JPS63235449A - High tensile cold rolled steel plate for working and its production - Google Patents

Abstract

PURPOSE:To stably produce the titled plate at low cost by applying a steel piece having the specific compsn. consisting of C, Mn, P, S, N, Al, Ti, Zr and Fe to specified soaking, hot rolling, cold rolling and continuous annealing. CONSTITUTION:The steel piece consisting of, by weight, 0.008-0.06% C, 0.001-0.20% Mn, 0.025-0.2% P, <=0.01% S, 0.0005-0.0070% N, <=0.1% acid soluble Al, one or more kinds among 0.001-0.085% Ti and Zr except for the oxide and the balance Fe with inevitable impurities and satisfying the conditions of <=0.25% Mn+P 48(N/14+S/32-Mn/220)<=X<=48(N/14+S/32)+0.005 (where X=Ti+48/91Zr and when the value of S/32-Mn/220 comes to negative, it is calculated as zero) is soaked to >=1,000 deg.C, and is thereafter hot rolled at the Ar3 transformation point or above. The rolled plate is then wound at <=720 deg.C and above the temp. expressed by CT( deg.C)=(1+C+1.8Mn)X500. The hot rolled plate is thereafter subjected to descaling, is cold rolled at the draft ratio of 60-95% and is applied to the continuous annealing at the recrystallization temp. - the temp. of the AC3 transformation point.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a high-strength cold-rolled steel sheet for processing that is processed into various shapes and used, and furthermore, it is possible to reduce the cost of the copper-tensile cold-rolled steel sheet for processing. This invention relates to a method for stable production.

<Prior art and its problems> Conventionally, high-strength cold-rolled steel sheets for processing have generally been manufactured by box annealing low carbon A1 killed steel to which P and Mn are added as reinforcing elements. With the increasing demand for improved productivity in recent years, continuous annealing has come to be widely adopted in the production of cold-rolled steel sheets. However, when manufacturing high-strength cold-rolled steel sheets for processing, productivity has improved dramatically by introducing continuous annealing instead of box annealing, but this time, as material steel, "conventional low carbon^" It has been pointed out that it is extremely difficult to stably obtain the desired material properties when using "l-killed steel."

For example, in recent years, high-strength cold-rolled steel sheets for processing have come to be required to have "drawability," but low-carbon A1 killed steel can be continuously annealed to produce cold-rolled steel sheets with good drawability. In this case, it is necessary to wind up the copper strip into a coil at high temperature after completion of rolling in the hot rolling process to coarsen the cementite and precipitate aluminum nitride. At this time, due to high-temperature winding, the tip of the coil is rapidly cooled when it comes into contact with the winding shaft of the winder, while the rear end is quickly cooled by radiation, so the above purpose can be fully achieved. not achieved,
Therefore, the product characteristics of these parts are inferior to those of the central part of the coil, resulting in a lack of uniformity.

In addition, since the cooling rate in continuous annealing is significantly faster than that in box annealing, the carbon dissolved in solid solution during heating and soaking cannot be sufficiently precipitated during the cooling process and remains in the steel sheet, which may cause strain aging. There is also the problem of In particular, P and Mn in steel sheets
Steel sheets that contain a large amount of C tend to undergo severe strain aging even at room temperature.

However, as a reverse use of this phenomenon, P and M
A method for producing bake-hardenable cold-rolled steel sheets has also been proposed (Japanese Patent Application Laid-open No. 119734/1983), in which a steel plate processed to have good formability is strengthened by using n as a reinforcing element (Japanese Unexamined Patent Publication No. 119734/1983). Although it is explained that a high-strength steel plate with workability can be obtained, the steel plate obtained by this method has a bake hardening amount of 6 kgf/am or more and cannot be strain-aged at room temperature due to solid solute carbon or solid solute nitrogen. Therefore, it was not possible to eliminate the possibility that stretcher strain would occur during press molding, leading to molding defects.

