JP4580334B2 - Deep drawing high strength steel plate and hot dipped steel plate - Google Patents

Description

  The present invention relates to a deep drawing high-strength cold-rolled steel sheet applied to the field of automobiles and home appliances, a manufacturing method thereof, a hot-dip plated steel sheet and a manufacturing method thereof, and particularly to a deep drawing high-strength suitable for automobile fuel tank applications. The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof, a hot-dip plated steel sheet and a manufacturing method thereof.

  In recent years, steel sheets for automobiles have been increased in strength for the purpose of improving fuel efficiency by reducing vehicle body weight. Similarly, steel plates for fuel tanks are required to have excellent formability due to the complexity of fuel tanks due to the lighter tanks and more complicated vehicle body design due to the location of the fuel tanks. In order to satisfy such demands for both formability and high strength, IF (Interstitial Free) steel, in which carbonitride-forming elements such as Ti and Nb are added to ultra-low carbon steel, P, Si, Mn High-strength IF steel added with solid solution strengthening elements such as has been developed.

  However, since IF steel precipitates and fixes C and N as carbides with Ti and Nb, the crystal grain boundary becomes very clean, and secondary processing embrittlement is likely to occur due to grain boundary fracture after forming. There is a point. Furthermore, in the case of high-strength IF steel, there is a problem that secondary work embrittlement is promoted because the intragranular strength is strengthened by the solid solution strengthening element and the relative grain boundary strength is significantly reduced.

  The fuel tank is used by press-molding the upper surface and the lower surface separately, and joining them by seam welding or laser welding. However, even if the strength of the steel plate is increased, there is a problem that the strength of the welded joint cannot be improved to a height commensurate with the increase in strength of the steel plate. At the same time, since the fuel tank is an important safety part, it is necessary to improve the fracture resistance against the collision in the winter in the low temperature area. There is a concern about the brittle fracture of the weld due to low temperature impact.

  Among these problems, several methods have been proposed for the purpose of avoiding secondary work embrittlement. For example, in the disclosed technique disclosed in Patent Document 1, based on Ti-added IF steel, in order to avoid deterioration of secondary work embrittlement resistance due to segregation of grain boundaries, the amount of P addition is reduced as much as possible, and Mn, A technique for manufacturing a high-tensile steel sheet excellent in deep drawability and secondary work brittleness resistance by adding a large amount of Si has been proposed. In addition, the disclosed technique disclosed in Patent Document 2 proposes a technique for increasing grain boundary strength and increasing secondary work brittleness resistance by adding B in addition to Ti and Nb using an ultra-low carbon steel sheet. Has been. In the disclosed technique disclosed in Patent Document 2, the optimum B addition amount is found in consideration of improving the secondary work brittleness resistance and preventing an increase in load during hot rolling due to a delay in recrystallization of austenite grains. ing.

Several proposals have been made for the purpose of improving weldability. For example, in the disclosed technique disclosed in Patent Document 3, an ultra-low carbon steel sheet to which Ti or Nb is added is carburized during annealing to form a martensite or bainite structure on the surface layer, thereby improving spot weldability. Patent Document 4 intends to increase the strength of a spot welded joint by adding Cu to an extremely low carbon steel and widening a heat-affected zone during welding. Patent Document 5 adds Mg to steel, forms Mg oxide and Mg sulfide, and refines the welded part and heat-affected zone by the pinning effect, thereby improving the formability of the welded part. This is a technology for preventing the deterioration of the material.
JP-A-5-59491 JP-A-6-57373 JP-A-7-188777 JP-A-8-291364 JP 2001-288534 A

However, in the disclosed techniques of Patent Documents 1 and 2, workability and secondary work brittleness resistance are good, but the problem that weld joint efficiency is low remains. Since the method described in Patent Document 3 is carburized during annealing, the rate of plate feed, atmospheric gas composition, and temperature are not constant in actual production equipment, so the amount of carburization changes and the material variation between the steel plates to be produced is large. Become. For this reason, it becomes difficult to produce a stable steel sheet. The method described in Patent Document 4 has a problem in that since a large amount of Cu is added, surface defects due to Cu frequently occur and the yield decreases. Furthermore, the method described in Patent Document 5 is effective in arc welding with a relatively slow cooling rate after welding, but has a drawback that the effect is not recognized in seam welding with a high cooling rate. Moreover, in the thin steel plate, not only is there no technique for improving the toughness of the welded portion, but there is no problem regarding the toughness of the welded portion.

  Therefore, the present invention has been devised in view of the above-described problems, has a tensile strength of 380 MPa or more and less than 590 MPa, has press formability applicable to automotive steel plates, particularly fuel tank applications, and The object is to propose a high-strength cold-rolled steel sheet for deep drawing, a hot-dip plated steel sheet, and a method for producing them, which have excellent weld joint efficiency such as secondary work brittleness resistance, seam welding, and laser welding.

