KR930009981B1 - Cold-roll steel sheet or hot dip galvanized cold-rolled steel sheets for deep drawing - Google Patents

Abstract

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Description

Cold rolled steel sheet or hot dip galvanized cold rolled steel sheet for deep drawing

1, 3, 5, 7 is a diagram showing the arrangement of the solid solution carbon through the thickness direction, the internal friction of the steel sheet samples prepared by grinding at the steel sheet thickness with respect to 1/10 of the steel sheet of Example 1-4 It is given by the conversion from the internal friction value.

1 is a distribution diagram for a third steel sheet according to Example 1. FIG.

3 is a distribution diagram of the third steel plate according to the second embodiment.

5 is a distribution diagram of the seventh steel plate according to the third embodiment.

7 is a distribution diagram of the seventh steel plate according to the fourth embodiment.

2, 4, 6 and 8 show the mechanical properties of the steel sheet containing 0.02% or less of P in the first to fifth steel sheets, the seventh and eighth steel sheets, and the first to fifth steel sheets according to the embodiment. The relationship between the property and (Ti * / 48 + Nb / 93) / (C / 12) is shown.

9 is a diagram showing the relationship between the amount of solid solution carbon from the steel surface to the thickness of 100 μm in the thickness direction, the γ-value and the adhesion of the galvanized coating in Example 5. FIG.

The present invention relates to a hot-dip galvanized cold rolled steel sheet excellent in deep drawing and plating adhesion. The cold rolled steel sheet according to the present invention has excellent resistance to cold embrittlement or bake hardenability, and in particular, the cold rolled steel sheet according to the present invention has excellent deep drawability and excellent zinc coating. The present invention relates to a hot-dip galvanized cold-rolled steel sheet having plating adhesion.

[Private Technology]

Cold rolled steel sheets used in automotive parts and external panels of electric devices are particularly required to have better moldability and good corrosion resistance in recent years.

Addition of carbonitride, which forms a component such as titanium (Ti) or niobium (Nb) into ultra low carbon steel for the purpose of stabilizing the carbon and nitrogen components in the steel to produce a cold rolled steel sheet that meets the above requirements. A method has been proposed to obtain a (111) structure which is advantageous for plating and deep drawing of steel sheet by preparing a short material or compound.

However, the ultra low carbon steel, in which C and N elements are combined inside the steel, is sufficiently stabilized by carburized nitride forming elements such as Ti and Nb. However, cold working after press forming causes a fragile tissue breakdown. Have In addition, phosphorus-added steels also have the problem that they promote brittleness of grain boundaries and precipitate at grain boundaries. This is due to the stabilization of solid nitrogen or solid carbon (Solid-solute C) inside the steel, which makes the grain boundary weak by preventing the precipitation of carbon into the ferrite grain boundary.

In particular, in the case of hot-dip galvanized steel sheet, the hot-dip galvanizes more susceptible by penetrating the grain boundary which is easily vulnerable. This conventional hot dip galvanized cold rolled steel sheet suffers from the problem of powdering or flaking during press molding, which weakens the adhesion of the plating coating.

Therefore, as a means for solving the above problems in the grain boundary, attempts have been made to allow dissolved carbon (C) and solid solution nitrogen (N) to remain in the steel by melting the steel by pre-addition control of Ti and Nb. According to this method, even though the steel contains residual solid solute C or nitrogen as a component, the dissolved carbon or solid nitrogen are poor because they substantially damage the γ-value and ductility of the steel. Press molding makes it difficult to avoid. In other words, the resistance to press molding and cold work weakness is not compatible with each other. In addition, it is practically and technically impossible to separate such small amounts of dissolved carbon or nitrogen in the river during the forced tank stage. In this regard, the following proposals have been made. For example, in order to improve resistance to cold-work embrittlement in deep drawable steel sheets, Ti and Nb are added to the steel sheet in advance to stabilize the C component to form a carburized layer on the surface of the steel sheet. After cold rolling, carburization is carried out through open-coil annealing (see Japanese Patent Laid-Open No. 63-38556). In this method, however, since carburization is applied during the period of batch annealing, too high a high concentration of carbide layer is formed in the surface layer of the steel (average C amount in the carburizing layer: 0.02 to 0.10%), and between the surface layer and the center layer. There are ferrites with different particle sizes at. Moreover, the batch annealing method itself is disadvantageous in that the productivity is not high and the mechanical properties are not uniform in the steel plate width direction and the rolling direction.