Means for Solving the Problems> From the above-mentioned viewpoints, the present inventors have developed a material with uniform properties that has excellent workability and appropriate bake hardenability and is sufficiently satisfactory for drawing processing. We conducted extensive research to find a means to stably provide cold-rolled steel sheets for processing even if a continuous annealing process was adopted.In particular, we focused on the drawability and cold-strain aging resistance of low-carbon Al-killed cold-rolled steel sheets by continuous annealing. In the process of investigating the influence of trace alloy components in steel and manufacturing conditions on the properties of steel, they came to the following findings (a) to (f). That is, even if the carbon content in the (al steel) is 0.008% or more (hereinafter, % representing the component ratio is expressed as weight %),
When the amount of solid solute Mn in the steel is sufficiently low, a cold rolled steel sheet with good drawability can be produced by continuous annealing without high temperature winding in the hot rolling process.

In other words, by reducing the amount of solid solute Mn in the steel, coarsening of cementite during coiling in the hot rolling process is promoted, and redissolution of cementite during recrystallization annealing is delayed to reduce the amount of solid solute carbon during recrystallization. It is possible to reduce the recrystallization suppressing effect by reducing the interaction between solid solution carbon and Mn.

(b) P is added as a means of manufacturing high-strength steel based on low carbon Al quilt steel, but P and h in steel have a close relationship with each other in terms of the amount of bake hardening of annealed materials. Therefore, not only solid solution Mn but also Mn
By regulating the contents of both P and P, the amount of bake hardening of the annealed material can be suppressed to a range in which room temperature strain aging does not occur.

In other words, since solute carbon and Mn tend to be used in combination with each other, the presence of a large amount of Mn in steel slows down the precipitation of cementite during overaging treatment during continuous annealing, resulting in cold strain aging. becomes more likely to occur. and,
There is an interaction between solute carbon and P that is opposite to the interaction between solute carbon and Mn, and as P increases, the interaction between solute carbon and Mn becomes stronger. thing.

(C1 On the other hand, by forming MnS on Mn as well, re
Since Mn has a useful effect of inhibiting the formation of low melting point sulfides such as S and preventing hot embrittlement of steel, it is necessary to add Mn in a larger amount than S. Reducing the amount of Mn to an extent that does not adversely affect performance is a measure that cannot be adopted in practice.

(d) However, if sulfide-forming elements such as Ti and Zr are added to the low-carbon Al-killed steel described above in a strictly specified range, hot embrittlement can be prevented without increasing Mn throughout the steel. to be possible.

Figure 1 shows the effects of Ti, Zr and M on the hot workability of steel.
It is a graph showing the influence of the amount of n. Here, hot workability is determined by a bar test of 10 cranes in diameter obtained by melting steel with the composition shown in Table 1 in an experimental vacuum melting furnace, hot forging it into a bar steel, and cutting it. The above test piece was first heated at 1250°C for 10 minutes using a Grieple tester, then cooled to 950°C, and at that temperature the strain rate was set at 1 se.
A tensile test was carried out at c-', and the cross-sectional shrinkage rate was determined and evaluated.

Figure 1 also shows that even in low-Mn steel, if appropriate amounts of Ti and Zr are added, the cross-sectional shrinkage rate increases to 50% or more, making it possible to sufficiently prevent cracking during hot rolling. it is obvious.

(e) Furthermore, if Ti or Zr is added, TiS
Alternatively, since TiN and ZrN are formed before ZrS is formed, sufficient Ti and Z to prevent hot embrittlement may be present.
Addition of r naturally eliminates the recrystallization suppressing effect of solid t@N.

(f> In other words, it is as strong as S or stronger than Mn (
By adding appropriate amounts of Ti and Zr, which react to form sulfides, it is possible to prevent hot embrittlement of the steel and reduce the amount of Mn dissolved in the steel. If the amount of solid solute Mn in the steel is reduced while limiting the total amount of Mn, room temperature strain aging can be sufficiently suppressed even if the amount of P necessary to achieve the desired strength is secured. To be able to stably produce high-strength steel sheets with uniform characteristics over the entire length of the coil, which have excellent drawability and room temperature strain aging resistance even through continuous annealing.

This invention was made based on the above knowledge, and a high-strength cold-rolled steel sheet for processing is made of: C: 0.008 to 0.06%, Mn: 0.0
0i ~0.20%.

P: 0.025-0.2%, S: 0.01% or less.

