JP2009013488A - High strength cold-rolled steel and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength cold-rolled steel excellent in press molding and to provide a manufacturing method thereof. <P>SOLUTION: A steel raw material, which contains C, Mn, Al, N and B by mass% and has a composition satisfying (Mn+1,300×B)≥2.0 (where Mn, B each denotes a contained amount (by mass%)), is heated to 1,000°C or more and subjected to finishing rolling at a finishing-rolling-out-side temperature of 800°C or more, then wound at 750°C or less to obtain a hot rolled plate. After that, a cold rolling process and an annealing process are performed in which a cold-rolled steel is heated at a temperature between (an Ac<SB>1</SB>point) and (an Ac<SB>3</SB>point plus 50°C) and cooled to 350°C or lower at an mean cooling rate of 5°C/s or more. As a result, a complex composition is produced which includes a ferrite phase of 95.0 to 99.5% by volume rate and a low-temperature production phase of 0.5 to 5.0% by volume rate. Thus, the cold-rolled steel obtained has a low yield ratio of 55% or less, a strength-ductility balance of 16,000 MPa% or more, and a strength-hole expanding ratio balance of 38,000 MPa% or more. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention mainly relates to a high-tensile cold-rolled steel sheet having a tensile strength of 340 MPa or more and generally 500 MPa or less, and a method for producing the same, which is suitable mainly for parts in the fields of electric appliances, building materials, and automobiles. The present invention relates to a high-tensile steel plate and a method for producing the same. Here, the “steel plate” includes a steel plate and a steel strip.

  In general, in the electric field and further in the automobile field, a soft steel sheet having a tensile strength of 270 MPa is widely used. However, in recent years, the use of high-tensile steel sheets as raw materials has also been increasing in this field. For example, in the automotive field, there is a strong demand for improving the fuel efficiency of automobiles from the viewpoint of protecting the global environment, and in order to reduce the weight of members, it is effective to apply a high-strength steel sheet to reduce the thickness. The application of high strength steel sheets is increasing. Further, 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 plates as materials for use is increasing. Recently, in the field of home appliances, as the competition for sales has intensified, there has been an increasing demand for cost reduction of materials, and there is also a demand for reduction in transportation costs. The tendency to apply is getting stronger. Further, 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 a high-tensile steel plate as a material.

  However, since many parts made of steel plate are formed by press working, the high-tensile steel plate used is required to have excellent press formability. As a steel sheet excellent in formability, various composite structure steel sheets have already been developed. A representative example of a composite steel sheet is a steel sheet having a composite structure of soft ferrite and hard martensite, but this type of steel sheet produced by gas jet cooling after continuous annealing has a low yield ratio and strength. -It is said that it is a steel plate having excellent ductility balance and excellent seizure curability. At the same time, since the yield point elongation is also low, the occurrence of surface defects on a non-uniform pattern can be prevented. In particular, with the recent expansion of the application of high-strength steel sheets, the demand for press formability has become higher, the ductility normally required is high, and not only the yield ratio YR is low, but also high hole expandability is required. There are many cases.

As a method for producing this type of steel sheet, although it is a plated steel sheet, for example, Patent Document 1 discloses that C: 0.005 to 0.15%, Mn: 0.3 to 3.0%, Mo: 0.05 to 1.0%, or Cr: 0.05 to 1.0. Is annealed at least once at a temperature not lower than the Ac ₁ transformation point and not higher than the Ac ₃ transformation point, cooled, and then heated to a temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point. The temperature range from this heating temperature to at least the plating bath temperature is cooled at a critical cooling rate or higher according to the alloy element content, and then hot dip galvanizing is performed. There has been proposed a method for producing a hot-dip galvanized high-tensile steel sheet excellent in workability, which is cooled at a critical cooling rate or higher according to the amount. According to the technique described in Patent Document 1, a composite structure of ferrite and martensite is formed, a low yield ratio of 55% or less is exhibited, and excellent workability is exhibited. Furthermore, plating property and powdering resistance It is said that it is possible to manufacture hot-dip galvanized high-tensile steel sheets with excellent properties.

  For such a problem, Patent Document 2 proposes a low yield ratio high strength steel plate with excellent shape freezing property, containing N: 0.03 to 2.0% and martensite volume ratio of 3 to 30%. Has been. In the technique described in Patent Document 2, the above-described N content can be secured by processing (annealing) in an atmosphere containing 2% or more of ammonia in a temperature range of 550 to 800 ° C. after hot rolling. N is effective in stabilizing the austenite phase. In the technique described in Patent Document 2, a steel sheet structure is a composite structure containing a martensite phase without adding a large amount of alloy elements such as Mo, Mn, and Cr. And you can.