In addition, in order to improve phosphatability (Japanese Patent Application Laid-Open No. 1-42331), a method of containing only a very small surface layer of dissolved carbon (C) and dissolved nitrogen (N) has been proposed. According to this method, however, resistance to cold work embrittlement is not taken into consideration, and therefore, it is impossible to carry out carburizing necessary to improve resistance to cold work embrittlement.

Similarly, in producing a deep drawing steel sheet by adding Ti and Nb, a method of carburizing after recrystallization annealing after cold rolling has also been proposed. This method, however, focuses on giving greater strength through mass deposition of carbides or nitrides, but it does not consider the improvement of resistance to cold work embrittlement. The nitriding operation is long and after annealing, the carburizing and nitriding operations tend to be excessively large and uneven, so it goes without saying that quality deterioration and productivity are low and the process is complicated.

In addition to the above-mentioned problems in the prior art, so-called bake hardnabi, which requires an increaseable property of the yield stress of the steel sheet after paint baking

lity) There was a problem.

In order to satisfy the above-mentioned properties, a method of adding Ti in an amount less than the C atom equivalent has been proposed for the purpose of separating the solid solution C (Japanese Patent Application No. 61-2732). However, this method also substantially deteriorates the γ-value of the solid solution C and N, and even if it contains the remaining solid solution C and N in the steel component, resulting in a very low formability. In other words, the moldability and the bake hardening ability are incompatible with each other.

In addition, the above-described process promotes carburization in the annealing process (Japanese Patent Laid-Open No. 63-38556), and the process for improving the addability also does not take into account the bake curing ability, thereby improving the bake curing ability. It is supposed to be impossible.

In addition, in the case of ultra-low carbon steel, even when carbonitride forming elements such as Ti and Nb sufficiently stabilizes C and N, it is not possible to obtain a bake hardening ability.

In addition, if the target value is too high by the step containing the solid solution (C), the aging property is deteriorated. On the contrary, if the target value is too low, the bake hardening ability cannot be obtained. Under compulsory legislation, it is very difficult to control the amount of carbon dioxide remaining.

[Summary of invention]

The present invention has been made in an attempt to solve the above-mentioned problems in the prior art, and an object thereof is to provide a cold rolled steel sheet having excellent deep drawing property and a hot molten zinc plated cold rolled steel sheet for manufacturing ultra low carbon steel containing Ti or Nb. Both of them have extremely good deep drawing and resistance to cold working embrittlement or bake hardening ability, and also provide an excellent deep drawing hot-dip galvanized cold rolled steel sheet having very good zinc coating.

In order to solve the above problems, the present inventors have completed the present invention as a result of studying the distribution of chemical components and solid solution (C) contained in steel.

In the present invention, C: 0.01% or less, Si: 0.2% or less, Mn: 0.05 to 1.0%, P: 0.10%, S: 0.02% or less, soluble aluminum (sol.Al) 0.005 to 0.08%, N: It contains 0.006% or less, and one or two kinds of Ti and Nb are defined by the following formula (1): the effective Ti amount (hereinafter referred to as Ti ^* ) and the relationship between the Nb amount and the C amount ) In the range satisfying 0.003, if necessary

Contains% or less of boron (B),

Ti ^* = (total total) Ti − [(48/32) × S + (48/14) × N]... … (One)

1≤ (Ti ^{* /} 48 + Nb / 93 / (C / 12) ≤4.5 …………………… (2)

A steel having a composition composed of the remaining Fe and inevitable impurities, having a concentration gradient such that the amount of solid solution carbon (C) decreases in the plate thickness direction from the surface to the center by carburizing from the surface to the center by carburizing treatment, The maximum amount of solid solution carbon (C) concentration in the plate thickness ratio of 1/10 of the surface layer is 15 ppm, and the solid carbon content of the entire steel sheet is 2 to 10 mass (weight) ppm. A cold rolled steel sheet or hot dip galvanized cold rolled steel sheet having dedrawing property and plating adhesion with bake hardenability is disclosed.

In addition, the present invention has the same chemical composition as the chemical composition, has a concentration gradient so that the amount of solid solution carbon in the plate thickness direction from the surface to the central portion by the carburizing treatment, and has a concentration gradient, C) A hot dip galvanized cold rolled steel sheet having excellent deep drawing and plating adhesion is disclosed, with a maximum amount of 60 ppm by weight and a solid carbon content of 5 to 30 ppm throughout the steel sheet.