The high-strength steel sheet for deep drawing according to the present invention is, by weight, C: 0.0005 to 0.005%, Si: 0.05 to 0.50%, Mn: 1.2 to 3.0%, P : 0.005-0.05%, Ti: 0.02-0.080%, B: 0.0005-0.0050%, Al: 0.10-0.90%, N: 0.0010-0 .0080%, the balance is Fe and inevitable impurities, N is included as the inevitable impurities, P content (%) is [P], Ti content (%) is [Ti], N When the content (%) is [N], the T ^* represented by the following formula (1) is less than 0.04%, and the absolute value of the tensile strength TS (MPa) is P% by weight. The absolute value is 1 × 10 ⁴ times or more.

The manufacturing method of the high-strength steel sheet for deep drawing which concerns on this invention is weight%, C: 0.0005-0.005%, Si: 0.05-0.50%, Mn: 1.2-3.0 %, P: 0.005-0.05%, Ti: 0.02-0.080%, B: 0.0005-0.0050%, Al: 0.10-0.90%, N: 0.00. 0010 to 0.0080%, the balance is Fe and inevitable impurities, N is included as the inevitable impurities, P content (%) is [P], Ti content (%) is [Ti , When the N content (%) is [N], the step of obtaining a slab by continuously casting molten steel having a composition of T ^* less than 0.04% represented by the formula (1), and the slab A hot rolling coil is obtained by hot rolling at a finishing temperature of Ar3 temperature or higher and a winding temperature of 750 ° C. or lower; A step of cold rolling the steel sheet at a cold rolling rate of 50% or more to obtain a cold rolled coil having a predetermined thickness, and a step of annealing the cold rolled coil at a temperature equal to or higher than a recrystallization temperature. The absolute value of the tensile strength TS (MPa) of the rolled steel sheet is 1 × 10 ⁴ times or more the absolute value of the weight percent of P.

The hot-dip galvanized steel sheet according to the present invention is, by weight, C: 0.0005 to 0.005%, Si: 0.05 to 0.50%, Mn: 1.2 to 3.0%, P: 0.00. 005 to 0.05%, Ti:
0.02 to 0.080%, B: 0.0005 to 0.0050%, Al: 0.10 to 0.90%, N: 0.0010 to 0.0080%, the balance being Fe and inevitable When N is included as an inevitable impurity, the P content (%) is [P], the Ti content (%) is [Ti], and the N content (%) is [N] T ^* represented by the formula (1) is less than 0.04%, and the absolute value of the tensile strength TS (MPa) is 1 × 10 ⁴ times or more the absolute value of the weight% of P. Features.

The manufacturing method of the hot-dip galvanized steel sheet according to the present invention is, in wt%, C: 0.0005 to 0.005%, Si: 0.05 to 0.50%, Mn: 1.2 to 3.0%, P : 0.005-0.05%, Ti: 0.02-0.080%, B: 0.0005-0.0050%, Al: 0.10-0.90%, N: 0.0010-0 .0080%, the balance is made of Fe and inevitable impurities, N is included as the inevitable impurities, P content (%) is [P], Ti content (%) is [Ti], N When the content (%) is [N], a step of continuously casting a molten steel having a composition represented by the following formula (A) having a T ^* of less than 0.04% to obtain a slab, and finishing the slab A step of hot rolling at a temperature of Ar3 temperature or higher and a coiling temperature of 750 ° C. or lower to obtain a hot rolled coil; Cold rolling the steel sheet at a cold rolling rate of 50% or more to form a cold rolled coil having a predetermined thickness, annealing the cold rolled coil at a temperature higher than the recrystallization temperature, Characterized in that it has a step of hot dipping on the surface of the rolled coil, and the absolute value of the tensile strength TS (MPa) of the hot dipped steel sheet is 1 × 10 ⁴ times or more the absolute value of the weight% of P. To do.

  According to the present invention, conventionally, P, which has been regarded as a solid solution strengthening element, is reduced to a tensile strength and a specific relationship or less, the content of Mn and Al is increased, and the contents of Ti, P and N are specified. Since the relationship is satisfied, a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized steel sheet having excellent press formability and having excellent secondary work brittleness resistance and weld joint efficiency are obtained. This effect makes it possible to increase the strength of the steel sheet and to improve fuel consumption by reducing the weight of the vehicle body. In particular, the fuel tank can be made lighter and the vehicle body design can be made more complex. This effect is extremely large industrially.

  BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the best mode for carrying out the present invention will be described in detail by taking a deep drawing high-strength cold-rolled steel sheet (hereinafter simply referred to as a cold-rolled steel sheet) as an example. Hereinafter, the weight% in the composition is simply described as%. The joint efficiency refers to the strength and toughness of the welded portion in the welded joint where the cold-rolled steel plate is welded.

  The inventors of the present invention have a high strength cold-rolled steel sheet and a high-strength molten steel that have excellent press formability, which has been extremely difficult with the prior art, and also have excellent secondary work brittleness resistance, seam welding, and laser weld joint efficiency. As a result of intensive studies to obtain a plated steel sheet, steel content has been reduced in order to reduce the P content, which has been regarded as a solid solution strengthening element, as much as possible, and to increase the Mn and Al contents, while at the same time improving the workability. When the amount of Ti that fixes C and N in the inside is specified, and the relationship between the contents of Ti, N, and P satisfies specific conditions, seam welding, laser welded joint efficiency and secondary work brittleness resistance are reduced. I found that it could be improved further.

  Hereinafter, the reason for adding each component constituting the cold-rolled steel sheet of the present invention and the reason for limiting the numerical values will be described.

The cold-rolled steel sheet of the present invention is, by weight, C: 0.0005 to 0.005%, Si: 0.05 to 0.50%, Mn: 1.2 to 3.0%, P: 0.005. -0.05%, Ti: 0.02-0.080%, B: 0.0005-0.0050%, Al: 0.10-0.90%, the balance from Fe and inevitable impurities When N is included as the inevitable impurity, the P content (%) is [P], the Ti content (%) is [Ti], and the N content (%) is [N], T ^* represented by (A) is less than 0.04%, and the absolute value of the tensile strength TS (MPa) is 1 × 10 ⁴ times or more the absolute value of the weight% of P.

  At this time, Ni: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0% are further included as selective elements for strength adjustment. May be.

C: 0.0005 to 0.005%
C is the most basic element that controls the hardenability and strength of steel, and is an extremely important element in the present invention. That is, C is an extremely effective element for combining with Ti to form a carbide and achieving high strength. However, if C is added in excess of 0.005%, workability will be reduced, and even when Ti required for fixing C will be added, the efficiency of seam welding and laser welded joints will be reduced. Therefore, the C content is 0.005% or less. Moreover, when extremely high workability is required, the C content is preferably 0.0020% or less. On the other hand, in the cold-rolled steel sheet of the present invention, even if the C content is low, it can be supplemented to some extent by other strengthening methods. However, when the C content is less than 0.0005%, it is difficult to ensure the strength, and in order to reduce the C content to less than 0.0005%, an increase in decarburization cost during steelmaking is caused. Therefore, the C content is set to 0.0005% or more.

Si: 0.05 to 0.50%
Si is an element generally known as a solid solution strengthening element. However, when the Si content increases, specifically, when the Si content exceeds 0.50%, the hot dipping property is impaired. Therefore, in the present invention, Si is added in a range of 0.50% or less. On the other hand, when the Si content is reduced, specifically, when the Si content is less than 0.05%, the strength of the steel sheet is decreased. Therefore, the Si content is set to 0.05% or more.

Mn: 1.2 to 3.0%
Mn is an element that improves hardenability as well as Si and increases material strength by solid solution strengthening, and is intended to increase the strength of the cold-rolled steel sheet of the present invention for the purpose of improving secondary workability brittleness resistance. It is one of the important elements. Mn has a mechanism for increasing the strength by refining the structure and a mechanism for increasing the strength by solid solution strengthening. However, when the Mn content is less than 1.2%, the effect of addition cannot be obtained. On the other hand, if the Mn content exceeds 3.0%, the in-plane anisotropy of the r value, which is an index of deep drawability, increases, and press formability is impaired. Therefore, the Mn content is set to 1.2 to 3.0%. In addition, the preferable range of Mn content is 1.4 to 2.0%, and thereby, the strength and formability of the steel sheet can be further increased.

P: 0.005 to 0.05%
P is an element effective in increasing strength by solid solution strengthening with little deterioration in workability even when added. However, P is an element that segregates at the grain boundaries to deteriorate the secondary work brittleness resistance and causes solidification segregation in the welded portion, thereby reducing the weld joint efficiency. Therefore, in the present invention, the P content is set to 0.05% or less in order to prevent segregation to grain boundaries. The lower limit of the P content does not need to be specified. However, in order to make the P content less than 0.005%, the refining cost becomes high and it is difficult to ensure the strength. The amount is 0.005% or more.