Further, Patent Document 3 includes, in mass%, C: 0.02 to 0.06%, Si: 0.2% or less, Mn: 1.5 to 2.5%, Cr: 0.03 to 0.5%, Mo: 0 to 0.5%, and Mn, The total amount of Cr and Mo is 1.8 to 2.5%, sol.Al: 0.01 to 0.10%, P, S, N are adjusted to an appropriate range, or a composition containing B is further used, and the structure is a composite structure. An alloyed hot-dip galvanized steel sheet using a base steel sheet characterized by the above has been proposed. The steel sheet manufactured by the technique described in Patent Document 3 has a relatively low C content, a composite content of alloy elements such as Mn, Cr or Mo, or further contains B to stabilize the austenite phase. This is an alloyed hot-dip galvanized high-tensile steel sheet that is easy to form and has excellent formability with a yield point of 300 N / m ² or less.
Japanese Unexamined Patent Publication No. 2000-109966 JP 2002-20834 A JP 2001-303184 A

  However, in the technique described in Patent Document 1, it is necessary to add a large amount of alloying elements such as Mo, Mn, and Cr, which greatly contribute to the improvement of hardenability, in order to stably obtain a composite structure. There was a problem that the manufacturing cost of this type of steel plate increased and it was economically disadvantageous. This tendency becomes remarkable in the region where the tensile strength is 440 MPa or less. This is because in such a tensile strength region, unless a large amount of Cr or Mo, which greatly contributes to the improvement of hardenability, is added, a composite structure steel plate cannot be obtained stably.

  Moreover, in the technique described in Patent Document 2, annealing in an atmosphere containing ammonia is expensive, and in order to perform annealing in an atmosphere containing ammonia, a large-scale modification of existing annealing equipment is performed. The problem of economic efficiency was left. Further, the technique described in Patent Document 3 has a problem that ductility is likely to be lowered when B is further contained in addition to a composite containing alloy elements such as Mn, Cr, or Mo. In addition, as an essential problem of the high-strength steel sheet having a composite structure, there is a problem that although the low yield ratio and the high strength and ductility balance can be obtained, the hole expandability is slightly lowered.

  The present invention advantageously solves the above-mentioned problems of the prior art, and does not actively add expensive alloy elements such as Cr and Mo, and is excellent in press formability, 340 MPa class to 440 MPa class high-tensile cold-rolled steel sheet And it aims at proposing the manufacturing method.

  In order to achieve the above-mentioned object, the present inventors do not include a large amount of expensive alloy elements such as Cr and Mo, and the steel sheet structure is composed of a composite composed of a soft ferrite phase and a hard martensite phase. In view of the need to secure a stable organization, we have intensively studied various factors that influence the formation of such a composite tissue. As a result, paying attention to B, which has not been actively used so far, by adjusting the content of B and Mn, which are elements that stabilize the austenite phase, to an appropriate range, alloys such as Cr and Mo Even without the addition of elements, the cooling rate after annealing is low, and even under conditions where it was difficult to obtain a conventional composite structure, the above-mentioned composite structure can be formed stably, and a low yield ratio of 55% or less is stable. It was found that a high-tensile cold-rolled steel sheet having excellent formability can be obtained.

First, the experimental results on which the present invention is based will be described.
In mass%, 0.02% C-0.01% Si-0.01% P-0.001% S-0.03% Al-0.004% N is the basic component, and B is 0.0001-0.0028% and Mn is 1.0-2.O%. Steel having a composition changed in the range was melted to obtain a sheet bar. These sheet bars were heated to 1250 ° C. and soaked, and then hot rolled with a thickness of 4.Omm by hot rolling of 3 passes adjusted so that the finish rolling finish temperature was 900 ° C. The obtained hot-rolled sheet was subjected to heat treatment (600 ° C. × 1 h) corresponding to the coil winding process after finishing rolling. Subsequently, cold rolling with a reduction ratio of 80% was performed to obtain a cold rolled sheet with a thickness of 0.8 mm. Next, these cold-rolled plates were heated to 780 ° C. and held for 60 s, and then cooled with gas or water or the like so that the average cooling rate to 300 ° C. was 10 ° C./s. Washed. After pickling, temper rolling was performed. The rolling reduction of temper rolling was O.5%.

  A JIS No. 5 tensile test piece was taken from the obtained steel sheet with the direction perpendicular to the rolling direction as the length direction of the test piece, and subjected to a tensile test in accordance with the provisions of JIS Z 2241. Tensile properties (yield strength) YS, tensile strength TS, elongation El), yield ratio YR (= (yield strength YS / tensile strength TS) x 100%), and strength-ductility balance TS x El The obtained results are shown in FIGS. 1 and 2 in relation to (Mn + 1300 × B).

1 and 2, even if Cr, Mo, etc. are not added (Mn + 1300 × B) is adjusted to 2.O or more, the cooling arrest after annealing is as slow as 1O ° C / s. The present inventors have found that a cold-rolled steel sheet having a low yield ratio of 55% or less and an excellent strength-ductility balance of 16000 MPa% or more can be obtained.
Further, a hole expansion test was performed on these steel plates, the hole expansion ratio λ was measured, and the strength-hole expansion ratio balance TS × λ was calculated. As a result, it was found that an excellent value of 38000 MPa% or more was exhibited. In the hole expansion test, specimens (size: thickness 0.8mm x 80mm x 80mm) are taken from these steel plates, and the initial hole diameter is 10mmφ and the die inner diameter is in accordance with the provisions of JFST1001. 10.2 mmφ, clearance was 12.5% of the plate thickness, the hole diameter when the crack penetrated the plate thickness was determined, and the hole expansion ratio λ defined by the following equation was obtained.

