JP2001335841A - Method for producing hot dip galvanized steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a hot dip galvanized steel sheet having good and uniform characteristics even in the case of being coiled at a low temperature. SOLUTION: A continuously cast slab containing, by mass, 0.01 to 0.2% C, <=0.1% Si and 0.05 to 2% Mn is rolled down in the temperature range of the Ar3 point to Ax( deg.C) given by the formula (1), Ax( deg.C)=Ar3+30+103×Al-104×N (wherein, Al is the content (%) of acid soluble Al, and N is the content (%) of N) in the poststage within a hot finish rolling stage at a draft of >=20% of the final sheet thickness, is coiled at <=650 deg.C, is cold-rolled at a draft of 50 to 85% and is thereafter subjected to hot dip galvanizing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet which are formed into various shapes by press working or the like and are used for structural members of automobiles.

[0002]

2. Description of the Related Art When manufacturing a thin steel sheet based on a low-carbon Al-killed steel, it is important to reduce the fluctuation of the properties of the steel sheet in the hot rolling process due to the fluctuation of the temperature in the longitudinal direction. The causes of the fluctuations are generally as follows. (1) When the slab is heated in the soaking furnace, the part in contact with the skid (the girder supporting the slab) in the furnace is less likely to be heated, and the temperature is lower than the part not in contact. (2) After roughly rolling the slab hot and forming a rough rolled material,
The rear end of the rough rolled material is cooled by radiation before hot finish rolling. (3) At the time of winding, the leading end of the steel sheet is cooled in contact with the shaft of the winder, and the rear end is cooled faster than the central part of the steel sheet by radiation after winding.

Among the causes of the above fluctuations, the one having the greatest effect is the fluctuation accompanying the winding in (3). In particular, when the winding temperature is a high temperature exceeding 650 ° C., this variation becomes remarkable. This is because there is a difference between the precipitation form of cementite and AlN precipitated in the steel sheet after winding. In other words, in the portion where the winding temperature is low, fine precipitation of cementite and insufficient precipitation of AlN result in poor growth of crystal grains during subsequent cold rolling or recrystallization annealing, resulting in poor ductility and deep drawing. A cold-rolled steel sheet and a hot-dip galvanized steel sheet having portions with poor properties will be manufactured.

To solve these problems, Japanese Patent Laid-Open No. 59-16 / 1984
Japanese Patent No. 2227 discloses that the winding temperature at both ends in the longitudinal direction of the strip is higher than the winding temperature at the central portion in the longitudinal direction, and the winding temperature at both ends and the winding length at that temperature are fixed. A method of controlling according to an equation is disclosed. Japanese Patent Application Laid-Open No. 5-43946 discloses that the leading end and the trailing end of the coil each have a length of 3 mm.
%, The winding temperature is set to 680 to 850 ° C, and the winding at the leading end and the tail end is cooled from 550 ° C or higher to a cooling rate of 3 ° C / min or lower by winding so as not to loosen. A method for producing a characteristic cold-rolled steel sheet or continuous galvanized steel sheet is disclosed.

However, since the tip is pressed against the shaft of the winder at a high temperature, the tip is rapidly cooled and hardened, and the portion wound immediately outside is soft at the high temperature. A dent is formed in the first to third turns that are close to the. The reason for this is that the quenched hard tip is pushed into a soft steel plate that is wrapped around and pressed.

In this portion, a phenomenon similar to that of the strain relief annealing occurs, and abnormal grain growth is likely to occur. Since such a portion is easily broken during cold rolling, even if the characteristics are good,
It has to be cut off before cold rolling. In addition, at the leading end and the trailing end, as the winding temperature increases, the growth of scale after winding is promoted, and not only the pickling property deteriorates, but also Si and the like in the steel sheet harmful to hot-dip galvanizing property. Has the drawback that the surface quality of the hot-dip galvanized steel sheet is degraded. Regarding the slab heating as the cause of the above-mentioned fluctuation (1), Japanese Patent Laid-Open Publication No.
Fine Mn by heating at a low temperature of 0 ° C. or more
A method of manufacturing a cold-rolled steel sheet that reduces the number of S and improves deep drawability and aging characteristics is disclosed. Here, the necessity of lowering the slab heating temperature with the decrease in the S content is also shown.