Ti: 0.02 to 0.080%
Ti has a strong affinity with N and C, forms carbonitrides during solidification, and reduces N and C dissolved in the steel, thereby improving workability. However, when the Ti content is less than 0.02%, this effect cannot be obtained. On the other hand, when the Ti content increases, specifically, when the Ti content exceeds 0.080%, the strength and toughness of the welded portion of the welded joint, that is, the welded joint efficiency deteriorates. Therefore, the Ti content is 0.02 to 0.080%.

B: 0.0005 to 0.0050%
B is an element that increases the grain boundary strength and improves the secondary work brittleness resistance by segregating at the grain boundaries. However, when the B content is less than 0.0005%, the effect cannot be obtained. On the other hand, when the B content increases, specifically, when the B content exceeds 0.0050%, not only the addition effect is saturated, but also the recrystallization temperature becomes high and high temperature annealing is required. In addition, the manufacturing cost is increased, the workability is deteriorated, and the weld joint efficiency is deteriorated. Therefore, the B content is set to 0.0005 to 0.0050%. In addition, the preferable range of B content is 0.0010 to 0.0030%.

Al: 0.10-0.90%
It is well known that Al is used as a deoxidizer when melting steel. The inventors of the present invention improve the seam weld joint efficiency by setting the Al addition amount to 0.10% or more of the amount necessary for ordinary deoxidation. On the other hand, when the Al addition amount increases, the surface quality of the steel sheet deteriorates, so the upper limit of the Al addition amount was set to 0.90%. In addition, the preferable range of this Al is 0.20-0.60 as conditions on which the outstanding seam welding is obtained. The reason why the lower limit of the Al content is set to the above value will be described in detail later.

N: 0.0010 to 0.0080%
N is an element that is inevitably mixed during the refining of steel, but forms nitrides of Ti and Al and does not adversely affect workability, but deteriorates weldability. For this reason, it is necessary to make N amount 0.0080% or less. On the other hand, reducing the N content to 0.0010% or less increases the manufacturing cost, so the lower limit was made 0.0010%.

The absolute value of the tensile strength TS (MPa) is 1 × 10 ⁴ times or more the absolute value of the weight% of P. P is an element effective in increasing strength by solid solution strengthening with little deterioration in workability. Are known. For this reason, in the past, a large amount of P was added to obtain high strength. However, when P is added beyond a specific amount relative to the strength, the secondary work brittleness resistance and weld joint efficiency deteriorate rapidly. I found out. For this reason, the absolute value of the tensile strength TS (MPa) needs to be 1 × 10 ⁴ times or more the absolute value of the weight% of P. In general, the tensile strength is basically influenced by the steel components and secondarily by the production conditions. When designing steel components, it is necessary to reduce the amount of P added as much as possible and increase the content of Si and Mn within a range that does not adversely affect weldability, workability, secondary work brittleness, and plating properties. In production, it is preferable not to perform so-called high-temperature annealing, which lowers the strength, does not increase the coiling temperature during hot rolling, and sets the annealing temperature to 850 ° C. or higher.

T ^* : Less than 0.04% The present inventors have found that when the value of T ^* defined by the following mathematical formula (2) increases, the weld joint efficiency deteriorates. In the following mathematical formula (2), [Ti] is the Ti content (%), [N] is the N content (%), and [P] is the P content (%). When the value of T ^* is 0.04% or more, the joint efficiency particularly deteriorates at a low temperature, the temperature at which a brittle fracture fracture surface is raised, and the toughness of the welded portion deteriorates. For the above reasons, the value of T ^* is set to less than 0.04%.

  Furthermore, in the cold-rolled steel sheet of the present invention, at least one element selected from Ni, Cr and Mo can be added as necessary. Hereinafter, the reason for adding these elements and the reason for limiting the numerical values will be described.

Ni: 0.01 to 1.0%
Ni is an element effective for increasing the strength, like Mn. However, when the Ni content is less than 0.01%, the effect cannot be obtained. On the other hand, if Ni is added in an amount exceeding 1.0%, the manufacturing cost is increased. Therefore, when adding Ni as a selection element, it is set as 0.01 to 1.0% of range.

Cr: 0.01 to 1.0%
Cr is an element effective for achieving high strength without degrading workability. However, if the Cr content is less than 0.01%, the effect cannot be obtained. On the other hand, if the Cr content exceeds 1.0%, the production cost is increased and the plating property is hindered. Therefore, when adding Cr as a selection element, it is set as 0.01 to 1.0% of range.

Mo: 0.01 to 1.0%
Mo is an element that enhances the strength of the steel sheet by solid solution strengthening. Further, the present inventors have found that workability is improved by adding Mo. However, when the Mo content is less than 0.01%, these effects cannot be obtained. On the other hand, if the Mo content exceeds 1.0%, the effect is saturated and the manufacturing cost is increased. Therefore, in the present invention, Mo is added in the range of 0.01 to 1.0% particularly when workability is required.