λ (%) = ((Dr−Do) / Do) × 100
(Where Dr: hole diameter when crack penetrates plate thickness (mm), Do: initial hole diameter (mm))
The details of the mechanism by which a cold-rolled steel sheet having such a low yield ratio, high strength-ductility balance, and high strength-hole expansion ratio balance is unknown at this stage. I think as follows.

In order to generate a bainitic ferrite phase and a martensite phase that are low-temperature generation phases (hard phases) after annealing and cooling, it is effective to improve the hardenability of the steel, and for that purpose, expensive Mo and Cr It is necessary to contain a large amount. This is because, in carbon steel, when it does not contain a hardenability-improving element, after annealing in the two-phase region (ferrite plus austenite) or the low-temperature region of the austenite region, the austenite phase decomposes and cools when cooled. This is because a coarse carbide Fe ₃ C tends to be formed, and bainitic ferrite and martensite phases are not generated. On the other hand, in the case of a composition containing appropriate amounts of B and Mn as in the present invention, both B and Mn are elements that stabilize the austenite phase. Stabilized, hard to form carbide during cooling after annealing, austenite phase is stable up to low temperature range, that is, hard second phase is formed at low temperature range, hard second phase is likely to remain, forming a predetermined composite structure It is estimated that it can be done. That is, by adjusting and containing B and Mn so that an appropriate amount, that is, (Mn + 1300 × B) value is 2.0 or more, the above-mentioned austenite phase can be effectively stabilized, and a predetermined composite It is thought that an organization can be easily formed.

In the present invention, attention has been paid to B and Mn as elements for stabilizing the austenite phase, but B is considered to have a large effect of stabilizing the austenite phase even in a small amount as compared with Mn, and the weighting of the B content compared to the Mn content. As a result of examining the experimental results, it was found that a low yield ratio and a high strength-ductility balance can be obtained by setting the (Mn + 1300B) value to 2.0 or more.
In addition, it is possible to obtain an excellent balance between strength and hole expandability by adjusting the steel sheet to contain B and Mn in appropriate amounts, that is, (Mn + 1300 × B) value is 2.0 or more. Although the mechanism is unknown, the present inventors consider as follows. That is, it is presumed that due to the effect of segregating at the grain boundaries of B as described above, the ferrite formation temperature was lowered and the ferrite phase was hardened, and the hardness difference between the ferrite phase and the martensite phase was reduced. The hole expansion ratio λ tends to increase as the hardness difference between the martensite phase and the ferrite phase decreases.

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) By mass%, C: 0.01 to 0.05%, Si: 0.4% or less, Mn: 1.0 to 2.0%, P: 0.08% or less, S: 0.005% or less, Al: 0.02 to 0.10%, N: 0.005% Hereinafter, B: 0.0006 to 0.0030% is included, and Mn and B are expressed by the following formula (1)
(Mn + 1300 × B) ≧ 2.0 (1)
(Where, Mn, B: content of each element (mass%))
The composition is composed of the balance Fe and unavoidable impurities, and the structure is composed of a ferrite phase with a volume ratio of 95.0 to 99.5% and a low-temperature generation phase with a volume ratio of 0.5 to 5.0%. High tensile cold-rolled steel sheet with a tensile strength of 340 MPa or more.
(2) A high-strength cold-rolled steel sheet having a plating layer on the steel sheet surface in (1).
(3) In manufacturing a cold-rolled steel sheet by sequentially performing a hot rolling process, a cold rolling process, and an annealing process on the steel material, the steel material is mass%, C: 0.01 to 0.05%, Si: 0.4% or less, Mn: 1.0 to 2.0%, P: 0.08% or less, S: 0.005% or less, Al: 0.02 to 0.10%, N: 0.005% or less, B: 0.0006 to 0.0030%, and Mn, B is the following formula (1)
(Mn + 1300 × B) ≧ 2.0 (1)
(Where, Mn, B: content of each element (mass%))
And a steel material having a composition comprising the balance Fe and inevitable impurities, and the hot rolling step heats the steel material to a heating temperature of 1000 ° C. After rolling into a sheet bar, the sheet bar is subjected to finish rolling at a finish rolling exit temperature of 800 ° C. or higher, and a winding temperature of 750 ° C. or lower to obtain a hot rolled sheet, during the rolling process is a process for the cold-rolled sheet subjected to cold rolling to the heat-rolled plate, wherein the annealing step, the cold rolled sheet (Ac ₁ transformation point) ~ (Ac ₃ transformation point + 50 ° C.) Of high-tensile cold-rolled steel sheet with a tensile strength of 340 MPa or more, characterized in that it is a process of cooling to a temperature range of 350 ℃ or less at an average cooling rate of 5 ℃ / s after heating to a temperature in the range Method.
(4) The method for producing a high-tensile cold-rolled steel sheet according to (3), wherein the steel sheet surface is subjected to an electrogalvanizing treatment following the annealing step.
(5) In (3), instead of the annealing step, after the cold-rolled sheet is heated to a temperature in the range of (Ac ₁ transformation point) to (Ac ₃ transformation point + 50 ° C.), the average cooling rate: 5 After cooling to 500 ° C or less at ℃ / s and applying hot dip galvanizing treatment, the average cooling rate: annealing to hot dip galvanizing treatment process to cool to a temperature range of 5 ° C / s or more and 350 ° C or less A method for producing a high-tensile cold-rolled steel sheet.
(6) In (3), instead of the annealing step, after the cold-rolled sheet is heated to a temperature in the range of (Ac ₁ transformation point) to (Ac ₃ transformation point + 50 ° C.), the average cooling rate: 5 Annealing-alloy that is cooled to 500 ° C or less at ℃ / s and galvanized, then alloyed, and then cooled to an average cooling rate of 5 ° C / s to 350 ° C A method for producing a high-tensile cold-rolled steel sheet, characterized in that the hot-dip galvanizing process is performed.