N: 0.0005-0.0070%, acid soluble AJ:
Less than 0.1%.

One or more of Ti and Zr (excluding those contained as oxides); 0.001 to 0.085% in total.

By configuring the composition to satisfy the following conditions: Pa and unavoidable impurities: remaining, and the amount of Mn P: 0.25% or less, excellent drawability and moderate strain aging properties are not observed at room temperature. It has the following characteristics: c: 0.008 to 0.06%, Mn: 0.
001-0.20%.

P: 0.025-0.2%, S: 0.01% or less.

N: 0.0005-0.0070%, acid-soluble Ag:
011% or less.

One or more of Ti and Zr (excluding those contained as oxides): o, ooi to 0.085% in total.

Steel slabs (slab slabs produced by continuous casting, or obtained by ingot-forming method, etc.) that consist of Fe and unavoidable impurities: the remainder, and satisfy the following conditions: 71+P: 0.25% or less (It may be any steel billet made by blooming and rolling a steel ingot), 10
After soaking to 00℃ or higher, hot rolling in Ar at a temperature higher than the transformation point, then coiling at a temperature lower than 720"C and higher than the temperature expressed by the formula %, and after descaling 60~
Ac
By heating the coil to a temperature below the transformation point and continuously annealing it, the coil can be heated to a temperature range below the transformation point and then annealed to produce a coil that exhibits excellent drawability without room temperature strain aging and moderate bake hardenability uniformly over the entire length of the coil. Another feature of the present invention is that it enables stable production of tensile cold-rolled steel sheets.

Next, in this invention, the reason why the chemical composition of the steel and the manufacturing conditions of the steel plate are limited as described above will be explained.

8) Composition 1) C C is an element that is inevitably accompanied in steel, but if its content exceeds 0.06%, the volume percentage of cementite is too large and it is not necessary for cold rolled steel sheets for processing. On the other hand, ductility cannot be obtained.
When the content is o, oos% ungrooved, continuous annealing is applied to suppress room temperature strain aging to a level that poses no practical problem unless carbonization is further reduced to an extremely low level of less than 0.003%. This will result in a severe industrial disadvantage. Therefore, although the C content is determined to be 0.008 to 0.06%, it is preferably adjusted to 0.008 to 0.02%.

ii) Mn Mn content is one of the important factors in this invention, and is preferably as low as possible. However, it is extremely difficult to stably and inexpensively reduce the content to less than 0.001% using current steelmaking technology. On the other hand, Mn
If the content exceeds 0.20%, the effect of promoting cementite coarsening during winding in the hot rolling process cannot be obtained, and furthermore, coexistence with solid solution carbon increases the recrystallization suppressing effect, which is favorable for drawability. It becomes difficult to obtain recrystallized texture. Therefore, the Mn content is 0.001-0.20
%.

1ii) P P has the effect of strengthening steel, but its content is 0.0
Below 25%, the desired reinforcement cannot be achieved, while 0.2%
If the content exceeds P.
If the content exceeds 0.25% in total with the Mn amount, the strain aging property of the steel sheet becomes significant, making it unsuitable for cold-rolled steel sheets for processing. Therefore, the P content was determined to be 0.025 to 0.2%, and the total content of P and Mn was determined to be 0.25% or less.

Note that FIG. 2 is a graph schematically showing the content ranges of Mn and P defined in the present invention.

1v) S S is one of the inevitable impurities that accompany steel,
TiJ than Mn? :j has a strong tendency to bond with Zr, and therefore an increase in S content leads to an increase in the amount of Ti and Zr added, so its content was limited to 0.01% or less, which is an acceptable range.

v) N An impurity that inevitably accompanies NtI steel, and cannot be easily and stably suppressed to less than 0.0005% using current steelmaking technology. And the N content is 0.007
If it exceeds 0%, the amount of Ti or Zr added will increase and the manufacturing cost of the steel plate will increase. Therefore, the N content is limited to 0.0005 to 0.0070%, but preferably 0.00%. It is best to adjust it to 0.030% or less.