In addition, the remainder in the cold-rolled steel sheet of the present invention, that is, components other than the above-described elements are Fe and inevitable impurities.
In the present invention, conventionally, the content of P, which has been regarded as a solid solution strengthening element, is reduced, the Mn content is increased, and the Ti content, the P content, and the N content satisfy a specific relationship. Thus, a cold-rolled steel sheet having excellent press formability and excellent secondary work brittleness resistance and weld joint efficiency is obtained. Thereby, the strength of the steel plate can be increased, and fuel efficiency can be improved by reducing the weight of the vehicle body. In particular, the fuel tank can be reduced in weight and the vehicle body design can be complicated. This effect is extremely large industrially.

  In addition, in the cold-rolled steel sheet of the present invention, elements such as S, Cu, Sn, and Sb can be added in a normal range in addition to the elements described above, thereby impairing the characteristics of the present invention described above. It will never be.

  The cold-rolled steel sheet of the present invention can be used as a hot-dip plated steel sheet by providing a plating layer made of zinc, Al alloy, Sn, Sn—Zn alloy, or the like on the surface.

  Next, the reason why the lower limit of the Al content is limited to 0.10% as described above will be described. C: 0.0020 to 0.0030%, Si: 0.10 to 0.30%, Mn: 1.3 to 2.0%, P: 0.02 to 0.03%, Ti: 0.04 to 0.08%, N: 0.0030 to 0.0045%, B; 0.0020% and further changed to Al: 0.04% to 1.0% steel in a vacuum melting furnace After being melted and heated to 1250 ° C., it was hot rolled to a thickness of 3.7 mm at a finishing temperature of 890 to 910 ° C., forcibly cooled to 650 ° C., charged in a heating furnace maintained at 640 ° C., and cooled. This hot rolled sheet was descaled and then cold rolled to a thickness of 1.0 mm, annealed at 780 ° C. for 60 seconds, and subjected to 1.0% temper rolling. Two steel sheets were seam welded, and a peel test was performed at -60 ° C. with the shape shown in FIG. 1, and the peel strength was measured. The relationship between the peel strength at −60 ° C. and the Al content is shown in FIG. As shown in FIG. 2, it can be seen that when the Al content is 0.1% or more, the peel strength at −60 ° C. is 350 MPa or more and excellent seam weldability is exhibited.

Next, the manufacturing method of the cold rolled steel sheet of this invention is demonstrated. When manufacturing the cold-rolled steel sheet of the present invention, first, the raw materials are put into a converter or an electric furnace so as to have the above-described steel composition, vacuum degassing treatment is performed to form a slab, and this slab is subjected to hot rolling. Provide a hot-rolled coil. This hot-rolled coil is usually annealed after being cold-rolled after descaling and adjusted to a predetermined plate thickness. This annealed steel sheet is temper-rolled and further subjected to surface treatment such as electroplating, and then shipped.

  Under the present circumstances, what is necessary is just to select the heating temperature of hot rolling in the range which does not impair the operation of rolling, even if it sets what degree C, since the characteristic of this invention is not impaired. Since the workability of the steel sheet is impaired when the finishing temperature of hot rolling is below the Ar3 temperature, it is preferable to perform hot rolling at the Ar3 temperature or higher. When the hot rolling coiling temperature is higher than 750 ° C., the strength of the steel sheet after cold rolling annealing is lowered. Furthermore, when the cold rolling rate is 50% or less, the strength of the steel sheet after annealing is lowered and the deep drawing workability is deteriorated. A preferable range is 65 to 80% for the same reason.

  The cold-rolled steel sheet of the present invention is annealed after cold rolling, and the annealing temperature needs to be higher than the recrystallization temperature. On the other hand, when the annealing temperature increases, the strength of the steel sheet decreases. Also, when annealing by the continuous annealing method, even if there is an overaging treatment during cooling, it does not impair the characteristics of the present invention, whether or not the overaging treatment is carried out either will do.

  Moreover, when using the cold-rolled steel plate of this invention as a hot dipped steel plate, zinc, Al alloy, Sn, Sn-Zn alloy, etc. are hot-plated on the surface in the cooling process of the above-mentioned annealing process.

  Next, Example 1 of the present invention will be described. First, the steel composition slab shown in Table 1 below was heated and held at 1200 ° C, and then the hot rolling finishing temperature was 850 to 880 ° C and the coiling temperature was 600 to 650 ° C, and the plate thickness was 3.7 mm. A hot-rolled steel sheet was obtained, and after cold pickling, cold rolling was performed until the thickness became 1.2 mm. Next, the cold-rolled sheet was annealed in a cycle in which it was held at 800 ° C. for 65 seconds, and then 1.0% temper rolling was performed.

