JPH1096052A - Slab for high strength cold rolled steel sheet excellent in workability or hot-dip plated steel sheet, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a slab, used for high strength cold rolled steel sheet excellent in workability and hot-dip plated steel sheet, with high productivity. SOLUTION: This slab has a composition consisting of 0.01-0.07% C, <=1.0% Si, 0.5-3.0% Mn, 0.05-0.2% P, 0.0005-0.02% S, 0.005-0.10% Al, <=0.010% N, and the balance essentially Fe and further containing, if necessary, 0.03-0.5% Cu and 0.03-0.5% Ni. Moreover, in the mixing proportion between pseudo-bainite structure and polygonal ferrite structure, pseudo-bainite structure comprises <=40%. This slab can further contain 0.03-0.5% Cu and 0.03-0.5% Ni. At the time of production, the slab is cooled at <=50 deg.C/sec cooling rate while the slab after solidification is in the temp. region of 950-750 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a slab for a high-strength cold-rolled steel sheet or a hot-dip coated steel sheet which is suitable for a body part of an automobile requiring a high degree of workability, and a method for producing the same.

[0002]

2. Description of the Related Art With the growing movement to protect the global environment, there has been a stronger demand than ever before for automobiles to reduce fuel consumption in order to reduce emissions and save energy. One of the leading measures for reducing fuel consumption is to reduce the weight of the vehicle body, and various high-strength steel sheets have been proposed. A high-strength steel sheet is an effective material for improving the safety of a vehicle body. Above all, cold-rolled steel sheets used for structural components of automobile bodies are subjected to severe forming mainly by deep drawing, and the products themselves require high component strength as important safety components. Therefore, there is a demand for a cold rolled steel sheet or a hot-dip coated steel sheet which has much more excellent workability than the conventional one and has higher strength than the conventional one.

In order to meet the demand for high workability and high strength, a slab containing a large amount of solid solution strengthening elements such as Si, Mn, and P based on a low-carbon Al-killed steel for continuous annealing is used. High-strength cold-rolled steel sheets for automobiles with excellent workability have been manufactured. Moreover, structural parts of automobile bodies have high value as important security parts, and their demands are increasing year by year, and the situation is growing from small-quantity production to mass production like ordinary materials.

[0004]

In order to increase the strength, more solid solution strengthening elements such as Si, Mn, and P are added than before,
Moreover, in a slab manufactured by increasing the cooling rate during continuous casting in order to improve productivity, a hard pseudo-bainite structure, which has not been seen before, comes to be generated. The hard pseudo-bainite structure causes many defects such as surface cracks, and as a result, reduces the product yield. Therefore, although mass production is aimed at, on the contrary, a large increase in manufacturing cost is caused. The present invention has been devised in order to solve such a problem, and by specifying the composition of the slab and the cast structure, excellent workability can be achieved without inducing productivity or increasing the production cost. It is an object to provide a high-strength cold-rolled steel sheet and a hot-dip coated steel sheet.

[0005]

The slab for a high-strength cold-rolled steel sheet or hot-dip steel sheet according to the present invention has a C content of 0.01 to 0.07% by weight and a Si content of 1.0% in order to achieve the object. %, Mn: 0.5 to 3.0% by weight, P: 0.05 to
0.2% by weight, S: 0.0005 to 0.02% by weight, A
1: 0.005 to 0.10% by weight, N: 0.010% by weight or less, the balance being substantially composed of Fe, and a pseudo bainite structure at a mixing ratio of pseudo bainite structure-polygonal ferrite structure. Is 40% or less. This slab further comprises Cu:
0.03 to 0.5% by weight and Ni: 0.03 to 0.5% by weight. At the time of manufacture, when the slab after solidification is in a temperature range of 950 to 750 ° C.,
It is necessary to cool the slab at a sub-second cooling rate.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have found that, in a slab having components and compositions set according to the strength and corrosion resistance of a cold-rolled steel sheet and a hot-dip coated steel sheet, the cast structure of the continuously cast slab is such that The effects on the properties of galvanized steel sheets were investigated and studied. As a result, when the pseudo-bainite structure in the slab microstructure is regulated to 40% or less, a high-strength cold-rolled steel sheet and a hot-dip coated steel sheet with excellent workability can be manufactured without inducing productivity and increasing manufacturing costs. I found what I could do. Si, Mn, P to improve strength
Pseudo-bainite is liable to be produced when a steel material containing a large amount of a solid solution strengthening element such as is continuously cast. The amount of pseudo-bainite produced decreases as the cooling rate of the solidified slab decreases, but productivity is impaired at an excessively slow cooling rate. According to the study of the present inventors, when the cooling rate in the temperature range of 950 to 750 ° C. is set to 50 ° C./second or less, the mixing ratio of the pseudo bainite structure is regulated to 40% or less, and surface cracks caused by pseudo bainite are reduced. It has been found that a slab for a high-strength cold-rolled steel sheet and a hot-dip coated steel sheet which is suppressed and has excellent workability without lowering the productivity can be obtained.