  According to the present invention, the yield ratio: 55% or less, the strength ductility balance TS × EI: 16000 MPa ·% or more, the strength hole expansion ratio, while adding a small amount of expensive alloy elements and keeping the amount of alloy elements small 340 to 440 MPa class high-strength cold rolled steel sheet with balance TS x λ: 38000 MPa ·% or more, excellent press formability and excellent workability can be manufactured easily and inexpensively. Yes, and it has a remarkable industrial effect. Further, the high-tensile cold-rolled steel sheet according to the present invention is used for relatively light processing such as being formed into a pipe by light bending or roll forming, to one subjected to relatively severe drawing. There is also an effect that it is suitable for a wide range of applications.

BEST MODE FOR CARRYING OUT THE INVENTION

The steel sheet of the present invention is a 340 MPa class to 440 MPa class high-tensile cold-rolled steel sheet having a tensile strength of 340 MPa to approximately 500 MPa.
First, the reasons for limiting the composition of the steel sheet of the present invention will be described. Hereinafter, the mass% in the composition is simply referred to as%.
C: 0.01-0.05%
C is a strengthening element that increases the strength of the steel sheet, and is an element that has an action of concentrating the austenite phase and stabilizing the austenite phase, and is one of the important elements in the present invention. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, if the content exceeds 0.05%, the amount of low-temperature generation phase is increased, the strength becomes too high, and the desired strength and ductility cannot be secured. For this reason, C was limited to the range of 0.01 to 0.05%. Preferably, it is 0.02 to 0.05% from the viewpoint of maximizing the effect of stabilizing the austenite phase by B.

Si: 0.4% or less
Si is a useful strengthening element that can increase the strength of a steel sheet without reducing the ductility of the steel. Si also suppresses the formation of carbides during the annealing process and improves the stability of the untransformed austenite phase. Have the effect of In the present invention, Si does not need to be positively added, and the content may be zero. However, in order to obtain the effects as described above, it is desirable to contain 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 it is necessary to perform pickling treatment for a long time to ensure surface aesthetics, resulting in an increase in manufacturing costs. Will be invited. For these reasons, Si is limited to 0.4% or less. In addition, it is preferably 0.3% or less for use in which a more beautiful surface is required.

Mn: 1.0-2.0%
Mn is an element that improves hardenability, greatly contributes to increasing the strength of the steel sheet through hardenability, and also has the effect of concentrating in the austenite phase and contributing to stabilization of the austenite phase. Further, Mn is an effective element that binds to S and prevents hot cracking due to S, and is preferably contained according to the amount of S contained. In order to obtain such an effect, a content of 1.0% or more is required. On the other hand, when the content exceeds 2.0%, the above-described effects are saturated, and the workability and spot weldability are significantly reduced. For this reason, Mn was limited to the range of 1.0 to 2.0%. In addition, it is preferable to set it to 1.8% or less for the usage in which excellent moldability is required.

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, but a large content exceeding 0.08% Reduces low temperature toughness. For this reason, P was limited to 0.08% or less. In addition, it is preferable to make it 0.05% or less for usages that require excellent weldability and excellent toughness. From the viewpoint of weldability and toughness, it is more preferably 0.03% or less.

S: 0.005% or less S is an element that exists as a sulfide inclusion in steel and lowers the ductility and formability of the steel sheet, particularly the stretch flange formability, and it is desirable to reduce it as much as possible. If it is reduced to 0.005% or less, the adverse effect on stretch flange formability is acceptable. For this reason, S was limited to 0.005% or less. In addition, it is preferable to make it 0.003% or less for the usage which requires more excellent stretch flange formability and excellent weldability.

