JPH021217B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a high-strength cold-rolled steel sheet that can be produced in a hot-melt plating line or a continuous annealing furnace and has the property of being hardened during painting baking and has excellent deep drawability. In recent years, efforts have been made to increase the strength of thin steel sheets for automobiles, and in order to improve dent resistance, there is a strong desire for high-strength cooling steel sheets with paint-baking hardenability, especially high-strength hot-dip galvanized steel sheets. This bake hardening utilizes the fixation phenomenon of dislocations due to solid solution C and solid solution N, but supersaturated solid solution C in ferrite hardens the steel sheet, deteriorates workability, and requires advanced processing. For example, it does not withstand deep drawing processing, and also undergoes strain aging at room temperature, which is accompanied by the occurrence of stretch strain. On the other hand, as a product with improved deep drawing workability etc.
Ti-added steel is known, but since this Ti-added steel completely fixes the solid solute C and N in the steel to improve workability, it cannot be expected that the solid solute C and N will impart bake hardenability. do not have. Furthermore, in order to improve deep drawability, there is an attempt to add a small amount of Ti and leave a small amount of solid solution C in the steel to give it a weak bake hardenability (Japanese Patent Application Laid-Open No.
53-114747), this steel plate does not give any consideration to high strength, which is a basic characteristic among the above-mentioned required characteristics. In view of the above-mentioned circumstances, the present invention has been made with the object of increasing the strength of cold-rolled steel sheets, particularly hot-dip galvanized steel sheets, while providing paint-baking hardenability and deep drawability. That is, the present invention contains 0.003 to 0.02% by weight of C (hereinafter, % is expressed as % by weight), 0.1 to 1% of Si, and 0.05% of Mn.
~0.5%, P0.03~0.15%, Al0.01~0.06%,
Contains N0.006% or less, and contains one or more of Ti and Nb in the range of 0.4 to 0.8 according to the formula below, with the balance being iron and impurities. The paint has excellent bake hardenability and deep drawability. It is a high-strength cold-rolled steel plate. K=(effective Ti%/48+Nb%/93)/(C%/12+N%/
14) However, effective Ti is total Ti excluding Ti in the form of oxides and sulfides. In addition, in the present invention, in addition to the above, Cr0.1 to 0.4
%, rare earth elements 0.05% or less, and B 0.01% or less, contributing to further improvement of deep drawability. In the steel plate of the present invention, K in the above formula is set in the range of 0.4 to 0.8 with C0.003 to 0.02%, Si0.1 to 1%,
By containing 0.03 to 0.15% of P, the deterioration in deep drawability that is observed when K in the above formula is set to 1.0 or less and high strength is attempted with other elements such as Mn can be avoided. In other words, despite the presence of solid solution C in the steel sheet of the present invention, the presence of appropriate amounts of Si and P makes it possible to develop a good {111} recrystallized texture even in rapid annealing such as continuous annealing. be. Incidentally, since Si and P are solid solution strengthening elements, they contribute to increasing the strength of the steel sheet, and baking hardening can also be achieved by the remaining solid solution C. By the way, this solute carbon causes differences in the range and extent of the restraint region in rotation toward the final stable orientation near the grain boundaries due to differences in the mobility of the slip system and the grain boundary restraint force during cold rolling texture formation. The behavior discovered in the present invention is thought to deteriorate the {111} recrystallized texture because it causes (110) and (200) grains to nucleate earlier than (111) grains during recovery recrystallization. When Si and P are contained, even if solid solution C exists, the effect of solid solution C as described above is alleviated, and the cold rolling texture behaves as if it does not contain solid solution C. Therefore, it is considered that the {111} recrystallized texture can be developed even in rapid annealing. Next, the reason for limiting the chemical composition of the steel sheet of the present invention will be described. When the amount of C increases, the growth of ferrite grains is suppressed, the amount of TiC precipitated increases, and the recrystallization temperature becomes high. Excessive solid solution C tends to cause hardening of the temper-rolled steel sheet before press working and deteriorates deep drawability, so the upper limit is set at 0.02%. Therefore, in order to avoid containing excessive solid solution C, molten steel is first decarburized by vacuum degassing treatment, but with the current technology, 0.003
It is extremely difficult to stably reduce C to less than %, and this results in a rapid increase in the amount of oxygen in the steel, resulting in deterioration of formability. Therefore, the lower limit is set at 0.003%. Since N lowers the elongation property value and therefore deteriorates the elongation property in press working, etc., the less the content, the better, and the upper limit is set at 0.006%. Si, together with P, is an essential element in the present invention to improve deep drawability within the range of K0.4 to 0.8 in the above formula, and for this purpose, it must be contained in an amount of 0.1% or more. On the other hand, if it exceeds 1%, this effect will be saturated, which will not only deteriorate the deep drawability but also reduce the oxide scale generated in the gas cleaning process of the plating pretreatment when trying to obtain a hot-dip galvanized steel sheet. 1% because it will run out
is the upper limit. Mn has the effect of preventing hot embrittlement caused by S, and therefore needs to be present in an amount of 0.05% or more. On the other hand, since Mn is a reinforcing component, it can be added depending on the required strength of the steel if deep drawability is not taken into account. If Mn remains in the state, increasing the Mn content will significantly deteriorate deep drawability, so the upper limit is set at 0.5%. Like the above-mentioned Si, P is an essential element in the present invention, and its effect on paint bake hardenability is greater than on improving deep drawability. In other words, the residual solid solution C after annealing is generally
Although it is determined by the amount of C in the steel, the above formula K, or the cooling rate after annealing, it is impossible to always produce a steel ingot with the same chemical composition during melting, and even with continuous annealing, the annealing temperature It is extremely difficult to constantly cool down devices with different coil plate thicknesses and coil positions at a constant cooling rate. Therefore, if the amount of residual solid solute carbon varies due to differences in these conditions, if the amount is large, room temperature aging will occur, and if it is small, the required amount of paint bake hardening cannot be obtained. However, when P is contained in an amount of 0.03% or more, a substantially constant amount of bake hardening is always obtained regardless of the above manufacturing conditions. Needless to say, it does not age at room temperature. However, if the content exceeds 0.15%, the steel will become too hard and the yield ratio (yield stress/tensile strength) will increase, making press forming difficult, so the range should be 0.03 to 0.15%. Al prevents Ti from being lost due to oxidation or becoming nonmetallic inclusions (oxides) in steel, and also prevents N from being lost.
