JP5935844B2 - Cold-rolled steel sheet with excellent delayed fracture resistance of laser welds and method for producing the same - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet having a thickness of 0.4 mm or more and 3.0 mm or less that is suitable for use in automobiles, electric machines, etc., and in particular, excellent delayed fracture resistance of a laser welded part having a tensile strength of 980 MPa or more. The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof.

  In recent years, improvement in fuel efficiency of automobiles has become important from the viewpoint of global environmental protection, and weight reduction of the vehicle body has been promoted. For this, the most effective means is to increase the strength of the steel sheet used and to reduce the thickness. In addition, occupant safety improvement technology is also an important issue. Against this, increasing the strength of the steel sheet used is an effective measure. For the purpose of increasing the strength of such steel sheets, conventionally, hot rolling and subsequent continuous annealing conditions have been strictly controlled, and various alloy elements such as C and Mn have been added to the steel sheets. .

  On the other hand, when a cold-rolled steel sheet is used as a member for an automobile, a method is generally employed in which the steel sheets are joined together by welding and finished into a desired shape after forming. For this reason, in order to ensure excellent safety as the vehicle body structure, excellent mechanical characteristics are required not only for the cold-rolled steel sheet base material but also for the region including the weld metal and the weld heat affected zone.

  Generally, if the steel plate is made stronger and the plate thickness is made thinner, there is a problem that the rigidity of the automobile structure decreases, but as a means of overcoming this, instead of general spot welding (spot welding) Applying continuous welding. For example, the laser welding method has high welding accuracy and can obtain deep penetration with a small heat input, so that it is possible to minimize the heat effect on the base material. Moreover, since remote control by fiber transmission is easy, the application is expanding as an automatic continuous welding method of the member for motor vehicles. Particularly in recent years, since the load of press molding of automobile members can be reduced, the use of tailored blank technology for press molding after joining different steel types by laser welding in advance is increasing.

  However, in general, the laser welding method is characterized by melting and solidifying the steel sheet by rapid heating and quenching, so that the microstructure of the weld metal and the weld heat affected zone becomes a martensite structure, and the hardness is higher than that of the base metal. Elevated and easily brittle. Further, even when compared with the conventional spot welding method, the hardening of the weld metal and the weld heat-affected zone is remarkable.

Furthermore, in laser welding, unlike the conventional spot welding method, the welded portion of the steel sheet surface and back surface is exposed to the atmospheric environment during welding, so depending on the welding environment such as humidity and temperature, and the steel sheet surface properties before welding. Is characterized in that hydrogen is easily mixed into the weld metal during welding and the subsequent cooling is extremely fast, so that hydrogen is not easily released from the weld metal to the environment again. Therefore, the amount of hydrogen in the welded part by laser welding tends to be higher than the amount of hydrogen in the welded part by conventional spot welding.
In addition, hydrogen mixed in the weld metal diffuses into the weld heat affected zone in the vicinity of the weld metal with time, and promotes embrittlement of the weld heat affected zone.

  As described above, when a forming process such as a tailored blank is applied to a steel plate including a weld metal and a weld heat-affected zone, which is hardened and mixed with hydrogen, the weld metal and the vicinity thereof are formed by hydrogen embrittlement. There is a problem that a brittle fracture called a delayed fracture starting from the weld heat affected zone is likely to occur.

  In this regard, various high-strength steel sheets have been proposed in Patent Documents 1 to 5, but with sufficient economic efficiency and productivity, high tensile strength: 980 MPa or more and delayed fracture resistance of laser welds. At present, it has not been possible to achieve both sufficient improvement in characteristics.

JP 2011-111671 A JP 2011-111675 A Japanese Patent No. 3854506 Japanese Patent No. 3895986 Japanese Patent No. 4091894

  The present invention has been developed in view of the above-described present situation, and has excellent delayed fracture resistance of a laser welded portion having a tensile strength of 980 MPa or more without causing an increase in manufacturing cost or a decrease in productivity. An object is to provide a cold-rolled steel sheet together with its advantageous production method.