Al: 0.02-0.10%
Al 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 acquire such an effect, it is desirable to contain 0.02% or more, but if it contains more than 0.10%, the surface properties of the steel sheet will be lowered. For this reason, in this invention, Al was limited to 0.02 to 0.10%. From the viewpoint of the stability of the material, the content is preferably limited to O.02 to O.08%.

N: 0.005% or less N is an element that easily binds to B and forms BN. Particularly, when it is contained in a large amount exceeding 0.005%, the solid solution B, which is important in the present invention, is remarkably reduced, and the austenite phase is not formed. Easy to stabilize. For this reason, N is preferably reduced as much as possible by setting the upper limit of the content to 0.005%. In addition, it is preferable to set it as the range of 0.001 to 0.005% from a viewpoint of avoiding the sharp increase in the refining cost accompanying N reduction.

B: 0.0006-0.0030%
B is an element that plays an important role in the present invention. Along with the optimization of the Mn and B contents described later, by containing B in an amount of 0.0006% or more, a target composite structure can be stably formed, and excellent moldability can be exhibited. Note that when the content exceeds 0.0030%, the effect is saturated and the yield is reduced due to surface defects of the slab. For this reason, B was limited to the range of 0.0006 to 0.0030%.

Furthermore, in the present invention, Mn and B are included in the above range, and the following formula (1)
(Mn + 1300 × B) ≧ 2.0 (1)
(Where, Mn, B: content of each element (mass%))
Adjusted to include Both Mn and B are important as elements that stabilize the austenite phase. If Mn and B are contained so as to satisfy the above formula (1), expensive hardenability such as Mo and Cr can be improved. A desired composite structure can be secured without containing elements, and a cold-rolled steel sheet having a low yield ratio and excellent strength-ductility balance and strength-hole expansion ratio balance can be obtained. For this reason, in this invention, it was decided to contain Mn and B so that Formula (1) may be satisfied.

The balance other than the above components is Fe and inevitable impurities. As unavoidable impurities, Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1% or less, etc. are acceptable. It should be noted that Cr, Mo, Ca, REM, Zr, and the like can be contained as long as they are within the normal steel composition range.
Next, the reason for limiting the structure of the steel sheet of the present invention will be described.

  The steel sheet of the present invention is a composite structure steel sheet having a ferrite phase with a volume ratio of 95.0 to 99.5% relative to the whole structure as a main phase and a low-temperature generation phase with a volume ratio of 0.5 to 5.0% with respect to the entire structure as a second phase. If the ferrite phase, the main phase, is less than 95% by volume with respect to the entire structure, it becomes difficult to ensure high ductility, and press formability tends to decrease, which is required as a steel sheet for parts that require high workability. It becomes difficult to ensure the press formability. On the other hand, in order to utilize the advantages of the composite structure, the ferrite phase as the main phase needs to be 99.5% or less in volume ratio. For this reason, the ferrite phase as the main phase is limited to a volume ratio of 95.0 to 99.5%. It should be noted that the ferrite phase is preferably 97% or more by volume in applications where higher ductility is required.

  If the low-temperature generation phase as the second phase is less than 0.5% by volume, high press formability, that is, a yield ratio of 55% or less cannot be ensured. On the other hand, if the low temperature generation phase exceeds 5.0%, the ductility is remarkably reduced. For this reason, the low temperature generation phase as the second phase was limited to a range of 0.5 to 5.0% by volume ratio. It should be noted that the low-temperature product phase, which is the second phase, is preferably 1% or more in volume ratio for use in which further high press formability is required. The “low temperature generation phase” referred to here is a hard martensite phase and / or bainitic ferrite phase.

In the present invention, a composite structure composed of the above-described ferrite phase (main phase) and a low-temperature generation phase (second phase) is preferable, but it is inevitably formed in addition to the above-described main phase and second phase. The inclusion of other phases such as a pearlite phase of 2% or less in volume ratio (that is, the total amount of the main phase and the second phase is 98% or more in volume ratio) is acceptable.
In the cold rolled steel sheet of the present invention, a plating (surface treatment) layer may be formed on the surface of the steel sheet. Examples of the plated layer include a hot dip galvanized layer, an alloyed hot dip galvanized layer, and an electrical galvanized layer.

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. The manufacturing method of the steel material is not particularly limited, but after the molten steel having the above composition is melted by a normal melting method such as a converter, it can be made into a steel material such as a slab by a normal method such as a continuous casting method. preferable. Needless to say, the steel material (slab) may be manufactured by an ingot-making method or a thin slab casting method.

  The manufactured steel material (slab) is then subjected to a hot rolling process. Heating for hot rolling is a method in which the steel material is once cooled to room temperature according to the amount of heat stored in the steel material and then charged into a heating furnace for reheating, or left as a hot piece without cooling to room temperature. An energy saving process such as a method of charging in a heating furnace or a direct feed rolling / direct rolling method in which rolling is performed immediately after performing a slight heat retention can be applied without any problem.

