JPH0830245B2 - High-strength cold-rolled steel sheet for processing and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-strength cold-rolled steel sheet for working which is processed into various shapes and used, and further, the high-strength cold-rolled steel sheet for working is manufactured at low cost. The present invention relates to a method for stable production.

<Prior art and its problems> Conventionally, a high-strength cold-rolled steel sheet for working is generally manufactured by box annealing a low carbon Al killed steel to which P and Mn are added as strengthening 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, the productivity was dramatically improved by adopting continuous annealing instead of box annealing, but this time, as a raw material steel, the steelmaking costs increased so much. It has been pointed out that it is extremely difficult to stably obtain desired material properties by using a "conventional low carbon Al killed steel".

For example, in recent years, good "drawability" has been required even for high-strength cold-rolled steel sheets for working, but cold-rolled steel sheets with good drawability are obtained by continuously annealing low-carbon Al-killed steel. When manufacturing, it is necessary to wind the steel strip that has been rolled in the hot rolling process into a coil at a high temperature to coarsen cementite and precipitate aluminum nitride. At this time, because of the high temperature winding, "the coil tip is rapidly cooled when it comes into contact with the winding shaft of the winder, while the trailing end is quickly cooled by radiation, the above-mentioned object is sufficiently achieved. However, the product characteristics of these portions are inferior to those of the central portion of the coil, resulting in lack of uniformity. "

In addition, since the cooling rate in continuous annealing is significantly higher than that in box annealing, the carbon solid-dissolved during heating and soaking does not fully precipitate in the cooling process and remains in the steel sheet, which causes strain aging. There is also the problem of. Especially, P and Mn in the steel plate
Steel sheets containing a large amount of steel tend to undergo severe strain aging even at room temperature.

However, assuming that this phenomenon is used in reverse, P and Mn
A manufacturing method for bake hardenable cold rolled steel sheet, in which steel sheet processed with good workability is strengthened in the paint baking process, has been proposed (JP-A-56-119734), which provides excellent processing. Although it is explained that a high-strength steel sheet with good properties can be obtained, the steel sheet obtained by this method has a bake hardening amount of 6 kgf / mm ² or more and prevents room temperature strain aging due to solid solution carbon or solid solution nitrogen. It was not possible to eliminate the possibility that stretcher strain would occur during press molding, resulting in defective molding.

<Means for Solving Problems> From the viewpoints described above, the inventors of the present invention have excellent workability and appropriate bake hardenability, and have uniform properties that are sufficiently satisfactory for drawing. Then, the inventors conducted extensive research to find a means for stably providing a cold-rolled steel sheet for working, which is also advantageous in terms of price, even if a continuous annealing process is adopted. And, considering the advantages of low carbon Al-killed steel such as ultra-low carbon steel that does not cause a significant increase in steelmaking cost, the drawability and resistance of cold-rolled low-carbon steel sheet by continuous annealing In the process of investigating the influence of trace alloy components in steel and manufacturing conditions on room temperature strain aging, the following (a)-
The findings as shown in (f) have been obtained.

(A) The amount of carbon in steel exceeds the range of ultra-low carbon steel 0.008
% Or more (hereinafter,% representing the component ratio shall be% by weight), continuous annealing without hot coiling in the hot rolling process if the amount of solute Mn in steel is sufficiently low It is possible to produce cold-rolled steel sheets with good drawability, which is more advantageous in terms of steelmaking cost, and which is advantageous in terms of bake hardenability, even if the carbon content exceeds 0.010%. Stable production of cold-rolled steel sheet with good drawability is sufficiently possible. That is, by reducing the amount of solid solution Mn in the steel is promoted cementite coarsening during winding in the hot rolling process, as a result, the remelting of cementite during recrystallization annealing is delayed, so the carbon content is somewhat higher. It has been found that even in steel, the amount of solute carbon at the time of recrystallization is reduced, so that the interaction between solute carbon and Mn can be reduced and the recrystallization suppressing effect can be reduced.