Now, in order to achieve the above-mentioned problems, the inventors have conducted intensive research on various factors that determine the chemical composition, manufacturing method and microstructure of the steel sheet, and obtained the following knowledge.
(1) Tensile strength: In order to achieve 980 MPa or more, it is important to strictly adjust the chemical composition of the steel sheet and to properly control the mass% ratio (Ti / N) of Ti and N.
This is because by properly controlling Ti / N, grain refinement strengthening and precipitation strengthening due to the formation of TiN are manifested, and it is possible to secure Nb in a solid solution state during the annealing process. This is because the effect of delaying the progress of recrystallization during heating, which is manifested thereby, contributes to increasing the strength of the steel sheet.

(2) To achieve excellent delayed fracture resistance of laser welds, it is important to suppress excessive hardening and embrittlement of the weld metal and weld heat affected zone in laser welding. For that purpose, in the weld metal and the weld heat affected zone, in addition to reducing the solid solution N as much as possible and making the crystal grains finer, the solid solution B in the weld metal and the weld heat affected zone is reduced as much as possible. It becomes important.
Here, in order to reduce the solute B in the weld metal and the weld heat affected zone, it is effective that the steel sheet does not substantially contain B. If B is present in the steel sheet, it dissolves at the time of heating during welding, and the hardenability increases in the subsequent cooling process, leading to an excessive increase in hardness and embrittlement in the weld metal and weld heat affected zone.
Moreover, when Nb exists in a solid solution state in steel, NbC can be formed in a low temperature region during the cooling process during welding, and softening of the weld heat affected zone can be prevented. Further, when Nb is present as NbC in the steel, this NbC acts as a trapping site for diffusible hydrogen, and delayed fracture due to hydrogen can be suppressed.

(3) In order to effectively exhibit the effects as described above, it is necessary to appropriately control the presence of Ti and Nb in the cold-rolled steel sheet after annealing.
In addition, in order to obtain the desired state of Ti and Nb, it is necessary to precisely control the manufacturing conditions, particularly hot rolling conditions and annealing conditions, after strictly adjusting the component composition and Ti / N of the steel sheet. is important.
The present invention was completed after further studies based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.05 to 0.15%,
Si: 0.05-2.0%,
Mn: 1.2 to 3.5%
P: 0.05% or less,
S: 0.005% or less,
Al: 0.01 to 0.1%,
Nb: 0.010 to 0.070%,
Ti: 0.005-0.040% and N: 0.0005-0.0065%
In addition, the mass ratio of Ti and N: Ti / N is 2.5 or more and 7.5 or less, further mixing of B is suppressed to 0.0003% or less, and the balance is made of Fe and inevitable impurities. Having a steel composition,
While 70% or more of Ti in steel exists as precipitates, 15% or more and 65% or less of Nb in steel exists as solute Nb,
A cold-rolled steel sheet excellent in delayed fracture resistance of a laser weld, wherein the tensile strength is 980 MPa or more.

2. The steel sheet is further mass%,
Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
Cu: 1.0% or less,
1. Cold rolling excellent in delayed fracture resistance of the laser welded portion according to 1 above, containing one or more selected from Ni: 1.0% or less and V: 0.1% or less steel sheet.

3. When the steel material having the steel composition described in 1 or 2 is set to a temperature represented by the following formula (1), Ts is heated to a temperature range of (Ts-50) ° C. or higher and (Ts + 200) ° C. or lower to finish. Rolling end temperature: after hot rolling at 850 ° C. or higher, after winding at a temperature of 650 ° C. or lower, then after cold rolling, heated to a temperature range of 700 ° C. or higher and 900 ° C. or lower, In the cooling process, by cooling to a temperature range of 200 ° C. or more and 450 ° C. or less at an average cooling rate of 5 ° C./s or more and 50 ° C./s or less, and performing continuous annealing for 30 hours or more and 600s or less in the temperature range ,
While 70% or more of Ti in steel is present as precipitates, 15% to 65% of Nb in steel is present as solute Nb, and a cold-rolled steel sheet having a tensile strength of 980 MPa or more. A method for producing a cold-rolled steel sheet excellent in delayed fracture resistance of a laser weld.
Ts (° C.) = 6770 / [2.26-log ₁₀ {[% Nb] ×
([% C] +0.86 [% N])}]-273 (1)
Here, [% Nb], [% C] and [% N] indicate the contents (mass%) of Nb, C and N in the steel, respectively.