[0007] A decrease in the slab heating temperature reduces the effect of cooling by the skid, and therefore leads to uniformity of the properties of the steel sheet, but complete uniformity cannot be expected. Also in this manufacturing method, it is assumed that the steel sheet is wound at 650 ° C. or higher. Further, when the slab heating temperature is excessively lowered, the portion where the temperature is reduced most in the hot finish rolling step falls below the Ar ₃ transformation point, causing deterioration of characteristics. That is,
This method is not an effective method for steel having a low S content.

On the other hand, Japanese Unexamined Patent Publication No. 10-195542
The publication discloses a method of manufacturing a steel sheet that can obtain uniform and good characteristics in the longitudinal direction of the coil even when the winding temperature is 650 ° C. or less. In this method, for the purpose of completely depositing MnS, the slab is heated to 1150 ° C. or lower, then hot rough-rolled, and the rough-rolled material is temporarily cooled by 9%.
After the temperature is reduced to 50 ° C. or less, reheating is performed to 980 ° C. or more, and hot finish rolling is performed.

In this case, there is no danger that the portion having the largest temperature drop during the hot finish rolling falls below the Ar ₃ transformation point, and at the same time, the influence of the above-mentioned variation factor (2) can be reduced. However, the lower the S content, the more it becomes necessary to cool the rough rolled material to a lower temperature. Therefore, the energy efficiency is poor, and furthermore, Al in the cooling process after hot finish rolling
The effect of accelerating the precipitation of N cannot be expected.

[0010]

The content of S in low-carbon Al-killed steel mass-produced recently has been reduced due to the progress of steelmaking technology. In recent thin plate manufacturing processes,
After continuous casting, the slab is immediately charged into the heating furnace,
Unless excessive slab low-temperature heating is performed, improvement in characteristics cannot be expected.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems in the prior art, and its object is to use a low-carbon Al-killed steel having a relatively small S content.
An object of the present invention is to provide a method capable of producing a hot-dip galvanized steel sheet having good and uniform properties even at a relatively low winding temperature without excessively heating the slab at a low temperature.

[0012]

The gist of the present invention is as follows.

(1) In mass%, C: 0.01-0.2%,
Si: 0.1% or less, Mn: 0.05 to 2%, P: 0.1
%, S: 0.02% or less, acid-soluble Al: 0.005
-0.1%, N: 0.0005-0.008%, and when the continuous cast slab consisting of Fe and impurities is hot-rolled, at the subsequent stage in the finish rolling step, Ar is ₃ points or more.
In a temperature range below Ax (° C.) given by the following equation (1),
Manufacturing of a hot-dip galvanized steel sheet characterized by rolling down at a rolling reduction of 20% or more of the final sheet thickness, winding at 650 ° C or less, cold rolling at a rolling reduction of 50 to 85%, and then hot-dip galvanizing. Method. Ax (° C.) = Ar ₃ + 30 + 10 ³ × Al-10 ⁴ × N (1) where, Al: acid-soluble Al content (%) N: N content (%) The content represents% by mass.

In the hot rolling, the final thickness of 20 mm
The rolling reduction of not less than% means a value calculated by the following equation (2). R = (H1−H2) / H3 × 100 (2) where, R: reduction ratio (%) in hot rolling. H1: In the case of a rolling mill composed of a plurality of stands, the thickness (mm) of the steel sheet on the entrance side of the latter stand that first passes after the steel sheet reaches the corresponding temperature range. Note that, in the case of a seven-stand rolling mill, for example, at least four stands and subsequent stands are used in the latter-stage stand. In the case of a rolling mill composed of one stand, the thickness (mm) of the steel sheet on the entry side of the second-stage pass that the steel sheet first passes after reaching the corresponding temperature range. H2: In the case of a rolling mill composed of a plurality of stands, the steel sheet is in the corresponding temperature range, and the thickness (mm) of the steel sheet on the exit side of the subsequent stand that passes last. In the case of a rolling mill composed of one stand, the thickness (mm) of the steel sheet at the exit side of the second-stage pass in which the steel sheet finally passes in the corresponding temperature range. H3: Final thickness (mm) in hot rolling.