In addition, the present invention has the same chemical composition as well as the hot-dip galvanized cold rolled steel sheet excellent in deep drawing and plating adhesion by making the amount of solid solution carbon 10 ~ 100ppm within the range from the surface of the steel sheet to the thickness of 100㎛ Initiate.

The present invention is described in more detail below.

First, the reason for limiting the chemical composition of the steel sheet according to the present invention will be described.

Carbon (C) component: Increasing the amount of Ti and / or Nb added to stabilize carbon results in an increase in carbon content. The result is an increased amount of TiC and / or NbC precipitation, which in turn hinders grain growth and thus worsens γ-values. This inevitably leads to an increase in manufacturing costs. Therefore, it is necessary to keep carbon amount at 0.01 weight% or less. The lower limit of this carbon content is not particularly limited in the emphasis, but technically, it should be set at about 0.0003% by weight in actual steel manufacturing technology. It is preferable to make carbon content into 0.01 weight% or less. And it is good to make the lower limit into 0.0003 to 0.01 weight%.

Furthermore, as will be described later, in order to provide resistance to cold work brittleness, the steel sheet has a concentration gradient, which decreases as it goes through the thickness direction from the surface toward the center, and with the maximum concentration of solid carbon (C) 15 ppm (below) All mass mass ppm) at the surface layer area of the one-tenth gage ratio of 1/10, and also with the amount of solid solution carbon set at 2-10 ppm in all of the steel sheets. In order to separate the good bake hardenability, the steel, however, is allowed to have a maximum concentration of solid solution carbon up to 60 ppm in some 1/10 gauge ratios in addition to the concentration gradients described above, Maintain ˜30 ppm (mass ppm below all). In addition, in order to obtain good adhesion of the galvanized steel sheet coating, the amount of solid solution carbon at a depth of 100 μm from the steel sheet in the thickness direction should be set at 10 to 100 ppm (all mass ppm below). Any means may be applied to indicate such suitable conditions for the presence of dissolved carbon (C). However, in terms of productivity, it is desirable to create an atmosphere having a carbon potential in the annealing process before galvanizing.

Silicon (Si): Silicon (Si) is mainly used to deoxidize molten steel. Excessive addition of it deteriorates the surface condition, impairs galvanized adhesion, and impairs paintability. Therefore, the Si content should be kept below 0.2% by weight.

Manganese (Mn): Manganese (Mn) is added mainly to prevent the occurrence of hot shortness. However, if it is added at 0.05 wt% or less, the desired effect is not obtained. However, since the ductility deteriorates excessively when added, it is necessary to keep it in the range of 0.05 to 1.0% by weight.

Phosphorus (P): Phosphorus (P) has the effect of increasing the strength of the steel without deteriorating the γ-value. In the case of very low carbon steels, phosphorus has a carbon-like effect on the zincation reaction in improving the adhesion of the zinc coating. However, since phosphorus causes segregation at the grain boundary and cold work embrittlement, it needs to be adjusted to 0.10 wt% or less.

Sulfur (S): Sulfur (S) combines with Ti to form titanium emulsion (TiS). Therefore, if the sulfur content is increased, the Ti content for stabilizing C and N increases. In addition, the amount of the manganese emulsion (MnS) series increases the inclusion (inclusion) to make a long shape, thereby hurting the local ductility. Therefore, it is necessary to be 0.02 weight% or less.

Soluble aluminum: Aluminum is added for the purpose of deoxidizing molten steel. If this soluble aluminum is made into 0.005 weight% or less, the objective cannot be achieved. On the other hand, if the content exceeds 0.08% by weight, the deoxidation effect is saturated and the amount of aluminum oxide (Al ₂ O ₃ ) inclusions impairs formability. Therefore, it is necessary to maintain the amount of soluble aluminum in the range of 0.005 to 0.08% by weight.

Nitrogen (N): Nitrogen (N) combines with Ti to produce titanium nitride (TiN). Therefore, the amount required to stabilize C increases as the amount of N increases. In addition, increasing the amount of TiN precipitate inhibits the growth of the crystal and deteriorates the γ-value. Therefore a small amount of addition is preferred. N should be controlled to 0.006% by weight or less.

Titanium (Ti), niobium (Nb): These additives are used to stabilize C and N for the purpose of increasing γ-values. In order to achieve the objective value of this invention, these addition amounts need to be added within the range which satisfy | fills following Formula (2) which shows the correlation of Ti ^* , Nb amount, and C.