ADVANTAGE OF THE INVENTION According to this invention, the cold rolled steel plate excellent in the delayed fracture resistance of the laser weld part of tensile strength: 980 MPa or more can be obtained, without causing the increase in manufacturing cost or the fall of productivity.
In addition, by using the cold-rolled steel sheet of the present invention, it is possible to improve the production efficiency when manufacturing a steel structure such as an automobile and the safety for passengers of the automobile, and further contribute greatly to the reduction of the environmental load accompanying the improvement of fuel consumption. Can do.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel material is limited to the above range in the present invention will be described. In addition, although the unit of element content in the component composition of steel materials is “mass%”, hereinafter, unless otherwise specified, it is simply indicated by “%”.
C: 0.05 to 0.15%
C is the most important element for strengthening steel and has a high solid solution strengthening ability. In order to acquire such an effect, 0.05% or more of C is required. On the other hand, if the amount of C exceeds 0.15%, the martensite phase in the base material is increased and markedly hardened, and the hole expandability deteriorates. Further, when laser welding is performed, the weld metal and the weld heat affected zone are significantly hardened, and the delayed fracture resistance of the laser weld is deteriorated. For this reason, C amount is limited to 0.05 to 0.15% of range. Preferably it is 0.06 to 0.13% of range.

Si: 0.05-2.0%
Si is a necessary element in steelmaking that acts as a deoxidizing material. Moreover, Si has the effect of increasing the strength of a steel sheet by solid solution in steel and solid solution strengthening. In order to obtain such an effect, it is necessary to contain 0.05% or more of Si. On the other hand, if the amount of Si exceeds 2.0%, the weld metal and the weld heat affected zone become brittle, and the delayed fracture resistance deteriorates. For this reason, Si amount is limited to 0.05 to 2.0% of range. Preferably it is 0.10 to 1.60% of range.

Mn: 1.2 to 3.5%
Mn has the effect of increasing the hardenability of steel at a relatively low cost, and in order to ensure the base material strength of tensile strength: 980 MPa or more, the Mn content needs to be 1.2% or more. . On the other hand, if the amount of Mn exceeds 3.5%, the microsegregation of the base material becomes large. When laser welding is performed on this segregated portion, an excessive increase in hardness is caused and the occurrence of delayed fracture is promoted. For this reason, the amount of Mn is limited to the range of 1.2 to 3.5%. Preferably it is 1.3 to 3.3% of range.

P: 0.05% or less P is an element having a large solid solution strengthening ability, but promotes microsegregation together with Mn. For this reason, if the amount of P exceeds 0.05%, not only the base material becomes brittle, but also the grain boundary segregation part tends to be the starting point of delayed fracture. Therefore, it is desirable to reduce P as much as possible with 0.05% as the upper limit. However, excessive P reduction raises the refining cost and is economically disadvantageous, so the lower limit of P is preferably about 0.005%.

S: 0.005% or less S is segregated at the grain boundary to lower the ductility during hot rolling, so it is desirable to reduce 0.005% as much as possible.

Al: 0.01 to 0.1%
Al acts as a deoxidizer and is the most widely used element in the molten steel deoxidation process for steel sheets. Moreover, it has the effect which suppresses the embrittlement by solid solution N by fixing solid solution N in steel and forming AlN. In order to obtain such an effect, it is necessary to contain 0.01% or more of Al. On the other hand, if the Al content exceeds 0.1%, surface cracks during slab production are promoted. For this reason, the amount of Al is limited to the range of 0.01 to 0.1%. Preferably it is 0.02 to 0.07% of range.