In the hot rolling step, the heating temperature of the steel material is set to 1000 ° C. or higher, rough rolled into a sheet bar, and then finish rolling to the finish rolling exit temperature: 800 ° C. or higher is applied to the sheet bar, and the winding temperature is set. : It is preferable to set it as the process made into a hot-rolled sheet at 750 degreeC or less.
Steel material heating temperature: 1000 ℃ or more.
The heating temperature of the steel material (slab) is preferably 1000 ° C. or higher from the viewpoint of preventing an increase in the rolling load during hot rolling and homogenizing the microstructure. The upper limit of the heating temperature of the steel material (slab) is not particularly limited, but is preferably 1280 ° C. or less from the viewpoint of an increase in scale loss accompanying an increase in oxidized weight.

Finishing rolling exit temperature: 800 ° C. or more By setting the finishing rolling exit temperature to 800 ° C. or more, 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 cold rolling and annealing, increasing the risk of various problems occurring during press forming. . When the finish rolling exit temperature is as low as less than 800 ° C., the risk that the processed structure remains increases. When the finish rolling exit temperature is as low as less than 800 ° C, even if a high coiling temperature is used to avoid the remaining of the processed structure, coarse grains are generated and various problems occur during press forming. It will be. 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 about 1000 ° C. or less from the viewpoint of preventing the occurrence of scale flaws. In addition, it is preferable that the finish rolling exit temperature is set to 820 ° C. or more from the viewpoint of further improving the characteristics.

Winding temperature: 750 ° C. or lower After finishing rolling, the hot-rolled sheet is wound in a coil shape, but the winding temperature is preferably 750 ° C. or lower. When the winding temperature exceeds 750 ° C., the scale loss increases. For this reason, the winding temperature is preferably limited to 750 ° C. or lower. As the winding temperature decreases, the strength increases, but the lower limit of the winding temperature is not strictly limited in terms of material. However, when the coiling temperature is below 200 ° C., the shape of the steel sheet is significantly disturbed, and the risk of causing problems in actual use increases. In addition, the uniformity of the material tends to decrease. For this reason, it is desirable to limit the winding temperature to 200 ° C. or higher. For uses where even higher material uniformity is required, the winding temperature is more preferably 300 ° C. or higher.

  The obtained hot-rolled sheet is then subjected to a cold rolling process. The cold rolling process is basically a process in which a hot rolled sheet is cold rolled to form a cold rolled sheet. In the cold rolling process, it is preferable to subject the hot-rolled sheet to pickling and then cold rolling. However, if it is in a very thin scale, it is directly cold-rolled without performing pickling. It is also possible. 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 a common pickling method. Further, it is sufficient that the cold rolling can be a cold-rolled plate having a desired size and shape, and in the present invention, the cold-rolling conditions such as the rolling reduction are not particularly limited. The cold rolling reduction ratio is preferably 40% or more from the viewpoint of surface flatness and tissue uniformity.

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 Ac ₁ transformation point to (Ac ₃ transformation point + 50 ° C.), the average cooling rate: up to a temperature range of 350 ° C. or less at a cooling rate of 5 ° C./s or more. Cool and use a cold-rolled annealed sheet. The cold-rolled sheet is preferably annealed using a continuous annealing line or a continuous hot dip galvanizing line.

When the annealing temperature is less than the Ac ₁ transformation point, a hard low-temperature transformation phase is not formed after annealing. On the other hand, when the annealing temperature exceeds (Ac ₃ transformation point + 50 ° C) and becomes high, stabilization of the austenite phase by B and Mn is diluted, and a predetermined amount of hard low-temperature transformation phase is stable during cooling after annealing. It becomes difficult to obtain. For this reason, it is preferable to limit the annealing temperature to a temperature in the range of Ac ₁ transformation point to (Ac ₃ transformation point + 50 ° C.). Here, Ac ₁ transformation point, Ac ₃ transformation point is assumed to use the value obtained from the thermal expansion measurement. Further, the holding time at the above-described annealing temperature is preferably 10 to 120 s. If the holding time is less than 10 s, recrystallization and grain growth may not proceed sufficiently, and the formability tends to deteriorate significantly.If the holding time exceeds 120 s, the cost increases due to an increase in annealing time. Inevitable.

  After annealing, it is preferable to cool from the annealing temperature to a temperature range of 350 ° C. or less at an average cooling rate of 5 ° C./s or more. When the cooling rate is less than 5 ° C./s, the second phase cannot be a desired low-temperature generation phase. When the cooling is out of the above range, the untransformed austenite is decomposed into ferrite and cementite, and a desired low-temperature generation phase cannot be secured.

  Further, in the present invention, following the annealing step, an electrogalvanizing treatment, a hot dip galvanizing treatment, or an alloyed hot dip galvanizing treatment is performed, and a plating treatment step for forming a plating layer on the surface of the steel sheet is performed. Also good. The conditions for the electrogalvanizing treatment, the hot dip galvanizing treatment, or the alloying hot dip galvanizing treatment are not particularly limited, and any conventional treatment method can be applied. In hot dip galvanizing or alloying hot dip galvanizing treatment, in order to secure a predetermined amount of low-temperature formation phase, cooling after the treatment is performed at an average cooling rate of 5 ° C./s or higher and a temperature range of 350 ° C. or lower It is necessary to do until.