vi) Acid-soluble AI Acid-soluble Aj2 is added as a deoxidizing agent in advance to improve the yield of Ti and Zr when adding Ti or Zr after vacuum degassing the molten steel. However, its presence is sufficient because it means that deoxidation has been completed. However, if the content exceeds 0.1%, the steel becomes hard and ductility decreases, so the acid-soluble Al content is set at 0.1% or less.

vii) Ti and Zr These components both combine with S in the steel to prevent hot embrittlement of the steel, and also combine with N in the steel to eliminate the recrystallization suppressing effect. As a result, the effect of reducing the Mn content and increasing the amount of P added is produced, but if the total amount excluding those contained as oxides is less than o, ooi%. If the content exceeds 0.085% in total, a large amount of TiC and ZrC will precipitate and the effect of suppressing recrystallization will appear, making it preferable as a working steel sheet. result in no results. Therefore, the total content of Ti and Zr was limited to 0.001 to 0.085%.

Furthermore, when the Ti and Zr contents are set as X=Ti+-Zr, the reason is as follows. That is, if the value of X is less than the lower limit of the above formula, S, which does not bond with Mn, Ti, or Zr, will be present and FeS will be formed, causing a problem of hot embrittlement of the steel, and the value of If it becomes less than 0%, a part of the ON in the steel has to be fixed as AIN, and drawability cannot be obtained unless high-temperature winding is performed. On the other hand, if the value of X exceeds the upper limit of the above formula, TiC'
A large amount of 1PZrC precipitates, and the effect of suppressing recrystallization appears, resulting in an unfavorable result as a steel sheet for processing.

Note that FIG. 3 is a graph schematically showing the content ranges of Ti, Zr, and Mn defined in the present invention for steel containing 0.008% S and 0.0020% N.

B) Manufacturing conditions of steel plate The high-strength cold-rolled steel plate for working according to the present invention has the above-mentioned composition. When manufacturing the steel plate, the steel plate having the above-mentioned composition is heated to 1000°C. After soaking to the above temperature, hot rolling is carried out at a temperature above the Ars transformation point, then coiling is carried out at a temperature below 720°C and above the temperature expressed by the formula %, and after descaling, 60 to
Ac
The reason why it is preferable to adopt conditions of continuous annealing by heating to a temperature range below the transformation point is as follows.

i) Heating temperature of steel slab (slab) If the heating temperature is less than 1000°C, temperature unevenness will easily occur in the heated slab, and it will be difficult to complete hot rolling at Ar or above the transformation point. The heating temperature is 100
It is set as 0℃ or higher, but preferably 1100 to 1200℃
It is best to heat it up.

ii) Hot rolling finishing temperature When the hot rolling finishing temperature is lower than the Ar3 transformation point, the steel will be hot rolled in the α+γ region or α region, and the heat that would normally be lost during the γ→α transformation will be removed. The inter-rolling texture remains in the hot-rolled sheet, hindering the development of a recrystallized texture favorable for drawability.

iii) Coiling temperature If the coiling temperature (CT) is lower than the formula % formula %, cementite coarsening will be insufficient even in low Mn steel. During winding, the thickness of the scale increases, descaling performance deteriorates, and abnormal grain growth occurs, resulting in the generation of coarse grains.

Note that one of the features of this invention is that it is possible to produce cold rolled steel sheets with good drawability even when rolled at low temperatures.
It is preferable to wind it in a temperature range of 0 to 6.50°C.

Note that FIG. 4 is a graph schematically showing the range of Mn content and winding temperature defined in the present invention.

iv) Reduction ratio of cold rolling If the reduction ratio of cold rolling is less than 60%, the recrystallized aggregate m weave, which is favorable for drawability, will not develop, so a higher reduction ratio is preferable, but if the reduction ratio exceeds 95%, the opposite is true. Squeezability begins to deteriorate. Therefore, the reduction ratio of cold rolling is 60-95%.
It was determined that

■) Annealing temperature Since this annealing is recrystallization annealing, it goes without saying that it is necessary to heat it to an abnormal recrystallization temperature, but if it is heated to a temperature range exceeding the Ac = transformation point, it will transform to α-+7-α. As a result, the "recrystallization texture favorable for drawability" formed during the recrystallization process will be erased.
The annealing temperature must be kept below Ac, the transformation point.

