can be prevented. As a method for producing this type of steel sheet, for example, JP 2000-109966 A discloses that C: 0.005 to 0.15%, Mn: 0.3 to 3.0%, Mo: 0.05 to 1. 0%, or even Cr: 0. 05~ 1. the plated for mother board containing 0%, a _Cl transformation point or higher Ac ₃ transformation point of at least once at a temperature annealed, cooled, then a _Cl was heated to a temperature range of transformation point ~ Ac ₃ transformation point, the temperature range from the heating temperature of this to at least plated bath temperature, then cooled in response Ji was critical cooling rate or higher on the content of alloying elements, and then the molten zinc A manufacturing method for hot-dip zinc-plated high-tensile strength steel sheets with excellent workability is proposed in which plating is performed and the temperature range up to 300 ° C after plating is cooled at a critical cooling rate or higher according to the alloy element content. ing. According to the technique described in Japanese Unexamined Patent Publication No. 2000-109966, a composite structure of ferrite and martensite is formed, has a low yield ratio of 55% or less, and exhibits excellent workability. It is said that it will be possible to produce high-tensile strength steel sheets with excellent melting and resistance to powdering. However, in the technique described in Japanese Patent Application Laid-Open No. 2000-109966, in order to obtain a stable composite structure, an alloy element such as Mo, Mn, or Cr that greatly contributes to improving hardenability is used. There was a problem that it was necessary to add a large amount, and the production cost of this type of steel sheet increased, which was economically disadvantageous. In order to solve this problem, Japanese Patent Application Laid-Open No. 2002-20834 contains N: 0.03 to 2.0%, and the volume fraction of martensi potato is 3 to 30%. A low-yield-ratio high-strength steel sheet with excellent strength is proposed. In the technique described in Japanese Patent Application Laid-Open No. 2002-20834, the above-described N content is reduced by treating (annealing) in an atmosphere containing 2% or more of ammonia in a temperature range of 550 to 800 ° C. after hot rolling. It can be secured. N is effective in stabilizing the austenite phase. With the technique described in Japanese Patent Application Laid-Open No. 2002-20834, the copper plate structure can be martensite without adding a large amount of alloy elements such as Mo, Mn, and Cr. It is said that it can be a composite structure containing phases. JP 2002-146478 A discloses that C: 0.025 to 0.15%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.08% or less, S: 0.02% or less, A1: 0.02% or less, N: 0.0050 to 0.0250%, NZA1 is 0.30 or more, and N as a solid solution state is more than 0.0010%, A high-tensile cold-rolled steel sheet having a structure containing a martensite phase with an area ratio of 80% or more in the ferrite phase and an area ratio of 5% or more as the second phase has been proposed. The steel sheet described in Japanese Patent Application Laid-Open No. 2002-146478 has a tensile strength of 440 MPa or higher, a high r value (r value = Lankford value), excellent strain age hardening, and non-aging at room temperature. It is going to have. However, in the technique described in Japanese Patent Application Laid-Open No. 2002-20834, annealing in an atmosphere containing ammonia is expensive, and in order to perform annealing in an atmosphere containing ammonia, the existing annealing equipment There was a problem with the economy, such as requiring large-scale remodeling. Furthermore, as is clear from the examples, the technique described in Japanese Patent Application Laid-Open No. 2002-146478 is generally intended for the region where the tensile strength exceeds 500 MPa, and the strength is too high for electric machines and cans. Therefore, there is a problem that it is difficult to secure desired workability (particularly ductility). In addition, there was a problem in the manufacturing cost for carrying out box annealing after cold rolling and then continuous annealing. Disclosure of the invention

The present invention advantageously solves the problems of the prior art, has a composition equivalent to that of mild steel with the content of expensive alloy elements reduced as much as possible, and has excellent press-formability. 340 MPa class to 440 MPa class high-tensile cooling It aims at providing a rolled steel plate and its manufacturing method. In order to achieve the above-mentioned object, the present inventors have made a steel sheet structure composed of a soft ferrite phase and a hard martensite phase without containing a large amount of expensive alloy elements such as Cr and Mo. In view of the need to secure a stable organization, we conducted extensive research on various factors that influence the formation of such a composite tissue. As a result, focusing on N, which has not been actively used so far, by adjusting the N content to an appropriate range, annealing can be performed without containing a large amount of expensive alloy elements such as Cr and Mo. Even when the cooling rate is low and the composite structure is difficult to obtain, the composite structure as described above can be stably formed, and a low yield ratio of 55% or less can be stably secured to improve the moldability. We have found that we can make an excellent high-tensile cold-rolled copper sheet. First, the experimental results on which the present invention is based will be described.

