HIGH-STRENGTH STEEL SHEET FOR FORMING AND PRODUCTION THEREOF

Abstract

 A high-strength steel sheet having a low in-plane anisotropy, a low yield ratio and a high r value and being completely nonaging and resistant to softening caused by high-temperature heating is produced by heating a steel slab having a composition comprising 0.01 to 0.1 wt % of carbon, 0.1 to 1.2 wt % of silicon, at most 3.0 wt % of manganese, such an amount of titanium as to satisfy the relationship of (Ti%-1.5S%-3.43N%)/C% = 4 to 12,0.0005 to 0.005 wt % of boron, at most 0.1 wt % of aluminum, at most 0.1 wt % of phosphorus, at most 0.02 wt % of sulfur and at most 0.05 % of nitrogen at a temperature ranging from 1,100 to 1,280 °C, hot rolling the heated slab, and further, if necessary, cold rolling and annealing.

Description

TECHNICAL FIELD

[0001] This invention proposes high-strength steel sheets having a tensile strength of not less than 40 kgf/mm², which are mainly suitable for use in inner and outer panels of an automobile, as well as a method of producing the same.

[0002] High-strength steel sheets have widely been used from the old time as a steel sheet for use in body constituting members, outer panels and the like of the automobile in order to reduce the vehicle weight. Such high-strength steel sheets for the vehicles are required to have not only a good formability for press forming or the like but also a sufficient strength for ensuring a safeness of the automobile. In addition, it is urgently needed to more raise the strength under such a recent circumstance that total automobile exhaust gas emission regulation becomes considerably severer.

[0003] Furthermore, these steel sheets are frequently subjected to a heat treatment above 900°C or heated to a higher temperature by welding, brazing or the like for removing a work strain after the forming or improving a resistance to cold work embrittlement, so that they are desirable to have a property hardly causing the softening against the high temperature heating.

[0004] Moreover, it is demanded to develop steel sheets capable of being easily subjected to various platings from a viewpoint of rust prevention recently and particularly considered as an important feature.

BACKGROUND ART

[0005] As properties required in the high-tensile steel sheet for automobiles having an excellent formability, there are mentioned high ductility, high r-value, low yield ratio, small plane anisotropy in material and the like.

[0006] In this connection, large-scale formable cold rolled steel sheets having an improved rigidity (high Young's modulus) and a method of producing the same are disclosed in, for example, Japanese Patent laid open No. 57-181361, and a method of producing deep drawable cold rolled steel sheets having slow aging property and small anisotropy is disclosed in Japanese patent laid open No. 58-25436. In these methods, a slight amount of Nb, Ti or the like is added to a ultra-low carbon steel as a base, and continuous annealing conditions are controlled, and P having less material degradation and large solid-solution strengthening property is used as a component for raising tension. However, the tensile strength of such P-added ultra-low C steel is about 40 kgf/mm² at most, so that such a composition system based on the ultra-low C steel and added with the solid-solution strengthening component is clearly difficult to cope with the demand for raising the strength of the steel sheet together with the reduction of body weight of the automobile rapidly advanced in future.

[0007] The plane anisotropy considered to strongly demand in future is disclosed in the above Japanese Patent laid open No. 58-25436, but in this case the tensile strength is a low level of 30 kgf/mm².

[0008] In addition to the above steel sheets based on the ultra-low C steel and added with solid-solution strengthening P, there are transformation strengthening steel sheets (dual-phase steel sheets) and precipitation strengthening steel sheets as a high-tensile steel sheet having a different strengthening mechanism.

[0009] Among them, the transformation strengthening steel sheets are unsuitable for deep drawing because r-value is low though low yield ratio and excellent elongation can easily be obtained.

[0010] On the other hand, the precipitation strengthening steel sheets or so-called HSLA (high strength low alloy) steel sheets are steels added with Si, Mn, Nb and the like to utilize solid-solution strengthening of Si and Mn and the strengthening based on precipitation of carbonitride of Nb and the formation of fine crystal grains accompanied therewith, and are used for not only automobiles but also domestically electric appliance and the like. In this steel sheet, however, the yield ratio is high, so that the use conditions are restricted.