  Next, the tensile properties, the r value which is an index of deep drawing, the secondary work brittleness resistance and the welded joint efficiency of each of the steel plates of Examples and Comparative Examples produced by the above-described methods were investigated. Hereinafter, the evaluation method will be described.

  Tensile properties were evaluated based on tensile strength and elongation by collecting JIS No. 5 test pieces so that the tensile direction was parallel to the rolling direction and performing a tensile test. TS was 440 MPa or more and the elongation was 35% or more.

The r value was measured by collecting JIS No. 5 test pieces parallel to the rolling direction, 45 °, and perpendicularly, respectively. Evaluation of r value was performed by the average value of each direction. Average, when the r value of _r 0, 45 ° direction parallel r values in the rolling direction and _{r 45,} the r value of the direction perpendicular and _{r 90, r = (r 0} + 2 * r 45 + r 90) / 4 was obtained. The r value was 1.50 or higher.

  Secondary work brittleness resistance can be measured by blanking a steel plate (thickness: 1.2 mm) to a diameter of 100 mm, performing cylindrical drawing with a punch having an outer diameter of 50 mm, and placing the drawn cup on a truncated cone of 30 °. Under the above temperature conditions, a weight of 5 kg was dropped from a position of 1 m in height, and the lowest temperature at which the cup did not crack (secondary work brittleness resistance) was determined. Although this secondary work brittleness temperature changes with the plate | board thickness and test method of a steel plate, in this Example, -50 degrees C or less was set as the pass.

  The weld joint efficiency was evaluated by evaluating the strength and toughness of the welded portion by seam welding two steel plates (plate thickness: 1.2 mm). At that time, the strength of the welded portion was evaluated by the ratio of the shear tensile strength and the tensile strength of the base material (shear tensile strength / base tensile strength). Further, the toughness of the welded portion was obtained by seam welding the steel plates 1a and 1b having the shape shown in FIG. 1 to produce test pieces, and the welded portion 2 was subjected to a peel test at different temperatures. Then, the fracture surface was observed with an SEM (Scanning Electron Microscope), and the lowest temperature at which no brittle fracture surface was observed on the fracture surface was examined. In this example, a material having a shear tensile strength of 75% or more of the tensile strength of the base material and a material having a minimum temperature of −30 ° C. or less at which no brittle fracture surface occurs in the peel test were accepted. The above test results are shown in Table 2 below.

  As shown in Table 2 above, No. 1 is an example within the scope of the present invention. No. 1 steel plate and No. 1 steel plate. Steel plate No. 2 has excellent processing characteristics such as an r value of 1.6 or more and an elongation of 35% or more, and has a secondary work brittleness resistance of −60 ° C., resulting in a brittle fracture surface in the peel test of seam welding. In addition to the excellent temperature of -60 ° C., the peel strength at −60 ° C. is 400 MPa or more, which indicates that it has excellent seam weldability.

  No. Steel plate No. 3 is an example within the scope of the present invention in which Mo is added by 0.25%. This No. It can be seen that the steel plate No. 3 also has excellent processing characteristics and has excellent secondary work brittleness resistance and weld joint efficiency. No. Steel plate No. 4 is an example within the scope of the present invention in which 0.45% of Ni and 0.10% of Cr are added. It can be seen that this steel sheet also has good processing characteristics, secondary work brittleness resistance and weld joint efficiency. No. Steel plate No. 5 is an example within the scope of the present invention in which 0.45% of Cr was added. This steel sheet also has excellent processing characteristics, secondary work brittleness resistance and weld joint efficiency.

  No. Steel plate No. 6 is a comparative steel having an Al content of 0.035% and deviating from the lower limit of the range of the present invention. When the Al content is low, the workability and the secondary work brittleness resistance are good, but the temperature at which the brittle fracture surface does not occur in the peel test of seam welding is as high as 0 ° C., and the peel strength at 0 ° C. is also 278 MPa. It can be seen that the seam weldability is inferior compared to the examples within the scope of the present invention. No. Steel sheet No. 7 has a P content of 0.070%, which deviates from the scope of the present invention, and a comparison in which the absolute value of the P content deviates from the condition of less than 0.0001 times the absolute value of the tensile strength TS (MPa). It is an example. This No. Although the processing characteristics of the steel plate No. 7 are good, the cracking resistance at the vertical crack generation is 20 ° C. and the secondary work brittleness resistance is inferior. In addition, the peel strength of seam welding is as low as 225 MPa, and the temperature at which the brittle fracture surface does not occur is 20 It can be seen that seam weldability is inferior because it is high at ℃.