Hereinafter, alloy components, contents, manufacturing conditions, and the like specified in the present invention will be described. C: 0.01 to 0.07% by weight At the time of high-temperature winding in the hot rolling step, carbides such as Fe ₃ C are formed, and an effect of improving the Rankford value and elongation is exhibited. Such an effect becomes remarkable at a C content of 0.01% by weight or more. However, when a large amount of C exceeding 0.07% by weight is contained, the effect of improving the Rankford value and elongation is saturated. Si: 1.0% by weight or less The effect of strengthening the steel material is exhibited, and the amount of addition is determined according to the required strength. However, the Si content is 1.0% by weight.
If the ratio exceeds the range, the chemical conversion property, the plating property and the like tend to decrease.

Mn: 0.5 to 3.0% by weight Similar to Si, it has the effect of strengthening the steel material, and the amount of addition is determined according to the required strength. The effect of strengthening the steel material becomes remarkable at a Mn content of 0.5% by weight or more. However, when a large amount of Mn exceeding 3.0% by weight is contained, A
The r ₃ transformation point drops sharply, the recrystallization annealing becomes in the γ range,
Rankford value and growth are likely to decrease. P: 0.05 to 0.2% by weight Similar to Si, it has the effect of strengthening steel, and a necessary amount is added according to the target strength. However, if less than 0.05% by weight of P is added, the effect of strengthening the steel is insufficient. Conversely, if a large amount of P exceeding 0.2% by weight is added, workability, secondary workability, plating property, etc., deteriorate.

S: 0.0005-0.02% by weight Mn forms a sulfide with Mn and the like, affects the formation of carbide-based precipitates, and has the effect of improving the Rankford value. However, since it is a component that induces cracking during hot working, the upper limit is restricted to 0.02% by weight. Further, excessively reducing the S content requires a large cost for desulfurization and refining in the steel making process.
It was set to 05% by weight. Al: 0.005 to 0.10% by weight It is added as a deoxidizing agent and has an effect of fixing N. Such an effect is obtained when the Al content is 0.005% by weight or more.
It becomes remarkable in the content. However, when a large amount of Al exceeding 0.10% by weight is contained, the amount of oxide-based inclusions increases, and workability, surface properties, and the like deteriorate.

N: 0.010% by weight or less N is an unavoidable component and is fixed with Al or the like.
However, when a large amount of N exceeding 0.010% by weight is contained, it is required to increase the amount of Al necessary for fixing N, and the workability is deteriorated due to an increase in precipitates. Cu: 0.03 to 0.5% by weight An alloy component added as necessary, and has an effect of improving corrosion resistance. The effect of adding Cu is 0.03% by weight.
It becomes remarkable at the above content and becomes saturated at 0.5% by weight. Ni: 0.03 to 0.5% by weight Since it has an action of preventing hot brittleness caused by Cu, it is an effective alloy component in a system to which Cu is added in order to improve corrosion resistance. In order to obtain such an effect, Cu
Preferably, the Ni content is approximately the same as the Ni content,
Therefore, when adding Ni, the range is set to 0.03 to
Set to 0.5% by weight.

Cast structure: Mixed structure of pseudo-bainite-polygonal ferrite having a pseudo-bainite structure of 40% or less A molten steel having the components and compositions adjusted as described above is continuously cast into a slab. The obtained slab has a microstructure that changes to a pseudo bainite structure, a polygonal ferrite structure, or the like, which is one of the low-temperature transformation products, according to the cooling rate. The pseudo-bainite structure has a particularly high transition density and is hard, and may cause defects such as surface cracks in the slab. On the other hand, the polygonal ferrite structure has a low transition density and is soft, and has a high resistance to surface cracking. As a result of investigating and studying the effects of pseudo-bainite structure and polygonal ferrite structure on surface cracking, if the pseudo-bainite structure is restricted to 40% or less by a mixing ratio of pseudo-bainite structure-polygonal ferrite structure, the effect of pseudo-bainite structure is reduced. It was clarified that the slab was suppressed and no defects such as surface cracks were generated. Although the reason for this is not necessarily clear, it is presumed that the cause is that the soft polygonal ferrite structure is dominant.

Cooling conditions for slab: Cooling rate of 950 to 750 ° C. at a cooling rate of 50 ° C./sec or less In order to obtain a microstructure with a pseudo bainite structure of 40% or less, the slab after solidification must be 950 to 750 ° C. When in the temperature range, it is necessary to cool the slab at a cooling rate of 50 ° C./sec or less. This temperature range is in the two-phase range of ferrite and austenite, and is effective in generating a polygonal ferrite structure or a pseudo-bainite structure from the γ phase. When the temperature range deviates from specific conditions, the pseudo bainite phase 40
% Or less tissue cannot be obtained. At a cooling rate exceeding 50 ° C./sec, a structure having a pseudo bainite phase of 40% or less cannot be obtained in any steel component. However, since an excessively slow cooling rate causes a decrease in productivity, it is preferable to set the lower limit of the cooling rate to 5 ° C./sec. The slab having the composition and structure adjusted according to the present invention is hot-rolled,
It is used as a high-strength cold-rolled steel sheet that has been subjected to pickling and cold rolling processes and then recrystallized and annealed, or as a hot-dip coated steel sheet that has been hot-dip after recrystallization annealing.

[0013] In the hot rolling, a heating temperature of 1000 to 120
It is preferable to use a low-temperature heating of 0 ° C. and employ a high-temperature winding of 600 ° C. or higher, which is higher than the Ar ₃ transformation point of the finishing temperature. Low-temperature heating and high-temperature winding are effective in fixing C and N solidly dissolved in steel as carbides, nitrides, and carbonitrides such as Fe ₃ C and AlN to improve workability. It is. When hot rolling is performed at a finishing temperature that does not reach the Ar ₃ transformation point, a hot rolled texture that causes deterioration in workability is formed. The hot-rolled coil is subjected to descaling by pickling and then cold-rolled. In cold rolling, the rolling reduction is preferably set to 60% or more in order to obtain sufficient deep drawability. Recrystallization annealing after cold rolling is a continuous annealing process,
Alternatively, when hot-dip plating of Zn, Al, or the like is performed, the hot-dip plating is performed in a reduction annealing furnace in a hot-dip plating facility. The plating conditions are not particularly limited, and conditions generally used industrially are selected. Further, the cold-rolled steel sheet after annealing and the hot-dip coated steel sheet after hot-dip coating can be subjected to temper rolling of 5% or less.

[0014]

EXAMPLE After the molten steel was adjusted to the composition shown in Table 1 in a melting furnace, it was poured into a continuous casting mold, and was cooled at 950 to 750 ° C. in a cooling process after solidification.
The casting was performed by changing the cooling rate in the temperature range of 10 to 120 ° C./sec to produce a 50 kg steel ingot.

[0015]

The surface of the obtained steel ingot was observed with the naked eye, and the presence or absence of surface cracks was examined. Subsequently, the number of surface cracks was counted in a magnetic particle flaw detection test. Moreover, the microstructure of the steel ingot was observed using an optical microscope. As shown in Table 2, when the cooling rate of the steel ingot is regulated to 50 ° C./sec or less according to the present invention, the structure ratio of pseudo bainite is suppressed to 40% or less, and no surface cracking occurs. Was. On the other hand, in the comparative examples of Test Nos. 5 and 7 in which the cooling rate of the steel ingot exceeded 50 ° C./sec, the structure ratio of pseudo bainite was 40%.
And the surface cracks occurred frequently. Further, when the relationship between the occurrence of surface cracks and the microstructure and the cooling rate during casting was investigated, the relationship shown in FIG. 1 was established between the two. That is, the pseudo bainite structure in the microstructure
In the case where the pseudo bainite structure is 40% or less in the mixing ratio of the polygonal ferrite structure, and when the casting is performed by cooling the temperature range of 950 to 750 ° C. at a cooling rate of 50 ° C./sec or less after solidification, the surface cracks are generated. No generation was observed, indicating that a good slab was obtained.

[0017]

A steel ingot having no surface cracks was hot forged into a 35 mm thick steel slab, then heated to 1150 ° C. and hot rolled. The finishing temperature of the hot rolling was set at 860 to 900 ° C. and higher than the Ar ₃ transformation point. The finished plate thickness was set to 5 mm in consideration of the rolling reduction in the subsequent cold rolling. After finishing the hot rolling, the steel strip is charged into a salt bath heated to 650 to 680 ° C., heated to a predetermined temperature, and held for about 2 hours to perform a process corresponding to the winding of the hot rolled steel strip. did.
Subsequently, descaling pickling was performed, and a cold-rolled steel sheet having a thickness of 1.0 mm was formed by cold rolling at a rolling reduction of 80%. This cold-rolled steel sheet was continuously annealed at a temperature of 800 ° C. or higher than the recrystallization temperature.