Regarding the heat pattern of continuous annealing, after heating above the recrystallization temperature and below the Ac3 transformation point, 5
After holding for more than 2 seconds, cool down to a temperature range of 550 to 700 °C at a cooling rate of 5 °C/see or less, and then cool from that temperature to a temperature range of 300 to 500 °C; In case, after heating, it is slowly cooled from the temperature range of 350 to 550℃ to 350℃ over 2 minutes or more, and then further heated to 350℃.
It is preferable to cool down from 0.degree. C. to 250.degree. C. or less at a cooling rate of 5"C/sec or less.

It goes without saying that the steel plate manufactured in this manner is annealed, then skin-pass rolled, and then shipped.

Next, this invention will be explained using examples and comparing with comparative examples, but these examples merely show one example of the present invention, and the present invention should not be unduly limited thereby. Of course not.

<Example> Example 1 First, steel having the composition shown in Table 2 was melted by a conventional method, and then hot forged into a slab for experiment with a thickness of 30M.

Next, after heating these to 1250°C for 30 minutes, 11
It was hot rolled in a temperature range of 50 to 900°C to a thickness of 3 cranes.

At this time, as a winding simulation, immediately after hot rolling, the material was rapidly cooled to 700°C, 600°C, and 520°C, and after being inserted into an electric furnace maintained at each temperature for 30 minutes, the cooling rate was 20°C/sin. Furnace cooling treatment was performed.

Next, the obtained hot rolled steel sheet was pickled, cold rolled at a rolling reduction of 73%, and continuously annealed using an infrared heating type continuous annealing simulator.

As shown in Figure 5, the heat cycle during this annealing is as follows: heating to 820°C at a cooling rate of 10°C/sec, holding at that temperature for 40 seconds, and then cooling at a rate of 3°C to 650°C.
/sec, and further cooling rate is 100℃/s up to 400℃
EC, then slowly cooled from 400°C to 350°C at a cooling rate of 16°C/lll1n, over-aged for about 3 minutes, and cooled to 250°C at a cooling rate of 3°C/sec. I followed the water cooling process.

The cold-rolled steel sheet thus obtained was temper-rolled at an elongation rate of 1.2%, then formed into JIS No. S test pieces and subjected to a tensile test to measure mechanical properties and amount of bake hardening. did. As shown in FIG. 6, the amount of bake hardening was defined as the amount of increase in yield point when 2% prestrain was applied by tension and then tension was applied again.

The results are shown in FIGS. 7 to 10.

Figure 7 shows the temperature of 0.09%po, is%Mn material at 700°C.
Figure 7 shows the effect of the carbon content in the steel on the mechanical properties of the annealed material when it is rolled up.
It can be seen that the desired ductility cannot be obtained if it does not fall below kgf/■II2 and exceeds 0.06%.

In addition, Fig. 8 shows 0.09%P-0,15%Mn material at 7
Figure 8 shows the effect of P content on the mechanical properties of the annealed material when coiled at 00°C. From Figure 8, it is clear that if the P content exceeds 0.2%, the steel is too hard. It can be seen that it is unsuitable for processing.

FIG. 9 shows the results regarding the influence of Mn content and P content on the bake hardening amount of annealed material when 0.012% C material is wound at 700°C. It can be seen from the figure that if the Mn content and P content deviate from the conditions specified in the present invention, the bake hardening amount cannot be reduced to less than 6 kgf/area.The numerical values in Fig. 9 indicate the bake hardening amount. (kgf/1mm”).

Furthermore, Figure 10 shows the effect of Mn on the r value of the annealed material when the 0.012%C-0.09%P material is wound at 700°C.
Although the results regarding the influence of the Mn content and the winding temperature are shown, it is clear from FIG. 10 that a good r value cannot be obtained if the Mn content and the winding temperature deviate from the conditions specified in the present invention. I understand. Note that the numerical values in FIG. 10 are r values.

Example 2 A slab obtained by melting steel having the composition shown in Table 3 in a converter, subjecting a portion to vacuum degassing treatment, and then continuously casting the steel was 1
After heating to 200°C, it was hot rolled and wound at 650°C.