(B) Also, as described above, “addition of P” is performed as a means for producing high-strength steel based on low carbon Al killed steel, but P and Mn in steel are annealed material They have a close relationship with each other with respect to the amount of hardening. Therefore, the normal temperature strain aging does not occur in the amount of bake hardening of the annealed material only by controlling the contents of both Mn and P as well as the solid solution Mn. Be able to keep within range. In other words, since there is a tendency for solute carbon and Mn to bond with each other,
If a large amount of Mn is present, the precipitation of cementite in the overaging treatment of continuous annealing is delayed, and as a result, room temperature strain aging is likely to occur. Then, there is an interaction between the solid solution carbon and P that is opposite to the interaction between the solid solution carbon and Mn, and when P is increased, the result is that the interaction between the solid solution carbon and Mn is strengthened. I also knew that.

(C) On the other hand, the formation of MnS in Mn also has a useful action of preventing the formation of low melting point sulfides such as FeS and preventing hot embrittlement of steel, so the Mn content is inevitably higher than that of S. It must be added, and reducing the amount of Mn to the extent that it does not adversely affect the deep drawability of the continuous annealed material is a means that cannot be actually adopted.

(D) However, Ti and low carbon Al killed steel as described above
By adding a sulfide-forming element such as Zr in a strictly regulated specific range, it becomes possible to prevent hot brittleness without relying entirely on Mn.

FIG. 1 is a graph showing the effects of Ti, Zr and Mn contents on the hot workability of steel. Here, the hot workability is the first
Steel of the composition shown in the table is melted in a laboratory vacuum melting furnace,
Diameter 10 mm obtained by cutting after making steel bar by hot forging
It is the measurement of the rod-shaped test piece. That is, the above-mentioned test piece was first heated at 1250 ° C. for 10 minutes using a greeble tester, cooled to 950 ° C., and subjected to a tensile test at that temperature at a strain rate of 1 sec ⁻¹ to obtain and evaluate the cross-sectional shrinkage rate.

From Fig. 1 as well, even in the case of low Mn steel, when Ti or Zr is added in an appropriate amount, the cross-sectional shrinkage ratio increases to 50% or more, and it becomes possible to sufficiently prevent cracking during hot rolling. it is obvious.

(E) Furthermore, when Ti or Zr is added, TiN or ZrN is formed before TiS or ZrS is formed. Therefore, if Ti or Zr is added sufficiently to prevent hot embrittlement, it is naturally necessary. The effect of suppressing recrystallization by solid solution N should also disappear.

(F) That is, by adding an appropriate amount of Ti or Zr that reacts strongly with S to form a sulfide as much as or more than Mn, Mn to be dissolved in steel while preventing hot embrittlement of steel It is possible to reduce the amount of P, and if the amount of solid solution Mn in the steel is reduced while limiting the amount of P added with the total amount of Mn, the amount of P required to achieve the desired strength can be reduced. Even if secured, the room temperature strain aging is sufficiently suppressed, and even with continuous annealing, it is possible to stably manufacture a high-strength steel sheet with uniform properties over the entire length of the coil that is excellent in drawability and room temperature strain aging resistance. To become a.