It is effective in fixing and rendering harmless AlN.
For this reason, 0.01% or more is added. However, if the amount is too large, not only the effect will be saturated, but also the surface quality will deteriorate due to an increase in nonmetallic inclusions, or recrystallized grains will become finer, which is undesirable. For this reason 0.06
The upper limit is %. Ti improves deep drawability as the amount added increases, and in particular completely converts C and N in steel into carbonitrides [Ti(C,
It is known that deep drawability becomes extremely good by fixing the steel as N)]. However, this also entails a significant increase in manufacturing costs. Moreover, the amount of hardening due to seizure becomes zero. Therefore, in the present invention, which was naturally achieved by the combined addition of Si and P, it is possible to maintain a high degree of deep drawability, prevent hardening of the steel sheet until press working, and prevent the steel from baking hardening after working. Added. The amount of Ti is such that K in the above formula satisfies 0.4 to 0.8.
It is necessary to add a certain amount. On the other hand, an excessive amount of Ti that satisfies K in the above formula from 0.4 to 0.8 completely or almost completely fixes C and N, so that the hardening properties after press working, which is the objective of the present invention, cannot be expected. Moreover, the secondary formability deteriorates, and the melting loss of the refractory increases, which impairs workability, resulting in a large increase in costs. On the other hand, if Ti is added below K0.4, the amount of excess carbon remaining in the steel is too large, so even if Si and P are included, good deep drawing properties cannot be obtained and room temperature aging occurs. Nb has the same effect as Ti, and C
It can be replaced equivalently depending on the atomic concentration ratio with the total amount of Ti and N.There is no problem even if part or all of Ti is replaced with Nb, and when used as a base plate for hot-dip galvanizing,
Hot-dip galvanizing has many advantages in terms of alloying properties and adhesion. In addition, K in the above formula is the effective amount of Ti obtained by excluding Ti as oxides and sulfides from the total amount of Ti, the amount of Nb, and C.
This is an equation showing the atomic concentration ratio between the amount and the amount of N. In addition, in the present invention, in order to further improve deep drawability, Cr, REM, and B can be contained singly or in combination as necessary. Deep drawability is further improved by containing 0.1% or more of Cr. This effect is due to the fact that TiC precipitate particles tend to become coarser due to the presence of Cr. Further, Cr not only improves deep drawability but also has the function of making it difficult for room temperature aging to occur due to solid solution carbon that does not combine as Ti(C,N). In addition, in the present invention, since the amount of Ti is relatively small, there is no need for the content to exceed 0.4%. REM reduces the amount of S impurities and also contributes to improving deep drawability by controlling the shape of sulfide inclusions that are harmful to formability. Also, REM and B are
It has the effect of promoting grain growth after recrystallization and contributes to improving deep drawability. The contents of REM and B are respectively
0.05% or less and B0.01% or less are sufficient, and there is no need to contain more. Next, examples of the present invention will be described together with comparative examples. Smelted in a converter, subjected to vacuum degassing treatment to form ingots,
A steel billet manufactured by blooming is hot rolled, pickled, and then cold rolled at a reduction rate of 70% to produce a cold rolled plate with a thickness of 0.8 mm. It was annealed in a galvanizing line to obtain a hot-dip galvanized steel sheet. Tables 1 and 2 show the chemical composition of the cold-rolled steel sheet and its characteristic values. Steel types C to J and N to S are the steels of the present invention, and steel types A, B, and K to M are comparative materials that are outside the scope of the present invention. As is clear from the above results, the cold rolled steel sheet of the present invention has better deep drawability (value) than the comparative steel sheets except for steel type K, and increases the hardness after press forming through bake hardening. Strength can be significantly increased.
Incidentally, although steel type K has good deep drawability, it has neither room temperature aging nor bake hardening.

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Claims (1)

[Claims] 1 C0.003 to 0.02% by weight (the same applies hereinafter), Si0.1 to 1
%, Mn0.05~0.5%, P0.03~0.15%, Al0.01~
0.06%, N0.006% or less, and contains one or both of Ti and Nb in the range of 0.4 to 0.8 according to the following formula, with the balance being iron and impurities. Excellent bake hardenability and deep drawability. High strength cold rolled steel sheet. K=(effective Ti%/48+Nb%/93)/(C%/12+N%/
14) However, effective Ti is total Ti excluding Ti in the form of oxides and sulfides. 2 C0.003~0.02%, Si0.1~1%, Mn0.05~0.5
%, P0.03~0.15%, Al0.01~0.06%, N0.006% or less, and contains one or two of Ti and Nb in the range of 0.4~0.8 according to the following formula, and further contains Cr0 .1~0.4%,
A high-strength cold-rolled steel sheet with excellent bake hardenability and deep drawability, containing at least 0.05% of rare earth elements and 0.01% or less of B, with the balance being iron and impurities. K=(effective Ti%/48+Nb%/93)/(C%/12+N%/
14) However, effective Ti is total Ti excluding Ti in the form of oxides and sulfides.