(2) The method for producing a steel sheet according to the present invention is characterized in that (1)
In the method of the above, further, 0.0002
B may be contained up to 0.003%. (3) In the method of the above (1) or (2), after the rough rolling, the coarsely-rolled material is at a temperature of 965 ° C. or more, and the uneven temperature in the rough-rolled material is within 140 ° C. Heated to
When the finish rolling is performed, further effects can be obtained.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors investigated in detail the effects of the composition and hot rolling conditions on the properties of a low-carbon Al-killed steel rolled at a low temperature and hot rolled. at a reduction temperature is Ar ₃ transformation point or higher, and closer to the Ar ₃ transformation point,
(2) It was found that the deeper the hot rolling, the greater the final rolling reduction, the better the deep drawability.

As a result of a comparison between a material having the same composition and a material having good deep drawability and a material having a poor deep drawability, the former showed that cementite was coarsened and the amount of AlN precipitated was large. It became clear that the precipitation amount was also small. In order to clarify the reason, hot working tests were performed at various temperatures and rolling reductions using a compression hot working simulator, and the subsequent transformation behavior was investigated.

As a result, it was found that there is a correlation between the transformation temperature and the amount of AlN deposited. This is because, when a large strain is applied to a low-temperature austenite where recrystallization is unlikely to occur, ferrite transformation is promoted, and A
It is estimated that the precipitation of 1N was promoted. It is also presumed that the amount of austenite that finally transforms into cementite after winding becomes smaller at higher temperatures, and that cementite eventually becomes coarse.

Thereafter, the influence of the Al and N contents was also investigated. As a result, it became clear that the deeper the steel, the higher the Al content and the lower the N content, the better the deep drawability. Assuming that the same deep drawability is obtained, increasing the Al content has the same effect as increasing the final rolling temperature, and increasing the N content has the same effect as lowering the final rolling temperature. effective. Based on these results, the hot rolling conditions necessary for obtaining good deep drawability in a low-carbon Al-killed steel that has been cold-rolled by hot rolling are clarified from the relationship between the Al and N contents. I made it. The reasons for limiting the composition of the components and the reasons for limiting the rolling conditions will be described in detail.

(A) Reasons for limiting the component composition C: C is inevitably contained in steel,
If it is less than 1%, the normal temperature strain aging cannot be suppressed to a practically acceptable level by continuous annealing unless carbonization is extremely low.
On the other hand, if the content exceeds 0.2%, the volume ratio of cementite is too large, and the ductility required for a cold-rolled steel sheet for working cannot be obtained. Preferably, it is 0.01-0.035%,
More preferably, it is 0.015 to 0.025%.

Si: Si is inevitably contained in steel, and the smaller the content, the better. When the content increases, not only does the deep drawability of the steel sheet deteriorate, but also the reaction with molten zinc deteriorates, so the upper limit was made 0.1%. Preferably, 0.0
3% or less.

Mn: Mn is added to prevent S inevitably contained in steel from forming FeS and causing hot embrittlement. If it is less than 0.04%, the effect cannot be obtained. If it exceeds 2%, the effect of promoting cementite coarsening at the time of winding in the hot rolling step cannot be obtained, and
By coexisting with solid solution carbon, the effect of suppressing recrystallization becomes excessive, and a recrystallization texture preferable for deep drawability cannot be easily obtained. Therefore, it is set to 0.04 to 2%.

P: It has the effect of strengthening the steel sheet, and may be positively added to increase the tensile strength according to the purpose of use, or may be contained as inevitable impurities. However, if the content exceeds 0.1%, the steel becomes brittle, so this value is made the upper limit.

S: S is inevitably contained in steel, and the smaller the, the better. Although precipitated as MnS, even if there is too much MnS, the properties are deteriorated.
% Or less. Preferably it is 0.008% or less, more preferably 0.004% or less.

N: N is inevitably contained in steel, and the smaller the number, the better. The lower limit is 0.0005% that can be easily and stably manufactured by the current steelmaking technology.
On the other hand, if the content exceeds 0.008%, the necessary amount of Al to be added increases, and the production cost increases. Preferably, 0.00
It is at most 3%, more preferably at most 0.002%.