Nb: 0.010 to 0.070%
Nb is present as a solid solution Nb in the annealing heating after cold rolling, thereby producing a solution drag effect, and delaying the recrystallization of the processed structure generated by cold rolling, thereby allowing the steel plate after annealing to It is an important element for increasing strength. NbC produced in the hot rolling and annealing processes not only improves the toughness by refining the microstructure of the base metal, weld metal and weld heat affected zone, but also traps diffusible hydrogen trapped during welding. It contributes to the improvement of the delayed fracture resistance. In order to obtain such an effect, it is necessary to contain 0.010% or more of Nb. On the other hand, when the Nb content exceeds 0.070%, coarse carbonitride precipitates, which may promote surface cracking during slab production and may be a starting point for fracture. For this reason, the amount of Nb is limited to a range of 0.010 to 0.070%. Preferably it is 0.015 to 0.060% of range.

Ti: 0.005-0.040%
Ti is an important alloying element in the present invention, and has the effect of suppressing the coarsening of crystal grains in the base metal, the weld metal and the weld heat affected zone by fixing solid solution N to form TiN. , It has the effect of suppressing embrittlement by reducing the solid solution N. In addition, the formation of TiN effectively contributes to securing a predetermined amount of solid solution Nb through suppressing the formation of Nb nitride in the hot rolling and annealing processes, and increasing the strength of the steel sheet after annealing. In order to obtain such an effect, it is necessary to contain 0.005% or more of Ti. On the other hand, if the amount of Ti exceeds 0.040%, very hard and brittle TiC precipitates and promotes embrittlement. For this reason, Ti amount is limited to 0.005 to 0.040% of range. Preferably it is 0.010-0.035%.

N: 0.0005 to 0.0065%
N is contained in steel as an unavoidable impurity, but by adding an appropriate amount of Ti, TiN is formed, and the effect of suppressing the coarsening of crystal grains in the weld metal and weld heat affected zone during welding is expressed. To do. In order to obtain such an effect, the N content needs to be 0.0005% or more. On the other hand, when the N content exceeds 0.0065%, the aging resistance is remarkably lowered due to an increase in the solid solution N. For this reason, N amount is limited to 0.0005 to 0.0065% of range. Preferably it is 0.0010 to 0.0060%.

In the present invention, it is important to control the mass ratio of Ti and N: Ti / N appropriately with the above-described component composition.
Ti / N: 2.5 or more and 7.5 or less By controlling Ti / N within the above range, grain refinement strengthening and precipitation strengthening due to the formation of TiN appear. In addition, an appropriate amount of solute Nb can be ensured in the annealing process through the suppression of Nb nitride formation, and the effect of delaying the recrystallization progress during heating, which increases the strength of the steel sheet. Contribute to. Moreover, in the weld metal and weld heat affected zone by laser welding, it contributes to the reduction of solid solution N and the refinement of crystal grains, and prevents the weld metal and weld heat affected zone from becoming brittle.
Here, when Ti / N is less than 2.5, the solid solution N in the steel sheet increases and promotes embrittlement. On the other hand, when Ti / N exceeds 7.5, very hard and brittle TiC is generated in the steel sheet, and the ductility is lowered and the embrittlement becomes remarkable. For this reason, Ti / N is limited to the range of 2.5-7.5. Preferably it is the range of 3.0-7.0.

  Furthermore, in the cold-rolled steel sheet of the present invention, it is indispensable to suppress B contamination to 0.0003% or less. This is because if more than 0.0003% B is mixed in the steel, solid solution B is generated in the weld metal and weld heat affected zone by laser welding, causing excessive hardening and promoting embrittlement. This is because the destructive characteristics are deteriorated. For this reason, in the cold-rolled steel sheet of the present invention, the B content is suppressed to 0.0003% or less, preferably 0.0002% or less.

As mentioned above, although the basic component was demonstrated, in this invention, the 1 type (s) or 2 or more types selected from Cr, Mo, Cu, Ni, and V can be contained as needed.
Cr: 0.01 to 1.0%
Cr is an element that contributes to improving the strength of steel. In order to obtain such an effect, it is necessary to add 0.01% or more of Cr. On the other hand, if the amount of Cr exceeds 1.0%, not only does it promote embrittlement due to an excessive increase in strength, but it becomes economically disadvantageous. For this reason, the Cr content is in the range of 0.01 to 1.0%. Preferably it is 0.03 to 0.8% of range.

