RU2532689C2 - Cold-rolled steel plate having excellent quality of surface after forming, and its manufacturing method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: after forming, due to exclusion of formation of linear structures, steel containing the following (wt %) is made: 0.0005-0.0050 C, not more than 0.30 Si, not more than 0.50 Mn, not more than 0.050 P, not more than 0.020 S, 0.010-0.100 Ti and not more than 0.080 sol. Al, not more than 0.007 N and Fe and inevitable impurities are the rest provided that content of C, N, S and Ti meets the following ratio (I): ([%Ti]/48-[%N]/14-[%S]/32)/([%C]/12)≥1.00…(I), where [%M] represents content in the steel of element M (wt %); it is subject to hot rolling, etching, cold rolling, continuous annealing and cooling at the rate not exceeding 30˚C/s at the temperature interval of 400-200°C.

EFFECT: improving steel plate surface quality.

5 cl, 2 tbl

Description

FIELD OF THE INVENTION

The invention relates to a cold-rolled steel sheet suitable for use in exterior skin panels and other similar parts of automobiles and having excellent surface quality after stamping, as well as a method for its production.

State of the art

Recently, stricter requirements for the quality of the surface of the panels of the outer skin of cars. Surface defects affecting surface quality are broadly classified as being observed on the surface of the steel sheet at the stage of its production and those that appear after stamping on the press line or other similar stage of automobile manufacturing.

The first type of surface defects is determined relatively easily, and therefore it has little effect on the production of cars. In addition, measures to prevent them at the source material stage are known, such as are disclosed, for example, in JP-A-H09-296222.

On the other hand, since surface defects of the second type can be detected at an early stage after stamping parts or already at the stage of final control after installation on a car body, their impact on automotive production is very large.

To date, effective countermeasures that can serve as a means of preventing the manifestations of the latter type of surface defects are not known.

SUMMARY OF THE INVENTION

Problems Solved by the Invention

The invention was developed taking into account the above situation and should create cold rolled steel sheets having, in particular, excellent surface quality after stamping, and also provide a preferred method for their production.

Ways to solve the problem

The authors of this invention have conducted various studies of the mechanism of formation of defects that develop in the form of surface defects after stamping, and countermeasures to suppress them in order to solve the above problems.

As a result, it was found that, due to elongation during yield, a local inhomogeneous deformation develops in the steel sheets, which exhibits such surface defects during their annealing, which is the cause of the surface defect after stamping.

Thus, when a non-uniform deformation is caused in the steel sheet during annealing, the hardness in the non-uniformly deformed section is greater than in the undeformed section, and the number of deformations is small, therefore, the non-uniformly deformed section is extruded onto the part in the form of a convex section, and its external the view becomes unsatisfactory. In addition, subtle linear defects are evidently introduced that demonstrate a shape that extends obliquely at an angle of 45 ° relative to the longitudinal direction of the steel sheet.

Initially, it was assumed that such defects mentioned above did not appear in the so-called IF-steel (steel without interstitial atoms), in which C and N in solid solution are bound to Ti and Nb, since in this case there is usually no obvious elongation during flow. However, as a result of initial studies of the authors of this invention, it was found that a clear elongation during flow can be observed to a small extent in the absence of training after annealing, and, therefore, as a result of this elongation during yield in IF steel, surface defects of a similar kind are formed.

To suppress the manifestation of the surface defects mentioned above, it is sufficient to ensure the absence of such amounts of deformations that exceed the yield strength during annealing. In general, the equipment design and sheet passage requirements set conditions in the continuous annealing furnace that do not create stresses above the yield strength of the steel sheet. It is obvious that in practice local inhomogeneous deformations are introduced due to thermal stress during heating and cooling, which under certain conditions can exceed the yield strength of a steel sheet.

The authors of this invention conducted further studies of the factors affecting the formation of surface defects due to the aforementioned inhomogeneous deformation during annealing, and found that when during cooling after completion of recrystallization, the cooling rate exceeds a certain level within a certain temperature range that develops in the steel sheet thermal deformation increases, and stress exceeding the yield strength of the steel sheet leads to surface defects after stamping.