Also, using a continuous hot dip galvanizing line, annealing and hot dip galvanizing treatment are carried out continuously-hot dip galvanizing treatment process, or annealing, hot dip galvanizing treatment and alloying treatment are carried out continuously- An alloying hot dip galvanizing treatment step is preferred.
In the case where annealing and hot dip galvanizing treatment or alloying hot dip galvanizing treatment is 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 Ac ₁ transformation point to (Ac ₃ transformation point + 50 ° C.), and then the average cooling rate: 5 ° C./s or more After cooling to 500 ° C or lower at a cooling rate of 5 ° C and then performing hot dip galvanizing treatment to form a hot dip galvanized layer on the steel sheet surface, the average cooling rate: 350 ° C or lower at a cooling rate of 5 ° C / s or higher It is preferable to set it as the annealing-hot-dip galvanization process process cooled to.

Further, when annealing, hot dip galvanizing treatment and alloying treatment are carried out continuously, after the cold-rolled sheet is heated to a temperature in the range of Ac ₁ transformation point to (Ac ₃ transformation point + 50 ° C.), the average cooling rate : After cooling to 500 ° C. or less at a cooling rate of 5 ° C./s or more and performing hot dip galvanizing treatment to form a hot dip galvanized layer on the steel sheet surface, the hot dip galvanized layer is made an alloyed hot dip galvanized layer An alloying treatment is performed, and then an average cooling rate: an annealing-alloying hot dip galvanizing treatment step of cooling to a temperature range of 350 ° C. or less at a cooling rate of 5 ° C./s or more is preferable.

  In any case, as usual, after heating, it is cooled to the vicinity of the hot dip galvanizing bath temperature, specifically to 500 ° C. or less. In order to ensure, the average cooling rate: 5 ° C./s or more, and after the galvanizing treatment or alloying treatment, after the alloying treatment, the average cooling rate is 5 ° C./s to a temperature range of 350 ° C. or less. Cool at the above cooling rate. When the cooling rate is out of the above range, untransformed austenite is decomposed into ferrite and cementite, and a predetermined amount of low-temperature generation phase cannot be secured.

As described above, the cooling stop temperature before the hot dip galvanizing treatment is preferably 500 ° C. or lower, more preferably the plating bath temperature + 20 ° C. or lower. You may cool to below bath temperature.
In the present invention, after the annealing step, the annealing-hot dip galvanizing treatment step, or the annealing-alloyed hot dip galvanizing treatment step, the rolling reduction is performed for the purpose of shape correction or roughness adjustment according to a conventional method. : Temper rolling of about 0.2 to 1.5% may be performed.

  Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method. Subsequently, these slabs were subjected to a hot process under the conditions shown in Table 2 to obtain hot-rolled steel strips (hot-rolled sheets) having a thickness of 4.0 mm. Then, these hot-rolled steel strips (hot-rolled sheets) are pickled and subjected to a cold rolling process in which a cold rolling of a reduction ratio of 80% is performed to obtain a 0.8 mm-thick cold-rolled steel strip (cold-rolled sheets). It was. Subsequently, these cold-rolled sheets are subjected to an annealing process in a continuous annealing line and a hot dip galvanizing line, or further subjected to a hot dip galvanizing process and an alloyed hot dip galvanizing process, an annealing process, an annealing-hot dip galvanizing process, or an annealing process. An alloying hot dip galvanizing 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 plates were subjected to an annealing process in a continuous annealing line, then pickled and subjected to an electrogalvanizing treatment in an electrogalvanizing line.

The obtained steel strip (cold rolled sheet) was further subjected to temper rolling with a rolling reduction of 0.5%. In the hot dip galvanizing treatment and alloying hot dip galvanizing treatment, the plating bath temperature was set to 460 ° C., and the alloying treatment temperature was set to 500 ° C.
A specimen was collected from the obtained steel strip and subjected to structure observation, tensile test, hole expansion test, and plating test. The test method was as follows.
(1) Microstructure observation A structural specimen is taken from the obtained steel strip, and the cross section (C cross section) perpendicular to the rolling direction is polished and corroded with nital, and then optical microscope (magnification: × 400) or scanning type A microscopic tissue was imaged using an electron microscope (magnification: × 1000), the type of tissue was identified using an image analyzer, and the tissue fraction of each phase was determined.
(2) Tensile test JIS No. 5 tensile test piece was taken from the obtained steel strip so that the tensile direction was perpendicular to the rolling direction, and the tensile test was conducted in accordance with the provisions of JIS Z 2241. Tensile properties (yield strength YS, tensile strength TS, elongation El, yield elongation YS-EL) were determined. The yield ratio YR (= (YS / TS) × 100%) was calculated from the obtained YS and TS, and the strength-ductility balance TS × El was calculated from the obtained TS and El, respectively.
(3) Hole expansion test A hole expansion test piece (size 80 mm x 80 mm x plate thickness 0.8 mm) was sampled from the obtained steel strip, and a hole expansion test was performed. The hole expansion test is conducted in accordance with the provisions of the Japan Iron and Steel Federation standard JFS T 1001, and the initial hole diameter is 10 mmφ, the die inner diameter is 10.2 mmφ, the clearance is 12.5% of the plate thickness, and the crack penetrates the plate thickness. The hole expansion ratio λ defined by the following equation was obtained from the hole diameter Dr.