The present invention has been made based on the above-mentioned findings. "High-strength cold-rolled steel sheet for processing is C: 0.011 to 0.06%, Mn: 0.001 to 0.20%, P: 0.025 to 0.2%, S: 0.01% Below, N: 0.0005 to 0.0070%, acid-soluble Al: 0.1% or less, one or more of Ti and Zr (excluding those contained as oxides): 0.001 to 0.085% in total, Fe and unavoidable impurities: the rest And Mn + P: 0.25% or less, and By having a composition that satisfies the following conditions, it has excellent drawing workability without room temperature strain aging and appropriate bake hardenability. ” : 0.011 to 0.06%, Mn: 0.001 to 0.20%, P: 0.025 to 0.2%, S: 0.01% or less, N: 0.0005 to 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: Consist of the rest and Mn + P: 0.25% or less, and A steel slab having a composition that satisfies the following conditions (either a slab slab produced by continuous casting, or a slab obtained by slab-rolling a steel ingot obtained by an ingot making method, etc.)
After soaking at ℃ or more, hot rolling at Ar ₃ transformation point or more, then 720 ℃ or less, and the formula CT (℃) = {1 + C (wt%) + 1.8 × Mn (wt%)} × 60 to 95 after winding and descaling at a temperature above 500
% Cold rolling at a rolling reduction of more than recrystallization temperature and Ac ₃ transformation point or less to continuously anneal it, so that room temperature strain aging is not recognized and excellent drawability and moderate baking It is also characterized in that it is possible to stably manufacture a high-strength cold-rolled steel sheet for working, which exhibits hardness and uniformity over the entire length of the coil. "

Next, in the present invention, the reason why the composition of the steel and the conditions for manufacturing the steel sheet are limited as described above will be explained.

A) Component composition i) CC C is an element that is inevitably accompanied in steel, but if its content exceeds 0.06%, the volume fraction of cementite is too large and the ductility required for cold-rolled steel sheet for working is increased. On the other hand, on the other hand, if the content is less than 0.011%, it leads to an increase in cost and cannot meet one of the objects of the present invention.
When continuous annealing is applied, room temperature strain aging cannot be suppressed to a level where there is no practical problem unless extremely low carbonization is made even in the region of less than 0.003%, resulting in a great industrial disadvantage. . Therefore, the C content is set to 0.011 to 0.06%, but it is preferably adjusted to 0.011 to 0.02%.

ii) Mn Mn content is one of the important factors in the present invention, and it is preferable that the content is as low as possible. However, it is extremely difficult to stably and inexpensively set the content to less than 0.001% by the current steelmaking technology. On the other hand, when the Mn content exceeds 0.20%, the effect of promoting cementite coarsening at the time of winding in the hot rolling step cannot be obtained, and further, the co-presence with solid solution carbon increases the effect of suppressing recrystallization, thereby reducing drawability. A preferred recrystallization texture cannot be easily obtained. Therefore, the Mn content is limited to 0.001 to 0.20%.

iii) PP has the effect of strengthening steel, but its content is 0.02
If it is less than 5%, the desired strengthening cannot be achieved. On the other hand, if the content exceeds 0.2%, the steel sheet becomes too hard, and if the P content exceeds 0.25% in total with the Mn content, the room temperature strain aging Becomes noticeable, which makes it unsuitable for cold-rolled steel sheets for working. Therefore, the P content is set to 0.025 to 0.2%, and the total content of P and Mn is set to a value of 0.25% or less.

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

iv) S S is one of the unavoidable impurities that accompany in steel, but it has a stronger tendency to combine with Ti and Zr than Mn. Therefore, an increase in S content leads to an increase in the amount of Ti and Zr added. Therefore, the content is limited to an allowable range of 0.01% or less.

v) N N is also an impurity that is inevitably accompanied in steel and cannot be easily and stably suppressed to less than 0.0005% by the current steelmaking technology. When the N content exceeds 0.0070%, the amount of addition of Ti or Zr also increases and the manufacturing cost of the steel sheet rises.
Although it is limited to 0005 to 0.0070%, it is preferably adjusted to 0.0030% or less.

vi) Acid-soluble Al Acid-soluble Al is Ti or Zr after the molten steel is vacuum degassed.
When adding, it is to be added as a deoxidizer in advance in order to improve the yield of Ti and Zr, it is sufficient because it means that deoxidation is completely carried out if present even in a trace amount. . However, if the content exceeds 0.1%, the steel becomes hard and leads to a decrease in ductility.
The Al content was set to 0.1% or less.