The present invention is based on these discoveries identified above.

Thus, a brief description and essence of the invention are as follows.

1. Cold-rolled steel sheet having excellent surface quality after stamping, having a chemical composition containing 0.0005-0.0050 wt.% C, not more than 0.30 wt.% Si, not more than 0.50 wt.% Mn, not more than 0.050 wt.% P, not more than 0.020 wt.% S, 0.010-0.100 wt.% Ti, not more than 0.080 wt.% sol. Al (acid-soluble aluminum), not more than 0.0070 wt.% N and the rest is Fe and inevitable impurities, provided that the content of C, N, S and Ti satisfies the following ratio (I):

where [% M] represents the content in the steel of the element M (wt.%),

characterized in that the sheet does not form linear structures when a unidirectional tensile strain is applied to the test sample in the form of a strip in the rolling direction, and then the surface of the sample is cleaned with an emery stone.

2. The cold-rolled steel sheet according to paragraph 1, having excellent surface quality after stamping, which, in addition, contains at least one of 0.0003-0.0030 wt.% B and 0.003-0.100 wt.% Nb, and in the case the presence of Nb instead of equation (1), the following relation (2) holds:

where [% M] represents the content in the steel of the element M (wt.%).

3. The cold-rolled steel sheet according to paragraphs 1 or 2, having excellent surface quality after stamping, which also has a layer of a zinc coating applied to the surface of the steel sheet.

4. A method of manufacturing a cold rolled steel sheet having excellent surface quality after stamping, characterized in that the steel material having the chemical composition specified in paragraphs 1 or 2 is subjected to hot rolling, pickling, cold rolling and then continuous annealing, while this cold rolled the steel sheet during cooling during continuous annealing is cooled with a cooling rate not exceeding 30 ° C / s, within the temperature range of 400-200 ° C.

5. A method of manufacturing a cold rolled steel sheet according to paragraph 4, having excellent surface quality after stamping, which further comprises coating the surface of the steel sheet in order to obtain a zinc coating layer.

Effect of the invention

According to this invention, it is possible to create cold rolled steel sheets having excellent surface quality after stamping.

The implementation of the invention

The invention will now be described in more detail.

First of all, the reasons why the chemical composition of the steel sheet is limited in the present invention by the above boundaries will be described. In this case, percentages representing the following chemical composition mean mass percent, unless otherwise indicated.

C: 0.0005-0.0050%.

With an increase in the C content, the ability to deep draw and ductility deteriorate, and it is also difficult to provide stampability in the form of panels of the outer skin. Therefore, the upper limit of the content of C is defined as constituting 0.0050%, preferably 0.0040%. On the other hand, when the C content is below 0.0005%, the size of the crystalline grains becomes larger and a tendency to form chalk deposits on the surface of the steel sheet is manifested, therefore, the lower limit of the C content was determined to be 0.0005%.

Si: not more than 0.30%.

Si is an element with high efficiency in solid solutions, but with an increase in its content, the hardness of the steel sheet increases and surface defects that are due to the formation of scale are easily manifested. Therefore, the upper limit of the Si content is defined as being 0.30%, preferably 0.20%.

Mn: not more than 0.50%.

Mn is an element, the addition of which in excessive amounts leads to an increase in the hardness of the steel sheet. Therefore, the upper limit of the Mn content is defined as 0.50%, preferably 0.30%.

P: not more than 0.050%.

P increases the hardness of the steel sheet when added even in small quantities. Therefore, the upper limit of the content of P is limited to 0.050%, preferably 0.035%.

S: not more than 0.020%.

S, when its content becomes as high as P, degrades the toughness of the welded joints. Therefore, the upper limit of the S content is limited to 0.020%, preferably 0.015%.

Ti: 0.010-0.100%.