[0011] Then, the precipitation strengthening steel sheet will be described with reference to conventionally known references.

[0012] Japanese Patent Application Publication No. 54-27822 discloses a method of producing precipitation strengthening high-strength cold rolled steel sheets, and Japanese patent Application Publication No. 55-16214 discloses a method of producing deep drawable high-strength cold rolled steel sheets. In these steel sheets, the yield ratio exceeds 70% and a greater part of the sheets exhibit a high value of not less than 80%.

[0013] In Japanese Patent laid open No. 55-152128 is disclosed a method of producing low-yield ratio and high-strength cold rolled steel sheets having an excellent formability through continuous annealing as a method for the production of precipitation strengthening steel sheets, but the deep drawability is not mentioned at all.

[0014] As a low C level Ti-IF (interstitial free) steel, Japanese patent laid open No. 57-35662 discloses super-deep drawable cold rolled steel sheets having an excellent secondary formability, and also Japanese Patent laid open No. 60-92453 discloses brazable cold rolled steel sheets having an excellent drawability. However, the tensile strength described in Examples of the above Japanese Patent laid open No. 57-35662 is less than 40 kgf/mm² and does not arrive at the strength level aimed at the present invention because the tensile strength aimed at the present invention is not less than 40 kgf/mm². Further, in the present invention, Si is an essential component and is restricted to a range of 0.1-1.2 wt%, while the claim of the above Japanese Patent laid open No. 60-92453 does not disclose the use of Si and the Si content is not more than 0.09 wt% as seen from the description of Examples, which is essentially different from the present invention effectively utilizing the Si effect.

DISCLOSURE OF INVENTION

[0015] This invention is to propose high-strength steel sheets which are based on a low C steel having a C content larger than the conventional ultra-low C steel and have a tensile strength of not less than 40 kgf/mm² by attaining IF with Ti and carefully selecting additive components and are low in the yield ratio (less than 70%) as compared with the conventional precipitation strengthening steel and small in the plane anisotropy and hardly cause the softening due to abnormal grain growth by reheating treatment, as well as a method of producing the same.

[0016] The inventors have made various studies and experiments and found that an Si-added low C - high Ti steel composition is rendered into complete IF to provide a high-strength steel sheet having a low yield ratio and a small plane anisotropy, and as a result the invention has been accomplished.

[0017] That is, the essential features of the invention are as follows.

1. A formable high-strength steel sheet consisting essentially of
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt%,
   N: up to 0.005 wt%
and the balance being iron and inevitable impurities (First aspect).

2. A formable high-strength steel sheet, wherein a part of iron as the balance component of the first aspect is replaced with at least one of
   V: from 0.02 wt% to 0.2 wt%,
   Nb: from 0.02 wt% to 0.2 wt% and
   Zr: from 0.02 wt% to 0.2 wt% (Second aspect).

3. A formable high-strength steel sheet, wherein a part of iron as the balance component of the first or second aspect is replaced with at least one of
   Cr: from 0.05 wt% to 1.5 wt%,
   Ni: from 0.05 wt% to 2.0 wt%,
   Mo: from 0.05 wt% to 1.0 wt% and
   Cu: from 0.05 wt% to 1.5 wt% (Third and fourth aspects).

4. A formable high-strength steel sheet, wherein a part of iron as the balance component of the first, second, third or fourth aspect is replaced with Ca: from 0.0005 wt% to 0.005 wt% (Fifth, sixth, seventh and eighth aspects).

5. A method of producing formable high-strength steel sheets, which comprises using a slab of steel containing
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt% and
   N: up to 0.005 wt%
as a starting material, heating the slab at a temperature of not lower than 1100°C but not higher than 1280°C and then hot rolling to obtain a hot rolled steel sheet (Ninth aspect).

6. A method of producing formable high-strength steel sheets, wherein an electroplating or hot-dip plating is conducted followed by the hot rolling described in the ninth aspect (Tenth aspect).

7. A method of producing formable high-strength steel sheets, which comprises using a slab of steel containing
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt% and
   N: up to 0.005 wt%
as a starting material, heating the slab at a temperature of not lower than 1100°C but not higher than 1280°C, hot rolling, cold rolling and then annealing at a temperature above recrystallization temperature (Eleventh aspect).