Acid soluble Al: Al is deoxidized and N in steel
Is added to fix as aluminum nitride.
Since it is necessary to add 10 times or more of the N content in the ratio of mass%, the lower limit was made 0.005%. Further, if added in excess of 0.1%, nonmetallic inclusions increase and ductility is impaired, so this was made the upper limit.

B: B is added to fix N, which cannot be precipitated as AlN, as BN. However, B in the solid solution state is preferably added in an amount corresponding to the N content in the solid solution state in order to suppress ferrite transformation after hot finish rolling. If the content is less than 0.0002%, no effect is obtained, so this is set as the lower limit. Also, 0.00
If it exceeds 3%, the B compound of Fe precipitates before hot finish rolling, and not only does not form BN, but also impairs ductility. (b) Reasons for Limiting Rolling Conditions, etc. Temperature conditions up to hot finish rolling entry side: In the low-S steels targeted by the present invention, the effect of low-temperature heating of the slab is not so expected, so the heating temperature is not particularly limited. The temperature on the inlet side of the hot finish rolling mill is preferably lower, but from the necessity of completing the hot finish rolling in the austenite region,
The lower limit was 965 ° C. If the temperature variation in the plate at that time exceeds 140 ° C., the characteristic fluctuation after cold rolling and recrystallization annealing becomes large, so this was made the upper limit. Preferably 60 ° C
Within 30 ° C., more preferably within 30 ° C.

Hot finish rolling conditions: Hot finish rolling conditions are the most important component of the present invention. When hot rolling is performed in the ferrite region, a rolling texture that causes the formation of a recrystallized texture that deteriorates deep drawability is formed in the surface layer of the steel sheet. Therefore, the lower limit is the Ar ₃ transformation point. In order to promote the ferrite transformation and impart sufficient strain to exert the effect of the present invention, the rolling is performed in a temperature range of Ax (° C.) or less given by the above formula (1). When the strain applied by the reduction immediately before the ferrite transformation increases, the crystal grain size of the hot-rolled steel sheet decreases, and the deep drawability of the cold-rolled steel sheet as the final product improves. Also, if the applied strain increases due to the reduction,
The time during which the steel sheet stays in the high-temperature ferrite region in which Al diffuses rapidly is prolonged, and the precipitation of AlN is promoted to improve the deep drawability. From the results of the test, the rolling reduction required to obtain these effects was a result of 20% or more in the rolling reduction given by the above equation (2). Based on these, in the later stage of the finish rolling process, Ar ₃ points or more, the equation (1)
In the temperature range below Ax (° C) given by
The rolling is performed at a rolling reduction of 0% or more. Further, in the case of a large reduction, there is a possibility that the flatness of the steel sheet is reduced. Therefore, it is desirable that the reduction ratio given by the equation (2) be within 250%.

Hot Rolling Winding Temperature: When the hot rolling winding temperature exceeds 650 ° C., the difference in properties between the front and rear ends and the central portion of the steel sheet increases. Furthermore, since the scale of the hot-rolled sheet becomes thicker, elements such as Si are concentrated at the interface between the steel sheet and the scale and impair hot-dip galvanizing properties, the temperature is set as the upper limit. On the other hand, when the winding temperature decreases, the cooling mode of the steel sheet by the cooling water changes from the cooling by the film boiling to the cooling by the nucleate boiling, and the cooling becomes non-uniform, and the flatness of the hot-rolled steel sheet may be reduced. For these reasons, the temperature is preferably set to 450 to 600C, more preferably 500 to 550C.

Cold rolling reduction: When the rolling reduction is less than 50%, a recrystallized texture preferable for deep drawability is not formed.
On the other hand, if it exceeds 85%, another recrystallized texture that inhibits the deep drawability is generated.

The cold-rolled steel sheet is annealed and plated in a normal continuous hot-dip galvanizing line, and if necessary, is reheated and alloyed. And shipped.

[0032]

An embodiment of the present invention will be described. Note that this is an exemplification of the embodiment of the present invention, and the present invention is not limited to this.