Mass ^{_{0/0, 0. 011% C-}} 0. 001% N- 1. 5% Mn system (C system) composition, and 0. 001% C one 0 · 013% Ν- 1. 5% Mn -based ( N type) Two types of copper were melted to form a sheet bar. These sheet bars were heated to 1200 ° C, soaked, and finished into a hot rolled sheet with a thickness of 4.0 mm by hot rolling of 3 passes adjusted to a finish rolling finish temperature of 900 ° C. . The obtained hot-rolled sheet was subjected to heat treatment (600 ^ X11) corresponding to coil cutting after finishing rolling. Next, cold rolling with a rolling reduction of 75% was performed to obtain a cold rolled sheet having a thickness of 1.0 mm. Next, these cold-rolled plates were heated to 800 ° C, which is a two-phase region, and held for 60 s, and then averaged up to 300 ° C at various cooling rates in the range of 8 to 500 ° CZ s. A cooling annealing treatment was performed, followed by pickling treatment. In this experiment, temper rolling (skin pass mill) was not performed. From the obtained steel plate, a half-size tensile test piece with a parallel part length of JIS (Japanese Industrial Standards) No. 13 B test piece as 1 Z 2 with the direction perpendicular to the rolling direction as the length direction of the test piece. The samples were collected and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to determine the tensile properties. Figure 1 shows the results obtained. The yield ratio (%) was defined as (yield strength Z tensile strength) X 100 (%). Yield strength was defined as the falling yield point when a yield point was observed, and 0.2% resistance ΓΟΟΙ stress; when the yield point was not known. From Fig. 1, it can be seen that the N-based composition tends to have a lower yield strength (yield point), yield elongation, and lower yield ratio over a wider range of cooling rates after annealing than the C-based composition. I found out. In other words, by adopting an N-based composition, even if the cooling rate is slower than that of a C-based composition, the steel sheet structure is stably made into a composite structure composed of a soft ferrite phase and an appropriate amount of hard phase. And excellent moldability can be stably secured. The details of this cause are not known so far, but the present inventors consider as follows. When cooling a cold-rolled sheet in an annealing process that heats and cools a cold-rolled sheet in a two-phase region (ferrite + austenite) or a low-temperature region in the austenite region, the austenite phase easily decomposes into a ferrite phase and coarse cementite. Low-temperature tran sformation phase such as bainitic ferrite and manoletite is difficult to form. On the other hand, with the N-based composition, it is presumed that the austenite phase is stable up to low temperatures because it is difficult to form nitrides during cooling of the annealing treatment, and that low-temperature-generated phases such as marineite are more likely to be formed. Is done. Such stabilization of the austenite phase by N was found to become remarkable in the range where the NZC value was 0.40 or more by further experiments. The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.

(1) the mass ^{_{0/0, C:. 0.}} 001~0 05%, Si: 0. 4% or less, Mn:. 0. 5~2 0% , P: 0. 08% or less, S: 0 005% or less, A1: 0. 05% or less, N: 0. 0080 ～ 0.0 250% included, N in a solid solution state is 0.0050% or more, NZA1 is 0.30 or more, N, C Is 0.44 or more, the balance is composed of Fe and inevitable impurities, and the structure is 95.0 to 99.5% ferrite phase by volume and 0.5 to 5 by volume A high-tensile cold-rolled steel sheet having a tensile strength of 340 MPa or more, characterized by being a composite structure having a low-temperature generation phase of 0%.

(2) In (1), the steel sheet has a high tension cooling layer characterized by having any one of a hot-dip zinc plating layer, an alloyed hot-dip zinc plating layer, and an electrogalvanizing layer as a plating layer. Rolled steel sheet.

(3) In manufacturing a cold-rolled steel sheet by sequentially performing a hot rolling process, a cold rolling process, and an annealing process on a steel material, the steel material is, in mass%, C: 0.001 to 0.05%, Si: 0.4% or less, Mn: 0.5—2.0%, P: 0.08% or less, S: 0.005% or less, A1: 0.05% or less, N A steel material having a composition comprising 0.0.0080 to 0.0250 ^ο / _ο , ≧ 30.30, NZC ≧ 0.40, and the balance consisting of Fe and inevitable impurities. In the process, the steel material is heated to a heating temperature of 1000 or more and roughly rolled into a sheet bar, and then the sheet bar is subjected to finish rolling with a finish rolling exit temperature of 800 ° C or more. A temperature of 750 ° C. or lower to obtain a hot rolled sheet, and the cold rolling process is a process of subjecting the hot rolled sheet to pickling and cold rolling to obtain a cold rolled sheet, and the annealing The process is to remove the cold rolled sheet A _This is a process of heating to a temperature in the range from the _Cl transformation point to (Ac ₃ transformation point + 50 ° C) and then cooling to 350 or below at an average cooling rate of 5 ° C / s or higher. A method for producing a high-tensile cold-rolled steel sheet having a tensile strength of 340 MPa or more. (4) The method for producing a high-tensile cold-rolled steel sheet according to (3), wherein an electric dumbbelling process step of forming an electrogalvanized layer on the steel sheet surface is performed subsequent to the annealing process.