8. A method of producing formable high-strength steel sheets, wherein an electroplating or hot-dip plating is conducted followed by the annealing described in the eleventh aspect (Twelfth aspect).

[0018] The invention will be described below with respect to experimental results based on the invention.

[0019] Twelve cold rolled steel sheets of 0.70 mm in thickness having a chemical composition of C: 0.05 wt%, Mn: 0.5 wt%, Ti: 0.2 wt%, B: 0.0005 wt%, Al: 0.05 wt%, P: 0.01 wt%, S: 0.001 wt%, N: 0.0015 wt% and Si: variable within a range of 0-2.60 wt% are subjected to a box annealing at 700°C and then tensile properties are measured.

[0020] In Fig. 1 is shown a relation between tensile property values and Si content from the above measured results.

[0021] As seen from Fig. 1, when the Si content is within a range of 0.1-1.2 wt%, low YR - high El - high mean r-value are obtained. This effect of Si is considered owing to the action of Si purifying ferrite.

[0022] Then, a relation between C and Ti is examined with respect to steel sheets having a formability and hardly causing the softening at a high temperature.

[0023] Thirty two steel materials having a chemical composition of Si: 0.5 wt%, Mn: 0.3 wt%, B: 0.0012 wt%, Al: 0.04 wt%, P: 0.05 wt%, S: 0.010 wt% and C and Ti: variable are heated to 1200°C, hot rolled at a finish rolling temperature of 900°C and coiled at a temperature of 550°C to obtain hot rolled sheets of 3.00 mm in thickness. Further, a part of the hot rolled sheet is subjected to a descaling treatment, cold rolled at a rolling reduction of 75%, held at 800°C for 40 seconds, continuously annealed under a condition of cooling at a rate of 20°C/sec (no overaging), and subjected to a temper rolling at an elongation of 0.8% to obtain a cold rolled sheet of 0.75 mm in thickness.

[0024] After the thus obtained hot rolled sheets and cold rolled sheets are held at 1000°C for 1 hour and cooled at a rate of 5°C/sec, the crystal grain size of these sheets are measured. The measured results are shown in Figs. 2(a) and (b).

[0025] Figs. 2(a), (b) show a relation between C wt% and ratio of

exerting on the crystal grain size, from which it is understood that the crystal grain size number in both the hot rolled sheet and cold rolled sheet becomes larger when the ratio of effective *Ti wt%/C wt% is not less than 4, so that the ratio of the above value is sufficient as an effective *Ti amount for fixing C.

[0026] Even after the heat treatment at 1000°C, the coarsening of crystal grains is not caused at the C content of not less than 0.01 wt% and the ratio of effective *Ti wt%/C wt% of not less than 4, and the crystal grain size number is 7 or more.

[0027] Moreover, when the crystal grain size number is 7 or more, the crystal grains after the heating do not cause the softening.

[0028] From the above results, the C content of not less than 0.01 wt% and the ratio of effective *Ti wt%/C wt% of not less than 4 are required for preventing the abnormal grain growth (prevention of softening) in the reheating. This is guessed due to the fact that the resulting fine carbides of Ti are relatively stably existent even in the reheating and serve to suppress the abnormal grain growth.

[0029] Moreover, it has been confirmed from detailed experimental results that Si content largely affects the plane anisotropy and r-value.

[0030] Figs. 3(a), (b), (c) and (d) show pole figures measured on four steel sheets after a cold rolled steel sheet containing C: 0.05 wt%, Si: 0 wt%, 1.0 wt%, 1.5 wt% or 2.0 wt%, Mn: 0.01 wt%, Ti: 0.206 wt%, B: 0.0008 wt%, Al: 0.04 wt%, P: 0.01 wt%, S: 0.001 wt% and N: 0.0014 wt% is subjected to a box annealing at 720°C, in which each of Figs. 3(a), (b), (c) and (d) corresponds to each Si content of 0 wt%, 1.0 wt%, 1.5 wt% and 2.0 wt%. As seen from these figures, when the Si content is 1.0 wt% (b), a {111}<112> texture is strong and also a development of <100>//ND orientation becomes weak. This makes the plane anisotropy small and raises the r-value, from which the Si content is preferable to be around 1 wt%.