  No. Steel plate No. 8 is a comparative example having a C content of 0.019%, which is out of the scope of the present invention. It can be seen that this steel sheet has low elongation, poor workability, low peel strength, and poor seam weldability. No. Steel plate No. 9 is a comparative example out of the scope of the present invention, with a Ti content of 0.105% and Ti * of 0.0857%. This No. It can be seen that the steel plate No. 9 has inferior seam weldability compared to the examples of the present invention. No. Steel plate No. 10 is a comparative example having a Mn content of 3.50% and deviating from the upper limit of the range of the present invention. This No. Steel plate No. 10 has an r value of 1.15, which is inferior in processing characteristics, and inferior in secondary work brittleness resistance and seam weldability.

  No. Steel No. 11 is a comparative example in which the B content is 0.0065%, which is outside the upper limit of the range of the present invention. It can be seen that this steel sheet is inferior in workability, has a low peel strength in seam welding, and has a high temperature at 0 ° C. at which no brittle fracture surface is produced, resulting in inferior toughness of the weld.

A slab having a steel composition shown in Table 3 is heated and maintained at 1200 ° C., and then a hot-rolled steel sheet having a thickness of 3.7 mm under conditions of a hot rolling finishing temperature of 850 ° C. to 880 ° C. and a winding temperature of 600 ° C. to 650 ° C. And cold rolled to a thickness of 1.2 mm after pickling. The cold-rolled sheet was annealed at a temperature cycle of 800 ° C. for 65 seconds, and an aluminum alloy plating was applied during the cooling of the annealing. For hot-dip aluminum plating, a non-oxidizing furnace-reduction type line was used, and after plating, the amount of plating adhered was adjusted to 60 g / m ² on both sides by gas wiping, then cooled and subjected to zero spangle treatment. The plating bath composition at this time was Al: 90% and Si: 10%. After plating, a chromium ^{3 +} -based chromate treatment was performed, and 1.0% temper rolling was performed in-line.

  Next, the tensile properties, the r value that is an index of deep drawing, the secondary work brittleness resistance, and the seam weldability of each of the hot dip plated steel sheets of Examples and Comparative Examples produced by the above-described methods were investigated. The investigation method and evaluation criteria for each item were the same as in Example 1 described above. In addition to the above items, the condition of the plating layer surface (plating property) was examined visually. This plating property was evaluated based on the presence or absence of an unplated portion. The above evaluation results are shown in Table 4 below.

  As shown in Table 4 above, No. 1 is an example within the scope of the present invention. 1A and 2A steel sheets have excellent processing characteristics with an r value of 1.6 or more and an elongation of 35% or more, secondary work brittleness resistance is good at -50 and -60 ° C, and weld joint efficiency. It can be seen that (Seam Weldability) also has good characteristics and that the plating properties are also good. No. The 3A steel plate is an example within the scope of the present invention containing 0.25% of Mo. This No. The 3A steel sheet also has excellent processing characteristics, secondary processing brittleness resistance, good characteristics of welded joint efficiency, and excellent plating properties. No. The steel sheets of 4A and 5A are examples within the scope of the present invention in which Ni and Cr are added, respectively. This No. It can be seen that the 4A and 5A steel sheets also have excellent properties in terms of workability, secondary work brittleness resistance, weld joint efficiency, and plating properties.

  No. The 6A steel plate has an Al content of 0.035%, which is a comparative example outside the scope of the present invention. This No. The 6A steel sheet is excellent in workability and secondary work brittleness resistance, but the temperature at which a brittle fracture surface does not occur in the peel test of seam welding is as high as 0 ° C. and does not satisfy the object of the present invention. No. The 7A steel plate has a P content of 0.070%, which is a comparative example deviating from the upper limit of the range of the present invention. This No. The steel plate of 7A has poor secondary work brittleness resistance of 20 ° C., the welded joint properties are also poor compared to the examples within the scope of the present invention, and does not satisfy the target of the present invention. No. The 9A steel sheet is a comparative steel whose Ti content deviates from the scope of the present invention. Even when Al plating is performed, the characteristics satisfying all of workability, secondary work brittleness resistance, seam weldability, and plating property cannot be obtained as in the case of cold-rolled steel sheets. No. The steel sheet of 10A has a Mn content of No. 10A. The 11A steel plate is a comparative example in which the B content is out of the scope of the present invention. These plated steel sheets are inferior in workability, seam weldability, and plating properties and do not satisfy the object of the present invention.

It is sectional drawing which shows a peel test method. It is a figure which shows the relationship of the peel strength with respect to Al content.