(5) In manufacturing a cold-rolled copper sheet by sequentially performing a hot rolling process, a cold rolling process, and an annealing and hot-dip galvanizing process on a copper material, the copper material is added in mass%. ,

C: 0.001 to 0.05%, Si: 0.4% or less, Mn: 0.5 to 2.0%, P: 0.08% or less, S: 0.005% or less, A1: 0 0.5% or less, N: 0.0080 to 0.0250%, N / A1 is 0.30 or more, N / C is 0.40 or more, and the balance is composed of Fe and inevitable impurities. In the hot rolling step, the copper material is heated to a heating temperature of 1000 ° C. or higher, and is roughly compressed to form a sheet bar, and then the finish rolling exit temperature is applied to the sheet bar. It is a process in which finish rolling is performed to 800 ° C or higher, and a hot rolling sheet is formed at a cutting temperature of 750 ° C or lower, and the cold rolling process includes pickling and cold rolling on the hot rolled sheet. After the annealing and molten zinc plating process, the cold-rolled sheet is heated to a temperature in the range of Ac, transformation point to (Ac ₃ transformation point + 50 ° C). Cool down to 500 ° C or less at an average cooling rate of 5 ° CZ s or more Next, after applying the hot dip galvanizing process to form a molten dumbbell layer on the surface of the copper plate, it is cooled to 350 ° C or lower at an average cooling rate of 5 ° CZ s or higher. ^^ A method for producing a high-tensile cold-rolled steel sheet with a tensile strength of 340 MPa or more, characterized by a mating process.

(6) In manufacturing a cold-rolled steel sheet by sequentially performing a hot rolling process, a cold rolling process, and an annealing-alloyed molten zinc plating process on a steel material, the steel material is C: 0.001 to 0.05%, Si: 0.4% or less, Mn: 0.5 to 2.0%, P: 0.08% or less,

S: 0.005% or less, A1: 0.05% or less, N: 0.0080 to 0.0250% included, NZAI is 0.30 or more, N / C is 0.40 or more, and the remainder A steel material having a composition comprising Fe and unavoidable impurities, and the hot rolling step heats the steel material to a heating temperature of 1000 ° C. or higher, roughly rolls it into a sheet bar, and then the sheet. Finish rolling the bar to finish rolling with an exit temperature of 800 ° C or higher, rolling the bar at a winding temperature of 750 ° C or lower to form a hot-rolled sheet, and the cold rolling step includes the hot rolling Pickling and cold rolling on plate To form a cold-rolled sheet, wherein the annealing and alloying hot-dip galvanizing process heats the cold-rolled sheet to a temperature in the range of A _Cl transformation point to (Ac ₃ transformation point + 50 ° C). After cooling to 500 ° C or less at an average cooling rate of 5 ° C / s or more, and applying a hot dip galvanizing treatment to form a hot dip galvanized layer on the steel sheet surface, the hot dip galvanized layer An alloying treatment with an alloyed hot-dip zinc plating layer followed by an annealing-alloying hot-dip zinc soldering process with an average cooling rate of 5 Z s or higher and cooling to a temperature range of 350 ° C or lower A method for producing high-tensile cold-rolled steel sheets with a tensile strength of 340 MPa or more.

Brief Description of Drawings

Figure 1 is a graph showing the effect of steel sheet composition on the relationship between the cooling rate after annealing and the tensile properties. Mode for Carrying Out the Invention The steel sheet of the present invention is a high-tensile cold-rolled steel sheet having a tensile strength of 340 MPa to approximately 440 MPa, and a strength of 340 MPa to 440 MPa.

First, the reasons for limiting the composition of the steel sheet of the present invention will be described. Hereinafter, mass% in the composition is simply referred to as%.

C: 0.001 to 0.05%

C is a strengthening element that increases the strength of the steel sheet, and also has an action of concentrating in the austenite phase and stabilizing the austenite phase, and is an important element in the present invention. In order to obtain such an effect, it is necessary to contain 0.001% or more. If C is made less than 0.001%, it takes a long time for decarburization, resulting in high costs. On the other hand, if the content exceeds 0.05%, the amount of low-temperature formation phase is increased, the strength becomes too high, and the desired strength and ductility cannot be ensured. like this Therefore, C is limited to the range of 0.001 to 0.05%. In addition, Preferably it is 0.008 to 0.04%, More preferably, it is 0.01 to 0.035%, More preferably, it is 0.01 to 0.029%.

Si: 0.4% or less

Si is a useful strengthening element that can increase the strength of steel sheets without lowering the ductility of copper.In addition, Si suppresses the formation of carbides during the annealing process and improves the stability of the untransformed austenite phase. Have the effect of In order to obtain such an effect, the content is preferably 0.001% or more, and more preferably 0.01% or more. On the other hand, a content exceeding 0.4% adversely affects surface aesthetics such as surface properties and chemical conversion treatment properties, and performs pickling treatment for a long time to ensure surface aesthetics. It will be necessary, leading to high manufacturing costs. For these reasons, Si was limited to 0.4% or less. It should be noted that the content is preferably 0.3% or less for usages that require a more beautiful surface.

Mn: 0.5-2. 0%

Mn is an element that improves hardenability and contributes significantly to increasing the strength of the steel sheet, and also has the effect of concentrating to austenite and contributing to stabilization of the austenite phase. Mn is an effective element that binds to S and prevents hot tearing caused by S, and is preferably contained according to the amount of S contained. In order to obtain such an effect, it is necessary to contain 0.5% or more. On the other hand, if the content exceeds 2.0%, the above-described effects are saturated, and the workability and spot weldability are significantly reduced. For this reason, Mn is limited to the range of 0.5 to 2.0%. In applications where excellent formability is required, it is preferably 1.8% or less.

P: 0.08% or less

P is an element that has the effect of strengthening steel and can be contained in an amount of 0.005% or more depending on the desired strength. Reduces low temperature toughness. Therefore, P is limited to 0.08% or less. In applications where excellent weldability and excellent toughness are required, 0.05% or less is preferable. From the viewpoint of weldability and toughness, it is more preferably 0.03% or less.