vii) Ti and Zr Any of these components has the action of binding to S in the steel to prevent hot embrittlement of the steel and also to binding to N in the steel to eliminate the recrystallization suppressing effect. However, as a result, there is an effect that it is possible to reduce the Mn content and increase the P addition amount, but the total amount excluding those contained as oxides is less than 0.001%. The desired effect is not obtained in the above action, on the other hand, if the total content exceeds 0.085%, a large amount of TiC and ZrC are precipitated and the recrystallization suppressing effect due to these appears, resulting in an unfavorable result as a working steel sheet. . Therefore, the total content of Ti and Zr is 0.001 ~
Limited to 0.085%.

Furthermore, the Ti and Zr contents are And if Is adjusted to a range that satisfies.

The reason is as follows. That is, if the value of X is less than the lower limit of the above equation, S that does not combine with Mn, Ti or Zr
Exists, FeS is formed, and the hot embrittlement of the steel becomes a problem, and when the value of X is less than 0%, N in the steel is
A part of it must be fixed as AlN, and drawability cannot be obtained unless it is wound at high temperature. On the other hand, when the value of X exceeds the upper limit value of the above formula, TiC and ZrC are also precipitated in large amounts, and the recrystallization suppressing effect due to these appears, resulting in an unfavorable result as a steel sheet for working.

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

B) Steel plate manufacturing conditions The high-strength cold-rolled steel plate for processing according to the present invention is configured to have the above-described component composition. After soaking as above, hot rolling at Ar ₃ transformation point or higher, then 720 ℃ or lower, and the formula CT (℃) = {1 + C (wt%) + 1.8 x Mn (wt%)} x 500 Winding above the temperature represented by and after descaling 60 to 95
It is preferable to adopt a condition in which cold rolling is performed at a rolling reduction of%, and further, heating is performed in a temperature range of the recrystallization temperature or more and Ac ₃ transformation point or less and continuous annealing is performed. The reason is as follows.

i) Steel slab (slab) heating temperature If the heating temperature is less than 1000 ° C, temperature unevenness easily occurs in the heated steel slab, and it becomes difficult to complete hot rolling at the Ar ₃ transformation point or higher. The heating temperature was set to 1000 ° C or higher, but it is preferable to heat it to 1100 to 1200 ° C.

ii) Hot-rolling finishing temperature When the hot-rolling finishing temperature becomes lower than the Ar ₃ transformation point, the steel is hot-rolled in α + γ region or α region, and usually disappears with γ → α transformation. The hot-rolled texture remains in the hot-rolled sheet, hindering the development of a recrystallized texture that is favorable for drawability.

iii) Winding temperature If the winding temperature (CT) is lower than the formula {1 + C (wt%) + 1.8 × Mn (wt%)} × 500, cementite coarsening is insufficient even with low Mn steel. On the other hand, when wound in a temperature range of higher than 720 ° C., the scale becomes thick during winding, descaling property deteriorates, and abnormal grain growth occurs to generate coarse grains.

It is one of the features of the present invention that a cold-rolled steel sheet having a good drawability can be produced even at low temperature winding, but if possible, 60
It is preferable to wind in a temperature range of 0 to 650 ° C.

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

iv) Cold rolling reduction If the cold rolling reduction is less than 60%, a recrystallization texture that is favorable for drawability does not develop. Therefore, a higher reduction is preferable, but a reduction of more than 95% reduces the reduction. The quality will deteriorate. Therefore, the reduction ratio of cold rolling is set to 60 to 95%.

v) Annealing temperature Needless to say, since this annealing is recrystallization annealing, it needs to be heated above the recrystallization temperature, but when heated to a temperature range exceeding the Ac ₃ transformation point, α → γ → α is transformed. I mean that would erase the "preferred recrystallization texture in drawability" which was formed by the recrystallization process by annealing temperature must be kept below Ac ₃ transformation point.