Subsequently, the obtained hot-rolled conditioned plate was pickled and then rolled at a rolling reduction rate of 75.
% to produce a cold-rolled plate with a thickness of 0.8 m,
Continuous annealing consisting of holding at 760°C for 40 seconds and overaging treatment at 400°C for 3.5 minutes was performed, and the elongation rate was further increased to 1.0.
% temper rolling to produce cold rolled steel sheet products.

Table 4 shows the characteristic values of the obtained cold rolled steel sheet product.

This Table 4 also shows that cold-rolled steel sheet products manufactured under the conditions of the present invention exhibit excellent properties as high-strength cold-rolled steel sheets for processing, while those manufactured under conditions that deviate from the provisions of the present invention. clearly does not satisfy the desired properties.

Summary of Effects> As explained above, according to the present invention, it is possible to stably obtain high-strength cold-rolled steel sheets with excellent drawability and room-temperature aging resistance even by continuous annealing. It brings about useful effects.

[Brief explanation of the drawing]

Figure 1 shows the effects of Ti, Zr and Mn on hot ductility of steel.
A graph showing the influence of the content. FIG. 2 is a graph schematizing the content range of Mn and P specified in this invention. FIG. 3 is a graph showing the content range of Ti, Zr, and Mn specified in this invention. FIG. 4 is a graph schematically illustrating the range of Mn content and coiling temperature defined in this invention. FIG. 5 is a diagram illustrating a heat cycle for simulating continuous annealing. , Figure 6 is a drawing explaining the definition of the amount of bake hardening, and Figure 7 is a diagram showing the effect on the mechanical properties of the annealed material when 0.09%P-0,15%Mn material is wound at 700℃. The graph shown in Figure 8 showing the results regarding the influence of carbon content in steel is 0.09%.
Figure 9 is a graph showing the effect of P content on the mechanical properties of the annealed material when the P-0.15%Mn material is rolled up at 700°C.
Effect of Mn on the amount of bake hardening of annealed material when coiled at °C
Figure 10, a graph showing the results regarding the influence of P content and P content, shows that 0.012%C-0.09%P material was
It is a graph showing the results regarding the influence of the Mn content and the winding temperature on the r value of the annealed material when the material is wound at 0°C.

Claims (2)

[Claims] (1) Weight percentage: C: 0.008-0.06%, Mn: 0.001-0.
20%, P: 0.025-0.2%, S: 0.01% or less, N: 0.0005-0.0070%, acid-soluble Al:
0.1% or less, one or more of Ti and Zr (excluding those included as oxides): 0.001 to 0.085 in total
%, Fe and unavoidable impurities: the remainder, and Mn+P: 0.25% or less, and 48 (N/14+S/32-Mn/220)≦X≦48
(N/14+S/32)+0.005 [However, X=Ti
+(48/91)Zr, S/32-Mn/220
If the value of is negative, it is calculated as 0. A high-tensile cold-rolled steel sheet for processing, characterized in that it has a composition that satisfies the following conditions. (2) Weight percentage: C: 0.008-0.06%, Mn: 0.001-0.
20%, P: 0.025-0.2%, S: 0.01% or less, N: 0.0005-0.0070%, acid-soluble Al:
0.1% or less, one or more of Ti and Zr (excluding those included as oxides): 0.001 to 0.085 in total
%, Fe and unavoidable impurities: remaining, and Mn+P: 0.25% or less, and 48 (N/14+S/32-(4/55)Mn)≦X≦
48(N/14+S/32)+0.005 [However, X=
Ti+(48/91)Zr, S/32-(4/5
5) If the value of Mn is negative, it is calculated as 0. ]
A steel billet having a composition satisfying the following conditions is soaked at 1000°C or higher, then hot rolled at Ar_3 transformation point or higher, and then heated to 720°C or lower, and with the formula CT(°C)={1+C(wt%) )+1.8×Mn(wt
%)}×500, and after descaling, it is cold rolled at a reduction rate of 60 to 95%, and then continuously annealed by heating to a temperature range from the recrystallization temperature to the Ac_3 transformation point. A method for manufacturing a high-strength cold-rolled steel sheet for processing, characterized by: