KR100270395B1 - The manufacturing method forlow alloy composite structure type high strength cold rolling steel sheet with excellent press workability - Google Patents

Abstract

PURPOSE: A method for manufacturing high tensile strength cold rolled steel sheet having superior press processability is provided for use in automobile panel requiring low yield ratio(yield strength/tensile strength) and superior press processability. CONSTITUTION: The high tensile strength cold rolled steel sheet is manufactured by homogenizing an Al-killed steel comprising C 0.05-0.1wt.%, N 0.005wt.% or less, S 0.02wt.%, Mn 1.2-1.7wt.%, silicon 0.2-0.6wt.%, Al 0.03-0.06wt.%, B 0.0030-0.0080wt.%, a balance of Fe, and other inevitable impurities in the temperature range of 1100 to 1250deg.C where the equivalent of Mn is adjusted to over 1.72%; finish hot rolling at over Ar3 transformation point of 900 to 930deg.C; hot coiling the steel sheet in the temperature range of 600 to 700deg.C; cold rolling it at a reduction rate of 40 to 80%; continuous annealing the cold rolled steel sheet in the temperature range of 800 to 850deg.C where the continuous annealing process is controlled to form combined texture steel even in the severe quenching zone of which cooling rate is -40deg.C/sec or lower.

Description

Manufacturing method of low alloy composite structure high strength cold rolled steel sheet with excellent press formability

The present invention relates to a method for producing a high strength cold rolled steel sheet having excellent workability and no cracking during press working. In particular, low carbon aluminum-killed steels have manganese (Mn), silicon (Si) and boron (B). By adjusting the addition amount and the addition ratio of the steel, etc., the composite structure type high strength cold rolled steel sheet is manufactured, so that there is no yield point stretching phenomenon showing discontinuous yielding behavior in the cold rolled steel sheet, and the yield ratio (yield strength / tensile strength) is low, which is excellent in press workability. Therefore, it can be used as a steel sheet of a product that requires high processability such as inner and outer plates of automobile materials, and can be used for parts requiring high strength such as reinforcing materials of automotive steel sheets.

Conventional high strength cold-rolled steel sheet is added to the low-carbon aluminum-kilted steel by adding manganese (Mn), silicon (Si), phosphorus (P), such as solid solution strengthening element to improve the strength by using the solid solution strengthening effect of the substitution type element, Niobium (Nb), titanium (Ti), vanadium (V), and the like, which are precipitate forming elements, have been added to improve the strength using these precipitates to produce high strength cold rolled steel sheets.

However, in the case of solid solution hardened steel added with manganese (Mn), silicon (Si), and phosphorus (P), the strength increases but the elongation decreases.In addition, after annealing, the yield point elongates due to the presence of solid carbon. It exhibits behavior.

The discontinuous yielding behavior and high yield ratio (yield strength / tensile strength) caused by the yield point stretching result in stretcher strain defects on the surface of the steel sheet during the processing of cold rolled steel such as press work, and because of the high yield point, back) bad shape freezing.

Precipitation hardened steels containing niobium (Nb), titanium (Ti), vanadium (V), etc. also exhibit yield point stretching, and have low yield points and very low elongation at the same tensile strength. There is this.

Recently, manganese, chromium (Cr), molybdenum (Mo), etc., which are high reinforcing elements, are added to low carbon aluminum-kilted steel, and the cooling rate is corrected in the quenching zone of the continuous annealing process, thereby martensite and ferrite. Manufactures a composite tissue steel present at the same time.

However, this method has a disadvantage in that the manufacturing cost of the steel sheet is very large because a large amount of manganese, an element having a high reinforcing capacity, must be added. In addition, the martensite is produced when the cooling rate is very high in the quenching zone of the continuous annealing process. Additional cooling equipment is needed to increase the manufacturing cost.

In addition, chromium (Cr) and molybdenum (Mo) may be added together with manganese to form martensite at a low cooling rate, but in this case, it is difficult to adjust the composition in steelmaking by adding a large amount of chromium and molybdenum. Not only does the surface property of the steel sheet worsen, there is a disadvantage in that the manufacturing cost is greatly increased.

The present invention has been invented to solve the above problems in consideration of the above problems, by appropriately adjusting the addition amount and the addition ratio of manganese, silicon, boron (B) to the low-carbon aluminum-kilted (Al-killed) steel, high strength cold rolling There is no yield point stretching phenomenon showing discontinuous yielding behavior in steel sheet, and it has excellent shape freezing during press processing with low yield ratio, so it can be used as a high strength cold rolled steel sheet that requires high processability such as interior and exterior of automotive materials.

In addition, the steel produced by the present invention can be applied to parts requiring high strength, such as bumpers of automobiles because of the high tensile strength. In order to manufacture the composite steel, conventionally, the rapid cooling was performed in the quenching zone of the continuous annealing line or by adding a large amount of alloying elements. It is possible to manufacture composite tissue steel at -40 ℃ / sec cooling rate without additional installation of rapid cooling equipment in the quenching table, and to manufacture and provide high strength cold rolled steel sheet with excellent workability at low manufacturing cost. Is there.

1a, b, c, d is a graph showing the change in mechanical properties according to the amount of boron added.

In order to achieve the above object, the method for producing a low alloy composite structured high strength cold rolled steel sheet having excellent press formability of the present invention is excellent in processability, and in producing a high strength cold rolled steel sheet without cracking during press working, carbon by weight 0.05 ~ 0.1%, nitrogen 0.005% or less, sulfur 0.02%, manganese 1.2-1.7%, silicon 0.2-0.6%, aluminum 0.03-0.06%, boron 0.0030-0.0080%, but the manganese equivalent is adjusted to 1.72% or more, After the homogenization treatment of aluminum-kilted steel containing elements inevitably contained in the production of steel at about 1100 ~ 1250 ℃, the finish hot rolling temperature is 900 ~ 930 ℃ just above Ar3 transformation point, and hot roll winding has a temperature range of 600 ~ 700 ℃. After the cold rolling, the rolling reduction is carried out at 40 to 80%, the annealing is continuously performed in the range of 800 to 850 ° C, and the composite tissue steel is formed even at a cooling rate of -40 ° C / sec or less. Continuous annealing And that is characterized.

At this time, even in the quench zone of continuous annealing, the composite tissue steel should be manufactured even if the cooling rate is below -40 ℃ / sec.

In the production of steel sheet, the addition of manganese serves to inhibit the transformation of austenite into ferrite to form martensite, known as a hard phase in steel, to strengthen the material. In order to reinforce the material by the addition of manganese, very fast cooling equipment such as water cooling (cooling rate-2000 ° C / sec) in the quenching zone of the continuous annealing process is required, or a large amount of manganese must be added.

However, if a small amount of boron is added to the low carbon aluminum steel with manganese added so that the B / N ratio is 1 or more, some of the boron forms boron nitrite and has the effect of removing nitrogen from the steel. It segregates at the grain boundaries and serves to suppress the transformation of austenite into ferrite. Since the transformation of austenite to ferrite inhibits the martensite transformation, the mixture of martensite and ferrite at room temperature is achieved by using a continuous annealing facility at low cooling rate (below -40 ℃ / sec) in the quenching zone by adding a small amount of boron. Tissue steel can be produced.

Therefore, no additional equipment for rapid cooling in the quench zone of the low cooling annealing line is required. Composite steel exhibits continuous yielding behavior during plastic deformation, and has a low yield ratio. Therefore, in the case of the present invention steel, even though it is a high strength steel, press workability becomes very easy.

And since the martensite structure is formed at room temperature, the tensile strength is very high. Since the solid solution carbon of the room temperature ferrite structure is very small, a material having a very high elongation and work hardening ability is obtained.

According to the present invention, the amount of manganese and silicon and the ratio of boron and nitrogen are controlled and added, so that a composite structure of martensite and ferrite is formed at room temperature even at a low cooling rate of the quenching zone of a continuous annealing facility, and thus has excellent press workability. It relates to a method capable of producing a cold rolled steel sheet.

Hereinafter, the reason for numerical limitation in the present invention will be described in detail.

When the amount of carbon (C) is less than 0.05% by weight (hereinafter referred to as%), since the amount of carbon is not sufficiently concentrated on the austenite during annealing, the transformation from austenite to ferrite is changed to martensite. Becomes difficult. In this case, the material at room temperature exhibits a discontinuous yielding behavior in which yield point elongation occurs, resulting in low shape freezing during press working.

And since martensite is not produced at room temperature, the tensile strength is not high. When the amount of carbon is more than 0.1%, the amount of martensite present at room temperature increases, so the tensile strength increases because the amount of martensite present at room temperature increases, but the tensile strength increases, but the elongation decreases, thereby decreasing workability. .

When the amount of carbon is 0.1% or more, cracks are generated at the edges of the cast steel, so the amount of carbon is limited to 0.05 to 0.1%.

Manganese does not form martensite at room temperature when added below 1.2%. If martensite is not formed, it exhibits a discontinuous yield behavior during processing, and the press workability is very poor because the yield ratio is high and the shape freezing is bad.

When the amount of manganese is 1.7% or more, martensite is easily formed at room temperature, but when added simultaneously with boron, the fraction of martensite is increased at room temperature, and the tensile strength is greatly increased, but the elongation is decreased, thereby greatly decreasing workability.

In addition, the addition of a large amount of manganese implies a large increase in manufacturing cost, so that the high manganese composite steel is difficult to be used commercially. In general, sulfur is an element that is inevitably contained in the production of steel, so the range of addition was limited to 0.02% or less.

Silicon is known as a stabilizing element for ferrite that promotes ferrite transformation, but since it is a substitutional element, it plays a role in suppressing the transformation of austenite to ferrite. If the amount of silicon is less than 0.2%, it is difficult to form martensite at room temperature, and thus a composite steel of martensite and ferrite cannot be obtained. If the amount of silicon is 0.6% or more, the composite tissue steel can be obtained at room temperature, but the upper limit is limited to 0.6% because the weldability of the steel sheet is very deteriorated.

Aluminum is added for deoxidation in steel, but when the amount of aluminum is less than 0.03%, oxygen is present in the steel, so if manganese or boron oxide forming elements are added during steelmaking, it is very small. It is difficult to control the composition of boron added with. And when the amount of aluminum is more than 0.06%, the amount of aluminum is added more than necessary to increase the manufacturing cost, so the upper limit of the amount of aluminum was limited to 0.06%.

When boron is added to low carbon steel alone, boron combines with nitrogen present in the steel to form boron nitride (BN). When the addition amount of boron is less than 0.003%, it is difficult for boron to be in solid solution because it is difficult to sufficiently remove nitrogen present in the steel. Since boron, which has a solid solution state in the steel, is segregated at grain boundaries and suppresses the transformation of austenite to ferrite, the role of boron in forming a composite steel at room temperature is very important. Therefore, the lower limit of boron addition amount was limited to 0.003%. 1 shows the change in mechanical properties according to the amount of boron added. When the addition amount of boron is more than 0.003%, the yield point is eliminated, the yield ratio is 0.6 or less. This is because martensite is formed in the steel at room temperature, thereby generating an operation potential during plastic deformation.

Since the solid solution of boron in the steel segregates at the austenite grain boundary and suppresses transformation, martensite can be produced even at a cooling rate of -40 ° C / sec or lower in the quench section of continuous annealing. When the boron was more than 0.008%, the boron carbide was formed to precipitate strengthening, and the increase in solid boron caused the tensile strength to increase more than necessary and the elongation rapidly decreased, so the upper limit was set to 0.008%.

Since the nitrogen in the steel combines with boron to reduce the amount of solid solution boron, the amount of nitrogen must be reduced or the amount of boron is increased so that the atomic ratio between boron and nitrogen is always one or more. However, as the amount of nitrogen increases, the amount of boron to form solid solution boron is increased by increasing the amount of boron nitride, so the amount of nitrogen in the steel is limited to less than 0.005%.

The steel dissolved in the composition is subjected to homogenization treatment at about 1100-1250 ° C. When the homogenization treatment temperature is 1250 ° C or more, fine precipitates are mainly precipitated, resulting in poor workability. Therefore, the homogenization temperature range was limited to 1100 ~ 1250 ℃.

Homogenized specimens are finished hot rolled at 900 ~ 930 ℃ directly above Ar3 temperature, and wound at 660 ~ 700 ℃ to obtain fine hot rolled sheet structure, and nitrogen in the steel is combined with boron boron nitride. To form residual boron in the steel. The hot rolled coil temperature is higher than 600 ℃ to promote the production of aluminum nitride to increase the amount of solid boron present in the steel to reduce the amount of boron added.

When the coiling temperature is 700 ° C or higher, the grains become coarse and the size of the precipitate increases, making it difficult to obtain high strength in the cold steel sheet.

Therefore, the coiling temperature of the hot rolled steel sheet was limited to 600 ~ 700 ℃. If the reduction ratio does not have a significant effect on the material, but the reduction ratio is not sufficient, the lower limit ratio of the reduction ratio is 40% because the workability is low.As the reduction ratio increases, the grain size becomes finer and the machinability increases. When this ratio is 80% or more, the load on the rolling mill is greatly increased during cold rolling, so the upper limit of the reduction ratio is set to 80%.

The cold rolled steel sheet is maintained at an annealing temperature of 800 ° C. or higher for 30 seconds or more, and is subjected to slow cooling to 680 ° C., followed by rapid cooling at a cooling rate of -40 ° C./sec or lower, and 120 seconds in a 450 to 350 ° C. section. Continuous annealing for performing the above-described transient processing is performed.

Hereinafter, the present invention will be described in detail through examples.

Table 1 below shows the chemical components of the inventive steel and the comparative steel. The molten steel ingot was held at 1250 ° C. for one hour and then hot rolled. At this time, the hot rolling finish temperature was 900 ℃, the coiling temperature 620 ℃, cold rolling with a cold reduction rate of 55 ~ 75% and then annealing was carried out continuously annealing temperature 780, 800, 830 ℃. During continuous annealing, the cooling rate of the quench zone was -25 ° C / sec. The specimens after continuous annealing were subjected to a tensile test using a universal tensile tester.

Table 2 below shows the change of mechanical properties in the annealing temperature of the inventive steel and the comparative steel.

In Table 2, the sample steels 1 and 2 of the present invention show excellent workability while having a high strength of 50 kgf / mm 2 or more, elongation 30% or more, yield ratio 0.60 or less, yield strength and tensile strength of 1800 or more. have. Yield point elongation does not appear in the tensile curve, and yield ratio is below 0.6.

However, in the case of the comparative steel sample No. 3, since the addition amount of boron is not sufficient, the composite tissue steel is not formed because martensite was not produced to generate sufficient operating potential to eliminate yield point stretching at room temperature. Nitrogen in the river precipitates all of the boron as boron nitride, so there is no high boron. Therefore, since there is no element in the steel that delays the change of austenite to ferrite, a sufficient amount of martensite is not produced to exhibit continuous yield behavior at room temperature.

* Hot Rolling Condition: Slab Reheating Temperature: 1250 ℃, Hot Rolling Finishing Temperature: 900 ℃

Winding Temperature: 620 ℃

* Cold rolling condition: Cold rolling reduction rate: 55%

* Annealing condition: Annealing time: 30 seconds, Quenching speed: -25 ℃ / sec

* Yield Ratio = Yield Strength / Tensile Strength

In case of sample No. 4, the amount of manganese is not sufficient, so martensite is hardly produced under low cooling rate. In the quench zone of continuous annealing, the critical manganese equivalents that can produce martensite when the cooling rate is below -40 ℃ are reported as 1.72. Sample No. 4 has low manganese content, so martensite is hardly produced at 1.30 of manganese equivalent. Therefore, in this case, the composite structure steel is not formed at room temperature, and the shape freezing property worsens during press working.

In the case of sample No. 5, the calculated value of manganese equivalent is somewhat lower than that of manganese equivalent which can form martensite at room temperature under low cooling rate because no silicon is added to steel. Therefore, in this case, as in the case of Sample No. 4, the composite tissue steel is not formed at room temperature, and the press workability deteriorates.

In the case of Sample Nos. 6 and 7, manganese, silicon and phosphorus were added to form martensite at room temperature, but low cooling below -40 ℃ / sec because solid boron, which is a metamorphic inhibitor, does not exist in steel. Under speed, it is difficult to form composite tissue steel at room temperature.

Therefore, the tensile strength of the room temperature material is as high as 50kgf / ㎜ but the elongation is not high. In addition, the yield point elongation is shown, and the yield ratio is relatively high at about 0.7, so that the shape freezing property during the press working is relatively bad.

The present invention as described above by controlling the addition amount and the addition ratio of manganese, silicon, boron, etc. to the low-carbon aluminum-kilted steel to produce a composite high-strength cold-rolled steel sheet, there is no yield point stretching phenomenon showing a discontinuous yielding behavior in cold-rolled steel sheet It has a low yield ratio, which is excellent in press workability, and can be used as a steel sheet of a product that requires high processability such as interior and exterior of automotive materials, and can be used for parts requiring high strength, such as reinforcing materials for automotive steel sheets.

Claims (1)

In manufacturing high strength cold rolled steel sheet with excellent workability and no cracking during press working, carbon by weight 0.05 ~ 0.1%, nitrogen 0.005% or less, sulfur 0.02%, manganese 1.2 ~ 1.7%, silicon 0.2 ~ 0.6%, aluminum Add 0.03 ~ 0.06%, boron 0.0030 ~ 0.0080%, adjust manganese equivalent to 1.72% or more, and homogenize aluminum-killed steel containing elements inevitably contained in steel production at 1100 ~ 1250 ℃, then finish hot The rolling temperature is set at 900 to 930 ° C, which is directly above the Ar3 transformation point, and the hot rolled winding is performed at a temperature range of 600 to 700 ° C, followed by rolling at a cold reduction rate of 40 to 80%, and annealing at 800 to 850 ° C. A method of manufacturing a low alloy composite tissue type high strength cold rolled steel sheet having excellent press formability, characterized by performing continuous annealing, and performing continuous annealing to produce a composite steel even at a cooling rate of -40 ° C / sec or less.