Explanation of symbols

1a, 1b Steel plate 2 Welded part

Claims (8)

% By weight
C: 0.0005 to 0.005%,
Si: 0.05 to 0.50%,
Mn: 1.2-3.0%
P: 0.005 to 0.05%,
Ti: 0.02 to 0.080%,
B: 0.0005 to 0.0050%,
Al: 0.10-0.90%,
N: 0.0010 to 0.0080% is contained,
The balance consists of Fe and inevitable impurities, and contains N as the inevitable impurities,
When the P content (%) is [P], the Ti content (%) is [Ti], and the N content (%) is [N], T ^* represented by the following formula (A) is 0.00. Less than 04%,
A high-strength cold-rolled steel sheet for deep drawing, wherein the absolute value of the tensile strength TS (MPa) is 1 × 10 ⁴ times or more the absolute value of the weight% of P.

In addition,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
The high-strength cold-rolled steel sheet for deep drawing according to claim 1, containing Mo: 0.01 to 1.0%. % By weight
C: 0.0005 to 0.005%,
Si: 0.05 to 0.50%,
Mn: 1.2-3.0%
P: 0.005 to 0.05%,
Ti: 0.02 to 0.080%,
B: 0.0005 to 0.0050%,
Al: 0.10-0.90%,
N: 0.0010 to 0.0080% is contained,
The balance consists of Fe and inevitable impurities, and contains N as the inevitable impurities,
When the P content (%) is [P], the Ti content (%) is [Ti], and the N content (%) is [N], T ^* represented by the following formula (A) is 0.00. A step of continuously casting molten steel having a composition of less than 04% to obtain a slab;
Hot rolling the slab with a finishing temperature of Ar3 temperature or higher and a winding temperature of 750 ° C. or lower to obtain a hot rolled coil;
Cold rolling the hot rolled coil at a cold rolling rate of 50% or more to obtain a cold rolled coil having a predetermined thickness;
Annealing the cold-rolled coil at a temperature above the recrystallization temperature,
A method for producing a high-strength cold-rolled steel sheet for deep drawing, wherein the absolute value of the tensile strength TS (MPa) of the cold-rolled steel sheet is 1 × 10 ⁴ times or more the absolute value of P% by weight.

In the step of obtaining the slab, further by weight%,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
4. The method for producing a high strength cold-rolled steel sheet for deep drawing according to claim 3, wherein molten steel having a composition containing Mo: 0.01 to 1.0% is continuously cast. % By weight
C: 0.0005 to 0.005%,
Si: 0.05 to 0.50%,
Mn: 1.2-3.0%
P: 0.005 to 0.05%,
Ti: 0.02 to 0.080%,
B: 0.0005 to 0.0050%,
Al: 0.10-0.90%,
N: 0.0010 to 0.0080% is contained,
The balance consists of Fe and inevitable impurities, and contains N as the inevitable impurities,
When the P content (%) is [P], the Ti content (%) is [Ti], and the N content (%) is [N], T ^* represented by the following formula (A) is 0.00. Less than 04%,
A hot-dip galvanized steel sheet characterized in that the absolute value of the tensile strength TS (MPa) is 1 × 10 ⁴ times or more the absolute value of the weight% of P.

In addition,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
The hot-dip galvanized steel sheet according to claim 5, containing Mo: 0.01 to 1.0%. % By weight
C: 0.0005 to 0.005%,
Si: 0.05 to 0.50%,
Mn: 1.2-3.0%
P: 0.005 to 0.05%,
Ti: 0.02 to 0.080%,
B: 0.0005 to 0.0050%,
Al: 0.10-0.90%,
N: 0.0010 to 0.0080% is contained,
The balance consists of Fe and inevitable impurities, and contains N as the inevitable impurities,
When the P content (%) is [P], the Ti content (%) is [Ti], and the N content (%) is [N], T ^* represented by the following formula (A) is 0.00. A step of continuously casting molten steel having a composition of less than 04% to obtain a slab;
Hot rolling the slab with a finishing temperature of Ar3 temperature or higher and a winding temperature of 750 ° C. or lower to obtain a hot rolled coil;
Cold rolling the hot rolled coil at a cold rolling rate of 50% or more to obtain a cold rolled coil having a predetermined thickness;
Annealing the cold-rolled coil at a temperature equal to or higher than the recrystallization temperature, and performing a hot-dip plating on the surface of the cold-rolled coil in the cooling process;
A method for producing a hot-dip galvanized steel sheet, characterized in that the absolute value of the tensile strength TS (MPa) of the hot-dip galvanized steel sheet is 1 × 10 ⁴ times or more the absolute value of the weight% of P.

In the step of obtaining the slab, further by weight%,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
The method for producing a hot-dip galvanized steel sheet according to claim 7 , wherein molten steel having a composition containing Mo: 0.01 to 1.0% is continuously cast.

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