1≤ (Ti ^{* /} 48 + Nb / 93 / (C / 12) ≤4.5 …………………… (2)

Ti combines S and N to form TiS and TiN described above, respectively, and the addition amount thereof is determined by the following equation (1) in which the Ti effective amount is converted and calculated.

Ti * = (total total) Ti − [(48/32) × S + (48/14) × N]... … (One)

If the value of equation (2) is smaller than 1, C and N will not be sufficiently stabilized, and thus the γ-value will deteriorate. In addition, when this value exceeds 4.5, the effect of increasing the γ-value is saturated, and the solid solution Ti and / or Nb immediately stabilizes the carbon that is penetrated during the atmosphere annealing process in the continuous process immediately. This carbon stabilization prevents segregation at grain boundaries and also the presence of solid-solute C.

Boron (B): Boron (B) is an effective element for imparting resistance to cold work brittleness. In addition, the additives increase resistance to cold work embrittlement with the intention of improving the bake curing ability. However, if this additive exceeds 0.003% by weight, the effect is included and the γ-value worsens. Therefore, it is necessary to consider to keep the B content at 0.003% by weight or less. The amount of 0.0001% by weight or less has little effect of adding bronze. Therefore, the preferable boron addition amount is 0.0001 to 0.003 weight%.

Next, the steel sheet produced by the method of the present invention is not particularly limited in its manufacturing method, and may be an example of the following manufacturing method, which will be described below. Steel having the chemical composition described above is hot rolled by a conventional method. That is, rolling is carried out after heating to an austenite region at a temperature of 1000 ~ 1250 ℃. Cold rolling after hot rolling is performed in the range of 500 ~ 800 ℃ to stabilize the solid solution carbon and nitrogen as carbonitrides in the steel.

In cold rolling, it is necessary to achieve a total reduction of 60-90% in order to have a (111) structure useful for γ-values. After this cold rolling, a continuous annealing operation is performed by the carburizing atmosphere gas within a temperature range exceeding the recrystallization temperature for forming a (111) structure useful for γ-.

As is already known, this γ-value mainly depends on the structure of the steel 111 and is performed by completely stabilizing the solid solution C and N by the coiling treatment before recrystallization annealing. However, once recrystallization is complete and tissue is formed, C and N continue to penetrate but do not adversely affect γ-values. This annealing atmosphere should consist of carburized gas with a controlled carbon potential. This carbon is permeated from the carburizing gas atmosphere, but the above-mentioned TiC and NiC are not stabilized enough to cause segregation at grain boundaries.

This improves the cold workability and improves the adhesion of galvanized steel sheets, and the specific amount of solid solution carbon improves the bake hardening ability.

According to the present invention, the display effect is not required, but the display effect may occur near the plating bath temperature. In order to produce a galvanized cold rolled steel sheet, the steel sheet is continuously immersed in a hot zinc coating bath, and further alloyed if necessary.

In this case, any of the steel sheet manufacturing methods to be annealed is applied as long as it includes hot rolling in the ferritic zone, hot filling rolling, and thin slab casting and rolling.

Next, the relationship between the control amount of solid solution carbon, resistance to cold work brittleness, and bake hardening ability or zinc plating adhesion will be described below.

Cold workability It is easy to occur in ultra low carbon steel with Ti added. The reason is high purity grain boundary and low Fe-Fe bond strength at grain boundary. In addition, during hot dip galvanizing, Zn diffusion occurs at grain boundaries, further weakening the Fe-Fe bond. Therefore, the improvement of resistance to cold work brittleness is achieved only by hindering two factors that weaken the above-described Fe-Fe bond. The former and latter problems can be solved by segregation of carbon with respect to grain boundaries. Particularly in the latter case, the depth of Zn diffusion becomes equal or about 50 µm with respect to various grains, and the above-mentioned problem can be solved by deeply concentrated carburizing as described above through the thickness direction. An effective way to obtain the best resistance to cold embrittlement is to create a concentration gradient in the steel sheet so that the amount of dissolved carbon decreases in the thickness direction as the basketball boat is directed from the surface to the center, with a maximum solution at the 1/10 gauge ratio of the surface layer. The concentration value of carbon is set to 15 ppm. In addition, it was confirmed that brittle fracture occurs after the deep drawing in the surface layer, and thus the graininess of the surface layer is strengthened by the segregation of the solid solution to the grain boundary.