[0031] The reason why the chemical composition of the steel according to the invention is limited to the above ranges will be described below. C: 0.01-0.1 wt%

[0032] When the C content is less than 0.01 wt%, the target tensile strength of not less than 40 kgf/mm² is not obtained and also the sheet is apt to be softened at a high temperature. On the other hand, when it is not less than 0.1 wt%, if the sheet is produced by the continuous annealing method, the growth of crystal grains in the annealing rapidly decreases and the given ductility can not be obtained. Therefore, the carbon content is not less than 0.01 wt% but less than 1.0 wt%.
Si: 0.1-1.2 wt%
Si is an important element in the invention and has an effect of discharging C from ferrite and promoting precipitation of Ti carbide and agglomeration coarsening.

[0033] When the amount is less than 0.1 wt%, the above effect is not obtained, while when it exceeds 1.2 wt%, the ductility rapidly degrades owing to the solid-solution strengthening action of Si itself and also the r-value and various plating properties are degraded. Therefore, the Si content is limited to a range of 0.1 wt% to 1.2 wt%, but is it preferable within a range of 0.4 wt% to 1.0 wt% from a viewpoint of the improvement of plane anisotropy and r-value.
Mn: up to 3.0 wt%
   Mn is useful as a strengthening element for steel. However, when the amount exceeds 3.0 wt%, the sheet is too hardened and the ductility is considerably degraded. Therefore, the content is up to 3.0 wt%. Ti: ratio of effective *Ti (wt%)/C (wt%) of 4-12
   Ti is an important element in the invention and is required to fix C, S and N. When the effective *Ti is less than 4xC, C can not completely be fixed and the crystal grains are coarsen by reheating to cause the softening as previously mentioned. On the other hand, when the effective *Ti exceeds 12xC, Ti excessively solutes to degrade the properties and damage the surface quality of the steel sheet. Therefore, the content is within a range satisfying the ratio of effective *Ti/C of 4 to 12.





B: 0.0005-0.005 wt%
   B is required to improve the resistance to cold work embrittlement. When the amount is less than 0.0005 wt%, the effect is insufficient, while when it exceeds 0.005 wt%, the degradation of deep drawability becomes conspicuous. Therefore, the content is within a range of 0.0005 - 0.005 wt%.
Al: up to 0.1 wt%
   Al is an element useful for fixing O in steel to avoid the decrease of effective *Ti amount. However, when the amount exceeds 0.1 wt%, the effect is saturated. Therefore, the content is up to 0.1 wt%.
P: up to 0.1 wt%
   P is a very excellent solid-solution strengthening element. However, when the amount exceeds 0.1 wt%, the surface quality of the steel sheet is considerably degraded. Therefore, the content is up to 0.1 wt%. Moreover, it is preferable that P (wt%)/C (wt%) as a relation to C content is less than 1.5.
S: up to 0.02 wt%
   S results in the occurrence of cracking in the hot rolling, so that the content is up to 0.02 wt%.
N: up to 0.005 wt%
   As N amount becomes large, the effective *Ti amount decreases and the degradation of r-value and ductility is caused. Therefore, the content is favorable to be less, but its acceptable upper limit is 0.005 wt%.
   In addition to the above chemical composition according to the invention, at least one of V, Nb and Zr as a carbide forming element may be added. The effect is developed when each content is not less than 0.02 wt%, but when it exceeds 0.2 wt%, the degradation of ductility is caused. Therefore, the content of each of V, Nb and Zr is within a range of 0.02 wt% to 0.2 wt%. Similarly, at least one of Cr, Ni, Mo and Cu as a solid-solution strengthening element may be added. The effect is developed when each content is not less than 0.05 wt%, but when it is too large, the degradation of surface quality of the steel sheet is caused. Therefore, the content is limited to 0.05 wt% to 1.5 wt% in Cr, 0.05 wt% to 2.0 wt% in Ni, 0.05 wt% to 1.0 wt% in Mo and 0.05 wt% to 1.5 wt% in Cu.