Ti is a particularly important element for the present invention, since C which is present in solid solution can be bound in the form of carbide or carbonitride to provide a deep drawing ability that meets the requirements for the manufacture of cladding panels. When the Ti content is less than 0.010%, the desired effect is not achieved, while when it exceeds 0.100%, the effect is redundant and the hardness of the steel sheet increases, resulting in worsening stampability, so the Ti content is limited to 0.010-0.100%.

Sol. A1: not more than 0.080%, N: not more than 0.0070%.

N and sol. A1 does not impair the effect of the invention as long as they are contained in the same amounts as in conventional steels, therefore they are limited as follows: sol. Al: 0.080% and N: not more than 0.0070%, respectively.

To control the content of solid solution C, it is necessary that the content of C, N, S and Ti satisfy the following ratio (I):

$$([\%Ti]/48-[\%N]/\mathrm{fourteen}-[\%S]/32)/([\%C]/12)\ge 1.00...(\mathrm{one})$$

where [% M] represents the content in the steel of the element M (wt.%).

When the conditions of the above ratio are fulfilled, the solid solution C is completely bound in the form of TiC, or TiCN, or a complex carbide, or its carbonitride, whereby the ability to deep draw, which meets the requirements for the production of external cladding panels, can be achieved. If the conditions of this ratio are not satisfied, the ability to deep draw is deteriorated. In addition, it is desirable to ensure complete binding of the solid solution C, when the value of the left side of equation (1) is not lower than 1.20. On the contrary, when the magnitude of the left side of equation (1) exceeds 15.0, the binding effect of C in the solid solution is redundant, so it is preferable that it does not exceed 15.0.

Although the above description relates to the main components of the invention, if necessary, the following elements may also be properly contained.

B: 0.0003-0.0030%.

To improve the secondary resistance caused by the hardening of brittleness of parts obtained by deep drawing, B. is added. However, when the content of B is less than 0.0003%, the desired effect is not achieved, while in case of exceeding 0.0030% the steel sheet becomes harder and its stampability worsens. Therefore, the content of B is limited to 0.0003-0.0030%.

Nb: 0.003-0.100%.

Nb is an element capable, similarly to Ti, of binding of C in the form of carbonitride, with proper control of the amount added to C. When the Nb content is less than 0.003%, such control in solid solution C is difficult and the desired effect cannot be obtained, while in the case when it exceeds 0.100%, the amount of precipitated phase increases and ductility worsens, therefore, the Nb content is limited to 0.003 -0.100%.

In the case of the presence of Nb, instead of the above equation (1), the condition of the following relation (2) should preferably be satisfied:

, $$\{[\%Nb]/93-([\%Ti]/48-[\%N]/\mathrm{fourteen}-[\%S]/32\}/([\%C]/12)\ge 1.00...(2)$$

,

where [% M] represents the content in the steel of the element M (wt.%).

When the conditions of the above ratio are satisfied, the solid solution C completely binds in the form of TiC, TiCN, NbC, or NbCN, or a complex carbide, or its carbonitride, whereby the ability to deep draw corresponding to the requirements for the manufacture of external cladding panels can be achieved. If the conditions of this ratio are not satisfied, the ability to deep draw is deteriorated. In addition, it is desirable to ensure complete binding of solid solution C, when the value of the left side of equation (2) is not lower than 1.20. On the contrary, when the value of the left side of equation (2) exceeds 15.0, the binding effect of C in the solid solution is redundant, therefore, it is preferable that it does not exceed 15.0.

Also, in order to improve the formability, for example, the ability to deep draw or other similar properties, as well as surface quality, V, W, Cu, Ni, Sn, Cr, Mo, Sb, etc. can be added, the action of which is based on the suppression of surface enrichment by individual elements at the stages of production. The effect of the invention does not deteriorate when they are added in an amount not exceeding 0.5%. In addition, the effect of the invention does not worsen when Ca is added to control the shape of the inclusions, or when the upper limit of the O content is raised to expand an acceptable range of deoxidation levels in order to improve purification efficiency, provided that they are added in amounts not exceeding 30 hours ./mln and 50 ppm, respectively.