As a result, it was confirmed that a remarkable effect was obtained in spite of slight or zero grain boundary segregation at the center of the steel sheet thickness. If the amount of solid solution carbon in the surface layer exceeds 15 ppm, the average amount of solid solution carbon in the whole steel sheet portion exceeds 10 ppm, and as a result, the improvement in resistance to cold work brittleness is saturated. Moreover, when the average amount of solid solution carbon in the whole steel plate part is 2 ppm or less, the increase of the effective resistance to cold work brittleness will become impossible.

On the other hand, in the case of Ti-added ultra low carbon steel, it is impossible to obtain a bake hardenability due to the presence of residual solid solution carbon. However, this hardening capacity can only be achieved if the recrystallization is completed and thus the formation of the tissue is completed so that the solid carbon can be maintained to maintain a high γ-value. Furthermore, given a concentration gradient that reduces the amount of solid solution carbon from the surface of the steel sheet toward the center, in the thickness direction, and at the one-tenth gage thickness of the surface layer where hardening of the surface layer is most facilitated. When the maximum employment carbon concentration is set at 60 ppm, the panel's surface can withstand the excellent properties of the subpanel of the vehicle, for example, the strength of the panel, which can withstand a large degree of strength or the impact of a material such as a stone on the surface of the vehicle. It can bring about characteristics such as (dent resistance). It is desirable that the amount of solid solution carbon in the steel sheet surface layer not exceed 60 ppm, because it is not preferable to allow the amount of solid solution carbon in the entire steel sheet to exceed 60 ppm. This is because it will not be possible to reduce the amount of dissolved carbon in the steel sheet to less than 30ppm, which will lead to deterioration of mechanical properties.

If this is reversed, lowering the amount of solid solution carbon in the whole steel sheet to less than 5 ppm also causes a problem and makes it impossible to obtain a baking hardenability.

The present invention also seeks to improve the adhesion of galvanized coatings. It will be described in detail below.

The present invention also seeks to improve the adhesion of galvanized coatings. It will be described in detail below.

In order to improve the coating property of zinc coating, an appropriate amount of aluminum is usually added to the molten zinc bath depending on the type of steel sheet. In the molten zinc bath, Fe and Al first react as an initial reaction of galvanizing, and an Fe-Al intermatallic compound layer is formed on the inner surface between the hot dip zinc and the surface of the steel sheet. Thereafter, the zinc plating reaction including alloying of the zinc plating proceeds while being affected by this inner metal composite layer. When a uniform Fe-Al inner metal composite layer is formed on the inner surface, the composite layer is likely to be an obstacle to the interdiffusion between the galvanized steel and the base steel sheet, and the alloying of the zinc plated ensures good adhesion of the galvanized coating. It is evenly done.

However, when the grain boundary of the steel sheet is purified, Al in the bath penetrates into the activated grain boundary in order to lower the Al concentration in the vicinity of the grain boundary. Thus, the Al-Fe composite layer is formed near the grain boundaries of the steel sheet, from which the galvanized coating forms a so-called "outburst" structure which is rapidly alloyed and crumbly. This soon leads to a rapid and uniform galvanization of the alloy, which in turn worsens the adhesion of the galvanized coating.

This problem can be solved by increasing the amount of Al added to the zinc bath to some extent. However, increasing the amount of Al adds a dross in the bath, thereby creating a surface bond such as a crater, thereby decreasing productivity. Increasing the amount of Al is not a fundamental solution in solving the above problems.

The adhesion of galvanized coatings in ultra low carbon steels, such as steel sheets with Ti, is deteriorated because there is no segregation of carbon in the ferritic grain boundary that occurs due to the absence of solid carbon in the steel.

To solve this problem, it is necessary to carburize the river. By doing so, carbon can be present at grain boundaries near the surface of the steel sheet, and it is possible to prevent Al from spreading throughout the grain boundaries of the steel sheet as a base metal. In doing so, a uniform Fe-Al composite layer at the interface between the molten zinc and the steel sheet can be obtained, and the occurrence of "outburst" can be prevented.

The present invention realizes the improvement of adhesion of the galvanized coating through carburization in the annealing process without deteriorating the formability of the steel sheet like the base metal.