[0034] Furthermore, Ca may be added for controlling the shape of inclusion. The effect is developed when the content is not less than 0.0005 wt%, but when it exceeds 0.005 wt%, the effect is saturated and also the degradation of properties becomes conspicuous. Therefore, the content is within a range of 0.0005 wt% to 0.005 wt%.

[0035] According to the invention, the reason why low yield ratio is obtained though the strength is raised by using low C steel having a C content larger than ultra-low C steel is considered as follows.

[0036] That is, C, S and N are completely fixed to attain complete IF when the effective *Ti/C is not less than 4. As a result, it is considered that the dislocation anchoring effect is decreased to increase movable dislocation, whereby the low yield ratio is obtained.

[0037] Then, the invention will be described with respect to production step conditions.

[0038] At first, the steel-making is sufficient to be carried out according to the usual manner, so that the restriction of conditions is not particularly required.

[0039] When the slab heating temperature is lower than 1100°C, the formability is poor, while when it exceeds 1280°C, coarse grains appear to cause the scattering of properties at subsequent step. Therefore, the slab reheating temperature is within a range of not lower than 1100°C but not higher than 1280°C. Moreover, the continuously cast slab may be subjected to rough rolling immediately or after the temperature holding treatment of not lower than 1100°C but not higher than 1280°C without reheating or lowering temperature after continuous casting to lower than 1100°C from a viewpoint of energy-saving.

[0040] As to the finish temperature at hot rolling, when it is too high, the final structure becomes coarse, which is unfavorable for the ductility. On the other hand, when it is too low, the stretching of the structure becomes conspicuous and the rolling load rapidly increases, which are unfavorable in the operation. Therefore, the finish temperature is preferable within a range of above Ar₃ transformation temperature but not higher than Ar₃ transformation temperature + 100°C.

[0041] The coiling temperature after the hot rolling is sufficient within a range of from 400°C to 700°C from a view point of subsequent pickling and capacity of coiling machine.

[0042] In the cold rolling, the rolling reduction is preferable to be not less than 55% for providing a sufficient formability after the annealing.

[0043] The annealing after the cold rolling is required to have a temperature above recrsytallization temperature for the recrsytallization of the structure. However, it is preferable to be lower than A_c3 transformation temperature for avoiding the formation of composite texture after the annealing. The annealing process is not particularly restricted, but may be a continuous annealing process or a box annealing process.

[0044] As to the plating conditions, in case of electroplating, it is sufficient that the plating is conducted at a given amount to each of the hot rolled sheet and cold rolled sheet in the usual manner. In case of hot-dip galvanization, the continuous hot-dip galvanization line may be applied to the annealing step in addition to a single hot-dip galvanization line.

[0045] Furthermore, these steel sheets may be subjected to a temper rolling at a rolling reduction (%) equal to a sheet thickness (mm) within common sense for correction of sheet shape or the like.

[0046] Moreover, the steel sheet according to the invention after the annealing or plating may be subjected to a special treatment to improve phosphatability, weldability, press formability, corrosion resistance and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] 

Fig. 1 is a graph showing a relation of Si content to tensile properties;

Fig. 2(a) is a graph showing a relation between C content and *Ti/C (weight ratio) exerting on crystal grain size number after the hot rolled sheet is reheated at 1000°C;

Fig. 2(b) is a graph showing a relation between C content and *Ti/C (weight ratio) exerting on crystal grain size number after the cold rolled sheet is reheated at 1000°C;

Fig. 3(a) is a (200) pole figure of a steel sheet containing Si content of 0 wt%;

Fig. 3(b) is a (200) pole figure of a steel sheet containing Si content of 1 wt%;

Fig. 3(c) is a (200) pole figure of a steel sheet containing Si content of 1.5 wt%; and

Fig. 3(d) is a (200) pole figure of a steel sheet containing Si content of 2.0 wt%.