S: 0.005% or less

S is present in the steel as sulfide inclusions and is an element that lowers the ductility and formability of the steel sheet, especially the stretch flangeability, and should be reduced as much as possible. If it is reduced below, the adverse effect on stretch flangeability will be acceptable. Therefore, S is limited to 0.005% or less. It should be noted that the content is preferably 0.003% or less for uses that require superior stretch flange formability and excellent weldability.

A1: 0.05% or less

A1 is a useful element that acts as a deoxidizer, improves the cleanliness of the steel sheet, and contributes to the refinement of the steel sheet structure. In order to obtain such an effect, the content is preferably 0.001% or more, more preferably 0.005% or more. However, if the content exceeds 0.05%, the surface properties of the steel sheet are deteriorated. At the same time, it leads to a significant decrease in the amount of solid solution N, and the amount of hard low-temperature formation phase formed through stabilization of the austenite phase by solid solution N decreases, making it difficult to form the desired composite structure. It becomes. For this reason, in the present invention, A1 is limited to 0.05% or less. From the viewpoint of the stability of the material, the content is preferably 0.001 to 0.03%.

N: 0.0008 to 0.0250%

N, like C, has the effect of concentrating to the austenite phase and stabilizing the austenite phase, and is an element that effectively works to form a hard, low-temperature product phase during cooling in the annealing process. It is an important element in the present invention for exhibiting excellent press formability. N also has the effect of lowering the transformation point of steel. If you do not want to perform hot rolling that is greatly interrupted, the N additive is effective. In order to obtain such an effect, the content of 0.0008% or more is required. On the other hand, when the content exceeds 0.025%, the occurrence rate of internal defects in the steel sheet increases, and the occurrence of cracks during continuous forging becomes prominent. For this reason, N is limited to a range of 0.0008 to 0.0250%. From the viewpoint of improving the stability of the material and the yield in consideration of the entire manufacturing process, the content is preferably set to 0.0100 to 0.0180%. This N content has no adverse effect on the weldability of the copper plate. Solid solution N: 0.0050% or more

In order to stabilize the austenite phase, form a desired amount of hard low-temperature formation phase, and stably secure the desired composite structure, it is necessary to secure a solid solution state of 0.0050% or more. It is essential. In addition, when the cooling rate during the cooling of the annealing process is slow and it is necessary to secure a stable and hard low-temperature formation phase, the N amount in the solid solution state is preferably set to 0.0008% or more. . The upper limit may be the N amount at the solid solubility limit, but 0.020% is sufficient. “Solid-state N” here refers to the value obtained by subtracting the amount of precipitated N from the total amount of N in steel. The “precipitated N content” here refers to the chemical analysis of the N content in the residue extracted from the sample collected from the target copper plate by the controlled potential electrolysis method in the acetylacetone-based electrolyte. The amount of N determined by

N / A1: 0.30 or more

In the present invention in which a steel sheet having a desired composite structure is obtained through stabilization of the austenite phase by N, in order to secure N in a solid solution state of a predetermined amount or more, the content of A1 having an action of fixing N is set. It is important to limit to the predetermined range in relation to the N content. N is all N and A1 is all A1. In the present invention, the ratio of N content to A1 content, NZA1, is adjusted to 0.30 or more within the above range of N and A1. As a result, it was confirmed that N in a solid solution state could be ensured to be 0.0050% or more stably after annealing, and the target moldability could be secured. For this reason, NZA1 is 0.30 in the present invention. Limited to the above. In addition, Preferably it is 0.40 or more, More preferably, it is 0.50 or more. The upper limit is not specified, but in practice it is 4.

N / C: 0.40 or more

Both C and N are elements that stabilize the austenite phase, where N is all N and C is all C. As described above, the use of N as compared to C stabilizes the austenite phase, so that a desired amount of low-temperature product phase can be secured stably. In particular, by adjusting N C to 0.40 or more, the effect becomes remarkable. Therefore, in the present invention, NZC is limited to 0.40 or more. In addition, Preferably it is 0.50 or more. The upper limit is not specified, but it is 20 in practice. The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1% or less. It should be noted that Ca, REM. Zr, etc. can be contained as long as they are within the normal steel composition range. Next, the reasons for limiting the structure of this Tsukumei copper sheet will be explained.

The steel sheet of the present invention has a ferrite phase of 95.0 to 99.5% in volume ratio to the whole structure as the main phase, and a low temperature formation phase in volume ratio of 0.5 to 5.0% with respect to the whole structure as the second phase. It is a steel sheet with a composite structure. If the ferrite phase, which is the main phase, is less than 950% of the volume of the entire structure, it will be difficult to ensure high ductility, and the press formability will tend to decline, and for parts that require high workability. It becomes difficult to ensure the press formability required for steel sheets. On the other hand, in order to take advantage of the composite structure, the ferrite phase, which is the main phase, needs to be 99.5% or less by volume. For this reason, the ferrite phase, which is the main phase, was limited to a volume ratio in the range of 95.0 to 99.5%. For applications that require higher ductility, the ferrite phase is preferably 97.0% or more by volume. If the low temperature product phase, which is the second phase, is less than 0.5% by volume, the yield ratio is 55% or less, and high press formability cannot be secured. On the other hand, if the low temperature product phase exceeds 5.0%, the ductility is significantly reduced. For this reason, the low-temperature product phase, which is the second phase, was limited to the range of 0.5 to 5.0% by volume. In addition, for use in which even higher press formability is required, the low-temperature generation phase as the second phase is preferably 1% or more by volume. The term “low-temperature generation phase” here refers to a hard martensite phase and a Z or vinylite ferrite phase. In the steel sheet of the present invention, it is preferable to have a composite structure composed of the ferrite phase (main phase) and the low-temperature generation phase (second phase). However, in addition to the main phase and second phase described above, it is inevitable. A small amount (for example, about 2.0% or less by volume) of other phases such as pearlite phase is acceptable. In this case, the total amount of the main phase and the second phase is 98% or more by volume ratio.