The steel sheet, however, is based on steel having a special chemical composition. In this case, however, if the amount of solid solution carbon present in the portion of the surface of the steel sheet in the thickness direction of approximately 100 μm or less is 10 ppm or less, the adhesion of the galvanized coating is not sufficiently improved. On the other hand, if the amount of solid solution carbon exceeds 100 ppm, the aging property is weakened. This aging requires a decrease in speed for supplying the steel sheet in the continuous annealing process. This leads to a decrease in productivity. In order to solve this problem, it is necessary to adjust the amount of solid solution carbon in the range of 10 ~ 100ppm in the portion corresponding to the depth of 100㎛ from the steel plate surface in the thickness direction.

These and other objects of the present invention are believed to be better understood from the following description and drawings.

Best Example

Hereinafter, the cold rolled steel sheet or hot molten zinc cold rolled steel sheet of the present invention will be described in detail. First, a steel sheet having excellent resistance to cold work brittleness and bake hardening ability will be described.

Example 1

The ultra low carbon steel having the chemical composition shown in Table 1 was heated at 1150 ° C. for about 30 minutes for solution treatment, hot rolled at a finishing temperature of 890 ° C., and then coiled at 670 ° C.

After pickling, these steel sheets were cold rolled at a 75% area reduction rate. This cold rolled steel sheet was then continuously annealed for 40 seconds at a temperature of 780 ° C. under a carburizing gas atmosphere (or under a N ₂ -H ₂ gas atmosphere) for recrystallization annealing. The steel sheet thus treated was hot dip galvanized at 450 DEG C and finally subjected to 0.8% skin pass rolling.

The critical temperatures for the cold brittleness of the mechanical properties obtained by the above-described treatment, the ring of solid solution carbon (average value in the total steel thickness direction), and the cold rolled steel sheet subjected to hot dip galvanizing are as shown in the second table.

The brittle test is a barometer that determines the critical temperature for cold working brittleness of the steel sheet. The steel sheet is trimmed to a height of 35 mm, and the cup is prepared by forming a cup in a drawing of a total ratio of 2.7. In the refrigerator, the concubine located in the refrigerator is pushed into a conical punch so that the angle of the vertex is 40 °, and the critical temperature at which cracking does not occur is measured. This measured critical temperature measures the critical temperature not in the second operation. This measured critical temperature is the critical temperature determined for brittleness in the second operation.

As can be clearly seen in the second table, it can be seen that the steel of the present invention is significantly more durable against cold brittleness than the steel of the prior art so as not to impair the requirement for hot molten zinc for deep drawing cold steel sheet. have.

As a result of the distribution test of the solid solution carbon C in the thickness direction in the third steel according to the present invention, in the case of the carburized steel shown in FIG. 1 from the concentration distribution, the solid solution carbon as the thickness was moved from the surface of the steel sheet to the center part The amount is reduced. Furthermore, in carburized steel in gas boron (B), the amount of dissolved carbon at a part of the 1/10 gauge ratio of the surface layer is 15 ppm or less, and as shown in FIG. 2, the resistance to cold work brittleness is It was confirmed that it is improved without deteriorating γ-value.

On the other hand, as shown in FIG. 2, in the case of the steel having no chemical composition in the present invention or not meeting the requirements for the amount of solid solution carbon, that is, in the case of steel having a different chemical composition compared with the present invention, both are? It was found to be low in both teeth and resistance to cold work brittleness.

[Table 1] Chemical Composition of Test Steel (Weight%)

(Note 1) These values are outside the scope of the present invention.

(Note 2) X = (Ti ^* / 48 + Nb / 93) / (C / 12).

[Table 2] Mechanical properties and critical temperature for cold work brittleness

Example 2

The test steel having the chemical composition shown in Table 1 is obtained by cold-rolled steel sheet by 0.8% skin pass rolling after recrystallization annealing in a carburizing atmosphere or a N ₂ -H ₂ gas atmosphere through a continuous annealing process. The other requirements are the same as in Example 1.

The mechanical properties as the average value in the total steel thickness direction and the critical carbon content and the critical temperature for the cold work brittle temperature of the rolled steel sheet are obtained as shown in the third table.

As can be seen from the third table, it can be seen that the steel of the present invention has a resistance to cold work brittleness which is much better than that of conventional steel without being disposed on a cold rolled steel sheet for deep drawing.

In any case, the arrangement study was conducted through the thickness direction of the solid carbon content of the third steel according to the present invention in the third table. As shown in FIG. 3, the carburized steel was dissolved in the thickness direction toward the center from the surface. It has a concentration gradient that slows down the amount of carbon. In addition, in case of sedimentation using gas B, the amount of solid solution carbon at the 1/10 gauge rate portion of the surface is 15 ppm or less, which is accelerated as shown in FIG. 4 so that the resistance to cold work brittleness is γ-value. It is improved without deterioration.