BEST MODE FOR CARRYING OUT THE INVENTION

EXAMPLE

[0048] Each of 31 continuously cast slabs in total of 26 acceptable steels and 5 comparative steels each melted in a converter and having a chemical composition as shown in Tables 1 and 2 is hot rolled to a thickness of 3.2 mm in case of steel symbols O, P, Q and R or 2.8 mm in case of all other steels. Furthermore, a part of the steels is subjected to a hot-dip galvanization.

[0049] The mechanical properties, aging index AI and crystal grain size number after the heat treatment (reheating) are measured with respect to the thus obtained steel sheets.

[0050] The above hot rolling conditions and measured results are shown in Tables 3 and 4.

[0051] Further, a part of the above hot rolled sheets (in which the slab re-heating temperature is within the range defined in the invention) is descaled, cold rolled at a rolling reduction of 75% to a thickness of 0.80 mm or 0.70 mm, subjected to a continuous annealing or a box annealing and then subjected to a temper rolling at a rolling reduction of 0.80% or 0.70%. Moreover, a part of thus treated sheets is subjected to an electroplating or a hot-dip galvanization.

[0052] The mean r-value, mechanical properties including Δr as an indication of plane anisotropy, aging index AI and crystal grain size number after heat treatment are measured with respect to the thus obtained steel sheets.

[0053] The annealing conditions and measured results are shown in Tables 5 and 6.

[0054] In this case, each treating condition is as follows.

[0055] In the electroplating, Zn-Ni plating is carried out at a plating amount of 30 g/m².

[0056] In the hot-dip galvanization, Zn plating or Al plating is conducted, in which the Zn plating is carried out at a plating amount of 45 g/m² under conditions of bath temperature: 475°C, passing sheet temperature: 475°C, immersion time: 3 seconds and alloying temperature: 485°C, while the Al plating is carried out at a plating amount of 30 g/m² under conditions of bath temperature: 650°C, passing sheet temperature: 650°C and immersion time: 3 seconds.

[0057] The heat treatment (reheating) is conducted under such a condition that the sheet is heated to 950°C, held at this temperature for 30 minutes and slowly cooled at a rate of 5°C/sec.

[0058] As the testing conditions, JIS No. 5 specimen is used in the tensile test, while YS, TS and El are measured in the rolling direction.

[0059] The r-value is determined by measuring widths at three points of a center and both positions separated from the center by 12.5 mm in the longitudinal direction of the specimen under 15% strain, and the mean r-value and Δr are calculated according to the following equations, respectively.








   Moreover, r₀, r₉₀ and r₄₅ are r-values in the rolling direction (r₀), a direction of 45° with respect to the rolling direction (r₄₅) and a direction of 90° with respect to the rolling direction (r₉₀), respectively.

[0060] The AI value is determined from a difference of deformation stress before and after the aging when the sheet is previously tensioned under a strain of 7.5% and then subjected to an aging treatment at 100°C for 30 minutes.

[0061] As seen from Tables 3, 4, 5 and 6, in the acceptable examples according to the invention, the tensile strength of not less than 40 kgf/mm² is obtained even when the annealing process is box annealing or continuous annealing, and low yield ratio (not more than 70%) and high El and crystal grain size number after heat treatment of not less than 7 hardly causing the softening by reheating are exhibited. Furthermore, the cold rolled steel sheets exhibit excellent properties that the high mean r-value is shown, and also Δr as an indication of plane anisotropy is small, and the aging index AI is not more than 1 kgf/mm² to ensure complete non-aging property.

INDUSTRIAL APPLICABILITY

[0062] According to the invention, even in the low C steel level having the C content larger than the extreme-low C steel, the high-strength steel sheets having a small plane anisotropy, a low yield ratio and a complete non-aging property and hardly causing the softening by high temperature heating are obtained by completely fixing solid soluted C, S, N and the like, and further high-strength precipitation strengthening steels having a higher r-value are obtained as a cold rolled steel sheet. Therefore, the invention serves to enlarge the application of the precipitation strengthening steel sheet for automobile and the like from its utility.

Claims

1. A formable high-strength steel sheet consisting essentially of
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt%,
   N: up to 0.005 wt%
and the balance being iron and inevitable impurities.