In an experimental vacuum melting furnace, steel having the composition shown in Table 1 was melted.

[0034]

[Table 1] Here, the Ar ₃ transformation point in the table is a diameter of 8 mm and a height of 12 mm.
It was determined by the method shown below using a cylindrical sample of mm.

After heating the sample steel to 1250 ° C. for 5 minutes,
After cooling to 0 ° C. at a cooling rate of 10 ° C./s, it was kept at the same temperature for 30 seconds, and further reduced at a reduction of 30%.
Thereafter, the change in the height of the sample during the cooling at 10 ° C./s was determined by continuously measuring the change.

Ax was calculated by the above equation (1).

The target steel is subjected to hot forging and has a width of 100 mm,
An experimental slab having a thickness of 25 mm was used.

Next, these slabs were placed in an electric furnace at 125
After heating at 0 ° C. for 1 hour, rolling was performed in a three-pass manner in a temperature range of 1030 to 800 ° C. by an experimental hot rolling mill composed of one stand to obtain a hot-rolled steel sheet having a thickness of 3 mm.

As a simulation of winding, the hot-rolled steel sheet was immediately cooled with water spray to 700 to 50.
After cooling to a temperature of 0 ° C., splicing, charging into an electric furnace maintained at the same temperature, and further maintaining at that temperature for 1 hour,
The furnace was cooled at a cooling rate of 20 ° C./h.

The obtained hot-rolled steel sheet was pickled and had a thickness of 0.8 m.
m.

The thus obtained cold-rolled steel sheet is heated in an infrared heating furnace to 820 ° C. at a rate of 10 ° C./s, maintained at the same temperature for 40 seconds, and cooled at 10 ° C./s. After cooling at a rate of 450 ° C. and holding for 30 seconds, reheating at a rate of 50 ° C./s to 500 ° C. and holding for 30 seconds,
It was cooled to room temperature at a cooling rate of 0 ° C./s.

After annealing, they were subjected to temper rolling at an elongation of 1.2%, and then subjected to a tensile test using a JIS No. 5 tensile test piece.

Table 2 shows the measurement results of the rolling conditions, winding temperature and mechanical properties for each test. The r value in the table indicates a measured value in the rolling direction.

[0044]

[Table 2] Table 3 shows test results when the hot rolling temperature, the rolling reduction, and the winding temperature were changed. The deviation r value in the table indicates a deviation from the r value of the steel A and the steel B in Table 2.

[0045]

[Table 3] FIG. 1 shows elongation at break and r for test numbers 1 to 5 when the final pass (third pass) of rolling was started at 870 ° C.
5 is a graph showing the effect of temperature Ax on the value.

In Test Nos. 1, 2, 3, and 5, the starting temperatures of the final pass corresponding to the reduction in the subsequent stage were steel A and steel A, respectively.
Since the B, C, and E temperatures are within the range of not less than the _three Ar points and not more than the temperature Ax, the r value exceeds 1.4 and is high, indicating good deep drawability. On the other hand, the test number 4 indicates that the temperature A of the steel D
Since x is lower than the starting temperature of the final pass, it is clear that the r value is low and good deep drawability cannot be obtained.

Test No. 6 is a case where steel F having a particularly low S content is used, and test No. 7 is a case where steel G containing B and B is used in a more preferable range. In each case, good deep drawability was obtained. Test No. 11 shows a case where the final pass start temperature has dropped to the Ar ₃ point or less due to a significant drop in the first pass start temperature. The r value has decreased.

Test number 10 indicates that the final pass start temperature is 90
This is the case where the temperature was raised to 0 ° C. Since the final pass start temperature is higher than the temperature Ax, the elongation at break is reduced.

FIG. 2 is a graph showing the influence of the final pass rolling reduction on the r value and the deviation r value.
The results of 8 and 9 are arranged. Note that the deviation r value represents a deviation from the test number 1 in which the final pass rolling reduction is 50%. It is apparent from the results of the drawing that good deep drawability can be obtained when the reduction ratio of the final pass corresponding to the reduction in the subsequent stage is 20% or more. On the other hand, when the rolling reduction of the final pass is less than 20%, both the deep drawability and the deviation r value are deteriorated. FIG. 3 is a graph showing the influence of the winding temperature on the r value and the deviation r value.
12, 13, and 14 are arranged. In addition,
The deviation r value indicates a deviation from the test number 2 in which the winding temperature is 600 ° C.