On the other hand, as shown in FIG. 3, in the case of the steel of comparative example without the chemical composition of the present invention or the steel which does not satisfy the requirements such as the amount of the solid solution carbon of the present invention as described above, It can be seen that it is inferior in resistance to cold work brittleness.

[Table 3] Cold work brittleness

Example 3

One-minute recrystallization annealing at 800 ° C under carburizing or (N ₂ -H ₂ ) gas atmosphere in the annealing process before cold-rolling galvanized test steel with chemical composition as shown in Table 1 After recrystallization annealing, hot dip galvanizing was performed at 450 ° C, and finally 0.8% scan pass rolling was performed.

The resulting mechanical properties, solid carbon content (average value in the direction of total steel thickness), ageing index (AI), and bake hardenability (BH) of the hot-dip galvanized steel sheet are shown in Table 4.

The age index was calculated by AI. AI was calculated using AI = σ ₂ −σ ₁ from the stress (σ ₂ ) at 10% stretching.

Bake hardening capacity was calculated by BH. BH is obtained by using a BH = -σ ₂ σ ₄ from stress (σ ₃₎ of 2% in the host retching stress (σ ₄₎ at the time of re-cast retching in 20 distributed aging at 170 ℃.

As is clear from the fourth table, the steel obtained according to the present invention has excellent bake hardenability comparable to that of the conventional steel and is not disposed under the conditions for the deep drawing hot dip galvanized cold rolled steel sheet. There is no problem at all and the aging is good steel.

Test results for the distribution of the amount of solid solution carbon C in the thickness direction in the steel No. 7 according to the present invention given in Table 4 show that the carburized steel has the amount It is decreasing accordingly. Moreover, when carburizing in gas B atmosphere, it was confirmed that the dissolved carbon concentration in the 1/10 gauge rate portion of the surface layer was 60 ppm or less, and the bake curing ability was improved without deteriorating the γ-value.

On the other hand, as shown in Table 4, the steels of the comparative examples which do not have a chemical composition made by the present invention also have a chemical composition of the present invention but which do not satisfy the conditions of the amount of solid solution carbon of the present invention. It can be seen that inferiority in both gamma -value and bace hardening ability.

[Table 4] Mechanical Properties, Aging Index (AI), Bake Hardening Capacity (BH)

Example 4

In Example 3, the test steel having the chemical composition shown in Table 1 was re-annealed under an (N ₂ -H ₂ ) gas atmosphere or under a carburized gas atmosphere and overaged for 3 minutes at 400 ° C. Finally, 1% skin pass rolling to obtain a cold rolled steel sheet. Other conditions were the same as those in Example 3.

The mechanical properties, the solid carbon amount (average value in the total steel thickness direction), the aging index (AI) and the bake hardening capacity (BH) of the thus obtained cold rolled steel sheet are as shown in Table 5.

As can be seen from the fifth table, the steel sheet produced according to the present invention has a very good bake hardenability compared to conventional steels, and is not disposed under the conditions as a cold rolled steel sheet for deep drawing and also has good aging. This is confirmed.

On the other hand, the results of testing the concentration gradient of the amount of solid solution carbon in the thickness direction of the seventh steel of the present invention is shown in Table 5. The carburized steel shown in FIG. 7 shows that the concentration gradient of the solid solution amount is made in the thickness direction toward the center from the surface. In addition, it can be seen that in the steel carburized under the atmospheric term of gas B, the dissolved carbon concentration at the site of 1/10 gauge ratio of the surface layer is 60 ppm or less, and the steel improves the bake hardening capacity without deteriorating the γ-value.

On the other hand, as shown in Table 5, both the steel as a comparative example without the chemical composition of the present invention and the steel as a comparative example without satisfying the conditions for the amount of the solid solution carbon of the present invention but without the chemical composition of the present invention are both γ. It is inferior in both the value and the bake hardening ability.

[Table 5] Mechanical Properties, Aging Index (AI), Bake Hardening Capacity (BH)

Next, a hot molten zinc cold rolled steel sheet having excellent galvanized coating adhesion by another embodiment of the present invention will be described below as an example.