In this case, the rest, in addition to the components described above, is represented by Fe and inevitable impurities.

Surface quality assessment of cold rolled steel sheet.

Next, a method for evaluating surface quality after stamping will be described. As mentioned above, surface defects detected after stamping can primarily be found at the stage of inspection of finished products after molding in the form of parts or later, after installation on a vehicle body, in contrast to surface defects that appear at the production stage, and therefore they have a very big impact on automobile manufacturing. The inventors of the present invention studied various methods for detecting defects in the form of strips arising from local plastic deformations during the manufacturing process, and found that such detection can be achieved in a simple and effective way by communicating a certain amount of deformations to the steel sheet and after grinding the surface with grinding a stone.

The optimal number of deformations is about 1-5%, because if it turns out to be too small or too large, the difference in deformation behavior becomes insignificant due to the difference in hardness between the plastically deformed section and the undeformed section. As a test sample, a sample in the form of a strip cut from a steel sheet in the rolling direction can be used. Since it is necessary to confirm the detection of a defect along the entire width of the product, it is advisable to maximize the possible expansion of the sample area within the limits established for tensile testing standards. In addition, when using a specimen with a longitudinal direction corresponding to the rolling direction, a linear structure can be estimated (strip defects).

In addition, strip-shaped defects that appear after tensile deformations assumed by the present invention are not so-called shear lines due to elongation at yield, but are defects due to the presence of a narrow section with higher hardness in the inner part of the steel sheet formed due to local plastic deformation introduced into the steel sheet during the manufacturing process. Shear lines are displayed in the form of strips with a width of 10 mm or more when the stretching of the sample in the form of a strip occurs, while the defect, which is the target for this invention, has a narrow linear shape with a width of not more than 5 mm.

Next, the production steps of the present invention will be described.

According to the invention, a steel material with the above chemical composition is cast, hot rolled, pickled, cold rolled, and then subjected to continuous annealing to obtain a cold rolled steel sheet. During continuous annealing of the invention, it is important, in particular, that the steel sheet is cooled during cooling at a cooling rate not exceeding 30 ° C / s, within the temperature range of 400-200 ° C.

According to the results of the studies carried out by the authors of this invention, the temperature range of 400-200 ° C is a temperature region that easily causes uneven deformation in the steel sheet due to changes in production conditions and thermal stress, since the yield strength is relatively small and the elongation during flow clearly manifests itself. In the temperature range exceeding 400 ° C, the yield strength is quite small, and an increase in the amount of dislocation easily occurs, and therefore, non-uniform deformation is not caused. On the other hand, in the temperature range below 200 ° C, the yield strength becomes quite high, and the deformation does not exceed the yield strength even in cases of the development of such a deformation.

The reason why the cooling rate is limited to no more than 30 ° C / s is because when the cooling rate exceeds the above value, the thermal stress generated by compression increases and locally exceeds the yield strength of the steel sheet, leading to uneven deformation. On the other hand, when the cooling rate becomes small, the deformation during cooling decreases, but if it turns out to be too small, an annealing line that is too long is required, and therefore it should preferably be at least 5 ° C / s.

The production method is not specifically limited in any way and can be carried out by conventional methods, except that cooling in the temperature range of 400-200 ° C. during cooling during the continuous annealing described above is the aforementioned controlled cooling. For example, a method for producing a slab by blooming or continuous casting and continuous hot rolling can be applied, which are combined for rough rolling of blanks during hot rolling. Also, a negative effect on the effect of the invention is not exerted by heating in the temperature range of 200 ° C. when an induction heater is used in the hot rolling process.

As for other preferred conditions of production, it is preferable that the heating temperature of the steel material during hot rolling is 1150-1300 ° C, the finish temperature of rolling is 850-950 ° C and the temperature of winding the strip into a roll is 500-700 ° C, the degree of compression during cold rolling was 60-90% and the holding temperature during continuous annealing (or continuous galvanizing) was 800-900 ° C.