Regarding the heat pattern of continuous annealing, after heating above the recrystallization temperature and below the Ac ₃ transformation point, 5
After holding for at least 2 seconds, it cools to a temperature range of 550 to 700 ° C at a cooling rate of 5 ° C / sec or less, and then 300 to 500 ° C from that temperature.
Cooling rate up to the temperature range: 20 ~ 300 ℃ / sec, or
If necessary, heat from 350 to 550 ℃ to 350
It is preferable to gradually cool to 2 ° C over 2 minutes, and further cool to a temperature range of 350 ° C to 250 ° C at a cooling rate of 5 ° C / sec or less.

Needless to say, the steel sheet manufactured in this manner is temper-rolled after annealing and shipped.

Next, the present invention will be described by way of examples in comparison with comparative examples, but this example merely shows one example of the present invention, which unduly limits the present invention. Not surprisingly.

<Example> Example 1 First, a steel having a chemical composition as shown in Table 2 was melted by a conventional method, and then hot forged to obtain a 30 mm-thick experimental slab.

Then, after heating these to 1250 ℃ for 30 minutes, 1150 ~ 90
Hot rolling was performed in a temperature range of 0 ° C to a plate thickness of 3 mm. At this time, as a winding simulation, immediately after hot rolling, 700 ° C, 600
C. and 520.degree. C. were rapidly cooled, inserted into an electric furnace maintained at each temperature, held for 30 minutes, and then subjected to furnace cooling at a cooling rate of 20.degree. C./min.

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

As shown in Fig. 5, the heater cycle at the time of this annealing was as follows.
After holding for 0 seconds, cooling rate up to 650 ℃ is 3 ℃ / sec.
Cool down to 400 ℃ at 100 ℃ / sec, then 400 ℃
From 350 to 350 ° C., it was gradually cooled at a cooling rate of 16 ° C./min, overaged for about 3 minutes, cooled to 250 ° C. at a cooling rate of 3 ° C./sec, and then water-cooled.

The cold-rolled steel sheet thus obtained was temper-rolled at an elongation of 1.2%, molded into a JIS No. 5 test piece, and subjected to a tensile test to measure the mechanical properties and the amount of bake hardening. The bake-hardening amount was defined as the amount of increase in the yield point when a prestrain of 2% was applied by tension and then tension was applied again, as shown in FIG.

 The results are shown in FIGS.

FIG. 7 shows the results regarding the effect of the carbon content in the steel on the mechanical properties of the annealed material when 0.09% P-0.15% Mn material was wound at 700 ° C. From FIG. When the carbon content in steel is 0.011% or more, the bake hardening amount is stable and 6 kgf / mm.
² or less, while too much carbon in steel 0.06%
It can be seen that the desired ductility cannot be obtained when it exceeds.

Further, FIG. 8 shows the results regarding the influence of the P content on the mechanical properties of the annealed material when 0.09% P-0.15% Mn material was wound at 700 ° C. , P content of more than 0.2%, the steel is too hard and is unsuitable for working.

And, FIG. 9 shows the results regarding the influence of the Mn content and the P content on the bake hardening amount of the annealed material when the 0.012% C material is wound at 700 ° C. , M
It can be seen that when the n content and the P content deviate from the conditions specified in the present invention, the bake hardening amount cannot be set to 6 kgf / mm ² or less. The numerical value in FIG. 9 is the bake hardening amount (kgf / mm ² ).

Furthermore, FIG. 10 shows the results regarding the influence of the Mn content and the winding temperature on the r value of the annealed material when the 0.012% C-0.09% P material was wound at 700 ° C. It can be seen from FIG. 10 that if the Mn content and the winding temperature deviate from the conditions specified in the present invention, a good r value cannot be obtained. The numerical value in FIG. 10 is the r value.

Example 2 A slab obtained by smelting steel having the chemical composition shown in Table 3 in a converter, performing vacuum degassing treatment on a part of the steel, and continuously casting the slab,
After heating to 1200 ° C, it was hot rolled and wound at 650 ° C.