Example 5

The steel sheet of ultra low carbon steel having the chemical composition shown in Table 6 was heated to 1150 ° C for solution treatment for about 30 minutes, hot rolled at 890 ° C, coiled at 720 ° C, and cold rolled at 75% area reduction rate through pickling. To a 0.8 mm thick steel sheet. Subsequently, the steel sheet was continuously annealed at 780 ° C. for about 40 seconds in a hot hot dip galvanizing line to recrystallize it under a carburizing atmosphere or N ₂ -H ₂ gas atmosphere, and then cooled to 500 ° C., followed by hot hot dip galvanizing operation After performing the alloying treatment (alloying treatment) process at 600 ℃ 40 seconds.

The seventh table is obtained by mechanical properties, coating adhesion, ageing, and the amount of solid solution carbon of hot galvanized cold rolled steel sheet as described above.

To evaluate galvanic coating adhesion, the steel sheet was made 60 mm high with 5 mm high beads, stripped in three steps using a 50 mm wide die and 52 mm wide die. The adhesiveness was measured by the amount peeled off by classifying the losing state. That is, it evaluated as good (○), slightly poor (△), and bad (×).

In order to measure the amount of solid solution carbon, the amount of free carbon and the amount of carbide in the steel were separated. That is, the amount of free carbon is that of both surfaces polished to a thickness of 100 mm from the surface and that of the unpolished sample, and half of the difference between the two samples is the thickness of 100 μm where the solid solution carbon is measured in the direction of the steel sheet thickness from the surface It measured by the amount contained in it.

Aging castle were evaluated by AI index, AI index lower yield stress _{(lower yield stress) (σ 2} ) of the time by aging at 100 ℃ from the stress (σ ₂₎ at the time of retching host 100% re-cast retching after 1 hour It was carried out using the difference AI = (σ ₂ -σ ₁₎ with formula.

As can be seen from the seventh table, it is confirmed that all samples of the present invention have excellent galvanized coating properties as compared to conventional steels, but are not arranged under the conditions of deep drawing hot dip galvanized steel as cold rolled steel sheet. Can be.

9 shows the relationship between the amount of solid solution carbon present in the steel as shown in Table 7 and the adhesion of γ-value and zinc coating in the thickness direction from the surface of the steel sheet to 100 μm thickness.

From the above Tables 7 and 9, it can be seen that the steel of the present invention further improves the adhesion of the galvanized coating without deteriorating the γ-value by carburizing.

[Table 6] Chemical Composition of Test Steel (wt%)

x = (Ti ^* / 48 + Nb / 93) / (C / 12)

Ti ^* = total Ti-{(48/32) × S + (48/14) × N}

[Table 7]

Claims (4)

By weight%, C: 0.01% or less, Si: 0.2% or less, Mn: 0.05 to 1.0%, P: 0.10%, S: 0.02% or less, soluble Al (sol.Al) 0.005 to 0.08%, N: 0.006% It contains the following, Furthermore, the relationship between Ti amount (henceforth Ti ^* ) and Nb amount and C amount is contained in the range which satisfy | fills following Formula (2), Ti ^* = (total total) Ti − [(48/32) × S + (48/14) × N]... … … (One) 1≤ (Ti ^{* /} 48 + Nb / 93 / (C / 12) ≤4.5 ……………………… (2) The remainder of the steel has a composition composed of Fe and inevitable impurities, and has a concentration gradient such that the amount of solid solution carbon (C) decreases from the surface to the center by the carburizing process from the surface to the center by carburizing treatment. Secondary processing, characterized in that the maximum amount of the dissolved carbon (C) concentration of the plate thickness ratio portion of the surface layer 1/10 is 15 ppm by weight, and the dissolved carbon content of the entire steel sheet is 2-10 mass ppm. Hot-dip galvanized steel sheet having brittle deep drawing property and plating adhesion. The method of claim 1, wherein the carburizing treatment has a concentration gradient such that the amount of solid solution carbon decreases from the surface to the center portion in the plate thickness direction. The mass ppm is set to 5 to 30 ma of the dissolved carbon content of the whole steel sheet. Hot-dip galvanized steel sheet having deep drawing property and plating adhesion to ss ppm. The molten zinc having deep drawing property and plating adhesion, according to claim 1, characterized in that the amount of solid solution is contained in a range of 10 to 100 ppm by mass within the range of 100 µm in the thickness direction from the surface of the same chemical steel sheet. Cold rolled steel sheet. 4. The molten zinc cold rolled steel sheet according to claim 1, wherein the steel sheet further contains B: 0.003 wt% or less.