In the cold-rolled steel sheet of the present invention, a plating (surface treatment) layer of any one of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer is formed on the surface of the steel sheet. Also good. That is, the cold-rolled steel sheet of the present invention may be a cold-rolled copper sheet having a zinc-plated layer on the surface, a so-called zinc-plated cold-rolled steel sheet. Below, the preferable manufacturing method of this invention steel plate is demonstrated.

The cold-rolled steel sheet of the present invention is manufactured by sequentially performing a hot rolling process, a cold rolling process, and an annealing process on a steel material (slab). The manufacturing method of the steel material is not particularly limited, but it is preferable that the molten copper having the above composition is melted by a normal melting method such as a converter and then the steel material is formed by a normal method such as a continuous forging method. Needless to say, the copper material may be manufactured by an ingot forming method or a thin slab forging method. The manufactured steel material is then subjected to a hot rolling process. Depending on the amount of heat stored in the steel material, the heat for hot rolling can be cooled to room temperature and then charged into a heating furnace for reheating, or hot strips can be heated without cooling to room temperature. Either the method of charging into a heating furnace as it is, or the direct feed rolling method in which rolling is performed immediately after performing a slight heat retention, and the direct rolling method can be applied. The direct feed rolling method is one of the useful techniques to effectively secure N in the solid solution state. The heating temperature of the steel material is preferably 1000 ° C or higher.

If the heating temperature of the copper material is less than 1000 ° C, a predetermined amount of N in the solid solution state cannot be secured as the initial state. On the other hand, the upper limit of the heating temperature is not particularly specified, but it is preferably 1280 ° C or less from the viewpoint of an increase in scale loss accompanying an increase in the oxidation weight.

The heated steel material is roughly rolled into a sheet bar. The sheet bar is then rolled into a hot rolled sheet. Finish rolling is rolling in which the finish rolling temperature is 800 ° C or higher. By setting the finish rolling exit temperature to 800 ° C or higher, a hot rolled sheet having a uniform fine structure can be obtained. If the finish rolling exit temperature is less than 800 ° C, the resulting hot-rolled sheet structure becomes non-uniform, and the non-uniform structure remains even after annealing, causing various problems during press forming (for example, cracks in the non-uniform structure part). The risk of occurrence) increases. When the finish rolling exit temperature is as low as less than 800 ° C, coarse grains are generated and various problems occur during press forming even if a high milling temperature is used to avoid the remaining of the processed structure. Will occur. For this reason, the finish rolling exit temperature was limited to 800 ° C or higher. The upper limit of the finish rolling exit temperature is not particularly limited, but is preferably 1000 ° C. or less from the viewpoint of preventing the occurrence of scale flaws. From the viewpoint of further improving the characteristics, it is preferable to set the finish rolling outlet temperature to 820 ° C or higher. After finishing rolling, the hot-rolled sheet is scraped in a coil shape, but the scraping temperature is preferably 750 ° C or lower. As the milling temperature decreases, the strength tends to increase. In order to secure the tensile strength of 340 MPa or more, which is the target strength of the steel sheet of the present invention, it is preferable to set the cutting temperature to 750 ° C. or less. The lower limit of the staking temperature is not particularly limited, but it is preferably 200 ° C. or more from the viewpoint of the shape and material uniformity of the steel sheet. When the milling temperature is less than 200 ° C, the shape of the steel sheet is noticeably disturbed, and the risk of occurrence of defects in the subsequent processes increases, and the uniformity of the material tends to decrease. In addition, it is more preferable to set the scraping temperature to 300 ° C or more for the usage in which further uniformity of material is required. The obtained hot rolled sheet is then subjected to a cold rolling process. The cold rolling process is a process in which hot-rolled sheets are pickled and cold-rolled to form cold-rolled sheets. The pickling may be any method that can remove the scale on the surface of the hot-rolled sheet, and is not particularly limited, including ordinary pickling methods. Needless to say, when the scale on the surface of the hot-rolled sheet is extremely thin, cold rolling may be performed without pickling. Further, the cold rolling conditions such as the rolling reduction are not particularly limited as long as the cold rolling can be a cold rolled sheet having a desired size and shape. In view of the flatness of the surface and the uniformity of the structure, the cold rolling reduction rate is preferably 40% or more. The resulting cold-rolled sheet is then subjected to an annealing process. In the annealing process, after the cold-rolled sheet is heated to a temperature in the range of A _Cl transformation point to (Ac ₃ transformation point + 50 ° C), the average cooling rate: 350 ° C or less with a cooling rate of 5 ° C / s or more It is set as the process cooled to the temperature range. The annealing of the cold rolled sheet is preferably performed using a continuous annealing line or a continuous molten zinc plating line. The upper limit of the cooling rate is not specified in terms of improving material properties, but a cooling rate of 50 ° C / s is realistic for ordinary cooling equipment. If the annealing temperature is less than the _ACl transformation point, a hard low-temperature phase is not formed after annealing. On the other hand, if the annealing temperature exceeds (Ac ₃ transformation point + 50 ° C) Since the stabilization of the tenite phase is diluted, it becomes difficult to stably form a predetermined amount of a hard low-temperature product phase during cooling after annealing. For this reason, it is preferable that the annealing temperature is a temperature in the range of A _Cl transformation point to (A c ₃ transformation point + 50 ° C.). As the transformation point, the value obtained from the thermal expansion measurement shall be used. The holding time at the annealing temperature is preferably 10 to 120 s. If the holding time at the annealing temperature is less than 10 s, recrystallization and grain growth may not proceed sufficiently, and formability will deteriorate. On the other hand, if the holding time is longer than 120 s, the economic efficiency is lowered with the increase of the annealing time.