The mechanical properties of the obtained cold rolled steel sheet were measured according to JIS.
It measured using the No. 5 tensile test piece. The Rankford value was measured by a three-point method after 15% tensile prestrain was applied, and the L direction (rolling direction), the D direction (45 degrees in the rolling direction), and the C direction (direction perpendicular to the rolling direction). )] ((R _L + 2r _D + r _C ) / 4). Also,
1. Drawing ratio using a blank punched to 90 mm in diameter
After forming a flat-bottom cylindrical cup with a diameter of 33 mm by three-stage drawing of 7, a punch with a tip angle of 60 degrees is pushed in from the upper part of the cylinder while being immersed in a refrigerant of various temperatures composed of liquid nitrogen and an organic solvent, so that brittle cracks do not occur. The lowest temperature was measured.
This measurement temperature was defined as a secondary working crack generation temperature. As shown in Table 3, the cold rolled steel strip manufactured from the steel ingot of which the cooling rate is controlled according to the present invention has excellent 0.2% proof stress, tensile strength and elongation, and has a Rankford value of 1 as shown in Table 3. .
It can be seen that the workability is excellent when the embrittlement temperature for secondary working cracks is -100 ° C or less. On the other hand, even when the cooling rate of the slab is 50 ° C./sec or less as specified in the present invention, in Test No. 15 which does not satisfy the conditions regarding the composition, the Rankford value is low and the secondary working crack embrittlement temperature is high. Therefore, the material was liable to cause secondary working cracks.

[0020]

Example 2 Molten steel having the composition shown in Table 4 was refined in a converter and a degassing furnace, and the thickness was 250 mm while changing the cooling speed of the slab by adjusting the drawing speed and the amount of cooling water during continuous casting. A slab with a unit weight of 13 tons was manufactured.

[0022]

The obtained slab was examined for surface cracks, microstructure, etc. in the same manner as in Example 1. As can be seen from the survey results in Table 5, the cooling rate of the steel
When the temperature was regulated to 0 ° C./second or less, it was found that the structure ratio of pseudo bainite was suppressed to 40% or less, and no surface cracking occurred. On the other hand, in the comparative examples of Test Nos. 20 and 25 in which the cooling rate of the steel ingot exceeded 50 ° C./sec, the structure ratio of pseudo bainite greatly exceeded 40%, and surface cracks occurred frequently.

[0024]

In the comparative example in which the surface cracks occurred in the slab stage, since the process passing after the hot rolling could not be performed, only the slab having no surface cracks was heated to 1150 ° C. in a heating furnace.
Hot rolled. Finishing temperature of hot rolling is 860-900
° C and above the Ar ₃ transformation point, the finished plate thickness to 5 mm,
The winding temperature was set at 650-680 ° C. The hot-rolled steel strip was passed through a continuous pickling line equipped with a hydrochloric acid-based pickling solution tank, descaled, sent to a cold rolling mill, and cold-rolled at a cold rolling rate of 80%. The obtained cold-rolled steel sheet having a thickness of 1.0 mm was sent to an annealing step and annealed at a recrystallization temperature or higher to obtain a product of a cold-rolled steel sheet. Some steel strips were subjected to reduction annealing at a recrystallization temperature or higher and passed through a continuous hot-dip galvanizing line with a plating bath temperature of 450 ° C. to obtain products of hot-dip galvanized steel sheets. The mechanical properties of each of the obtained products were examined in the same manner as in Example 1. As can be seen from the survey results in Table 6, each of the product steel strips has excellent mechanical properties such as 0.2% proof stress and tensile strength, and has high elongation, Rankford value and low secondary work cracking embrittlement temperature. From the results, it can be seen that it has good workability.

[0026]

[0027]

As described above, in the present invention, by using a steel material having a specified composition and controlling the microstructure of a cast slab and the cooling rate during continuous casting, productivity is reduced and A high-strength cold-rolled steel sheet and a hot-dip coated steel sheet excellent in deep drawing can be produced without increasing the production cost.

[Brief description of the drawings]

Fig. 1 Effect of cooling rate during casting on microstructure and occurrence of surface cracks

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Teruo Tanaka 11-1 Showa-cho, Kure-shi, Hiroshima Pref.

Claims (3)

[Claims] C: 0.01 to 0.07% by weight, Si:
1.0% by weight or less, Mn: 0.5 to 3.0% by weight, P:
0.05-0.2% by weight, S: 0.0005-0.02
Wt%, Al: 0.005 to 0.10 wt%, N: 0.
010% by weight or less, with the balance substantially consisting of Fe, and a cast structure having a pseudo bainite structure-polygonal ferrite structure in a mixed ratio of 40% or less by pseudo-bainite structure. Slab for high strength cold rolled steel sheet or hot-dip coated steel sheet. 2. The slab for a high-strength cold-rolled steel sheet or hot-dip coated steel sheet according to claim 1, further having a composition containing 0.03 to 0.5% by weight of Cu and 0.03 to 0.5% by weight of Ni. . 3. The high-strength cold-rolling according to claim 1, wherein the slab is cooled at a cooling rate of 50 ° C./second or less when the slab after solidification is in a temperature range of 950 to 750 ° C. A method for producing a slab for a steel sheet or a hot-dip coated steel sheet.