2. A formable high-strength steel sheet consisting essentially of
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt%,
   N: up to 0.005 wt%,and
at least one of
   V: from 0.02 wt% to 0.2 wt%,
   Nb: from 0.02 wt% to 0.2 wt% and
   Zr: from 0.02 wt% to 0.2 wt%,
and the balance being iron and inevitable impurities.

3. A formable high-strength steel sheet consisting essentially of
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt%,
   N: up to 0.005 wt%, and
at least one of
   Cr: from 0.05 wt% to 1.5 wt%,
   Ni: from 0.05 wt% to 2.0 wt%,
   Mo: from 0.05 wt% to 1.0 wt% and
   Cu: from 0.05 wt% to 1.5 wt%,
and the balance being iron and inevitable impurities.

4. A formable high-strength steel sheet consisting essentially of
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt%,
   N: up to 0.005 wt%, and
at least one of
   V: from 0.02 wt% to 0.2 wt%,
   Nb: from 0.02 wt% to 0.2 wt% and
   Zr: from 0.02 wt% to 0.2 wt%, and
at least one of
   Cr: from 0.05 wt% to 1.5 wt%,
   Ni: from 0.05 wt% to 2.0 wt%,
   Mo: from 0.05 wt% to 1.0 wt% and
   Cu: from 0.05 wt% to 1.5 wt%,
and the balance being iron and inevitable impurities.

5. A formable high-strength steel sheet consisting essentially of
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Ca: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt%,
   N: up to 0.005 wt%
and the balance being iron and inevitable impurities.

6. A formable high-strength steel sheet consisting essentially of
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Ca: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt%,
   N: up to 0.005 wt%,and
at least one of
   V: from 0.02 wt% to 0.2 wt%,
   Nb: from 0.02 wt% to 0.2 wt% and
   Zr: from 0.02 wt% to 0.2 wt%,
and the balance being iron and inevitable impurities.

7. A formable high-strength steel sheet consisting essentially of
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective ^*Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Ca: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt%,
   N: up to 0.005 wt%, and
at least one of
   Cr: from 0.05 wt% to 1.5 wt%,
   Ni: from 0.05 wt% to 2.0 wt%,
   Mo: from 0.05 wt% to 1.0 wt% and
   Cu: from 0.05 wt% to 1.5 wt%,
and the balance being iron and inevitable impurities.

8. A formable high-strength steel sheet consisting essentially of
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Ca: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt%,
   N: up to 0.005 wt%, and
   at least one of
   V: from 0.02 wt% to 0.2 wt%,
   Nb: from 0.02 wt% to 0.2 wt% and
   Zr: from 0.02 wt% to 0.2 wt%, and
at least one of
   Cr: from 0.05 wt% to 1.5 wt%,
   Ni: from 0.05 wt% to 2.0 wt%,
   Mo: from 0.05 wt% to 1.0 wt% and
   Cu: from 0.05 wt% to 1.5 wt%,
and the balance being iron and inevitable impurities.

9. A method of producing formable high-strength steel sheets, which comprises using a slab of steel containing
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt% and
   N: up to 0.005 wt%
as a starting material, heating the slab at a temperature of not lower than 1100°C but not higher than 1280°C and then hot rolling to obtain a hot rolled steel sheet.

10. A method of producing formable high-strength steel sheets, wherein an electroplating or hot-dip plating is conducted followed by the hot rolling described in claim 9.

11. A method of producing formable high-strength steel sheets, which comprises using a slab of steel containing
   C: from 0.01 wt% to less than 0.1 wt%,
   Si: from 0.1 wt% to 1.2 wt%,
   Mn: up to 3.0 wt%,
   Ti: a ratio of effective *Ti (wt%) represented by the following equation to the above C (wt%), i.e.








   B: from 0.0005 wt% to 0.005 wt%,
   Al: up to 0.1 wt%,
   P: up to 0.1 wt%,
   S: up to 0.02 wt% and
   N: up to 0.005 wt%
as a starting material, heating the slab at a temperature of not lower than 1100°C but not higher than 1280°C, hot rolling, cold rolling and then annealing at a temperature above recrystallization temperature.

12. A method of producing formable high-strength steel sheets, wherein an electroplating or hot-dip plating is conducted followed by the annealing described in claim 11.

Drawing