Subsequently, the hot-rolled steel sheet thus obtained was pickled and then the rolling reduction ratio: 75
% Cold-rolled to produce a cold-rolled sheet with a thickness of 0.8 mm, then 760
Continuous annealing consisting of holding at 40 ° C for 40 seconds and overaging at 400 ° C for 3.5 minutes was performed, and further temper-rolled at an elongation of 1.0% to obtain a cold-rolled steel sheet product.

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

Also from Table 4, the cold-rolled steel sheet product manufactured according to the conditions of the present invention exhibits excellent characteristics as a high-strength cold-rolled steel sheet for working, whereas the manufacturing conditions deviate from the regulations of the present invention. It is clear that does not meet the desired properties.

<Summary of effects> As described above, according to the present invention, industrially useful effects such as the ability to stably obtain a high-strength cold-rolled steel sheet excellent in drawability and room temperature aging resistance even by continuous annealing are brought about. .

[Brief description of drawings]

1 is a graph showing the effect of Ti, Zr and Mn contents on the hot ductility of steel, FIG. 2 is a graph schematically showing the Mn and P content ranges specified in the present invention, 3 FIG. 4 is a graph schematically showing the Ti, Zr, and Mn content ranges specified in the present invention. FIG. 4 is a graph schematically showing the Mn content and winding temperature ranges specified in the present invention. Fig. 5 is a diagram showing a heat cycle for simulating continuous annealing, Fig. 6 is a drawing explaining the definition of bake hardening amount, and Fig. 7 is a 0.09% P-0.15% Mn material wound at 700 ° C. Fig. 8 is a graph showing the results of the effect of the amount of carbon in steel on the mechanical properties of the annealed material when taken, Fig. 8 is an annealed material when the 0.09% P-0.15% Mn material was wound at 700 ° C. Fig. 9 is a graph showing the results of the effect of P content on the mechanical properties of C, Fig. 9 shows the case of winding 0.012% C material at 700 ℃. 10 is a graph showing the results of the effects of the Mn content and the P content on the bake hardening amount of the annealed material, and FIG. 10 is an annealed material when the 0.012% C-0.09% P material is wound at 700 ° C. 3 is a graph showing the results of the effects of the Mn content and the winding temperature on the r-value of.

Claims (2)

[Claims] [Claim 1] C: 0.011 to 0.06%, Mn: 0.001 to 0.20%, P: 0.025 to 0.20%, S: 0.01% or less, N: 0.0005 to 0.0070%, Acid soluble Al: 0.1% Below, one or more of Ti and Zr (excluding those contained as oxides): 0.001 to 0.085% in total, Fe and unavoidable impurities: consisting of the rest, and Mn + P: 0.25% or less, and A high-strength cold-rolled steel sheet for working, characterized in that it has a composition that satisfies the following conditions. 2. C: 0.011 to 0.06%, Mn: 0.001 to 0.20%, P: 0.025 to 0.20%, S: 0.01% or less, N: 0.0005 to 0.0070%, Acid soluble Al: 0.1% Below, one or more of Ti and Zr (excluding those contained as oxides): 0.001 to 0.085% in total, Fe and unavoidable impurities: consisting of the rest, and Mn + P: 0.25% or less, and The steel slabs with the composition satisfying the following conditions are soaked at 1000 ° C or higher and hot-rolled at the Ar ₃ transformation point or higher, then 720 ° C or lower, and the formula CT (° C) = {1 + C (wt %) + 1.8 x Mn (wt%)} x 500
Winding above the temperature represented by and after descaling 60 to 95
A method for producing a high-strength cold-rolled steel sheet for working, comprising cold rolling at a rolling reduction of%, further heating in a temperature range not lower than the recrystallization temperature and not higher than the Ac ₃ transformation point and continuously annealing.