Mo: 0.01 to 1.0%
Mo is an element that contributes to improving the strength of steel. In order to obtain such an effect, it is necessary to add 0.01% or more of Mo. On the other hand, if the amount of Mo exceeds 1.0%, not only does it promote embrittlement due to an excessive increase in strength, but it becomes economically disadvantageous. For this reason, Mo amount is taken as 0.01 to 1.0% of range. Preferably it is 0.03 to 0.8% of range.

Cu: 1.0% or less Cu is an element that contributes to improving the strength of steel. However, if the amount of Cu exceeds 1.0%, hot brittleness is caused and the surface properties of the steel sheet are deteriorated. For this reason, the amount of Cu shall be 1.0% or less.

Ni: 1.0% or less Ni is an element that contributes to improving the strength of steel. However, if the amount of Ni exceeds 1.0%, the effect is saturated and economically disadvantageous. Therefore, the Ni content is 1.0% or less.

V: 0.1% or less V is an element that contributes to improving the strength of steel. However, if the amount of V exceeds 0.1%, the base metal ductility is deteriorated. For this reason, the amount of V is made into 0.1% or less.

  Among the component compositions in the steel sheet of the present invention, components other than those described above are Fe and inevitable impurities.

As mentioned above, although the component composition in the steel plate of this invention was demonstrated, in this invention, it is very important to control the presence form in steel of Ti and Nb appropriately.
Ratio of Ti present as precipitates in steel: 70% or more In the annealing process, the structure is refined by Ti precipitates, and the hole expandability of the finally obtained cold rolled steel sheet is improved. Moreover, when Ti exists as a precipitate in the cold-rolled steel sheet after annealing, coarsening of the crystal grains in the weld heat affected zone due to the welding heat history during welding is suppressed, and the fracture strength of the weld zone is improved. In order to obtain such an effect, 70% or more of Ti in the steel needs to be present as precipitates. Preferably it is 75% or more. Further, the upper limit of the ratio of Ti present as precipitates in the steel is not particularly specified, but when it becomes 100%, the toughness is greatly deteriorated due to the remaining solid solution N, so it may be made less than 100%. Preferably, it is less than 98%.
The form of the precipitate is mainly TiN alone or a composite precipitate of TiN and other precipitates, but Ti oxide or Ti carbide is less than 10% of the total number of Ti-based precipitates. If there is, the effect is negligible even if mixed.

Ratio of Nb present as solid solution Nb in steel: 15% or more and 65% or less When Nb is present in a solid solution state, it is effective in increasing the strength of steel in the annealing process due to the effect of suppressing recrystallization during heating. While contributing, it has the effect of suppressing softening of the weld heat affected zone.
In order to obtain such an effect, 15% or more of Nb in steel needs to exist as solute Nb. Preferably it is 20% or more.
On the other hand, when the ratio of Nb existing as solid solution Nb in the steel exceeds 65%, the above effect is saturated, NbC in the steel is reduced, and the action as a diffusible hydrogen trap site is weakened. Delayed fracture resistance decreases. For this reason, the ratio of Nb which exists as solid solution Nb in steel needs to be 65% or less. Preferably it is 60% or less.
In addition, as a presence form of Nb in steel other than solid solution Nb, Nb carbide, Nb carbonitride, etc., such as NbC, are mentioned.

  Next, the manufacturing method of this invention is demonstrated. In addition, the temperature of the steel plate in manufacturing conditions shall mean the surface temperature of a steel plate.