The invention may also include a production method comprising a coating step for forming a zinc-based coating layer on the surface of the steel sheet. A zinc-based coating layer of pure zinc or a zinc alloy (Zn-Fe, Zn-Ni, Zn-Al or the like) can be obtained on the surface of a steel sheet by electrical plating or galvanizing. In the case of galvanizing, annealing and coating can be separate steps, or annealing and coating can be carried out continuously (for example, continuous galvanizing).

According to the invention, even in the case of a coated steel sheet obtained by electric coating, or a primed coated steel sheet or subsequently galvanized steel sheet, the effect of the invention does not deteriorate even if its surface is subjected to a grease treatment or film coating.

Examples

The invention is further described in detail based on the following examples.

Steel, the chemical composition of which is shown in Table 1, was melted and continuously cast to obtain a slab that was hot rolled under conditions of a heating temperature of 1200 ° C, a finish temperature of finished rolling of 900 ° C, and a strip winding temperature of 600 ° C. After that, the hot-rolled steel sheet was etched and cold rolled with a reduction ratio of 75% to form a 0.75 mm thick cold-rolled steel sheet. Further, it was subjected to continuous annealing or continuous galvanizing under the conditions shown in table 2, to obtain a cold-rolled steel sheet or galvanized steel sheet. After that, the sheet was trained with compression of 0.3%. Galvanizing conditions: the temperature of the galvanizing bath 460 ° C, the concentration of Al in the galvanizing bath in the case of performing alloying treatment of 0.13% or 0.2% in the absence of alloying treatment, the amount of coating: 45 g / m ² on each one-sided surface (double-sided coating), alloying temperature: 480-500 ° С and alloying degree (wt.% Fe) 10%.

Then, a sample in the form of a strip 150 mm long and 30 mm wide with a longitudinal direction in the rolling direction was taken from the roll (steel strip) over the full width, and a strain of 1%, 3%, and 5% was applied to it (the direction of extension in the longitudinal direction) in a tensile testing machine (traverse speed 10 mm / min). After that, the sample in a pre-stretched state was placed on a flat table and cleaned with an emery stone to assess the presence or absence of linear structures (strip-shaped defects) (Table 2 respectively shows: o — absence of a defect and X — presence of a defect). A visual study was made of the presence or absence of defects with the marking of the results of the observation of defects in the form of a strip in one or more places with an X.

Regarding the mechanical properties, the tensile strengths TS and the total elongation of EL were measured by conducting a tensile test of specimen JIS No. 5 (crosshead speed 10 mm / min). Tensile tests were performed on a sample taken along the rolling direction.

In addition, the ability to deep draw was evaluated by measuring the values of r - values (crosshead speed 10 mm / min, the use of preliminary deformation of 15%) of the JIS test specimen No. 5 in the directions 0 °, 45 ° and 90 ° relative to the rolling direction, then there are r0, r45 and r90 values, and then calculating the average value (r0 + 2 * r45 + r90) / 4.

The results thus obtained are also presented in table 2.