When the winding temperature is 650 ° C. or less, 700 ° C.
The absolute value of the r value is not as high as when the
As shown by the values, the fluctuation of the r value due to the winding temperature is small, and the uniformity of the r value in the coil plane is ensured. On the other hand, when the temperature exceeds 650 ° C., although the r value is high, the winding temperature becomes non-uniform, so that it is inevitable that the temperature falls below 650 ° C. at the edge portion of the steel sheet, etc. Become.

FIG. 4 is a graph showing the influence of the first pass start temperature on the elongation at break, the r value and the deviation r value.
Test numbers 1, 15, 16, 17, and 18 are arranged. In this test, the first pass start temperature was changed by changing the temperature of the experimental slab to simulate the effect of non-uniform temperature in the steel sheet surface in the actual machine. For example, when 1030 ° C. is used as a reference for the start temperature of the first pass, temperature non-uniformity is within 140 ° C. (± 70 ° C.).
Non-uniform r value can be reduced to within approximately 0.3. In addition, the non-uniform temperature
By setting the temperature at 0 ° C. (within ± 30 ° C.), the unevenness of the r value can be reduced to within approximately 0.1.

[0052]

As described in detail above, according to the manufacturing method of the present invention, a hot-dip galvanized steel sheet having good and uniform deep drawability can be manufactured even at a relatively low winding temperature. It greatly contributes to cost reduction and quality improvement of plated steel sheets.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of temperature Ax on elongation at break and r-value when a final pass is started at 870 ° C.

FIG. 2 is a graph showing the influence of a final pass rolling reduction on an r value and a deviation r value.

FIG. 3 is a graph showing the influence of the winding temperature on the r value and the deviation r value.

FIG. 4. Elongation at break, first value on r value and deviation r value
9 is a graph showing the effect of the pass start temperature.

Claims (3)

[Claims] C .: 0.01 to 0.2% by mass, S:
i: 0.1% or less, Mn: 0.05 to 2%, P: 0.1%
Hereinafter, S: 0.02% or less, acid-soluble Al: 0.005 to
0.1%, N: 0.0005 to 0.008%, the balance being F
When hot-rolling a continuous cast slab consisting of e and impurities, the final sheet is formed at a temperature of not less than ₃ points and not more than Ax (° C.) given by the following formula (1) at a later stage in the finish rolling process. A method for producing a hot-dip galvanized steel sheet, comprising reducing the thickness at a reduction rate of 20% or more, winding at 650 ° C. or less, performing cold rolling at a reduction rate of 50 to 85%, and then performing galvanizing. Ax (° C.) = Ar ₃ + 30 + 10 ³ × Al-10 ⁴ × N (1) where: Al: content of acid-soluble Al (%) N: content of N (%) 2. In mass%, C: 0.01-0.2%, S
i: 0.1% or less, Mn: 0.05 to 2%, P: 0.1%
Hereinafter, S: 0.02% or less, acid-soluble Al: 0.005 to
0.1%, N: 0.0005 to 0.008%, B: 0.
When hot-rolling a continuous cast slab consisting of 0002 to 0.003%, the balance being Fe and impurities, at the subsequent stage in the finish rolling step, at _three or more Ar points, Ax (° C.) given by the following formula (1): ) In the following temperature range, reduce at a rolling reduction of 20% or more of the final plate thickness, wind at 650 ° C or less, and reduce the rolling
A method for producing a hot-dip galvanized steel sheet, comprising performing hot-dip galvanizing after cold rolling at 85%. Ax (° C.) = Ar ₃ + 30 + 10 ³ × Al-10 ⁴ × N (1) where: Al: content of acid-soluble Al (%) N: content of N (%) 3. After the rough rolling, the rough-rolled material is heated at a temperature of 965 ° C. or more so that the temperature in the rough-rolled material becomes 140 ° C. or less, and finish-rolled. The method for producing a galvanized steel sheet according to claim 1 or 2.