The molten steel having the above component composition is melted by a known method such as a converter or an electric furnace, and is made into a steel material such as a slab having a predetermined size by a known method such as a continuous casting method or ingot-bundling rolling method. . It goes without saying that treatments such as ladle refining and vacuum degassing may be added to the molten steel.
Subsequently, the obtained steel material was immediately or once cooled, heated to a temperature range of (Ts-50) ° C. or higher and (Ts + 200) ° C. or lower, and finished with a finish rolling at a temperature of 850 ° C. or higher. Then, it winds at 650 degrees C or less, and is set as a hot rolled sheet steel.
Ts is defined by the following equation (1).
Ts (° C.) = 6770 / [2.26-log ₁₀ {[% Nb] ×
([% C] +0.86 [% N])}]-273 (1)
Here, [% Nb], [% C] and [% N] indicate the contents (mass%) of Nb, C and N in the steel, respectively.

Heating temperature: (Ts-50) ° C. or more and (Ts + 200) ° C. or less Carbonitride containing coarse Nb crystallized during melting of the steel material does not contribute to high strength of the steel sheet, and hydrogen penetrates during laser welding. It does not work well as a trap site. For this reason, the coarse Nb-based crystallized material is once dissolved in steel in the heating stage before hot rolling, and then again in the process of rolling, cooling, annealing, etc., again with fine Nb carbides and charcoal. It is important to deposit as nitride.
If the heating temperature is less than (Ts-50) ° C., the heating is not sufficient, so that the Nb-based crystallized substance does not sufficiently dissolve in the steel, and the strength after annealing is insufficient. On the other hand, when the temperature exceeds (Ts + 200) ° C., the above effect is saturated, and the Ti crystallized product is completely dissolved, and it becomes difficult to make an appropriate amount of Ti present as a precipitate after annealing. In addition, it is economically disadvantageous because the yield decreases due to an increase in scale-off as the fuel cost for heating increases. Accordingly, the heating temperature is set to (Ts-50) ° C. or higher and (Ts + 200) ° C. or lower. Preferably, it is (Ts−20) ° C. or higher and (Ts + 170) ° C. or lower.

Finish rolling end temperature: 850 ° C. or more When the finish rolling end temperature is less than 850 ° C., not only the rolling efficiency is lowered, but also the rolling load is increased and the load on the rolling mill is increased. For this reason, finish rolling finish temperature shall be 850 degreeC or more.

Winding temperature: 650 ° C. or less When the winding temperature of the hot-rolled steel sheet exceeds 650 ° C., NbC precipitated during winding is excessively coarsened, so that it easily becomes brittle and tends to be a starting point of fracture. For this reason, the coiling temperature of a hot-rolled steel sheet needs to be 650 degrees C or less. Preferably it is 620 degrees C or less. Note that the lower limit of the coiling temperature of the hot-rolled steel sheet does not need to be specified in particular.

  Next, the obtained hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. Here, the conditions for cold rolling need not be specified, but in order to ensure a desired strength after annealing, the total rolling reduction is preferably 30% or more. On the other hand, in order to avoid an excessive load on the rolling mill, the total rolling reduction is preferably 80% or less.

And about the cold-rolled steel plate obtained as mentioned above, continuous annealing is given on the following conditions.
Heating temperature in continuous annealing: 700 ° C. or more and 900 ° C. or less When the heating temperature in continuous annealing is less than 700 ° C., the reverse transformation of austenite becomes insufficient, and the amount of hard martensite or bainite generated during subsequent cooling is insignificant. It becomes sufficient and the desired strength cannot be obtained. On the other hand, when it exceeds 900 ° C., coarsening of austenite grains becomes remarkable, and the hole expansibility of the base metal and the toughness of the weld heat affected zone deteriorate. For this reason, the heating temperature in continuous annealing shall be 700 degreeC or more and 900 degrees C or less. Preferably they are 720 degreeC or more and 880 degrees C or less.

  The holding time after heating is not particularly required, but is preferably held for 15 seconds or more in order to ensure a uniform temperature distribution and a stable microstructure. On the other hand, holding for a long time not only lowers the production efficiency but also causes coarsening of austenite grains, so the holding time is preferably 600 s or less.