After annealing, the average cooling rate is 5 from the above annealing temperature. It is preferable to cool to a temperature range of 350 ° C. or lower at a cooling rate of J / s. This means that the average cooling rate is at least 5 ° C / s until the steel plate temperature reaches 350 ° C. If the cooling rate is less than 5 ° CZ s, it is difficult to make the second phase the desired 'low-temperature generation phase'. If the cooling is out of the above range, the untransformed austenite decomposes into ferritic soot and cementite, making it difficult to secure the desired low-temperature formation phase. Further, in the present invention, following the above-described annealing step, an electrogalvanizing process, a molten zinc plating process, or an alloyed molten zinc plating process is performed to form a coating layer on the surface of the steel sheet. A sticking process may be performed. The conditions for the electrogalvanizing treatment, hot dip galvanizing treatment, or alloying hot dip galvanizing treatment are not particularly limited, and any conventional treatment method can be applied. In the hot dip galvanizing or galvannealed hot dip treatment, the cooling after the treatment is performed at an average cooling rate of 5 ° C / s or more on average to secure a predetermined amount of low-temperature formation phase. ° there must force ^s conduct C to below the temperature range. In addition, using the continuous hot-dip zinc plating line, annealing and hot-dip zinc plating process are performed in succession, or the hot-dip zinc plating process or annealing, hot-dip zinc plating process. Further, it is preferable to use an annealing / alloying / melting dumbbelling process step in which the alloying process is continuously performed.

When the annealing and the hot dip galvanizing treatment or the alloyed hot dip galvanizing treatment are continuously performed, the following steps are preferable. When annealing and hot dip galvanizing are performed continuously, the cold-rolled sheet is heated to a temperature in the range of A _Cl transformation point to (A c ₃ transformation point + 50 ° C), then the average cooling rate: 5 Cool at a temperature of 500 ° C or less at a cooling rate of ° CZ s or more, and then form a molten zinc adhesion layer on the steel sheet surface. After performing the molten zinc adhesion treatment, the average cooling rate is 5 ° CZ s or more. It is preferable to use an annealing-hot galvanizing treatment process that cools to a temperature range of 350 ° C or lower. Also, when annealing, hot dip galvanizing and alloying are performed continuously, the cold-rolled sheet is heated to a temperature in the range of A _Cl transformation point to (Ac ₃ transformation point + 50 ° C). Average cooling rate: After cooling to 500 ° C or less at a cooling rate of 5 ° CZ s or more, and performing a hot-dip galvanizing treatment to form a hot-dip galvanized layer on the steel sheet surface, the molten galvanized layer The alloyed molten zinc galvanized layer is then subjected to an alloying treatment, and then the average cooling rate is cooled to a temperature range of 350 ° C or lower at a cooling rate of 5 ° CZ s or higher. It is preferable to use a physical process. In any case, as usual, after heating, it is cooled to near the hot-dip zinc bath temperature, specifically below 500 ° C. The cooling rate at this time is reduced to a predetermined amount of low temperature. In order to secure the formation phase, the average cooling rate should be 5 ° CZ s or more, and after the hot dip galvanizing treatment or alloying treatment, after the alloying treatment, the average cooling to 350 ° C or less Cool at a cooling rate of at least 5 ° CZ s. When the average cooling rate is outside the above range, untransformed austenite decomposes into ferrite and cementite, making it difficult to secure a predetermined amount of low-temperature formation phase. Note that the cooling stop temperature before the hot dip galvanizing treatment is 500 ° C. or lower as described above, but more preferably, the hot dip bath temperature is + 20 ° C. or lower, and even if it is cooled to just above the plating bath temperature. It may be cooled to below the plating bath temperature (for example, (plating bath temperature-60 ° C)).