λ (%) = ((Dr−Do) / Do) × l00
(Here, Dr: hole diameter when the crack penetrates the plate thickness (mm), Do: initial hole diameter (mm))
(4) Plating property test About the steel strip which formed the plating layer on the surface, the steel plate surface was visually observed over the full length, the presence or absence of the non-plating defect was observed, and the plating property was evaluated.

The transformation point of each steel strip (steel plate) was determined by thermal expansion measurement.
The obtained results are shown in Table 3.

In all of the examples of the present invention, the yield ratio is 55% or less, the strength-ductility balance TS × El is 16000 MPa% or more, and the strength-hole expansion ratio balance TS × λ is 38000 MPa% or more. It is a high-tensile cold-rolled steel sheet with a strength TS of 340MPa to 440MPa with a strength TS of 340MPa to 500MPa. On the other hand, in a comparative example outside the scope of the present invention, the moldability is deteriorated.

It is a graph which shows the relationship between a yield ratio and (Mn + 1300B). It is a graph which shows the relationship between a strength-ductility balance and (Mn + 1300B).

Claims (6)

% By mass
C: 0.01 to 0.05%, Si: 0.4% or less,
Mn: 1.0 to 2.0%, P: 0.08% or less,
S: 0.005% or less, Al: 0.02 to 0.10%,
N: 0.005% or less, B: 0.0006 to 0.0030%
Mn and B are adjusted so as to satisfy the following formula (1), the composition is composed of the remaining Fe and inevitable impurities, and the structure is a ferrite phase with a volume ratio of 95.0 to 99.5%. A high strength 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 0.5 to 5.0% in volume ratio.
Record
(Mn + 1300 × B) ≧ 2.0 (1)
Here, Mn, B: Content of each element (mass%)   2. The high-tensile cold-rolled steel sheet according to claim 1, having a plating layer on the steel sheet surface. 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: 0.01 to 0.05%, Si: 0.4% or less,
Mn: 1.0 to 2.0%, P: 0.08% or less,
S: 0.005% or less, Al: 0.02 to 0.10%,
N: 0.005% or less, B: 0.0006 to 0.0030%
And Mn and B are adjusted so as to satisfy the following formula (1), and a steel material having a composition composed of the remaining Fe and inevitable impurities,
In the hot rolling step, the steel material is heated to a temperature of 1000 ° C. or higher, roughly rolled into a sheet bar, and then finished to a finish rolling exit temperature of the sheet bar: 800 ° C. or higher. It is a process of rolling and making the coiled hot rolled sheet at a coiling temperature of 750 ° C. or less,
The cold rolling step is a step of cold rolling the hot rolled sheet to form a cold rolled sheet,
The annealing step, after heating the cold rolled sheet to a temperature range of (Ac ₁ transformation point) ~ (Ac ₃ transformation point + 50 ° C.), an average cooling rate: 5 ° C. / s or higher at 350 ° C. below the temperature A method for producing a high-strength cold-rolled steel sheet having a tensile strength of 340 MPa or more, characterized by being a step of cooling to an area.
Record
(Mn + 1300 × B) ≧ 2.0 (1)
Here, Mn, B: Content of each element (mass%)   4. The method for producing a high-tensile cold-rolled steel sheet according to claim 3, wherein the galvanizing treatment is performed on the steel sheet surface following the annealing step. Instead of the annealing step, the cold-rolled sheet is heated to a temperature in the range of (Ac ₁ transformation point) to (Ac ₃ transformation point + 50 ° C.), and then the average cooling rate is 5 ° C./s or more and 500 ° C. or less. 4. An annealing-hot dip galvanizing treatment step of cooling to a temperature range of 5 ° C./s or higher and 350 ° C. or lower after performing hot dip galvanizing treatment. The manufacturing method of the high tension cold-rolled steel sheet of description. Instead of the annealing process, after heating the cold-rolled sheet to a temperature range of (Ac ₁ transformation point) ~ (Ac ₃ transformation point + 50 ° C.), an average cooling rate: 5 ° C. / s or higher at 500 ° C. or less After cooling to galvanizing treatment, alloying treatment is performed, and then an average cooling rate: annealing to alloying galvanizing treatment step is performed to a temperature range of 5 ° C./s to 350 ° C. The method for producing a high-tensile cold-rolled steel sheet according to claim 3.

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