Average cooling rate: 5 ° C./s or more and 50 ° C./s or less If the average cooling rate in the cooling process after heating is less than 5 ° C./s, a soft ferrite phase is excessively generated during cooling, and the desired strength is obtained. Not only is it difficult to ensure, but also Nb reprecipitates excessively during cooling, making it difficult to secure a desired amount of solid solution Nb. Further, a coarse ferrite phase or pearlite phase is generated during the cooling, and the strength is lowered. On the other hand, if the average cooling rate after annealing exceeds 50 ° C./s, it becomes difficult to ensure the shape of the steel sheet. For this reason, the average cooling rate after annealing treatment shall be 5 degrees C / s or more and 50 degrees C / s or less. Preferably they are 7 degrees C / s or more and 45 degrees C / s or less.

Cooling stop temperature: 200 ° C. or higher and 450 ° C. or lower If the cooling stop temperature is lower than 200 ° C., the conveyance speed of the steel sheet is extremely reduced, which is not preferable in terms of production efficiency. On the other hand, when the cooling is stopped at a temperature exceeding 450 ° C., not only is a relatively soft bainite phase generated excessively after the cooling is stopped and it becomes difficult to secure a desired strength, but Nb is excessively re-deposited after the cooling is stopped. Therefore, it becomes difficult to secure a desired amount of solid solution Nb. In addition, a soft structure such as ferrite is excessively generated and the strength is insufficient. For this reason, cooling stop temperature shall be 200 degreeC or more and 450 degrees C or less. Preferably they are 230 degreeC or more and 420 degrees C or less.

Holding time in the cooling stop temperature region: 30 s or more and 600 s or less When the holding time in the cooling stop temperature region is less than 30 s, the temperature and the uniformity of the material in the steel sheet are lowered. On the other hand, when the holding time in the cooling stop temperature region exceeds 600 s, the manufacturing efficiency is lowered. For this reason, the holding time in the cooling stop temperature region is set to 30 s or more and 600 s or less.

  Steel having the component composition shown in Table 1 was melted in a converter and then smelted in a ladle and made into a steel slab by continuous casting. Subsequently, the steel slab was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. Thereafter, these hot-rolled steel sheets were subjected to cold rolling and continuous annealing under the conditions shown in Table 2 to obtain cold-rolled steel sheets.

  The cold-rolled steel sheet thus obtained was subjected to (1) precipitate residue analysis, (2) tensile test, and (3) laser welding test in the following manner.

(1) Extraction residue analysis of precipitates Samples for electrolytic extraction were collected from each cold-rolled steel sheet obtained as described above, and AA electrolyte solution (ethanol solution of acetylacetone tetramethylammonium chloride) was used for the test pieces. The residue was extracted by filtration.
The extracted residue was made up to a volume of 100 ml with pure water, and the amount of Ti was measured by high frequency inductively coupled plasma emission spectroscopy. The measured amount of Ti was defined as the amount of Ti present as a precipitate. Similarly, the amount of Nb in the extracted residue was measured, and the amount of solid solution Nb was calculated by subtracting the measured amount of Nb from the total amount of Nb contained in the test piece.
By dividing the Ti amount and solid solution Nb amount present as precipitates thus calculated by the total Ti amount and Nb amount contained in the test piece, respectively, the ratio of Ti present as precipitates in the steel and the steel The ratio of Nb existing as solid solution Nb was determined. These evaluation results are shown in Table 3.

(2) Tensile test A JIS No. 5 tensile test piece was taken from the direction perpendicular to the rolling direction, and the tensile strength (TS) and total elongation (El) were measured according to JIS Z 2241 (2011). These evaluation results are shown in Table 3. Here, it was determined that TS ≧ 980 MPa and El ≧ 13% or more were good.

(3) Laser welding test Using the cold-rolled steel sheet obtained as described above, NdYAG laser as the oscillator, argon as the shielding gas, laser power of 4.5 kW and welding speed of 5.5 m / min. Laser welding was performed. At this time, two identical steel plates of 1.6 mmt × 45 mmw × 90 mmL were butted together and laser welding was performed so that 90 mmL became a weld line.
After the laser welding, an Erichsen test and a cross-sectional test of the laser welded part were performed as follows using the obtained sample.