In the present invention, after the above-described annealing process, annealing-hot-dip zinc plating process, annealing-alloyed hot-dip zinc bonding process, elongation rate is adjusted for the purpose of shape correction and roughness adjustment according to conventional methods. A temper rolling of about 0.2 to 1.5% may be performed. From the viewpoint of lowering the yield ratio, it is preferable that the elongation is about 0.2 to 0.6%. Example

Molten steel having the composition shown in Table 1 was melted in a converter and slabs (steel materials) were formed by continuous forging. These slabs (steel materials) were subjected to a hot rolling process under the conditions shown in Table 2, (Hot rolled steel strip) (Thickness: 4.0 mm). Then, these hot-rolled sheets were pickled and cold-rolled by cold rolling with a rolling reduction of 80% to obtain cold-rolled sheets (cold-rolled steel strip) (sheet thickness: 0.8 mm). Next, these cold-rolled sheets are subjected to an annealing process in a continuous annealing line or a hot-dip zinc plating line, or further, a hot-dip zinc plating process, an alloyed molten dumbbell plating process, an annealing process, or an annealing-hot-dip zinc plating process. Process, Annealing-alloyed hot-dip galvanizing treatment process was performed. In the hot dip galvanizing treatment or alloying hot dip galvanizing treatment, the plating bath temperature was set to 460 ° C and the alloying treatment temperature was set to 500 ° C. Some steel sheets were subjected to an annealing process in a continuous annealing line, followed by an electrogalvanizing process in which the steel sheet was pickled and electrogalvanized in an electrogalvanizing line. The obtained steel plate (steel strip) was subjected to temper rolling with an elongation of 0.3% as shown in Table 2. For some steel sheets, the elongation was changed as shown in Table 2 in order to understand the effect of the elongation of temper rolling.

Specimens were collected from the obtained steel sheet, and microstructure observation, tensile test, plating property test, and measurement of N amount in a solid solution state were performed. The test method was as follows. (1) Tissue observation

Take a structural specimen from the obtained steel plate, and polish and corrode the cross section (C cross section) perpendicular to the rolling direction with nital, and image the microscopic structure using an optical microscope or scanning electron microscope. In addition to identifying the type of tissue using an image analyzer, the tissue fraction of each phase was determined and used as the volume fraction.

(2) Tensile test

From the obtained steel sheet, a tensile direction is taken JIS 5 No. Tensile test pieces such that the direction orthogonal to the rolling direction, and a tensile test according to the provisions of JIS Z 2241, tensile ^JP: resistance ( Yield strength YS, tensile strength TS, yield point elongation YS—EL) were determined. : Yield ratio YR (= (YS / TS) X 100%) was calculated from the obtained YS and TS.

(3) Plating property test

About the steel plate which formed the plating layer on the steel plate surface, the steel plate surface was visually observed over the entire length, the presence or absence of non-plating defects was observed, and the plating property was evaluated.

(4) Measurement of N amount in solid solution

A test piece for electrolytic extraction analysis was collected from the obtained steel plate, a residue was extracted by performing a potentiostatic electrolysis method in a cetylacetone-based electrolytic solution, and the amount of N in the residue was obtained by chemical analysis, and this was deposited. N amount. The value obtained by subtracting the amount of precipitated N from the total amount of N was taken as the amount of N in the solid solution state. The transformation point of each steel plate was obtained by thermal expansion measurement.

The results obtained are shown in Table 3.

**) Average cooling rate in temperature range from plating temperature to cooling stop temperature

***) a: Hot-dip zinc plating,: Electric zinc plating

*) F: Ferrite, BF: Paynictic ferrite, M: Martensite, B: Paynite, P: Pearlite

**) Low-temperature generation phase: payetic ferrite + martensite

Each of the inventive examples is a high-tensile cold-rolled steel sheet having a high tensile strength of 340 MPa or more and a low yield ratio of 55% or less and having excellent press formability. Also, the yield point elongation is small. In all of the inventive examples, no non-plating defect was generated, and no decrease in the tackiness was observed. In order to lower the yield ratio, it is effective to set the elongation of temper rolling to 0.2 to 0.6% (see steel plates No. 13 and No. 14). On the other hand, the comparative example outside the scope of the present invention has a high yield ratio exceeding 55%, a large yield point elongation, and a press formability is deteriorated. The range of the average r value in this application example was 0.7 to 1.2, and all were less than 1.3. In the present invention, even with such an average r value, a low yield ratio of 55% or less can be achieved stably over a wide range of annealing ^: later cooling rates as shown in FIG. There is no problem with the target formability (average r value was determined as follows: 1) From the obtained steel sheet, parallel to the rolling direction (0 °), 45 °, 90 ° Collect JIS No. 5 tensile test specimens with the direction as the pull direction. 2) Measure r value in each direction according to JIS Z 2254. ③ Calculate the average r value from the following formula: Average] ^ straight = (r. + 2 r ₄₅ + r ₉₀ ) / 4, where r ₀ is 0 °, r ₄₅ is 45 °, r ₉₀ is 90 ° r value. ) Industrial applicability

According to the present invention, a high-tensile cold-rolled steel sheet having a yield ratio of 55% or less and excellent press formability, which does not contain a large amount of expensive alloying elements, that is, a tensile strength of Cold rolled copper sheet of 340MPa or more and roughly 500MPa or less can be manufactured easily and inexpensively, and it has a remarkable industrial effect.