table 2 No. Steel type CAL / CGL Continuous Annealing Conditions The properties The presence or absence of strip defects Notes Holding Temperature (° C) The presence or absence of alloying treatment Maximum cooling rate at each stage (° C / s) TS (MPa) EL (%) r is the value 400-300 ° C less than 300-200 ° C less than 200-100 ° C one BUT Cgl 850 is present 10 10 twenty 276 fifty 2,3 about An example of the invention 2 BUT Cgl 850 is present 10 fifty fifty 278 51 2,4 X Comparative example 3 BUT Cgl 850 absent twenty twenty 40 279 fifty 2.2 about An example of the invention four AT CAL 850 40 thirty thirty 284 49 2,4 X Comparative example 5 AT CAL 850 thirty thirty fifty 274 fifty 2,5 about An example of the invention 6 AT Cgl 850 is present thirty thirty fifty 280 fifty 2,3 about An example of the invention 7 C Cgl 850 absent fifty fifty thirty 301 49 2.1 X Comparative example 8 D CAL 850 - twenty twenty 70 291 49 2.2 about An example of the invention 9 D Cgl 820 is present 5 fifteen thirty 299 49 2.0 about An example of the invention 10 E Cgl 850 absent fifteen 25 35 278 fifty 2.2 about An example of the invention eleven E Cgl 850 absent 25 35 45 279 fifty 2.1 X Comparative example 12 F Cgl 850 is present fifteen 90 90 277 fifty 2,5 X Comparative example 13 F CAL 800 - 35 10 fifteen 270 51 2,5 X Comparative example fourteen F CAL 800 - 25 25 25 271 51 2,5 about An example of the invention fifteen G Cgl 800 is present 25 25 45 288 fifty 22 about An example of the invention 16 G Cgl 800 is present 10 10 one hundred 284 fifty 2.1 about An example of the invention 17 N CAL 850 - 85 twenty twenty 275 51 2,4 X Comparative example eighteen N CAL 850 - 45 thirty thirty 278 51 2,4 X Comparative example 19 I Cgl 850 absent fifteen fifteen 40 277 51 2.2 about An example of the invention twenty J Cgl 850 is present 10 fifteen thirty 342 38 1.4 about Comparative example 21 TO Cgl 850 is present fifteen twenty 25 338 34 12 about Comparative example GAL - continuous annealing, CGL - continuous galvanizing

From table 2 it can be seen that by adjusting the cooling rate after continuous annealing, at which it is maintained within the temperature range of 400-200 ° C at a level of not more than 30 ° C / s, cold-rolled steel sheets and galvanized steel sheets having excellent the ability to deep draw with no defects in the form of strips even after stamping.

In addition, it was found that when the temperature of the steel sheet drops below 200 ° C, the yield strength of the steel sheet becomes large enough so that defects in the form of strips do not appear even when the cooling rate exceeds 30 ° C / s.

Industrial Applicability

According to the present invention, cold-rolled steel sheets can be produced and stably supplied, which are well suited for use in the exterior or interior panels of automobiles and have excellent surface quality after stamping, which makes it very important for industry.

Claims (5)

1. Cold-rolled steel sheet containing, wt.%:
C 0.0005-0.0050
Si no more than 0.30
Mn no more than 0.50
P no more than 0,050
S no more than 0,020
Ti 0.010-0.100
Al sol. no more than 0,080
N no more than 0,0070
Fe and unavoidable impurities - the rest,
provided that the content of C, N, S and Ti satisfies the following relationship (1):

where [% M] represents the content in the steel of the element M (wt.%). 2. The cold rolled steel sheet according to claim 1, on the surface of which a zinc coating layer is applied.
3 Cold-rolled steel sheet containing, wt.%:
C 0.0005-0.0050
Si no more than 0.30
Mn no more than 0.50
P no more than 0,050
S no more than 0,020
Ti 0.010-0.100
Al sol. no more than 0,080
N no more than 0,0070
at least
one element in 0.0003-0.0030
and Nb 0.003-0.100
Fe and unavoidable impurities - the rest,
provided that the content of C, N, S and Ti in the absence of Nb satisfies the following relation (1):

where [% M] represents the content in the steel of the element M (wt.%),
and the content of C, N, S and Ti in the case of the presence of Nb, satisfies the following relationship (2):

where [% M] represents the content in the steel of the element M (wt.%). 4. The cold-rolled steel sheet according to claim 3, on the surface of which a zinc coating layer is applied. 5. A method of manufacturing a cold-rolled steel sheet, comprising smelting steel with a composition according to any one of claims 1 or 3, hot rolling, pickling, cold rolling, continuous annealing and cooling, while cooling the sheet is carried out with a cooling rate not exceeding 30 ° C / s, within the temperature range of 400-200 ° C. 6. The method according to claim 5, in which a zinc coating layer is applied to the surface of the steel sheet.