Erichsen test The sample obtained as described above was stored in a constant temperature and humidity chamber at 25 ° C and 60% humidity immediately after laser welding, and taken out after 24 hours.
Then, an Erichsen test was conducted as a test for delayed fracture characteristics in accordance with JIS Z 2247 (2006). Specifically, the punch was pushed in such a way that the punch was in contact with the center of the weld line of the sample, and the depth of the dent when a crack occurred was measured to obtain an Erichsen value. Here, it was judged that an Erichsen value of 5 mm or more is excellent in delayed fracture resistance of a laser welded portion.

-Sectional test After laser welding, the welded section cut out perpendicular to the weld bead was polished, and then subjected to Nital corrosion to obtain a specimen for hardness measurement.
In accordance with JIS Z 2244 (2009), at a test force of 0.9807 N, at a position 0.4 mm below the steel sheet surface, from one base metal at a 0.25 mm pitch parallel to the steel sheet surface, the weld heat affected zone, weld metal The Vickers hardness test was carried out on the weld heat affected zone and the other base metal, and it was judged that those having a maximum value of Vickers hardness of less than HV500 were excellent in the delayed fracture resistance of the laser weld zone.
These evaluation results are also shown in Table 3.

As shown in Table 3, in all of the inventive examples, the tensile strength of the base material is 980 MPa or more, the Erichsen value is 5 mm or more, and the maximum value of Vickers hardness is less than HV500. Delayed fracture characteristics were obtained. In all the inventive examples, the total elongation of the base material was 13% or more.
On the other hand, in the comparative example, it could not be said that at least one of the tensile strength and total elongation of the base material and the maximum value of Erichsen value and Vickers hardness in the laser welding test was not sufficient.

Claims (3)

% By mass
C: 0.05 to 0.15%,
Si: 0.05-2.0%,
Mn: 1.2 to 3.5%
P: 0.05% or less,
S: 0.005% or less,
Al: 0.01 to 0.1%,
Nb: 0.010 to 0.070%,
Ti: 0.005-0.040% and N: 0.0005-0.0065%
In addition, the mass ratio of Ti and N: Ti / N is 2.5 or more and 7.5 or less, further mixing of B is suppressed to 0.0003% or less, and the balance is made of Fe and inevitable impurities. Having a steel composition,
While 70% or more of Ti in steel exists as precipitates, 15% or more and 65% or less of Nb in steel exists as solute Nb,
A cold-rolled steel sheet excellent in delayed fracture resistance of a laser weld, wherein the tensile strength is 980 MPa or more. The steel sheet is further mass%,
Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
Cu: 1.0% or less,
2. One or more selected from Ni: 1.0% or less and V: 0.1% or less are contained. Cooling excellent in delayed fracture resistance of laser welds according to claim 1 Rolled steel sheet. When the steel material having the steel composition according to claim 1 or 2 is heated to a temperature range of (Ts-50) ° C. or higher and (Ts + 200) ° C. or lower when Ts is set to a temperature represented by the following formula (1), Finishing rolling finish temperature: after hot rolling at 850 ° C. or higher, winding at a temperature of 650 ° C. or lower, then cold rolling, and then heating to a temperature range of 700 ° C. or higher and 900 ° C. or lower, in the course of cooling, by performing the average to a cooling rate of 5 ° C. / s or higher 50 ° C. / s or less at 200 ° C. or higher 450 ° C. or less of the temperature range is cooled, continuous annealing for holding 30s or 600s following time temperature range ,
While 70% or more of Ti in steel is present as precipitates, 15% to 65% of Nb in steel is present as solute Nb, and a cold-rolled steel sheet having a tensile strength of 980 MPa or more. A method for producing a cold-rolled steel sheet excellent in delayed fracture resistance of a laser weld.
Ts (° C.) = 6770 / [2.26-log ₁₀ {[% Nb] ×
([% C] +0.86 [% N])}]-273 (1)
Here, [% Nb], [% C] and [% N] indicate the contents (mass%) of Nb, C and N in the steel, respectively.