Claims

The scope of the claims

1. By mass%

C: 0.001-0.05%, Si: 0.4% or less,

Mn: 0.5 to 2.0%, P: 0.08% or less,

S: 0.005% or less, A1: 0.05% or less,

N: 0.0080 to 0.0250%

And N in a solid solution state is 0.0050% or more, NZA1 is 0.30 or more, N, C is 0.40 or more, the balance is composed of Fe and inevitable impurities, and the structure has a volume ratio of 95.0 A high-tensile cold-rolled steel sheet having a tensile strength of 340 MPa or more, characterized by a composite structure having a ferrite phase of ˜99.5% and a low-temperature formation phase of 0.5 to 5.0% by volume.

2. The high tension according to claim 1, which has any one of a hot-dip zinc plating layer, an alloyed hot-dip zinc plating layer, and an electrogalvanizing layer as a plating layer on the surface of the steel sheet. Cold rolled steel sheet.

3. In manufacturing a cold-rolled steel sheet by sequentially performing a hot rolling process, a cold rolling process, and an annealing process on a steel material,

% By mass

C: 0.001 to 0.05%, Si: 0.4% or less,

Mn: 0.5-2.0%, P: 0.08% or less,

S: 0.005% or less, A1: 0.05% or less,

N: 0.0080 to 0.0250%

, N / A1 is 0.30 or more, NZC is 0.40 or more, and the balance is Fe and a composition consisting of unavoidable impurities, and the hot rolling process comprises 1000 steel After heating to a heating temperature of ° C or higher and rough rolling into a sheet bar, the The sheet bar is subjected to a finish rolling at a finish rolling outlet temperature of 800 ° C. or higher, and a scraping temperature is set to 750 ° C. or lower to form a hot rolled sheet, the cold rolling step comprising: The hot-rolled sheet is pickled and cold-rolled to form a cold-rolled sheet, and the annealing step is performed at a temperature in the range of A _Cl transformation point to (Ac ₃ transformation point + 50 ° C). The method of manufacturing a high-tensile cold-rolled steel sheet with a tensile strength of 340 MPa or more, characterized in that it is cooled to a temperature range of 350 or less at an average cooling rate of 5 ° C / s or more after .

4. The method for producing a high-tensile cold-rolled steel sheet according to claim 3, wherein an electric zinc plating process for forming an electrozinc plating layer on the steel sheet surface is performed subsequent to the annealing process. .

5. When manufacturing a cold-rolled copper sheet by sequentially performing a hot rolling process, a cold rolling process, and an annealing and hot-dip zinc plating process on a steel material,

% By mass

C: 0.001—0.05%, Si: 0.4% or less,

Mn: 0.5 to 2.0%, P: 0.08% or less,

S: 0.005% or less, A1: 0.05% or less,

N: 0.0008 to 0.0250%

NZA1 is 0.30 or more, NZC is 0.40 or more, and the balance is Fe and an unavoidable impurity composition, and the hot rolling step is performed by After heating to a heating temperature of C or higher and rough rolling into a sheet bar, the sheet par is subjected to finish rolling with a finish rolling exit temperature of 800 ° C or higher, and the winding temperature is 750 ° C or lower. A hot-rolled sheet, the cold-rolling process is a process of subjecting the hot-rolled sheet to pickling and cold-rolling to form a cold-rolled sheet, and the annealing and hot-dip galvanizing process includes After the cold-rolled sheet is heated to a temperature in the range of A _Cl transformation point to (Ac ₃ transformation point + 50 ° C), it is cooled to 500 ° C or less at an average cooling rate of 5 ° CZ s or more. After applying the hot dip galvanizing process to form a hot dip galvanized layer on the steel sheet surface, Annealing that cools to a temperature range of 350 ° C or lower at an average cooling rate of 5 ° CZ s or higher. Production method.

6. In manufacturing a cold-rolled steel sheet by sequentially performing a hot rolling process, a cold rolling process, and an annealing-alloyed molten zinc plating process on a steel material,

% By mass

C: 0.001 to 0.05%, Si: 0.4% or less,

Mn: 0.5 to 2.0%, P: 0.08% or less,

S: 0.005% or less, A1: 0.05% or less,

N: 0.0008 to 0.0250%:

And N / A1 is 0.30 or more, NZ C is 0.40 or more, and the balance is Fe and an inevitable impurity composition, and the hot rolling step includes the steel material After heating to 1000 ° C or higher and extending it into a sheet bar, the sheet bar is subjected to finish rolling with a finish rolling exit temperature of 800 ° C or higher, and the milling temperature is 750 ° C or lower The cold rolling step is a step of subjecting the hot rolled plate to pickling and cold rolling to form a cold rolled plate, and the annealing-alloying-melting hot metal plating The processing step is to heat the cold-rolled sheet to a temperature in the range of Ac / transformation point to (Ac ₃ transformation point +50), and then to an average cooling rate of 5 ° C / s or more to 500 ° C or less After cooling and applying a molten zinc plating process to form a molten zinc plating layer on the surface of the copper plate, the molten zinc plating layer is alloyed. It is characterized by an alloying treatment for forming a hot-dip zinc plating layer, followed by an annealing process for cooling to a temperature range of 350 or lower at an average cooling rate of 5 ° CZ s or higher. A high-tensile cold-rolled steel sheet with a tensile strength of 340 MPa or more.